We examine potential critical thresholds for biome shift in the Amazonian tropical forest by forcing a potential vegetation model with prescribed climate anomalies and projections from global and regional climate models and different levels of CO2 fertilization effect under the SRES A2 scenario (2070–2099). The results indicate that tropical forests might be replaced by seasonal forests or savanna over eastern Amazonia with temperature increases of 2–3°C (4–5°C), when CO2 fertilization effect is not considered (partially considered), depending on precipitation anomaly. A precipitation decrease greater than 30% would trigger the shift from tropical forest to drier biomes, such as savanna and shrubland in southeastern Amazonia. The projected decrease in precipitation during the dry season and the increase of temperature are the main mechanisms driving calculated biome changes. However, biome changes are considerably smaller when the optimum fertilization effect is included.