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Late twentieth-century trends in the structure and dynamics of South American forests

Abstract:

Widespread recent changes in the ecology of old-growth tropical forests have been documented, in particular an increase in stem turnover (pan-tropical), and an increase in above-ground biomass (neotropical). Whether these changes are synchronous and whether changes in growth are also occurring is not known. This chapter reports assesses changes from fifty long-term plots from across South America spanning 1971-2002. The key findings are significant increases in: basal area (BA: sum of the cross-sectional areas of all trees in a plot) (by approximately 0.10 square meters per hectare per year); stand-level BA growth; stand-level BA mortality; stem density (about 0.94stems per hectare per year); stem recruitment; and stem mortality. The gain terms (BA growth, stem recruitment) consistently exceeded the loss terms (BA loss, stem mortality) throughout, suggesting that whatever process is driving these changes was already acting before the plot network was established. Long-term, simultaneous increases in growth, BA and stem density imply a continent-wide increase in resource availability which is affecting productivity and forest dynamics. Changes in incoming solar radiation, increases in atmospheric concentrations of CO2, and temperature increases, may all have increased resource supply over recent decades, accelerating growth and dynamics in the world's largest tropical forest.