DEVELOPMENT OF A PHYSIOLOGICALLY MECHANISTIC MODEL FOR USE AT THE ALPINE TREELINE ECOTONE

Tree growth within the alpine treeline ecotone (ATE) is limited by a suite of environmental processes that are not included in general forest ecosystem process models. In this study, I explore the use of a one such model, FOREST-BGC, at four ATE sites in Glacier National Park, Montana. FOREST-BGC is modified to incorporate the effects of snow redistribution, frozen soil, and winter desiccation. Net primary productivity (NPP) and carbon balance are modeled at the four ATE sites. Under the carbon-balance hypothesis, carbon balance within the ATE should be near zero; however, the unmodified FOREST-BGC routinely predicts carbon balances greatly in excess of zero. The hydrologic modifications (snow redistribution and effects of frozen soil) produce the greatest reductions in NPP and carbon balance at the modeled sites. The incorporation of winter desiccation in the model has less effect than the hydrologic modification. To improve predictions of NPP and carbon balance at ATE locations, further modifications of FOREST-BGC are required.