Abstract
Climatic sensitivity of white spruce (Picea glauca (Moench) Voss) was examined growing in association with trembling aspen (Populus tremuloides Michx.) at their southern limit of distribution in a transitional ecotone between the southern boreal forest and northern prairie region. The study was carried out in the Spruce Woods Provincial Park (SWPP) located in southwestern Manitoba, Canada. The dry regional climate restricted trembling aspen growth during the growing season via moisture deficiency and temperature induced drought stress. Warm, mild winters also negatively affected radial growth of trembling aspen. Growth of white spruce was moderated by conditions within the aspen stands as radial growth patterns showed low variability from year to year, a low common growth signal, and a stronger response to temperature than to precipitation. Nonetheless, the dry regional climate still restricted growth of white spruce during the growing season via temperature induced drought stress. The findings of the study for white spruce support the stress gradient hypothesis in which facilitative interactions between tree species are expected under harsher environmental conditions.
Highlights
Mixed species forest stands generally have higher overall long-term productivity rates due to niche segregation of competing tree species, in terms of differences in functional traits such as shade tolerance [1,2]
In the Spruce Woods Provincial Park (SWPP), the vegetation is classified as aspen parkland, which represents a transitional ecotone between the southern boreal forest and northern prairie [35,36]
White spruce showed few periods of reduced radial growth according to a running average
Summary
Mixed species forest stands generally have higher overall long-term productivity rates due to niche segregation of competing tree species, in terms of differences in functional traits such as shade tolerance [1,2]. Shade tolerant species (e.g., white spruce: Picea glauca (Moench) Voss) can develop in the understory of a shade intolerant species (e.g., trembling aspen: Populus tremuloides Michx.) which established earlier during the phase of stand establishment which eventually leads to canopy stratification [3]. In hot and dry environments for tree growth, such as the southern limit of tree distributions gradating with grassland ecosystems, direct sunlight can raise temperatures and lead to increased rates of evapotranspiration [5]. In these harsh environments, trees can promote facilitative interactions whereby increased tree cover and shade can help reduce the rate of evapotranspiration
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