Variation in leaf conductance of silver birch: effects of irradiance, vapour pressure deficit, leaf water status and position within a crown

Leaf conductance decreased if Ψx fell below certain values after the noon, while the sun foliage experienced greater negative water potentials than the shade foliage. In a diurnal scale the influence of bulk leaf water potential on gL altogether was rather weak. The lower-canopy foliage exhibited more conservative water-use behaviour, having lower maximum stomatal conductance and greater sensitivity to Ψx bringing about a smaller responsiveness to VPD than the upper canopy foliage. The mean GT was 1.7-1.8 times bigger (P < 0.001) for the upper canopy, compared to the lower canopy; the shade foliage responded more sensitively to changes in GT; there were steeper water potential gradients between the soil and lower-canopy leaves; gL in the lower-canopy foliage was more strictly controlled by leaf water status-the evidence suggesting that the shade foliage may be hydraulically more constrained. We set up a hypothesis that stomatal conductance at the base of the live crown is limited not only by low light availability but also by plant's inner hydraulic constraints.