Forests have taken up a large fraction of CO2 emitted by humans, substantially slowing the rise of atmospheric CO2 levels. The Pacala, Hedin, and Medvigy research groups are investigating how future forest storage of carbon will be impacted by increasing CO2 levels and climate change. Their insights are leading to new tools that will be incorporated into LM3, the land model component of GFDL’s Earth System Model, to improve its representation of forest processes and predictions of future climate.
Impacts of resource limitation on CO2 “fertilization”
The impact of increased CO2 on forest carbon storage, or “CO2 fertilization,” remains controversial. Estimates of terrestrial carbon uptake made over a decade ago commonly ignored nutrient limitation and predicted large carbon sinks in response to increasing atmospheric CO2 . Coupled carbon-nutrient models were subsequently developed and predicted far smaller carbon sinks, but are inconsistent with field-based CO2 fertilization experiments that seem to affirm the original larger estimates. To address this inconsistency, Caroline Farrior, Ray Dybzinski, and Steve Pacala are using models of allocation and carbon storage in forests to investigate how plants’ competitive responses impact carbon sinks in nutrient and water-limited environments.
The group found two main impacts of increasing atmospheric CO2 on forest carbon storage. First, plants will shift allocation patterns in such a way that they will use all additional carbon provided. Because wood has a low nitrogen-to-carbon ratio and is always beneficial to plants in competition for light, nitrogen-limited plants under high CO2 increase their investment in wood, sacrificing a small amount of nitrogen in leaves or fine roots in order to make use of extra carbon in wood.
If plants are also water-limited, enhanced CO2 benefits carbon sinks further. Increased water-use efficiency resulting from increased CO2 frees plants from water-limitation and intense competition for water belowground. As a result, plants decrease investment in fine roots, allowing even greater storage of carbon as woody biomass. Taken together, the team finds that nitrogen limitation does not work to decrease carbon storage in forests, and, if water is scarce, that belowground limitation is actually a driver of increases in carbon storage.
As these competitive allocation patterns are critical to carbon allocation, the team has built the machinery to include them into the Earth System Model at GFDL (in collaboration with Ensheng Weng, Elena Shevliakova, and Sergey Malyshev).
Nitrogen fixation and carbon storage in tropical forests
In tropical forests, legume trees that fix nitrogen can play a large role in combating nutrient limitation and enhancing carbon storage. The Hedin and Medvigy groups are investigating this phenomenon using both field observations and numerical models.
In a recent paper published in Nature, lead author and postdoctoral scholar Sarah Batterman reported that nitrogen-fixing species can help overcome ecosystem-scale deficiencies in nitrogen that emerge during periods of rapid biomass accumulation in tropical forests, and they consequently have a strong impact on the ability of tropical forests to sequester CO2 . Over a 300-year period in Panama, such nitrogen-fixing tree species accumulated carbon up to nine times faster per individual than their non-fixing neighbors.
Postdoctoral scholar Jennifer Levy has been using measurements to develop a regional-scale model relating tropical forest carbon accumulation to nitrogen, land use, and climate change that distinguishes between nitrogen-fixing and non-fixing trees. The model has been used to realistically predict forest carbon accumulation, nitrogen fixation, and the proportion of trees that are capable of nitrogen fixation. This property allows researchers, for the first time, to isolate the control of nitrogen-fixing trees on carbon accumulation in re-growing tropical forests. As next steps, they plan to use the regional-scale model to inform the development of the nitrogen cycle in GFDL’s global-scale land model, LM3. Using LM3, they will be able to understand how their results scale up from particular regions to the entire globe.
Understanding forest responses to drought
The global impact of drought on the carbon cycle is also a critical unknown that the Earth System Model at GFDL is uniquely poised to address, because of its unique treatment of species diversity and competition in forests. Adam Wolf of the Pacala group is working on multiple fronts to improve understanding of the diverse strategies plants use to deal with water limitation.
Water is the pre-eminent scarce resource for plants, and they compete desperately to attain it. Wolf is working to understand the extent of water “theft” belowground in a combination of field experiment, lab investigation, and modeling work (Figure 2). The field experiment takes place in the Silas Little Experimental Forest in central New Jersey. In this experiment, shelters are built around focal trees to divert rain water, and isotopically labeled water is applied instead. This labeled water can be traced to the different trees that take it up, allowing a quantification of the degree to which plants can “hoard” water in private fiefdoms, or instead compete in an open access commons. Wolf is using this work to recast models of plant responses to low humidity as plant responses to water status, which should be a better indicator of soil water availability.
The benefit for this understanding will be an improved understanding for how plants, especially forests, deal with water limitation, particularly to avoid drought-induced mortality. This work directly leads to improvements to the LM3 land surface model in its treatment of water limitation and forest mortality.
Changes in biomass burning and their impact on the carbon cycle
Biomass burning contributes large amounts of greenhouse gases and aerosols to the atmosphere every year. Wildfires are not the only source of this burning, however. People also use fire to manage land, in ways that can differ significantly from burning patterns in less-managed areas. Sam Rabin has helped develop a method that estimates the fraction of observed fire occurring on different land-use types, which has resulted in the first regional and global estimates of pasture burning extent and seasonality (Figure 3). Using these estimates, he is building a model that will simulate burning in croplands and pastures, as well as undisturbed and secondary lands, that will be incorporated in the GFDL Earth System Model. When development is complete, Rabin will be able to: answer questions about how fire has influenced the distribution of biomes around the world, explore how changing climate and land use will affect fire and therefore vegetation in the future, and assess the consequences of these changes for global climate.