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

Abstract

This paper incorporates the cumulative effect of water stress, in a phenomenological model of stomatal conductance of upper canopy foliage (g_su) for Eucalyptus globulus and Eucalyptus nitens. Maximum observed stomatal conductance of irrigated trees was moderated by three functions describing the response of g_su to light, air temperature and vapour pressure deficit. This model explained 70% of the variation in g_su of irrigated trees using the same parameter values for both species. Daily total conductance was calculated for rainfed (G_su,R) and irrigated (G_su,I) trees. Linear relationships between the ratio of G_su,R/G _su,I (f(W)) and pre-dawn water potential explained only 46 and 30%, respectively, of variation in f(W) for E. globulus and E. nitens. The average value of pre-dawn water potential for the preceding x days (S_ψx) was calculated for a range of values of x. When x was 8 for E. globulus and 20 for E. nitens a single exponential decay function accounted for 70% of variation in f(W) for both species. This result and the number of days over which it was necessary to calculate (S_ψx) were consistent with previous studies of the leaf water relations, canopy and sapwood development, and stomatal behaviour of the two species.