Diversity and compositional stability of riparian understory plant communities following logging in Boreal Plain watersheds

Background/Question/Methods

In the boreal forest, understory vegetation accounts for the majority of floristic diversity and affects overstory succession, 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. Moreover, as a reflection of resource quantity and habitat heterogeneity, the contribution of understory vegetation in riparian forests to overall diversity and productivity greatly exceeds their spatial extent across the landscape. Despite this critical importance, effects of logging on understory plant communities in riparian forests are still poorly understood. Through a carefully controlled field experiment, we explored temporal changes in diversity and compositional stability (i.e., the measure of change in community membership and abundance), along a gradient of streamside to upland, in Canadian boreal headwater stream sites harvested with and without a 30 m buffer. Data were collected during the summer before harvesting (Yr 0), and three summers after harvesting (Yrs 1, 5 and 7). Non-metric multidimensional scaling was used to ordinate vegetation plots using species cover, and then we measured vector length in the ordination space to quantify floristic dissimilarity (i.e., inverse of compositional stability) of each sample plot over time.

Results/Conclusions

The altered environment following stand-replacing disturbance, such as increased light, soil temperatures and decreased soil moisture is favourable for most vascular plants but exceeds the tolerance of many boreal bryophytes. After an initial decrease, vascular species richness and abundance of understory vegetation was highest in the cut-to-shores sites, relative to the less disturbed treatments (i.e., buffer and uncut), but only in the transition and upland sites (10 to 30 m from shore). Alternatively, richness and abundance of non-vascular species decreased following cut-to-shore treatment. However, this response was much less pronounced in the streamside communities (0-10 m). Following cut-to-shore, compositional stability of boreal understory plant communities, including vascular and non-vascular species, decreased from streamside to upland, but not in the uncut sites. This gradient was positively and negatively correlated with pre-harvest species richness and bryophyte cover, respectively. The relative stability of streamside plants is a reflection of the life-history strategies, productivity, and diversity of the community, imparted by hydrological processes and disturbance regimes.