Below-ground processes in gap models for simulating forest response to global change

Gap models have a rich history of being used to simulate individual tree in teractions that impact species diversity and patterns of forest succession. Questions arise, however, as to whether these same models can be used to s tudy the response of forest structure and composition under a changing clim ate. In contrast to many process-based models, gap models have traditionall y been based on rather descriptive representations of species-specific grow th processes. Opportunities now exist to expand upon these simple empirical relationships with more mechanistic descriptions of growth, the response o f growth to environmental variables, and competition among species for avai lable light, water, and nutrient resources. In this paper, we focus on seve ral areas of below-ground research with the potential to improve the utilit y of gap models for predicting forest composition in response to a changing climate. Specific areas for model improvement include (1) improved descrip tions of the soil environment for seed germination and subsequent seedling establishment, (2) multi-layer representations of soil water and nutrient a vailability, (3) more accurate information on biomass allocation to roots a nd root distribution within the soil profile, (4) improved treatment of int er- and intra-specific competition for available soil resources, (5) increa sed consideration of spatial processes as related to land-surface hydrology , and (6) improved attention to above- and below-ground interactions. This list is meant to stimulate discussion and provide guidance for future field research and model development. As an example of how increased attention t o below-ground processes could help address intra-specific competition for water among trees of differing size classes, the gap model LINKAGES was mod ified to include a sub-model of multi-layered soil hydrology. It was then u sed to examine the impact of root distribution within soils on the simulate d drought response of seedlings, saplings, and mature trees. An annual simu lation of soil water content for a deciduous forest in eastern Tennessee sh owed that seedlings whose roots were restricted to the upper 20-cm of the s oil experienced far more 'drought days' than did saplings and larger trees that otherwise had access to deeper soil water reserves. We recognize that models of forest succession cannot include mechanistic detail on all potent ial below-ground processes and that there are obvious tradeoffs between mod el simplicity and more sophisticated parameterizations. We conclude, howeve r, that feedbacks among global environmental change, seed germination and s eedling establishment, above- and below-ground carbon allocation, root dist ribution within the soil profile, and soil water and nutrient dynamics will be critically important for predicting forest dynamics and ecosystem funct ion in the 21st century. As a result, steps should now be taken to ensure t hat these processes are represented in future gap models.