VARIATION IN SHADE TOLERANCE OF DOUGLAS-FIR, WESTERN HEMLOCK, AND WESTERN RED CEDAR IN COASTAL BRITISH-COLUMBIA

This exploratory study examined the variation in shade tolerance of Douglas fir, western hemlock. and western red cedar in coastal British Columbia. To analyze the light environment-growth performance relationship, canopy transmittance, height increment, diameter at the base of the leader, vigor, vegetation competition, and ground surface materials data were obtained for naturally established or planted seedlings from 50 study sites which were located across a wide range of climatic and soil moisture conditions. Relationships between the percent of above-canopy light (in the photosynthetically active wavelengths (400-700 nm) associated with each seedling and its relative height increment and leader diameter were examined for individual study sites and groups of sites according to tree species and soil moisture regimes. Results indicated occurrence of both inter- and intraspecific variation in shade tolerance. Western red cedar performed relatively better at lower light levels than western hemlock, and western hemlock better than Douglas fir. The trend of increasing light required to achieve a given level of relative growth performance with increasing available soil moisture was consistent across all three species. It is concluded that the method developed has good utility but more rigorous studies are needed to define light environment-growth performance relationships and to explain ecological and physiological mechanisms involved.