COMPETITION AND FACILITATION ON ELEVATION GRADIENTS IN SUB-ALPINE FORESTS OF THE NORTHERN ROCKY-MOUNTAINS, USA

The relative importance of competition and facilitation has been hypot hesized to change with variation in abiotic conditions. 1 examined the relative importance of competition and facilitation along elevation g radients in the northern Rocky Mountains where Pinus albicaulis and Ab ies lasiocarpa dominate the overstory. Al lower elevations and in more sheltered sites, A. lasiocarpa seedlings, saplings, and trees were no t spatially associated with mature P. albicaulis, whereas at high-elev ation sites along exposed ridges near timberline A. lasiocarpa were hi ghly aggregated around mature P, albicaulis. I also compared growth ra tes of A. lasiocarpa trees before and after the death of adjacent P. a lbicaulis to growth rates of A. lasiocarpa in the same years but adjac ent to living trees. Ln the Bitterroot Mts. a. lasiocarpa responded po sitively 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 site s 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 wi th P. albicaulis than A. lasiocarpa seedlings. At the high-elevation s ite in the Bitterroots, growth rates of large A. lasiocarpa were signi ficantly lower in open microsites than when trees were adjacent to eit her living or dead P. albicaulis. In contrast, growth rates of small s aplings did not differ among these microsites. Stronger facilitative e ffects on mature trees than on seedlings or saplings may develop becau se the winter snowpack protects small A. lasiocarpa from blowing ice a nd snow. After trees grow above the snowpack shelter From large P, alb icaulis may be crucial. These results emphasize the importance of stud ying interspecific interactions over a range of conditions; in these f orests both positive and negative interactions occur between A. lasioc arpa and P. albicaulis, but their relative importance depends on abiot ic conditions and plant life history stage.