Growth of planted tree seedlings in response to ambient light levels in northwestern interior cedar-hemlock forests of British Columbia

Insights into field-planted conifer seedling growth were gained by fitting height and diameter growth to relative irradiance over the growing season using Michaelis-Menten functions. There was little difference among tree species (Abies lasiocarpa (Hook.) Nutt., Picea glauca (Moench) Voss x Picea sitchensis (Bong.) Carr., Pinus contorta Dougl. ex Loud., Thuja plicata Donn ex D. Don, Tsuga heterophylla (Raf.) Sarg.) in response to ambient light. No significant differences in whole-plant compensation paints were observed among species but the ranking of species' compensation points was consistent with their shade-tolerance ranking. Five years after planting, total size and recent growth rates varied little among species from low to high light, implying an absence of trade-offs in low- and highlight growth strategies. Thuja plicata had the greatest response to increased light under deep shade (<20% relative irradiance). All species increased growth above 40% relative irradiance, with no clear whole-plant light saturation point evident under field conditions. Growth rates at high light were broadly overlapping and varied considerably within species. As expected, Pinus contorta growth exceeded that of other species above 70% relative irradiance, but it also exhibited high growth rates at low light. Greatest variability among species was at intermediate light levels (30-70% relative irradiance) where careful matching of tree species to light environment can maximize growth rates.