Shift from trait convergence to divergence along old-field succession.

Aims: Ecological theories predict that assembly processes shape communities so that co-existing species may either be functionally more dissimilar ("divergence") or more similar ("convergence") than expected by chance. Two important factors that are rarely considered in combination are spatial scale and successional stage. Our aim is to identify different functional patterns during succession across spatial scales and to discuss the likely underlying assembly processes. We expect to find convergence due to environmental filtering at early succession, especially at the largest scales, and a tendency towards divergence driven by competitive interactions as succession unfolds and at smaller spatial scales. Location: Protected Landscape Area “Bohemian Karst”, Czech Republic. Methods: We studied three succession stages (two, eight and 55 years after abandonment). Each field was sampled using 40 1-m² quadrats containing 100 pins in a regular grid. We analysed trait dispersion at several combinations of grain and extent and characterised trait similarity using phylogeny and four traits: specific leaf area, leaf dry matter content, height, and seed mass analysed individually and combined using Gower distance (multi-trait dissimilarity). Results: In the youngest field, three of the traits individually and the multi-trait dissimilarity showed convergence regardless of scale. Phylogeny showed convergence at most spatial scales of the youngest stage. The other two fields showed greater divergence predominantly in the oldest field at the smallest spatial scale. Conclusions: The results are mainly congruent with theoretical expectations and provide a valuable example of directional changes from trait convergence to divergence along succession. The scale-invariant convergence in the early stages of succession seems to be the result of environmental filtering and weaker competitive exclusion. At later stages and particularly at smaller scales, divergence becomes more common, likely because competition limits the trait similarity of species. These results highlight the importance of predictable (non-random) changes in ecological succession. [ABSTRACT FROM AUTHOR]