Influence of harvesting on understory vegetation along a boreal riparian-upland gradient

Abstract

Management of riparian forests, and how they respond to disturbance, continues to be a focus of interest in the literature. Earlier studies on riparian plant community assembly following harvesting in the boreal forest have focused merely on highly contrasting microhabitats within a landscape, for example, stream-bank riparian habitat or upland habitat. Sustaining biodiversity and evaluating the success of riparian management requires an understanding of plant community assembly following overstory harvesting across the landscapes, e.g., along the entire riparian-upland gradient, and how they recover over time. Using pre- and post-harvest data, we quantified how riparian harvesting along a disturbance gradient affects understory plant species diversity, abundance, turnover, and composition. We also asked how these disturbance–response relationships vary from stream edge to uplands. We expect changes in the plant community will be greater and recovery to be slower with increased disturbance severity. Based on the ecology of riparian versus upland, we also expect harvesting to exert a stronger control with increasing distance from the stream channel through the colonization of early successional species and extirpation of extant species. We found that disturbance severity (i.e., from cut-to-shore) from harvesting exerted strong controls on the dynamics of understory vegetation in boreal riparian forests, which was still evident seven years after the disturbance event. However, the dynamic responses strongly differed with the distance from the stream channel. Specifically, streamside communities harvested with or without a 30m riparian buffer, were maintained to a condition similar to uncut forests. However, upland communities were less resistant to overstory harvest and subsequently colonized by early successional species present in pre-harvest riparian plots. Furthermore, vascular and non-vascular plants exhibited contrasting responses in their richness, abundance, turnover, and composition. Our results indicate that streamside understory vegetation is inherently more resistant to stand-replacing disturbance than upland assemblages.