Spatial scale and intraspecific trait variability mediate assembly rules in alpine grasslands

Abstract

Summary 1.Assembly of grassland communities has long been scrutinized through the lens of functional diversity. Studies generally point to an overwhelming influence of climate on observed patterns of functional diversity, despite experimental evidence demonstrating the importance of biotic interactions. We postulate that this is because most observational studies neglect both scale dependencies of assembly processes and phenotypic variation between individuals. Here, we test for changes in the importance of abiotic filtering and biotic interactions along a stress gradient by explicitly accounting for different scales. In addition to quantifying intraspecific trait variability (ITV), we also vary the two components of spatial scale, including grain (i.e. community size) and extent (i.e. the geographical area that defines the species pool). 2.We sampled 20 grassland communities in ten sites distributed along a 975-m elevation gradient. At each site, we measured seven functional traits for a total of 2020 individuals at different spatial grains. We related community functional diversity metrics to the main environmental gradient of our study area, growing season length (GSL), and assessed the dependence of these relationships on spatial grain, spatial extent and ITV.3. At large spatial grain and extent, the imprint of environmental filtering on functional diversity became more important with increasing stress (i.e. functional diversity decreased with shorter GSL). At small spatial grain and extent, we found a convex relationship between functional diversity and GSL congruent with the hypothesis that competition is dominant at low-stress levels while facilitative interactions are dominant at high-stress levels (i.e. high functional diversity at both extremes of the stress gradient). Importantly, the effect of intraspecific variability on assembly rules was noticeable only at small spatial grain and extent.4. Synthesis. Our study reveals how the combination of abiotic stress and biotic interactions shapes the functional diversity of alpine grasslands at different spatial scales, and highlights the importance of phenotype variation between individuals for community assembly processes at fine spatial scale. Our results suggest that studies analysing trait-based assembly rules but ignoring ITV and focusing on a single spatial scale are likely to miss essential features of community diversity patterns.

Disentangling the processes driving plant assemblages in mountain grasslands across spatial scales and environmental gradients

“…The signal of limiting similarity was only weakly linked to elevation with the strongest effect visible on SLA and VH for which limiting similarity increased with elevation. This result is apparently contradictory to the stress gradient hypothesis, which predicts an increase in limiting similarity in productive lowlands in which plant competition is expected to be more intense (Bertness & Callaway, ; Chalmandrier et al, ; Chamberlain, Bronstein, & Rudgers, ; He, Bertness, & Altieri, ). An alternative interpretation, as presented above, is that the observed patterns of trait divergence at the plot level were linked to habitat heterogeneity within plots rather than the direct effects of competition‐driven limiting similarity.…”

Forest community assembly is driven by different strata‐dependent mechanisms along an elevational gradient

“…Consistent with our predictions, the predictive power of the relationships between environmental variables and estimates of diversity across forest strata were stronger at the community scale than at the neighbourhood scale. This result likely arises from larger spatial scales having a higher sensitivity to species turnover as well as shifts in phylogenetic relatedness and traits along the environmental gradient (Cavender‐Bares et al, ; Chalmandrier et al, ). Collectively, these findings support the general hypothesis that environmental filters have stronger effects under more stressful conditions (Cavender‐Bares et al, ), and that the effect of environmental filtering on community structure may be expected at larger (e.g.…”

“…While at relatively larger scales (i.e. community level), diversity is often driven by environmental variables such as climate and resource availability (Chalmandrier et al, ). Given this relationship, we predict that the relative effects of environmental variables on community diversity will be larger at the community scale compared to the effects of these variables at the neighbourhood scale.…”

Multi‐trophic β‐diversity mediates the effect of environmental gradients on the turnover of multiple ecosystem functions

“…Subalpine grasslands dominate the bottom of the gradient while sparsely vegetated alpine meadows characterize higher elevations. Ten sites were sampled in summer 2012 along the same south‐facing slope, and they were separated by an 100 m elevation difference and an average distance of 340 m. Each site consisted of two 10 m × 10 m plots with homogeneous vegetation (see Chalmandrier et al, , for more details).…”