Daily, seasonal and annual patterns of transpiration from a stand of remnant vegetation dominated by a coniferous Callitris species and a broad-leaved Eucalyptus species

Quantifying water use of native vegetation is an important contribution to understanding landscape ecohydrology. Few studies provide long-term (more than one growing season) estimates of water use and even fewer quantify interseasonal and interannual variation in transpiration. Globally, changes in land use are significantly altering landscape ecohydrology, resulting in problems such as dryland salinity and excessive groundwater recharge. Estimating stand water use is complex in multispecies forests, due to the differences in relationships among sapwood area, basal area and tree size for co-occurring species. In this article, we examine seasonal and interannual variation in transpiration rate of the tree canopy of two co-occurring species (a conifer Callitris glaucophylla J. Thompson & L.A.S. Johnson and a broad-leaved Eucalyptus crebra F. Muell.) in an open woodland in eastern Australia. Evapotranspiration of understorey species was measured using an open-top chamber, and tree water use was measured using heat-pulse sap flow sensors. Annual stand transpiration was 309 mm in 2003, a year of below average rainfall, and 629 mm in 2004, a year with higher-than-average rainfall. Despite an almost doubling (522 vs. 1062 mm) of annual rainfall between 2003 and 2004, annual tree water use remained a constant fraction (59%) of rainfall, indicative of compensatory mechanisms linking annual rainfall, leaf area index and tree water use. Deep drainage was estimated to be 4% of rainfall (20.8 mm) in 2003 and 2% (21.2 mm) in 2004, indicating that this native woodland was able to minimize deep drainage despite large interannual variability in rainfall.