Patterns of growth dominance in forests of the Rocky Mountains, USA

Abstract

We used data from 142 stands in Colorado and Wyoming, USA, to test the expectations of a model of growth dominance and stand development. Growth dominance relates the distribution of growth rates of individual trees within a stand to tree sizes. Stands with large trees that account for a greater share of stand growth than of stand mass exhibit strong growth dominance. Stands with large trees that contribute less to stand growth than to stand mass show reverse growth dominance. The four-phase model predicts that forests move from a period of little dominance (Phase 1), with trees accounting for similar contributions to stand growth and stand mass. Phase 2 is a period of strong growth dominance, where larger trees account for a disproportionately large amount of total stand growth. Growth dominance declines during Phase 3 as growth of the larger trees slows. A final Phase 4 shows reverse growth dominance when the growth of larger trees is less than their proportional contribution to total stand mass. The datasets supported the expectation of reverse growth dominance in old forests of ponderosa pine, Engelmann spruce and subalpine fir, lodgepole pine, and mixed stands of aspen and conifers. Pure aspen stands did not show reverse growth dominance. An age sequence of lodgepole pine failed to show the expected Phase 2 period of strongly developed growth dominance. Future work needs to combine quantitative descriptions of patterns in growth dominance with experimental manipulations of resource supplies and environmental conditions to connect forest dynamics at the scales of individual trees, groups of trees, and stands.