Effects of shade on morphology, chlorophyll concentration, and chlorophyll fluorescence of four Pacific Northwest conifer species

Four species of Pacific Northwest conifer seedlings (ponderosa pine [Pinus ponderosa Dougl. ex Laws.], Douglas-fir [Pseudotsuga menziesii (Mirb.) France], western redcedar [Thuja plicata Donn ex D. Donn], and western hemlock [Tsuga heterophylla (Raf.) Sarg.]) were planted in individual pots and grown under shade-cloth shelters that provided four levels of shade (0% [full sunlight], 35%, 55%, and 75%) for approximately 30 weeks. Height growth was recorded every 2 weeks. Initial and final seedling morphology and chlorophyll concentrations were measured. Chlorophyll fluorescence was measured monthly. All species responded similarly to shade. Although height growth was greatest under 75% shade and least in 0% shade, total biomass production was significantly lower and shoot:root ratio significantly higher in 75% shade than in 0% shade. As the treatment shade level increased from 0% to 75%, F-v/F-m was significantly lower while chlorophyll concentrations were significantly higher. These results indicate that photochemical efficiency of all four species was lower under higher shade. Morphological and physiological responses differed greatly among species and corresponded with their degree of shade tolerance.