Key Relationship Emerges in Modeling Plant Response to CO2 Increase
Few current-generation climate models include dynamic vegetation; that is, the behavior of plants to shift their habitats in response to environmental changes, for instance, stress from a hotter climate, water shortages, or limited nutrients such as nitrogen. Using a model that simulated the coupled dynamic vegetation and the nitrogen cycle, researchers found that these processes and their interactions are critically important for understanding how plants around the globe respond to the increase of atmospheric CO2 and its associated warming, leading to changes in the carbon cycle through feedback loops.
Among the climate models that contributed to the Intergovernmental Panel on Climate Change’s fifth assessment report, only a handful include dynamic vegetation, and even fewer incorporate the nitrogen cycle. Findings from this study show that dynamic vegetation and the nitrogen cycle play a key role in understanding the carbon cycle in the future. The analysis also revealed that differences in vegetation cover simulated with and without dynamic vegetation can propagate to differences in the nitrogen cycle, illustrating a specific example of model development reducing uncertainty in projecting future carbon-cycle and climate changes.
Terrestrial ecosystems play a key role in the Earth’s carbon cycle, directly interacting with CO2 in the atmosphere. Higher atmospheric CO2 concentrations help plants photosynthesize more efficiently. Although the latter may counter CO2 emissions, warming that results from the higher CO2 concentrations can increase heat stress on plants and speed up the decomposition of carbon in litter and soil to increase CO2 emissions. At the same time, faster carbon decomposition increases nitrogen available to plants and enhances their uptake of atmospheric CO2. These various interactions between plants, CO2, and climate produce feedback loops that control the global carbon cycle of the future. Although vegetation cover change is among the key processes controlling the carbon cycle, model assumptions about vegetation cover affecting the strength of the feedback loops, especially when the carbon and nitrogen cycles are considered jointly, previously have not been carefully quantified. Scientists at the U.S. Department of Energy’s (DOE) Pacific Northwest National Laboratory and their collaborators used version 4 of the Community Land Model including a dynamic vegetation model to simulate vegetation cover change in response to both the evolving carbon and nitrogen cycle. Model experiments showed a significant difference in the potential strength of the carbon-cycle feedback loop between simulations with dynamic vegetation cover and simulations with static vegetation cover that does not respond to climate change. Their research showed that the difference came from interaction between dynamic vegetation and the nitrogen cycle. As some plant types require more nitrogen than others, the spatial coverage of the nitrogen-demanding plants is reduced in dynamic vegetation simulations relative to simulations with static vegetation cover, leading to differences in plant response to CO2 and the strength of the feedback loops.