DIURNAL HYSTERESES OF STOMATAL AND BULK SURFACE CONDUCTANCES IN RELATION TO VAPOR-PRESSURE DEFICIT IN A COOL-TEMPERATE WETLAND

In order to evaluate the stomatal control on the canopy evapotranspira tion, diurnal variations in stomatal conductance (g(s)) of individual plants and bulk surface conductance (G(s)) of the canopy were measured independently in the Sarobetsu mire of northern Japan. During July-Au gust 1993 and 1994, g(s) was measured for four species (Sasa palmata, Moliniopsis japonica, Myrica gale L. var. tomentosa and Ilex crenata v ar. paludosa) growing in the mire. In July and August 1995, G(s) was d etermined using Penman-Monteith equation. In this equation, evapotrans piration rate was determined by the Bowen ratio/energy balance method. Diurnal hysteresis curves of g(s) in relation to leaf-to-air vapor pr essure deficit (nu) followed different courses from day to day accordi ng to the magnitude of nu, and daily maximum g(s) was correlated negat ively with daily maximum nu except for Ilex crenata var. paludosa. Sim ilar clockwise hysteresis curves were observed between G(s) and Vapor pressure deficit at the mean evaporative surface (D-0), and daily maxi mum G(s) decreased with daily maximum D-0 in midsummer. In particular, on days when daily maximum D-0 increased up to 1.9 kPa, the large dec rease in G(s) decreased the evapotranspiration rate, despite high irra diance levels. Thus, stomatal control by individual plant leaves stron gly affects evapotranspiration from the canopy and prevents excessive water loss on days when vapor pressure deficit is high, even when soil water is plentiful. On the rising limb of the diurnal course of the t ranspiration rate (T-r), increases in T-r with change in nu, Delta T-r /Delta nu varied from day to day, in particular for M. gale L. var, to mentosa. Further, a negative relationship was found between Delta T-r/ Delta nu and the rate of increase of nu for unit change in time. We be lieve that the change of the daily course of g(s) and T-r was caused b y the response of stomata to the increased rate of change of nu. Thus, in order to predict the diurnal variation of g(s), the magnitude and tendency of nu for each day must be considered. (C) 1998 Elsevier Scie nce B.V. All rights reserved.