Simulating the Impacts of Disturbances on Forest Carbon Cycling in North America: Processes, Data, Models, and Challenges
Disturbances disrupt the forest structures and alter forest resources, substrate availability, or the physical environment. Understanding disturbances and their impacts is critical for a better quantification of North American carbon dynamics. We reviewed the status and major challenges in simulating the impacts of disturbances on forest carbon cycling in North America from the perspectives of process understanding, disturbance characterization, and modeling, and found some major issues. First, we do not have adequate continentally-consistent disturbance databases to support the NACP missions. Community effort, in collaborating with the modeling community, should be designated to systematically define international standards for disturbance databases with explicit inclusion of uncertainty measurements, adequate spatial and temporal resolutions, and explicit and quantifiable linkages with model simulations. Second, there is a need to develop effective and comprehensive process-based procedures or algorithms that can be used to quantify the immediate and long-term impacts of disturbances on forest succession, soils, microclimate, and cycles of carbon, water, and nutrients. These algorithms should be evaluated not only on the carbon cycle but also on the simulations of vegetation succession, alterations of soil and microclimate, and water and nutrients dynamics because adequate simulations of carbon dynamics rely on the correct quantification of the changes of soil and vegetation conditions. Third, the scaling challenges have rarely been addressed at the continental scale. We do not understand which processes and properties at a given temporal or spatial scales are critical while others can be simplified when we try to balance the North American carbon budget. Previous and current model simulations at the continental scale did not incorporate a full suite of disturbances information (because they do not exist as described above), and therefore provided little insight on the relative importance of various disturbances at the continent and regional scales on the carbon cycle. Fourth, our capability of simulating the occurrences and severity of disturbances under climate change and management futures are very limited. Fifth, uncertainty exists in every step of the effort to simulating the impacts of disturbances on carbon dynamics. Procedures are needed to quantify the uncertainty of model inputs or data layers, model parameters, and model structures, and their impacts on model simulations. Working together, modelers and data layer developers can identify the most uncertain areas and develop working hypotheses to reduce the uncertainty.