Treefalls and the Coexistence of Conifers in Subalpine Forests of the Central Rockies

Differences in replacement patterns and frequencies of treefalls were investigated for subalpine fir (Abies lasiocarpa) and Englemann spruce (Picea engelmannii) in the Colorado Front Range. In seven old—growth unlogged stands, data were collected on age and size of trees, frequencies of fallen trees, growth rates of trees, and sizes and abundances of occupants of treefall gaps. Subalpine fir accounted for 74.5% of the potential successors in the 125 treefall gaps sampled. The much greater abundance of subcanopy trees of subalpine fir implies more frequent recruitment into the canopy. However, its greater recruitment rate into the canopy is approximately balanced by its greater loss from the canopy; even though subalpine fir accounted for only 37% of the canopy trees it accounted for 76% of the fallen trees. The consistently lower frequency of Engelmann spruce as treefalls and its great longevity compared to subalpine fir imply a lower adult mortality rate for the spruce. Consequently, the greater proportion of young subalpine fir does not imply that it will gradually replace Engelmann spruce in old—growth stands unaffected by large—scale exogenous disturbance. The results of this study provide empirical support for the coexistence of ecologically similar species by means of different life history strategies.