Stomatal conductance and photosynthesis in a mature Scots pine forest. I, Diurnal, seasonal and spatial variation in shoots.
In measurements made on a 45-yr-old stand at Thetford Forest, East Anglia, UK, from Mar. to Oct. 1976, shoot conductance declined during the day as water vapour saturation deficit increased, but there was little evidence of stomatal closure as a result of internal water deficits in spite of drought conditions during summer months. Max. conductance (0.5-0.7 cm/s) was obtained in Sep. and Oct. Photosynthesis was closely related to changing incident quantum flux density, except at large saturation deficits when rates became CO2-limited as a result of stomatal closure. Max. photosynthesis rates were 0.55 mg/m2 per second. Rates of photosynthesis and conductance declined with depth in the canopy and varied diurnally with aspect. The results showed that most CO2 assimilation occurred in the upper half of the canopy; it is concluded that water losses from transpiration could be significantly reduced, with minimal effect on growth, by pruning the lower parts of the tree crown.