Edge effects of wildfire and riparian buffers along boreal forest streams.
Abstract
Clearcutting and wildfire are the two major edge-creating disturbances in boreal forests. While clearcutting retains at least a 30 m buffer, wildfires burn close to streams killing most of the trees and potentially creating a different edge structure. Different edge structures are likely to support different plant assemblages. The riparian buffer and fire edge structures and their effects on plant assemblages are unknown, but they have implications for forest management that aims to harvest trees and conserve biodiversity by emulating natural disturbance. We hypothesized that environmental filters created by post-fire residual structures at the fire edge will support a different plant assemblage than the buffer edge. We further hypothesized that the fire edge with a weaker environmental filter because of post-fire residual structures and proximity to streams will result in lower depth of edge effects (DEEs) and magnitude of edge effects (MEEs) than buffer edges. We determined the structure of canopy trees, understorey cover and the near-ground microclimate by sampling 576 (5×10 m) and 1820 (1×1 m) quadrats along 96 transects beside 24 streams near Thunder Bay, Canada. We determined DEEs and MEEs by comparing edge variables with reference forests. We used repeated measures/factorial ANOVAS with Tukey's post hoc tests to determine DEEs and MANOVA for MEEs. The average microclimatic DEEs extended 8 m into the buffer but only 2.5 m from the fire edge. Similarly, the DEEs for plant life-forms extended 20 m from the buffer edge and 5 m from the fire edge. At the fire edge, the structural MEEs were significantly higher, but the microclimatic MEEs were lower than the buffer edge. We found no significant difference in the overall life-form MEEs, but shrubs, shade-tolerant herbs and grasses were increased at the buffer edge and decreased at the fire edge. Synthesis and applications. We discovered that the ecological structure at buffer and fire edges in boreal forests creates different environmental filters supporting different plant assemblages. Lower structural and microclimatic DEEs and MEEs at the fire edge result from (i) edge location, (ii) intact shrub layer and (iii) disturbance-resilient riparian vegetation. We suggest that replacing the conventional sharp edges of the riparian clear-cut buffer with 'feathered' edges by selective harvesting of trees will create wider edges mimicking wildfire legacy and will help to emulate natural disturbance for conserving biodiversity.