Growth of Thuja plicata seedlings along a light gradient

Abstract

To improve understanding of light – growth response relationships in western red cedar, seedlings were grown on three environmentally comparable sites representing nearly optimum growth conditions for the species in coastal British Columbia. On each site, the seedlings were planted on transects extending from a clearcut to the interior of an adjacent old-growth stand. After three growing seasons, seedling mortality, vigour, growth characters, and foliar nutrients were compared across five light classes and between study sites using analysis of variance and related to the percentage of above-canopy light available to each seedling using regression analysis. Three-year mortality was ≤ 27%, and most mortality (≥ 85%) occurred at ≤ 20% of above-canopy light. Seedling growth improved consistently with increasing light; however, the actual growth response at > 40% of above-canopy light varied with study site. The pattern of variation in the growth characters suggested that planted seedlings were able to acclimate morphologically to varying light conditions in three structural traits typically associated with shade adaptation: specific leaf area, height to caliper ratio, and aboveground to root biomass ratio. Specific leaf area and height to caliper ratio decreased and aboveground to root biomass ratio increased with increasing light. This study does not support the contention that under optimal moisture and nutrient conditions carbohydrate deficiency increases the allocation of net biomass production to aboveground structures. We consider western red cedar in the study area to be a. very shade-and exposure-tolerant species but not one that is specifically adapted to an understory environment. Key words: Thuja plicata seedlings, light – growth response relationships, specific leaf area, height to caliper ratio, aboveground to root biomass ratio, shade tolerance.