ECOPHYSIOLOGY OF SHADE NEEDLES OF PICEA-GLAUCA SAPLINGS IN RELATION TO REMOVAL OF COMPETING HARDWOODS AND DEGREE OF PRIOR SHADING

We studied shade needles of Picea glauca (Moench) Voss saplings that w ere growing in 15, 45 or 90% shade from competing vegetation at northe rn boreal forest sites in Alberta and Saskatchewan. At each site, in l ate May or early June 1990, all hardwoods were removed within a 2-m ra dius of each of eight saplings in each shade treatment (released sapli ngs), and eight saplings in each shade treatment were left as controls . Light-saturated net assimilation (NA), stomatal and mesophyll conduc tance and water use efficiency of one-year-old needles were measured f our times during the 1990 growing season and in the spring of 1991. Th ere was a trend of increased photosynthetic capacity within one week f ollowing release. By August 1990 and in the following spring, NA was h igher 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 conduct ance to CO2. 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.