LEAF CONDUCTANCE AND CO2 ASSIMILATION OF LARIX GMELINII GROWING IN ANEASTERN SIBERIAN BOREAL FOREST

In July 1993, we measured leaf conductance, carbon dioxide (CO2) assim ilation, and transpiration in a Larix gmelinii (Rupr.) Rupr, ex Kuzen forest in eastern Siberia. At the CO2 concentration of ambient air, ma ximum values (mean of 10 highest measured values) for CO2 assimilation , transpiration and leaf conductance for water vapor were 10.1 mu mol m(-2) s(-1), 3.9 mmol m(-2) s(-1) and 365 mmol m(-2) s(-1), respective ly. The corresponding mean values, which were much lower than the maxi mum values, were 2.7 mu mol m(-2) s(-1), 1.0 mmol m(-2) s(-1) and 56 m mol m(-2) s(-1). The mean values were similar to those of Vaccinium sp ecies in the herb layer. The large differences between maximum and act ual performance were the result of structural and physiological variat ions within the tree crowns and between trees that reduced maximum ass imilation and leaf conductance by about 40 and 60%, respectively. Thus , maximum assimilation and conductance values averaged over the canopy were 6.1 mu mol m(-2) s(-1) and 146 mmol m(-2) s(-1), respectively. D ry air caused stomatal closure, which reduced assimilation by an addit ional 26%. Low irradiances in the morning and evening had a minor effe ct (-6%). Daily canopy transpiration was estimated to be 1.45 mm day(- 1), which is higher than the value of 0.94 mm day(-1) measured by eddy covariance, but similar to the value of 1.45 mm day(-1) calculated fr om the energy balance and soil evaporation, and less than the value of 2.1 mm day(-1) measured by xylem flux. Daytime canopy carbon assimila tion, expressed on a ground area basis, was 0.217 mol m(-2) day(-1), w hich is higher than the value measured by eddy flux (0.162 mol m(-2) d ay(-1) including soil respiration). We discuss the regulation of leaf gas exchange in Larix under the extreme climatic conditions of eastern Siberia (temperature > 35 degrees C and vapor pressure deficit > 5.0 kPa).