Bibliography - Stephen W. Pacala

1. Alvarez, Ramón A., Stephen W. Pacala, James J. Winebrake, W. Chameides, and Steven P. Hamburg, 2012: Greater focus needed on methane leakage from natural gas infrastructure. Proceedings of the National Academy of Sciences of the United States of America, doi:10.1073/pnas.1202407109
   [ Abstract ]

   Natural gas is seen by many as the future of American energy: a fuel that can provide energy independence and reduce greenhouse gas emissions in the process. However, there has also been confusion about the climate implications of increased use of natural gas for electric power and transportation. We propose and illustrate the use of technology warming potentials as a robust and transparent way to compare the cumulative radiative forcing created by alternative technologies fueled by natural gas and oil or coal by using the best available estimates of greenhouse gas emissions from each fuel cycle (i.e., production, transportation and use). We find that a shift to compressed natural gas vehicles from gasoline or diesel vehicles leads to greater radiative forcing of the climate for 80 or 280 yr, respectively, before beginning to produce benefits. Compressed natural gas vehicles could produce climate benefits on all time frames if the well-to-wheels CH₄ leakage were capped at a level 45—70% below current estimates. By contrast, using natural gas instead of coal for electric power plants can reduce radiative forcing immediately, and reducing CH₄ losses from the production and transportation of natural gas would produce even greater benefits. There is a need for the natural gas industry and science community to help obtain better emissions data and for increased efforts to reduce methane leakage in order to minimize the climate footprint of natural gas.

2. Bohlman, S. A., and Stephen W. Pacala, 2012: A forest structure model that determines crown layers and partitions growth and mortality rates for landscape-scale applications of tropical forests. Journal of Ecology, British Ecological Society, 100, doi:10.1111/j.1365-2745.2011.01935.x 508-518
   [ Abstract ]

   1. We present a model to quantify tropical forest structure and explain variance in dynamic rates (growth and mortality) that is computationally simple and can be applied to landscape-scale forest inventory and, potentially, remote sensing-derived data. 2. The model is a modification of the perfect plasticity approximation (PPA) based on tree allometry, tree locations and sizes. The model quantifies crown area index (CAI) (number of crowns per unit ground area) and assigns trees to crown layers, which determines the expected number of crowns above each tree and thus its light environment. 3. The structural model, parameterized and tested for the Barro Colorado Island, Panama 50-ha forest dynamics plot using data from forest inventories and stereo aerial photographs, reproduces most canopy and understorey structural and dynamic properties. The PPA model worked as well or better than a computationally intensive, spatially explicit model. A single allometry for all trees worked equally well as functional group or species allometries. Models of growth and mortality were always improved by adding crown layers as defined by the PPA model. 4. The mean CAI of the 50-ha plot was 3.1 with low variance. The observed variance was lower than when tree locations were randomized, which drastically lowered the variance in tree density per plot, indicating that there are regulating forces towards a small range of crown area indices. 5. Synthesis. A number of simplifying characteristics in structure were uncovered with the PPA structural model applied to a tropical forest: species allometries were not needed despite the high species diversity in the forest; the model worked on a range of plot sizes; and the variance in CAI was surprisingly low, suggesting regulatory mechanisms. The PPA structural model can be used to develop a fully dynamic simulation model for tropical forests. The ability of the simulation model to predict temporal changes in landscape patterns of biomass, dynamic rates, and species and/or functional group composition will provide validation for the partitioning of dynamic rates by crown layers in the PPA structural model.

3. Chisholm, Ryan A., and Stephen W. Pacala, 2011: Theory predicts a rapid transition from niche-structured to neutral biodiversity patterns across a speciation-rate gradient. Theoretical Ecology, Springer, doi:10.1007/s12080-011-0113-5 195-200
   [ Abstract ]

   A central challenge in community ecology is to predict patterns of biodiversity with mechanistic models. The neutral model of biodiversity is a simple model that appears to provide parsimonious and accurate predictions of biodiversity patterns in some ecosystems, even though it ignores processes such as species interactions and niche structure. In a recent paper, we used analytical techniques to reveal why the mean predictions of the neutral model are robust to niche structure in high diversity but not lowdiversity ecosystems. In the present paper, we explore this phenomenon further by generating stochastic simulated data from a spatially implicit hybrid niche-neutral model across different speciation rates. We compare the resulting patterns of species richness and abundance with the patterns expected from a pure neutral and a pure niche model. As the speciation rate in the hybrid model increases, we observe a surprisingly rapid transition from an ecosystem in which diversity is almost entirely governed by niche structure to one in which diversity is statistically indistinguishable from that of the neutral model. Because the transition is rapid, one prediction of our abstract model is that high-diversity ecosystems such as tropical forests can be approximated by one simple model-the neutral model-whereas low-diversity ecosystems such as temperate forests can be approximated by another simple model-the niche model. Ecosystems that require the hybrid model are predicted to be rare, occurring only over a narrow range of speciation rates.

4. Dybzinski, Ray, Caroline Farrior, Adam Wolf, Peter B. Reich, and Stephen W. Pacala, 2011: Evolutionarily Stable Strategy Carbon Allocation to Foliage, Wood, and Fine Roots in Trees Competing for Light and Nitrogen: An Analytically Tractable, Individual-Based Model and Quantitative Comparisons to Data. American Naturalist, 177(2), doi:10.1086/657992 153-166
   [ Abstract ]

   We present a model that scales from the physiological and structural traits of individual trees competing for light and nitrogen across a gradient of soil nitrogen to their community-level consequences. The model predicts the most competitive (i.e., the evolutionarily stable strategy [ESS]) allocations to foliage, wood, and fine roots for canopy and understory stages of trees growing in oldgrowth forests. The ESS allocations, revealed as analytical functions of commonly measured physiological parameters, depend not on simple root-shoot relations but rather on diminishing returns of carbon investment that ensure any alternate strategy will underperform an ESS in monoculture because of the competitive environment that the ESS creates. As such, ESS allocations do not maximize nitrogen-limited growth rates in monoculture, highlighting the underappreciated idea that the most competitive strategy is not necessarily the "best," but rather that which creates conditions in which all others are "worse." Data from 152 stands support the model’s surprising prediction that the dominant structural trade-off is between fine roots and wood, not foliage, suggesting the "root-shoot" trade-off is more precisely a "root-stem" trade-off for long-lived trees. Assuming other resources are abundant, the model predicts that forests are limited by both nitrogen and light, or nearly so.

5. Lichstein, J W., and Stephen W. Pacala, 2011: Local diversity in heterogeneous landscapes: quantitative assessment with a height-structured forest metacommunity model. Theoretical Ecology, Springer, 4, doi:10.1007/s12080-011-0121-5 269-281
   [ Abstract ]

   "Mass effects," in which "sink populations" of locally inferior competitors are maintained by dispersal from "source populations" elsewhere in the landscape, are thought to play an important role in maintaining plant diversity. However, due to the complexity of most quasirealistic forest models, there is little theoretical understanding of the strength of mass effects in forests. Here, we develop a metacommunity version of a mathematically and computationally tractable height-structured forest model, the Perfect Plasticity Approximation, to quantify the strength of mass effects (i.e., the degree of mixing of locally dominant and subordinate species) in heterogeneous landscapes comprising different patch types (e.g., soil types). For realistic levels of inter-patch dispersal, mass effects are weak at equilibrium (i.e., in the absence of disturbance), even in some cases where differences in growth, mortality, and fecundity rates between locally dominant and subordinate species are too small to be reliably detected from field data. However, patch-scale transient dynamics are slow following catastrophic disturbance (in which post-disturbance initial abundances are determined exclusively by immigration) so that at any given time, subordinate species are present in appreciable numbers in most patches. Less severe disturbance regimes, in which some seeds or individuals survive the disturbance, should result in faster transient dynamics (i.e., faster approach to the low-diversity equilibrium). Our results suggest that in order for mass effects to play an important role in tree coexistence, niche differences must be strong enough to prevent neutral drift, yet too weak to be reliably detected from field data.

6. Pan, Yude, R. A. Birdsey, Jingyun Fang, R. A. Houghton, Pekka Kauppi, Oliver L. Phillips, Anatoly Shvidenko, Simon L. Lewis, Josep G. Canadell, Philippe Ciais, Robert B. Jackson, and Stephen W. Pacala, et al., 2011: A Large and Persistent Carbon Sink in the World's Forests. Science, 333, doi:10.1126/science.1201609 988-993
   [ Abstract ]

   The terrestrial carbon sink has been large in recent decades, but its size and location remain uncertain. Using forest inventory data and long-term ecosystem carbon studies, we estimate a total forest sink of 2.4 ± 0.4 petagrams of carbon per year (Pg C year ^-1) globally for 1990 to 2007. We also estimate a source of 1.3 ± 0.7 Pg C year ^-1 from tropical land-use change, consisting of a gross tropical deforestation emission of 2.9 ± 0.5 Pg C year ^-1 partially compensated by a carbon sink in tropical forest regrowth of 1.6 ± 0.5 Pg C year ^-1. Together, the fluxes comprise a net global forest sink of 1.1 ± 0.8 Pg C year ^-1, with tropical estimates having the largest uncertainties. Our total forest sink estimate is equivalent in magnitude to the terrestrial sink deduced from fossil fuel emissions and land-use change sources minus ocean and atmospheric sinks.

7. Chisholm, Ryan A., and Stephen W. Pacala, 2010: Niche and neutral models predict asymptotically equivalent species abundance distributions in high-diversity ecological communities. Proceedings of the National Academy of Sciences of the United States of America, 107(36), doi:10.1073/pnas.1009387107
   [ Abstract ]

   Afundamental challenge inecology is tounderstandthemechanisms that govern patterns of relative species abundance. Previous numerical simulations have suggested that complex niche-structured models produce species abundance distributions (SADs) that are qualitatively similar to those of very simple neutral models that ignore differences between species. However, in the absence of an analytical treatment of niche models, one cannot tell whether the two classes of model produce the same patterns via similar or different mechanisms. We present an analytical proof that, in the limit as diversity becomes large, a strong niche model give rises to exactly the same asymptotic form of SAD as the neutral model, and we verify the analytical predictions for a Panamanian tropical forest data set. Our results strongly suggest that neutral processes drive patterns of relative species abundance in high-diversity ecological communities, even when strong niche structure exists. However, neutral theory cannot explain what generates high diversity in the first place, and it may not be valid in low-diversity communities. Our results also confirm that neutral theory cannot be used to infer an absence of niche structure or to explain ecosystem function.

