GRADATION IN NUTRIENT COMPOSITION AND PHOTOSYNTHETIC PATHWAYS ACROSS THE RESTINGA VEGETATION OF BRAZIL

The restinga comprises coastal vegetation formations which dominate th e Atlantic seaboard of Brazil. Exposed sand ridges and associated lago on systems have poorly developed soils subject to pronounced water def icits. Distinct vegetation zones support a high diversity of life form s, and a comparative study has been undertaken to investigate interact ions between degree of exposure, nutrient supply and photosynthetic pa thway (C-3, or CAM) in selected species across the restinga. A number of species occurring throughout the restinga were chosen as representa tive species of different life forms, comprising C-3 pioneer shrubs (E ugenic rotundifolia and Erythroxylum ovalifolium), impounding (tank) t errestrial bromeliad (Neoregelia cruenta: CAM) and the atmospheric epi phyte (Tillandsia stricta: CAM). Comparisons of plant and soil nutrien t composition, and airborne deposition were conducted for each zone. S oil nutrient content and organic matter were closely related, reaching a maximum in zone 4, the seaward face of the inner dune. Salt concent ration in leaves was independent of atmospheric deposition for the ter restrial species, in contrast to the atmospheric epiphyte T. stricta. In the slack area, vegetation formed characteristic ''islands'' with t he soil beneath enriched in nutrients, suggesting a complex interplay between plants and soil during the development of vegetation successio n. Here, two additional trees were investigated, C-3 and CAM members o f the Clusiaceae, respectively Clusia lanceolata and C, fluminensis. S table isotope composition of nitrogen (delta(15)N) was generally more negative (depleted in N-15) in plants with low total nitrogen content. This was exemplified by the atmospheric bromeliad, T. stricta, with a n N content of 2.91 g/kg and delta(15)N Of -12.3 per mil. Stable isoto pes of carbon (delta(13)C) were used to identify the distribution of p hotosynthetic pathways, and while the majority of bromeliads and orchi ds were CAM, analysis of the soil organic matter suggested that C-3 pl ants made the major contribution in each zone of the restinga. Since d elta(13)C of plant material also suggested that water supply was optim al in zone 4, we conclude that succession and high diversity in the re stinga is dependent on exposure, edaphic factors, and perhaps a critic al mass of vegetation required to stabilize nutrient relations of the system.