CANOPY CONDUCTANCE OF PINUS-TAEDA, LIQUIDAMBAR-STYRACIFLUA AND QUERCUS-PHELLOS UNDER VARYING ATMOSPHERIC AND SOIL-WATER CONDITIONS

Sap flow, and atmospheric and soil water data were collected in closed -top chambers under conditions of high soil water potential for saplin gs of Liquidambar styraciflua L., Quercus phellos L. and Finals taeda L., three co-occurring species in the southeastern USA. Responses of c anopy stomatal conductance (g(t)) to water stress induced by high atmo spheric water vapor demand or transpiration rate were evaluated at two temporal scales. On a diurnal scale, the ratio of canopy stomatal con ductance to maximum conductance (g(t)/g(t,max)) was related to vapor p ressure deficit (D), and transpiration rate per unit leaf area (E-1). High D or E-1 caused large reductions in g(t)/g(t,max) in L. styracifl ua and P. taeda. The response of g(t)/g(t,max) to E-1 was light depend ent in L. styraciflua, with higher g(t)/g(t,max) on sunny days than on cloudy days. In both L. styraciflua and Q. phellos, g(t)/g(t,max) dec reased linearly with increasing D (indicative of a feed-forward mechan ism of stomatal control), whereas g(t)/g(t,max) of P. taeda declined l inearly with increasing E-1 (indicative of a feedback mechanism of sto matal control). Longer-term responses to depletion of soil water were observed as reductions in mean midday g(t)/g(t,max), but the reduction s did not differ significantly between species. Thus, species that emp loy contrasting methods of stomatal control may show similar responses to soil water depletion in the long term.