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Intransitivity increases plant functional diversity by limiting dominance in drylands worldwide.

ABSTRACT: 1. Biotic interactions are key determinants of plant community structure. Indirect interactions such as intransitivity (i.e. in the absence of competitive hierarchies among species) have been hypothesized to benefit diversity within plant communities. However, their effect on functional diversity remains scarcely explored in real communities. Here we develop a novel approach to infer intransitivity from plant spatial patterns and functional traits (height and specific leaf area), and quantify its effect on different components of plant diversity along environmental gradients in 100 drylands from all continents except Antarctica. 2. We first calculated the spatial association pattern for all perennials to infer competition between species. Trait values were used as a proxy of competitive hierarchies to infer the direction of these interactions. We used multiple regression models to evaluate how intransitivity responds to environmental variables (mean annual temperature and precipitation, precipitation seasonality, soil pH, sand content and woody cover). We also used confirmatory path analysis to evaluate the effects of intransitivity on species richness and evenness, trait dispersion and functional diversity. 3. Intransitivity mostly responded to climatic variables, and significantly increased with precipitation scarcity and seasonality. We found that intransitivity had significant effects on functional diversity, mostly by promoting plant community evenness. However, the dominance of woody vegetation (steppes vs. shrublands) modulated this effect. Synthesis:Intransitivity increased the functional diversity of drylands, particularly under high rainfall seasonality, by limiting functionally dominant species. Our findings specify how intransitivity structures the functional diversity of dryland vegetation worldwide. Intransitivity may be particularly important in ecosystems where the availability of abiotic resources changes over time, thereby breaking down inherent competitive hierarchies between plant species. Neglecting intransitivity will bias our estimation of the impacts of biotic interactions on plant communities, a fundamental issue to fully understand how plant communities will respond to ongoing environmental changes.

SUBMITTER: Saiz H

PROVIDER: S-EPMC6287709 | biostudies-literature | 2019 Jan

REPOSITORIES: biostudies-literature

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Project description:The relationship between biodiversity and ecosystem function has increasingly been debated as the cornerstone of the processes behind ecosystem services delivery. Experimental and natural field-based studies have come up with nonconsistent patterns of biodiversity-ecosystem function, supporting either niche complementarity or selection effects hypothesis. Here, we used aboveground carbon (AGC) storage as proxy for ecosystem function in a South African mistbelt forest, and analyzed its relationship with species diversity, through functional diversity and functional dominance. We hypothesized that (1) diversity influences AGC through functional diversity and functional dominance effects; and (2) effects of diversity on AGC would be greater for functional dominance than for functional diversity. Community weight mean (CWM) of functional traits (wood density, specific leaf area, and maximum plant height) were calculated to assess functional dominance (selection effects). As for functional diversity (complementarity effects), multitrait functional diversity indices were computed. The first hypothesis was tested using structural equation modeling. For the second hypothesis, effects of environmental variables such as slope and altitude were tested first, and separate linear mixed-effects models were fitted afterward for functional diversity, functional dominance, and both. Results showed that AGC varied significantly along the slope gradient, with lower values at steeper sites. Species diversity (richness) had positive relationship with AGC, even when slope effects were considered. As predicted, diversity effects on AGC were mediated through functional diversity and functional dominance, suggesting that both the niche complementarity and the selection effects are not exclusively affecting carbon storage. However, the effects were greater for functional diversity than for functional dominance. Furthermore, functional dominance effects were strongly transmitted by CWM of maximum plant height, reflecting the importance of forest vertical stratification for diversity-carbon relationship. We therefore argue for stronger complementary effects that would be induced also by complementary light-use efficiency of tree and species growing in the understory layer.

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Project description:AimThe global spread of woody plants into grasslands is predicted to increase over the coming century. While there is general agreement regarding the anthropogenic causes of this phenomenon, its ecological consequences are less certain. We analyzed how woody vegetation of differing cover affects plant diversity (richness and evenness) and multiple ecosystem functions (multifunctionality) in global drylands, and how this changes with aridity.Location224 dryland sites from all continents except Antarctica widely differing in their environmental conditions (from arid to dry-subhumid sites) and woody covers (from 0 to 100%).MethodsUsing a standardized field survey, we measured the cover, richness and evenness of perennial vegetation. At each site, we measured 14 ecosystem functions related to soil fertility and the build-up of nutrient pools. These functions are critical for maintaining ecosystem function in drylands.ResultsSpecies richness and ecosystem multifunctionality were strongly influenced by woody vegetation, with both variables peaking at relative woody covers (RWC) of 41-60%. This relationship shifted with aridity. We observed linear positive effects of RWC in dry-subhumid sites. These positive trends shifted to hump-shaped RWC-diversity and multifunctionality relationships under semiarid environments. Finally, hump-shaped (richness, evenness) or linear negative (multifunctionality) effects of RWC were found under the most arid conditions.Main conclusionsPlant diversity and multifunctionality peaked at intermediate levels of woody cover, although this relationship became increasingly positive under wetter environments. This comprehensive study accounts for multiple ecosystem attributes across a range of woody covers and environmental conditions. Our results help us to reconcile contrasting views of woody encroachment found in current literature and can be used to improve predictions of the likely effects of encroachment on biodiversity and ecosystem services.

Project description:Plant-plant interactions are driven by environmental conditions, evolutionary relationships (ER) and the functional traits of the plants involved. However, studies addressing the relative importance of these drivers are rare, but crucial to improve our predictions of the effects of plant-plant interactions on plant communities and of how they respond to differing environmental conditions. To analyze the relative importance of -and interrelationships among- these factors as drivers of plant-plant interactions, we analyzed perennial plant co-occurrence at 106 dryland plant communities established across rainfall gradients in nine countries. We used structural equation modeling to disentangle the relationships between environmental conditions (aridity and soil fertility), functional traits extracted from the literature, and ER, and to assess their relative importance as drivers of the 929 pairwise plant-plant co-occurrence levels measured. Functional traits, specifically facilitated plants' height and nurse growth form, were of primary importance, and modulated the effect of the environment and ER on plant-plant interactions. Environmental conditions and ER were important mainly for those interactions involving woody and graminoid nurses, respectively. The relative importance of different plant-plant interaction drivers (ER, functional traits, and the environment) varied depending on the region considered, illustrating the difficulty of predicting the outcome of plant-plant interactions at broader spatial scales. In our global-scale study on drylands, plant-plant interactions were more strongly related to functional traits of the species involved than to the environmental variables considered. Thus, moving to a trait-based facilitation/competition approach help to predict that: 1) positive plant-plant interactions are more likely to occur for taller facilitated species in drylands, and 2) plant-plant interactions within woody-dominated ecosystems might be more sensitive to changing environmental conditions than those within grasslands. By providing insights on which species are likely to better perform beneath a given neighbour, our results will also help to succeed in restoration practices involving the use of nurse plants.

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Intransitivity increases plant functional diversity by limiting dominance in drylands worldwide.

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