8. Lichstein, J W., J. Dushoff, Kiona Ogle, Anping Chen, D. W. Purves, J. P. Caspersen, and Stephen W. Pacala, 2010: Unlocking the forest inventory data: relating individual tree performance to unmeasured environmental factors. Ecological Applications, (20(3)), doi:10.1890/08-2334.1 684-699
   [ Abstract ]

   Geographically extensive forest inventories, such as the USDA Forest Service's Forest Inventory and Analysis (FIA) program, contain millions of individual tree growth and mortality records that could be used to develop broad-scale models of forest dynamics. A limitation of inventory data, however, is that individual-level measurements of light (L) and other environmental factors are typically absent. Thus, inventory data alone cannot be used to parameterize mechanistic models of forest dynamics in which individual performance depends on light, water, nutrients, etc. To overcome this limitation, we developed methods to estimate species-specific parameters (θ_G) relating sapling growth (G) to L using data sets in which G, but not L, is observed for each sapling. Our approach involves: (1) using calibration data that we collected in both eastern and western North America to quantify the probability that saplings receive different amounts of light, conditional on covariates x that can be obtained from inventory data (e.g., sapling crown class and neighborhood crowding); and (2) combining these probability distributions with observed G and x to estimate θ_G using Bayesian computational methods. Here, we present a test case using a data set in which G, L, and x were observed for saplings of nine species. This test data set allowed us to compare estimates of θ_G obtained from the standard approach (where G and L are observed for each sapling) to our method (where G and x, but not L, are observed). For all species, estimates of θ_G obtained from analyses with and without observed L were similar. This suggests that our approach should be useful for estimating light-dependent growth functions from inventory data that lack direct measurements of L. Our approach could be extended to estimate parameters relating sapling mortality to L from inventory data, as well as to deal with uncertainty in other resources (e.g., water or nutrients) or environmental factors (e.g., temperature).

9. Pacala, Stephen W., et al., 2010: Verifying Greenhouse Gas Emissions: Methods to Support International Climate Agreements. Committee on Methods for Estimating Greenhouse Gas Emissions; National Research Council, National Academy of Sciences, 124pp.
10. Sarmiento, Jorge, M. N. Gloor, N. Gruber, C. Beaulieu, A. R. Jacobson, S. E. Mikaloff-Fletcher, Stephen W. Pacala, and K. B. Rodgers, 2010: Trends and regional distributions of land and ocean carbon sinks. Biogeosciences, www.biogeosciences.net/7/2351/2010/, 7, doi:10.5194/bg-7-2351-2010 2351-2367
    [ Abstract ]

    We show here an updated estimate of the net land carbon sink (NLS) as a function of time from 1960 to 2007 calculated from the difference between fossil fuel emissions, the observed atmospheric growth rate, and the ocean uptake obtained by recent ocean model simulations forced with reanalysis wind stress and heat and water fluxes. Except for interannual variability, the net land carbon sink appears to have been relatively constant at a mean value of −0.27 PgC yr⁻¹ between 1960 and 1988, at which time it increased abruptly by −0.88 (−0.77 to −1.04) PgC yr⁻¹ to a new relatively constant mean of −1.15 PgC yr⁻¹ between 1989 and 2003/7 (the sign convention is negative out of the atmosphere). This result is detectable at the 99% level using a t-test. The land use source (LU) is relatively constant over this entire time interval. While the LU estimate is highly uncertain, this does imply that most of the change in the net land carbon sink must be due to an abrupt increase in the land sink, LS = NLS – LU, in response to some as yet unknown combination of biogeochemical and climate forcing. A regional synthesis and assessment of the land carbon sources and sinks over the post 1988/1989 period reveals broad agreement that the Northern Hemisphere land is a major sink of atmospheric CO₂, but there remain major discrepancies with regard to the sign and magnitude of the net flux to and from tropical land.

11. Chakravarty, Shoibal, A. Chikkatur, H. de Coninck, Stephen W. Pacala, Robert H. Socolow, and M. Tavoni, 2009: Sharing global CO₂ emission reductions among one billion high emitters. Proceedings of the National Academy of Sciences of the United States of America, 106(29), doi:10.1073/pnas.0905232106 11884-11888
    [ Abstract ]

    We present a framework for allocating a global carbon reduction target among nations, in which the concept of ‘‘common but differentiated responsibilities’’ refers to the emissions of individuals instead of nations. We use the income distribution of a country to estimate how its fossil fuel CO₂ emissions are distributed among its citizens, from which we build up a global CO₂ distribution. We then propose a simple rule to derive a universal cap on global individual emissions and find corresponding limits on national aggregate emissions from this cap. All of the world’s high CO₂ emitting individuals are treated the same, regardless of where they live. Any future global emission goal (target and time frame) can be converted into national reduction targets, which are determined by ‘‘Business as Usual’’ projections of national carbon emissions and incountry income distributions. For example, reducing projected global emissions in 2030 by 13 GtCO₂ would require the engagement of 1.13 billion high emitters, roughly equally distributed in 4 regions: the U.S., the OECD minus the U.S., China, and the nonOECD minus China. We also modify our methodology to place a floor on emissions of the world’s lowest CO₂ emitters and demonstrate that climate mitigation and alleviation of extreme poverty are largely decoupled.

12. Chakravarty, Shoibal, A. Chikkatur, H. de Coninck, Stephen W. Pacala, M. Tavoni, and Robert H. Socolow, 2009: Reply to Grubler and Pachauri: Developing national obligations from individual emissions. Proceedings of the National Academy of Sciences of the United States of America, 106(43 E124), doi:10.1073/pnas.0911102106
13. Gerber, S., L.O. Hedin, Michael Oppenheimer, Stephen W. Pacala, and E. Shevliakova, 2009: Nitrogen Cycling and Feedbacks in a Global Dynamic Land Model. Global Biogeochemical Cycles, http://www.agu.org/journals/pip/gb/2008GB003336-pip.pdf, doi:10.1029/2008GB003336
    [ Abstract ]

    Global anthropogenic changes in carbon (C) and nitrogen (N) cycles call for modeling tools that are able to address and quantify essential interactions between N, C, and climate in terrestrial ecosystems. Here, we introduce a prognostic N cycle within the Princeton-GFDL LM3V land model. The model captures mechanisms essential for N cycling and their feedbacks on C cycling: N limitation of plant productivity, the N dependence of C decomposition and stabilization in soils, removal of available N by competing sinks, ecosystem losses that include dissolved organic and volatile N, and ecosystem inputs through biological N fixation.
    Our model captures many essential characteristics of C-N interactions, and is capable of broadly recreating spatial and temporal variations in N and C dynamics. The introduced N dynamics improves the model’s short term NPP response to step changes in CO₂. Consistent with theories of successional dynamics, we find that physical disturbance induces strong C-N feedbacks, caused by intermittent N loss and subsequent N limitation. In contrast, C-N interactions are weak when the coupled model system approaches equilibrium. Thus, at steady state many simulated features of the carbon cycle, such as primary productivity and carbon inventories are similar to simulations that do not include C-N feedbacks.

14. Lichstein, J W., C. Wirth, H. S. Horn, and Stephen W. Pacala, 2009: Biomass Chronosequences of United States Forests: Implications for Carbon Storage and Forest Management. Old-growth forests, 207, doi:10.1007/978-3-540-92706-8_14 301-341
    [ Abstract ]

    A variety of mechanisms have been identified that may result in late-successional declines in forest biomass, including synchronous mortality of even-aged early-successional cohorts, increased susceptibility of mature forests to wind or insect damage, and, in some systems, reduced stature of late-successional species. We used data from the United States (US) Forest Service’s Forest Inventory and Analysis (FIA) program, and a literature database on old-growth biomass, to quantify late-successional biomass trajectories in different US forest types. Our results suggest that late-successional biomass declines are rare in US forests. Thus, in most cases, there is no conflict between maximizing carbon storage in forest biomass and protecting or restoring old-growth forests.

15. Ogle, Kiona, and Stephen W. Pacala, 2009: A modeling framework for inferring tree growth and allocation from physiological, morphological, and allometric traits. Tree Physiology, 29, doi:10.1093/treephys/tpn051 587-605
    [ Abstract ]

    Predictions of forest succession, diversity and function require an understanding of how species differ in their growth, allocation patterns and susceptibility to mortality. These processes in turn are affected by allometric constraints and the physiological state of the tree, both of which are coupled to the tree’s labile carbon status. Ultimately, insight into the hidden labile pools and the processes affecting the allocation of labile carbon to storage, maintenance and growth will improve our ability to predict tree growth, mortality and forest dynamics. We developed the ‘Allometrically Constrained Growth and Carbon Allocation’ (ACGCA) model that explicitly couples tree growth, mortality, allometries and labile carbon. This coupling results in (1) a semi-mechanistic basis for predicting tree death, (2) an allocation scheme that simultaneously satisfies allometric relationships and physiology- based carbon dynamics and (3) a range of physiological states that are consistent with tree behavior (e.g., healthy, static, shrinking, recovering, recovered and dead). We present the ACGCA model and illustrate aspects of its behavior by conducting simulations under different forest gap dynamics scenarios and with parameter values obtained for two ecologically dissimilar species: loblolly pine (Pinus taeda L.) and red maple (Acer rubrum L.). The model reproduces growth and mortality patterns of these species that are consistent with their shade-tolerance and succession status. The ACGCA framework provides an alternative, and potentially improved, approach for predicting tree growth, mortality and forest dynamics. Keywords: Acer rubrum, carbon allocation, carbon reserves, carbon storage, growth model, labile carbon, loblolly pine, Pinus taeda, red maple, retranslocation, shade-tolerance, succession, tree mortality.

16. Sarmiento, Jorge, M. N. Gloor, N. Gruber, C. Beaulieu, A. R. Jacobson, S. M. Fletcher, Stephen W. Pacala, and K. B. Rodgers, 2009: Trends and Regional Distributions of Land and Ocean Carbon Sink. Biogeosciences, http://www.biogeosciences-discuss.net/6/10583/2009/bgd-6-10583-2009.html, (6), 10583-10624
    [ Abstract ]

    We show here a new estimate of the variability and long-term trends in the net land carbon sink from 1960 onwards calculated from the difference between fossil fuel emissions, the observed atmospheric growth rate, and the ocean uptake obtained by 5 recent ocean model simulations forced with reanalysis wind stress and heat and water fluxes. The net land carbon sink appears to have increased by −0.88 (−0.77 to −1.04) PgCyr−1 after 1988/1989 from a relatively constant mean of −0.27 PgCyr−1 before then to −1.15 PgCyr−1 thereafter (the sign convention is negative out of the atmosphere). This result is significant at the 1% critical level. The increase in net land 10 uptake is partially compensated by a reduction in the expected oceanic uptake, which we estimate from model simulations as about 0.35 (0.26 to 0.49) PgCyr−1. This implies that the atmospheric growth rate must have decreased by about −0.53 (−0.51 to −0.55) PgCyr−1 (equivalent to −0.25 ppm yr−1) below what would have been projected if the ocean uptake had continued to grow at the rate expected from a constant 15 climate model and if the net land uptake had continued at its pre-1988/1989 level. A regional synthesis and assessment of the land carbon sources and sinks over the post 1988/1989 period reveals broad agreement that the northern hemisphere land is a major sink of atmospheric CO2, but there remain major discrepancies with regard to the sign and magnitude of the net flux to and from tropical land.

