Gas exchange and isotopic signature of mangrove species in Southern Brazil

The study measured leaf gas exchange parameters and abundance of stable isotopes of C and N of three mangrove species (Rhizophora mangle L., Avicennia schaueriana Stapf & Leechm. ex Moldenke. and Laguncularia racemosa (L.) Gaertn) to investigate how photosynthetic capacity and water use efficiency characterize salt tolerance along an environmental gradient, in the Guaratuba Bay Estuary, Brazil, around 25°S. Along the estuary, parallel to the river, nine 50 m² plots were marked: three plots at the fringe, three plots at the intermediary zone (around 150 m from the fringe), and three plots at the interior zone (around 350 m from the fringe), to represent the zonation pattern expressed by mangrove species. We used gas exchange parameters to calculate intrinsic water use efficiency (A_max/g_s, WUEi). Long-term water use efficiency was calculated based on δ¹³C values, and δ¹⁵N values were related to source of N. Avicennia schaueriana showed the capacity to maintain high WUEi, even with high g_s under higher salinities. The foliar δ¹³C of L. racemosa, was lower than the other species and negatively correlated to pore-water salinity. Rhizophora mangle gas exchange parameters did not correlate to the measured soil variables. Although the difference was not significant in R. mangle, δ ¹⁵N values indicate that fringe mangroves may use a marine source of N. How species respond to changes in pore-water salinity is expressed in both gas exchange parameters and isotopic signature through the floodplain gradient.