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

Abstract

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.

Wood Basic Density in Large Trees: Impacts on Biomass Estimates in the Southwestern Brazilian Amazon

“…The twenty species with the highest coverage values (basal area per hectare) were selected [36]. The number of individuals sampled (224) was determined according to the sample size (n) for a population considered to be infinite using Equation (1). The variable used to determine sample size was volume (m 3 ).…”

“…Sterculia apetala 5 35.08 1 This refers to the last section sampled along the stem, which can be Section B, C, or D depending on the species. 2 The negative value indicates a decrease in density along the stem (Figure 4).…”

“…Large trees play a crucial role in forest conservation, contributing significantly to the hydrological and carbon cycles, as well as sustaining biodiversity and carbon storage [1]. Storage in large trees (≥50 cm diameter at breast height (DBH)) represents 40 to 60% of the forest's dry aboveground biomass.…”

“…Storage in large trees (≥50 cm diameter at breast height (DBH)) represents 40 to 60% of the forest's dry aboveground biomass. Due to their specific architecture, these trees are particularly important for issues related to climate change, given their disproportionate contribution to the total forest biomass [1][2][3]. The lack of data on large trees can be attributed to the high cost of sampling and to legal restrictions, because extracting these trees from the forest in Brazil is only permitted with the approval of a forest management plan [4].…”