Measuring and Modeling Stomatal and Aerodynamic Conductances of Mountain Birch: Implications for Treeline Dynamics

Abstract

Stomatal conductances of mountain birch leaves were determined by field measurements using a steady-state porometer under varying weather and radiation conditions. The stomatal conductances were related to atmospheric environmental parameters (water vapor deficit, air temperature, soil moisture potential, and photoactive radiation) by a nonlinear regression equation where the fit to the data set was optimized. By applying a 3-layer canopy model for evapotranspiration, with the actual measurements of the stomatal conductance inserted, characteristic bulk aerodynamic conductances for the exchange between the birch trees and the ambient air were determined empirically by comparison with the evapotranspiration from cut birches using a potometric technique. Finally, the 3-layer evapotranspiration model, with the stomatal submodel incorporated, was tested against the evapotranspiration measurements using a total of 27 cut birches on 54 d.

Water vapor pressure deficit has a dominating effect on the variations of stomatal conductance. For air temperatures below about + 13°C, there is a significant decreasing trend of the conductance. The ecological implications of stomatal conductance and the seasonal transpiration near the altitudinal treeline are discussed.