No abstract

Amazonian forests are the largest and most diverse in the tropics, and much of the mystery surrounding their ecology can be traced to attempts to understand them through tiny local inventories. In this paper we bring together a large number of such inventories scattered across immense areas of western Amazonia in order to address simple questions about the distribution and abundance of tropical tree species in lowland terra firme forests there. The goal is to describe patterns of commonness and rarity at local (1 ha), landscape (∼104 km2), and regional (>106 km2) scales, and to fuse the results into a more complete picture of how tropical tree communities are structured. We present estimates of landscape‐scale densities for ∼1400 taxa, based on data from tree plots scattered over large tracts of terra firme forest in eastern Ecuador and southeastern Peru. A database of morphological, ecological, and other traits of >1000 of these species compiled from the taxonomic literature is then used to explore how species that are common in the inventories differ from species that are rare. Although most species show landscape‐scale densities of <1 individual/ha, most trees in both forests belong to a small set of ubiquitous common species. These common species combine high frequency with high local abundance, forming predictable oligarchies that dominate several thousand square kilometers of forest at each site. The common species comprising these oligarchies are a nonrandom subset of the two floras. At both sites a disproportionate number of common species are concentrated in the families Arecaceae, Moraceae, Myristicaceae, and Violaceae, and large‐statured tree species are more likely to be common than small ones. Nearly a third of the 150 most common tree species in the Ecuadorean forest are also found among the 150 most common tree species in the Peruvian forest. For the 254 tree species shared by the two data sets, abundance in Ecuador is positively and significantly correlated with abundance ∼1400 km away in Peru. These findings challenge popular depictions of Amazonian vegetation as a small‐scale mosaic of unpredictable composition and structure. Instead, they provide additional evidence that tropical tree communities are not qualitatively different from their temperate counterparts, where a few common species concentrated in a few higher taxa can dominate immense areas of forest. We hypothesize that most Amazonian forests are dominated at large scales by oligarchies similar in nature to the ones observed in Ecuador and Peru, and we argue that the patterns are more indicative of regulation of relative abundances by ecological factors than of nonequilibrium chance‐based dynamics. The paper concludes with a discussion of the practical applications of predictable oligarchies over large areas of unexplored forest.

Previous work has shown that tree turnover, tree biomass and large liana densities have increased in mature tropical forest plots in the late twentieth century. These results point to a concerted shift in forest ecological processes that may already be having significant impacts on terrestrial carbon stocks, fluxes and biodiversity. However, the findings have proved controversial, partly because a rather limited number of permanent plots have been monitored for rather short periods. The aim of this paper is to characterize regional-scale patterns of 'tree turnover' (the rate with which trees die and recruit into a population) by using improved datasets now available for Amazonia that span the past 25 years. Specifically, we assess whether concerted changes in turnover are occurring, and if so whether they are general throughout the Amazon or restricted to one region or environmental zone. In addition, we ask whether they are driven by changes in recruitment, mortality or both. We find that: (i) trees 10 cm or more in diameter recruit and die twice as fast on the richer soils of southern and western Amazonia than on the poorer soils of eastern and central Amazonia; (ii) turnover rates have increased throughout Amazonia over the past two decades; (iii) mortality and recruitment rates have both increased significantly in every region and environmental zone, with the exception of mortality in eastern Amazonia; (iv) recruitment rates have consistently exceeded mortality rates; (v) absolute increases in recruitment and mortality rates are greatest in western Amazonian sites; and (vi) mortality appears to be lagging recruitment at regional scales. These spatial patterns and temporal trends are not caused by obvious artefacts in the data or the analyses. The trends cannot be directly driven by a mortality driver (such as increased drought or fragmentation-related death) because the biomass in these forests has simultaneously increased. Our findings therefore indicate that long-acting and widespread environmental changes are stimulating the growth and productivity of Amazon forests.

No abstract

AimThe accurate mapping of forest carbon stocks is essential for understanding the global carbon cycle, for assessing emissions from deforestation, and for rational land-use planning. Remote sensing (RS) is currently the key tool for this purpose, but RS does not estimate vegetation biomass directly, and thus may miss significant spatial variations in forest structure. We test the stated accuracy of pantropical carbon maps using a large independent field dataset.LocationTropical forests of the Amazon basin. The permanent archive of the field plot data can be accessed at: http://dx.doi.org/10.5521/FORESTPLOTS.NET/2014_1MethodsTwo recent pantropical RS maps of vegetation carbon are compared to a unique ground-plot dataset, involving tree measurements in 413 large inventory plots located in nine countries. The RS maps were compared directly to field plots, and kriging of the field data was used to allow area-based comparisons.ResultsThe two RS carbon maps fail to capture the main gradient in Amazon forest carbon detected using 413 ground plots, from the densely wooded tall forests of the north-east, to the light-wooded, shorter forests of the south-west. The differences between plots and RS maps far exceed the uncertainties given in these studies, with whole regions over- or under-estimated by > 25%, whereas regional uncertainties for the maps were reported to be < 5%.Main conclusionsPantropical biomass maps are widely used by governments and by projects aiming to reduce deforestation using carbon offsets, but may have significant regional biases. Carbon-mapping techniques must be revised to account for the known ecological variation in tree wood density and allometry to create maps suitable for carbon accounting. The use of single relationships between tree canopy height and above-ground biomass inevitably yields large, spatially correlated errors. This presents a significant challenge to both the forest conservation and remote sensing communities, because neither wood density nor species assemblages can be reliably mapped from space.