- Water use of forests along elevation gradients in the Berchtesgaden National Park (2005)
- Forests are major sources of water vapor to the atmosphere and have an important influence on both regional and global climate. Understanding of the water exchange between the vegetation and the atmosphere, and recognition of factors to predict the water fluxes is necessary to estimate the changes in water use according to possible climate change. In this study the processes affecting the transpiration of Norway spruce (Picea abies) in the Berchtesgaden National Park along an elevation gradient were investigated. Sapflow and meteorological measurements were carried out during the summer of 2002. For each stand, structural differences (tree height, circumference, diameter (DBH), tree density, basal area) were also analyzed. For the experiment, three Norway spruce stands – with similar inclination, exposition, LAI and soil conditions, but situated at different elevations a.s.l in the Berchtesgaden National Park were selected. The transpiration rates of spruce were compared with those of other species: Fagus sylvatica, Larix decidua and Pinus mugo. In each stand, six to fifteen trees were selected for sapflow measurements. Xylem sap flux was observed with constant-heating-method according to Granier. Sapflow rates of individual trees were scaled up to the stand level according to the cross-sectional sapwood area of the DBH class (for spruce and beech) or total cross-sectional sapwood area (for larch). Within the sparse Larix decidua stand, both the sapflow in trees and water loss from understory vegetation were measured. Strong correlations between transpiration rates and maximum VPD and daily integrated PPFD were found for the experimental species. For the Norway spruce stands, needle nutrient concentrations were also determined. During the 2002 growing season (May- September), the seasonal patterns of tree canopy transpiration were similar for the different experimental stands. The highest water use was observed in June and July for spruce and dwarf-pine, and in July and August for beech. The seasonal changes in canopy transpiration and conductance, and the variation between spruce stands, may be in part explained by differences in meteorological conditions. Canopy conductance was calculated from canopy transpiration (Ec) and vapor pressure deficit (VPD). Absolute rates of seasonal canopy transpiration within experimental Picea abies sites differed greatly from 78 to 165 mm. The highest transpiration rates occurred in the lowest situated stand and they decreased with increasing elevation. The seasonal water loss from the Fagus sylvatica site (78 mm) was similar to the water loss at the highest situated spruce site. The seasonal transpiration of Pinus mugo site reached 44 mm. Transpiration of Larix decidua was only measured successfully during June and during a few days in July. A strong decrease of canopy conductance (gt) with increasing VPD was observed for all stands. The highest maximum canopy conductance for spruce, 20 mm s-1, was calculated for the 630 m site, while for the 1040 m and the 1360 m sites the maximum gtmax reached 15 mm s-1. In comparison the maximum canopy conductance for the beech site was higher than for the spruce site at the same elevation a.s.l. (630 m) and reached 30 mm s-1. The lowest maximum gt, 10 m s-1, was estimated for dwarf-pine shrub. For the examination of controls on spruce forest water use that were found along the elevation gradient, the GAS-FLUX model was used. Model simulations support two alternative hypotheses, namely that stomatal patchiness as well as carboxylation capacity (Vcmax) may have had an influence on stand water use. The modelled transpiration with changes in Vcmax was similar to estimated transpiration rates obtained by changing the “portion of leaves that are active” (stomatal patchiness). The developed model can be used for estimation of water use for other similar mountainous locations or for examination of ecosystem sensitivities to environmental changes (nitrogen supply, air temperature). The current work should stimulate further research on the detailed study of e.g. stomatal patchiness and its influence on transpiration rates, or to test the results at other sites.