Environmental variables influencing the carbon balance at the alpine treeline: a modeling approach

Growth of coniferous trees at alpine treeline locations has traditionally been viewed as primarily temperature dependent. In this study, we use a physiologically mechanistic process model, ATE-BGC (Alpine Treeline Ecotone BioGeochemical Cycles), to study the effects of a suite of environmental variables on the carbon balance of krummholz growth forms of subalpine Abies lasiocarpa under environmental conditions representative of treeline locations in Glacier National Park, Montana, USA. We perform a sensitivity analysis of ATE-BGC to determine the relative effects of temperature, insolation, snow, rain, soil depth, leaf area index, winter injury and elevation on krummholz carbon balance. Our results indicate that ATE-BGC carbon balance estimates are most highly influenced by temperature and winter injury. Leaf area index, soil depth, rainfall, insolation and snowfall follow temperature and winter injury in their effect on carbon balance. Due to the sensitivity of the model to moisture related variables, we propose that local irregularities in the location of the alpine treeline ecotone are the result of localized xeric conditions.