Geological control of floristic composition in Amazonian forests

Abstract

Aim Conservation and land-use planning require accurate maps of patterns in species composition and an understanding of the factors that control them. Substantial doubt exists, however, about the existence and determinants of largearea floristic divisions in Amazonia. Here we ask whether Amazonian forests are partitioned into broad-scale floristic units on the basis of geological formations and their edaphic properties.Location Western and central Amazonia.Methods We used Landsat imagery and Shuttle Radar Topography Mission (SRTM) digital elevation data to identify a possible floristic and geological discontinuity of over 300 km in northern Peru. We then used plant inventories and soil sampling to document changes in species composition and soil properties across this boundary. Data were obtained from 138 sites distributed along more than 450 km of road and river. On the basis of our findings, we used broad-scale Landsat and SRTM mosaics to identify similar patterns across western and central Amazonia.Results The discontinuity identified in Landsat and SRTM data corresponded to a 15-fold change in soil cation concentrations and an almost total change in plant species composition. This discontinuity appears to be caused by the widespread removal of cation-poor surface sediments by river incision to expose cation-rich sediments beneath. Examination of broad-scale Landsat and SRTM mosaics indicated that equivalent processes have generated a north-south discontinuity of over 1500 km in western Brazil. Due to similarities with our study area, we suggest that this discontinuity represents a chemical and ecological limit between western and central Amazonia.Main conclusions Our findings suggest that Amazonian forests are partitioned into large-area units on the basis of geological formations and their edaphic properties. The evolution of these units through geological time may provide a general mechanism for biotic diversification in Amazonia. These compositional units, moreover, may correspond to broad-scale functional units. The existence of large-area compositional and functional units would suggest that protected-area, carbon sequestration, and other land-use strategies in Amazonia be implemented on a region-by-region basis. The methods described here can be used to map these patterns, and thus enable effective conservation and management of Amazonian forests.

Abrupt Change in Forest Height along a Tropical Elevation Gradient Detected Using Airborne Lidar

“…Nevertheless, elevation gradients are also edaphic, with soil development controlled not only by climate, but also by spatial variation in bedrock types [17,18], and erosion. Soil variation is generally recognized as important to understanding vegetation patterns in tropical forests [19][20][21][22], but the role of soils along elevation gradients is not yet well understood.…”

Biodiversity from mountain building

“…Along the western margin of South America, the Andes grew into a high mountain range, whereas farther east the continent subsided and the Amazon Basin formed. Mountain and basin formation, as well as erosion, marine incursions and orographic precipitation 13,15 , together provide the nutrient-rich sediments and soils that now sustain the high species richness of Amazonia 16,17 . From its formation about 10.5 million years ago 18 , the Amazon River opened up entirely new habitats both on land and at sea.…”

Large-scale climatic and geophysical controls on the leaf economics spectrum

“…Elevated foliar N in both Andean valley and lowland floodplain forest canopies is associated with highfertility sediment deposition from montane sources (21). Northwestern high-N substrates have dual origin: exposed Pebas soils dating to the early Miocene (22), and more recent basaltic soils derived from volcanic flows starting in Ecuadorian Andes (23).…”