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Tree

Source: Wikipedia, the free encyclopedia.

angiosperm
) tree

In

angiosperms or hardwoods; of the rest, many are gymnosperms
or softwoods. Trees tend to be long-lived, some reaching several thousand years old. Trees have been in existence for 370 million years. It is estimated that there are some three trillion mature trees in the world.

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. Below the ground, the roots branch and spread out widely; they serve to anchor the tree and extract moisture and nutrients from the soil. Above ground, the branches divide into smaller branches and shoots. The shoots typically bear leaves, which capture light energy and convert it into sugars by photosynthesis
, providing the food for the tree's growth and development.

Trees usually reproduce using seeds. Flowers and fruit may be present, but some trees, such as conifers, instead have pollen cones and seed cones. Palms, bananas, and bamboos also produce seeds, but tree ferns produce spores instead.

Trees play a significant role in reducing

mythologies
.

Definition

coniferous
tree showing idealised vertical and horizontal sections. A new layer of wood is added in each growing season, thickening the stem, existing branches and roots.

Although "tree" is a term of common parlance, there is no universally recognised precise definition of what a tree is, either

herbaceous plants such as papaya and bananas are trees in this broad sense.[2][6]

A commonly applied narrower definition is that a tree has a woody trunk formed by

growing tip.[4][7] Under such a definition, herbaceous plants such as palms, bananas and papayas are not considered trees regardless of their height, growth form or stem girth. Certain monocots may be considered trees under a slightly looser definition;[8] while the Joshua tree, bamboos and palms do not have secondary growth and never produce true wood with growth rings,[9][10] they may produce "pseudo-wood" by lignifying cells formed by primary growth.[11] Tree species in the genus Dracaena, despite also being monocots, do have secondary growth caused by meristem in their trunk, but it is different from the thickening meristem found in dicotyledonous trees.[12]

Aside from structural definitions, trees are commonly defined by use; for instance, as those plants which yield lumber.[13]

Overview

The tree growth habit is an

botanists, making tree diversity and ranges poorly known.[21]

The majority of tree species are

monocots
.

Wood gives structural strength to the trunk of most types of tree; this supports 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. Trees, as relatively tall plants, need to draw water up the stem through the xylem from the roots by the suction produced as water evaporates from the leaves. If insufficient water is available the leaves will die.[25] 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. Because this growth ruptures the epidermis of the stem, woody plants also have a cork cambium that develops among the phloem. The cork cambium gives rise to thickened cork cells to protect the surface of the plant and reduce water loss. Both the production of wood and the production of cork are forms of secondary growth.[26]

Trees are either

Larix and Pseudolarix) are deciduous, dropping their needles each autumn, and some species of cypress (Glyptostrobus, Metasequoia and Taxodium) shed small leafy shoots annually in a process known as cladoptosis.[5] The crown is the spreading top of a tree including the branches and leaves,[29] while the uppermost layer in a forest, formed by the crowns of the trees, is known as the canopy.[30] A sapling is a young tree.[31]

Many tall palms are herbaceous

adventitious roots.[33]

Distribution

The number of trees in the world, according to a 2015 estimate, is 3.04 trillion, of which 1.39 trillion (46%) are in the tropics or sub-tropics, 0.61 trillion (20%) in the temperate zones, and 0.74 trillion (24%) in the coniferous boreal forests. The estimate is about eight times higher than previous estimates, and is based on tree densities measured on over 400,000 plots. It remains subject to a wide margin of error, not least because the samples are mainly from Europe and North America. The estimate suggests that about 15 billion trees are cut down annually and about 5 billion are planted. In the 12,000 years since the start of human agriculture, the number of trees worldwide has decreased by 46%.[34][35][36][37]

In suitable environments, such as the

Ulva Island, New Zealand, forest is the more-or-less stable climatic climax community at the end of a plant succession, where open areas such as grassland are colonised by taller plants, which in turn give way to trees that eventually form a forest canopy.[38][39]

Conifers in the Swabian alps

In

cool temperate regions, conifers often predominate; a widely distributed climax community in the far north of the northern hemisphere is moist taiga or northern coniferous forest (also called boreal forest).[40][41] Taiga is the world's largest land biome, forming 29% of the world's forest cover.[42] The long cold winter of the far north is unsuitable for plant growth and trees must grow rapidly in the short summer season when the temperature rises and the days are long. Light is very limited under their dense cover and there may be little plant life on the forest floor, although fungi may abound.[43] Similar woodland is found on mountains where the altitude causes the average temperature to be lower thus reducing the length of the growing season.[44]

Where rainfall is relatively evenly spread across the seasons in temperate regions,

temperate broadleaf and mixed forest typified by species like oak, beech, birch and maple is found.[45] Temperate forest is also found in the southern hemisphere, as for example in the Eastern Australia temperate forest, characterised by Eucalyptus forest and open acacia woodland.[46]

In tropical regions with a

baobab are well adapted to living in such areas.[48]

Parts and function

Roots

red pine
(Pinus resinosa) with spread of roots visible, as a result of soil erosion