BELOW-GROUND PROCESSES IN GAP MODELS FOR SIMULATINGFOREST RESPONSE TO GLOBAL CHANGESTAN D. WULLSCHLEGER1, ROBERT B. JACKSON2, WILLIAM S. CURRIE3,ANDREW D. FRIEND4,YIQILUO5, FLORENT MOUILLOT6, YUDE PAN7andGUOFAN SHAO81Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge,TN 37831-6422, U.S.A.E-mail: [email protected] of Botany and Nicholas School of the Environment, Phytotron Building,Duke University, Durham, NC 27708, U.S.A.3Appalachian Laboratory, Frostburg, MD 21532-2307, U.S.A.4Center for Environmental Prediction, Rutgers University, New Brunswick, NJ 08901, U.S.A.at NASA Goddard Institute for Space Studies, New York, NY 10025, U.S.A.5Department of Botany and Microbiology, University of Oklahoma,Norman, OK 73019-0245, U.S.A.6Dynamique Réactionnelle des Ecosystèmes, Analyse spatiale et Modélisation Unit, CEFE-CNRS,F-34293 Montpellier cedex 5, France and Univ. Corse, Cevaren, 20250 Corte, France7USDA Forest Service, Global Change Research, Newtown Square Corporate Campus,Newtown Square, PA 19073, U.S.A.8Department of Forestry and Natural Resources, Purdue University, West Lafayette,IN 47907-1159, U.S.A.Abstract. Gap models have a rich history of being used to simulate individual tree interactions thatimpact species diversity and patterns of forest succession. Questions arise, however, as to whetherthese same models can be used to study the response of forest structure and composition under achanging climate. In contrast to many process-based models, gap models have traditionally beenbased on rather descriptive representations of species-specific growth processes. Opportunities nowexist to expand upon these simple empirical relationships with more mechanistic descriptions ofgrowth, the response of growth to environmental variables, and competition among species for avail-able light, water, and nutrient resources. In this paper, we focus on several areas of below-groundresearch with the potential to improve the utility of gap models for predicting forest compositionin response to a changing climate. Specific areas for model improvement include (1) improveddescriptions of the soil environment for seed germination and subsequent seedling establishment,(2) multi-layer representations of soil water and nutrient availability, (3) more accurate informationon biomass allocation to roots and root distribution within the soil profile, (4) improved treatmentof inter- and intra-specific competition for available soil resources, (5) increased consideration ofspatial processes as related to land-surface hydrology, and (6) improved attention to above- andbelow-ground interactions. This list is meant to stimulate discussion and provide guidance forfuture 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 differingsize classes, the gap model LINKAGES was modified to include a sub-model of multi-layered soilhydrology. It was then used to examine the impact of root distribution within soils on the simulateddrought response of seedlings, saplings, and mature trees. An annual simulation of soil water con-tent for a deciduous forest in eastern Tennessee showed that seedlings whose roots were restrictedto the upper 20-cm of the soil experienced far more ‘drought days’ than did saplings and largerClimatic Change 51: 449–473, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.450 STAN D. WULLSCHLEGER ET AL.trees that otherwise had access to deeper soil water reserves. We recognize that models of forestsuccession cannot include mechanistic detail on all potential below-ground processes and that thereare obvious tradeoffs between model simplicity and more sophisticated parameterizations. We con-clude, however, that feedbacks among global environmental change, seed germination and seedlingestablishment, 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 andecosystem function in the 21st century. As a result, steps should now be taken to ensure that theseprocesses are represented in future gap models.1. IntroductionSimulation models are now routinely used to assess the response of terrestrialecosystems to regional and global environmental change (VEMAP members, 1995;King et al., 1997; Neilson and Drapek, 1998; Pan et al., 1998; Potter et al., 1998).These models have historically varied in complexity from simple ‘black box’ rep-resentations of the biosphere (Oeschger et al., 1975) to highly detailed land-surfaceschemes operating in tandem with equally complicated global circulation models(Sellers et al., 1986; Foley et al., 1998). Experience has shown that the degree ofcomplexity incorporated into such models is driven largely by how the model willbe used and, as might be expected, there currently exists considerable diversityin the soil, plant, and atmospheric processes represented within specific models(Jackson et al., 2000).Among the many categories of models used to simulate the potential responseof ecosystems to climate change, individual-based models of forest succession or‘gap’ models have both a unique history and application (Bugmann, 2001). Thetraditional emphasis of gap models in predicting species composition and suc-cessional dynamics under current climatic conditions has resulted in fairly simpleparameterizations for a range of above- and below-ground processes compared tomore detailed biophysical models (Shugart et al., 1992; Shugart and Smith, 1996).Many of these simplistic descriptions remain in current-day gap models, althoughthere has been an increasing debate as to whether more mechanistic detail shouldbe included in forest gap models (Pacala et al., 1993; Bugmann et al., 1996; Hurtt etal., 1998). Central to this debate is whether simple formulations included in manygap models can be used to study the response of forest structure and compositionunder a changing climate.Attempts to incorporate stomatal, photosynthetic, and energy exchange dynam-ics into physiology-based models of forest succession have, for example, beensuccessful (Martin, 1992; Friend et al., 1997). The below-ground components ofgap models have not, however, benefited from the same degree of model improve-ment. Pastor and Post (1986) incorporated decomposition and nutrient cyclinginto LINKAGES and were