Bibliography - P. R. Moorcroft
- 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.
- 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.
- 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.
- 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.
- 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. - 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.
- 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.
Direct link to page: http://cmi.princeton.edu/bibliography/results.php?author=4478
