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

Leaf conductance was measured periodically from early spring until October for three Pinuscontorta Dougl. saplings in Western Montana on a harsh site exhibiting low temperatures, high absolute humidity deficits, and substantial soil moisture depletion. Springtime maximum daily leaf conductance was correlated with 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%; however, overall seasonal transpiration was increased by only 8%. Summer humidity and predawn water potential controls were found necessary to prevent lethal water stress.