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October 27, 2003
Seattle, Washington, USA

Effects of Canopy Representation on Carbon Balance Simulations at Treeline

David M. Cairns^1
1Department of Geography, Texas A&M University, 3147 TAMU, College Station, TX, 77845-3147, USA, Phone 979-845-2783, Fax 979-862-4487, cairns@tamu.edu

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. There is a wealth of data indicating how major vegetation types have changed their distribution in response to Holocene vegetation changes. One major vegetation feature that has changed location through the Holocene is the forest-tundra boundary found in both Arctic and alpine locations. As the treeline is approached, the forest canopy thins and the physiognomy of the trees changes from an arboreal growth form to a mat-like growth form called krummholz. Full-sized upright trees, dwarfed trees and krummholz are often found within a short distance of each other. The primary difference between the tree types at treeline locations is the canopy structure. Krummholz trees tend to have their foliage concentrated at the top of the canopy, whereas dwarf-trees have more foliage near the bottom of the canopy. The canopy structure influences the distribution of light within the tree canopies and also has an effect on the temperatures within the canopy.

This study reports on the measurement and modeling the effects of these gradients within treeline canopies. During the summers of 2000 and 2001 vertical gradients in light and temperature were measured within krummholz and dwarf tree canopies of Abies lasiocarpa and Pinus contorta at the forest-tundra boundary (treeline) in northwestern Montana. The magnitudes of the gradients differ between the canopy forms.

The location of the forest-tundra boundary should be controlled in part by carbon balance. Trees at locations beyond the boundary should not be able to maintain positive carbon balances. Therefore, by predicting carbon balance across the landscape a potential treeline can be predicted. The location of this treeline will be influenced by canopy structure if the magnitudes of the gradients in light and temperature within the different canopy types are great enough. Simulations of carbon balance using a physiologically mechanistic model (ATE-BGC) for different canopy types indicate that there are differences in predicted treeline position using the two canopy types. Gradients in light are the most important. Temperature gradients can be important, but have much less effect on location and spatial pattern of the treeline ecotone.

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