17. Shevliakova, E., Stephen W. Pacala, S. Malyshev, G. C. Hurtt, P.C.D. Milly, J. P. Caspersen, L. T. Sentman, J. P. Fisk, C. Wirth, and C. Crevoisier, 2009: Carbon Cycling Under 300 Years of Land-Use Change: The Importance of the Secondary Vegetation Sink. Global Biogeochemical Cycles, doi:10.1029/2007GB003176
    [ Abstract ]

    We have developed a dynamic land model (LM3V) able to simulate ecosystem dynamics and exchanges of water, energy, and CO₂ between land and atmosphere. LM3V is specifically designed to address the consequences of land use and land management changes including cropland and pasture dynamics, shifting cultivation, logging, fire, and resulting patterns of secondary regrowth. Here we analyze the behavior of LM3V, forced with the output from the Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric model AM2, observed precipitation data, and four historic scenarios of land use change for 1700–2000. Our analysis suggests a net terrestrial carbon source due to land use activities from 1.1 to 1.3 GtC/a during the 1990s, where the range is due to the difference in the historic cropland distribution. This magnitude is substantially smaller than previous estimates from other models, largely due to our estimates of a secondary vegetation sink of 0.35 to 0.6 GtC/a in the 1990s and decelerating agricultural land clearing since the 1960s. For the 1990s, our estimates for the pastures’ carbon flux vary from a source of 0.37 to a sink of 0.15 GtC/a, and for the croplands our model shows a carbon source of 0.6 to 0.9 GtC/a. Our process-based model suggests a smaller net deforestation source than earlier bookkeeping models because it accounts for decelerated net conversion of primary forest to agriculture and for stronger secondary vegetation regrowth in tropical regions. The overall uncertainty is likely to be higher than the range reported here because of uncertainty in the biomass recovery under changing ambient conditions, including atmospheric CO₂ concentration, nutrients availability, and climate.

18. Smith, D., A. C. Schuerger, M. Davidson, Stephen W. Pacala, C. Bakermans, and T. C. Onstott, 2009: Survivability of Psychrobacter Cryohalolentis K5 Under Simulated Martian Surface Conditions. Astrobiology, 9(2), doi:10.1089/ast.2007.0231 221-228
    [ Abstract ]

    Spacecraft launched to Mars can retain viable terrestrial microorganisms on board that may survive the interplanetary transit. Such biota might compromise the search for life beyond Earth if capable of propagating on Mars. The current study explored the survivability of Psychrobacter cryohalolentis K5, a psychrotolerant microorganism obtained from a Siberian permafrost cryopeg, under simulated martian surface conditions of high ultraviolet irradiation, high desiccation, low temperature, and low atmospheric pressure. First, a desiccation experiment compared the survival of P. cryohalolentis cells embedded, or not embedded, within a medium/salt matrix (MSM) maintained at 25°C for 24 h within a laminar flow hood. Results indicate that the presence of the MSM enhanced survival of the bacterial cells by 1 to 3 orders of magnitude. Second, tests were conducted in a Mars Simulation Chamber to determine the UV tolerance of the microorganism. No viable vegetative cells of P. cryohalolentis were detected after 8 h of exposure to Mars-normal conditions of 4.55 W/m2 UVC irradiation (200–280 nm), −12.5°C, 7.1 mbar, and a Mars gas mix composed of CO2 (95.3%), N2 (2.7%), Ar (1.6%), O2 (0.2%), and H2O (0.03%). Third, an experiment was conducted within the Mars chamber in which total atmospheric opacities were simulated at ô = 0.1 (dust-free CO2 atmosphere at 7.1 mbar), 0.5 (normal clear sky with 0.4 = dust opacity and 0.1 = CO2-only opacity), and 3.5 (global dust storm) to determine the survivability of P. cryohalolentis to partially shielded UVC radiation. The survivability of the bacterium increased with the level of UVC attenuation, though population levels still declined several orders of magnitude compared to UVC-absent controls over an 8 h exposure period.

19. Tilman, D., Robert H. Socolow, J. A. Foley, J. Hill, Eric Larson, L. R. Lynd, Stephen W. Pacala, J. Reilly, Timothy Searchinger, C. Sommerville, and Robert H. Williams, July 2009: Beneficial Biofuels - The Food, Energy, and Environment Trilemma. Science, Washington, D.C., American Association for the Advancement of Science, 325(5938), doi:10.1126/science.1177970 270-271
    [ Abstract ]

    Exploiting multiple feedstocks, under new policies and accounting rules, to balance biofuel production, food security, and greenhouse-gas reduction.
    Dramatic improvements in policy and technology are needed to meet global demand for both food and biofuel feedstocks.

20. Tol, R.S.J., Robert H. Socolow, and Stephen W. Pacala, 2009: Understanding Long-Term Energy Use and Carbon Dioxide Emissions in the USA. Journal of Policy Modeling, 31, doi:10.1016/j.jpolmod.2008.12.002 425-445
    [ Abstract ]

    Energy is at the core of some of the greatest environmental and geopolitical challenges of our time. Cheap and plentiful energy – deemed necessary for our current standard of living – can at the moment only be supported by oil and coal, which pollutes the air, changes the climate, and, in the case of oil and gas, comes from unstable regions. Besides stimulating less polluting energy sources, it is important to improve the overall energy efficiency of the economy through technological, behavioural and other changes. For that, one needs to understand how and why energy use has changed in the past. This paper contributes to that.

21. Purves, D. W., J W. Lichstein, N. Strigul, and Stephen W. Pacala, August 2008: Predicting and understanding forest dynamics using a simple tractable model. Proceedings of the National Academy of Sciences of the United States of America, www.pnas.org cgi doi 10.1073 pnas.0807754105, 105(44), 17018-17022,
    [ Abstract ]

    The perfect-plasticity approximation (PPA) is an analytically tractable model of forest dynamics, defined in terms of parameters for individual trees, including allometry, growth, and mortality. We estimated these parameters for the eight most common species on each of four soil types in the US Lake states (Michigan, Wisconsin, and Minnesota) by using short-term (<15-year) inventory data from individual trees. We implemented 100-year PPA simulations given these parameters and compared these predictions to chronosequences of stand development. Predictions for the timing and magnitude of basal area dynamics and ecological succession on each soil were accurate, and predictions for the diameter distribution of 100-year-old stands were correct in form and slope. For a given species, the PPA provides analytical metrics for early-successional performance (H₂₀, height of a 20-year-old open-grown tree) and late-successional performance (Z^*, equilibrium canopy height in monoculture). These metrics predicted which species were early or late successional on each soil type. Decomposing Z^*, showed that (i) succession is driven both by superior understory performance and superior canopy performance of late-successional species, and (ii) performance differences primarily reflect differences in mortality rather than growth. The predicted late-successional dominants matched chronosequences on xeromesic (Quercus rubra) and mesic (codominance by Acer rubrum and Acer saccharum) soil. On hydromesic and hydric soils, the literature reports that the current dominant species in old stands (Thuja occidentalis) is now failing to regenerate. Consistent with this, the PPA predicted that, on these soils, stands are now succeeding to dominance by other latesuccessional species (e.g., Fraxinus nigra, A. rubrum).

22. Purves, D. W., and Stephen W. Pacala, 2008: Predictive Models of Forest Dynamics. Science, Vol. 320(no. 5882), doi:10.1126/science.1155359 1452 - 1453
    [ Abstract ]

    Dynamic global vegetation models (DGVMs) have shown that forest dynamics could dramatically alter the response of the global climate system to increased atmospheric carbon dioxide over the next century. But there is little agreement between different DGVMs, making forest dynamics one of the greatest sources of uncertainty in predicting future climate. DGVM predictions could be strengthened by integrating the ecological realities of biodiversity and height-structured competition for light, facilitated by recent advances in the mathematics of forest modeling, ecological understanding of diverse forest communities, and the availability of forest inventory data.

23. Strigul, N., D. Pristinski, D. W. Purves, J. Dushoff, and Stephen W. Pacala, 2008: Scaling from Trees to Forests: Tractable Macroscopic Equations for Forest Dynamics. Ecological Monographs, 78(4), doi:10.1890/08-0082.1 523-525
    [ Abstract ]

    Individual-based forest simulators, such as TASS and SORTIE, are spatial stochastic processes that predict properties of populations and communities by simulating the fate of every plant throughout its life cycle. Although they are used for forest management and are able to predict dynamics of real forests, they are also analytically intractable, which limits their usefulness to basic scientists. We have developed a new spatial individual-based forest model that includes a perfect plasticity formulation for crown shape. Its structure allows us to derive an accurate approximation for the individual-based model that predicts mean densities and size structures using the same parameter values and functional forms, and also it is analytically tractable. The approximation is represented by a system of von Foerster partial differential equations coupled with an integral equation that we call the perfect plasticity approximation (PPA). We have derived a series of analytical results including equilibrium abundances for trees of different crown shapes, stability conditions, transient behaviors, such as the constant yield law and self-thinning exponents, and two species coexistence conditions.

24. Crevoisier, C., E. Shevliakova, M. N. Gloor, C. Wirth, and Stephen W. Pacala, 2007: Drivers of fires in the boreal forests: data constrained design of a prognostic model for burned area for use in dynamic global vegetation models. Journal of Geophysical Research, 112(D24112), doi:10.1029/2006JD008372
    [ Abstract ]

    Boreal regions are an important component of the global carbon cycle because they host large stocks of aboveground and belowground carbon. Since boreal forest evolution is closely related to fire regimes, shifts in climate are likely to induce changes in ecosystems, potentially leading to a large release of carbon and other trace gases to the atmosphere. Prediction of the effect of this potential climate feedback on the Earth system is therefore important and requires the modeling of fire as a climate driven process in dynamic global vegetation models (DGVMs). Here, we develop a new data-based prognostic model, for use in DGVMs, to estimate monthly burned area from four climate (precipitation, temperature, soil water content and relative humidity) and one humanrelated (road density) predictors for boreal forest. The burned area model is a function of current climatic conditions and is thus responsive to climate change. Model parameters are estimated using a Markov Chain Monte Carlo method applied to on ground observations from the Canadian Large Fire Database. The model is validated against independent observations from three boreal regions: Canada, Alaska and Siberia. Provided realistic climate predictors, the model is able to reproduce the seasonality, intensity and interannual variability of burned area, as well as the location of fire events. In particular, the model simulates well the timing of burning events, with two thirds of the events predicted for the correct month and almost all the rest being predicted 1 month before or after the observed event. The predicted annual burned area is in the range of various current estimates. The estimated annual relative error (standard deviation) is twelve percent in a grid cell, which makes the model suitable to study quantitatively the evolution of burned area with climate.

