Evaporation and transpiration from forests in Central Europe relevance of patch-level studies for spatial scaling

Spatial scaling from patch to the landscape level requires knowledge on the effects of vegetation structure on maximum surface conductances and evaporation rates. The following paper summarizes results on atmospheric, edaphic, and structural controls on forest evaporation and transpiration observed in stands of Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and European beech (Fagus sylvatica). Forest canopy transpiration (E-c) was determined by tree sapflow measurements scaled to the stand level. Estimates of understory transpiration and forest floor evaporation were derived from lysimeter and chamber measurements. Strong reduction of E-c due to soil drought was only observed at a Scots pine stand when soil water content dropped below 16% v/v. Although relative responses of E-c on atmospheric conditions were similar, daily maximum rates of E-c (E-cmax) could differ more than 100% between forest patches of different structure (1.5-3.0 mm d(-1) and 2.6-6.4 mm d(-1) for spruce and beech, respectively). A significant decrease of E-cmax per leaf area index (E-cmax/LAI) with increasing stand age was found for monocultures of Norway spruce, whereas no pronounced changes in E-cmax/LAI were observed for beech stands. It is concluded that structural effects on E-cmax can be specified and must be considered for spatial scaling from forest stands to landscapes. Hereby, in conjunction with LAI, age-related structural parameters are important for Norway spruce stands. Although compensating effects of tree canopy layers and understory on total evaporation of forests were observed, more information is needed to quantify structure-function relationships in forests of heterogenous structure.