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 A formação de anéis de crescimento anuais tem sido confirmada para diversas espécies de mangue na última década, dentre as quais Rhizophora mangle. Contudo, o registro de anéis anuais para esta espécie foi feito em região com alta sazonalidade hídrica, um fator de indução de anéis anuais em espécies tropicais amplamente reconhecido. Nesse sentido, o presente estudo teve como objetivo verificar a ocorrência de anéis de crescimento anuais em R. mangle nos manguezais de Guaratiba (Rio de Janeiro, sudeste do Brasil), região com baixa sazonalidade hídrica. Para isso, a técnica de datação cruzada foi aplicada em dez árvores coletadas com idade conhecida (sete anos). Os anéis de crescimento são caracterizados pela alternância entre uma camada com baixa densidade de vasos (lenho inicial) e outra com alta densidade de vasos (lenho tardio). A análise de regressão múltipla indicou que o padrão de variação de largura dos anéis de crescimento é determinado por precipitação, excedente hídrico, déficit hídrico e armazenamento hídrico. A análise de datação cruzada confirmou a existência de anéis de crescimento anuais no lenho de R. mangle em Guaratiba. Essa descoberta em região com sazonalidade hídrica pouco pronunciada aumenta o potencial dendrocronológico desta espécie e sugere a influência de outros fatores (ex. comportamento fenológico) como indutores complementares para a formação dos seus anéis de crescimento desta espécie.

ABSTRACT The formation of annual growth rings has been confirmed for several mangrove species in the last decade, among which is the Rhizophora mangle. However, the record of annual rings for this species was made in a region with high hydric seasonality, a widely recognized induction factor of annual rings in tropical species. In this sense, the present study aimed to verify the occurrence of annual growth rings in R. mangle in the mangroves of Guaratiba (Rio de Janeiro, Southeastern Brazil), a region with low hydric seasonality. For this purpose, the crossdating technique was applied in ten trees collected with known age (seven years). The growth rings are characterized by alternating layers of low vessel density (earlywood) and high vessel density (latewood). Multiple regression analysis indicated that growth rings width variation is driven by precipitation, water surplus, water deficit and water storage. Crossdating analysis confirmed the existence of annual growth rings in the R. mangle in Guaratiba. This discovery in a region with low hydric seasonality increases the dendrocronological potential of this species and suggests the importance of biological factors (eg. phenological behavior) as complementary inductors for the formation of growth rings in this species.