Abstract
We studied shade needles of Picea glauca (Moench) Voss saplings that were growing in 15, 45 or 90% shade from competing vegetation at northern boreal forest sites in Alberta and Saskatchewan. At each site, in late May or early June 1990, all hardwoods were removed within a 2-m radius of each of eight saplings in each shade treatment (released saplings), and eight saplings in each shade treatment were left as controls. Light-saturated net assimilation (NA), stomatal and mesophyll conductance and water use efficiency of one-year-old needles were measured four times during the 1990 growing season and in the spring of 1991. There was a trend of increased photosynthetic capacity within one week following release. By August 1990 and in the following spring, NA was higher in released trees than in controls. The increase in NA appeared to be related to increased stomatal conductance to water vapor and to increased foliar nitrogen and resulting increases in mesophyll conductance to CO(2). There was no measurable effect of degree of shading prior to release on NA following release. Foliage of the released saplings appeared capable of rapid acclimation to the open conditions.
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