One sixth of Amazonian tree diversity is dependent on river floodplains

Amazonia's floodplain system is the largest and most biodiverse on Earth. Although forests are crucial to the ecological integrity of floodplains, our understanding of their species composition and how this may differ from surrounding forest types is still far too limited, particularly as changing inundation regimes begin to reshape floodplain tree communities and the critical ecosystem functions they underpin. Here we address this gap by taking a spatially explicit look at Amazonia-wide patterns of tree-species turnover and ecological specialization of the region's floodplain forests. We show that the majority of Amazonian tree species can inhabit floodplains, and about a sixth of Amazonian tree diversity is ecologically specialized on floodplains. The degree of specialization in floodplain communities is driven by regional flood patterns, with the most compositionally differentiated floodplain forests located centrally within the fluvial network and contingent on the most extraordinary flood magnitudes regionally. Our results provide a spatially explicit view of ecological specialization of floodplain forest communities and expose the need for whole-basin hydrological integrity to protect the Amazon's tree diversity and its function.

Consistent patterns of common species across tropical tree communities

Trees structure the Earth's most biodiverse ecosystem, tropical forests. The vast number of tree species presents a formidable challenge to understanding these forests, including their response to environmental change, as very little is known about most tropical tree species. A focus on the common species may circumvent this challenge. Here we investigate abundance patterns of common tree species using inventory data on 1,003,805 trees with trunk diameters of at least 10 cm across 1,568 locations1-6 in closed-canopy, structurally intact old-growth tropical forests in Africa, Amazonia and Southeast Asia. We estimate that 2.2%, 2.2% and 2.3% of species comprise 50% of the tropical trees in these regions, respectively. Extrapolating across all closed-canopy tropical forests, we estimate that just 1,053 species comprise half of Earth's 800 billion tropical trees with trunk diameters of at least 10 cm. Despite differing biogeographic, climatic and anthropogenic histories7, we find notably consistent patterns of common species and species abundance distributions across the continents. This suggests that fundamental mechanisms of tree community assembly may apply to all tropical forests. Resampling analyses show that the most common species are likely to belong to a manageable list of known species, enabling targeted efforts to understand their ecology. Although they do not detract from the importance of rare species, our results open new opportunities to understand the world's most diverse forests, including modelling their response to environmental change, by focusing on the common species that constitute the majority of their trees.

Mapping density, diversity and species-richness of the Amazon tree flora

Using 2.046 botanically-inventoried tree plots across the largest tropical forest on Earth, we mapped tree species-diversity and tree species-richness at 0.1-degree resolution, and investigated drivers for diversity and richness. Using only location, stratified by forest type, as predictor, our spatial model, to the best of our knowledge, provides the most accurate map of tree diversity in Amazonia to date, explaining approximately 70% of the tree diversity and species-richness. Large soil-forest combinations determine a significant percentage of the variation in tree species-richness and tree alpha-diversity in Amazonian forest-plots. We suggest that the size and fragmentation of these systems drive their large-scale diversity patterns and hence local diversity. A model not using location but cumulative water deficit, tree density, and temperature seasonality explains 47% of the tree species-richness in the terra-firme forest in Amazonia. Over large areas across Amazonia, residuals of this relationship are small and poorly spatially structured, suggesting that much of the residual variation may be local. The Guyana Shield area has consistently negative residuals, showing that this area has lower tree species-richness than expected by our models. We provide extensive plot meta-data, including tree density, tree alpha-diversity and tree species-richness results and gridded maps at 0.1-degree resolution.

More than 10,000 pre-Columbian earthworks are still hidden throughout Amazonia

Indigenous societies are known to have occupied the Amazon basin for more than 12,000 years, but the scale of their influence on Amazonian forests remains uncertain. We report the discovery, using LIDAR (light detection and ranging) information from across the basin, of 24 previously undetected pre-Columbian earthworks beneath the forest canopy. Modeled distribution and abundance of large-scale archaeological sites across Amazonia suggest that between 10,272 and 23,648 sites remain to be discovered and that most will be found in the southwest. We also identified 53 domesticated tree species significantly associated with earthwork occurrence probability, likely suggesting past management practices. Closed-canopy forests across Amazonia are likely to contain thousands of undiscovered archaeological sites around which pre-Columbian societies actively modified forests, a discovery that opens opportunities for better understanding the magnitude of ancient human influence on Amazonia and its current state.

Giants of the Amazon: How does environmental variation drive the diversity patterns of large trees?

For more than three decades, major efforts in sampling and analyzing tree diversity in South America have focused almost exclusively on trees with stems of at least 10 and 2.5 cm diameter, showing highest species diversity in the wetter western and northern Amazon forests. By contrast, little attention has been paid to patterns and drivers of diversity in the largest canopy and emergent trees, which is surprising given these have dominant ecological functions. Here, we use a machine learning approach to quantify the importance of environmental factors and apply it to generate spatial predictions of the species diversity of all trees (dbh ≥ 10 cm) and for very large trees (dbh ≥ 70 cm) using data from 243 forest plots (108,450 trees and 2832 species) distributed across different forest types and biogeographic regions of the Brazilian Amazon. The diversity of large trees and of all trees was significantly associated with three environmental factors, but in contrasting ways across regions and forest types. Environmental variables associated with disturbances, for example, the lightning flash rate and wind speed, as well as the fraction of photosynthetically active radiation, tend to govern the diversity of large trees. Upland rainforests in the Guiana Shield and Roraima regions had a high diversity of large trees. By contrast, variables associated with resources tend to govern tree diversity in general. Places such as the province of Imeri and the northern portion of the province of Madeira stand out for their high diversity of species in general. Climatic and topographic stability and functional adaptation mechanisms promote ideal conditions for species diversity. Finally, we mapped general patterns of tree species diversity in the Brazilian Amazon, which differ substantially depending on size class.

