ANALYSIS OF GROWTH AND LIGHT INTERCEPTION OF BALSAM FIR AND WHITE BIRCH SAPLINGS FOLLOWING PRECOMMERCIAL THINNING

A precommerical thinning was conducted on young balsam fir (Abies balsamea (L) Mill) and white birch (Betula papyrifera Marsh) trees. Changes in light environment and growth response of the trees were followed during the next 2 growing seasons. The relative growth rate (RGR) of thinned balsam firs increased during both the first and the second growing season. This increase in growth was attributed to a greater net assimilation rate (NAR) which was associated with a higher level of light availability. Thinning tended to positively affect the RGR of white birch during the first summer following treatment but not during the second growing season. Similar fluctuations were noted for NAR even though light levels remained high for thinned white birch trees during both the first and the second growing season. Balsam fir produced more sapwood per unit of additional leaf area than controls during the first summer following treatment but no differences were observed during the second. The sapwood area growth to leaf area growth ratios of thinned and control white birches were similar during both the first and the second summer following thinning. Thus the sapwood area-leaf area relationship appears to be more stable following abrupt changes in environmental conditions for the indeterminate growth species, white birch, than for the determinate growth species, balsam fir.