Local Hydrological Conditions Explain Floristic Composition in Lowland Amazonian Forests

Tree species occurring in Amazonian wetland forests consistently show broader range sizes and niche breadths than trees in upland forests

Generally, species with broad niches also show large range sizes. We investigated the relationship between hydrological niche breadth and geographic range size for Amazonian tree species seeking to understand the role of habitat specialization to Amazonian wetlands and upland forests on the current distribution of tree species. We obtained 571,092 valid occurrence points from GBIF and SpeciesLink to estimate the range size and the niche breadth of 76% of all known Amazonian tree species (5150 tree species). Hydrological niche breadth was measured on different unidimensional axes defined by (1) total annual precipitation; (2) precipitation seasonality; (3) actual evapotranspiration; and (4) water table depth. Geographic range sizes were estimated using alpha-hull adjustments. General linear models were used to relate niche breadth to range size while contrasting tree species occurring and not occurring in wetlands. The hydrological niche breadth of Amazonian tree species varied mostly along the water table depth axis. The average range size for an Amazonian tree species was 751,000 km2 (median of 154,000 km2 and standard deviation of 1,550,000 km2). Niche breadth-range size relationships for Amazonian tree species were positive for all models, and the explanatory power of the models improved when including whether a species occurred in wetlands or in terrestrial uplands. Wetland species had steeper positive slopes for the niche breadth-range size relationship, and consistently larger range sizes for a given niche breadth. Amazonian tree species varied strongly in hydrological niche breadth and range size, but most species had narrow niche breadths and range sizes. Our results suggest that the South American riverscape may have been acting as a corridor for species dispersal in the Neotropical lowlands.

The other side of tropical forest drought: do shallow water table regions of Amazonia act as large-scale hydrological refugia from drought?

Tropical forest function is of global significance to climate change responses, and critically determined by water availability patterns. Groundwater is tightly related to soil water through the water table depth (WT), but historically neglected in ecological studies. Shallow WT forests (WT < 5 m) are underrepresented in forest research networks and absent in eddy flux measurements, although they represent c. 50% of the Amazon and are expected to respond differently to global-change-related droughts. We review WT patterns and consequences for plants, emerging results, and advance a conceptual model integrating environment and trait distributions to predict climate change effects. Shallow WT forests have a distinct species composition, with more resource-acquisitive and hydrologically vulnerable trees, shorter canopies and lower biomass than deep WT forests. During 'normal' climatic years, shallow WT forests have higher mortality and lower productivity than deep WT forests, but during moderate droughts mortality is buffered and productivity increases. However, during severe drought, shallow WT forests may be more sensitive due to shallow roots and drought-intolerant traits. Our evidence supports the hypothesis of neglected shallow WT forests being resilient to moderate drought, challenging the prevailing view of widespread negative effects of climate change on Amazonian forests that ignores WT gradients, but predicts they could collapse under very strong droughts.

Ground-herb communities of terra firme riparian forests of the lower Tapajós River in the Brazilian Amazon

Abstract We conducted an inventory of the fern, lycophyte and non-palm monocotyledon ground-herbs of terra firme riparian forests in the lower Tapajós region of the Brazilian Amazon. Eight 1.5 × 250 m plots, totaling 0.3 hectares, were surveyed along the watersheds of the Cupari and Curuá-Una rivers, located at the Tapajós National Forest, Pará, Brazil. To characterize the ground-herb community, we calculated species richness, abundance and Fisher’s alpha for each plot. To analyze turnover, we compared composition among plots by pairwise Bray-Curtis distance. In total, we sampled 3,130 individuals, 58 species, 27 genera and 20 families of riparian ground-herbs. Marantaceae (14 spp) was the richest family and Poaceae the most abundant family (738 individuals). The fern Triplophyllum glabrum (Tectariaceae) was the most frequent species, observed in 87.5 % of plots. Ground-herbs communities in the studied area have high species turnover, making it necessary to invest time and resources to adequately characterize and manage riparian habitats. The ground-herb community composition observed in the riparian zone here resembles that of other non-riparian forested sites in the Amazon with the plant families Marantaceae, Pteridaceae and Poaceae generally being the most commonly represented in the Amazonian ground-herb stratum. We highlight the importance of herb inventories, especially in conservation units.

Differential effects of soil waterlogging on herbaceous and woody plant communities in a Neotropical savanna

The impacts of soil properties and fire regime on Neotropical savannas are well-known, but the importance of hydrological regime for plant species assembly has received less attention. Here, we assessed changes in diversity patterns of herbaceous and woody communities along a water table gradient in a fire-excluded Neotropical savanna. We found that increased waterlogging of soils was associated with declines in both herbaceous and woody species richness. Woody species richness decreased once the water table depth is less than 4 m and no woody species occurred once water table depth was less than 23 cm. Herbaceous communities remained species rich until the shallowest water table depth, where there is flooding at some point in the year, and even there, over a dozen species occurred. Woody species that occurred in areas with shallower water tables were a nested subset of those in areas with deeper water tables. In contrast, herbaceous communities showed turnover over the hydrological gradient, with distinct species specialized for different water table levels. However, we found that those specialists are restricted to few evolutionary lineages, evidenced by increased phylogenetic clustering over the water table gradient in herbaceous communities. We suggest that evolutionarily conserved hydrological niches define the herbaceous layer over the hydrological gradient, whereas only generalist woody species persist under high water tables. Our findings show that the effect of soil waterlogging differs between the herbaceous and woody layer of savannas, indicating that these communities will respond differently to shifts in the hydrological regime under future environmental change.

