Amazonian functional diversity from forest canopy chemical assembly

Abstract

Patterns of tropical forest functional diversity express processes of ecological assembly at multiple geographic scales and aid in predicting ecological responses to environmental change. Tree canopy chemistry underpins forest functional diversity, but the interactive role of phylogeny and environment in determining the chemical traits of tropical trees is poorly known. Collecting and analyzing foliage in 2,420 canopy tree species across 19 forests in the western Amazon, we discovered (i) systematic, community-scale shifts in average canopy chemical traits along gradients of elevation and soil fertility; (ii) strong phylogenetic partitioning of structural and defense chemicals within communities independent of variation in environmental conditions; and (iii) strong environmental control on foliar phosphorus and calcium, the two rock-derived elements limiting CO2 uptake in tropical forests. These findings indicate that the chemical diversity of western Amazonian forests occurs in a regionally nested mosaic driven by long-term chemical trait adjustment of communities to large-scale environmental filters, particularly soils and climate, and is supported by phylogenetic divergence of traits essential to foliar survival under varying environmental conditions. Geographically nested patterns of forest canopy chemical traits will play a role in determining the response and functional rearrangement of western Amazonian ecosystems to changing land use and climate.

Keywords: Amazon basin; biological diversity; chemical phylogeny; community assembly; leaf traits.

Fig. 1.

Changes in average canopy foliar traits along a 3,500-m Andes to Amazon elevation gradient for (A) all sites on all soil types and (B) a subset of sites on high-fertility soils. The lines are ordinary least squares regression fits for each trait after normalization of the data to their elevation gradient mean values (site mean − gradient mean)/gradient SD (SI Methods). *Linear regression fits to foliar data that are significant at the P < 0.05 level. Car, carotenoid; Chl, chlorophyll.

Fig. 2.

Partitioning of the variance for each tree canopy chemical trait into phylogenetic (family/genus/species), site, and unexplained residual components for (A) all sites on all soil types and (B) a subset of sites on high-fertility soils. The site component incorporates variation in soils, geology, topography, and tree and foliage selection among other factors. Unexplained residuals are comprised of measurement error and other nonsite-related sources of uncertainty.

Fig. 3.

Mean intra- and interspecific variations in tree canopy foliar traits along the Andes to Amazon elevation gradient for (A and B) all sites on all soil types and (C and D) a subset of sites on high-fertility soils (SI Methods). These regressions are computed using averaged coefficients of variation (CVs) on chemical data collected along the elevation gradient.