25. Lichstein, J W., J. Dushoff, S. A. Levin, and Stephen W. Pacala, 2007: Intraspecific variation and species coexistence. American Naturalist, 170(6), doi:10.1086/522937 807-818
    [ Abstract ]

    We use a two-species model of plant competition to explore the effect of intraspecific variation on community dynamics. The competitive ability (“performance”) of each individual is assigned by an independent random draw from a species-specific probability distribution. If the density of individuals competing for open space is high (e.g., because fecundity is high), species with high maximum (or large variance in) performance are favored, while if density is low, species with high typical (e.g., mean) performance are favored. If there is an interspecific mean-variance performance trade-off, stable coexistence can occur across a limited range of intermediate densities, but the stabilizing effect of this trade-off appears to be weak. In the absence of this trade-off, one species is superior. In this case, intraspecific variation can blur interspecific differences (i.e., shift the dynamics toward what would be expected in the neutral case), but the strength of this effect diminishes as competitor density increases. If density is sufficiently high, the inferior species is driven to extinction just as rapidly as in the case where there is no overlap in performance between species. Intraspecific variation can facilitate coexistence, but this may be relatively unimportant in maintaining diversity in most real communities.

26. Pacala, Stephen W., et al., 2007: The North American Carbon Budget Past and Present. The First State of the Carbon Cycle Report (SOCCR), http://www.treesearch.fs.fed.us/pubs/13120,
    [ Abstract ]

    North America is currently a net source of carbon dioxide to the atmosphere, contributing to the global buildup of greenhouse gases in the atmosphere and associated changes in the Earth’s climate. In 2003, North America emitted nearly two billion metric tons of carbon to the atmosphere as carbon dioxide. North America’s fossil-fuel emissions in 2003 (1856 million metric tons of carbon ± 10% with 95% certainty) were 27% of global emissions. Approximately 85% of those emissions were from the United States, 9% from Canada, and 6% from Mexico. The combustion of fossil fuels for commercial energy (primarily electricity) is the single largest contributor, accounting for approximately 42% of North American fossil emissions in 2003. Transportation is the second largest, accounting for 31% of total emissions. There are also globally important carbon sinks in North America. In 2003, growing vegetation in North America removed approximately 500 million tons of carbon per year (± 50%) from the atmosphere and stored it as plant material and soil organic matter. This land sink is equivalent to approximately 30% of the fossil-fuel emissions from North America. The imbalance between the fossil-fuel source and the sink on land is a net release to the atmosphere of 1350 million metric tons of carbon per year (± 25%). Approximately 50% of North America’s terrestrial sink is due to the regrowth of forests in the United States on former agricultural land that was last cultivated decades ago, and on timberland recovering from harvest. Other sinks are relatively small and not well quantified with uncertainties of 100% or more. The future of the North American terrestrial sink is also highly uncertain. The contribution of forest regrowth is expected to decline as the maturing forests grow more slowly and take up less carbon dioxide from the atmosphere. But, how regrowing forests and other sinks will respond to changes in climate and carbon dioxide concentration in the atmosphere is highly uncertain. The large difference between current sources and sinks and the expectation that the difference could become larger if the growth of fossil-fuel emissions continues and land sinks decline suggest that addressing imbalances in the North American carbon budget will likely require actions focused on reducing fossil-fuel emissions. Options to enhance sinks (growing forests or sequestering carbon in agricultural soils) can contribute, but enhancing sinks alone is likely insufficient to deal with either the current or future imbalance. Options to reduce emissions include efficiency improvement, fuel switching, and technologies such as carbon capture and geological storage. Implementing these options will likely require an array of policy instruments at local, regional, national, and international levels, ranging from the encouragement of voluntary actions to economic incentives, tradable emissions permits, and regulations. Meeting the demand for information by decision makers will likely require new modes of research characterized by close collaboration between scientists and carbon management stakeholders.

27. Purves, D. W., J W. Lichstein, and Stephen W. Pacala, 2007: Crown Plasticity and Competition for Canopy Space: A New Spatially Implicit Model Parameterized for 250 North American Tree Species. PLOS one, (9), doi:10.1371/journal.pone.0000870
    [ Abstract ]

    Background. Canopy structure, which can be defined as the sum of the sizes, shapes and relative placements of the tree crowns in a forest stand, is central to all aspects of forest ecology. But there is no accepted method for deriving canopy structure from the sizes, species and biomechanical properties of the individual trees in a stand. Any such method must capture the fact that trees are highly plastic in their growth, forming tessellating crown shapes that fill all or most of the canopy space. Methodology/Principal Findings. We introduce a new, simple and rapidly-implemented model–the Ideal Tree Distribution, ITD–with tree form (height allometry and crown shape), growth plasticity, and space-filling, at its core. The ITD predicts the canopy status (in or out of canopy), crown depth, and total and exposed crown area of the trees in a stand, given their species, sizes and potential crown shapes. We use maximum likelihood methods, in conjunction with data from over 100,000 trees taken from forests across the coterminous US, to estimate ITD model parameters for 250 North American tree species. With only two free parameters per species–one aggregate parameter to describe crown shape, and one parameter to set the so-called depth bias–the model captures between-species patterns in average canopy status, crown radius, and crown depth, and within-species means of these metrics vs stem diameter. The model also predicts much of the variation in these metrics for a tree of a given species and size, resulting solely from deterministic responses to variation in stand structure. Conclusions/Significance. This new model, with parameters for US tree species, opens up new possibilities for understanding and modeling forest dynamics at local and regional scales, and may provide a new way to interpret remote sensing data of forest canopies, including LIDAR and aerial photography.

28. Hurtt, G. C., S. Frolking, M. G. Fearon, B. Moore III, E. Shevliakova, S. Malyshev, Stephen W. Pacala, and R. A. Houghton, 2006: The underpinnings of land-use history: three centuries of global gridded landuse transitions, wood-harvest activity, and resulting secondary lands. Global Change Biology, 12(7), doi:10.1111/j.1365-2486.2006.01150.x 1208-1299
    [ Abstract ]

    To accurately assess the impacts of human land use on the Earth system, information is needed on the current and historical patterns of land-use activities. Previous global studies have focused on developing reconstructions of the spatial patterns of agriculture. Here, we provide the first global gridded estimates of the underlying land conversions (land-use transitions), wood harvesting, and resulting secondary lands annually, for the period 1700–2000. Using data-based historical cases, our results suggest that 42–68% of the land surface was impacted by land-use activities (crop, pasture, wood harvest) during this period, some multiple times. Secondary land area increased 10–44 X 10⁶km²; about half of this was forested. Wood harvest and shifting cultivation generated 70–90% of the secondary land by 2000; permanent abandonment and relocation of agricultural land accounted for the rest. This study provides important new estimates of globally gridded land-use activities for studies attempting to assess the consequences of anthropogenic changes to the Earth’s surface over time.

29. Moorcroft, P. R., Stephen W. Pacala, and M. A. Lewis, 2006: Potential role of natural enemies during tree range expansions following climate change. Journal of Theoretical Biology, 241(3), doi:10.1016/j.jtbi.2005.12.019 601-616
    [ Abstract ]

    Recent investigations have shown how chance, long-range dispersal events can allow tree populations to migrate rapidly in response to changes in climate. However, this apparent solution to Reid’s paradox applies solely within the context of single species models, while the rapid migration rates seen in pollen records occurred within multispecies communities. Ecologists are therefore presented with a new challenge: reconciling the macroscopic dynamics of spread seen in the pollen record with the rules and interactions governing plant community assembly. A case that highlights this issue is the rapid spread of Beech during the Holocene into a landscape already dominated by a close competitor, Hemlock. In this study, we analyse a simple model of plant community assembly incorporating competition for space and dispersal dynamics, showing how, even when a species is capable of rapid migration into an empty landscape, the presence of an ecologically similar competitor causes Reid’s paradox to re-emerge because of the dramatic slowing effect of competitive interactions on a species’ rate of spread. We then show how the answer to the question of how tree species dispersed rapidly into occupied landscapes may lie in secondary interactions with host-specific pathogens and parasites. Inclusion of host-specific pathogens into the simple community assembly model illustrates how tree species undergoing range expansions can temporarily outstrip specialist predators, giving rise to a transient Jansen–Connell effect, in which the invader acts as temporary ‘super-species’ that spreads rapidly into communities already occupied by competitors at rates consistent with those observed in the paleo-record.

30. Socolow, Robert H., and Stephen W. Pacala, 2006: A Plan to Keep Carbon in Check. Scientific American, http://www.scientificamerican.com/carbon/0906050.pdf, 295(3), 50-57
    [ Abstract ]

    Humanity can emit only so much carbon dioxide into the atmosphere before the climate enters a state unknown in recent geologic history and goes haywire. Climate scientists typically see the risks growing rapidly as CO₂ levels approach a doubling of their pre-18thcentury value. To make the problem manageable, the required reduction in emissions can be broken down into “wedges”—an incremental reduction of a size that matches available technology.

31. Tol, R.S.J., Robert H. Socolow, and Stephen W. Pacala, 2006: Understanding Long-Term Energy Use and Carbon Dioxide Emissions in the USA. Fondazione Eni Enrico Mattei Working Papers, doi:10.1016/j.jpolmod.2008.12.002
    [ Abstract ]

    We compile a database of energy uses, energy sources, and carbon dioxide emissions for the USA for the period 1850-2002. We use a model to extrapolate the missing observations on energy use by sector. Overall emission intensity rose between 1850 and 1917, and fell between 1917 and 2002. The leading cause for the rise in emission intensity was the switch from wood to coal, but population growth, economic growth, and electrification contributed as well. After 1917, population growth, economic growth and electrification pushed emissions up further, and there was no net shift from fossil to non-fossil energy sources. From 1850 to 2002, emissions were reduced by technological and behavioural change (particularly in transport, manufacturing and households), structural change in the economy, and a shift from coal to oil and gas. These trends are stronger than electrification, explaining the fall in emissions relative to GDP.

32. Livnat, A., Stephen W. Pacala, and S. A. Levin, 2005: The Evolution of Intergenerational Discounting in Offspring Quality. American Naturalist, http://www.journals.uchicago.edu/doi/pdf/10.1086/428294?cookieSet=1, 165(3), 311-321
    [ Abstract ]

    Intergenerational effects occur when an individual’s actions affect not only its own survivorship and reproduction but also those of its offspring and possibly later descendants. In the presence of intergenerational effects, short-term and long-term measures of success (such as the expected numbers of surviving offspring and of farther descendants, respectively) may be in conflict. When such conflicts occur, life-history theory normally takes long-termmeasures to predict the outcome of selection. This ignores the fact that, because traits change in time—through mutation, sex, and recombination— long-term relations disintegrate. We study this issue with numerical simulations and analytical models combining intergenerational effects and evolutionary change. In the models, the parental investment per offspring, as well as the total reproductive effort, stand for investments in future generations. The models show that the rate of evolutionary change determines the level of those investments. Higher rates of mutation and of sexual as opposed to parthenogenetic reproduction favor lower parental investment per offspring and lower total reproductive effort. It follows that the level of investment of ancestors in descendants responds to the genetic relatedness between the generations of the lineage, in a manner unaccounted for by preexisting theory.

