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Is the efficiency of sunfleck utilization an important determinant of shade tolerance? : a case study with Western redcedar and coastal Douglas-fir in the understory of red alder McKinnon, L. M.


To determine whether efficient photosynthetic utilization of sunflecks may be an important determinant of shade tolerance among sympatric conifer species, transient and steady-state photosynthetic responses to light were characterized for two conifers of contrasting shade tolerance growing under a closed canopy of red alder {Alnus rubra Bong.). Western redcedar {Thuja plicata Donn) is a very shade tolerant species which can survive for extended periods in deep understory shade, whereas coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) exhibits poorer survival and is considered shade intolerant. The requirement for photosynthetic induction initially limited photosynthesis in both species by about 70%. Given constant high light, times to reach 50% and 90% of full photosynthetic induction were 3.5 and 31.8 minutes for redcedar and 2.2 and 19.8 minutes for Douglasfir. However, despite requiring more time to become fully induced, redcedar attained similar or higher absolute photosynthetic rates (on both area and mass bases) as Douglas-fir during the same induction time-course. Little loss of induction occurred in either species during 2 and 5 minutes in shade, but redcedar maintained induction better than Douglas-fir after 15 and 30 minutes. This was, however, unlikely to be important on sunny days when the time between most sunflecks was <30 seconds and only rarely exceeded 5 minutes. When given a fluctuating light regime consisting of five 30 second lightflecks (simulated sunflecks) separated by 2 minute intervals of low light, redcedar achieved a similar induction state as Douglas-fir because it maintained a high rate of stomatal opening between lightflecks which compensated for its longer initial lag period for stomatal opening. Lightfleck use efficiency and the contribution of net post-illumination C 0 2 fixation to the overall carbon gain attributable to lightflecks did not differ significantly between species, nor did absolute carbon gain or rates of carbon gain in fluctuating light. However, while the two species did not clearly separate on the basis of photosynthetic induction characteristics, gas exchange measurements under steady-state conditions indicated that redcedar had an approximately two-fold lower foliar dark respiration rate and light compensation point than Douglas-fir. It is concluded that interspecific differences in the efficiency of sunfleck utilization are much less likely to be important in explaining the contrasting shade tolerance of these species than differences in dark respiration.

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