Forest - Definition and Types

On a global scale, forests are shaped by the amount of solar radiation and precipitation, both of which are influenced by latitude. These climatic conditions determine what organisms can survive in an area and have helped shape the evolution of forests for millions of years. Based on latitude, there are three types of forests: boreal, temperate, and tropical.

Boreal forests, found farthest north, experience long, cold winters with short growing seasons. Temperate forests, located in the mid-latitudes, have four distinct seasons. Tropical forests, found along the equator, experience high temperatures, long growing seasons, and harbor incredible amounts of biodiversity. 

Forests support humans on local and regional scales by providing ecosystem services like pollination, climate regulation, and soil conservation.1 Despite the value of intact forests for human well-being, forests around the world are threatened by human activities, according to the Food and Agriculture Organization of the United Nations (FAO).

Definition

What Is a Forest?

A forest is an ecosystem dominated by trees. According to the parameters established by the FAO, an area must cover at least half a hectare, or about one and a quarter acres, to be considered a forest. The trees in the area must also be able to grow to heights above 16 feet and have a canopy that covers at least 10% of the sky. 

Despite the precise definition laid out by the FAO, there is still controversy over what constitutes a forest. One issue with the organization's interpretation is that it does not differentiate between natural and planted forests. According to a study by leading forest ecologists published in the journal Ambio, because the current forest definition does not distinguish between forest types, it can be difficult to monitor changes in the amount of forest.

1. Boreal forest

Boreal forests, or taiga, are found between 50 and 60 degrees latitude in North America, Asia, and Europe. Beneath boreal forests is land shaped by glaciers that left a legacy in the geology, hydrology, and soils of the area. Boreal forests’ bitter cold climate makes it difficult for life, leading to low species diversity compared to temperate and tropical forests. 

The plants and animals that do live in boreal forests are specially adapted to cope with short growing seasons and cold temperatures. Due to their vastness and remoteness, boreal forests are important storers of carbon.

Of the three forest types, boreal forests have the shortest growing season, about 130 days.3 Boreal forests tend to have shallow, acidic, nutrient-poor soils. Conifers are the most abundant type of tree, although there are some well-adapted deciduous trees, such as willows, poplars, and alders, as well. Prominent species include black and white fir, jackpine, balsam fir, and tamarack. In the understory, blueberry and cranberry bushes provide high-energy food for wildlife. 

Types of Boreal Forest

Open Canopy Boreal: Also known as lichen woodland, open canopy boreal forests occur at higher latitudes and have lower species diversity.

Closed Canopy Boreal: Found at lower latitudes, closed canopy boreal forests have richer soil and denser tree stands that allow little light to reach the forest floor. Less harsh conditions, however, lead to greater species diversity.

2. Temperate forest

Temperate forests are located at mid-latitudes, which gives them their characteristic four seasons. Very few patches of old-growth temperate forest remain; the zone is dominated by secondary forests.8 As of 2020, temperate forests accounted for 16% of the Earth’s total forest cover.

Temperate forests are inhabited by species adapted for seasonality. Deciduous trees like maples, hickories, oaks, and many others drop their leaves and become dormant in the fall and winter to save energy. Bears, bobcats, squirrels, and deer make their homes in temperate forests and can store food, adapt their diet, or hibernate to cope with the lack of nutritious foods in the winter. 

Although temperate forests have seasonality in common, they vary widely in yearly precipitation and temperature. Annual temperatures range from -22 F to 86 F depending on location and season. Temperate forests receive an average of 30 to 59 inches of rain per year.9 Soils are generally fertile, with a thick layer of organic matter from which plants can extract nutrients to grow. 

Temperate forests are home to many endangered species. In the U.S., 12 mammal species listed as Endangered by the Fish and Wildlife Service live in temperate forest habitats. The red wolf, native to the temperate forests of eastern North Carolina, is listed as Critically Endangered by the IUCN. The northern spotted owl was federally listed as Endangered in 1990 and is currently considered Threatened.10 These birds of prey prefer the old-growth forest habitat of Washington, Oregon, and California, which has continued to decline in recent decades. 

Types of Temperate Forest

Deciduous Forest: This forest type is dominated by deciduous trees, which lose their leaves during colder months and enter a period of dormancy. 

Coniferous Forest: This biome has a higher proportion of evergreen, cone-producing trees. 

Temperate Rainforest: With moderate temperatures, these forests report extremely high amounts of precipitation — 140 to 167 inches per year.

3. Tropical forest

Located between the Tropics of Cancer and Capricorn at 23 degrees north and south, tropical forests are some of the most biodiverse ecosystems on Earth. These forests cover only a tenth of the surface of the planet, yet harbor half of all species. They are also some of the most threatened by human activities.

Tropical forests have relatively stable conditions that have allowed life to thrive. They are the warmest and rainiest forests on Earth, with temperatures ranging between 68 F and 77 F, with 79 to 394 inches of rain annually.

Tropical forests are known for their extraordinary biodiversity. The Amazon rainforest, for example, is home to 10% of the world's described species.

The diversity of tropical forests makes them very efficient at processing nutrients. Dead and decaying matter is quickly broken down by decomposers and almost instantly taken up by another organism. This makes tropical forest soils nutrient-poor. To cope with poor soils, many tropical trees have adapted shallow root systems that spread across the forest floor and can more easily take in nutrients.

Many charismatic tropical forest species are threatened with extinction. For example, the African forest elephant, found in West and Central Africa, is listed as Critically Endangered by the IUCN due to habitat loss and poaching. Primates live almost exclusively in the tropics, and most live in tropical forests. In some Brazilian forests, as many as 13 kinds of primates live in the same area.

Human activities such as logging, land clearing for agriculture, and poaching are a threat to the future of tropical forests. In 2020 alone, over 12 million hectares of tropical forests were lost, according to the World Resources Institute.

Types of Tropical Forest

Evergreen Rainforest: Often thought of as “real” rainforest, these are the wettest (~80 inches of rain per year) and most biodiverse tropical forests. 

Tropical Moist Forest: Further from the equator than evergreen rainforests, tropical moist forests experience less rainfall overall and bigger differences between seasons.

Tropical Dry Forest: Receive very little rain between four and six months out of the year. Plants and animals have specific adaptations to deal with this period of water scarcity. 

Mangrove: Coastal tropical forests with trees adapted to live in brackish water with changing levels. Mangroves protect the coast from storms and act as nurseries for aquatic species.

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