33. Purves, D. W., and Stephen W. Pacala, 2005: Ecological drift in niche-structured communities: neutral pattern does not imply neutral process. Biotic Interactions in the Tropics, Chapter 5, 107-138
    [ Abstract ]

    We can define a neutral community as one in which all species, and so all individuals, are equivalent, in the sense that they are interchangeable at all times and under all conditions. In contrast, we can define a structured community as one in which species are not equivalent, and species-specified differences affect the population dynamics, and therefore the behaviour of the community. This distinction is an important one, because in a neutral community the biodiversity, as measured by species richness and abundance patterns, has nothing to do with the biogeochemical functioning of the community (e.g. carbon fixation and nutrient-cycling). In fact, in a truly neutral community one could eliminate all but one species without affecting the biogeochemical functioning of the community at all. In contrast, much of the species-specific variation in biological traits observed in reality has direct relevance for the functioning of the community. For example, the short-term carbon uptake of a forest depends on the growth rates of the individual trees, and the long-term carbon storage depends on adult life-span and wood density, and there is wide species-specific variation in these traits. In niche-structured communities, the biodiversity and functioning are intimately linked, and some combination of at least some species is required to maintain the functioning of the community. In the most highly structured community possible there is no equivalence between any of the species, which is the so-called “one species one niche” idea so prevalent in the history of ecology: in such a community, removing just one species has a significant impact on the dynamics and functioning of the community. Which of these two pictures of communities – neutral or structured – is nearer to the truth? Is it “one species one niche” or “all species one niche”? This is the neutral vs structure debate, and it continues apace because there is good evidence for both sides of the argument.

34. Sandin, S. A., and Stephen W. Pacala, 2005: Demographic theory of coral reef fish populations with stochastic recruitment: comparing sources of population regulation. American Naturalist, 165(1), doi:10.1086/426674 107-119
    [ Abstract ]

    The effects of three forms of density-dependent regulation were explored in model coral reef fish populations: top-down (predation), bottom-up (competition for food), and pelagic (non-reefbased mechanisms) control. We describe the demographic responses of both biomass and numbers of adult fish, predicting the mean and the variance of temporal fluctuations resulting from stochastic recruitment of juveniles. We find that top-down control acts by suppressing variability of numbers of fish, which in turn suppresses the variability of biomass. Bottom-up control has no effect on fluctuations of numbers of fish, though it strongly reduces fluctuations of biomass. Because fecundity of fish is directly linked to body mass, the regulation of biomass tightly regulates reproductive output independently of the number of individuals in the population. Finally, populations under pelagic control experience bounded fluctuations of biomass and numbers directly proportional to the bounded fluctuations of recruitment. The demographic signatures predicted from both bottom-up and pelagic control are consistent with current evidence supporting the recruitment limitation hypothesis in reef fish ecology. We propose tests to discriminate the dominant mode of density-dependent regulation using qualitative trends in time series demographic data across environmental clines.

35. Sandin, S. A., and Stephen W. Pacala, 2005: Fish aggregation results in inversely density dependent predation on continuous coral reefs. Ecology, 86(6), doi:10.1890/03-0654 1520-1530
    [ Abstract ]

    Spatially density-dependent predation is a leading hypothesis describing mechanisms of population regulation in coral reef fish. However, studies supporting this hypothesis predominantly have been conducted on small, isolated patch reefs. Here, we searched for evidence of spatially density-dependent predation on the continuous reefs of the Netherlands Antilles in a study of a dominant planktivore, the blue chromis (Chromis cyanea). Across space, we quantified both the patterns of loss from site-attached aggregations of C. cyanea through time and the behavioral reaction of predators to these aggregations. Looking across C. cyanea densities, we found that loss from aggregations was not characteristic of direct density dependence, but instead was commonly inversely related to density. Individual C. cyanea in larger aggregations were less likely to be lost from the group than were individuals in smaller aggregations. Thus, the observed density dependence increased spatial heterogeneity of C. cyanea. Predators showed behaviors that were consistent with these demographic patterns. Using remote videography, we quantified predator visitation and strike rates across a range of C. cyanea aggregation sizes. Predators consistently visited and struck at individuals in C. cyanea aggregations in a pattern that was strongly inversely density dependent, suggesting that aggregation is an effective means of minimizing per capita risk of predation for prey reef fish. Differences in spatial distribution of resources for predators (i.e., prey fish) between continuous and patch reef habitats may explain the difference between these results and those of previous studies on patch reefs.

36. Baidya Roy, S., Stephen W. Pacala, and R. L. Walko, 2004: Can large wind farms affect local meteorology? Journal of Geophysical Research, 109(D19101), doi:10.1029/2004JD004763
    [ Abstract ]

    The RAMS model was used to explore the possible impacts of a large wind farm in the Great Plains region on the local meteorology over synoptic timescales under typical summertime conditions. A wind turbine was approximated as a sink of energy and source of turbulence. The wind farm was created by assuming an array of such turbines. Results show that the wind farm significantly slows down the wind at the turbine hub-height level. Additionally, turbulence generated by rotors create eddies that can enhance vertical mixing of momentum, heat, and scalars, usually leading to a warming and drying of the surface air and reduced surface sensible heat flux. This effect is most intense in the early morning hours when the boundary layer is stably stratified and the hub-height level wind speed is the strongest due to the nocturnal low-level jet. The impact on evapotranspiration is small.

37. Hurtt, G. C., R. Dubayah, J. Drake, P. R. Moorcroft, Stephen W. Pacala, J. B. Blair, and M. G. Fearon, 2004: Beyond Potential Vegetation: Combining Lidar Data and a Height Structured Model for Carbon Studies. Ecological Applications, 14(3), doi:10.1890/02-5317 873-883
    [ Abstract ]

    Carbon estimates from terrestrial ecosystem models are limited by large uncertainties in the current state of the land surface. Natural and anthropogenic disturbances have important and lasting influences on ecosystem structure and fluxes that can be difficult to detect or assess with conventional methods. In this study, we combined two recent advances in remote sensing and ecosystem modeling to improve model carbon stock and flux estimates at a tropical forest study site at La Selva, Costa Rica (10°25' N, 84°00' W). Airborne lidar remote sensing was used to measure spatial heterogeneity in the vertical structure of vegetation. The ecosystem demography model (ED) was used to estimate the consequences of this heterogeneity for regional estimates of carbon stocks and fluxes. Lidar data provided substantial constraints on model estimates of both carbon stocks and net carbon fluxes. Lidar-initialized ED estimates of aboveground biomass were within 1.2% of regression-based approaches, and corresponding model estimates of net carbon fluxes differed substantially from bracketing alternatives. The results of this study provide a promising illustration of the power of combining lidar data on vegetation height with a heightstructured ecosystem model. Extending these analyses to larger scales will require the development of regional and global lidar data sets, and the continued development and application of height structured ecosystem models.

38. Keith, D. W., J. F. De Carolis, D. C. Denkenberger, D. H. Lenschow, S. Malyshev, Stephen W. Pacala, and P. J. Rasch, 2004: The Influence of Large-scale Wind-power on Global Climate. Proceedings of the National Academy of Sciences of the United States of America, 101(46), doi:10.1073/pnas.0406930101 16115-16120
    [ Abstract ]

    Large-scale use of wind power can alter local and global climate by extracting kinetic energy and altering turbulent transport in the atmospheric boundary layer. We report climate-model simulations that address the possible climatic impacts of wind power at regional to global scales by using two general circulation models and several parameterizations of the interaction of wind turbines with the boundary layer. We find that very large amounts of wind power can produce nonnegligible climatic change at continental scales. Although large-scale effects are observed, wind power has a negligible effect on global-mean surface temperature, and it would deliver enormous global benefits by reducing emissions of CO₂ and air pollutants. Our results may enable a comparison between the climate impacts due to wind power and the reduction in climatic impacts achieved by the substitution of wind for fossil fuels.

39. Pacala, Stephen W., and J. P. Caspersen, et al., 2004: Forest Inventory Data Falsify Ecosystem Models of CO₂ Fertilization. ,
    [ Abstract ]

    We analyze tree growth data from Wisconsin forest inventories completed in 1968, 1983, 1996 and 2002. These show that the rate of forest growth decreased steadily over the period, in contrast to the increases predicted by CO₂ fertilization models. Measured growth rate changed an average of -0.27% y^-1 (95% confidence range: -0.05% to -0.49% y^-1), whereas the prediction for CO₂ fertilization is 0.16% y^-1 (corresponding to a ß of 0.36). The high statistical precision is due both to large sample sizes and positive correlations among the growth rates from different time periods within the same plot. Decreased growth occurred in stands of all ages, and so our results are not caused by age-related declines in growth (although highly significant age-related declines were also detected).
    Data allowing a direct examination of growth rates over several decades are available only for Wisconsin, but Caspersen et al. (2000) introduced an indirect method for detecting past changes in growth rate using only two sequential inventories. This method was criticized by Joos et al. (2002), who claimed that it lacked the statistical power to falsify state-of–the-art ecosystem models of CO₂ fertilization. We explain both the sound points and the critical errors in Joos et al.’s argument, introduce a transparent and analytically tractable version of Caspersen et al.’s method, and check its ability to detect the decreasing growth rates in the Wisconsin data. The results show that the indirect method accurately characterizes the past changes that actually occurred, and has sufficient statistical power to falsify CO₂ fertilization models, including the model in Joos et al. (2002).
    We discuss the implications of decreasing Wisconsin growth rates, together with other reasons for skepticism about the future magnitude of CO₂ fertilization. In particular, the steep reductions in fossil fuel emissions required to stabilize atmospheric CO₂ at 500 ppm must begin more than a decade sooner if the predictions of the CO₂ fertilization models in the IPCC Third Assessment (Prentice et al. 2001) are incorrect. The difference between a terrestrial carbon sink that grows because of CO₂ fertilization, and one that shrinks because it is caused by recovery from past land use, is the difference between the luxury of a substantial delay and the need to act now.

