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Tropical dry forests have high diversity and plant abundance of potentially biologically nitrogen fixing (BNF) legume species, attributed to the ecological advantage of fixation. However, there are few estimates of N quantities annually fixed, hindering the understanding of factors that control BNF, like low phosphorus availability. The quantities of N fixed in three dry forest (caatinga) fragments of the semiarid Brazilian northeastern region with different legume plant proportions were determined and seedlings of Mimosa tenuiflora were grown with phosphorous fertilized soil from the fragments to verify if lack of fixation was due to the absence of rhizobia populations or P deficiency. The vegetation of all areas was dominated by legume plants, mainly potentially nodulating ones, despite the relatively high soil N availability. M. tenuiflora was the most abundant nodulating legume in all fragments, with annual leaf productions from 800 to 1400 kg ha-1. BNF amounts were low (1.4, 18 and 3.6 kg ha-1 year-1 in the mature caatinga of Petrolina and in the mature and regenerating caatingas of São João, respectively) considering the high proportions of potentially nodulating plants (33, 61 and 82% of total plant basal area), because 80, 10 and 70% of these plants were not fixing Electronic supplementary material The online version of this article (

O aumento na concentração de CO2 atmosférico e seu potencial em causar mudanças climáticas globais renovaram o interesse no ciclo de carbono e em estratégias de seqüestro de C. Balanços de C têm sido estimados para diferentes regiões e ecossistemas do mundo mas são escassos no semiárido nordestino. Este semi-árido estende-se por 1 milhão de km2, dos quais estima-se que 40, 30, 15 e15% são ocupados com caatinga, pastos nativos, pastos plantados e lavouras, com estoques de biomassa de 47, 15, 2 e 1 Mg ha-1, totalizando cerca de 1200 Tg de C, e produzindo 9, 8, 8 e 5 Mg ha-1 ano-1, totalizando 400 Tg de C ano-1.As concentrações de C no solo foram estimadas em 9,25 e 5 g C kg solo-1 , nas profundidades de 0-20 e 20-100 cm, totalizando 8,9 Pg de C. A abertura anual de novas roças equivale a 2,4 x 106 ha, correspondendo a 48 x 106 Mg de biomassa e 24 Tg de C. O consumo anual de lenha corresponde a 17 x 106 Mg de biomassa, o de forragem a 47 x 106 Mg e a produção agrícola a 15 x 106 Mg. Não é possível fechar um balanço entre perdas e ganhos de C por falta de informação sobre o balanço na derrubada e queima da caatinga para a formação de roçados e pastos e na regeneração da vegetação nativa em áreas nas quais estas atividades foram descontinuadas. Há um potencial inaproveitado na venda de créditos de C nas áreas em regeneração. Palavras chave: uso da terra, biomassa, raízes, C no solo, lenha, forragem  Stocks and Fluxes of Carbon in Semiarid Northeast Brazil: Preliminary Estimates  ABSTRACT The increase in atmospheric CO2 concentration and its potential to cause climatic changes brought a new interest in carbon cycle and C sequestration strategies. Carbon balances have been estimated for several regions and ecosystem in the world but are scarce for the semiarid northeastern region. This region covers one million km2, of which caatinga, native pastures, planted pastures and crops occupy 40, 30, 15 and 15%, with biomass stocks of 47, 15, 2 and 1 Mg ha-1, totaling about 1200 Tg of C, and producing 9, 8, 8 and 5 Mg ha-1 year-1, totaling 400 Tg of C year-1. Soil C concentrations were estimated as 9.25 e 5 g C kg soil-1 , in the layers of 0-20 and 20-100 cm depth, totaling 8,9 Pg of C. The opening of new crop fields is equivalent to 2.4 x 106 ha, corresponding to 48 x 106 Mg biomass and24 Tg of C. Firewood consumption corresponds to 17 x 106 Mg biomass, forage consumption to 47 x 106 Mg and agricultural production to 15 x 106 Mg. It is not possible to close a balance of C gains and losses due to lack of information on the balance of native vegetation slash and burn to establish new crop fields and native vegetation regrowth with the abandonment of these fields. There is an unused potential in the marketing of C credits in the areas of regenerating caatinga. Key words: land use, biomass, soil C, firewood, forage

