Rapid decay of tree-community composition in Amazonian forest fragments
William F. Laurance*,
,
, Henrique E. M. Nascimento, Susan G. Laurance*, Ana Andrade, José E. L. S. Ribeiro
, Juan Pablo Giraldo¶, Thomas E. Lovejoy, Richard Condit*, Jerome Chave||, Kyle E. Harms*,**, and Sammya D'Angelo
*Smithsonian Tropical Research Institute, Apartado Postal 2072, Balboa, Republic of Panamá; Biological Dynamics of Forest Fragments Project and Department of Botany, National Institute for Amazonian Research (INPA), CP 478, Manaus, AM 69011-970, Brazil; ¶Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138; ||Laboratoire Evolution et Diversité Biologique, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 5174, Université Paul Sabatier, 31062 Toulouse, France; and **Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803
Communicated by Gretchen C. Daily, Stanford University, Stanford, CA, October 12, 2006 (received for review August 1, 2006)
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.
edge effects | floristic composition | forest dynamics | habitat fragmentation | tree communities
PNAS | December 12, 2006 | vol. 103 | no. 50 | 19010-19014. OPEN ACCESS ARTICLE.
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The rainforests of central Amazonia contain some of the most biologically diverse tree communities ever encountered, averaging >250 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-km2 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 forest fragmentation on one of the world's most diverse tree floras.
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