Validating space-for-time substitution in a new-growth coastal dune forest

Questions: Can trends derived from chronosequences (space) be compared with those derived from repeated surveys (time) to validate the use of space-for-time substitution? Can this approach provide insight into the dynamics of vegetation in rehabilitating forests? Location: Rehabilitating coastal dune forest in KwaZulu-Natal, South Africa. Methods: We surveyed sapling and adult trees in seven study sites with similar abiotic conditions that were rehabilitated after mining. Sites ranged in age from 1 to 24 yr in 2001 and we repeated the surveys in 2005 and 2010. We related patterns of change in community attributes (e.g. species composition, turnover, taxonomic and functional diversity) to site age. We compared functional diversity indices (e.g. functional dispersion and evenness) with a random community to infer the ecological processes that structure the tree community during forest development. We focused on the magnitude and direction of change predicted by the 2001 chronosequence with the values returned from subsequent surveys. Results: Species composition changed during forest development. Species richness increased and species turnover decreased. Null models indicated that early stages of forest development had lower functional diversity than expected from a random community, indicating convergence in reproductive traits in early succession. Late stages of forest development showed higher functional diversity. Apart from species composition, the rest of the community attributes recorded from repeated surveys confirmed the predictions made by the chronosequence. Conclusions: The trajectory of plant community succession observed in space across a chronosequence was supported by the site-specific findings derived from long-term monitoring. In the case of our study system (rehabilitating dune forests), space-for-time substitution works. The main drivers of species co-existence changed from environmental filtering and/or dispersal limitation to niche complementarity as these forests developed.