A forest is a large area dominated by trees.
Forests are the dominant terrestrial ecosystem of Earth, and are distributed across the globe.
Forests account for 75% of the gross primary productivity of the Earth's biosphere, and contain 80% of the Earth's plant biomass.
Etymology
The word forest comes from Middle English, from Old French forest "forest, vast expanse covered by trees";
first introduced in English as the word for wild land set aside for hunting without the necessity in definition for the existence of trees.
Forests
Forests can be classified in different ways and to different degrees of specificity.
One such way is in terms of the biome in which they exist, combined with leaf longevity of the dominant species (whether they are evergreen or deciduous).
Another distinction is whether the forests are composed predominantly of broadleaf trees, coniferous (needle-leaved) trees, or mixed.
Boreal forests occupy the subarctic zone and are generally evergreen and coniferous.
Temperate zones support both broadleaf deciduous forests (e.g., temperate deciduous forest) and evergreen coniferous forests (e.g., temperate coniferous forests and temperate rainforests).
Warm temperate zones support broadleaf evergreen forests, including laurel forests.
Tropical and subtropical forests include tropical and subtropical moist forests, tropical and subtropical dry forests, and tropical and subtropical coniferous forests.
Physiognomy classifies forests based on their overall physical structure or developmental stage (e.g. old growth vs. second growth).
Forests can also be classified more specifically based on the climate and the dominant tree species present, resulting in numerous different forest types (e.g., Ponderosa pine/Douglas-fir forest).
Evolution
The first known forests on Earth arose in the Late Devonian (approximately 380 million years ago), with the evolution of Archaeopteris.
Archaeopteris was a plant that was both tree-like and fern-like, growing to 10 metres (33 ft) in height.
Archaeopteris quickly spread throughout the world, from the equator to subpolar latitudes.
Archaeopteris formed the first forest by being the first known species to cast shade due to its fronds and forming soil from its roots.
Archaeopteris was deciduous, dropping its fronds onto the forest floor.
The shade, soil, and forest duff from the dropped fronds created the first forest.
The shed organic matter altered the freshwater environment, slowing it down and providing food. This promoted freshwater fish.
Components
A forest consists of many components that can be broadly divided into two categories that are biotic (living) and abiotic (non-living) components. The living parts include trees,
shrubs, vines, grasses and other herbaceous (non-woody) plants, mosses, algae, fungi, insects, mammals, birds, reptiles, amphibians,
and microorganisms living on the plants and animals and in the soil.
Layers
A forest is made up of many layers. Starting from the ground level and moving up, the main layers of all forest types are the forest floor, the understory and the canopy. The emergent layer exists in tropical rainforests. Each layer has a different set of plants and animals depending upon the availability of sunlight, moisture and food.
Forest floor contains decomposing leaves, animal droppings, and dead trees. Decay on the forest floor forms new soil and provides nutrients to the plants. The forest floor supports ferns, grasses, mushroom and tree seedlings.
Understory is made up of bushes, shrubs, and young trees that are adapted to living in the shades of the canopy.
Canopy is formed by the mass of intertwined branches, twigs and leaves of the mature trees. The crowns of the dominant trees receive most of the sunlight. This is the most productive part of the trees where maximum food is produced. The canopy forms a shady, protective "umbrella" over the rest of the forest.
Emergent layer exists in the tropical rain forest and is composed of a few scattered trees that tower over the canopy.
