Correlations of stomatal conductance with hydraulic and chemical factors in several deciduous tree species in a natural habitat

Recent research in whole-plant stomatal physiology, conducted largely with potted plants in controlled environments, suggests that stomatal conductanc e (g(s)) might be more closely linked to plant chemical variables than to h ydraulic variables. To test this in a field situation, seasonal g(s) was ex amined in relation to a number of plant and environmental variables in 11 t emperate, deciduous forest tree species. Stomatal conductance was generally better correlated with environmental variables (air temperature, vapor pre ssure deficit, PPFD) than with plant variables, and slightly better correla ted with plant hydraulic variables (shoot water and osmotic potentials) tha n with plant chemical variables (xylem sap ABA concentration, xylem sap pH) . We examined a model, developed previously for maize, which describes regu lation of g(s) by xylem sap ABA concentration with leaf water status acting to modify stomatal sensitivity to the ABA signal. This model explained sli ghtly more variation in seasonal g(s) in the forest trees than did single p lant variables but not more variation than most single environmental variab les. Response surface models, especially those incorporating environmental variables, were more consistently successful at explaining g(s) across spec ies.