Stomatal Conductance and Photosynthesis in a Mature Scots Pine Forest. I. Diurnal, Seasonal and Spatial Variation in Shoots

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Abstract
Water use by coniferous plantations can lead to water shortages in regions where water is a scarce resource. If water losses are to be reduced by silvicultural practice which minimal effects on growth, it is essential to know where in the canopy are to be found the main sources of water and sinks for CO2. A null-balance porometer was used to measure the stomatal conductance of current-year shoots from different levels, aspects, trees and age-classes in the canopy of a plantation forest of Scots pine (P. sylvestris) in eastern England [UK]. Concurrent measurements of photosynthesis were made on adjacent shoots by measuring the uptake of 14CO2 over 30 s. Measurements were made in 1976 in each of the months from March to Oct. Shoot conductance declined during the day as the water vapor saturation deficit increased, but there was little evidence of stomatal closure as a result of internal water deficits, in spite of the drought conditions experienced during the summer months. Maximum values of conductance (0.5-0.7 cm s-1) were obtained in Sept. and Oct. Photosynthetic rates were closely related to the changing incident quantum flux density except at large saturation deficits when rates became CO2 limited as a result of stomatal closure. Maximum rates of photosynthesis were 0.55 mg m-2 s-1. Rates of photosynthesis and conductances of shoots declined with depth in the canopy and varied diurnally with aspect. There were two age-classes of shoot present but it was not possible to establish consistent ratios of performance between them. The variation of photosynthesis and conductance between trees was considerably less than that within a tree. These results demonstrate that most CO2 assimilation occurs in the upper half of the canopy and suggest that water losses in transpiration could be significantly reduced, with minimal effects on growth, by pruning the lower parts of the tree crowns.