Gradation in Nutrient Composition and Photosynthetic Pathways Across the Restinga Vegetation of Brazil

Abstract

Abstract:The restinga comprises coastal vegetation formations which dominate the Atlantic seaboard of Brazil. Exposed sand ridges and associated lagoon systems have poorly developed soils subject to pronounced water deficits. Distinct vegetation zones support a high diversity of life forms, and a comparative study has been undertaken to investigate interactions between degree of exposure, nutrient supply and photosynthetic pathway (C3, or CAM) in selected species across the restinga. A number of species occurring throughout the restinga were chosen as representative species of different life forms, comprising C3 pioneer shrubs (Eugenia rotundifolia and Erythroxylum ovalifolium), impounding (tank) terrestrial bromeliad (Neoregelia cruenta: CAM) and the atmospheric epiphyte (Tillandsia stricta: CAM). Comparisons of plant and soil nutrient composition, and airborne deposition were conducted for each zone. Soil nutrient content and organic matter were closely related, reaching a maximum in zone 4, the seaward face of the inner dune. Salt concentration in leaves was independent of atmospheric deposition for the terrestrial species, in contrast to the atmospheric epiphyte T. stricta. In the slack area, vegetation formed characteristic “islands” with the soil beneath enriched in nutrients, suggesting a complex interplay between plants and soil during the development of vegetation succession. Here, two additional trees were investigated, C3 and CAM members of the Clusiaceae, respectively Clusia lanceolata and C. fluminensis. Stable isotope composition of nitrogen (δ15N) was generally more negative (depleted in 15N) in plants with low total nitrogen content. This was exemplified by the atmospheric bromeliad, T. stricta, with an N content of 2.91 g/kg and δ15N of −12.3 per mil. Stable isotopes of carbon (δ13C) were used to identify the distribution of photosynthetic pathways, and while the majority of bromeliads and orchids were CAM, analysis of the soil organic matter suggested that C3 plants made the major contribution in each zone of the restinga. Since δ13C of plant material also suggested that water supply was optimal in zone 4, we conclude that succession and high diversity in the restinga is dependent on exposure, edaphic factors, and perhaps a critical mass of vegetation required to stabilize nutrient relations of the system.