Quantifying the effect of cumulative water stress on stomatal conductance of <i>Eucalyptus globulus</i> and <i>Eucalyptus nitens</i> : 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 Gsu,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.