Riparian vegetation recovery in a 23 year chronosequence of clear-cuts along boreal headwater streams

Abstract

Recovery of understory vegetation after clearcut harvesting has received much attention but little is known about the recovery of riparian vegetation after forest harvesting along headwater streams, despite the critical role vegetation plays in the protection of biodiversity and water quality. We investigated the impacts of clearcut harvesting on riparian plant communities and their recovery along headwater streams in boreal mixedwood forests of northwestern Ontario, Canada. We studied 30 headwater streams (width 1–3 m), 24 in clearcut sites harvested 3–23 years previously and 6 streams in reference 90–100 year-old mature (unharvested) forests. Each stream was studied at two locations: (i) within clearcut areas and (ii) within the downstream unharvested locations (at the buffer zones) of the larger stream into which the headwater streams flow. We quantified species richness, abundance, diversity and evenness of riparian understory vegetation at each site. We used a nested model and Nonmetric Multidimensional Scaling (NMS) to examine the effect of years since clearcut (age) on understory plant community change. We assessed the number of original, newly recruited and locally eliminated species based on species presence/absence data from the sampled quadrats in clearcuts, downstream unharvested forests and reference forests. We found that clearcut harvesting had no immediate impact on overall species richness or diversity, but it was associated with changes in species composition. The post-harvest habitats were often dominated by ruderal invasive species. Some late-seral species were locally eliminated and showed no sign of recovery 23 years after harvesting. We conclude that although clearcutting along headwater streams does not affect the overall species diversity, it changes species composition by locally eliminating some understory plants, which in turn may cause significant change in the ecological services they provide. Hence, it is important to recognize the sensitivity of headwater systems during forest harvesting and subsequent vegetation recovery. Avoiding ground disturbance in head water systems during forest management, careful selective/partial harvesting in riparian areas and seeding of fast growing native plants immediately after clearcutting may help mitigate potential adverse effects of forest management in headwater systems.