Evaluation of competitive effects of green alder, willow and other tall shrubs on white spruce and lodgepole pine in Northern Alberta

Abstract

In boreal forests of western Canada, lodgepole pine ( Pinus contorta Dougl. ex. Loud.) and white spruce ( Picea glauca (Moench) Voss) often grow together with numerous tall shrubs such as green alder ( Alnus crispa (Ait.) Pursh) and little-tree willow (Salix spp .). In an area south of Grande Prairie, Alberta, Canada, we examined the effects of shrubs, herbs and other trees on nutrient and light availability and growth of white spruce and lodgepole pine. For white spruce the best competition measure (tested against volume increments of the past 3 years) was visually estimated % ground cover times the height of the competitor (VCHT) with light (DIFN) ranking in third place. For lodgepole pine, DIFN was the best competition measure for predicting volume increment and the best competition index was again VCHT. Taller conifers had a stronger competitive effect than tall shrubs, with their effect on white spruce being larger than that on lodgepole pine. Comparison of the effects of alder and willow indicates that both species are having negative effects on growth of pine and spruce on these sites with green alder having a stronger influence on spruce than pine due to the smaller size of the spruce. Herbaceous cover was having more effect on white spruce than lodgepole pine and may be related to both their height and differences in rooting depth. The results did not show any significant relationship ( p > 0.05) between nitrogen availability and cover of alder and willow and between conifer foliar nutrient concentrations and cover of alder and willow. Our study suggests that the current Alberta regeneration standards may be overestimating the effect that competing vegetation has on conifer growth since significant growth reduction occurs only at high shrub densities.