Consequences of shrub clearing on the richness of aquatic vegetation in oligotrophic seasonal pools in Southern France

The characteristic vegetation of Mediterranean seasonal oligotrophic pools is dominated by aquatic and amphibious annual plant species. Temporary flooded pools in southern France have experienced during the last decades a progressive decline of grazing, notably by sheep, favouring the encroachment of shrubs (Ulmus minor and Fraxinus angustifolia). A major consequence of this colonisation is the regression of the aquatic and amphibious annual plant species, resulting from shading by dominant vegetation and from litter accumulation. A restoration experiment has been established on 6 pools in southern France testing the impact of shrub clearing on vegetation dynamics. The objective of this project was to evaluate the feasibility of the restoration of the community dominated by aquatic and amphibious annuals and to identify indicators for success (restoration of the target vegetation). This project hypothesized that recruitment from seed bank is the dominant mechanism for regeneration and that seed bank of the target species persisted in shrub-dominated areas of the pools. The seed bank was analysed by the germination method. Our analysis showed that seed banks for the annual amphibious and aquatic species were present in shrub dominated areas of the pools. Shrub clearing was followed in 2 pools by a significant increase in the richness of the vegetation in aquatic annual species. Among the other 4 pools the abundance of perennial herbaceous species and of annual terrestrial species were significantly negatively correlated with the number of aquatic annual species. Preliminary indicators for the success of restoration by shrub clearing are proposed based on existing vegetation, seed banks and hydrology. The importance of perennial grasses suggest that grazing combined with shrub clearing should achieve better results for the restoration of these annual dominated temporary pools.