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.

The extent to which pre-Columbian societies altered Amazonian landscapes is hotly debated. We performed a basin-wide analysis of pre-Columbian impacts on Amazonian forests by overlaying known archaeological sites in Amazonia with the distributions and abundances of 85 woody species domesticated by pre-Columbian peoples. Domesticated species are five times more likely than nondomesticated species to be hyperdominant. Across the basin, the relative abundance and richness of domesticated species increase in forests on and around archaeological sites. In southwestern and eastern Amazonia, distance to archaeological sites strongly influences the relative abundance and richness of domesticated species. Our analyses indicate that modern tree communities in Amazonia are structured to an important extent by a long history of plant domestication by Amazonian peoples

Forest fragmentation is considered a greater threat to vertebrates than to tree communities because individual trees are typically long-lived and require only small areas for survival. Here we show that forest fragmentation provokes surprisingly rapid and profound alterations in Amazonian tree-community composition. Results were derived from a 22-year study of exceptionally diverse tree communities in 40 1-ha plots in fragmented and intact forests, which were sampled repeatedly before and after fragment isolation. Within these plots, trajectories of change in abundance were assessed for 267 genera and 1,162 tree species. Abrupt shifts in floristic composition were driven by sharply accelerated tree mortality and recruitment within Ϸ100 m of fragment margins, causing rapid species turnover and population declines or local extinctions of many large-seeded, slow-growing, and old-growth taxa; a striking increase in a smaller set of disturbance-adapted and abiotically dispersed species; and significant shifts in tree size distributions. Even among old-growth trees, species composition in fragments is being restructured substantially, with subcanopy species that rely on animal seed-dispersers and have obligate outbreeding being the most strongly disadvantaged. These diverse changes in tree communities are likely to have wide-ranging impacts on forest architecture, canopy-gap dynamics, plant-animal interactions, and forest carbon storage.edge effects ͉ floristic composition ͉ forest dynamics ͉ habitat fragmentation ͉ tree communities T he rainforests of central Amazonia contain some of the most biologically diverse tree communities ever encountered, averaging Ͼ250 species that attain a diameter of at least 10 cm (measured at breast height or above any buttresses) per hectare (1, 2). These communities are also being cleared and fragmented at alarming rates as a result of large-scale cattle ranching, slash-and-burn farming, rapid soya expansion, industrial logging, and wildfires (3-8). Because tree communities are crucial components of forest ecosystems (9) and sustain a wide variety of dependent animal species (10, 11), their persistence in fragmented landscapes will ultimately have a major impact on tropical biodiversity.We evaluated the most extensive dataset ever collected on tree-community dynamics in fragmented forests, obtained from the Biological Dynamics of Forest Fragments Project, the world's largest and longest-running experimental study of habitat fragmentation (12, 13). Within a 1,000-km 2 landscape, data were collected in 40 1-ha plots arrayed across nine forest fragments ranging from 1 to 100 ha in area and in control sites in nearby intact forest (see Methods). A key advantage of our experiment is that all study plots in fragmented and intact forests were sampled both before isolation of the fragments and at regular intervals thereafter, greatly increasing confidence in our findings. Our analysis, based on a two-decade study of nearly 32,000 trees, provides uniquely detailed insights into the impact of f...

Abstract. The effects of habitat fragmentation on diverse tropical tree communities are poorly understood. Over a 20-year period we monitored the density of 52 tree species in nine predominantly successional genera (Annona, Bellucia, Cecropia, Croton, Goupia, Jacaranda, Miconia, Pourouma, Vismia) in fragmented and continuous Amazonian forests. We also evaluated the relative importance of soil, topographic, forest dynamic, and landscape variables in explaining the abundance and species composition of successional trees. Data were collected within 66 permanent 1-ha plots within a large (ϳ1000 km 2 ) experimental landscape, with forest fragments ranging from 1 to 100 ha in area.Prior to forest fragmentation, successional trees were uncommon, typically comprising 2-3% of all trees (Ն10 cm diameter at breast height [1.3 m above the ground surface]) in each plot. Following fragmentation, the density and basal area of successional trees increased rapidly. By 13-17 years after fragmentation, successional trees had tripled in abundance in fragment and edge plots and constituted more than a quarter of all trees in some plots. Fragment age had strong, positive effects on the density and basal area of successional trees, with no indication of a plateau in these variables, suggesting that successional species could become even more abundant in fragments over time.Nonetheless, the 52 species differed greatly in their responses to fragmentation and forest edges. Some disturbance-favoring pioneers (e.g., Cecropia sciadophylla, Vismia guianensis, V. amazonica, V. bemerguii, Miconia cf. crassinervia) increased by Ͼ1000% in density on edge plots, whereas over a third (19 of 52) of all species remained constant or declined in numbers. Species responses to fragmentation were effectively predicted by their median growth rate in nearby intact forest, suggesting that faster-growing species have a strong advantage in forest fragments.An ordination analysis revealed three main gradients in successional-species composition across our study area. Species gradients were most strongly influenced by the standlevel rate of tree mortality on each plot and by the number of nearby forest edges. Species composition also varied significantly among different cattle ranches, which differed in their surrounding matrices and disturbance histories. These same variables were also the best predictors of total successional-tree abundance and species richness. Successional-tree assemblages in fragment interior plots (Ͼ150 m from edge), which are subjected to fragment area effects but not edge effects, did not differ significantly from those in intact forest, indicating that area effects per se had little influence on successional trees. Soils and topography also had little discernable effect on these species.Collectively, our results indicate that successional-tree species proliferate rapidly in fragmented Amazonian forests, largely as a result of chronically elevated tree mortality near forest edges and possibly an increased seed rain from successional p...

