Linking intra- and inter-site trait variability to plant strategies: ruderals drive trait variability at high productivity only

A slightly version of this chapter has been submitted. ABSTRACT Separation of trait values into intra-site (alpha component) and inter-site (beta component) variability is a powerful tool to study plant strategies and drivers of plant trait variation. Here we relate both sources of variation with the competitor-stress tolerator – ruderal (CSR) classification to understand trait patterns across and within sites. Alpha and beta values were calculated for six traits (specific leaf area, leaf nitrogen concentration, leaf phosphorus concentration, maximum height, leaf size and stem specific density) measured in The Netherlands and Ukraine (71 sites, 389 samples of 159 species). Data spanned all growth forms and common mesic to wet habitats in temperate Europe. The relation between CSR and alpha and beta trait values was quantified with weighted least squares ANOVA and Tukey post hoc tests. Variation in leaf economy traits, changes in leaf size concomitant with plant height and plant traits related to the vertical partitioning of light within communities occurred consistently across growth forms and location, while stem density relations varied with growth form, environmental limitation and disturbance regime. In agreement with the CSR theory, beta trait values of competitors and stress tolerators were separated only along gradients representing productivity. Ruderals had deviating trait values (alpha values) only at the productive end of the gradient (beta values). Ruderals' patterns reflected the effects of disturbance in grasslands, floodplains and sites in early states of succession, while in old forests the ruderal classification did not allow to tease apart effects of disturbance and shade tolerance. By using trait gradient analysis, we have shown that understanding trait variation requires considering how plant strategies arise across growth forms, species and locations, as well as the role of disturbance and shade tolerance as drivers of trait variability within sites, particularly when productivity is high.