Performance of two Picea abies ( L . ) Karst . stands at different stages of decline III . Canopy transpiration of green trees

The water relations of Picea abies in a healthy stand with green trees only and a declining stand with trees showing different stages of needle yellowing were investigated in northern Bavaria. The present study is based on observations of trees differing in their nutritional status but apparently green on both sites in order to identify changes in the response pattern which might be caused by atmospheric concentrations of air pollutants and could lead to the phenomenon of decline. Transpiration was measured as water flow through the hydroactive xylem using an equilibrium mass-flow measurement system. Total tree transpi-ration was monitored diurnally, from July 1985 until Oc-tober 1985 at both sites. The relationship between transpira-tion and meteorological measurements indicated that tran-spiration was a linear function of the vapor pressure deficit. No differences in transpiration of green trees were observed between the two sites. Canopy transpiration was 57%-68% of total throughfall and 41%-54% of total rainfall. Due to this positive water balance, soil water potential at 10 and 20 cm depths remained close to-0.02 MPa (max.-0.09 MPa) for most of the summer. Soil water potential was correlated with the difference between the weekly precipitation and transpiration. No differences in the water relations of apparently healthy trees in the two P. abies stands were observed. It is concluded that differences between green trees at the two sites in terms of nutrient relations or growth rate cannot be explained by changes in whole-tree transpiration or soil water status. An understanding of the water relations of a tbrest stand is a necessary component of an assessment of stand vigor. The stands of Picea abies considered in this study had large differences in growth rate (Oren et al. 1987a). The stand with the higher growth rate appeared healthy while the other stand had extensive visible needle yellowing. The visibly damaged stand receives significantly higher H + inputs as a result of higher sulphate and nitrate depqsition (Hantschel et al. 1985). Growth differences between the stands and needle yellowing are associated with the nutritional status of trees in the stands (Oren et al. 1987 b; Meyer Offprint requests to: E.-D. Schulze et al. 1987). Since the uptake of several important plant nutrients is strongly related to the rate of transpiration (Kirkby and Pilbeam 1984), it is the aim of this study to consider to what extent transpiration and soil water regimes of the two stands were correlated with …