Aspect and slope influence plant community composition more than elevation across forest–tundra ecotones in subarctic Canada

Abstract

Questions How does tree line community composition vary between elevations, aspects and slope angles in the alpine subarctic and what are the specific abiotic factors governing this variability? How do species richness and rates of community turnover vary from low to high elevation across the forest–tundra ecotone? What do the results indicate about future vegetation change? Location Kluane Region, southwest Yukon, Canada. Methods We surveyed plant communities and measured key abiotic variables across forest–tundra ecotones in six alpine valleys, each with a north‐ and a south‐facing slope, in two mountain ranges of southwest Yukon. We used NMS to identify patterns in plant community composition and infer which abiotic variables drive these patterns. We calculated species richness and community dissimilarity at regular elevational intervals to assess trends in richness and rates of community turnover within the ecotone. Results Plant communities varied more with aspect and slope angle than they did with elevation. Aspect‐related differences were driven by warmer soil temperatures and deeper active layers on south‐ compared to north‐facing slopes, while differences related to slope angle occurred only on north‐facing slopes and were driven by soil moisture. Species richness increased with elevation on north‐facing slopes and showed no trend with elevation on south‐facing slopes. Rates of community turnover were higher on south‐facing than north‐facing slopes. Conclusions Plant community composition within the forest–tundra ecotone is driven primarily by soil temperature and, to a lesser extent, soil moisture, both of which vary more in relation to aspect and slope angle than they do between forest and tundra elevations. We recommend that models of vegetation change in subarctic alpine regions address the possibility of change occurring at different rates and in different directions depending on the topographic characteristics of each slope.

Limited prospects for future alpine treeline advance in the Canadian Rocky Mountains

“…At our study sites, mean growing season soil temperatures did not decrease with increasing elevation as expected, but rather were coldest in the forest zone and warmest in the transition, with alpine temperatures falling between the two. Other studies (Bader, Rietkerk, & Bregt, ; Dearborn & Danby, ) have also found soil temperatures to be lower in forested relative to alpine areas, attributing the reduction in temperature to the effect of shading by forest canopies. Although based on only one year of monitoring data, the negative correlation between establishment and growing season temperatures indicates that low mean soil temperatures are not likely the primary abiotic control on regeneration beyond the treeline in our study area.…”

Controlling factors of plant community composition with respect to the slope aspect gradient in the Qilian Mountains

“…Therefore, permanent plots capturing variation in plant community composition due to major topographic features are needed. These plots should be surveyed and monitored dynamically at each study site (Dearborn and Danby 2017), as long-term experiments can provide invaluable datasets for identifying how species composition changes both spatially and temporally (Cleland et al 2013, Collins et al 2016.…”

“…). Dearborn and Danby () showed that plant communities varied more significantly with the slope aspect than they did with elevation. Likewise, Sternberg and Shoshany () found that the vegetation structure within a specific site had changed significantly in the short distance separating the north and south‐facing slope aspects, which corresponded to the patterns observed in two other sites with respect to the rainfall gradient.…”

“…Despite the studies mentioned above, detailed evidence-based studies on the influence of topographic factors on plant community composition, which are especially important for mountainous environments, are only partially available, due in part to the high variability in plant community composition resulting from irregular local topography in mountainous regions (Dearborn and Danby 2017). However, as mountains are considered to be among the ecosystems most threatened by climate change (owing to mountaintop extinction; Bertrand et al 2011), the lack of detailed knowledge about plant community composition in these ecosystems can impede the interpretation and prediction of plant distributions (Lavorel and Garnier 2002).…”

Positive interaction facilitates landscape homogenization by shrub expansion in the forest–tundra ecotone

“…We focused on the assembly rules in a forest–tundra ecotone and considered the ecological processes that are potentially at play in vegetation changes. Previous studies (e.g., Trant et al, ) have suggested that abiotic environmental conditions directly mediate vegetation displacement in the forest–tundra ecotone, and thus, these conditions may significantly affect landscape changes in response to changes in climate, which are expected to progress further (Dearborn & Danby, ). In our study, however, we found only weak evidence that tundra communities are significantly mediated by environmental factors.…”