Aim To assess the relative roles of geologically defined terrain types (environmental heterogeneity) and a major river (physical dispersal barrier) as predictors of ecological structuring and biogeographical differentiation within Amazonian forests.Location Western Brazilian Amazonia, where the Juru a river and its terraces cross a 1000-km-long boundary between two geological formations (the Solimões and Ic ß a Formations). MethodsWe sampled a 500-km stretch of the Juru a with 71 transects (5 m by 500 m) that spanned both the river and the geological boundary. All transects were inventoried for pteridophytes (ferns and lycophytes) and Melastomataceae, and a subset of 39 transects also for palms and Zingiberales. Three surface soil samples were collected from each transect. The data were analysed using ordinations, regression trees, indicator species analyses and Mantel tests. ResultsAll plant groups showed congruent species turnover between geologically defined terrain types, but little evidence of isolation by the river or geographical distance. Soil cation concentration differed between the Solimões Formation and other terrain types and emerged as the main explanatory factor for species turnover. A large proportion of the plant species were significant indicators for specific parts of the soil cation concentration gradient, and these edaphic associations were congruent with those found in other parts of Amazonia. Pteridophytes had a larger proportion of species in the cation-rich soils than the other plant groups did, and palms had a higher proportion of generalists. Main conclusionsThe geological boundary between the Solimões and Ic ß a formations is confirmed as significant floristic turnover zone. As it runs in a north-south orientation for more than 1000 km, the edaphic differences associated with this boundary have wide-ranging implications for speciation and biogeographical patterns in Amazonia.

Aim To map and interpret floristic and geoecological patterns across the Amazon basin by combining extensive field data with basin‐wide Landsat imagery and climatic data. Location Amazonia. Taxon Ground truth data on ferns and lycophytes; remote sensing results reflect forest canopy properties. Methods We used field plot data to assess main ecological gradients across Amazonia and to relate floristic ordination axes to soil base cation concentration, Climatologies at High Resolution for the Earth's Land Surface Areas (CHELSA) climatic variables and reflectance values from a basin‐wide Landsat image composite with generalized linear models. Ordination axes were then predicted across all Amazonia using Landsat and CHELSA, and a regional subdivision was obtained using k‐medoid classification. Results The primary floristic gradient was strongly related to base cation concentration in the soil, and the secondary gradient to climatic variables. The Landsat image composite revealed a tapestry of broad‐scale variation in canopy reflectance characteristics across Amazonia. Ordination axis scores predicted using Landsat and CHELSA variables produced spatial patterns consistent with existing knowledge on soils, geology and vegetation, but also suggested new floristic patterns. The clearest dichotomy was between central Amazonia and the peripheral areas, and the available data supported a classification into at least eight subregions. Main conclusions Landsat data are capable of predicting soil‐related species compositional patterns of understorey ferns and lycophytes across the Amazon basin with surprisingly high accuracy. Although the exact floristic relationships may differ among plant groups, the observed ecological gradients must be relevant for other plants as well, since surface reflectance recorded by satellites is mostly influenced by the tree canopy. This opens exciting prospects for species distribution modelling, conservation planning, and biogeographical and ecological studies on Amazonian biota. Our maps provide a preliminary geoecological subdivision of Amazonia that can now be tested and refined using field data of other plant groups and from hitherto unsampled areas.

Aim: To evaluate the relative importance of climatic versus soil data when predicting species distributions for Amazonian plants and to gain understanding of potential range shifts under climate change.Location: Amazon rain forest. Methods:We produced species distribution models (SDM) at 5-km spatial resolution for 42 plant species (trees, palms, lianas, monocot herbs and ferns) using species occurrence data from herbarium records and plot-based inventories. We modelled species distribution with Bayesian logistic regression using either climate data only, soil data only or climate and soil data together to estimate their relative predictive powers. For areas defined as unsuitable to species occurrence, we mapped the difference between the suitability predictions obtained with climateonly versus soil-only models to identify regions where climate and soil might restrict species ranges independently or jointly.Results: For 40 out of the 42 species, the best models included both climate and soil predictors. The models including only soil predictors performed better than the models including only climate predictors, but we still detected a drought-sensitive response for most of the species. Edaphic conditions were predicted to restrict species occurrence in the centre, the north-west and in the north-east of Amazonia, while the climatic conditions were identified as the restricting factor in the eastern Amazonia, at the border of Roraima and Venezuela and in the Andean foothills.Main conclusions: Our results revealed that soil data are a more important predictor than climate of plant species range in Amazonia. The strong control of species ranges by edaphic features might reduce species' abilities to track suitable climate conditions under a drought-increase scenario. Future challenges are to improve the quality of soil data and couple them with process-based models to better predict species range dynamics under climate change. K E Y W O R D SAmazon rain forest, Bayesian logistic regression, cation exchange capacity, climate change, ecological niche models, soil factors, SoilGrids, species distribution models, species range, tropical soils Fittkau, Junk, Klinge, & Sioli, 1975;Higgins et al., 2011;Sombroek, 2000;Tuomisto & Poulsen, 1996). However, studies focused on the Amazonian rain forests and others also found in other biomes). Species occurrence records were obtained from two sources: plot-based inventories and herbarium records. To ensure data consistency, we targeted species that are easy to identify in the field. We included only species that had more than 20 presence records (see further details: Table S1, Appendix S1 in Supporting Information). 28 km at the equator) into two climatic variables: annual precipitation and dry season length, defined as the maximum consecutive number of months with <100 mm of precipitation. | Environmental dataWe used four remote sensing variables that describe terrain and forest structure properties: elevation, percentage tree cover, per- | Modelling frameworkTo evalu...

