Canopy conductance and water use in Eucalyptus plantations

Previous studies have shown leaf water efficiency (transpiration per unit of leaf area) to be similar among trees and stands of different Eucalyptus species in similar soil and climatic conditions. On the other hand, leaf water efficiency may vary widely among stands in different regions. The present study compares young plantations of a single species (Eucalyptus camaldulensis Denhn.) on two sites with contrasting conditions in Australia and Pakistan, to examine the nature of differences in leaf water efficiency and the factors that cause it. Tree water use was measured on up to 28 trees per site over 12 months using heat pulse equipment with a 30 minute sampling interval and logging period of 3-4 weeks per tree. Observations of climate and soil parameters were also recorded automatically at 30-minute intervals. Water use over a 12-month period was 1160 mm at Pacca Anna (Pakistan) and 310 mm at Girgarre (Australia). This is predominantly a re sult of higher sap flux density (and hence leaf water efficiency) at Pacca Anna. Sap flux density at Girgarre reached its maximum value at a daily mean vapour pressure deficit of approximately 1.2 kPa, but at Pacca Anna sap flux density continued to increase at daily mean vapour pressure deficit of up to 6 kPa. Greater hydraulic conductivity of the sandy soil at Pacca Anna, or higher rhizospheric conductance into a more extensive root system, also do not account for the observed differences in the vapour pressure deficit at which transpiration is a maximum. The results imply that the slope of the decline in stomatal conductance with increasing vapour pressure deficit differs between sites, although maximum stomatal conductance is the same. Variations in leaf water efficiency between sites may arise from the influence of environmental factors such as air or soil temperature on stomatal functions, possibly mediated by hormonal responses.