Global warming has a disproportionately large impact on high elevation ecosystems, where temperatures are rising, snowpack is receding and summer droughts are becoming more common.  Near the Rocky Mountain Biological Laboratory in the Elk Mountains of Colorado, experimental heating has shown shifts in the carbon cycle due to increased dominance of Sagebrush (Artemisia tridentata ssp. vaseyana, a characteristic species of the Great Basin Desert) at the expense of subalpine forbs.  In order to determine whether an increase in Sagebrush abundance should be expected near its elevational limit with climate change, I examined the effects of both heating and neighboring subalpine forbs on survival of Sagebrush seedlings in the summer of 2009.  Two experimental treatments were applied in a fully factorial design:  heating via small open-top chambers, and neighbor-removal via repeated above-ground clipping.  

Results/Conclusions
The open top chambers effectively heated the soil at 5 cm by 3.11^oC daytime and 1.22^oC nighttime (both p < 0.0001).  End of summer mortality was highest for seedlings with the neighbors removed, regardless of heating treatment (43% without heating and 35% with heating), while mortality was relatively low in treatments with the surrounding canopy intact (7% without heating and 14% with heating).  Surviving seedlings without neighbors grew to only half the height of those with neighbors intact.  These results suggest that facilitation by neighboring subalpine forbs is critical to the survival of Sagebrush seedlings under current climatic conditions and under warmer conditions.  Therefore, predictions of range increase of Sagebrush, and corresponding changes in carbon cycling, that do not take species interactions into account are likely to overpredict range expansion of this species with climate change.