Trees are an important component of the biosphere. They dominate many regions of the world as natural forests or plantations. They are not only important economically through the multiple uses of wood, but also for climate change mitigation because they capture and store carbodioxide, one of the major greenhouse gases. They also play an important role in the regulation of the hydrological cycle. Recent studies on tree mortality and climate change show that some of the world's forest ecosystems may already be responding with increasing tree mortality rates in response to climatic warming and drought, even in ecosystems that are not normally considered water-limited.