Influence of riparian harvesting on boreal understory vegetation
MacDonald, Rebecca Louise
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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 disturbance-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.