Simulating carbon balance at treeline for krummholz and dwarf tree growth forms
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Modeling vegetation systems has become one of the most powerful methods available for predicting the response of modern vegetation assemblages to future changes in climate. The alpine treeline is a landscape feature that is coupled to climate and is expected to change location with changes in climate. In the Rocky Mountains, treeline environments are dominated by two distinct growth forms. Dwarf trees are stunted trees that retain an arboreal growth form and krummholz are trees that have assumed a mat-like growth form. Models of treeline dynamics have not taken into account these differences. In this study, I investigate the effects of canopy type on carbon balances predicted using ATE-BGC. ATE-BGC is a forest process model that has been developed specifically for treeline environments. The canopies of dwarf trees and krummholz differ with regard to the vertical distribution of leaf area index (LAI) and light within the canopies. The thermal regimes of the two canopy types also differ vertically. These differences within and between canopy types have significant effects on simulated carbon balance. Dwarf tree canopies are simulated to have higher carbon balances for all leaf area index values above 4 and carbon balances for both canopy types decrease with increases in LAI. The incorporation of vertical changes in the thermal regime does not have a significant impact on carbon balance for either canopy type. The results show that canopy type is an important factor to consider when modeling the alpine treeline. The results also suggest avenues for future modification to the ATE-BGC model. The inclusion of a mechanism to allow for a physiognomic switch (dwarf tree to krummholz or krummholz to dwarf tree) would benefit future simulations of treeline response to climate change. 2005 Elsevier B.V. All rights reserved.
