The Tree reference article from the English Wikipedia on 24-Apr-2004
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Tree

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This article is about the biological organisms known as trees. For other meanings of the word see tree (disambiguation).

Image:Baobab.jpg
Baobab tree in South-Africa

Table of contents
1 Trees
2 Morphology
3 Major tree genera
4 Life stages
5 Culture
6 See also

Trees

A tree can be defined as a large woody perennial plant. Though there is no set definition of size, it is generally at least 4.5 m (15 ft) high at maturity, and with branches supported on a single main stem. Trees are important components of the natural landscape and significant elements in landscaping. Compared with most other forms of plants, trees are long-lived. A few species of trees grow to over 100 m (300 ft) tall and some live for several millennia.

Morphology

The component parts of a tree are the roots, trunk(s), branches, twigs and leaves. Tree stems consist mainly of support and transport tissues (xylem and phloem). In fact, wood consists of xylem cells, and the bark is primarily made of phloem. As a tree grows, it creates growth rings, which can be counted in temperate climates to determine the age of the tree, and used to date cores or even lumber taken from trees in the past, using the science of dendrochronology. The roots of a tree are generally embedded in earth, providing anchorage for the above-ground biomass and absorbing water and nutrients from the soil. Above ground, the trunk gives height to the leaf-bearing branches, aiding in competition with other plant species for sunlight. In many trees the arrangement of the branches optimize exposure of the leaves to sunlight.

Not all trees have the plant organs mentioned above. For examples: most palmss are not branched, the saguaro cactus of North America has no functional leaves, tree ferns do not have bark, etc. Based on their rough shape and size, all of these are nonetheless generally regarded as trees. Indeed, sometimes size is the most important consideration. A plant form that is similar to a tree, but generally having smaller, multiple trunks and/or branches that arise near the ground, is called a shrub. However, no sharp differentiation between shrubs and trees is possible. Given their small size, Bonsai plants would not technically be 'trees', but one should not confuse reference to the form of a species with the size or shape of individual specimens. A spruce seedling does not fit the definition of a tree, but all spruces are trees. Bamboos by contrast, do show most of the characteristics of trees, yet are perhaps strangely rarely called trees.

A small group of trees growing together is called a grove or copse, and a landscape covered by a large area of trees is called a forest. Several biotopes are defined largely by the trees that inhabit them, for example, rainforest and taiga; see ecozones. Large, but scattered trees with grassland (usually grazed or burned over periodically) in between is called savanna.

Major tree genera

Trees occur in many diverse families of plants, and thus show a wide variety of leaf types and shapes, bark, flowers, fruit, etc. The earliest trees were probably tree ferns, which grew in vast forests. Later the conifers, ginkgos, cycads and other gymnosperms appeared. Most species of trees today are flowering plants and conifers. The list below gives some examples of well known trees and how they are typically classified.

Flowering plants (Magnoliophyta)

Dicotyledons (Magnoliopsida)

Image:Davidia1.jpg
Nyssaceae: a Dove tree in flower

Monocotyledons (Liliopsida)

Conifers

Public domain photo from www.nps.gov/redw/
Coast redwood, the tallest tree species on earth; a Conifer

Ginkgos

Cycads

Ferns

Life stages

The life cycles of trees, especially conifers, are divided into the following stages in
forestry for survey and documentation purposes:
  1. Seed
  2. Seedling: the above ground part of the embryo that sprout from the seed
  3. Sapling: After the seedling reaches 1m tall, and until it reaches 7cm in stem diameter
  4. Pole: young trees from 7-30cm diameter
  5. Mature tree: over 30cm diameter, reproductive years begin
  6. Old tree: dominate old growth forest; height growth slows greatly, with majority of productivity in seed production
  7. Overmature: dieback and decay become common
  8. Snag: standing dead wood
  9. Log/debris: fallen dead wood

Tree diameters are measured at height of between 1.3-1.5m above the highest point on the ground at its base. The 7cm diameter definition is economically based, from the smallest saleable stem size (for paper production, etc), and the 30cm diameter is the smallest base diameter for sawlogs. Each stage may be uniquely perceptive to different pathogens and suitable for especially adapted arboreal animals.

Culture

Trees often serve as important symbols in mythologies and religions. Examples are Yggdrasil in the Norse Mythology, the Christmas Tree that is derived from Germanicic mythology, the Tree of Knowledge of Judaism and Christianity, and the Bodhi tree in Buddhism. In some religions, such as Hinduism, trees are said to be the homes of tree spirits.

See also


Tree is the digit 3 in the NATO phonetic alphabet. It is pronounced as such, to prevent possible accidental confusion with other digits.