Primary succession and plant functional traits on an oceanic island

Abstract

The discipline of functional island biogeography and studies on plant ecological strategies shaping island assemblages have emerged only recently. Due to volcanic activity, primary succession is an important natural process driving ecosystem dynamics on oceanic islands. However, our knowledge about plant functional traits in explaining the mechanism of ecological succession on these islands characterized by impoverished and disharmonic, but endemic-rich floras, is limited. We investigated the distribution of plants with respect to nine functional traits related to resource use, reproduction, and growth as well as their overall functional diversity during primary succession using a chronosequence on 10 lava flows of La Palma, Canary Islands. We analysed changes of single traits with age using community-weighted trait means and applied ordination techniques to evaluate changes in trait composition. Based on ecological succession theory, we tested several hypotheses for changes of functional traits along the successional gradient. We also analysed the role of endemic and introduced plants in structuring successional communities. Both the single-trait and trait compositional approach revealed that the composition of plants displaying core traits related to leaf economics and dispersal ability shifted with substrate age showing a change from acquisitive to conservative traits and from anemochory to zoochory along the gradient. These shifts were entirely driven by endemic shrubs, while other growth forms contributed to the increase in species and functional richness with lava age but little to vegetation structure. Introduced species were almost absent in successional trajectories. Functional dispersion increased whereas trait turnover was constant and low during succession. Synthesis. Our island system reflects a directional succession governed by the selection of functional traits related to environmental conditions and acquisitive-conservative strategies as well as to dispersal capacity in early stages followed by a gradual modification of the environment and an expansion of the niche space linked to functional divergence in later stages. Major changes in functional traits occurred five hundred years after the eruptions indicating that primary succession in this dry environment is very slow. The dominance of endemic shrubs and the lack of introduced plants at all successional stages highlight the importance of evolutionary processes in shaping species ecological strategies linked to disturbances like volcanism on this oceanic archipelago.