Natural regeneration triggers compositional and functional shifts in soil seed banks.

Secondary forests emerging during traditional shifting cultivation practices are increasingly recognized for their fulfillment of ecosystem services and mitigation potential of climate change and biodiversity losses. The soil seed bank as a recruit reservoir is a limiting factor for natural forest regeneration of such secondary forests and is decisive for the formation and restitution of the post-disturbance community. The aim of this study was to compare the composition of the soil seed bank along a natural regeneration chronosequence from the Caxiuanã National Forest, eastern Amazon, including old-growth reference sites. We sampled standing vegetation, soil properties and soil seed banks and compared the density and species richness of different life forms among different regeneration stages. Using nonmetric, multiple scaling, we compared the composition of the soil seed bank among different regeneration stages and with standing vegetation composition. Furthermore, we outlined the influence of stand age, vegetation structure and soil properties on the density, richness and functional characterization of the soil seed bank using mixed effect models. The soil seed bank was dominated by herb seeds in all regeneration stages, and the density and richness of tree seeds increased with regeneration time and recovery of vegetation structure. Seed bank composition changed gradually with regeneration advance and differed from standing vegetation, containing a high amount of allochthonous seeds, especially in older stands. This observation highlights the importance of dispersal and habitat connectivity for the natural regeneration of these secondary forests. Shifts in soil seed bank composition towards slow-growing, animal-dispersed, non-pioneer species with larger, recalcitrant seeds in older regeneration stands indicate changes in vegetation composition along succession. Thus, our data indicate the importance of connectivity for forest regeneration and long fallow periods (> 40 years) to increase the performance of ecosystem services, resilience and stability of secondary forests arising during shifting cultivation practices.

Impacts of hydroelectric dams on alluvial riparian plant communities in Eastern Brazilian Amazonian.

The major rivers of the Amazon River basin and their biota are threatened by the planned construction of large hydroelectric dams that are expected to have strong impacts on floodplain plant communities. The present study presents forest inventories from three floodplain sites colonized by alluvial riparian vegetation in the Tapajós, Xingu and Tocantins River basins in eastern Amazonian. Results indicate that tree species of the highly specialized alluvial riparian vegetation are clearly distinct among the three river basins, although they are not very distinct from each other and environmental constraints are very similar. With only 6 of 74 species occurring in all three inventories, most tree and shrub species are restricted to only one of the rivers, indicating a high degree of local distribution. Different species occupy similar environmental niches, making these fragile riparian formations highly valuable. Conservation plans must consider species complementarily when decisions are made on where to place floodplain forest conservation units to avoid the irreversible loss of unique alluvial riparian vegetation biodiversity.

O desmatamento na Amazônia e a importância das áreas protegidas / Deforestation in the Amazon and the importance of protected areas

O modelo da ocupação demográfica da Amazônia legalnos últimos cinqueta anos tem levado a níveis significativos de desmatamento, resultante de múltiplos fatores, tais como a abertura de estradas pioneiras, o crescimento das cidades, a ampliação de pecuária extensiva, a acelerada exploração madeireira e a crescente agricultura intensiva de monoculturas. A área cumulativa desmatada na Amazônia legal brasileira chegou a cerca de 653.000 km² em 2003, correspondendo a 16,3 por cento da região. Este estudo visou a determinar o desmatamento dentro e fora dos atuais Unidades de Conservação (UC) e Terras Indígenas (TI) na Amazônia legal, nos estados de Mato Grosso, Rondônia e Pará, que, juntos, corresponderam por mais de 90 por cento do desmatamento observado entre 2001 e 2003. Os resultados mostraram que o desmatamento foi cerca de dez a vinte vezes menor dentro das Unidades de Conservação e Terras Indígenas do que em áreas contíguas fora delas. Isto demonstra a importância dessas áreas protegidas para diminuir o processo do desmatamento nos três estados. Isto refuta a hipótese generalizada de que estas áreas não cumprissem a sua função principal na conservação e uso racional dos recursos na Amazônia legal.

Effects of river level fluctuation on plant species richness, diversity, and distribution in a floodplain forest in Central Amazonia.

River levels in Central Amazonia fluctuate up to 14 m annually, with the flooding period ranging from 50 to 270 days between the rising and falling phases. Vast areas of forest along the rivers contain plant species that are well adapted to annual flooding. We studied the effect of flooding level on tree species richness, diversity, density, and composition in lake, river, and stream habitats in Jaú National Park, Brazil. 3051 trees >10 cm diameter (at 1.3 m diameter at breast height, dbh) were measured and identified in 25 10 m × 40 m randomly selected plots in each habitat. Ordination methods and analysis of variance results showed that forested areas near lakes had significantly lower species richness of trees than riverine and streamside habitats. Plot species richness and diversity were strongly negatively correlated with the water level and duration of flooding. The drier (stream) habitat had more total species (54 species of trees) and more unique species of trees (6 tree species) than the riverine (52 tree species; 3 unique species) and lake (33 tree species; 3 unique species) habitats. Species composition overlap among habitats was surprisingly high (42.6-60.6% overlap), almost one-third of the species were found in all three habitat types, and few species were unique to each habitat. We conclude that: (1) duration of flooding has a strong impact on species richness, diversity and plant distribution patterns; (2) most species are adapted to a wide range of habitats and flood durations; and (3) while flood duration may decrease local diversity, it also creates and maintains high landscape-scale diversity by increasing landscape heterogeneity.