Comparison of interception, forest floor evaporation and transpiration in Pinus radiata and Eucalyptus globulus plantations

Studies of evapotranspiration (ET) processes in forests often only measure one component of total ET, most commonly interception. This study examined all three components of annual ET (interception, evaporation from the forest floor and transpiration) and the correlations between them at 18 plantation forest sites in two species. All plantations had closed canopies, and sparse or no understorey. Single-sided leaf area index averaged 3.5 (standard deviation 0.5) in Eucalyptus globulus Labill. and 6.1 (0.8) in Pinus radiata D.Don. Measurements included annual totals of rainfall in the open and under the canopy, stem flow (four sites only), evaporation from the forest floor and transpiration by the overstorey. Interception (I) averaged 19% (+/- 4.9) of annual rainfall in E.globulus compared with 31% (+/- 11.1) in P.radiata. However, higher annual interception in P.radiata did not result in higher total ET because annual evaporation from the forest floor (E) averaged 29% (+/- 4.9) of rainfall in E.globulus but only 15% (+/- 3.5) in P.radiata. Hence, the relative contribution of annual I plus E to ET did not differ significantly between the two species, averaging 48% (+/- 7.3) of annual rainfall in E.globulus compared with 46% (+/- 11.8) in P.radiata. As reported previously, transpiration did not differ significantly between the two species either, but was strongly related to depth-to-groundwater. In closed canopy plantations, mean annual ET did not differ between the two species. We conclude that when grown in plantations under similar soil and climatic conditions, conifer and broad-leaved tree species can have similar annual ET, once the canopy of the plantation has closed. Lower average annual interception in broad-leaved trees was offset by higher soil evaporation. These results highlight the importance of measuring all components of ET in studies of vegetation water use. Copyright (c) 2014 John Wiley & Sons, Ltd.