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High levels of evaporation and transpiration that forests produce throughout the year contribute to a wetter at mospheric boundary layer than would be the case with nonforest. This surfaceatmosphere coupling is more im portant where largescale factors for rainfall formation are weaker, such as in central and eastern Amazonia. Near the Andes, the impact of at least modest deforestation is less dramatic because the general ascending motion of airmasses in this area induces high levels of rainfall in addition to that expected from local evaporation and transpiration.

Where might the tipping point be for deforestation generated degradation of the hydrological cycle? The very first model to examine this question (2) showed that at about 40% deforestation, central, southern and eastern Amazonia would experience diminished rainfall and a lengthier dry season, predicting a shift to savanna vegeta tion to the east.