Aim A major problem for conservation in Amazonia is that species distribution maps are inaccurate. Consequently, conservation planning needs to be based on other information sources such as vegetation and soil maps, which are also inaccurate. We propose and test the use of biotic data on a common and relatively easily inventoried group of plants to infer environmental conditions that can be used to improve maps of floristic patterns for plants in general. Location Brazilian Amazonia. Methods We sampled 326 plots of 250 m × 2 m separated by distances of 1–1800 km. Terrestrial fern individuals were identified and counted. Edaphic data were obtained from soil samples and analysed for cation concentration and texture. Climatic data were obtained from Worldclim. We used a multivariate regression tree to evaluate the hierarchical importance of soils and climate for fern communities and identified significant indicator species for the resultant classification. We then tested how well the edaphic properties of the plots could be predicted on the basis of their floristic composition using two calibration methods, weighted averaging and k‐nearest neighbour estimation. Results Soil cation concentration emerged as the most important variable in the regression tree, whereas soil textural and climatic variation played secondary roles. Almost all the plot classes had several fern species with high indicator values for that class. Soil cation concentration was also the variable most accurately predicted on the basis of fern community composition (R2 = 0.65–0.75 for log‐transformed data). Predictive accuracy varied little among the calibration methods, and was not improved by the use of abundance data instead of presence–absence data. Conclusions Fern species composition can be used as an indicator of soil cation concentration, which can be expected to be relevant also for other components of rain forests. Presence–absence data are adequate for this purpose, which makes the collecting of additional data potentially very rapid. Comparison with earlier studies suggests that edaphic preferences of fern species have good transferability across geographical regions within lowland Amazonia. Therefore, species and environmental data sets already available in the Amazon region represent a good starting point for generating better environmental and floristic maps for conservation planning.

To determine the effect of rivers, environmental conditions, and isolation by distance on the distribution of species in Amazonia. Location: Brazilian Amazonia. Time period: Current. Major taxa studied: Birds, fishes, bats, ants, termites, butterflies, ferns + lycophytes, gingers and palms. We compiled a unique dataset of biotic and abiotic information from 822 plots spread over the Brazilian Amazon. We evaluated the effects of environment, geographic distance and dispersal barriers (rivers) on assemblage composition of animal and plant taxa using multivariate techniques and distance- and raw-data-based regression approaches. Environmental variables (soil/water), geographic distance, and rivers were associated with the distribution of most taxa. The wide and relatively old Amazon River tended to determine differences in community composition for most biological groups. Despite this association, environment and geographic distance were generally more important than rivers in explaining the changes in species composition. The results from multi-taxa comparisons suggest that variation in community composition in Amazonia reflects both dispersal limitation (isolation by distance or by large rivers) and the adaptation of species to local environmental conditions. Larger and older river barriers influenced the distribution of species. However, in general this effect is weaker than the effects of environmental gradients or geographical distance at broad scales in Amazonia, but the relative importance of each of these processes varies among biological groups.

Highlights • The river network in central Amazonia has changed radically during the last 100 ky. The river Japurá (Caquetá) disconnected from the Rio Negro and became a tributary of the Amazon only about 1000 years ago, and the upper reaches of both the Juruá and the Madeira have been tributaries of the Purus. • Shifting tributary arrangements create a dynamic biogeographic landscape where dispersal barriers and dispersal routes for both terrestrial and aquatic organisms are more ephemeral than has been thought. • The Madeira approximately follows a geological boundary, so species distributional limits might depend either on the riverine barrier effect or on habitat selection. Furthermore, the upper Madeira formerly flowed into the Purus, so the lower Madeira has a shorter history as a large river. • We provide a map of apparent former positions of four large central Amazonian rivers to encourage the testing of dynamic biogeographical hypotheses.

1.A negative relationship between stand biomass and the density of stems is expected to develop during the self-thinning process in resource-limited forests; this leads to a large proportion of the total biomass occurring in large trees. Nevertheless, frequent disturbance regimes can reduce self-thinning and the accumulation of large trees. 2. We investigated size-density relationships and the contribution of large trees (dbh ≥ 70 cm) to stand biomass in 55 1-ha plots along a 600 km transect in central-southern Amazonia. The effects of natural-disturbance gradients (frequency of storms and soil characteristics) and seasonality on forest-structure components (density of stems and mean individual mass) and stand biomass were examined. 3. Contrary to self-thinning predictions, stand biomass increased in forests with higher stem densities. Large trees contained only an average of 5% of stand biomass, and half of the stand biomass was represented by small trees with diameters < 27 cm. These findings indicate that persistent or strong disturbance plays a critical role in forest structure and biomass in the central-southern Amazon. Frequent storms were identified as an important source of disturbance in the region. Forests under higher frequency of storms had trees with lower individual mass and higher stem packing. More physically restrictive soils seem to magnify the effects of exogenous disturbances limiting individual tree size. 4. Forests in areas with longer dry seasons had lower stem densities; however, individual mass was higher in these areas. These structural components of biomass seem to counterbalance each other in generating total stand biomass. Seasonality affected forest structural components but not stand biomass. 5.Synthesis. Forests of central-southern Amazonia are not resource limited and accumulate most part of their biomass in small-to mid-sized trees. The effects of environmental gradients on specific structural components of stand biomass differ such that strong positive effects on one component can mitigate strong negative effects on other component. Future work on the determinants of stand biomass should investigate forest structure and the contributions of individual components to stand biomass.

Aim:To disentangle the influence of environmental factors at different spatial grains (regional and local) on fern and lycophyte species richness and to ask how regional and plot-level richness are related to each other.Location: Global.