We define afforestation as both reestablishing forests on land that has been without forest cover for some time and the establishment of forest on land that has not previously been forested (note that some entities involved in carbon markets and reporting use different definitions for this term). Afforestation can remove substantial CO2 from the atmosphere. Between 1850 and 2000, global land-use change resulted in the release of 156,000 teragrams (1 teragram = 1 billion kilograms) of carbon to the atmosphere, mostly from deforestation. This amount is equivalent to 21.9 years of global fossil fuel CO2 emissions at the 2003 level.
The rate of carbon storage in tree growth varies with species, climate, and management, ranging widely from about 3 to 20 megagrams (1 megagram = 1,000 kilograms) per hectare per year. In the continental United States, the highest potential growth rates are found in the Pacific Northwest, the Southeast, and the south central United States. Much land currently in pasture and agricultural use in the eastern United States and in the Great Lake states will naturally revert to forests if left fallow, while reestablishing forests in many western forests requires tree planting.
The benefits of afforestation are enhanced where forests include a substantial proportion of native species. Planting native species or allowing natural succession to re-create the forest that historically occupied the site will yield the greatest benefits for species diversity and wildlife habitat and the lowest risk for unintended consequences. Because native species often grow more slowly than exotics or trees selected for improved growth, restoration of the historical ecosystem may yield lower carbon accumulation rates than other forest reestablishment practices. Planting monocultures of non-native or native improved-growth species on historical forest land will likely yield greater carbon accumulation rates but fewer benefits in terms of biodiversity.
Afforestation can have negative consequences, too. Planting forests where they were not present historically can have drawbacks, such as lower species diversity (if trees are planted in native grassland), changes in water table, and a higher energy absorption compared to the native ecosystem. In addition, afforestation generally reduces stream flow regardless of the ecosystem type, because trees use more water than grass or crops. Conversion of agricultural or grazing lands to forest reduces revenue from agricultural products. If afforestation efforts include the addition of nitrogen fertilizer, emissions of nitrous oxide (a greenhouse gas roughly 300 times as powerful as CO2) will increase.
Other srategies include:
Related to Increasing Afforestation to Increase Carbon Storage: