Temporal and spatial changes in plant dune diversity in urban resorts

We investigated the temporal and spatial changes in the floristic composition and abundance in sand dunes along a coastal strip in the province of Buenos Aires (Argentina). Firstly, we analysed changes over a span of 70 years, comparing 41 of our own inventories carried out in 14 beach resorts in 2005-2006 with 18 inventories made in the first half of the XXth century (1930-1940) in the same study area. We grouped sampled plots into four categories for comparison, taking into account the time period (1930-1940 or 2005-2006) and location at each end of the surveyed coastal strip in the northern or southern 'tourist' sectors. Secondly, we analysed the effect of afforestation with exotic trees on extant sand dune vegetation for the period 2005-2006. A total of 74 plant inventories were carried out in afforested and natural dunes at the same resorts. In both comparisons we contrasted plant richness, percentage plant cover, similarity and diversity. A cluster analysis was used to classify categories taking into account plant abundance and composition. Significant differences in total diversity were observed between sampling years, suggesting an increase in diversity at the present time due to urbanization and recreation related activities. Nevertheless, no single exotic plant species was clearly dominant across the sampled sites, suggesting that these coastal areas are, up to now, resistant to alien invasion. Diversity indexes discriminated by plant groups indicating different habitat conditions and exotics were shown to be more sensitive to existing changes than to total richness and cover.