Search

Biodiversity maintenance in human-modified landscapes largely depends on spatial variations in species composition (β-diversity), but the impact of human disturbance on β-diversity remains poorly understood. We examined how taxonomic and phylogenetic β-diversity of woody plant communities in the Brazilian Caatinga dry forest respond to two emerging threats—chronic anthropogenic disturbance and water scarcity. We separately assessed diversity metrics that give a disproportionate weight to rare species, and metrics weighted by dominant species. We recorded a total of 5118 individuals from 104 species across 19 0.1-ha plots into a 21,430-ha human-modified landscape exposed to chronic disturbances and a high variation in climatic water deficit. At the landscape scale, β-diversity was higher when considering rare species than when focusing on dominant species, especially for the phylogenetic dimension of β-diversity. Water deficit was the primary driver of plant taxonomic and phylogenetic β-diversity, followed by the number of cattle and inter-site isolation; however, rare species seem to depend more strongly on these factors than the dominant ones. Therefore, preserving as much forest as possible, including areas exposed to different disturbance level and climate variables, is critical to prevent the loss of rare species and maintain the compositional differentiation of biotic assemblages. Such forest cover should be maintained in a large number of forest patches scattered through the landscape to preserve biologically and functionally distinct components of this ecosystem, avoid landscape-scale biotic homogenization, and thus favor its ecological resilience.

The proliferation of disturbance-adapted species in human-modified landscapes may change the structure of plant communities, but the response of biodiversity to human disturbances remains poorly understood. We examine the proliferation of the palm,Syagrus coronata, in disturbed forest stands and its impacts on the structure of vascular epiphyte assemblages in a human-modified landscape of Brazilian Caatinga dry forest. First, we comparedS. coronatadensity between old-growth and regenerating forest stands. We then surveyed vascular epiphytes on 680 phorophytes (S. coronataand non-palm/control species) across five habitat types with different disturbance levels. There was an eight-fold increase inS. coronatadensity in regenerating areas compared with in old-growth forest.Syagrus coronotasupported richer epiphyte assemblages at local (i.e. per palm) and landscape (i.e. pooling all palms) scale than control phorophytes, supporting more than 11 times the number of species of control phorophytes at both scales. Epiphyte assemblages were more abundant, species-rich and dominated by abiotically dispersed species in forest sites with intermediate disturbance levels (regenerating forest stands). More than simply operating as an exclusive phorophyte for more than 90% of the epiphyte species we recorded here,S. coronatafavours epiphyte persistence and structures their assemblages across human-modified landscapes of the Caatinga forest.

Question: Losses in species richness and functional diversity of tree assemblages have often been observed in Neotropical fragmented landscapes, but the way in which tree phylogenetic diversity responds to forest insularization over decadal timescales remains unclear. Here, we examine how tree phylogenetic diversity and structure have responded to profound landscape alteration and subsequent non-random species losses within forest islands created within a vast hydroelectric reservoir. Location: Balbina Hydroelectric Dam Reservoir, Central Brazilian Amazonia. Methods: We used a large tree data set including 435 subplots of 0.05 ha each in 34 forest islands of variable sizes and three continuous forest sites in the Balbina Hydroelectric Dam, which contained 11,000 stems representing 365 species and 188 genera. In each subplot we quantified tree phylogenetic diversity and structure and investigated their relationship with landscape structure metrics and the proportion of species within specific functional groups. Results: We uncovered a reduction in phylogenetic diversity in small (

Background: Habitat loss and fragmentation have been argued to drastically alter the composition of tree assemblages inhabiting small forest fragments but the successional trajectory experienced by such edge-affected habitats remains controversial.Aims: Here we examine whether small fragments (3.4–91.2 ha) support seedling assemblages more similar to those in 10–70-year-old secondary forests than to those in mature forests, in order to infer to what extent fragments move toward early successional systems.Methods: Using 59 0.1-ha plots distributed in a fragmented landscape of Brazilian Atlantic forest, we evaluated species richness and functional and taxonomic composition of seedling assemblages in 20 small forest fragments, 19 stands of secondary forest and 20 stands of mature forests in the interior of an exceptionally large fragment (ca. 3500 ha).Results: Small fragments presented the least species-rich seedling assemblages (17.2 ± 5.7 species), followed by secondary (22.5 ± 5.3), and mature forest (28....

