Nigel C. A. Pitman

Drought sensitivity of the Amazon rainforest

Amazon forests are a key but poorly understood component of the global carbon cycle. If, as anticipated, they dry this century, they might accelerate climate change through carbon losses and changed surface energy balances. We used records from multiple long-term monitoring plots across Amazonia to assess forest responses to the intense 2005 drought, a possible analog of future events. Affected forest lost biomass, reversing a large long-term carbon sink, with the greatest impacts observed where the dry season was unusually intense. Relative to pre-2005 conditions, forest subjected to a 100-millimeter increase in water deficit lost 5.3 megagrams of aboveground biomass of carbon per hectare. The drought had a total biomass carbon impact of 1.2 to 1.6 petagrams (1.2 × 1015 to 1.6 × 1015 grams). Amazon forests therefore appear vulnerable to increasing moisture stress, with the potential for large carbon losses to exert feedback on climate change.

DOMINANCE AND DISTRIBUTION OF TREE SPECIES IN UPPER AMAZONIAN TERRA FIRME FORESTS

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. n nAlthough 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. n nThe 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. n nThese 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.

TREE SPECIES DISTRIBUTIONS IN AN UPPER AMAZONIAN FOREST

Not a single tree species distribution in the Amazon basin has been reliably mapped, though speculation regarding such distributions has been extensive. We present data from a network of 21 forest plots in Manu National Park, Peru, totaling >36 ha and sited over an area of -400 km2, to explore how tree species are distributed across upper Amazonia at a variety of spatial scales. For each of 825 tree species occurring in the plots we asked three questions: (1) Does the species have a large or small geographic range? (2) Is the species restricted to a single forest type, or is it found in several? (3) Is the species locally abundant anywhere or is it scarce everywhere? The answers served to classify a subset of species under Rabinowitzs classification scheme for rare species. Three main conclusions emerged. First, the great majority of tree species at Manu are geographically widespread. Every species identified to date occurs elsewhere in South America, outside the department of Madre de Dios; more than two-thirds of them have been collected 1500 km away in Amazonian Ecuador. Second, 15-26% of species appear to be restricted to a single forest type, when forest types are defined by historical river dynamics (i.e., terra firme forest, mature floodplain forest, swamp forest, and primary successional floodplain forest). The proportion of restricted species declined with increasing sampling effort, mak- ing 15% a more reliable figure. Third, while 88% of species occurred at densities of 1.5 individuals/ha. Extrapolating these results provides a first guess at how tree species are distributed across the western portion of the Amazon basin. We conclude with the suggestion that most tree species in the region are habitat generalists occurring over large areas of the Amazonian lowlands at low densities but large absolute population sizes.

A COMPARISON OF TREE SPECIES DIVERSITY IN TWO UPPER AMAZONIAN FORESTS

We inventoried two Amazonian tree communities separated by ;1400 km of continuous lowland tropical forest, in an effort to understand why one was more diverse than the other. YasuniNational Park, near the equator in eastern Ecuador, has one of the most diverse tree communities in the world. Manu National Park, at 128 S in Perus Madre de Dios region, is only moderately diverse by upper Amazonian standards. Following the field inventories, a database of morphological, ecological, and other traits was compiled from the taxonomic literature for 1039 species from the plots. Our goals were (1) to describe how terra firme tree communities at the two sites differed in composition, diversity, and structure; (2) to characterize the extra species responsible for the higher diversity at Yasuni ´; and (3) to assess, in the light of those observations, some explanations for why forests near the equator are so diverse. Yasunihas ;1.4 times as many tree species as Manu at all three spatial scales we examined: local (1 ha), landscape (,10 000 km 2 ), and regional (,100 000 km 2 ). Yasuni ´ samples contain more families and genera, more individual trees per unit area, and a larger proportion of small trees. Tree species at Yasunihave smaller stature, larger leaves, larger seeds, and smaller geographic and altitudinal ranges than those at Manu, and dispropor- tionate increases in species diversity are observed within the Myrtaceae, Lauraceae, Me- lastomataceae, and several other families. Community structures were strikingly similar, with the same species (Iriartea deltoidea, a palm) dominating both sites at identical den- sities. Common species at Yasunioccur at the same densities as equally ranked species at Manu, but there are substantially more very rare species at Yasuni ´. The poorer tree flora is not a nested subset of the richer tree flora, though a majority of species in each inventory do occur at the other site. Several models that offer explanations for geographic variation in tropical tree species diversity are assessed in light of these data. Most do a poor job of accounting for the patterns revealed by the inventories. We speculate that the most important factor in pro- ducing the higher diversity in Yasuniis its rainier, aseasonal climate, and we discuss two specific rainfall-related mechanisms that appear to be supported by the data: (1) year-round water availability allowing more species to persist in the understory at Yasuniand (2) a newly described mixing effect related to the higher stem density there.

