Impacts of seasonal air and soil temperatures on photosynthesis in Scots pine trees

Seasonal courses of light-saturated rate of net photosynthesis (A(360)) and stomatal conductance (g(s)) were examined in detached 1-year-old needles of Scots pine (Pinus sylvestris L.) from early April to mid-November. To evaluate the effects of soil frost and low soil temperatures on gas exchange, the extent and duration of soil frost, as well as the onset of soil warming, were manipulated in the field. During spring, early summer and autumn, the patterns of A(360) and g(s) in needles from the control and warm-soil plots were generally strongly related to daily mean air temperatures and the frequency of severe frosts. The warm-soil treatment had little effect on gas exchange, although mean soil temperature in the warm-soil plot was 3.8 degreesC higher than in the control plot during spring and summer, indicating that A(360) and g(s) in needles from control trees were not limited by low soil temperature alone. In contrast, prolonged exposure to soil temperatures slightly above 0 degreesC severely restricted recovery of A(360) and especially g(s) in needles from the cold-soil treatment during spring and early summer; however, full recovery of both A(360) and g(s) occurred in late summer. We. conclude that inhibition of A(360) by low soil temperatures is related to both stomatal closure and effects on the biochemistry of photosynthesis, the relative importance of which appeared to vary during spring and early summer. During the autumn, soil temperatures as low as 8 degreesC did not affect either A360 or g(s).