Susan G. Laurance

RAIN FOREST FRAGMENTATION AND THE DYNAMICS OF AMAZONIAN TREE COMMUNITIES

Few studies have assessed effects of habitat fragmentation on tropical forest dynamics. We describe results from an 18-yr experimental study of the effects of rain forest fragmentation on tree-community dynamics in central Amazonia. Tree communities were assessed in 39 permanent, 1-ha plots in forest fragments of 1, 10, or 100 ha in area, and in 27 plots in nearby continuous forest. Repeated censuses of >56000 marked trees (≥10 cm diameter at breast height) were used to generate annualized estimates of tree mortality, damage, and turnover in fragmented and continuous forest. On average, forest fragments exhibited markedly elevated dynamics, apparently as a result of increased windthrow and microclimatic changes near forest edges. Mean mortality, damage, and turnover rates were much higher within 60 m of edges (4.01, 4.10, and 3.16%, respectively) and moderately higher within 60–100 m of edges (2.40, 1.96, and 2.05%) than in forest interiors (1.27, 1.48, and 1.15%). Less-pronounced changes in mortality and turnover rates were apparently detectable up to ∼300 m from forest edges. Edge aspect had no significant effect on forest dynamics. Tree mortality and damage rates did not vary significantly with fragment age, suggesting that increased dynamics are not merely transitory effects that occur immediately after fragmentation, while turnover rates increased with age in most (8/9) fragments. These findings reveal that fragmentation causes important changes in the dynamics of Amazonian forests, especially within ∼100 m of habitat edges. A mathematical “core-area model” incorporating these data predicted that edge effects will increase rapidly in importance once fragments fall below ∼100–400 ha in area, depending on fragment shape. Accelerated dynamics in fragments will alter forest structure, floristic composition, biomass, and microclimate and are likely to exacerbate effects of fragmentation on disturbance-sensitive species.

Impacts of roads and linear clearings on tropical forests

Linear infrastructure such as roads, highways, power lines and gas lines are omnipresent features of human activity and are rapidly expanding in the tropics. Tropical species are especially vulnerable to such infrastructure because they include many ecological specialists that avoid even narrow (<30-m wide) clearings and forest edges, as well as other species that are susceptible to road kill, predation or hunting by humans near roads. In addition, roads have a major role in opening up forested tropical regions to destructive colonization and exploitation. Here, we synthesize existing research on the impacts of roads and other linear clearings on tropical rainforests, and assert that such impacts are often qualitatively and quantitatively different in tropical forests than in other ecosystems. We also highlight practical measures to reduce the negative impacts of roads and other linear infrastructure on tropical species.

Relationship between soils and Amazon forest biomass: a landscape-scale study

Above-ground dry biomass of living trees including palms was estimated in 65 1 ha plots spanning a 1000 km 2 landscape in central Amazonia. The study area was located on heavily weathered, nutrient-poor soils that are widespread in the Amazon region. Biomass values were derived by measuring the diameter-at-breast-height (DBH) of all10 cm trees in each plot, then using an allometric equation and correction factor for small trees to estimate total tree biomass. Detailed information on soil texture, organic carbon, available water capacity, pH, macro- and micro-nutrients, and trace elements was collected from soil surface samples (0‐20 cm) in each plot, while slope was measured with a clinometer. Biomass estimates varied more than two-fold, from 231 to 492 metric tons ha ˇ1 , with a mean of 356 47 tons ha ˇ1 . Simple correlations with stringent (p < 0.006) Bonferroni corrections suggested that biomass was positively associated with total N, total exchangeable bases, K a ,M g 2a , clay, and organic C in soils, and negatively associated with Zn a , aluminum saturation, and sand. An ordination analysis revealed one major and several minor soil gradients in the study area, with the main gradient discriminating sites with varying proportions of clay (with clayey soils having higher concentrations of total N, organic C, most cations, and lower aluminum saturation and less sand). A multiple regression analysis revealed that the major clay-nutrient gradient was the only significant predictor, with the model explaining 32.3% of the total variation in biomass. Results of the analysis suggest that soil-fertility parameters can account for a third or more of the variation in above-ground biomass in Amazonian terra-firme forests. We suggest that, because the conversion of forest to pasture tends to reduce the nitrogen, clay, organic carbon, and nutrient contents of soils, forests that regenerate on formerly cleared lands may have lower biomass than the original forest, especially in areas with low soil fertility. # 1999 Elsevier Science B.V. All rights reserved.

