Compositional stability of boreal understorey vegetation after overstorey harvesting across a riparian ecotone

Abstract

Questions: Understanding factors that contribute to the stability of an ecosystem following harvesting is central to predicting responses of boreal ecosystems to increasing human disturbances. While the response of understorey vegetation to harvesting is well understood for upland sites, little is known about compositional stability of riparian understorey vegetation. We examined how compositional stability changes with or without harvesting along an upland to streamside gradient and tested whether compositional stability is affected by pre-harvest species diversity and composition.Location: Lower Foothills sub-region of the Boreal Plain, ca. 20 km northwest of Whitecourt, Alberta, CA.Methods: We repeatedly sampled understorey vegetation of four winter-harvested and four unharvested sites in western Canadian boreal forest. Species covers were measured during the summer prior to harvesting in 2003 (year 0) and in 2004, 2008 and 2010 (1, 5 and 7 yrs after, respectively). We used nonmetric multidimensional scaling to ordinate plots using species covers, and measured vector length in the ordination space to quantify floristic dissimilarity of each plot between year 0 and years 1, 5 and 7, an inverse measure of compositional stability.Results: Floristic dissimilarity between year 0 and subsequent years was significantly greater in harvested than unharvested sites for all subsequent sampling years. Furthermore, along the upland to stream gradient, floristic dissimilarity was larger on upland than stream sites. Additional analyses revealed that floristic dissimilarity was related negatively to pre-harvesting species richness and evenness, but positively to bryophyte cover.Conclusions: Our results demonstrate that understorey compositional stability is strongly influenced by harvesting, and streamside communities are more stable than upland communities. Our results indicate that compositional stability of understorey vegetation in response to harvesting is associated positively with pre-disturbance species richness and evenness, but negatively with bryophyte dominance.

Evaluating Short-Term Impacts of Forest Management and Microsite Conditions on Understory Vegetation in Temperate Fir-Beech Forests: Floristic, Ecological, and Trait-Based Perspective

“…Overall, changes in species composition (quantified by species turnover; see Supplementary Figure S1) were more driven by colonization than by the process of species loss. These results are different from those of the authors of [34], who found that the boreal understory communities with high species richness improved compositional stability following overstory cutting through less species loss than the communities with low species richness, whereas the number of colonizing species was similar in both species-rich and species-poor communities. In the case of our study, species-rich communities were more densely populated with less empty space, that is, had higher total understory cover (Pearson's correlation coefficient between species richness and total cover: r = 0.63, p < 0.001), thereby preventing potential colonizing species from entering the community after the disturbance.…”

“…For each plot, compositional shift was measured as the Euclidean distance (vector length) between the two points, that is, pre-treatment and post-treatment position of sample unit (plot) in the NMDS ordination space. Compositional resistance and stability are inversely related to the Euclidean distance: resistance is maximal when the sample unit maintains its initial position in ordination space, that is, the Euclidean distance equals 0 [34]. For the trait-based analysis, we also calculated the Euclidean distance (from sample scores) between the initial and post-treatment position of each plot in the PCA ordination space.…”

“…Furthermore, human-induced disturbances affect plant communities for decades, but the extent of temporal change in community composition may depend more on environmental gradients and community attributes [32]. In the context of community resistance and the stability of forest understory, a compositional response to canopy reduction or complete removal may be strongly influenced by pre-disturbance community features, such as species richness and evenness [33,34].…”

N‐dimensional hypervolumes to study stability of complex ecosystems

“…This is an important drawback, since both the stability of ecosystem functions and of ecosystem structure and composition can be important aspects in terms of management planning and policy making for complex ecosystems, especially if several types of habitats exist and ecotone dynamics can change (MacDonald et al . ).…”

“…However, studies rarely investigated the impact of disturbances on the stability of ecosystem function and of biodiversity together (but see Steudel et al 2012). This is an important drawback, since both the stability of ecosystem functions and of ecosystem structure and composition can be important aspects in terms of management planning and policy making for complex ecosystems, especially if several types of habitats exist and ecotone dynamics can change (MacDonald et al 2015).…”