Atmospheric carbon dioxide records indicate that the land surface has acted as a strong global carbon sink over recent decades1, 2, with a substantial fraction of this sink probably located in the tropics3, particularly in the Amazon4. Nevertheless, it is unclear how the terrestrial carbon sink will evolve as climate and atmospheric composition continue to change. Here we analyse the historical evolution of the biomass dynamics of the Amazon rainforest over three decades using a distributed network of 321 plots. While this analysis confirms that Amazon forests have acted as a long-term net biomass sink, we find a long-term decreasing trend of carbon accumulation. Rates of net increase in above-ground biomass declined by one-third during the past decade compared to the 1990s. This is a consequence of growth rate increases levelling off recently, while biomass mortality persistently increased throughout, leading to a shortening of carbon residence times. Potential drivers for the mortality increase include greater climate variability, and feedbacks of faster growth on mortality, resulting in shortened tree longevity5. The observed decline of the Amazon sink diverges markedly from the recent increase in terrestrial carbon uptake at the global scale1, 2, and is contrary to expectations based on models. (Résumé d'auteur

The occurrence of cerrado (as tree and shrub savanna is called in Brazil) and forest formations side by side is common at the southern margin of the Brazilian Amazonian Forest, and previous studies have demonstrated the advance of forests over cerrado areas. The aim of the present study is to provide an accurate documentation of the transition process between the two major biomes. Tree data (≥ 5 cm diameter at 0.3 m above soil level) from three plots of cerrado sensu stricto lying near three of cerradão (the taller, denser form of cerrado) were inventoried starting in 2002 in an area of 1.5 ha made up of 150 subplots of 10 × 10 m (50 in each area). This showed that the most important species of the cerradão were invading areas previously occupied by smaller, lower forms of cerrado (although it is sometimes difficult to define which are ‘forest’ and which ‘cerrado’ species as many are flexible in size – for instance Emmotum nitens can often be intermediate, establishing in cerrado that develops into cerradão and on to forest). Some typical species such as Eriotheca gracilipes and Emmotum nitens, established since the first inventories, have increased their populations (between 27 and 210%). Tachigali vulgaris, a typical, weedy, adventive species of the Cerrado–Amazonian Forest transition, showed the largest increase in abundance in areas of cerrado sensu stricto (between 100 and 1200%), and is probably the most important pioneer species in the initial advance of the forest into cerrado at the Southern Amazonian border.

Fire can change the species composition, diversity, and structure of savanna vegetation, thus altering growth and mortality rates. Such changes in the woody vegetation of burned savanna forest were evaluated over four years in comparison to unburned savanna forest. All woody plants with a diameter at breast height > 10 cm were measured in 100 permanent plots. Six months later, 38 of these plots were burned. Three and a half years later, all surviving individuals were re-sampled. Species richness, diversity, and the number of individuals did not change in the burned plots, although they had significantly higher (p < 0.05) increases in basal area and mortality rates (5.1% year -1 ) than the unburned plots (3.0% year -1 ). Tachigali vulgaris had the greatest post-fire increase in basal area (53%). The results indicate that fire alters the dynamics and structure of the savanna forest, excluding the less fire-tolerant species and smaller individuals ( 15cm). Tachigali vulgaris is a key species for the recovery of savanna forest biomass due to its considerable post-fire gains in basal area, at least over the short term due to its short life cycle. It follows that frequent burning of savanna forest would result in a marked change in the species composition and structure of its woody vegetation.

ResumoO objetivo do presente estudo foi testar a hipótese de que remanescentes de florestas, mesmo em pequenas unidades de conservação e sujeitos a perturbações intermediárias, são resilientes. Para isso, as distribuições de diâmetros e alturas da comunidade e das espécies mais abundantes de um cerradão (14°42'02,3"S e 52°21'02,6"W) foram analisadas e comparadas em intervalos de dois a três anos em um período de oito anos. Em 2002Em , 2005Em , 2008 e 2010 foram medidas todas as árvores vivas com diâmetro ≥ 5 cm a 0,3 m do solo em 50 parcelas de 10 x 10 m. Mesmo com variações significativas na densidade, a distribuição de diâmetros e alturas da comunidade manteve o padrão de "J-reverso" e unimodal, respectivamente, indicando recrutamento contínuo e poucas mudanças na estrutura ao longo dos anos inventariados, corroborando a hipótese do presente estudo. Entre as espécies analisadas foram observados três padrões de distribuição diamétricas que provavelmente refletem diferentes estratégias de ocupação da floresta. Hirtella glandulosa foi a espécie mais apta a explorar os recursos do ambiente, apresentando as maiores abundâncias e indivíduos distribuídos em todas as classes de diâmetros. Palavras-chave: Distribuição de diâmetros; Fogo; Mato Grosso; Seca; Unidades de conservação Biotemas, 26 (3): 49-62, setembro de 2013 ISSNe 2175-7925 http://dx. Abstract Resilience of a cerradão subjected to intermediate disturbance in the Cerrado-Amazonia transition,Mato Grosso, Brazil. The present study examined the hypothesis that remnant forests in small protected areas may be resilient to intermediate disturbances. We analyzed the diameter and height distributions of the tree community in a typical cerradão (14°42'02.3"S and 52°21'02.6"W) and determined which species were most abundant, sampling at two-to three-year intervals over eight years. In 2002 and 2010, we measured all live trees with a diameter ≥ 5 cm at 0.3 m above ground in 50 plots of 10 x 10 m. Although significant variation was observed in the density of the trees and the distributions of their diameters and heights, the community maintained the "reverse-J" and unimodal patterns for these distributions, respectively. These results indicate continuous recruitment and little change in the structure of the community over the study period, supporting our hypothesis. Three different patterns of diametric distribution were observed among the analyzed species, likely reflecting different forest occupation strategies. Hirtella glandulosa was the speciesmost able to exploit its environment, as it possessed the greatest overall abundance and was represented by individuals in all diameter classes.

A number of studies have focused on the species composition and diversity of the woody vegetation of the ''Cerrado'', but few comparative data are available on the flora or the structure of different physiognomies and vegetation strata, precluding more general assessments of variations within the biome. We aimed to evaluate the diversity, species composition, and structure of the upper (trunk diameter at 30 cm above the ground-D 30 C 10 cm), middle (D 30 C 5 and \10 cm) and lower (D 30 C 2 and \5 cm) strata of two plots of typical ''Cerrado'' (TC1 and TC2) and two of sparse ''Cerrado'' (SC1 and SC2) in the Serra Azul State Park in Barra do Garças, Mato Grosso, Brazil, discussing possible implications for the conservation of the biodiversity of the ''Cerrado''. We sampled 3658 individuals representing 75 species and 29 families. The density and diversity of species were higher in the TC plots, irrespective of the stratum analyzed. In all four plots, the density of the middle and lower strata was higher than that of the upper one, although species richness tended to be higher in the middle and upper strata. Species composition was highly similar between SC and TC plots. The same species were dominant in all four plots, and no indicator species were identified for either type of vegetation, indicating that the SC plots represent a subset of the TC ones. Given this, it is important that the selection of areas for conservation units includes both types of habitat, considering their distinct compositions and structural characteristics. This would contribute considerably to the preservation of the diversity of the local flora.