A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

BIOMASS DYNAMICS OF AMAZONIAN FOREST FRAGMENTSWilliam F. LauranceSmithsonian Tropical Research Institute, Panama; and Biological Dynamics ofForest Fragments Project, National Institute for Amazonian Research, Manaus,Brazil (laurancew@tivoli.si.edu)Habitat fragmentation affects aboveground biomass in Amazonian forests, withpotentially important implications for carbon storage and greenhouse gasemissions. I describe the dynamics of aboveground-biomass by combining longterm(>20 years) data on mortality, growth, and recruitment of large (>10 cmdiameter) trees with detailed measurements of nearly all other live and deadplant material in fragmented and continuous Amazonian forests.The key process altering biomass dynamics in fragmented forests is thechronically elevated mortality of large trees, which apparent results frommicroclimatic changes and increased wind turbulence near forest edges. This inturn accelerates the production of necromass (dead material) and leads tosignificantly increased wood debris and litter on the forest floor. Near forestedges, frequent canopy disturbance increases the amount of light in theunderstorey, resulting in accelerated tree recruitment, significantly higherbiomass of small trees, and higher liana densities. Surprisingly, the estimatedannual turnover of necromass increases significantly near forest edges,suggesting that decomposition is occurring more rapidly in fragmented thancontinuous forests.These results reveal that habitat fragmentation fundamentally alters thedistribution and dynamics of aboveground biomass in Amazonian forests. Therate of carbon cycling probably increases sharply, both because long-livedcanopy and emergent trees decline in favor of shorter-lived successional treesand lianas, and because necromass production and turnover both appear toincrease. Carbon storage in live vegetation also declines because smallsuccessional trees and lianas (which typically have low wood density) storesubstantially less carbon than do large, old-growth trees. Finally, the decline andrapid decay of live biomass in forest fragments probably leads to substantialatmospheric carbon emissions, above and beyond that resulting fromdeforestation per se.