Partitioning of forest evapotranspiration: The impact of edge effects and canopy structure

The magnitude of small scale variability in tree transpiration was explored in a structurally heterogeneous Norway spruce (Picea abies (L.) H. Karst.) plantation in western Denmark. The trees are arranged in a distinctive small scale mosaic (0.25 ha) of young open-canopy stands interspaced with older mature closed-canopy stands. Tree transpiration was measured in the open and closed canopy stands using Granier type thermal dissipation probes; forest floor evapotranspiration (ET) was estimated by monitoring the weight of cut out sections of forest floor; and total ET was estimated using the eddy covariance (EC) method. We show that (a) canopy structure had a major impact on transpiration rate allowing the open-canopy stands to transpire at approximately 30% higher rate than the closed-canopy stands; (b) within the open-canopy stand there was a significant relation between tree size and sap-flux density; and (c) within the closed-canopy stands there was an edge effect with trees next to access roads and aisles being responsible for a disproportionately large part of the stand transpiration. Through careful scaling, taking into account the observed variability, it was possible to get good agreement on dry days between independent measures of transpiration plus forest floor ET and the EC estimate of total forest ET. On average transpiration and forest floor ET amounted to 86% of the EC estimate on dry days.