Size and Reproductive Traits Rather than Leaf Economic Traits Explain Plant-Community Composition in Species-Rich Annual Vegetation along a Gradient of Land Use Intensity

Abstract

Agricultural land use imposes a major disturbance on ecosystems worldwide, thus greatly modifying the taxonomic and functional composition of plant communities. However, mechanisms of community assembly, as assessed by plant functional traits, are not well known for dryland ecosystems under agricultural disturbance. Here we investigated trait responses to disturbance intensity and availability of resources to identify the main drivers of changes in composition of semiarid communities under diverging land use intensities. The eastern Mediterranean study region is characterized by an extended rainless season and by very diverse, mostly annual communities. At 24 truly replicated sites, we recorded the frequency of 241 species and the functional traits of the 53 most common species, together with soil resources and disturbance intensity across a land use gradient ranging from ungrazed shrubland to intensively managed cropland (six land use types). Multivariate RLQ analysis (linking functional traits, sites and environmental factors in a three-way ordination) and fourth corner analysis (revealing significant relations between traits and environmental factors) were used in a complementary way to get insights into trait-environment relations. Results revealed that traits related to plant size (reflecting light absorption and competitive ability) increased with resource availability, such as soil phosphorus and water holding capacity. Leaf economic traits, such as specific leaf area (SLA), leaf nitrogen content (LNC), and leaf dry matter content showed low variation across the disturbance gradient and were not related to environmental variables. In these herbaceous annual communities where plants grow and persist for just 3–5 months, SLA and LNC were unrelated, which together with relatively high SLA values might point to strategies of drought escape and grazing avoidance. Seed mass was high both at higher and lower resource availability, whereas seed number increased with the degree of disturbance. The strong response of size and reproduction traits, and the missing response of leaf economic traits reveal light interception and resource competition rather than resource acquisition and litter decomposition as drivers of plant community composition. Deviations from trait relationships observed in commonly studied temperate ecosystems confirm that climatic conditions play a fundamental role by filtering species with particular life forms and ecological strategies.

Mechanisms of seed mass variation along resource gradients

“…; Dirks et al . ; Santini et al . ), we are not aware of any previous report of unimodal relationships between resource availability and the variance in seed mass, probably because most previous studies have only tested for linear trends (Bernard‐Verdier et al .…”

“…; Dirks et al . ; Santini et al . ), theoretical studies attempting to explain the observed patterns are extremely rare (Tilman ; Adler et al .…”

“…Understanding the causes and consequences of seed mass variation along resource gradients is particularly important because seed mass is a major determinant of individual fitness and community structure (Venable & Brown 1988;Westoby et al 2002;Turnbull et al 2004;Maron et al 2012;Diaz et al 2016). Still, while patterns of seed mass variation along soil resource gradients have been documented in numerous studies (Pakeman et al 2008;May et al 2013;Carmona et al 2015;Lhotsky et al 2016;Dirks et al 2017;Santini et al 2017), theoretical studies attempting to explain the observed patterns are extremely rare (Tilman 1988;Adler et al 2013). Furthermore, all previous studies focused on changes in community mean, ignoring variation in community variance.…”

“…The unimodal relationship in CWV is attributed to the fact that, under intermediate resource levels, neither largeseeded species, nor small-seeded species have a strong competitive advantage. Thus, intermediate levels of soil resources act as an 'equalising mechanism' that reduces fitness differences among large-seeded and small-seeded species, thereby allowing co-occurrence of species with a wider range of seed mass than either high or low resource levels Many studies of herbaceous communities have documented an increase in community seed mass with increasing soil resource availability as predicted by the model (Manning et al 2009;Harel et al 2011;Bernard-Verdier et al 2012;May et al 2013;Dirks et al 2017;Santini et al 2017). However, this pattern could be also attributed to other mechanisms (e.g.…”

Trait–performance relationships of grassland plant species differ between common garden and field conditions

“…Similarly, LES traits were found to have a subordinate role in community assembly as response to land‐use. For example, Dirks, Dumbur, Lienin, Kleyer, and Grünzweig () found that size and reproduction traits rather than leaf economic traits drove the composition of Mediterranean annual vegetation along a land‐use intensity gradient. This emphasizes the general importance of traits concerning clonal growth and vegetative reproduction for plant performance (Klimešová, Tackenberg, & Herben, ).…”