Effects of overtopping vegetation on light availability and growth of Engelmann spruce (Picea engelmannii) seedlings

To examine the effects of overtopping vegetation on solar irradiance reaching Engelmann spruce (Picea engelmannii Parry) seedlings and on crop seedling performance, neighborhood studies were established at six sites in mixed shrub-herb and fireweed communities of the Interior Cedar-Hemlock zone of southern British Columbia. These communities were dominated by mixtures of fireweed (Epilobium angustifolium L.), thimbleberry (Rubus parviflorus Nutt.), red raspberry (Rubus idaeus L.), and (or) bracken fern (Pteridium aquilinum (L.) Kuhn). Three hundred and eighty-eight planted Engelmann spruce seedlings, between 1 and 5 years of age, served as plot centers for measurements in 1990 and 1991. Vegetation had been clipped around 109 of these seedlings in 1988, 1989, and 1990. Seedling diameter and height were measured in the autumn of 1990 and 1991. Vegetation cover, vegetation height, and the fraction of full sunlight reaching the top branches of each spruce seedling were measured in midsummer of 1991. A simple competition index (CI), based on visually estimated percent cover, height of each species present within 1.26 m of the crop seedling, and crop-seedling height, can be used to estimate the fraction of full sunlight reaching crop seedlings. Measurements of transmittance through vegetation canopies are correlated with this competition index and could be useful as a separate, objective assessment of competition levels. Seedling growth was related to both seedling-needle biomass (at the beginning of the year) and either CI or the amount of light reaching crop seedlings during midsummer. Growth increased as needle biomass or irradiance reaching the seedling increased and decreased as CI increased. Since CI can be calculated from simple field measurements and since seedling growth is related to CI, this index has potential utility in evaluating the need for operational release treatments. Transmittance measurement provides an equally useful, and potentially more objective approach to evaluating the need for release treatment. Results from this study suggest that cumulative effects of competition result at least in part from effects on growth of seedling leaf mass.