Relative control of air temperature and water status on seasonal transpiration of Pinuscontorta

Abstract

Leaf conductance was measured periodically from early spring until Oct. for 3 P. contorta Dougl. saplings in Western Montana [USA] on a harsh site exhibiting low temperatures, high absolute humidity deficits, and substantial soil moisture depletion. Springtime maximum daily leaf conductance was correlated with a previous night minimum air temperature and a seasonal temperature summation (R2 = 0.93). Summer leaf conductance was correlated with predawn leaf water potential and absolute humidity deficit (R2 = 0.87). To assess the significance of these leaf conductance controls to the seasonal water balance of a forest stand on this site, regression equations relating leaf conductance to environmental parameters were incorporated into a stand hydrologic model. Removing spring air temperature controls of leaf conductance was estimated to increase transpiration in May and June by 47%; overall seasonal transpiration was increased by only 8%. Summer humidity and predawn water potential controls were necessary to prevent lethal water stress.