Trait-based assembly rules across climatic gradients of European grasslands

Background & Aims: Trait-based studies of species assembly reveal mechanisms of species coexistence. Are these mechanisms similar within European grasslands? Are these mechanisms influenced by site-specific disturbance history? We investigated fine-scale patterns of trait-based community assembly in European grasslands across continental gradients within the framework of the BiodivERsA project SIGNAL. The gradient extends from the mesic grasslands in France and Germany (P: 1200 to 750 mm/year; T: 8 to 7°C respectively), to intermediate ones in Italy and Bulgaria (P: 880 to 560 mm/year; T: 12,1 to 10,2°C respectively), to the xeric ones in Turkey and Hungary (P: 715 to 550 mm/year; T: 16,7 to 10,5°C respectively). The sites also differ in management, disturbance history, geology and edaphic factors. They represent common types of grasslands of the respective study country. Methods: Fine-scale patterns of species combinations (rooting individuals) were sampled in 2.80 m x 0.40 m blocks, subdivided into 448 microquadrats of 25 cm2 (5 cm x 5 cm). For representing within site heterogeneity, six blocks were sampled at each site. Three important plant traits (specific leaf area, plant height and seed mass) were assessed (from literature and direct measurements) for all recorded vascular herbaceous species. Based on these traits, the functional diversity (Rao index) was calculated for each microquadrats and compared to a null model. We used Schamp’s methods (Schamp 2008) for randomization (keeping the abundances of species and the local species richness as in the field and assigning traits at random to each species combinations). Deviation of functional diversity from random expectation was interpreted as trait divergence or trait convergence. Main Results & Interpretations: Several occurrences of trait-based assembly rules could be detected across countries. The strongest deviations from randomness in terms of seed mass were found in the Turkish grassland (driest one). In Bulgaria, Hungary, France and Germany, by contrast, a convergence of seed mass was seen. Strong convergence of plant heights appeared in Germany and France, in tall mesic grasslands, while this trait showed random patterns in dry grasslands. The Turkish grassland showed convergence for SLA. Similar tendency occurred at the German and the Italian sites while France and Bulgaria mostly showed random patterns. Based on our first results, we conclude that assembly rules can be contrasting and context dependent (cf. Wellstein et al. 2014) at different grassland sites and climatic differences are often masked by local factors (e.g. by disturbance regime or soil heterogeneity). For example Italian grasslands showed both convergence and divergence of seed mass even at nearby blocks indicating heterogeneous environmental conditions and complex site history.