Traits differentially adapt plant species to particular conditions generating compositional shifts along environmental gradients. As a result, communityâscale trait values show concomitant shifts, termed traitâenvironment relationships. Traitâenvironment relationships are often assessed by evaluating communityâweighted mean (CWM) traits observed along environmental gradients. Regressionâbased approaches (CWMr) assume that local communities exhibit traits centred at a single optimum value and that traits do not covary meaningfully. Evidence suggests that the shape of traitâabundance relationships can vary widely along environmental gradientsâreflecting complex interactionsâand traits are usually interrelated. We used a model that accounts for these factors to explore traitâenvironment relationships in herbaceous forest plant communities in Wisconsin (USA). We built a generalized linear mixed model (GLMM) to analyse how abundances of 185 species distributed among 189 forested sites vary in response to four functional traits (vegetative heightâVH, leaf sizeâLS, leaf mass per areaâLMA and leaf carbon content), six environmental variables describing overstorey, soil and climate conditions, and their interactions. The GLMM allowed us to assess the nature and relative strength of the resulting 24 traitâenvironment relationships. We also compared results between GLMM and CWMr to explore how conclusions differ between approaches. The GLMM identified five significant traitâenvironment relationships that together explain ~40% of variation in species abundances across sites. Temperature appeared as a key environmental driver, with warmer and more seasonal sites favouring taller plants. Soil texture and temperature seasonality affected LS and LMA; seasonality effects on LS and LMA were nonlinear, declining at more seasonal sites. Although often assumed for CWMr, only some traits under certain conditions had centred optimum traitâabundance relationships. CWMr more liberally identified (13) traitâenvironment relationships as significant but failed to detect the temperature seasonalityâLMA relationship identified by the GLMM.
Turnover in species composition and the dominant functional strategies in plant communities across environmental gradients is a common pattern across biomes, and is often assumed to reflect shifts in trait optima. However, the extent to which communityâwide trait turnover patterns reflect changes in how plant traits affect the vital rates that ultimately determine fitness remain unclear. We tested whether shifts in the communityâweighted means of four key functional traits across an environmental gradient in a southern California grassland reflect variation in how these traits affect species' germination and fecundity across the landscape. We asked whether models that included traitâenvironment interactions help explain variation in two key vital rates (germination rates and fecundity), as well as an integrative measure of fitness incorporating both vital rates (the product of germination rate and fecundity). To do so, we planted seeds of 17 annual plant species at 16 sites in cleared patches with no competitors, and quantified the lifetime seed production of 1360 individuals. We also measured community composition and a variety of abiotic variables across the same sites. This allowed us to evaluate whether observed shifts in communityâweighted mean traits matched the direction of any traitâenvironment interactions detected in the plant performance experiment. We found that commonly measured plant functional traits do help explain variation in species responses to the environmentâfor example, highâSLA species had a demographic advantage (higher germination rates and fecundity) in sites with high soil Ca:Mg levels, while lowâSLA species had an advantage in low Ca:Mg soils. We also found that shifts in communityâweighted mean traits often reflect the direction of these traitâenvironment interactions, though not all traitâenvironment relationships at the community level reflect changes in optimal trait values across these gradients.
- NSF-PAR ID:
- 10369730
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Ecology
- Volume:
- 110
- Issue:
- 4
- ISSN:
- 0022-0477
- Format(s):
- Medium: X Size: p. 833-844
- Size(s):
- ["p. 833-844"]
- Sponsoring Org:
- National Science Foundation
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Abstract Synthesis . Although GLMM represents a more methodologically complex approach than CWMr, it identified a reduced set of traitâenvironment relationships still capable of accounting for the responses of forest understorey herbs to environmental gradients. It also identified separate effects of mean and seasonal temperature on LMA that appear important in these forests, generating useful insights and supporting broader application of GLMM approach to understand traitâenvironment relationships. -
Abstract Environmental gradients act as potent filters on species distributions driving compositional shifts across communities. Compositional shifts may reflect differences in physiological tolerances to a limiting resource that result in broad distributions for tolerant species and restricted distributions for intolerant species (i.e. a nested pattern). Alternatively, tradeâoffs in resource use or conflicting species' responses to multiple resources can result in complete turnover of species along gradients.
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We determined whether functional and taxonomic composition was nested or turned over completely and whether community mean traits and species composition were more strongly driven by P or moisture.
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Turnover characterized the taxonomic composition of tree communities. Geographic distances explained a larger fraction of variation for taxonomic composition than for functional composition, and community mean traits were more strongly driven by P than moisture.
Synthesis . Our results offer weak support for the tolerance hypothesis for tree communities in central Panama. Instead, we observe functional and taxonomic turnover reflecting tradeâoffs and conflicting species' responses to multiple abiotic factors including moisture, soil phosphorus and potentially other correlated variables (e.g. light). -
Abstract Spatial partitioning is a classic hypothesis to explain plant species coexistence, but evidence linking local environmental variation to spatial sorting, demography and species' traits is sparse. If coâoccurring species' performance is optimized differently along environmental gradients because of trait variation, then spatial variation might facilitate coexistence.
We used a system of four naturally coâoccurring species of
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Abstract Under fire suppression, many tropical savannas transform into forests. Forest expansion entails changes in environmental variables and plant community structure. We hypothesized that forest expansion into savanna results in a shift in communityâweighted mean functional traits from stress tolerance to competitiveness, with generalist species having trait values intermediate between those of specialists of savanna and forest habitats.
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Plain Language Summary can be found within the Supporting Information of this article. -
Abstract Plant species can show considerable morphological and functional variation along environmental gradients. This intraspecific trait variation (ITV) can have important consequences for community assembly, biotic interactions, ecosystem functions and responses to global change. However, directly measuring ITV across many species and wide geographic areas is often infeasible. Thus, a method to predict spatial variation in a speciesâ functional traits could be valuable.
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