Deforestation refers to the removal of forest or stands of trees. The trees may be used for construction, or wood and paper, or they may simply be removed so that the land can be used for other purposes, including agricultural activities or urbanization. Reforestation is the growth of new forests, either without human influence by simply restricting human activity, or by the human re-planting of trees. Humans have engaged in deforestation for thousands of years, but industrialization over the last 300 years, and especially over the last 70 years, has led to extensive deforestation. In some parts of the world land use change has also led to substantial reforestation.
Deforestation in the Amazon Rainforest (left) and volunteers planting redwood trees in San Francisco (right). Credits: Flickr user Matt Zimmerman (left photo), National Park Service (right photo)
Deforestation and reforestation impact the Earth system in a variety of ways, including:
- The amount of greenhouse gases in the atmosphere. When trees grow they take carbon dioxide out of the atmosphere and transfer it into their wood, leaves, bark and roots. The carbon is returned to the atmosphere when downed trees are left to rot, or if the trees are burnt, which is a common means of deforestation. Fires also release nitrous oxide and airborne particles into the atmosphere, and reduce air quality. Thus, deforestation typically releases carbon dioxide, unless all the material is used for construction, or for paper products.
- The distribution of habitats, which affects species populations, ranges, biodiversity, and interactions of organisms that depend on forests for food and shelter. Deforestation often fragments ecosystems and can cause species extinctions, while reforestation can increase local biodiversity and species populations.
- Soil quality. Deforestation can destabilize soils, increase erosion, and reduce the nutrient levels in terrestrial ecosystems. This, in turn, can decrease agricultural productivity. Increasing erosion can decrease water quality by increasing sediment and pollutants in rivers and streams.
- How much and how fast water cycles through the biosphere and atmosphere. Trees return a significant proportion of precipitation to the atmosphere via a process called transpiration. Thus, deforestation decreases the amount of water cycling through the biosphere and atmosphere, while reforestation increase the amount of water cycling.
- The color of forests affects how much sunlight is reflected by the Earth’s surface. Dark colored forests absorb more sunlight than light colored agricultural fields, thus deforestation can have a cooling effect, but not enough to offset the warming effects of greenhouse gases in the atmosphere.
Can you think of additional cause and effect relationships between deforestation and reforestation and other parts of the Earth system?
Visit the agricultural activities, urbanization, and resource extraction pages to learn more about how processes and phenomena related to land use affect global climate and ecosystems.