Edge effects are major drivers of change in many fragmented landscapes, but are often highly variable in space and time. Here we assess variability in edge effects altering Amazon forest dynamics, plant community composition, invading species, and carbon storage, in the world's largest and longest-running experimental study of habitat fragmentation. Despite detailed knowledge of local landscape conditions, spatial variability in edge effects was only partially foreseeable: relatively predictable effects were caused by the differing proximity of plots to forest edge and varying matrix vegetation, but windstorms generated much random variability. Temporal variability in edge phenomena was also only partially predictable: forest dynamics varied somewhat with fragment age, but also fluctuated markedly over time, evidently because of sporadic droughts and windstorms. Given the acute sensitivity of habitat fragments to local landscape and weather dynamics, we predict that fragments within the same landscape will tend to converge in species composition, whereas those in different landscapes will diverge in composition. This ‘landscape-divergence hypothesis’, if generally valid, will have key implications for biodiversity-conservation strategies and for understanding the dynamics of fragmented ecosystems.

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The sensitivity of tropical forest carbon to climate is a key uncertainty in predicting global climate change. Although short-term drying and warming are known to affect forests, it is unknown if such effects translate into long-term responses. Here, we analyze 590 permanent plots measured across the tropics to derive the equilibrium climate controls on forest carbon. Maximum temperature is the most important predictor of aboveground biomass (−9.1 megagrams of carbon per hectare per degree Celsius), primarily by reducing woody productivity, and has a greater impact per °C in the hottest forests (>32.2°C). Our results nevertheless reveal greater thermal resilience than observations of short-term variation imply. To realize the long-term climate adaptation potential of tropical forests requires both protecting them and stabilizing Earth’s climate.

Abstract. Habitat fragmentation affects aboveground biomass in Amazonian forests, with potentially important implications for carbon storage and greenhouse gas emissions. We assessed the dynamics of aboveground-biomass stocks by combining long-term (10-19 yr) data on mortality, damage, growth, and recruitment of large (Ն10 cm diameter at breast height [dbh]) trees with measurements of nearly all other live and dead plant material (seedlings, saplings, small trees, palms, lianas, downed wood debris, snags, litter) in 50 1-ha plots in fragmented and continuous Amazonian forests.The key process altering biomass dynamics in fragmented forests is the chronically elevated mortality of large trees, which apparently results from microclimatic changes and increased wind turbulence near forest edges. This, in turn, accelerates the production of necromass and leads to significantly increased wood debris and litter on the forest floor. Near forest edges, frequent canopy disturbance increases the amount of light in the understory, resulting in accelerated tree recruitment, significantly higher biomass of small (5-10 cm dbh) trees, and higher liana densities. Surprisingly, the estimated annual turnover of wood debris increases significantly near forest edges, suggesting that decomposition is occurring more rapidly in fragmented than continuous forests.These results reveal that habitat fragmentation fundamentally alters the distribution and dynamics of aboveground biomass in Amazonian forests. The rate of carbon cycling probably increases sharply, both because long-lived canopy and emergent trees decline in favor of shorter-lived successional trees and lianas, and because necromass production and turnover both appear to increase. Carbon storage in live vegetation also declines because small successional trees and lianas (which typically have low wood density) store substantially less carbon than do large, old-growth trees. Finally, the decline and rapid decay of live biomass in forest fragments may produce substantial atmospheric carbon emissions, above and beyond that resulting from deforestation per se.