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 interactions that impact species diversity and patterns of forest succession. Questions arise, however, as to whether these same models can be used to study the response of forest structure and composition under a changing climate. In contrast to many process-based models, gap models have traditionally been based on rather descriptive representations of species-specific growth processes. Opportunities now exist to expand upon these simple empirical relationships with more mechanistic descriptions of growth, the response of growth to environmental variables, and competition among species for available light, water, and nutrient resources. In this paper, we focus on several areas of below-ground research with the potential to improve the utility of gap models for predicting forest composition in response to a changing climate. Specific areas for model improvement include (1) improved descriptions of the soil environment for seed germination and subsequent seedling establishment, (2) multi-layer representations of soil water and nutrient availability, (3) more accurate information on biomass allocation to roots and root distribution within the soil profile, (4) improved treatment of inter- and intra-specific competition for available soil resources, (5) increased 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 to below-ground processes could help address intra-specific competition for water among trees of differing size classes, the gap model LINKAGES was modified to include a sub-model of multi-layered soil hydrology. It was then used to examine the impact of root distribution within soils on the simulated drought response of seedlings, saplings, and mature trees. An annual simulation of soil water content for a deciduous forest in eastern Tennessee showed that seedlings whose roots were restricted to the upper 20-cm of the soil 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 potential below-ground processes and that there are obvious tradeoffs between model simplicity and more sophisticated parameterizations. We conclude, however, that feedbacks among global environmental change, seed germination and seedling establishment, above- and below-ground carbon allocation, root distribution within the soil profile, and soil water and nutrient dynamics will be critically important for predicting forest dynamics and ecosystem function in the 21st century. As a result, steps should now be taken to ensure that these processes are represented in future gap models.