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We investigated recent ecotone dynamics in the forest-grassland mosaics of southwestern Yukon. Our objectives were to determine ( i) if forests are encroaching into grasslands, ( ii) if rate and extent of encroachment varies by region or with topographic setting, and ( iii) if encroachment is related to climate change and variability. Dendroecological techniques were used to obtain dates of establishment for 1847 trees (trembling aspen ( Populus tremuloides Michx.) and white spruce ( Picea glauca (Moench) Voss)) sampled from 28 sites divided between two different regions and three topographic settings. Generalized linear modeling was used to identify relationships between climate and tree establishment. Results show that encroachment of forest, particularly aspen trees, into grasslands has been nearly ubiquitous on flat terrain and on south-facing slopes in both regions over the last 60-80 years. In contrast, spruce-dominated ecotones on north-facing slopes experienced little change. Aspen establishment was positively associated with spring temperatures and precipitation, although evidence suggests that other factors such as soil moisture interact with climate to mediate the timing and rate of tree encroachment. These results indicate that transformation of grasslands to aspen-dominated forest is an additional, but previously unexplored, element of the widespread ecosystem changes currently being experienced in northwestern North America. [ABSTRACT FROM AUTHOR]