Habitat-related error in estimating temperatures from leaf margins in a humid tropical forest

Leaf margin characters are strong predictors of mean annual temperature (MAT) in modern plant communities and widely used tools for reconstructing paleoclimates from fossil floras. However, the frequency of nonentire-margined species may vary dramatically between different habitats of the same forest. In this paper we explore the potential for this habitat variation to introduce error into temperature reconstructions, based on field data from a modern lowland forest in Amazonian Ecuador.The data show that the provenance of leaves can influence temperature estimates to an important degree and in a consistent direction. Woody plants growing along lakes and rivers underestimated MAT by 2.5°-5°C, while those in closed-canopy forest provided very accurate predictions. The high proportion of liana species with toothed leaves in lakeside and riverside samples appears to be responsible for a large part of the bias. Samples from closed-canopy forest that included both lianas and trees, however, were more accurate than tree-only or liana-only samples.We conclude that paleotemperature reconstructions based on leaf margin characters will be misleading to the extent that fossilization provides a better record of certain habitats than others. The preponderance of lake and river deposits in the angiosperm fossil record suggests that underestimation of mean annual paleotemperature may be common.

Fast demographic traits promote high diversification rates of Amazonian trees

The Amazon rain forest sustains the worlds highest tree diversity, but it remains unclear why some clades of trees are hyperdiverse, whereas others are not. Using dated phylogenies, estimates of current species richness and trait and demographic data from a large network of forest plots, we show that fast demographic traits – short turnover times – are associated with high diversification rates across 51 clades of canopy trees. This relationship is robust to assuming that diversification rates are either constant or decline over time, and occurs in a wide range of Neotropical tree lineages. This finding reveals the crucial role of intrinsic, ecological variation among clades for understanding the origin of the remarkable diversity of Amazonian trees and forests.

Evolutionary heritage influences Amazon tree ecology

Lineages tend to retain ecological characteristics of their ancestors through time. However, for some traits, selection during evolutionary history may have also played a role in determining trait values. To address the relative importance of these processes requires large-scale quantification of traits and evolutionary relationships among species. The Amazonian tree flora comprises a high diversity of angiosperm lineages and species with widely differing life-history characteristics, providing an excellent system to investigate the combined influences of evolutionary heritage and selection in determining trait variation. We used trait data related to the major axes of life-history variation among tropical trees (e.g. growth and mortality rates) from 577 inventory plots in closed-canopy forest, mapped onto a phylogenetic hypothesis spanning more than 300 genera including all major angiosperm clades to test for evolutionary constraints on traits. We found significant phylogenetic signal (PS) for all traits, consistent with evolutionarily related genera having more similar characteristics than expected by chance. Although there is also evidence for repeated evolution of pioneer and shade tolerant life-history strategies within independent lineages, the existence of significant PS allows clearer predictions of the links between evolutionary diversity, ecosystem function and the response of tropical forests to global change.