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 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) fo r 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.