Sap flow of birch and Norway spruce during the European heat and drought in summer 2003

In this study the hydrological regime of Norway spruce (Picea abies) and birch (Betula pendula) growing on heavy soils in the south east of Austria was analysed. Results from the year 2003 characterised by an extremely hot and dry summer are presented in this paper. Due to the extreme weather conditions the soil water content in August 2003 was very low (0.10–0.25 m3 m−3) in the topsoil (0–50 cm) with no explicit difference between both tree species.Sap flow was measured in 11 trees of each of both tree species using the heat field deformation (HFD) method. Only the qualitative variability of sap flow was analysed so that data of sap flow per section (SFS) were used. The SFS of the birch trees was about three times higher than that of the spruce trees. In June 2003 the daily SFS of the birch trees averaged 0.87 kg cm−1 and of the spruce trees 0.23 kg cm−1. Due to the proceeding drought in August 2003 these values were reduced to 0.39 kg cm−1 and 0.11 kg cm−1, respectively. A comparison of daily potential evapotranspiration (PET) with daily SFS values showed that the transpiration of the trees followed the atmospheric demand in June and in the second half of July. In August 2003 the SFS depended solely on the soil water content. Birch trees began to transpire earlier in the morning and could keep their maximum SFS longer during the midday hours than the spruce trees, when sufficient soil water was available. As the drought stress proceeded, significant differences in the diurnal variations of SFS were detected for both tree species. Most of the investigated birch trees could maintain their common daily SFS dynamic whereas spruce trees showed a different pattern. One part of the spruce trees showed a midday depression of SFS, whereas the others closed their stomata during the whole day.