40. Pacala, Stephen W., and Robert H. Socolow, 2004: Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies. Science, 305, doi:10.1126/science.1100103 968-972
    [ Abstract ]

    Humanity already possesses the fundamental scientific, technical, and industrial know-how to solve the carbon and climate problem for the next half-century. A portfolio of technologies now exists to meet the world’s energy needs over the next 50years and limit atmospheric CO₂ to a trajectory that avoids a doubling of the preindustrial concentration. Every element in this portfolio has passed beyond the laboratory bench and demonstration project; many are already implemented somewhere at full industrial scale. Although no element is a credible candidate for doing the entire job (or even half the job) by itself, the portfolio as a whole is large enough that not every element has to be used. The debate in the current literature about stabilizing atmospheric CO₂ at less than a doubling of the preindustrial concentration has led to needless confusion about current options for mitigation. On one side, the Intergovernmental Panel on Climate Change (IPCC) has claimed that “technologies that exist in operation or pilot stage today” are sufficient to follow a less-thandoubling trajectory “over the next hundred years or more” [(1), p. 8]. On the other side, a recent review in Science asserts that the IPCC claim demonstrates “misperceptions of technological readiness” and calls for “revolutionary changes” in mitigation technology, such as fusion, space-based solar electricity, and artificial photosynthesis (2). We agree that fundamental research is vital to develop the revolutionary mitigation strategies needed in the second half of this century and beyond. But it is important not to become beguiled by the possibility of revolutionary technology. Humanity can solve the carbon and climate problem in the first half of this century simply by scaling up what we already know how to do. 13

41. Purves, D. W., J. P. Caspersen, P. R. Moorcroft, G. C. Hurtt, and Stephen W. Pacala, 2004: Human-induced Changes in U.S. Biogenic Volatile Organic Compound Emissions: evidence from long-term forest inventory data. Global Change Biology, 10(10), doi:10.1111/j.1365-2486.2004.00844.x 1737-1755
    [ Abstract ]

    Volatile organic compounds (VOCs) emitted by woody vegetation influence global climate forcing and the formation of tropospheric ozone. We use data from over 250 000 re-surveyed forest plots in the eastern US to estimate emission rates for the two most important biogenic VOCs (isoprene and monoterpenes) in the 1980s and 1990s, and then compare these estimates to give a decadal change in emission rate. Over much of the region, particularly the southeast, we estimate that there were large changes in biogenic VOC emissions: half of the grid cells (1° X 1°) had decadal changes in emission rate outside the range -2.3% to +16.8% for isoprene, and outside the range 0.2–17.1% for monoterpenes. For an average grid cell the estimated decadal change in heatwave biogenic VOC emissions (usually an increase) was three times greater than the decadal change in heatwave anthropogenic VOC emissions (usually a decrease, caused by legislation). Leaf-area increases in forests, caused by anthropogenic disturbance, were the most important process increasing biogenic VOC emissions. However, in the southeast, which had the largest estimated changes, there were substantial effects of ecological succession (which decreased monoterpene emissions and had location-specific effects on isoprene emissions), harvesting (which decreased monoterpene emissions and increased isoprene emissions) and plantation management (which increased isoprene emissions, and decreased monoterpene emissions in some states but increased monoterpene emissions in others). In any given region, changes in a very few tree species caused most of the changes in emissions: the rapid changes in the southeast were caused almost entirely by increases in sweetgum (Liquidambar styraciflua) and a few pine species. Therefore, in these regions, a more detailed ecological understanding of just a few species could greatly improve our understanding of the relationship between natural ecological processes, forest management, and biogenic VOC emissions.

42. Sandin, S. A., and Stephen W. Pacala, 2004: Regulation in populations of coral reef fish: an exploration of models and data. Proceedings of the Ninth International Coral Reef Symposium, Bali, Indonesia, http://cmbc.ucsd.edu/content/1/docs/Sandin_Pacala_final_9ICRS.pdf, 455-462
    [ Abstract ]

    The study of population regulation in reef fish populations is confounded by large amounts of stochasticity obscuring patterns in the field. We analyze a series of population models, comparing equilibrial solutions under conditions of top-down and bottom-up regulation. We also treat patterns of population variance that will be expected as recruitment variance is propagated through to adult populations. We find that predators affect reef fish populations by absorbing recruitment variance across space and through time. Food limitation will instead reduce fluctuations of adult fish biomass through time. Our model suggests that the study of regulation cannot be conducted through counting fish alone, but requires measurement of biomass simultaneously. By surveying fish across natural and human-created clines of recruitment and mortality, we can focus fieldwork on testing focused predictions of regulation on coral reefs.

43. Socolow, Robert H., R. Hotinski, J. B. Greenblatt, and Stephen W. Pacala, December 2004: Solving the Climate Problem: Technologies Available to Curb CO₂ Emissions. Environment, http://www.princeton.edu/~cmi/resources/CMI_Resources_new_files/Environ_08-21a.pdf, 46(10), 8-19
    [ Abstract ]

    The atmosphere’s concentration of carbon dioxide (CO₂) has increased by more than 30 percent over the last 250 years, largely due to human activity. Two-thirds of that rise has occurred in the past 50 years.1 Unless there is a change, the world will see much higher CO₂ levels in the future—levels that are predicted to lead to damaging climate change. Fortunately, many carbon mitigation strategies are available to set the world on a new path, one that leads to a lower rate of CO₂ emissions than is currently expected. The environmental community is currently playing a prominent role in the development of the CO₂ policies that will elicit these strategies. Until a few years ago, the environmental community was almost exclusively interested in policies that promote renewable energy, conservation, and natural sinks. More recently, it has begun to explore alliances with traditional energy supply industries on the grounds that to establish the pace required to achieve environmental goals, parallel action on many fronts is required.

44. Socolow, Robert H., Stephen W. Pacala, and J. B. Greenblatt, September 2004: Wedges: Early Mitigation with Familiar Technology. Proceedings of the 7th International Conference on Greenhouse Gas Control Technologies, (GHGT-7), http://www.princeton.edu/~cmi/research/Vancouver04/socolow%20poster.pdf,
    [ Abstract ]

    If the world is willing to accept a tripling of the pre-industrial atmospheric CO₂ concentration, significant carbon mitigation can be delayed for most of the next half century. If the world is to be put on a path to avoid a doubling, however, a monumental mitigation effort needs to start now. To convey the magnitude of the effort, we introduce the “wedge” as the unit of mitigation: a wedge is an activity that creates 1 GtC/y of carbon emission reductions in 2054, relative to a world unconcerned about global carbon emissions. To pursue 500 ppm stabilization, the task for the next 50 years is to achieve about seven wedges by avoiding about 175 billion tons of carbon emissions.

45. Baidya Roy, S., G. C. Hurtt, C. P. Weaver, and Stephen W. Pacala, 2003: Impact of historical land cover change on the July climate of the United States. Journal of Geophysical Research, 108(D24), doi:10.1029/2003JD003565
    [ Abstract ]

    We used the Regional Atmospheric Modeling System (RAMS) model to investigate the possible impact of land cover change on the July climate of the coterminous United States over the last 290 years. Vegetation data were estimated using the Ecosystem Demography model. The observed change in land cover leads to a weak warming along the Atlantic coast and a strong cooling of more than 1 K over the Midwest and the Great Plains region. The precipitation signal is weaker and shows some reduction in the Midwest because of changes in the patterns of large-scale moisture advection.

46. Bolker, B. M., Stephen W. Pacala, and C. Neuhauser, 2003: Spatial dynamics in model plant communities: what do we really know? American Naturalist, 162(2), doi:10.1086/376575 135-148
    [ Abstract ]

    A variety of models have shown that spatial dynamics and small-scale endogenous heterogeneity (e.g., forest gaps or local resource depletion zones) can change the rate and outcome of competition in communities of plants or other sessile organisms. However, the theory appears complicated and hard to connect to real systems. We synthesize results from three different kinds of models: interacting particle systems, moment equations for spatial point processes, and metapopulation or patch models. Studies using all three frameworks agree that spatial dynamics need not enhance coexistence nor slow down dynamics; their effects depend on the underlying competitive interactions in the community. When similar species would coexist in a nonspatial habitat, endogenous spatial structure inhibits coexistence and slows dynamics. When a dominant species disperses poorly and the weaker species has higher fecundity or better dispersal, competition-colonization tradeoffs enhance coexistence. Even when species have equal dispersal and per-generation fecundity, spatial successional niches where the weaker and faster-growing species can rapidly exploit ephemeral local resources can enhance coexistence. When interspecific competition is strong, spatial dynamics reduce founder control at large scales and short dispersal becomes advantageous. We describe a series of empirical tests to detect and distinguish among the suggested scenarios.

47. Levin, S. A., and Stephen W. Pacala, 2003: Ecosystem Dynamics. Handbook of Environmental Economics, North Holland, Amsterdam, Elsevier, 1, doi:10.1016/S1574-0099(03)01007-6 61-95
    [ Abstract ]

    From ecosystems we derive food and fiber, fuel and pharmaceuticals. Ecosystems mediate local and regional climates, stabilize soils, purify water, and in general provide a nearly endless list of services essential to life as we know it. To understand how to manage these services it is essential to understand how ecological communities are organized and how to measure the biological diversity they contain. Ecological communities are comprised of many species, which are in turn made up of large numbers of individuals, each with their own separate ecological and evolutionary agendas. Not all species are equal as regards their role in maintaining the functioning of ecosystems or their resiliency in the face of stress. This chapter explains how ecosystems evolve and function as complex adaptive systems. It examines ecological systems at scales from the small to the large, from the individual to the collective to the community, from the leaf to the plant to the biosphere (including the global carbon cycle). It reviews theoretical and empirical models of ecosystem dynamics, which are highly nonlinear and contain the potential for qualitative and irreversible shifts. It considers applications to forests, fisheries, grasslands, and freshwater lakes.

48. Pacala, Stephen W., E. Bulte, J. A. List, and S. A. Levin, 2003: False Alarm over Environmental False Alarms. Science, 301, doi:10.1126/science.1086646 1187-1188
    [ Abstract ]

    We live in an uncertain world, in which action and prudence must be continually juggled. Caution can be costly, but indifference to serious risks can be disastrous. In all aspects of life, we weigh risks and benefits, invoking measures to insure against events that threaten what is most important to us, while gambling with those that can be tolerated. In matters of our environment, science has the responsibility to inform these decisions, and society must find ways to identify the appropriate level of circumspection. By any calculation, we must protect ourselves against a wide variety of events that individually have low probabilities of occurrence, and still feel good when they have not occurred. We must rely on environmental science to alert us to an even wider set of possible disasters, many of very low probability, so that we as citizens can decide which cause us most worry, and which mandate action.

49. Chesson, P., Stephen W. Pacala, and C. Neuhauser, 2002: Environmental Niches and Ecosystem Functioning. The Functional Consequences of Biodiversity: Empirical Progress and Theoretical Extensions (MPB-33), Princeton University Press - Princeton, NJ, http://eebweb.arizona.edu/faculty/chesson/Peter/Reprints/2001_Environ, 213-245
    [ Abstract ]

    The idea that the response of an organism to components of its physical environment, as distinct from the availability of resources, is an important component of its niche. Differences between species in their environmental niches may promote their coexistence. The coexistence mechanism involved is the storage effect, augmented in the case of spatial variation by fitness density covariance. Environmental niches are temporal niches if they refer to temporally varying aspects of the physical environment, or spatial niches if they refer to spatially varying aspects of the physical environment, or spatio-temporal niches, if they refer simultaneously to variation in both space and time.

