Influence of riparian harvesting on boreal understory vegetation

In the boreal forest, understory vegetation accounts for the majority of floristic diversity and affects overstory succession and productivity, nutrient cycling, and wildlife habitat. In riparian forests, understory vegetation also plays an important functional role by reducing erosion, stabilizing the stream channel, and regulating water quality and quantity. Despite this critical importance, effects of harvesting on understory plant communities in riparian forests are still poorly understood. This study uses data from two boreal ecozones in Canada to quantify how overstory harvesting affects understory vegetation communities and how isturbance-response relationships vary from stream edge to uplands.;Using data collected during the summer before harvesting (Yr 0), and three summers after harvesting (Yrs 1, 5 and 7) from headwater stream sites on the Boreal Plain study area (Central Alberta), compositional stability (i.e., the measure of change in community membership and abundance) was measured following disturbance. Non-metric multidimensional scaling was used to ordinate vegetation plots using species cover, and then measured vector length in the ordination space to quantify floristic dissimilarity (i.e., inverse of compositional stability) of each sample plot. Streamside understory plant communities were compositionally more stable relative to upland communities following clearcut harvesting. Compositional stability was positively related to pre-harvest species richness and negatively related to pre-harvest bryoid cover. Changes in species diversity, abundance and turnover following harvesting were also examined. Streamside communities harvested with or without a 30 m riparian buffer, were maintained to a condition similar to uncut forests. However, upland communities were less resistant to overstory harvest, relative to streamside communities, and subsequently colonized by grasses, forbs, and tall shrubs; many of which were present in pre-harvest riparian plots.;Mechanical site preparation (i.e., scarification) exacerbates environmental changes following overstory harvest by destroying above- and below-ground plant parts and coarse woody debris and removing more of the forest floor. The response of boreal understory plant communities to overstory harvesting with and without scarification was examined on 24 small stream sites on Boreal Shield (Northwestern Ontario) watersheds. Species composition was not strongly affected by harvesting when forest floor and soil disturbance is minimal; but it was following scarification. However, this effect varied with habitat. First, the riparian and upland plots that were harvested and scarified diverged from the uncut plant communities; however this effect was stronger in the upland communities. Second, changes within the community following scarification were not detectable in the transitional communities; these communities retain species of both riparian and upland communities and thus their flora has lower site fidelity and likely tolerates a wider range of environmental conditions. Scarification profoundly alters the substrate by breaking up coarse woody debris and decreasing the depth of organic matter, which destroys forest floor feathermosses and creates a habitat suitable for early colonizing species (i.e., grasses).;This study indicates that streamside understory plant communities are inherently more resilient to stand-replacing disturbance than upland assemblages. I attribute this to the higher moisture availability and shading from tall shrubs associated with riparian habitat that buffers the effect of harvesting on shade-tolerant species in streamside plots. Thus, the response to harvesting is not as marked as it is in the uplands. This study highlights that understory disturbance response relationships can differ within a small spatial scale (i.e., 30 m) based on habitat characteristics, imparted by hydrological processes and disturbance regimes that drive community composition.