Climate-driven treeline movement and forest encroachment into alpine meadows has been documented at many locations, and may influence surface albedo, snow melt patterns, carbon stores, and species habitat. Dynamic vegetation model scenarios suggest large-scale changes in response to climate, but treeline movement and meadow encroachment are modified by a variety of environmental factors at local spatial scales. The relative importance and interactive effects of these fine-scale environmental controls on tree establishment is poorly understood. The objective of this study was to quantify patterns of tree establishment in a subalpine forest/meadow parkland in relation to topographic variability of late season snow persistence, distance from potential seed sources, climate variation, and a debris flow disturbance event. The study occurred in the subalpine parkland of the Mount Jefferson Wilderness, Oregon, U.S.A. Field plots were established and geo-referenced in relation to LiDAR-derived micro-topographic gradients, distances to overstory trees (potential seed sources), and recent debris flows. Late season snow depth and seedling/sapling abundance by species (predominantly Tsuga mertensiana and Abies amabilis) were measured. Seedling/sapling dates of establishment were determined by tree coring.  

Results/Conclusions

Late season snow depth was lowest on ridge tops, greatest in basins and depressions, negatively associated with distance from overstory canopy, and lowest within the debris flows. Seedling/sapling abundance was inversely associated with snow depth, with greater abundance on high topographic features and closer to overstory trees. Tsuga mertensiana recruitment was not related to distance to potential seed sources but Abies amabilis was. Seedling/sapling abundance was not different between undisturbed versus debris flow plots. Seedling/sapling establishment was positively associated with annual and spring temperature, and negatively associated with annual snowfall. Climate and topography had an interactive effect, with trees establishing on higher topographic positions during periods of high snow and lower temperatures, while establishing on lower topographic positions during periods of reduced snowfall and higher temperatures. Results of this study suggest there are strong topographic, overstory canopy, seed source, and climate controls of tree establishment at fine spatial scales. These controls restrict the spatial extent of tree establishment in alpine meadows even when climate conditions are favorable for establishment. Results suggest large scale climate-driven models of vegetation change may overestimate treeline movement and meadow decline by not accounting for fine- scale topographic and biotic controls which limit tree establishment.