- Canopy transpiration of beech forests in Northern Bavaria – Structure and function in pure and mixed stands with oak at colline and montane sites (2007)
- Mixed stands of broadleaved deciduous tree species are increasing in importance in Bavaria and all of Germany, especially in the context of climate change, forest conversion and sustainable forestry. Very few investigations to date have quantified the canopy transpiration of mixed forest stands, and particularly of mixed stands of European beech and sessile oak. This study shows results of transpiration on different spatial-structural scales (within-tree, whole-tree and stand level) as estimated from xylem sap flow measurements, in two mixed stands in the colline-submontane Steigerwald, a pure beech plot in the Steigerwald and a montane pure beech stand in the Fichtelgebirge in Northern Bavaria. Both the short-term control on transpiration via environmental drivers and longer-term control via structural drivers were analysed. Assessed structural characteristics were stem diameter, tree height, stand density, basal area, sapwood area and leaf area index LAI. At the within-tree level the axial xylem sap flow density, Js, declined radially from outer to inner sapwood in beech. This decrease was stronger in small trees than in large trees. Js at any sapwood depth increased with stem diameter. In oak a significant correlation of Js with stem diameter was only observed in the oak-rich stand. Js in the outer sapwood was the same order of magnitude in beech and oak at a given stem diameter. The seasonal maximum daily whole-tree water use, Qt max, of all studied beech trees was influenced positively but non-linearly by stem diameter, leaf area and sapwood area, whereas in oaks such correlations were weaker. The maximum tree water use per unit leaf area increased with tree height in beech. In oak, values were smaller and followed an opposite trend. At the stand level and at comparable LAI or basal area, maximum daily and seasonal rates of canopy transpiration for the mixed stands in the Steigerwald were 2.3 to 2.7 mm per day and 166 to 217 mm per season, while the highest rates, observed in the pure beech plot in the Steigerwald, reached up to 4.4 mm per day and 349 mm per season. Among the mixed stands, Ec increased with a higher contribution of beech to stand level LAI or sapwood area, As. Maximum daily canopy transpiration, Ec max, increased with increasing LAI. Ec max also increased with As and saturated at the highest values of As observed in the pure beech plots. The pattern of the highest values of maximum diurnal canopy conductance among the sites and species studied reflected that of Ec max. At the higher elevation site in the Fichtelgebirge beech trees with comparable stem diameter achieved rates of Js and Qt max similar to those at the lower elevation site in the Steigerwald. Also at the stand level, the two sites with similar LAI compared well regarding Ec max. The high elevation stand transpired about 120 mm less over the whole season. The results show that systematic relationships exist between structure and function in pure beech stands and in mixed stands with oak; at the within-tree, the whole-tree and the stand level.