Plant regenerative strategies after a major disturbance : The case of a riverine wetland restoration.
Résumé
In aquatic habitats, both vegetative and sexual reproduction are involved in vegetation maintenance. Restoration projects carried out in such habitats rarely consider the role of both sexual reproduction (seeds) and vegetative (vegetative propagules or clonal fragments) components of propagule banks in vegetation regeneration. Moreover, changes in the seed bank due to the restoration itself have rarely been studied. The aim of this paper is to answer the following questions. Does restoration consisting of sediment removal in a cut-off meander lead to a change in the propagule bank (both sexual and vegetative propagules), and does the vegetation regeneration involve vegetative propagules created by plant breakage during the disturbance? Vegetation was visually surveyed three times during a 15-year period before the restoration and two years afterwards in both the restored zone and an upstream undisturbed zone, The propagule bank was characterized just before and one year after the restoration in the restored zone by sampling of sediment cores and propagule germination in a greenhouse. The impact of restoration on the propagule bank was very pronounced. Sprouting propagules were three times more abundant in post-restoration samples than in pre-restoration samples. The ratio of seeds / vegetative propagules in the bank was lower in the year following restoration. Links between propagule bank and established vegetation were assessed through calculation of correlation and similarity indices. We observed a large difference between established vegetation and the propagule bank before restoration, suggesting that recruitment from the propagule bank was low before restoration. Restoration enhanced the recruitment from rhizomes and vegetative fragments, which were highly correlated with the established vegetation. The pre-eminent role of some vegetative propagules in the recolonization and the increasing abundance of an invasive species are related to the season (June) in which restoration was carried out. Impact of restoration on both the propagule bank composition and environmental conditions (especially those promoting or inhibiting germination or sprouting) thus has to be considered in further vegetation management work.