FIRE, LOGGING, AND OVERSTORY AFFECT UNDERSTORY ABUNDANCE, DIVERSITY, AND COMPOSITION IN BOREAL FOREST

Abstract

Understory vegetation plays a critical role in boreal ecosystems. Despite this, quantitative evaluation of the factors controlling understory vegetation abundance, diversity, and composition in the most diverse boreal forest region in North America is lacking. This study examined the dynamics of understory vegetation of stands of fire origin and tested effects of overstory composition and logging vs. fire on the understory vegetation dynamics in Ontario, Canada. Understory vegetation communities were sampled in 68 stands of conifer, mixed-wood, and deciduous overstory type ranging from 7 to 201 years postfire for stands of fire origin, and from 7 to 31 years for stands of logging origin. For stands of fire origin, total cover and species richness followed similar trends for the three overstory types and were highest in the intermediate-aged stands (72–90 years). Trends in cover and richness, however, differed significantly for vascular and nonvascular plant groups. Vascular cover and species richness were generally higher under deciduous stands, and lower on older stands, while nonvascular species richness was highest under conifer stands and increased with time since fire. Neither species richness nor compositional turnover was higher under mixed-wood stands; mixed-wood stands were compositionally intermediate to conifer and deciduous stands. Multivariate analysis using multiple-response permutation procedures indicated that understory communities were compositionally distinct for all overstory types and showed no convergence with increasing time since fire. Compared with postfire stands of similar ages, post-logged stands had similar total understory cover and richness. Vascular cover and richness, however, were higher on post-logged stands, and nonvascular cover and richness were lower. Stands of logging and fire origin were compositionally distinct for all overstory types and ages. Compositional differences appeared to be driven by higher preestablished rhizomatous species and few pyrophilic species on post-logged sites. Understory vegetation communities in the central boreal shield appear to support the intermediate disturbance hypothesis. Understory richness, however, was not negatively associated with high cover values as predicted by the intermediate disturbance hypothesis. Moreover, richness appears to be highest on sites with high light availability, suggesting that boreal understory communities are influenced more by plant tolerances for low resources, rather than by competition.