Quantifying the effect of cumulative water stress on stomatal conductance of Eucalyptus globulus and Eucalyptus nitens: a phenomenological approach

This paper incorporates the cumulative effect of water stress, in a phenome nological model of stomatal conductance of upper canopy foliage (g(su)) for Eucalyptus globulus and Eucalyptus nitens. Maximum observed stomatal condu ctance of irrigated trees was moderated by three functions describing the r esponse of g(su) to light, air temperature and vapour pressure deficit. Thi s model explained 70% of the variation in g(su) of irrigated trees using th e same parameter values for both species. Daily total conductance was calcu lated for rainfed (G(su,R)) and irrigated (G(su,I)) trees. Linear relations hips between the ratio of G(su,R)/G(su,I) (f(W)) and pre-dawn water potenti al explained only 46 and 30%, respectively, of variation in f(W) for E, glo bulus and E. nitens. The average value of pre-dawn water potential for the preceding x days (<(S-Psi x)over bar> was calculated for a range of values of x. When x was 8 for E. globulus and 20 for E. nitens a single exponentia l decay function accounted for 70% of variation in f(W) for both species. T his result and the number of days over which it was necessary to calculate (<(S-Psi x)over bar>) were consistent with previous studies of the leaf wat er relations, canopy and sapwood development, and stomatal behaviour of the two species.