Soil fertility and flood regime are correlated with phylogenetic structure of Amazonian palm communities

BACKGROUND AND AIMS

Identifying the processes that generate and maintain biodiversity requires understanding of how evolutionary processes interact with abiotic conditions to structure communities. Edaphic gradients are strongly associated with floristic patterns but, compared with climatic gradients, have received relatively little attention. We asked (1) How does the phylogenetic composition of palm communities vary along edaphic gradients within major habitat types? and (2) To what extent are phylogenetic patterns determined by (a) habitat specialists, (b) small versus large palms, and (c) hyperdiverse genera?

METHODS

We paired data on palm community composition from 501 transects of 0.25 ha located in two main habitat types (non-inundated uplands and seasonally inundated floodplains) in western Amazonian rain forests with information on soil chemistry, climate, phylogeny and metrics of plant size. We focused on exchangeable base concentration (cmol+ kg-1) as a metric of soil fertility and a floristic index of inundation intensity. We used a null model approach to quantify the standard effect size of mean phylogenetic distance for each transect (a metric of phylogenetic community composition) and related this value to edaphic variables using generalized linear mixed models, including a term for spatial autocorrelation.

KEY RESULTS

Overall, we recorded 112 008 individuals belonging to 110 species. Palm communities in non-inundated upland transects (but not floodplain transects) were more phylogenetically clustered in areas of low soil fertility, measured as exchangeable base concentration. In contrast, floodplain transects with more severe flood regimes (as inferred from floristic structure) tended to be phylogenetically clustered. Nearly half of the species recorded (44 %) were upland specialists while 18 % were floodplain specialists. In both habitat types, phylogenetic clustering was largely due to the co-occurrence of small-sized habitat specialists belonging to two hyperdiverse genera (Bactris and Geonoma).

CONCLUSIONS

Edaphic conditions are associated with the phylogenetic community structure of palms across western Amazonia, and different factors (specifically, soil fertility and inundation intensity) appear to underlie diversity patterns in non-inundated upland versus floodplain habitats. By linking edaphic gradients with palm community phylogenetic structure, our study reinforces the need to integrate edaphic conditions in eco-evolutionary studies in order to better understand the processes that generate and maintain tropical forest diversity. Our results suggest a role for edaphic niche conservatism in the evolution and distribution of Amazonian palms, a finding with potential relevance for other clades.

Habitat filtering of six coexisting Heliconia species in a lowland tropical rain forest in Amazonian Ecuador

Herbaceous plants are often under-studied in tropical forests, despite their high density and diversity, and little is known about the factors that influence their distribution at microscales. In a 25-ha plot in lowland Amazonian rain forest in Yasuní National Park, Ecuador, we censused six species of Heliconia (Heliconiaceae) in a stratified random manner across three topographic habitat types. We observed distribution patterns consistent with habitat filtering. Overall, more individuals occurred in the valley (N = 979) and slope (N = 847) compared with the ridge (N = 571) habitat. At the species level, Heliconia stricta (N = 1135), H. spathocircinata (N = 309) and H. ortotricha (N = 36) all had higher abundance in the valley and slope than ridge. Further, H. vellerigera (N = 20) was completely absent from the ridge. Conversely, H. velutina (N = 903) was most common in the drier ridge habitat. The two most common species (H. stricta and H. velutina) had a reciprocal or negative co-occurrence pattern and occurred preferentially in valley versus ridge habitats. These results suggest that taxa within this family have different adaptations to the wetter valley versus the drier ridge and that habitat partitioning contributes to coexistence.

Different environmental gradients affect different measures of snake β-diversity in the Amazon rainforests

Mechanisms generating and maintaining biodiversity at regional scales may be evaluated by quantifying β-diversity along environmental gradients. Differences in assemblages result in biotic complementarities and redundancies among sites, which may be quantified through multi-dimensional approaches incorporating taxonomic β-diversity (TBD), functional β-diversity (FBD) and phylogenetic β-diversity (PBD). Here we test the hypothesis that snake TBD, FBD and PBD are influenced by environmental gradients, independently of geographic distance. The gradients tested are expected to affect snake assemblages indirectly, such as clay content in the soil determining primary production and height above the nearest drainage determining prey availability, or directly, such as percentage of tree cover determining availability of resting and nesting sites, and climate (temperature and precipitation) causing physiological filtering. We sampled snakes in 21 sampling plots, each covering five km², distributed over 880 km in the central-southern Amazon Basin. We used dissimilarities between sampling sites to quantify TBD, FBD and PBD, which were response variables in multiple-linear-regression and redundancy analysis models. We show that patterns of snake community composition based on TBD, FBD and PBD are associated with environmental heterogeneity in the Amazon. Despite positive correlations between all β-diversity measures, TBD responded to different environmental gradients compared to FBD and PBD. Our findings suggest that multi-dimensional approaches are more informative for ecological studies and conservation actions compared to a single diversity measure.

