Disturbance effects on structure and tree species composition of Pinuscontorta forests in western Montana

Abstract

To what extent are stand structure and tree species composition affected by the nature of stand-initiating disturbances and other disturbances that cause significant tree mortality? I documented recent disturbance history and tree species composition, density, spatial pattern, and age structure in 48 stands dominated by Pinuscontorta Dougl. ex Loud. ssp. latifolia (Engelm.) Critchfield in western Montana. Stand modal ages ranged from 8 to 267 years, and sites were sampled across a range of elevations and aspects. Disturbance histories included stand-replacing fires (N = 25), partial burns (N = 8), clear-cutting (N = 7), and other disturbances (N = 8). All young stands (modal tree age < 23 years) had unimodal age structures; however, within-stand ranges in tree ages were greater following cutting than burning. Young fire-origin stands were more strongly dominated by P. contorta than young clearcut-origin stands. For older stands, within-stand variability in tree ages was greatest after nonfire disturbances and lowest after stand-replacing burns, while tree density was highest after such bums, and tree species diversity did not differ among disturbance types. The primary axis of variation in age structures across all stands, as revealed by principal components analysis, related to within-stand variability in tree ages. Compositional and structural stand features were not correlated with this axis.