50. Hixon, M. A., Stephen W. Pacala, and S. A. Sandin, 2002: Population regulation: historical context and contemporary challenges of open vs. closed systems. Ecology, 83(6), doi:10.1890/0012-9658(2002)083[1490:PRHCAC]2.0.CO;2 1490-1508
    [ Abstract ]

    By definition, a population is regulated if it persists for many generations with fluctuations bounded above zero with high probability. Regulation thus requires density- dependent negative feedback whereby the population has a propensity to increase when small and decrease when large. Ultimately, extinction occurs due to regulating mechanisms becoming weaker than various disruptive events and stochastic variation. Population regulation is one of the foundational concepts of ecology, yet this paradigm has often been challenged, during the first half of the 20th century when the concept was not clearly defined, and more recently by some who study demographically open populations. The history of ecology reveals that earlier manifestations of the concept focused mostly on competition as the mechanism of population regulation. Because competition is often not evident in nature, it was sometimes concluded that population regulation was therefore also absent. However, predation in the broadest sense can also cause density dependence. By the 1950s, the idea that demographic density dependence was essential (but not sufficient) for population regulation was well established, and since then, challenges to the general concept have been short lived. However, some now believe that metapopulations composed of demographically open local populations can persist without density dependence. In particular, some recent manifestations of the Recruitment Limitation Hypothesis all but preclude the possibility of regulation. The theory of locally open populations indicates that persistence always relies on direct demographic density dependence at some spatial and temporal scale, even in models reportedly demonstrating the contrary. There is also increasing empirical evidence, especially in marine systems where competition for space is not self evident, that local density dependence is more pervasive than previously assumed and is often caused by predation. However, there are currently insufficient data to test unequivocally whether or not any persistent metapopulation is regulated. The challenge for more complete understanding of regulation of metapopulations lies in combined empirical and theoretical studies that bridge the gap between smaller scale field experiments and larger scale phenomena that can presently be explored solely by theory.

51. Kinzig, A. P., Stephen W. Pacala, and G. D. Tilman, 2002: Looking Back, Peering Forward (editors. The Functional Consequences of Biodiversity: Experimental Progress and Theoretical Extensions, Princeton, NJ, Princeton University Press, 314-329
    [ Abstract ]

    Does biodiversity influence how ecosystems function? Might diversity loss affect the ability of ecosystems to deliver services of benefit to humankind? Ecosystems provide food, fuel, fiber, and drinkable water, regulate local and regional climate, and recycle needed nutrients, among other things. An ecosystem's ability to sustain functioning may depend on the number of species residing in the ecosystem--its biological diversity--but this has been a controversial hypothesis. There are many unanswered questions about how and why changes in biodiversity could alter ecosystem functioning. This volume, written by top researchers, synthesizes empirical studies on the relationship between biodiversity and ecosystem functioning and extends that knowledge using a novel and coordinated set of models and theoretical approaches. These experimental and theoretical analyses demonstrate that functioning usually increases with biodiversity, but also reveals when and under what circumstances other relationships between biodiversity and ecosystem functioning might occur. It also accounts for apparent changes in diversity-functioning relationships that emerge over time in disturbed ecosystems, thereby addressing a major controversy in the field. The volume concludes with a blueprint for moving beyond small-scale studies to regional ones--a move of enormous significance for policy and conservation but one that will entail tackling some of the most fundamental challenges in ecology. In addition to the editors, the contributors are Juan Armesto, Claudia Neuhauser, Andy Hector, Clarence Lehman, Peter Kareiva, Sharon Lawler, Peter Chesson, Teri Balser, Mary K. Firestone, Robert Holt, Michel Loreau, Johannes Knops, David Wedin, Peter Reich, Shahid Naeem, Bernhard Schmid, Jasmin Joshi, and Felix Schläpfer.

52. Pacala, Stephen W., and A. P. Kinzig, 2002: Introduction to Theory and the Common Ecosystem Model. The Functional Consequences of Biodiversity: Experimental Progress and Theoretical Extensions, Princeton, NJ, Princeton University Press, 169-174
    [ Abstract ]

    Does biodiversity influence how ecosystems function? Might diversity loss affect the ability of ecosystems to deliver services of benefit to humankind? Ecosystems provide food, fuel, fiber, and drinkable water, regulate local and regional climate, and recycle needed nutrients, among other things. An ecosyste's ability to sustain functioning may depend on the number of species residing in the ecosystem--its biological diversity--but this has been a controversial hypothesis. There are many unanswered questions about how and why changes in biodiversity could alter ecosystem functioning. This volume, written by top researchers, synthesizes empirical studies on the relationship between biodiversity and ecosystem functioning and extends that knowledge using a novel and coordinated set of models and theoretical approaches. These experimental and theoretical analyses demonstrate that functioning usually increases with biodiversity, but also reveals when and under what circumstances other relationships between biodiversity and ecosystem functioning might occur. It also accounts for apparent changes in diversity-functioning relationships that emerge over time in disturbed ecosystems, thereby addressing a major controversy in the field. The volume concludes with a blueprint for moving beyond small-scale studies to regional ones--a move of enormous significance for policy and conservation but one that will entail tackling some of the most fundamental challenges in ecology. In addition to the editors, the contributors are Juan Armesto, Claudia Neuhauser, Andy Hector, Clarence Lehman, Peter Kareiva, Sharon Lawler, Peter Chesson, Teri Balser, Mary K. Firestone, Robert Holt, Michel Loreau, Johannes Knops, David Wedin, Peter Reich, Shahid Naeem, Bernhard Schmid, Jasmin Joshi, and Felix Schläpfer.

53. Pacala, Stephen W., A. P. Kinzig, and G. D. Tilman, 2002: The Transition from Sampling to Complementarity. The Functional Consequences of Biodiversity: Experimental Progress and Theoretical Extensions, Princeton, NJ, Princeton University Press, 151-166
    [ Abstract ]

    Does biodiversity influence how ecosystems function? Might diversity loss affect the ability of ecosystems to deliver services of benefit to humankind? Ecosystems provide food, fuel, fiber, and drinkable water, regulate local and regional climate, and recycle needed nutrients, among other things. An ecosyste's ability to sustain functioning may depend on the number of species residing in the ecosystem--its biological diversity--but this has been a controversial hypothesis. There are many unanswered questions about how and why changes in biodiversity could alter ecosystem functioning. This volume, written by top researchers, synthesizes empirical studies on the relationship between biodiversity and ecosystem functioning and extends that knowledge using a novel and coordinated set of models and theoretical approaches. These experimental and theoretical analyses demonstrate that functioning usually increases with biodiversity, but also reveals when and under what circumstances other relationships between biodiversity and ecosystem functioning might occur. It also accounts for apparent changes in diversity-functioning relationships that emerge over time in disturbed ecosystems, thereby addressing a major controversy in the field. The volume concludes with a blueprint for moving beyond small-scale studies to regional ones--a move of enormous significance for policy and conservation but one that will entail tackling some of the most fundamental challenges in ecology. In addition to the editors, the contributors are Juan Armesto, Claudia Neuhauser, Andy Hector, Clarence Lehman, Peter Kareiva, Sharon Lawler, Peter Chesson, Teri Balser, Mary K. Firestone, Robert Holt, Michel Loreau, Johannes Knops, David Wedin, Peter Reich, Shahid Naeem, Bernhard Schmid, Jasmin Joshi, and Felix Schläpfer.

54. Pacala, Stephen W., October 2002: Global Constraints on Reservoir Leakage. Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies (GHGT-6), 267-272
    [ Abstract ]

    A crucial unresolved problem about geological sequestration is that some receptor reservoirs will leak stored CO₂. At local scales, leaks might endanger shallower drinking water supplies or public health. At global scales, leaks might be large enough to make sequestration ineffective. This paper focuses on the global-scale problem and uses models of carbon storage reservoirs and natural carbon sinks to calculate constraints on reservoir leakage. It assumes fossil fuel consumption at a level that would that would lead to an atmospheric CO₂ concentration of 750 ppm and then calculates the sequestration and leakage limits that would reduce the maximum concentration to 450 or 550 ppm. The surprising result is that leakage limits are much less severe than expected because of heterogeneity among reservoirs. In some cases, the reduction from 750 to 450 ppm would be possible even with a mean leakage rate of 1% per year or more. The results imply that economic considerations or local risks are likely to constrain allowable leakage rates more tightly than impacts of leakage on global atmospheric CO₂.

55. Keeling, R. F., H. B. Wilson, and Stephen W. Pacala, 2001: Deterministic limits to stochastic spatial models of natural enemies. American Naturalist, 159(1), doi:10.1086/324119 57-80
    [ Abstract ]

    Stochastic spatial models are becoming an increasingly popular tool for understanding ecological and epidemiological problems. However, due to the complexities inherent in such models, it has been difficult to obtain any analytical insights. Here, we consider individual-based, stochastic models of both the continuous time Lotka-Volterra system and the discrete-time Nicholson-Bailey model. The stability of these two stochastic models of natural enemies is assessed by constructing moment equations. The inclusion of these moments, which mimic the effects of spatial aggregation, can produce either stabilizing or destabilizing influences on the population dynamics. Throughout, the theoretical results are compared to numerical models for the full distribution of populations, as well as stochastic simulations.

56. Moorcroft, P. R., G. C. Hurtt, and Stephen W. Pacala, 2001: A Method for Scaling Vegetation Dynamics: the Ecosystem Demography Model (ED). Ecological Monographs, 71(4), doi:10.1890/0012-9615(2001)071[0557:AMFSVD]2.0.CO;2 557-586
    [ Abstract ]

    The problem of scale has been a critical impediment to incorporating important fine-scale processes into global ecosystem models. Our knowledge of fine-scale physiological and ecological processes comes from a variety of measurements, ranging from forest plot inventories to remote sensing, made at spatial resolutions considerably smaller than the large scale at which global ecosystem models are defined. In this paper, we describe a new individual-based, terrestrial biosphere model, which we label the ecosystem demography model (ED). We then introduce a general method for scaling stochastic individual-based models of ecosystem dynamics (gap models) such as ED to large scales. The method accounts for the fine-scale spatial heterogeneity within an ecosystem caused by stochastic disturbance events, operating at scales down to individual canopy-tree-sized gaps. By conditioning appropriately on the occurrence of these events, we derive a size and age-structured (SAS) approximation for the first moment of the stochastic ecosystem model. With this approximation, it is possible to make predictions about the large scales of interest from a description of the fine-scale physiological and population-dynamic processes without simulating the fate of every plant individually. We use the SAS approximation to implement our individual-based biosphere model over South America from 15° N to 15° S, showing that the SAS equations are accurate across a range of environmental conditions and resulting ecosystem types. We then compare the predictions of the biosphere model to regional data and to intensive data at specific sites. Analysis of the model at these sites illustrates the importance of fine-scale heterogeneity in governing large-scale ecosystem function, showing how population and community-level processes influence ecosystem composition and structure, patterns of above ground carbon accumulation, and net ecosystem production.

