Uncovering multiscale effects of aridity and biotic interactions on the functional structure of Mediterranean shrublands

Abstract

Summary 1.Habitat filtering (HF, trait convergence) and niche differentiation (ND, trait divergence) are known to impact upon plant community structure. Both processes integrate individual responses to the abiotic environment and biotic interactions. Thus, it is difficult to clearly identify the underlying abiotic and biotic factors that ultimately impact community structure by looking at community-level patterns of trait divergence or convergence alone. 2. We used a functional trait-based and multiscale approach to assess how biotic interactions and aridity determine the functional structure of semi-arid shrublands sampled along a large aridity gradient in Spain. At the regional scale, we investigated functional differences among species (axes of specialization) to identify important traits for community assembly. At the community scale, we evaluated the relative impact of HF and ND on community structure using a null model approach. Finally, at the plant neighbourhood scale, we evaluated the impact of biotic interactions on community structure by investigating the spatial patterns of trait aggregation. 3. The shrub species surveyed can be separated along four axes of specialization based on their above-ground architecture and leaf morphology. Our community scale analysis suggested that the functional structure of semi-arid communities was clearly non-random, HF and ND acting independently on different traits to determine community structure along the aridity gradient. At the plant neighbourhood scale, the spatial distribution of species was also clearly not random, suggesting that competition and facilitation impacted on the observed changes in the functional diversity of shrubland communities along the aridity gradient. 4. Synthesis: Our results demonstrated that HF and ND acted simultaneously on independent traits to jointly determine community structure. Most importantly, our multiscale approach suggested that competition and facilitation interplayed with aridity to determine this structure. Competition appeared to be constant along the aridity gradient and explained the high functional diversity observed in semi-arid shrublands. Facilitation affected subordinate and rare species and, thus, may act to enhance the biodiversity of these ecosystems. Finally, the framework employed in our study allows moving forward from the examination of patterns to the development of mechanistic traitbased approaches to study plant community assembly.

Cascading effects from plants to soil microorganisms explain how plant species richness and simulated climate change affect soil multifunctionality

“…The trait variables obtained per species and mesocosm were normalized using a log-transformation to reduce the impact of species with extreme trait values. To represent each axis of variation, we selected one trait for each PCA component as a functional marker of specialization (Gross et al, 2013). We used a PCA for these analyses because different traits are associated with different niche axes (Butterfield & Suding, 2013), and to avoid selecting highly correlated traits for further analyses (Supporting information Table S3).…”

“…This response could be related to initial "sampling effect" (Tilman, Lehman, & Thomson, 1997); that is, treatments with higher species richness had higher probabilities of having keystone species. Alternatively, this response may have occurred during the experiment and may reflect an effect of biotic interactions (Gross et al, 2013). Since tall species are better competitors than short species for space, the latter can be outcompeted.…”

Grass strategies and grassland community responses to environmental drivers: a review

“…Here, we propose in future studies to consider multiple spatial scales in combination with the multi-dimensionality of traits to better associate trait patterns with assembling rules (see Gross et al 2013a, b as an example).…”

“…In addition to the former competition-based process of species stabilization, facilitation is also able to promote the coexistence of subordinate to rare species with dominant species within a community. For instance, Gross et al (2013a) observed that facilitation interplayed with an aridity gradient to enhance subordinate and rare species and the biodiversity of semi-arid Mediterranean shrublands. All together, niche-based processes may jointly operate in a given community promoting different statuses of species.…”

Water availability and species identity control shrub colonization in abandoned semiarid steppes

“…Different shrub assemblages, which are the result of long-term processes such as habitat filtering and niche differentiation (Gross et al, 2013), may exhibit contrasting colonization dynamics. Differences in life-history traits among species (e.g.…”