Amazon tree dominance across forest strata

Abstract

The forests of Amazonia are among the most biodiverse plant communities on Earth. Given the immediate threats posed by climate and land-use change, an improved understanding of how this extraordinary biodiversity is spatially organized is urgently required to develop effective conservation strategies. Most Amazonian tree species are extremely rare, but a small number are common across the region. Indeed, just 227 "hyperdominant" species account for more than 50% of all individuals > 10 cm dbh. Yet, the degree to which the phenomenon of hyperdominance is sensitive to tree size, the extent to which the composition of dominant species changes with size-class, and how evolutionary history constrains tree hyperdominance, all remain unknown. Here, we use a unique floristic dataset to show that,

Biogeographic history and habitat specialization shape floristic and phylogenetic composition across Amazonian forests

“…Compared to other 1-ha samples, our plots included a wider range of size classes, and thus include small understory trees (and some shrubs) and juveniles of large trees in addition to the largest emergent canopy trees. In addition, the fact that our transects are spread out over a larger area means that the most common tree species (which often can have clumped distributions [Condit et al 2000]) are less likely to show strong dominance patterns and are more likely to capture rare species than in analyses from 1-ha plot networks (see Draper et al 2021).…”

Forest tree species distribution for Europe 2000-2020: mapping potential and realized distributions using spatiotemporal Machine Learning

“…at the scale of 500-1000 m. For example, in Brazil, it has been estimated that about 220 forest tree species cover most of the land and represent over 95 of the biomass i.e. so called "hyper-dominant species" (Draper et al, 2021). Scaling up approach described in this paper to help produce objective predictions and help monitor forest dynamics and support re-forestation efforts across globe is our next frontier.…”

Fragmentation disrupts the seasonality of Amazonian evergreen forests

“…Recent studies in Amazonian forests have shown that leaf area increases in the understory occur under maximal irradiance conditions when the upper canopy layer is partially deciduous during the dry season 38,39 , as diffuse and direct solar radiation in the understory can increase linearly with decreasing upper canopy plant area 56 . The dominant species in the understory of Amazonian forests are distinct from upper canopy dominants, and are differentiated and more complex in functional strategies 37,68 . Understory trees have xylem that is more embolism resistant and can tolerate more negative water potentials in the dry season without risking hydraulic failure compared to upper canopy trees, which tend to be more vulnerable to drought-induced embolism 32 .…”

“…These results We also tested the hypothesis suggested by Smith and colleagues 19 that the lower and upper strata of the vegetation have asynchronous changes in plant area during the dry season by comparing PAD on October 16 th with PAD on June 24 th in these strata. Species, functional and phylogenetic composition of the understory are distinct from the upper canopy in Central Amazonian forests 32,37 . While the understory is comprised of lower branches, seedlings, shade-tolerant and embolism resistant trees and shrubs, lianas, acaulescent palms and saplings of young adult trees, the upper canopy is made up of adult predominantly shade-tolerant species, including tall and emergent trees and lianas.…”

“…Seasonal variations in leaf quantity and leaf area across evergreen Amazonian forests have frequently been considered negligible or small 4,12,21,36 . However, spaceborne remote sensing approaches tend to detect only trees that dominate the upper canopy, thereby obtaining more information from those species that are adapted to more stressful conditions such as high solar radiation and temperatures and low air humidity 37 . LiDAR-based observations may provide fresh insights into the interacting factors controlling vegetation dynamics and have more recently shown that leaf phenology in Amazonian forests is stratified over canopy positions and conditions 19,38 .…”