Photosynthetic light response and growth analysis of competitive regeneration after partial cutting in a boreal mixed stand

Abstract.

The ecophysiological development of two competitive species naturally regenerated after partial cutting was studied in a boreal mixed stand of eastern Canada. Photosynthetic light response curve and growth analyses were performed on balsam fir [Abies balsamea (L.) Mill.] and trembling aspen (Populus tremuloides Michx.) natural regeneration growing at different light intensities. Five levels of light intensity (14, 21, 26, 30 and 76% of full light) were obtained by applying the following treatments: three partial cutting intensities (removal of 35, 50 and 65% of the initial basal area), a clearcut (removal of 100% of the initial basal area) and an uncut control. Leaves of aspen suckers showed higher values of photosynthetic capacity, apparent quantum yield, light compensation point and dark respiration in comparison to fir seedlings at all light levels. In response to light availability, photosynthetic plasticity of aspen was greater than that of fir. Aspen was less able to maintain a positive net photosynthesis at low light than fir, and this could reduce its survivorship, particularly at 14% and 21% of full light. Leaf area ratio of balsam fir was higher than for aspen at all light levels, but both species showed a general increase in leaf area ratio as percent of full light decreased. Above-ground relative growth rate of aspen was higher than for fir at all light levels. At 14% and 21% of full light, aspen suckers had higher height-to-stemwood weight ratios than at higher light intensities. This higher allocation to height growth formed etiolated seedlings that can hinder the long-term survivorship of aspen in low light.