Biased-corrected richness estimates for the Amazonian tree flora

Amazonian forests are extraordinarily diverse, but the estimated species richness is very much debated. Here, we apply an ensemble of parametric estimators and a novel technique that includes conspecific spatial aggregation to an extended database of forest plots with up-to-date taxonomy. We show that the species abundance distribution of Amazonia is best approximated by a logseries with aggregated individuals, where aggregation increases with rarity. By averaging several methods to estimate total richness, we confirm that over 15,000 tree species are expected to occur in Amazonia. We also show that using ten times the number of plots would result in an increase to just ~50% of those 15,000 estimated species. To get a more complete sample of all tree species, rigorous field campaigns may be needed but the number of trees in Amazonia will remain an estimate for years to come.

Rarity of monodominance in hyperdiverse Amazonian forests

Tropical forests are known for their high diversity. Yet, forest patches do occur in the tropics where a single tree species is dominant. Such “monodominant” forests are known from all of the main tropical regions. For Amazonia, we sampled the occurrence of monodominance in a massive, basin-wide database of forest-inventory plots from the Amazon Tree Diversity Network (ATDN). Utilizing a simple defining metric of at least half of the trees ≥ 10 cm diameter belonging to one species, we found only a few occurrences of monodominance in Amazonia, and the phenomenon was not significantly linked to previously hypothesized life history traits such wood density, seed mass, ectomycorrhizal associations, or Rhizobium nodulation. In our analysis, coppicing (the formation of sprouts at the base of the tree or on roots) was the only trait significantly linked to monodominance. While at specific locales coppicing or ectomycorrhizal associations may confer a considerable advantage to a tree species and lead to its monodominance, very few species have these traits. Mining of the ATDN dataset suggests that monodominance is quite rare in Amazonia, and may be linked primarily to edaphic factors.

Floristic and structural comparisons between woody communities of two seasonal forest fragments in the Tocantins river basin and other remnants of this forest physiognomy in Brazil

This work describes the woody layer composition and structure in two seasonal forest fragments in the Tocantins river basin and compares them to other remnants of this forest physiognomy in Brazil. The survey was carried out by using 17 plot samples (20 × 50 m) located in Palmeirópolis, state of Tocantins, and in Minaçú, state of Goiás. All woody individuals showing diameters > 5 cm, at 1.30 cm above ground level, were recorded. The higher floristic similarity of these forest remnants compared with other closer seasonal forests did not show a distinct pattern. The floristic composition was more similar to that of a deciduous seasonal forest in the Paranã valley, and more dissimilar to other forests in this same valley. This result and a higher dissimilarity related to the southern forests in Goiás suggest that the forest fragments showed a floristic composition and structure typical of lowland seasonal forests in the Tocantins river basin. The results also indicated that the forest fragments have distinct floristic compositions with a relatively similar structure and diversity.

Vascular Flora of the Tocantins River Middle Basin, Brazil

This study provides a checklist of the phanerogams and pteridophytes of the Tocantins river middle basin, in northern Goiás state and southern Tocantins state, Brazil. Herbarium samples were collected from 2000 to 2009 and this floristic survey recorded 1572 species from 135 families. The most species-rich families were Fabaceae (217), Poaceae (116), Asteraceae (88), Euphorbiaceae (65), Orchidaceae (58) and Malpighiaceae (56). Furthermore, 14 endangered species and 31 rare species were recorded, mainly associated with the campos rupestres in the Veadeiros Plateau region. The flora mainly from the phytophyisiognomies cerrado stricto sensu, campo rupestre (“rocky fields”), mata de galeria (“gallery forest”), mata ciliar (“riverine forest”) and semi-deciduous seasonal forest comprised typical species of the mid-western Cerrado floristic province, such as the most widely known woody plants Anadenanthera colubrina (Vell.) Brenan (popular name angico), Aspidosperma subincanum Mart. ex A. DC. (guatambú), Astronium fraxinifolium Schott ex Spreng. (gonçalo-alves), Callisthene fasciculata Mart. (pau-jacaré), Dipteryx alata Vogel (barú), Guazuma ulmifolia Lam. (mutamba) and Magonia pubescens A. St.-Hil. (tingui). This study is the first to record a wide floristic list of this important region of central Brazil.

Angiosperms from the Araripe National Forest, Ceará, Brazil

This study provides a species list of the angiosperms from the Araripe National Forest (ANF) in the south part of Ceará state, northeastern Brazil. The floristic survey recorded 188 species from 55 Angiosperm families. The most species-rich families were Fabaceae (31), Rubiaceae (17), Asteraceae (14), Bignoniaceae (10), Myrtaceae (9), Euphorbiaceae (7), Apocynaceae and Malpighiaceae (7) and Annonaceae and Solanaceae (5). The flora of the ANF is composed of typical species from the vegetation physiognomies of Cerrado sensu lato such as cerradão and cerrado sensu stricto and it is influenced by the northeastern phytogeographic Cerrado province. Furthermore, there are influences of ecotones between these physiognomies, seasonal forests and carrascos within the Caatinga biome. In addition to four threatened species that were recorded, the ANF also harbors species that represent important incomes for local communities because of the non-timber forest products, such as Dimorphandra gardneriana Tul. and Caryocar coriaceum Wittm.