Changes in Amazonian Forest Biomass, Dynamics, and Composition, 1980–2002

© 2009 by the American Geophysical Union. All rights reserved. Long-term, on-the-ground monitoring of forest plots distributed across Amazonia provides a powerful means to quantify stocks and fluxes of biomass and biodiversity. Here we examine the evidence for concerted changes in the structure, dynamics, and functional composition of old-growth Amazonian forests over recent decades. Mature forests have, as a whole, gained biomass and undergone accelerated growth and dynamics, but questions remain as to the long-term persistence of these changes. Because forest growth on average exceeds mortality, intact Amazonian forests have been functioning as a carbon sink. We estimate a net biomass increase in trees > 10 cm diameter of 0.62 ± 0.23 t C ha -1 a -1 through the late twentieth century. If representative of the wider forest landscape, this translates into a sink in South American old-growth forest of at least 0.49 ± 0.18 Pg C a -1 . If other biomass and necromass components also increased proportionally, the estimated South American old-growth forest sink is 0.79 ± 0.29 Pg C a -1 , before allowing for possible gains in soil carbon. If tropical forests elsewhere are behaving similarly, the old-growth biomass forest sink would be 1.60 ± 0.58 Pg C a -1 . This bottom-up estimate of the carbon balance of tropical forests is preliminary, pending syntheses of detailed biometric studies across the other tropical continents. There is also some evidence for recent changes in the functional composition (biodiversity) of Amazonian forest, but the evidence is less comprehensive than that for changes in structure and dynamics. The most likely driver(s) of changes are recent increases in the supply of resources such as atmospheric carbon dioxide, which would increase net primary productivity, increasing tree growth and recruitment, and, in turn, mortality. In the future the growth response of remaining undisturbed Amazonian forests is likely to saturate, and there is a risk of these ecosystems transitioning from sink to source driven by higher respiration (temperature), higher mortality (drought), or compositional change (functional shifts toward lighterwooded plants). Even a modest switch from carbon sink to source for Amazonian forests would impact global climate, biodiversity, and human welfare, while the documented acceleration of tree growth and mortality may already be affecting the interactions of thousands of plant and millions of animal species.