Laurance et al. (2004) report observations of changes in Amazonian rainforests in regions not impacted by deforestation or other human disturbance. Rates of tree recruitment, mortality and growth have changed over the last two decades. These changes may impact the ability of Amazonia to serve as a major carbon sink for elevated levels of carbon dioxide. Causes of these changes may be the higher atmospheric levels of CO2 (which favor some species for enhanced growth) or other accompanying environmental conditions.
Reference
Laurance, W.F., et al., 2004: Pervasive alteration of tree communities in undisturbed Amazonian forests. Nature, 428, 171-175.