The Water Relations of Pinus sylvestris

The water potential (β) in ten-year-old Scots pine trees (Pinus sylvestris L.) from four different latitudinal provenances ranging from 57° N to 67° N, growing in close proximity to each other, was measured in a pressure chamber during five selected periods between May 1972 and March 1973. Diurnal and seasonal patterns of φ are presented in relation to irradiance, air temperature and vapour pressure deficit (D) for one cloudy and one clear day in each experimental period. The largest daily amplitude in φ was found at the beginning and end of summer, indicating a larger resistance to water flow from soil to needles. As the soil water potentials (measured as pre-dawn values) were, at the same time, the highest for the year, it is suggested that these changes in resistance from period to period mainly take place in the trees. Plots of φ against D during clear days, showed marked hysteresis as the result of the simultaneous influence on φ of several environmental factors. Close linear relations resulted when φ was plotted against potential evaporation rate (calculated from the Pennman-Monteith formula). The slopes of these regression lines, essentially the flow resistance, showed marked seasonal variations, with the largest resistance found at the beginning and end of summer. In most periods the water relations of the trees from different provenances were strikingly similar. A continuous change in the water relations of the remote provenances towards the situation for the “home-provenance” is indicated by the experimental results. It is concluded that trees from different latitudes after ten years of growth have about the same chance as the home provenance to survive periods during which their water balance might become critical.