Establishing baseline biodiversity data prior to hydroelectric dam construction to monitoring impacts to bats in the Brazilian Amazon

The modification of Amazonian rivers by the construction of megaprojects of hydroelectric dams has widely increased over the last decade. Robust monitoring programs have been rarely conducted prior to the establishment of dams to measure to what extent the fauna, and its associated habitats may be affected by upcoming impacts. Using bats as models, we performed analyses throughout the area under the influence of the Santo Antônio hydroelectric dam, Southwestern Brazilian Amazonia before its construction to estimate how the fauna and its associated habitats would be affected by the upcoming impacts. We surveyed bats in 49 plots distributed along the areas going to be inundated by the dam and those remaining dry. As predictors for the species distribution, we tested the variables of vegetation structure and topography. Species composition largely differed between the dry plots and the plots located in areas that will be flooded, and this was strongly associated with the variables of forest basal area and elevation. Vegetation-related variables also had strong influence on the guilds distribution. The flooding of lower elevations areas is expected to negatively affect the species number and abundance of frugivorous species. In contrast, it is likely that animalivores will be less vulnerable to dam-induced flooding, since they were abundant in the areas not expect to be inundated. We urge for the implementation of studies to predict impacts caused by large hydroelectric dams, including tests of the influence of the local conditions that shape diversity to avoid massive losses of the biota, and to build preventive monitoring and management actions.

Impact of sedimentary processes on white-sand vegetation in an Amazonian megafan

Amazonian white-sand vegetation has unique tree communities tolerant to nutrient-poor soils of interest for interpreting processes of adaptation in neotropical forests. Part of this phytophysionomy is confined to Late Quaternary megafan palaeo-landforms, thus we posit that sedimentary disturbance is the main ecological factor controlling tree distribution and structuring in this environment. In this study, we characterize the topographic trend of one megafan palaeo-landform using a digital elevation model and verify its relationship to the forest by modelling the canopy height with remote sensing data. We also compare the composition and structure (i.e. canopy height and diameter at breast height) of tree groups from the outer and inner megafan environments based on the integration of remote sensing and floristic data. The latter consist of field inventories of trees ≥ 10 cm dbh using six (500 × 20 m) plots in várzea, terra firme and igapó from the outer megafan and 20 (50 × 20 m) plots in woodlands and forests from the inner megafan. The unweighted pair group method with arithmetic mean (UPGMA) and the non-metric multidimensional scaling (NMDS) were applied for clustering and dissimilarity analyses, respectively. The megafan is a sand-dominated triangular wetland with a topographic gradient of < 15 cm km−1, being more elevated along its axis. The outer megafan has a higher number of tree species (367), taller canopy height (mean of 14.1 m) and higher diameter at breast height (mean of 18.2 cm) than the white-sand forest. The latter records 89 tree species, mean canopy height of 8.4 cm and mean diameter at breast height of 15.3 cm. Trees increase in frequency closer to channels and toward the megafan's axis. The flooded and nutrient-poor sandy megafan substrate favoured the establishment of white-sand vegetation according to the overall megafan topography and morphological heterogeneities inherent to megafan sub-environments.

Ground-Vegetation Clutter Affects Phyllostomid Bat Assemblage Structure in Lowland Amazonian Forest

Vegetation clutter is a limiting factor for bats that forage near ground level, and may determine the distribution of species and guilds. However, many studies that evaluated the effects of vegetation clutter on bats have used qualitative descriptions rather than direct measurements of vegetation density. Moreover, few studies have evaluated the effect of vegetation clutter on a regional scale. Here, we evaluate the influence of the physical obstruction of vegetation on phyllostomid-bat assemblages along a 520 km transect in continuous Amazonian forest. We sampled bats using mist nets in eight localities during 80 nights (3840 net-hours) and estimated the ground-vegetation density with digital photographs. The total number of species, number of animalivorous species, total number of frugivorous species, number of understory frugivorous species, and abundance of canopy frugivorous bats were negatively associated with vegetation clutter. The bat assemblages showed a nested structure in relation to degree of clutter, with animalivorous and understory frugivorous bats distributed throughout the vegetation-clutter gradient, while canopy frugivores were restricted to sites with more open vegetation. The species distribution along the gradient of vegetation clutter was not closely associated with wing morphology, but aspect ratio and wing load differed between frugivores and animalivores. Vegetation structure plays an important role in structuring assemblages of the bats at the regional scale by increasing beta diversity between sites. Differences in foraging strategy and diet of the guilds seem to have contributed more to the spatial distribution of bats than the wing characteristics of the species alone.