Abstract
Understanding how functional and phylogenetic patterns vary among scales and along ecological gradients within a given species pool is critical for inferring community assembly processes. However, we lack a clear understanding of these patterns in stressful habitats such as Mediterranean high mountains where ongoing global warming is expected to affect species fitness and species interactions, and subsequently species turnover. In this study, we investigated 39 grasslands with the same type of plant community and very little species turnover across an elevation gradient above the treeline at Sierra de Guadarrama National Park in central Spain. In particular, we assessed functional and phylogenetic patterns, including functional heterogeneity, using a multi-scale approach (cells, subplots, and plots) and determined the relevance of key ecological factors (i.e., elevation, potential solar radiation, pH, soil organic carbon, species richness, and functional heterogeneity) that affect functional and phylogenetic patterns at each spatial scale. Overall, at the plot scale, coexisting species tended to be more functionally and phylogenetically similar. By contrast, at the subplot and cell scales, species tended to be more functionally different but phylogenetically similar. Functional heterogeneity at the cell scale was comparable to the variation across plots along the gradient. The relevance of ecological factors that regulate diversity patterns varied among spatial scales. An increase in elevation resulted in functional clustering at larger scales and phylogenetic overdispersion at a smaller scale. The soil pH and organic carbon levels exhibited complex functional patterns, especially at small spatial scales, where an increase in pH led to clustering patterns for the traits related to the leaf economic spectrum (i.e., foliar thickness, specific leaf area, and leaf dry matter content). Our findings confirm the presence of primary environmental filters (coldness and summer drought at our study sites) that constrain the regional species pool, suggesting the presence of additional assembly mechanisms that act at the smallest scale (e.g., micro-environmental gradients and/or species interactions). Functional and phylogenetic relatedness should be determined using a multi-scale approach to help interpret community assembly processes and understand the initial community responses to environmental changes, including ongoing global warming.
Highlights
Coexistence theory considers that community assembly is a process regulated by multiple mechanisms, both stochastic and deterministic, which act at different spatial scales and that are subject to different ecological factors (Palmer, 1994; Chesson, 2000)
A total of 35 independent linear models were drawn as a result of considering standardized effect size (SES)–MPD values for all traits together, each individual trait, and phylogeny at Overall, we identified significant deviations in the functional and phylogenetic clustering–overdispersion patterns depending on the spatial scale and null model
Our multi-scale approach applied to a single plant community type with a low species turnover highlights the importance of evaluating functional and phylogenetic patterns at different spatial scales to understand the current community assembly
Summary
Coexistence theory considers that community assembly is a process regulated by multiple mechanisms, both stochastic and deterministic, which act at different spatial scales and that are subject to different ecological factors (Palmer, 1994; Chesson, 2000). Environmental filtering, which is considered to act at relatively large scales, is expected to lead to a community where species are more functionally similar or closely related (i.e., functional and phylogenetic clustering; Weiher and Keddy, 1995; Leibold, 1998; Webb, 2000) because filters will select species that can thrive under the local conditions (but see Gerhold et al, 2015). Trait clustering may be linked to biotic competitive processes at small spatial scales when the dominance of closely related species excludes weaker competitors (Chesson, 2000; Mayfield and Levine, 2010)
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