Vegetation recolonisation of a Mediterranean temporary pool in Morocco following small-scale experimental disturbance

Disturbances are key factors in the dynamics and species richness of plant communities. They create regeneration niches allowing the growth of new individuals in patches submitted to lower intensity of competition. In Mediterranean temporary pools, the intense summer drought constitutes for communities a large-scale disturbance whose intensity varies along the topographical and hydrological gradient between the centre and the edges. In this context, the importance of small-scale disturbance, such as those created by trampling and rooting herbivores in temporary pools, is poorly known. The recolonisation of small bare patches of a woodland temporary pool in western Morocco was studied experimentally in the field. The experiment was carried out using nine small control plots and nine experimental plots (sterilisation of the soil) distributed along the topographical gradient (centre, intermediate and edge zones). The area covered by plant species, and the water levels, were recorded for the plots over two successive hydrological cycles (2006/2007 and 2007/2008). The effects of natural history traits (size of seeds, presence or absence of dispersal mechanisms and annual/perennial) on the success of recolonisation of individual species were analysed. The results show that the experimental plots were rapidly recolonised. The community composition apparently was affected by the very dry conditions during the first year of the experiment, when annual species were largely absent and the clonal perennial species (Bolboschoenus maritimus and Eleocharis palustris) were dominant in the centre and intermediate zones, whilst not a single species colonised the edge zone. In the second year, less dry hydrological conditions allowed annual plants to appear in all three zones. After 2 years, the species composition of the vegetation in the experimental plots was similar to that of the unsterilised (control) plots. The abundance of plants in the centre zone was identical for experimental and control plots; in the intermediate and edge zones, the species' abundance was lower in the experimental plots than in the control plots, suggesting an incomplete return to the reference condition (control state). Differences in abundance of species were uncorrelated with the size of seeds or to the annual/perennial nature of the plants, but were particularly dependent on the hydrological conditions, which favoured lateral colonisation by perennials (runners, rhizomes). These results show that recovery from the minor disturbances can be rapid in Mediterranean temporary pools.