A change in global climate would be accompanied by shifts in climatic zones, thereby altering the suitability of a region for the growth of distinctive species. Trees in particular have long reproductive cycles, and many species may not be able to respond to the climatic changes quickly enough. A shift in climatic zones not only affects the vegetation but also affects the incidence of tree pests such as insects and diseases. These pests have less difficulty in migrating with their climatic zones than vegetation and may damage tree species with lower immunity.
As well as the effects of temperature and precipitation variations, and changes to weather patterns, forest growth may also respond to increased atmospheric concentrations of carbon dioxide. Studies with immature forest plantations suggest that an increase in atmospheric carbon dioxide would be beneficial to tree growth. The elevated carbon dioxide concentrations enhance photosynthesis rates with increased utilisation of carbon dioxide. This is called the carbon fertilisation effect. As a consequence of carbon fertilisation, water use efficiency may also increase. Increase in growth rates however, would vary enormously within ecosystems and between species. In general, it is expected that the negative impacts of climate change on forests will have a greater impact than any positive effect due to an increase in growth rates as a result of elevated atmospheric carbon dioxide concentrations. With unmitigated emissions of greenhouse gases, substantial dieback of tropical forests and tropical grasslands is predicted to occur by the 2080s, especially in northern South America and central southern Africa. If emissions are reduced enabling atmospheric carbon dioxide concentrations to stabilise at 550 ppm (double the pre-industrial level), this loss would be substantially reduced, even by the 2230s. Considerable growth of forests is predicted to occur in North America, northern Asia and China.
As well as the effects on forests themselves, climate change is expected to influence societies and economies dependent upon forestry. Forest products make up the third most valuable international commodity after oil and gas. Trade is expected to increase in the 21st century along with demand, particularly in the developing countries. Global warming may well affect the development of such developing economies, particularly if current rates of deforestation remain unchecked and the unsustainable management of forests continues. As a consequence, societies dependent upon the income, food and shelter their forests provide them may well face increasing stresses due to crop failures, soil nutrient depletion and the effects of extreme weather events in the years to come.