RESUMO O efeito de borda causa uma série de alterações na estrutura da vegetação, microclima, dinâmica e composição de espécies, tornando esse ambiente bastante distinto do interior. Neste trabalho, foi analisada a influência da distância do limite florestal na estrutura e dinâmica (2006 e 2009) do dossel e do sub-bosque de um fragmento com 14 ha localizado na Floresta Atlântica em Igarassu, PE. A borda externa, formada há cerca de 35 anos, foi definida do limite florestal até 50 m, o ambiente intermediário de 50 e 100 m e o interior a 150 m. Para amostragem do dossel (indivíduos com diâmetro do caule a 1,30 m do solo > 4,77 cm) foram alocadas 10 parcelas de 10 × 10 m por ambiente e do sub-bosque (indivíduos com diâmetro do caule ao nível do solo entre 0,96 cm de diâmetro do caule ao nível do solo e diâmetro do caule a 1,30 m do solo < 4,77 cm) em sub-parcelas de 5 × 5 m instaladas no vértice superior e lado esquerdo das parcelas do dossel. Os valores de densidade e área basal do dossel foram semelhantes ao longo do gradiente borda -interior, enquanto no sub-bosque foram maiores no interior. A borda do dossel teve o maior incremento médio em diâmetro e foi o único ambiente a apresentar um recrutamento (número absoluto) maior que a mortalidade. No sub-bosque houve maior recrutamento que mortalidade em todos ambientes. A distância do limite florestal não influenciou a estrutura nem a dinâmica.Palavras-chave: dossel, efeito de borda, sub-bosque, vegetação ABSTRACT Edge effect produces a series of modifications on vegetation structure, microclimate, dynamics and species composition, making this environment very distinct from forest interior. In this study, the influence of distance from the forest border on the structure and dynamics (2006 and 2009) of canopy and understory assemblages was analyzed in a 14 ha fragment of Atlantic Rainforest in Igarassu, PE, Brazil. The edge, formed about 35 years ago, was defined as area between the forest margins up to 50 m inwards; the intermediate environment was considered from 50 m to 100 m; and the interior environment beyond 150 m from the edge line. In order to sample the canopy (individuals with stem diameter > 4.77 cm at 1.30 m from the ground) ten 10 × 10 m plots were allocated in each environment; for sampling the understory (individuals with stem diameter at ground level > 0.96 cm up to stem diameter at 1.30 m < 4.77 cm) sub-plots of 5 × 5 m were located at the apex and the left side of the canopy plots. Density and basal area of canopy were similar along the edge -interior gradient, whereas the understory had greater values in the interior. For the canopy, the edge showed the highest average increment in diameter and was the only environment to provide recruitment (absolute) higher than mortality. For the understory, in all environments, recruitment was greater than mortality. Distance from forest margins did not influence forest structure and dynamics.

& Key message Potentially nodulating Leguminosae species had high density and biomass in young stands of forests in Northeast Brazil, except in a dry forest on a low-fertility soil. & Context Leguminosae species, especially potentially nodulating ones, appear to be important components of tropical secondary forests, although information is still scarce concerning their contributions to richness and biomass under different environmental conditions. & Aims To evaluate the contribution of Leguminosae to the aboveground biomass and density of young and old forest stands in six sites, with different climate and soil conditions, in northeastern Brazil. & Methods The biomass of plants was estimated using allometric equations in ten plots in one young and one old forest stand in each site. Leguminosae were separated into nodulating and non-nodulating species based on literature. & Results Leguminosae, mainly nodulating species, displayed the largest number of species among all families and the largest biomass and density in young stands. There was more similarity in Leguminosae species composition between young and old stands in dry than in humid forests. & Conclusion Nodulating species make considerable contributions to the biomass of the initial successional stages of forests in Northeast Brazilconfirming the importance of the family for natural forest regeneration, especially in dry environments.