Carbon balance modeling at the alpine treeline ecotone in Glacier National Park, Montana

The alpine treeline ecotone (ATE) is the zone of transition between low elevation subalpine forest and high elevation tundra characterized by a gradual reduction in tree stature from upright trees to a shrub-like krummholz growth. The purpose of this study is to evaluate the carbon balance hypothesis of ATE location, which states that the ATE marks the location on a mountain side above which trees cannot maintain a positive carbon balance. Carbon balance is the sum of carbon influx to a vegetation system due to photosynthesis and the outflows of carbon due to respiration and direct carbon loss (e.g. senescence and injury). To evaluate the carbon balance hypothesis I use a simulation modeling approach. I simulate carbon balance values for ATE locations in Glacier National Park (GNP), Montana, using a forest process model, ATE-BGC (Alpine Treeline Ecotone - BioGeochemical Cycles).; ATE-BGC is a physiologically mechanistic, process based model specifically designed to represent conditions at the ATE. ATE-BGC is derived from an existing general forest process model (FOREST-BGC). I modified FOREST-BGC to include a representation of the krummholz canopy, a winter injury process, a new soil temperature model, and constraints on the length of the growing season based on the occurrence of late growing season frosts and the spring recovery of winter photosynthetic dormancy.; Simulation runs of ATE-BGC for landscapes in GNP show that the position of the ATE is not predicted well by carbon balance alone. At many sites, ATE-BGC predicts that trees could maintain positive carbon balances at locations 10 m upslope of the current ATE. Furthermore, when ATE-BGC is run for 11 year period beginning with 1950-1960 and ending with 1981-1991, the location of the zero carbon balance point on the landscape migrates over time. In contrast, the location of the ATE in GNP has been stable for at least the past 20 years. The inability of ATE-BGC to accurately predict the location of the ATE over time shows that the location of the ATE is not tightly coupled to changes in climate.