Tree planting is the process of transplanting tree seedlings, generally for forestry, land reclamation, or landscaping purposes.
Tree planting is carried out in many different parts of the world,Because trees remove carbon dioxide from the air as they grow, tree planting can be used as a geoengineering technique to remove CO2 from the atmosphere.
Government agencies, environmental organisations and private trusts carry out tree planting for conservation and climate change mitigation. Landcare Research use planted forests for their EBEX21 system for greenhouse gas emissions mitigations.
Climate scientists believe that human-induced global deforestation is responsible for 18-25% of global climate change. Trees sequester carbon through photosynthesis, converting carbon dioxide and water into molecular dioxygen (O2) and plant organic matter, such as carbohydrates. Hence, forests that grow in area or density and thus increase in organic biomass will reduce atmospheric CO2 levels.
Compared to less vegetated lands, forests affect climate in three main ways:
Cooling the Earth by functioning as carbon sinks, and adding water vapor to the atmosphere and thereby increasing cloudiness.
Warming the Earth by absorbing a high percentage of sunlight due to the low reflectivity of a forest's dark surfaces. This warming effect, or reduced albedo, is large where evergreen forests, which have very low reflectivity, shade snow cover, which is highly reflective.
To date, most tree planting offsets strategies have taken only the first effect into account. A study published in December 2005 combined all these effects and found that tropical forestation has a large net cooling effect, because of increased cloudiness and because of high tropical growth and carbon sequestration rates.
However, this study confirmed earlier findings that reforestation of colder regions probably results in global warming. According to Ken Caldeira, a study co-author from the Carnegie Institution for Science, "To plant forests outside of the tropics to mitigate climate change is a waste of time."
His premise that grassland reflects more sun, keeping temperatures lower, is, however, applicable only in arid regions. A well-watered lawn, for example, is as green as a tree, but absorbs far less CO2. Deciduous trees also have the advantage of providing shade in the summer and sunlight in the winter; so these trees, when planted close to houses, can be utilized to help increase energy efficiency of these houses.
However, the science linking trees and climate change is largely unsettled, and trees remain a controversial source of offsets.
The type of tree planted may have great influence on the environmental outcomes. Planting the wrong kind of trees, such as monocultures of eucalyptus where they are not native species, can devastate the lands of the local people. However, it is often much more profitable to outside interests to plant non-native fast-growing trees, such as eucalyptus or pine, even though the environmental and biodiversity benefits of such monoculture plantations are not comparable to native forest, and such offset projects are frequently objects of controversy.
To promote the growth of native ecosystems, many environmentalists advocate only indigenous trees be planted. A practical solution is to plant tough, fast-growing native tree species which begin rebuilding the land. Planting non-invasive trees that assist in the natural return of indigenous species is called "assisted natural regeneration."