PHYSIOLOGICAL-RESPONSES OF PAPER BIRCH TO THINNING IN BRITISH-COLUMBIA

The influence of thinning on the net photosynthetic rate (A), specific leaf area (SLA), photosynthetically active radiation (PAR), water use efficiency (WUE) and nitrogen use efficiency (NUE) of 9 to 13-year-ol d paper birch (Betula papyrifera Marsh.) stands was studied during the 1992 and 1993 growing seasons. Study sites were located in the Interi or Cedar Hemlock (ICH) zone within the moist, warm subzone (ICHmw) in the Kamloops Forest Region, British Columbia, Canada. The birch stands were thinned in June 1991, from initial densities which ranged betwee n 11 000 and 31 000 stems ha(-1), to control (unthinned), 400, 1000 an d 3000 stems ha(-1). Significant physiological responses to thinning w ere observed during the subsequent two growing seasons. Photosynthetic rate of trees in the thinned stands increased significantly compared with that in unthinned trees. Photosynthetic rates between 11 :00 and 14:00 h on cloudless days in the second growing season were 2.2, 7.9, 10.6 and 16.4 mu mol CO2 m(-2) s(-1) for unthinned, 3000, 1000 and 400 stems ha(-1), respectively, This increase was attributed to increases in PAR and foliage nitrogen concentration and a decrease in SLA. Amon g the three thinning densities, the 400 stems ha(-1) treatment had the best physiological responses to thinning. Thinning these stands to 10 00 stems ha(-1) appears more appropriate, however, when timber managem ent objectives, such as improvement of stem quality, are considered.