Photosynthetic capacity and intrinsic water-use efficiency of Rhizophora mangle at its southernmost western Atlantic range

Abstract

The southernmost presence of Rhizophora mangle in the western Atlantic coast occurs in coastal wetlands between 27 and 28°S in the State of Santa Catarina, Brazil. We selected mangrove communities at the estuary of the River Tavares, Florianopolis, and the Sonho Beach, Palhosa, for measurement of photosynthetic performance and intrinsic water-use efficiency of R. mangle and coexisting individuals of Avicennia schaueriana and Laguncularia racemosa, during the spring season. We used gas-exchange techniques and isotopic signatures of C and N to estimate instantaneous water-use and carboxylation efficiency (CE), long-term water-use efficiency, and potential N limitation. Results showed that R. mangle had significantly lower photosynthetic rates but similar conductance values as the other two species resulting in lower intrinsic water use (WUEi) and CE. WUEi and CE were positively correlated in L. racemosa and A. schaueriana, but not in R. mangle. At each site, δ13C values of A. schaueriana were consistently higher than those for the other species, indicating that these species are subjected to contrasting water stress conditions. Leaf concentrations of C were lower, whilst those of N were always higher in A. schaueriana, indicating accumulation of salts and nonprotein N-compounds in leaves. Nitrogen concentrations and moderate positive δ15N values indicated that plant growth at the study sites was not inhibited by nutrient deficiency, and was not influenced by urban residual waters. Lower photosynthetic rates and values of CE of R. mangle compared to the other two species may constitute constraining factors preventing this species from establishing at higher latitudes.