Climatic response of Picea glauca seedlings in a forest-prairie ecotone of western Canada

We examined radial and height growth-climate relationships of juvenile white spruce in three contrasting microenvironments within a prairie-forest ecotone of western Canada. The three microenvironments were (1) the understory of mature trembling aspen (Populus tremuloides Michx.) groves, (2) the understory of mature white spruce (Picea glauca (Moench) Voss) tree islands, and (3) the open prairie outside the influence of mature trees. Interannual patterns in radial and height growth from each of the three environments were related to the regional climate (temperature and precipitation). The growth-climate relationships identified indicated that growth of juvenile white spruce was conditioned primarily by direct moisture stress during the wettest month of the year, namely June precipitation in the current growing season (t). The growth of juvenile white spruce also responded secondarily to moisture stress indirectly induced by high temperatures in June (t). The results support the general understanding that the southern limit of conifer species in western Canada is controlled mainly by moisture stress either directly by low precipitation or indirectly due to temperature-induced drought stress.