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Research to date on Amazonian swamps has reinforced the impression that tree communities there are dominated by a small, morphologically specialized subset of the regional fl ora capable of surviving physiologically challenging conditions. In this paper, using data from a large-scale tree inventory in upland, floodplain, and mixed palm swamp forests in Amazonian Ecuador, we report that tree communities growing on well-drained and saturated soils are more similar than previously appreciated. While our data support the traditional view of Amazonian swamp forests as lowdiversity tree communities dominated by palms, they also reveal four patterns that have not been well documented in the literature to date: 1) tree communities in these swamp forests are dominated by a phylogenetically diverse oligarchy of 30 frequent and common species; 2) swamp specialists account for < 10% of species and a minority of stems; 3) most tree species recorded in swamps (> 80%) also occur in adjacent well-drained forest types; and 4) many tree species present in swamps are common in well-drained forests (e.g. upland oligarchs account for 34.1% of all swamp stems). These observations imply that, as in the temperate zone, the composition and structure of Amazonian swamp vegetation are determined by a combination of local-scale environmental filters (e.g. plant survival in permanently saturated soils) and landscape-scale patterns and processes (e.g. the composition and structure of tree communities in adjacent non-swamp habitats, the dispersal of propagules from those habitats to swamps). We conclude with suggestions for further research to quantify the relative contributions of these factors in structuring tree communities in Amazonian swamps. [ABSTRACT FROM AUTHOR]

Aim The aim of this study was to assess the effect of Holocene fire activity and drought on the mesic forests of the Upper Beni and the extent of ecotone migration in south-western Amazonia during the mid-Holocene. An additional goal was to address the hypothesis of Amazonia as a manufactured landscape prior to the arrival of European colonizers in the New World in ad 1492. Location Lakes Chalalán and Santa Rosa, Upper Beni, Bolivian Amazon. Methods Holocene vegetation and fire activity were reconstructed based on pollen and microcharcoal records from sediments of lakes Chalalán and Santa Rosa. Chronologies were based on accelerator mass spectrometry radiocarbon analyses, and vegetation changes were summarized using detrended correspondence analysis. The normalized difference vegetation index was used to classify and determine the extent of the modern savanna-forest ecotone using MODIS satellite image data. Results Mesic evergreen forests persisted in the Upper Beni throughout the Holocene. Fire was frequent during the early and mid-Holocene and temporally consistent in both records. Ordination of pollen data showed an increase in forest change during the late Holocene when fire activity was asynchronous in the two records. Main conclusions The Upper Beni sites were not reached by expanding savannas during periods of major environmental change, suggesting forest resilience and a degree of ecotone stability. We associated the largest observed change in these forests with late Holocene fires that were most probably ignited by humans. We found little evidence supporting widespread pre-1491 cultural landscapes. [ABSTRACT FROM AUTHOR]

Aim To determine if recurrent fire accounts for bamboo-dominated forests that cover about 180,000 km2 of western Amazonia. Location Western Amazonia, including Los Amigos and Cocha Cashu, Peru, and Acre, Brazil. Methods We identified bamboo (B+) and closed-canopy forests (B−) using a combination of MODIS imagery, Landsat TM imagery and field surveys. Local-scale and landscape-scale patterns of historical bamboo dynamics were analysed by collecting soil cores from B+ and B− sites across three regions. Soil charcoal within cores was used to document previous fire, and phytoliths were employed to reconstruct vegetational patterns through time. Results Fire occurred in B+ and B− sites with approximately equal frequencies in each region sampled. Between regions, fire signals were most prevalent at Los Amigos, with Cocha Cashu and Acre sites typically containing only trace amounts of charcoal. Vegetational state remained constant through time in both B+ and B− forests of Cocha Cashu and Acre. The persistence of bamboo through time is less clear at Los Amigos, with bamboo presence changing both spatially and temporally. Main conclusions Fire does not appear to be the driving mechanism behind current bamboo distributions, although it may facilitate bamboo invasion. Once established in an area, bamboo persistently dominates the vegetation structure over historical time-scales at some sites (Cocha Cashu and Acre), but not at others (Los Amigos), which is likely to be a function of patch origin. Increasing human activity, including fire and deforestation, combined with predicted Amazonian drought, may allow bamboo to expand from its current distribution and replace typical Amazonian closed-canopy forests. These structural changes in the forests have important implications for carbon storage, as Amazonian forests are currently the largest terrestrial carbon sink in the world. [ABSTRACT FROM AUTHOR]

The history of landscape alteration in Amazonia by humans prior to the arrival of Europeans remains poorly understood. Estimates of human population size at the time of European contact vary by several million, and the trajectories of cultural and agricultural development are equally uncertain. The extent to which human populations altered Amazonian ecosystems is a function of population size, land management style, radius of influence of each subpopulation, and the temporal development of settlements. Here we report evidence that relates to the temporal and spatial scales of human disturbance in western Amazonia. Three lake sediment records and 94 terrestrial soil cores located 0–12 km from the adjacent lake were used to examine scales of pre-Columbian human impacts at two sites in western Amazonian forests. Lake sediment and soil charcoal records indicate discontinuous and localized burning around the study areas. Terrestrial soil phytolith and nutrient analyses reveal a range in disturbance intensity between the two sites but contain no evidence of large-scale forest clearing or anthropogenic soil enrichment. Our data suggest that while all of the settings examined were occupied or used, the halo of influence around each was limited. It should not be assumed that intensive landscape transformations by prehistoric human populations occurred throughout Amazonia or that Amazonian forests were resilient in the face of heavy historical disturbance. [ABSTRACT FROM AUTHOR]

The influence of pre-Columbian human populations on Amazonian ecosystems is being actively debated. The longstanding view that Amazonia was only minimally impacted by human actions has been challenged, and a new paradigm of Amazonia as a ‘manufactured landscape’ is emerging. If such disturbance was the norm until just 500 years ago, Amazonian ecosystems could be far more ecologically resilient to disturbance than previously supposed. Alternatively, if the ‘manufactured landscape’ label is an overstatement, then policy that assumes such resilience may cause substantial and long-lasting ecological damage. We present paleoecological data suggesting a middle path, in which some areas were heavily modified, but most of Amazonia was minimally impacted. Bluffs adjacent to main river channels and highly seasonal areas appear to have been the most extensively settled locations. Away from areas where humans lived, their influence on ecosystems was very local. Consequently, we see no evidence suggesting that large areas at a distance from rivers or in the less seasonal parts of Amazonia were substantially altered by human activity. Extrapolating from sites of known human occupation to infer Amazon-wide landscape disturbance may therefore potentially lead to unrealistic projections of human impact and misguided policy. [ABSTRACT FROM AUTHOR]

Investigates the distribution and abundance of tropical tree species found in terra firme forests in the Upper Amazon River Region. Environmental heterogeneity and community heterogeneity; Extrapolations to other regions; Implications for forest dynamics; Traits associated with commonness and rarity.