Whether aging forest fragments are able to recover original assemblages or progressively move toward impoverished successional stages remains as an open question. This study tested the hypothesis that seedling assemblages in forest fragments differ from those across mature forest stands and examined to what extent the uncovered patterns supported the notion that edge-affected habitats tend to support impoverished tree assemblages dominated by pioneer species. We contrasted a series of small forest remnants (3–91 ha) to old-growth stands located in the largest (ca 3500 ha) and best preserved forest remnant in northeastern Brazil and found that tree seedling assemblages inhabiting forest fragments exhibited reduced species richness (up to 50%) at different spatial scales in comparison to seedling assemblages in mature forest and adult assemblages in both fragments and mature forest stands. Moreover, ordination analyses clearly segregated fragment seedling assemblages in taxonomic/functional terms and segregation correlated to the richness of pioneer species. Seedlings of pioneer species and those bearing medium-sized seeds (0.6–1.5 cm) increased in fragments, whereas large-seeded species (1.5–3.0 cm) were reduced by more than a half. Such a multiple-scale replacement of the old-growth flora by pioneers was also confirmed by an indicator species analysis and the resulting pioneer indicator species. Our results suggest that small forest fragments support impoverished and distorted seedling assemblages. This floristic/functional drift implies that forest remnants or edge-affected habitats tend to be dominated by a small set of pioneer tree species rather than supporting a substantial portion of the old-growth flora as do mature forest stands. Resumo Se os fragmentos florestais sao capazes de naturalmente restaurar as assembleias originais ou tendem a suportar apenas assembleias sucessionais empobrecidas permanece uma questao aberta. Neste estudo nos examinamos a hipotese que as assembleias de plântulas em fragmentos florestais diferem daquelas em areas de floresta madura e analisamos em que medida os padroes encontrados suportam a ideia que habitats afetados pelos efeitos de borda tendem a apresentar assembleias de arvores empobrecidas e dominadas por especies pioneiras. Nos comparamos uma serie de pequenos fragmentos florestais (3–91 ha) com areas de floresta madura localizadas no maior remanescente de floresta Atlântica no nordeste do Brasil (ca. 3500 ha). As assembleias de plântulas de especies arboreas nos pequenos fragmentos apresentaram uma reducao na riqueza de especies (ate 50%) em diferentes escalas espaciais em comparacao com as assembleias de plântulas em areas de floresta madura e as assembleias de arvores adultas nos pequenos fragmentos e areas de floresta madura. Plântulas de especies pioneiras e com sementes de tamanho medio (0.6–1.5 cm) apresentaram um aumento na riqueza relativa nos pequenos fragmentos, enquanto especies com grandes sementes (>1.5–3.0 cm) apresentaram uma reducao maior que 50 por cento. Analises de ordenamento e de especies indicadoras tambem revelaram a substituicao da flora tipica de floresta madura por plantas pioneiras nos pequenos fragmentos. Nossos resultados sugerem que pequenos fragmentos florestais suportam uma flora de plântulas empobrecida. Essa deriva floristica/funcional implica que os remanescentes florestais ou habitats afetados pelos efeitos de borda tendem a ser dominados por um pequeno grupo de arvores pioneiras ao inves de suportarem uma parte substancial da flora tipica de floresta madura.

The biodiversity value of human-modified landscapes has become a central question in the tropical forest conservation biology, yet the degree to which plant populations and communities are restructured in response to environmental change remains unclear. Here, we address tree species density in a fragmented Atlantic forest landscape to test the hypothesis that tree assemblages inhabiting edge-dominated forest habitats approach typical conditions of early successional systems. Seedlings and adults from 141 tree species were sampled across 39 0.1-ha plots: 19 in small fragments (55 % of all tree species exhibiting higher densities in small fragments than in mature forest, particularly pioneers (>60 % of all species). Seedlings and adults of these proliferating species differed from species exhibiting population declines in terms of wood density and seed size, respectively. Additionally, pioneers were more abundant than shade-tolerant species, as were hardwood species in the case of seedlings. Tree species showing highest population increases consisted largely of long-lived, light-demanding canopy species bearing soft or hardwood and small-to-medium-sized seeds. Tree assemblage structure also differed in terms of forest habitats with small forest fragments supporting few rare species, whereas the most rapidly proliferating species were much more widespread and abundant in fragments. However, 60 % of all adult pioneer species recorded in small fragments were not recorded as seedlings in this habitat type, although both seedling and adult assemblages were dominated by pioneer species. Edge-dominated tree assemblages are likely to experience long-term shifts toward greater dominance of long-lived, pioneer canopy species.