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Accueil du site → Doctorat → Australie → 1996 → Physiological responses to drought of Eucalyptus globulus and Eucalyptus nitens in plantations.

University of Tasmania (1996)

Physiological responses to drought of Eucalyptus globulus and Eucalyptus nitens in plantations.

White Don

Titre : Physiological responses to drought of Eucalyptus globulus and Eucalyptus nitens in plantations.

Auteur : White Don

Université de soutenance : University of Tasmania

Grade : Doctor of Philosophy (PhD) 1996

Résumé
Eucalyptus globulus Labill. and E. nitens (Deane and Maiden) Maiden are the predominant hardwood plantation species in southern Australia. This thesis describes some physiological strategies exhibited by these species in response to drought as a means of determining their suitability for establishment on water limited sites. To this end a 2 ha experimental plantation was established on a low rainfall site (ca. 515 mm. a-1 ) in August 1990. The plantation was divided into irrigated and rainfed blocks so that the effects of soil drought could be separated from those of diurnal and seasonal climatic variation. Between November 1991 and April 1993 pre-dawn leaf water potential (Ymax) was significantly lower in the rainfed than the irrigated treatment for six defined periods or stress cycles. At the end of this period, when Ymax was -2.37 and -2.34 MPa in the rainfed E. globulus and E. nitens respectively, leaf osmotic potential and bulk elastic modulus were still not significantly affected by water stress. At this time a significant interspecific difference was evident in the shape of the desorption isotherms. In the region of positive turgor the mean slope of these relationships was significantly greater in E. nitens (14.5 MPa) than E. globulus (9.3 MPa) resulting in turgor loss at a significantly higher relative water content in E. nitens (0.86) than E. globulus (0.79). This interspecific difference in leaf water relations was independent of soil water status. Allometric relationships between leaf area and sapwood area were developed by destructive sampling in July 1992 and July 1993. These relationships were used with monthly growth data to plot the course of leaf area index (L*) between August 1991 and April 1993. L* was significantly lower in E. nitens than E. globulus throughout the study and was significantly reduced by water stress after November 1992. During the 1992193 growing season the L* of the rainfed E. globulus increased rapidly after rewatering, resulting in a stepped pattern of leaf area development which was not observed in E. nitens. At the end of the experiment, in April 1993, L* of the irrigated and rainfed treatments were, respectively, 8.3 and 6.1 in E. globulus and 6.9 and 4.3 in E. nitens. Daily maximum stomatal conductance of both E. globulus and E. nitens was significantly reduced when Ymax was <-0.55 MPa. Reduction in stomatal conductance (gs) as Ymax decreased was greater in E. nitens than E. globulus. After rewatering, stomatal conductance was slower to recover in E. nitens than E. globulus. These differences caused a significant species by water stress interaction. A phenomenological model of stomatal conductance of upper canopy foliage (gsu) was developed for the irrigated trees. The maximum gsu observed (387 mmol m‑2 S-1) was attenuated with normalised functions of total solar radiation (Q), air temperature (T) and vapour pressure deficit (D). This model explained 70% of the variation in gsu. The effect of water stress on the daily total of gsu was accommodated by predicting the ratio of total daily stomatal conductance of rainfed to that of irrigated trees as a function of the cumulative water stress integral for the preceding x days (SYx, where x was eight days for E. globulus and 20 days for E. nitens. Using SYx as an independent variable accommodated the longer residual effect of water stress in E. nitens than E. globulus. The model explained 75 and 73% of the residual variation in the daily total of gsu of E. nitens and E. globulus after the direct effects of Q, T and D were removed. E. globulus and E. nitens responded to drought by stomatal closure rather than by adjustment of the osmotic or elastic properties of their leaves. This response was stronger in E. nitens than E. globulus so that on sites where water stress is moderate and seasonal, E. globulus will probably grow more rapidly than E. nitens. Another major outcome of this thesis is quantification of the different stomatal responses of the two species at the canopy scale in a way that allows transpiration to be calculated from relatively easily obtained variables. Calculation of canopy conductance as gc’ predicts a much lower value of g, for foliage at the bottom of high L* canopies than was measured. This suggests that as L* increased the irrigated canopies of both species became more water use efficient. It is argued that if the effects of water stress on g, and L* are considered together then long term water stress will reduce the water use efficiency of E. nitens more than that of E. globulus.

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