Broadcast burning has persistent, but subtle, effects on understory composition and structure: Results of a long-term study in western Cascade forests

Approaches to forest management have changed markedly in the Pacific Northwest in recent decades, yet legacies of past management persist on the landscape. Following clearcut logging, woody residues were typically burned to reduce future fire hazard, create planting spots, facilitate natural recruitment, and retard growth of competing vegetation. We asked whether legacies of broadcast burning persist in the forest understory during the early stages of stand closure, how they manifest structurally or compositionally, whether they are altered by subsequent management (pre-commercial thinning), and how they vary with gradients in the physical environment or stand structure. We used data from 44 pairs of burned and unburned plots sampled 37–42 years after clearcut logging of mature and old-growth forests in western Oregon and Washington. Burning had persistent, but mostly subtle effects on community structure and composition. Burned plots had greater cover of early-seral species and lower cover of woody forest species. Among the latter, tall shrubs showed consistently negative responses to burning. Smaller-statured woody and herbaceous species were more variable in their responses, reflecting a greater diversity of regenerative traits. For some understory attributes, effects of burning were contingent on thinning history. For example, burning enhanced species richness and reduced species evenness, but only in unthinned sites. Conversely, burning increased the spatial variability of tall shrub cover, but only in thinned sites. In some instances, thinning compounded the effects of burning, resulting in a four-fold increase in cover of early-seral herbs and a 50% reduction in cover of forest tall shrubs relative to plots that received neither treatment. For most understory attributes, regional variation in the magnitude of the burn effect was not easily explained by burn severity, site environment, or stand structure. Collectively, our results highlight the persistent, but highly variable, effects of broadcast burning on the post-harvest development of the forest understory. They suggest that, where it is feasible, controlled burning can be used in regeneration-harvest units to achieve a variety of management objectives, with minimal effect on the forest understory.