The alpine-treeline ecotone is one of the most conspicuous land-cover boundaries and is controlled by climate, yet few studies have evaluated treeline responses to experimental manipulations of climate.   Establishment of tree seedlings is a demographic bottleneck for tree populations at treeline, and seedlings are thus a point of climate sensitivity for treelines.  To determine the climate sensitivity of tree seedlings to climate, we performed a field experiment at treeline in the Teton Mountains USA, in which conifer seedlings were subjected to a range of experimental temperatures, supplemental precipitation, and manipulations of snow cover.  Temperature and precipitation treatments were replicated in six locations spanning a 1 km ridgeline at treeline. 

Results/Conclusions

The first month of seedling germination and initial establishment occurred in drought, caused in part by an early snowmelt.  No seedlings emerged or survived without supplemental watering or overhead cover provided by herbs.  Herb cover was highly patchy in this treeline landscape.  Thereafter, watering and warming had relatively minor effects on seedlings, and primarily only in physiological performance of spruce.  In contrast, seedlings of subalpine fir did not survive in any treatments, whereas seedlings of whitebark pine were most abundant and also unresponsive to the treatments.  The replicate block (ie. landscape position) in which the seedling was located in was a major source of variability.  Duration of snow cover varied strongly across the blocks.  Furthermore, the snowcover-manipulation experiment elicited the strongest responses from seedlings.  Climate change effects are not likely to be manifest equivalently on plant communities, even within a horizontal, 1 km band of treeline on one mountain.  Landscape factors such as the topography influencing snowpack, tree species present, and herb cover patterns could be used to predict sensitivity of tree establishment to future trends in warming or precipitation.