Facilitation and Competition on Gradients in Alpine Plant Communities

Abstract

We conducted a neighbor removal experiment in natural alpine plant communities of the southwestern Alps to test for the relative importance of competitive and facilitative interactions along elevational and topographical gradients. The experimental sites were chosen to encompass most of the floristic diversity observed along gradients of elevation and topography, which are the two main ecological gradients associated with alpine plant communities in the western Alps. The effects of neighbor removal on the survival, aboveground biomass, and reproduction of five target species were tested at each of six experimental sites. Using biomass data, we calculated relative competitive index (RCI) and log response ratio (LRR) as measures of interaction strength and direction. We found highly significant shifts from strong competitive effects in low and sheltered sites to strong facilitative responses in high and exposed sites. When experimental results were integrated with gradient analyses, we found that the responses of particular alpine plant species to neighbor removal generally depended on the species' position on elevational and topographical gradients. When neighbors were removed from around target species at experimental sites that were lower in elevation than the distributional mean of the target species, biomass generally increased. When neighbors were removed from around target species at experimental sites that were higher in elevation than the distributional mean of the target species, biomass decreased. In other words, facilitation appeared to allow species from lower elevations to move up the gradient, but competition at low elevations appeared to restrict species from higher elevations from moving down the gradient. In high and exposed sites, experimental evidence for facilitation was coupled to small‐scale spatial associations among species, but spatial disassociation was not coupled to experimental evidence for competition at any sites. We conclude that the distribution and abundance of many species in high‐elevation communities of the western Alps appears to be enhanced by neighbors, and that species continua commonly observed along environmental gradients are the result of both negative and positive plant interactions.

Facilitation among plants in alpine environments in the face of climate change

“…These results suggest that, under the effects of accelerated warming, the absence of an important nurse species in a new alpine area targeted for upward migration will have a negative effect on plant diversity by impeding the establishment of associated plants (e.g., Myrosmodes sp., Lupinus microphyllus ; Figure 3). This upholds the hypothesis that the future of (alpine) biodiversity could be partly dependent on facilitative and mutualistic interactions among organisms, required to avoid “chains of extinction” (Choler et al, 2001; Brooker et al, 2008; Bellard et al, 2012). …”

“…Alpine plant communities have been widely used by ecologists over the last two decades to infer patterns and mechanisms of plant–plant interactions, in particular because mountain environments provide abrupt stress variations along elevation gradients (Körner, 2007). Studies conducted in these environments have provided major contributions to the definition and further refinements of the SGH (Choler et al, 2001; Callaway et al, 2002; Maestre et al, 2009; He et al, 2013). In most alpine environments, greater facilitation is observed at higher elevation, i.e., in more stressful conditions—readers are referred to the specific cases in dry alpine environments reported by Cavieres et al (2006) and Michalet et al (2014) where two opposing stress gradients were found.…”

Riders in the sky (islands): Using a mega‐phylogenetic approach to understand plant species distribution and coexistence at the altitudinal limits of angiosperm plant life

“…An additional criticism of the community phylogenetic approach is that phylogenetic clustering may also be generated by interspecific competition when community structure is mainly shaped by competitive hierarchy fitness differences (Mayfield & Levine, 2010). In high-alpine environments, however, it is widely expected that competitive interactions will be reduced and that plant-plant interactions will be dominantly positive through ecological facilitation (Choler et al ., 2001). Therefore, alpine areas are well suited for studying the mechanisms of biodiversity origins and species coexistence using a comparative community phylogenetic approach.…”

The shape of things to come: woodland herb niche contraction begins during recruitment in mesic forest microhabitat

“…Thus, when species abundance peaks at intermediate resources to form a unimodal curve [2,3], this pattern may represent a self-limiting optimum [4][5][6] or a contraction owing to negative biotic interactions that overwhelm the species where resources are high [1,7,8]. Conversely, positive biotic interactions can extend the fundamental niche beyond that of the realized niche by enhancing access to limiting resources [9][10][11]. This requires, however, that resource conditions are optimal for both mutualists, and failed or limited facilitation can result in contracted response patterns similar to those created by competitive interactions [12,13].…”