Background/Question/Methods
Bryophytes perform important ecological roles in boreal forests, but some species may be prone to extirpation after forest harvesting. Forest management practices that retain canopy trees and forest floor substrates such as decayed wood are presumed to enhance the persistence of some forest biota by protecting habitat and improving growing conditions. As these practices become more widely implemented we need to evaluate the effectiveness of residual forest structure in preserving bryophytes. We examined bryophyte (liverwort and moss) abundance in 24 forest stands (each 10 ha) of mixed and coniferous canopy composition in the Lower Boreal–Cordilleran Ecoregion of northern Alberta, Canada. Forests were harvested along a gradient (10, 50, 75, and 100 percent) of dispersed green-tree retention five to six years prior to our sampling, with minimal disturbance to forest floor substrates important to bryophytes. Three replicate stands, which were similar in pre-harvest ecological site classification, understory vegetation composition, and age of fire-origin, were sampled for each retention level in each forest type. Bryophytes were enumerated using large (1256 m²) plots to ensure the capture of uncommon species, and data were analyzed using multivariate ordination and generalized linear mixed models to elucidate species responses along the retention gradient.
Results/Conclusions
Forest type and amount of canopy retention significantly influenced the composition of bryophytes and response of individual species, and need to be considered when managing forests for bryophyte conservation. As canopy retention decreased, species composition became increasingly different from that found in intact (control) forests. The distinct taxonomic groups, liverworts and mosses, responded differently to forest harvesting. Richness of liverworts decreased rapidly with canopy removal with pronounced losses at the lowest levels (10 and 50 percent) of retention. In comparison, richness of mosses was not affected by canopy removal. This was partly because of turnover in species along the gradient of partial harvesting. Bryophyte species that were significant indicators of a particular retention level were found almost exclusively for intact forests and were mostly epixylic liverworts that are restricted to decayed wood. Results suggest that declining levels of canopy retention create progressively degraded growing conditions for bryophytes (especially liverworts) through loss of forest moisture. Management practices that promote low levels (10 and 50 percent) of dispersed canopy retention are expected to be least effective at preserving sensitive bryophytes in the short-term after harvesting.