57. Pacala, Stephen W., G. C. Hurtt, P. R. Moorcroft, and J. P. Caspersen, 2001: Carbon storage in the US caused by land use change. The Present and Future of Modeling Global Environmental Change, Terra Scientific Publishing, Tokyo, Japan, http://www.terrapub.co.jp/e-library/toyota/pdf/145.pdf, 145-172
    [ Abstract ]

    Here we examine the cause, size and future of the U.S. carbon sink. To estimate the size of the U.S. carbon sink we review a comprehensive land-based analysis of the carbon sink in the coterminous U.S. For the 1980s, the sink is between 1/3 and 2/3 PgC y^-1, and is split approximately evenly between forest and non-forest sectors. The non-forest sink is caused by fire suppression on non-forested lands, sediment burial in reservoirs, alluvium and colluvium, and agricultural practices.
    The forest sink has been attributed to changes in land use and the enhancement of plant growth by CO₂ fertilization, N deposition and climate change. To estimate the relative contribution of land use and growth enhancement in forest ecosystems, we use forest inventory data from five states spanning a latitudinal gradient in the eastern U.S. Land use is the dominant factor governing the rate of carbon accumulation in forests in these states, with growth enhancement contributing far less than previously reported. The estimated fraction of above-ground net ecosystem production due to growth enhancement is 2.0 ± – 4.4%, with the remainder due to land use.
    To forecast the future of the U.S. carbon sink, we used the Ecosystem Demography Model (ED). We first modeled carbon sources and sinks from 1700–1990, and then projected patterns to 2100. Our projections indicate that the land-use portion of the U.S. carbon sink will decrease in the future, with a half-life of approximately 50 years, as U.S. ecosystems gradually equilibrate with current patterns of natural and anthropogenic disturbance.
    Inventories of terrestrial carbon storage in the coterminous United States (the U.S. minus Alaska and Hawaii) appear to support the conclusion that the sink is small (4). However, inventories in the Northern Hemisphere have been able to account for only a third or less of the 1–2 PgC y^-1 indicated strongly by other lines of evidence (5). Moreover, the two primary groups of U.S. inventory studies strongly disagree about cause of the U.S. sink. Houghton and his colleagues (6) used historical records of land use change, timber production, soil conservation, wildfire rates, and simple models of carbon gains and losses in vegetation, soils and wood products. They estimated a sink for the coterminous U.S. averaging 0.39 PgC y^-1 from 1950–1990, and caused primarily by increases in crop productivity and changes in the management of agricultural soils (0.15 PgC), and by fire suppression on non-forested land (0.13 Pg C). They concluded that the entire forest sector contributed only 0.07–0.12 PgC y^-1.
    The U.S. Forest Service (USFS) estimated a coterminous U.S. sink averaging 0.33 PgC y^–1 from 1952–1992, using census and tree measurement data from the over 100,000 plots in their Forest Inventory and Analysis (FIA) network, together with models of soil carbon and the fate of wood products (7). Although 0.33 Pg is close to 0.39 Pg, the entire USFS estimate is for the forest sector. If all of the carbon identified in both (6) and (7) were real, then the annual sink for the coterminous U.S. would be 0.60–0.65 PgC (0.33 from (7) plus 0.39 from (6) minus the forest sector estimates from (6)) and thus the overlap between the two estimates is only 11–20% of the total (0.07/0.65 to 0.12/0.60).
    The FIA data base shows that the increase of carbon in trees has remained remarkably steady from 1952 to 1992, at approximately 0.10 + 0.02 PgC y^-1, because regrowth in the eastern half of the U.S. consistently exceeds harvest by about 0.1 PgC (7). This value is approximately double the increase in living forest carbon modeled in (6). To first order, differences among published inventories based on FIA data are caused by differences in the modeling of all forms of dead organic matter (including slash, wood products, standing dead trees, and soil carbon). For example, one study produced an estimate of only 0.08 PgC y^-1 for the 1980’s, because its modeling assumptions led to negligible accumulation of nonliving carbon (8). In contrast, the assumptions behind the USFS estimate of 0.33 PgC y^-1 implied that soil carbon accumulated twice as fast as living carbon in trees. In addition, no comprehensive inventory has as yet included the carbon sink caused by sediment burial in reservoirs, alluvium and colluvium and by the transport of carbon into the oceans by rivers. At least one study suggests that sediment burial and river transport may be significant (9). Finally, no comprehensive inventory accounts for net export of carbon in agricultural and wood products.
    Ecosystem models provide the final source of information about the terrestrial carbon sink and generally produce small estimates for the coterminous U.S. For example, the models in the recently published VEMAP comparison produced estimates of 0.08 + 0.02 PgC y^-1 for the period from 1980–1993 (10). However, none of these models includes the land use changes (i.e. agricultural abandonment, fire suppression, forest harvesting and regrowth, no-till agriculture) that play such a dominant role in the inventory analyses. The models focus instead on the effects of climate change and CO₂ and nitrogen fertilization.
    In what follows, we first summarize the results of a new inventory-based analysis of the coterminous U.S. carbon sink, which shows that the sink averages between one third and two thirds Pg annually (11). These estimates and smaller than the 0.81–0.84 PgC y^-1 in the controversial study (2) (see 11 for a discussion of the portion of estimates in (2) that correspond to the coterminous U.S.). However, they are significantly larger than the previously published range (one tenth to one third PgC y^-1). The analysis in (11) shows that approximately half the sink can be unambiguously ascribed to land use and management. We then summarize a recently published analysis (12) of the FIA data showing that the other half of the U.S. carbon sink is also overwhelmingly caused by land use and management.
    Given that human land use rather than CO₂ or nitrogen fertilization or climate change causes the sink, it is interesting to consider the sink’s future. We then turn to two additional studies. The first (13), introduces a new model that incorporates the sub grid-scale heterogeneity necessary to simulate land use. The second (14), applies this model for the past 300 years of land use in the coterminous U.S., and shows that the U.S. sink will decrease throughout the coming century. Unlike sinks caused by fertilization or climate change that might increase, the land use sink will decrease as U.S. ecosystems adjust to the altered disturbance regimes created by land use and management.

58. Pacala, Stephen W., G. C. Hurtt, D. Baker, P. Peylin, R. A. Houghton, R. A. Birdsey, L. Heath, E. T. Sundquist, R. F. Stallard, P. Ciais, P. R. Moorcroft, J. P. Caspersen, and E. Shevliakova, et al., 2001: Consistent Land- and Atmosphere-Based U.S. Carbon Sink Estimates. Science, 292, doi:10.1126/science.1057320 2316-2320
    [ Abstract ]

    For the period 1980-89, we estimate a carbon sink in the coterminous United States between 0.30 and 0.58 petagrams of carbon per year (petagrams of carbon = 10¹⁵ grams of carbon). The net carbon ßux from the atmosphere to the land was higher, 0.37 to 0.71 petagrams of carbon per year, because a net ßux of 0.07 to 0.13 petagrams of carbon per year was exported by rivers and commerce and returned to the atmosphere elsewhere. These land-based estimates are larger than those from previous studies (0.08 to 0.35 petagrams of carbon per year) because of the inclusion of additional processes and revised estimates of some component fluxes. Although component estimates are uncertain, about one-half of the total is outside the forest sector. We also estimated the sink using atmospheric models and the atmospheric concentration of carbon dioxide (the tracer-transport inversion method). The range of results from the atmosphere-based inversions contains the land-based estimates. Atmosphere- and land-based estimates are thus consistent, within the large ranges of uncertainty for both methods. Atmosphere-based results for 1980-89 are similar to those for 1985-89 and 1990-94, indicating a relatively stable U.S. sink throughout the period.

59. Rees, M., R. C. Condit, M. Crawley, Stephen W. Pacala, and G. D. Tilman, 2001: Long-Term Study of Vegetation Dynamics. Science, 293, doi:10.1126/science.1062586 650-655
    [ Abstract ]

    By integrating a wide range of experimental, comparative, and theoretical approaches, ecologists are starting to gain a detailed understanding of the long-term dynamics of vegetation. We explore how patterns of variation in demographic traits among species have provided insight into the processes that structure plant communities. We find a common set of mechanisms, derived from ecological and evolutionary principles, that underlie the main forces shaping systems as diverse as annual plant communities and tropical forests. Trait variation between species maintains diversity and has important implications for ecosystem processes. Hence, greater understanding of how Earth’s vegetation functions will likely require integration of ecosystem science with ideas from plant evolutionary, population, and community ecology.

60. Schimel, D., J. J. House, K. A. Hibbard, P. Bousquet, P. Ciais, P. Peylin, B. H. Brasswell, M. J. Apps, D. Baker, A. Bondeau, J. Canadell, G. Churkina, W. Cramer, A. S. Denning, C. B. Field, P. Friedlingstein, C. L. Goodale, M. Heimann, R. A. Houghton, J. M. Melillo, B. Moore III, D. Murdiyarso, I. Noble, and Stephen W. Pacala, et al., 2001: Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems. Nature, 414, doi:10.1038/35102500 169-172
    [ Abstract ]

    Knowledge of carbon exchange between the atmosphere, land and the oceans is important, given that the terrestrial and marine environments are currently absorbing about half of the carbon dioxide that is emitted by fossil-fuel combustion. This carbon uptake is therefore limiting the extent of atmospheric and climatic change, but its long-term nature remains uncertain. Here we provide an overview of the current state of knowledge of global and regional patterns of carbon exchange by terrestrial ecosystems. Atmospheric carbon dioxide and oxygen data confirm that the terrestrial biosphere was largely neutral with respect to net carbon exchange during the 1980s, but became a net carbon sink in the 1990s. This recent sink can be largely attributed to northern extratropical areas, and is roughly split between North America and Eurasia. Tropical land areas, however, were approximately in balance with respect to carbon exchange, implying a carbon sink that offset emissions due to tropical deforestation. The evolution of the terrestrial carbon sink is largely the result of changes in land use over time, such as regrowth on abandoned agricultural land and fire prevention, in addition to responses to environmental changes, such as longer growing seasons, and fertilization by carbon dioxide and nitrogen. Nevertheless, there remain considerable uncertainties as to the magnitude of the sink in different regions and the contribution of different processes.

61. Hurtt, G. C., Stephen W. Pacala, P. R. Moorcroft, J. P. Caspersen, E. Shevliakova, R. A. Houghton, and B. Moore III, 0000: Projecting the Future of the U.S. Carbon Sink. Proceedings of the National Academy of Sciences of the United States of America, 99(3), doi:10.1073/pnas.012249999 1389-1394
    [ Abstract ]

    Atmospheric and ground-based methods agree on the presence of a carbon sink in the coterminous United States (the United States minus Alaska and Hawaii), and the primary causes for the sink recently have been identified. Projecting the future behavior of the sink is necessary for projecting future net emissions. Here we use two models, the Ecosystem Demography model and a second simpler empirically based model (Miami Land Use History), to estimate the spatio-temporal patterns of ecosystem carbon stocks and fluxes resulting from land-use changes and fire suppression from 1700 to 2100. Our results are compared with other historical reconstructions of ecosystem carbon fluxes and to a detailed carbon budget for the 1980s. Our projections indicate that the ecosystem recovery processes that are primarily responsible for the contemporary U.S. carbon sink will slow over the next century, resulting in a significant reduction of the sink. The projected rate of decrease depends strongly on scenarios of future land use and the long-term effectiveness of fire suppression.

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