Dynamic simulation of tree-grass interactions for global change studies

The objective of this study was to simulate dynamically the response of a c omplex landscape, containing forests, savannas, and grasslands, to potentia l climate change. Thus, it was essential to simulate accurately the competi tion for light and water between trees and grasses. Accurate representation of water competition requires simulating the appropriate vertical root dis tribution and soil water content. The importance of different rooting depth s in structuring savannas has long been debated. In simulating this complex landscape, we examined alternative hypotheses of tree and grass vertical r oot distribution and the importance of fire as a disturbance, as they influ ence savanna dynamics under historical and changing climates. MCl, a new dy namic vegetation model, was used to estimate the distribution of vegetation and associated carbon and nutrient fluxes for Wind Cave National Park, Sou th Dakota, USA. MCl consists of three linked modules simulating biogeograph y, biogeochemistry, and fire disturbance. This new tool allows us to docume nt how changes in rooting patterns may affect production, fire frequency, a nd whether or not current vegetation types and life-form mixtures can be su stained at the same location or would be replaced by others. Because climat e change may intensify resource deficiencies, it will probably affect alloc ation of resources to roots and their distribution through the soil profile . We manipulated the rooting depth of two life-forms, trees and grasses, th at are competing for water. We then assessed the importance of variable roo ting depth on ecosystem processes and vegetation distribution by running MC l for historical climate (1895-1994) and a GCM-simulated future scenario (1 995-2094). Deeply rooted trees caused higher tree productivity, lower grass productivity, and longer fire return intervals. When trees were shallowly rooted, grass productivity exceeded that of trees even if total grass bioma ss was only one-third to one-fourth that of trees. Deeply rooted grasses de veloped extensive root systems that increased N uptake and, the input of li tter into soil organic matter pools. Shallowly rooted grasses produced smal ler soil carbon pools. Under the climate change scenario, NPP and live biom ass increased for grasses and decreased for trees, and total soil organic m atter decreased. Changes in the size of biogeochemical pools produced by th e climate change scenario were overwhelmed by the range of responses across the four rooting configurations. Deeply rooted grasses grew larger than sh allowly rooted ones, and deeply rooted trees outcompeted grasses for resour ces. In both historical and future scenarios, fire was required for the coe xistence of trees and grasses when deep soil water was available to trees. Consistent changes in fire frequency and intensity were simulated during th e climate change scenario: more fires occurred because higher temperatures resulted in decreased fuel moisture. Fire also increased in the deeply root ed grass configurations because grass biomass, which serves as a fine fuel source, was relatively high.