1. Atmospheric Pollution
Deforestation is often cited as one of the major causes of the enhanced greenhouse effect. Trees and other plants remove carbon (in the form of carbon dioxide) from the atmosphere during the process of photosynthesis.
Both the decay and burning of wood releases much of this stored carbon back to the atmosphere deforestation also causes carbon stores held in soil to be released. Forests are stores of carbon and can be either sinks or sources depending upon environmental circumstances.
Mature forests can be net sinks of C02 (see Carbon dioxide sink and Carbon cycle). Deforestation caused by humans is estimated to contribute to one-third of all carbon dioxide. The water cycle is also affected by deforestation. Trees extract groundwater through their roots and release it into the atmosphere. When part of a forest is removed, the region cannot hold as much water and can result in a much drier climate.
Some forests are rich in biological diversity. Deforestation can cause the destruction of the habitats that support this biological diversity, thus causing contributing to the ongoing Holocene extinction event. Numerous countries have developed Biodiversity Action Plans to limit clear cutting and slash and burn agricultural practices as deleterious to wildlife and vegetation, particularly when endangered species are present.
3. Water Cycle and Water Resources
Trees and plants in general, affect the water cycle significantly;
(i) Their canopies intercept a proportion of precipitation, which is then evaporated back to the atmosphere (canopy interception);
(ii) Their litter stems and trunks slow down surface runoff.
(iii) Their roots create macropores – large conduits – in the soil that increase infiltration of water.
(iv) They contribute to terrestrial evaporation and reduce soil moisture via transpiration.
(v) Their litter and other organic residue change soil properties that affect the capacity of soil to store water.
As a result, the presence or absence of trees can change the quantity of water on the surface, in the soil or groundwater, or in the atmosphere. This in turn changes erosion rates and the availability of water for either ecosystem functions or human services.
The forest may have little impact on flooding in the case of large rainfall events, which overwhelm the storage capacity of forest soil if the soils are at or close to saturation.
4. Soil Erosion
Undisturbed forest has very low rates of soil loss, approximately 0.02 metric tons per hectare. Deforestation generally increases rates of soil erosion, by increasing the amount of runoff and reducing the protection of the soil from tree litter.
This can be an advantage in excessively leached tropical rain forest soils. Forestry operations themselves also increase erosion through the development of roads and the use of mechanized equipment.
China’s Loess Plateau was cleared of forest millennia ago. Since then it has been eroding, creating dramatic incised valleys, and providing the sediment that gives the Yellow River its yellow colour and that causes the flooding of the river in the lower reaches (hence the river’s nickname ‘China’s sorrow’).
Removal of trees does not always increase erosion rates. In certain regions of southwest US, shrubs and trees have been encroaching on grassland. The trees themselves enhance the loss of grass between tree canopies. The bare intercanopy areas become highly erodible. The US Forest Service, in Bandelier National Monument for example, is studying how to restore the former ecosystem, and reduce erosion, by removing the trees.
Tree roots bind soil together, and if the soil is sufficiently shallow they act to keep the soil in place by also binding with underlying bedrock. Tree removal on steep slopes with shallow soil thus increases the risk of landslides, which can threaten people living nearby. However most deforestation only affects the trunks of trees, allowing for the roots to stay rooted, negating the landslide.