Competition and facilitation on elevation gradients in subalpine forests of the northern Rocky Mountains, USA

The relative importance of competition and facilitation has been hypothesized to change with variation in abiotic conditions. 1 examined the relative importance of competition and facilitation along elevation gradients in the northern Rocky Mountains where Pinus albicaulis and Abies lasiocarpa dominate the overstory. Al lower elevations and in more sheltered sites, A. lasiocarpa seedlings, saplings, and trees were not spatially associated with mature P. albicaulis, whereas at high-elevation sites along exposed ridges near timberline A. lasiocarpa were highly aggregated around mature P, albicaulis. I also compared growth rates of A. lasiocarpa trees before and after the death of adjacent P. albicaulis to growth rates of A. lasiocarpa in the same years but adjacent to living trees. Ln the Bitterroot Mts. a. lasiocarpa responded positively to the death of adjacent P. albicaulis at low-elevation sites (7% increase), but negatively at high-elevation sites (24% decrease). This suggests that facilitation was more important at timberline sites characterized by abiotic extremes and competition was more important in more moderate abiotic conditions. At high-elevation sites in both mountain ranges, large A. lasiocarpa were 2-4 times more aggregated with P. albicaulis than A. lasiocarpa seedlings. At the high-elevation site in the Bitterroots, growth rates of large A. lasiocarpa were significantly lower in open microsites than when trees were adjacent to either living or dead P. albicaulis. In contrast, growth rates of small saplings did not differ among these microsites. Stronger facilitative effects on mature trees than on seedlings or saplings may develop because the winter snowpack protects small A. lasiocarpa from blowing ice and snow. After trees grow above the snowpack shelter From large P, albicaulis may be crucial. These results emphasize the importance of studying interspecific interactions over a range of conditions; in these forests both positive and negative interactions occur between A. lasiocarpa and P. albicaulis, but their relative importance depends on abiotic conditions and plant life history stage.