In some usages, the definition of a tree may be narrower, including only woody plants, only plants that are usable as lumber or only plants above a specified height. At its broadest, trees include the taller palms, the tree ferns, bananas and bamboo.
A tree typically has many secondary branches supported clear of the ground by the trunk. This trunk typically contains woody tissue for strength, and vascular tissue to carry materials from one part of the tree to another. For most trees it is surrounded by a layer of bark which serves as a protective barrier. Trees play a significant role in reducing erosion and moderating the climate. They remove carbon dioxide from the atmosphere and store large quantities of carbon in their tissues.
Flowers and fruit may also be present, but some trees such as conifers instead have pollen cones and seed cones, and others such as tree ferns produce spores instead. Trees have been in existence on the Earth for 370 million years. Trees are not a taxonomic group but are a number of plant species that have independently evolved a woody trunk and branches as a way to tower above other plants and make full use of the sunlight. Trees and forests provide a habitat for many species of animals and plants.
In its broadest sense, a tree is any plant with the general form of an elongated stem, or trunk, which supports the photosynthetic leaves or branches at some distance above the ground. Trees are also typically defined by height, with smaller plants being classified as shrubs, however the minimum height which defines a tree varies widely, from 10 m to 0.5 m. By these broadest definitions, large herbaceous plants such as papaya and bananas are trees, despite not being considered as trees under more rigorous definitions.
Another criterion often added to the definition of a tree is that it has a woody trunk. Such a definition excludes herbaceous trees such as bananas and papayas. Monocots such as bamboo and palms may be considered trees under such a definition. Despite being herbaceous and not undergoing secondary growth and never producing wood, palms and bamboo may produce “pseudo-wood” by lignifying cells produced through primary growth.
Trees exist in two different groups of vascular or higher plants, the gymnosperms and the angiosperms. Both groups are seed plants. The gymnosperm trees include conifers, cycads, ginkgophytes and gnetales. Angiosperm trees are also known as broad-leaved trees. Most angiosperm trees are eudicots, the “true dicotyledons”, so named because the seeds contain two cotyledons or seed leaves. A relatively smaller number of other angiosperm trees are paleodicots; these include Amborella, Magnolia, nutmeg, avocado, and others.
Wood gives structural strength to a tree stem which is used to support the plant as it grows larger. The vascular system of trees allows water, nutrients and other chemicals to be distributed around the plant, and without it trees would not be able to grow as large as they do. The three main parts of trees include the root, stem, and leaves; they are integral parts of the vascular system which interconnects all the living cells. In trees and other plants that develop wood, the vascular cambium allows the expansion of vascular tissue that produces woody growth.
In places where the climate is suitable, trees are the climax vegetation. In some of the cool temperate regions, conifers tend to predominate, but in much of the southern hemisphere, the tropics, or in warm-temperate climates, broad-leaved trees are more common. Shade tolerance in young trees varies between species, and may determine the pattern of forest succession.
More than half the species of terrestrial plants and animals on the Earth are thought to live in tropical rainforests even though these occupy just five percent of the land area. In tropical regions with a monsoon climate, where a drier part of the year alternates with a wet period, different species of broad-leaved trees dominate the forest, some of them being deciduous.