ABSTRACT Studies on belowground roots biomass have increasingly reported the importance of the contribution of this compartment in carbon stock maintenance in mangrove forests. To date, there are no estimates of this contribution in Brazilian mangrove forests, although the country has the second largest area of mangroves worldwide. For this study, trenches dug in fringing forests in Guaratiba State Biological Reserve (Rio de Janeiro, Brazil) were used to evaluate the contribution of the different classes of roots and the vertical stratification of carbon stock. The total carbon stock average in belowground roots biomass in these forests was 104.41 ± 20.73 tC.ha−1. From that, an average of 84.13 ± 21.34 tC.ha−1 corresponded to the carbon stock only in fine roots, which have diameters smaller than 5 mm and are responsible for over 80% of the total belowground biomass. Most of the belowground carbon stock is concentrated in the first 40 cm below the surface (about 70%). The root:shoot ratio in this study is 1.14. These estimates demonstrate that the belowground roots biomass significantly contributes, more than 50%, to the carbon stock in mangrove forests. And the mangrove root biomass can be greater than that of other Brazilian ecosystems.

Resumo No presente estudo, a variabilidade estrutural de uma floresta de mangue associada a uma planície hipersalina na Baía de Sepetiba (SE-Brasil) é analisada. A estrutura vegetal e a salinidade da água intersticial foram medidas em 32 parcelas estabelecidas ao longo de 3 transectos localizados desde a margem do estuário até a transição com a planície hipersalina. O desenvolvimento estrutural foi altamente variável, com altura média variando entre 0,54 m e 7,96 m, diâmetro médio entre 1,58 cm e 9,46 cm e densidade de troncos entre 2.733 troncos vivos.ha-1 e 106.667 troncos vivos.ha-1. A variabilidade da estrutura vegetal respondeu ao gradiente de aumento da salinidade da água intersticial à planície hipersalina, com a altura e o diâmetro médios diminuindo e a densidade de troncos aumentando no mesmo sentido. O seguinte padrão de dominância de espécies foi observado: Rhizophora mangle nas proximidades da margem do estuário; Laguncularia racemosa na transição com a planície hipersalina; e Avicennia schauerianna nas zonas intermediárias. O entendimento das características estruturais das florestas de mangue e suas relações com estressores naturais (ex. salinidade) contribuirá para a melhor compreensão da dinâmica das espécies ao longo de planícies costeiras associadas à planícies hipersalinas bem preservadas como em Guaratiba.

Abstract In this study, forest structure variability of a mangrove associated to a salt flat in Sepetiba Bay (SE-Brazil) was assessed. Forest structure and interstitial water salinity were measured in 32 plots established along three transects ranging from the margin of the estuary to the transition with the salt flat. Structural development was shown to be highly variable, with mean height ranging from 0.54 m to 7.96 m, mean diameter ranging from 1.58 cm to 9.46 cm and trunk density ranging from 2,733 live trunks.ha-1 to 106,667 live trunks.ha-1. Forest structure variability responded to the gradient of increasing interstitial water salinity towards the salt flat, with mean height and diameter decreasing and trunks density increasing progressively in the same direction. The following pattern of species dominance was observed: Rhizophora mangle near the estuary margin; Laguncularia racemosa at the transition with the salt flat; and Avicennia schaueriana on intermediate zones. The assessment of structural characteristics of mangrove forests and their relationships with natural stressors (e.g., salinity) contribute to better understand the dynamics of the species along coastal plains associated with preserved salt flats, such as the one found in Guaratiba.