Conservation: Rainforest fragmentation kills big trees

In tropical forests, large canopy and emergent trees are crucial sources of fruits, flowers and shelter for animal populations. They are also reproductively dominant and strongly influence forest structure, composition, gap dynamics, hydrology and carbon storage. Here we show that forest fragmentation in central Amazonia is having a disproportionately severe effect on large trees, the loss of which will have major impacts on the rainforest ecosystem.

Rainforest fragmentation kills big trees.

In tropical forests, large canopy and emergent trees are crucial sources of fruits, flowers and shelter for animal populations. They are also reproductively dominant and strongly influence forest structure, composition, gap dynamics, hydrology and carbon storage. Here we show that forest fragmentation in central Amazonia is having a disproportionately severe effect on large trees, the loss of which will have major impacts on the rainforest ecosystem.

Rapid decay of tree-community composition in Amazonian forest fragments

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 ≈100 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.

RAIN FOREST FRAGMENTATION AND THE PROLIFERATION OF SUCCESSIONAL TREES

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 (approximately 1000 km2) 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 (> or =10 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 >1000% 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 (>150 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 plants growing in nearby degraded habitats. The proliferation of fast-growing successional trees and correlated decline of old-growth trees will have important effects on species composition, forest dynamics, carbon storage, and nutrient cycling in fragmented forests.

Long-term decline of the Amazon carbon sink

Atmospheric carbon dioxide records indicate that the land surface has acted as a strong global carbon sink over recent decades, with a substantial fraction of this sink probably located in the tropics, particularly in the Amazon. Nevertheless, it is unclear how the terrestrial carbon sink will evolve as climate and atmospheric composition continue to change. Here we analyse the historical evolution of the biomass dynamics of the Amazon rainforest over three decades using a distributed network of 321 plots. While this analysis confirms that Amazon forests have acted as a long-term net biomass sink, we find a long-term decreasing trend of carbon accumulation. Rates of net increase in above-ground biomass declined by one-third during the past decade compared to the 1990s. This is a consequence of growth rate increases levelling off recently, while biomass mortality persistently increased throughout, leading to a shortening of carbon residence times. Potential drivers for the mortality increase include greater climate variability, and feedbacks of faster growth on mortality, resulting in shortened tree longevity. The observed decline of the Amazon sink diverges markedly from the recent increase in terrestrial carbon uptake at the global scale, and is contrary to expectations based on models.

Habitat fragmentation, variable edge effects, and the landscape-divergence hypothesis

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 worlds 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.

Tropical wildlife corridors: use of linear rainforest remnants by arboreal mammals

In fragmented landscapes, linear forest remnants have the potential to provide habitat and movement corridors for wildlife. We used systematic spotlighting surveys to sample arboreal mammals in 36 linear rainforest remnants in tropical Queensland, Australia. The effects of corridor width, height, isolation, elevation, and floristic composition on mammals were assessed with multiple regression models. Six species were recorded during 108 surveys. The most vulnerable species, the lemuroid ringtail possum (Hemibelideus lemuroides), was found only in remnants comprised of primary rainforest that were linked to large tracts of continuous forest. Two other species, the Herbert River ringtail possum (Pseudochirulus herbertensis) and striped possum (Dactylopsila trivirgata), also favored corridors that were linked to forest tracts or fragments, with the former favoring high-diversity forest (primary forest or mixed regrowth) over low-diversity (Acacia) regrowth. Three other species, the coppery brushtail possum (Trichosurus vulpecula), green ringtail possum (Pseudochirops archeri), and Lumholtzs tree-kangaroo (Dendrolagus lumholtzi), occurred in both isolated and non-isolated remnants and both primary forest and regrowth. Our findings suggest that linear forest remnants that are floristically diverse (not Acacia-dominated regrowth) and at least 30–40 m width can function as habitat and probably movement corridors for most arboreal mammals in this region. The lemuroid ringtail, however, apparently requires corridors of primary rainforest of at least 200 m in width. Because the lemuroid ringtail is highly vulnerable to forest fragmentation, faunal corridors in this region should be designed wherever possible to meet its ecological requirements.