Frontiers in Marine Science (Apr 2014)
Environmental change in a Mediterranean salt marsh wetland: ecological drivers of halophytes diversity along flooding frequency gradients
Abstract
Coastal wetlands are among most threatened ecosystems, owing to the intense human activity concentrated in shoreline areas together with the expected sea level rise resultant from climate change. Salt marshes are wetlands which are inundated twice daily by the sea, thus tightly dependent on frequency and duration of submergence. Identifying the factors that determine the diversity, distribution and abundance of halophyte species in salt marshes will help retaining their conservation status and adopt anticipate management measures, and this will ultimately contribute to preserve marshland biodiversity and ecological services. Reserva Natural de Castro Marim e Vila Real de Santo António (RNSCMVRSA) is a natural reserve located in South Eastern Portugal, comprising the tidal area of Guadiana River mouth. In spite of their great ecological value, salt marsh ecosystems in this region have suffered intense anthropic disturbance, namely hydrologic alterations and vegetation removal to gain soils for agriculture and salt intensive production. The present study aimed at characterizing the halophyte diversity in the RNSCMVRSA salt marshes and determining their major ecological correlates. The end-point is to implement, afterward, a sustainable cultivation of autochthonous halophyte plants, with economic value, in the abandoned saltpans and degraded rangelands. This project will contribute to the conservation of halophyte diversity, promote environmental requalification, and provide an economic alternative for local populations, enabling the reduction of unregulated harvest of halophyte plant populations. Field sampling strategy included a preliminary survey of local vegetation diversity and floristic inventories of halophyte communities in plots established across the existing environmental heterogeneity in order to span the whole variation gradients of the species presence and abundance. The abiotic characterization of halophyte communities included a topographic survey, soil analysis and water level monitoring carried out continuously during a whole year cycle. Biotic and abiotic data were compiled into matrices further analysed by means of multivariate (ordination and hierarchical clustering) and univariate analysis (logistic regression modelling), searching for the floristic community patterns and their ecological correlates. Our results showed that, in natural systems, the halophyte species are distributed along an elevation gradient, reflecting mostly flooding regime (intensity, duration and amplitude of submergence), and edaphic differences. Functional attributes such as plant life span and reproductive traits revealed closely related with the duration and amplitude of submergence. The few annual species present (Salicornia sp) seem to be associated to the longest submergence periods during growing period whilst require open soil for seed germination. Lower elevation perennial species showed a larger ecological plasticity than perennials associated to higher elevations. Finally, in semi-natural systems, derived from the abandoned saltpans, artificial conditions come about for the development of halophyte species at higher elevations from their natural elevation range. This result has a twofold interpretation, i) suggests an opportunity for the cultivation of the species and ii) points out upcoming challenges of a sea level rise scenario, given the lack of available inland migration areas, currently occupied by human settlements.
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