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Tree

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The coniferous Coast Redwood, the tallest tree species on earth.

A tree is a large, perennial, woody plant. Though there is no set definition regarding minimum size, the term generally applies to plants at least 6 m (20 ft) high at maturity and having secondary branches supported on a main stem or stems (see shrub for comparison). Most trees exhibit clear apical dominance, though this is not always the case (Mitchell, 1978).[1] Compared with most other plant forms, trees are long-lived. A few species of trees grow to 115 m (375 ft) tall and some can live for several thousand years.

Trees are an important component of the natural landscape due to their prevention of erosion and significant elements in landscaping and agriculture, both for their aesthetic appeal and their orchard crops (such as apples). Wood from trees is a common building material. Trees also play an intimate role in many of the world's mythologies (see trees in mythology). Trees have also been found to play an important role in producing oxygen and reducing carbon dioxide in the atmosphere, as well as moderating ground temperatures and increasing albedo. These traits could potentially help to alleviate Global warming.

Classification

 
An oak tree in Denmark

A tree is a plant form that occurs in many different orders and families of plants. Trees show a wide variety of growth forms, leaf type and shape, bark characteristics, and reproductive organs.

The earliest trees were tree ferns and horsetails, which grew in vast forests in the Carboniferous Period; tree ferns still survive, but the only surviving horsetails are not of tree form. Later, in the Triassic Period, conifers, ginkgos, cycads and other gymnosperms appeared, and subsequently flowering plants in the Cretaceous Period. Most species of trees today are flowering plants (Angiosperms)and conifers. The listing below gives examples of many well-known trees and how they are typically classified.

A small group of trees growing together is called a grove or copse, and a landscape covered by a dense growth of trees is called a forest. Several biotopes are defined largely by the trees that inhabit them; examples are rainforest and taiga (see ecozones). A landscape of trees scattered or spaced across grassland (usually grazed or burned over periodically) is called a savanna.

Morphology

 
Leaves are an important feature of trees. These are Beech leaves.
 
Tree roots anchor the structure and provide water and nutrients. The ground has eroded away around the roots of this young Pine tree.

The basic 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). Wood consists of xylem cells, and bark is made of phloem and other tissues external to the vascular cambium.

Trees may be broadly grouped into exogenous and endogenous trees according to the way in which their stem diameter increases. Exogenous trees, which comprise the great majority of modern trees (all conifers, and all broadleaf trees), grow by the addition of new wood outwards, immediately under the bark. Endogenous trees, mainly in the monocotyledons (e.g. palms), grow by addition of new material inwards.

As an exogenous tree grows, it creates growth rings. In temperate climates, these are commonly visible due to changes in the rate of growth with temperature variation over an annual cycle. These rings can be counted to determine the age of the tree, and used to date cores or even wood taken from trees in the past; this practice is known as the science of dendrochronology. In some tropical regions with constant year-round climate, growth is continuous and distinct rings are not formed, so age determination is impossible. Age determination is also impossible in endogenous trees.

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. It should be noted, however, that while ground nutrients are essential to a tree's growth the majority of its biomass comes from carbon dioxide absorbed from the atmosphere (see photosynthesis). 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 optimizes exposure of the leaves to sunlight.

All trees have all the plant organs or parts mentioned above. For example, most palm trees are not branched, the saguaro cactus of North America has no functional leaves, tree ferns do not produce bark, etc. Based on their general shape and size, all of these are nonetheless generally regarded as trees. Indeed, sometimes size is the more 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.

Life stages

 
A moss-covered tree.

The life cycles of trees, especially conifers, are divided into the following stages in forestry for survey and documentation purposes, though the boundaries between stages vary among foresters:

  1. Seed
  2. Seedling: the above-ground part of the embryo that sprouts from the seed
  3. Sapling: After the seedling reaches 1 m tall, and until it reaches 7 cm in stem diameter
  4. Pole: Trees from 7–30 cm diameter
  5. Mature tree: over 30 cm 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 1.3–1.5 m above the highest point on the ground at its base (known as "breast height"). The 7 cm diameter definition is economically based, from the smallest merchantable stem size (for paper production, etc). Each stage may be uniquely susceptible to different pathogens and suitable for especially adapted arboreal organisms.

Champion trees

The world's champion trees can be considered on several factors; height, trunk diameter or girth, total size, and age. It is significant that in each case, the top position is always held by a conifer, though a different species in each case; in most measures, the second to fourth places are also held by conifers.

Tallest trees

The heights of the tallest trees in the world have been the subject of considerable dispute and much (often wild) exaggeration. Modern verified measurement with laser rangefinders combined with tape drop measurements made by tree climbers, carried out by the U.S. Eastern Native Tree Society has shown that most older measuring methods and measurements are unreliable, often producing exaggerations of 5% to 15% above the real height. Historical claims of trees of 117 m, 130 m, and even 150 m, are now largely disregarded as unreliable, fantasy or outright fraud. The following are now accepted as the top five tallest reliably measured species:

  1. Coast Redwood Sequoia sempervirens: 115.55 m (379.1 ft.), Redwood National Park, California (Gymnosperm Database)
  2. Coast Douglas-fir Pseudotsuga menziesii: 100.3 m (329.1 ft.), Brummit Creek, Coos County, Oregon (Gymnosperm Database)
  3. Australian Mountain-ash Eucalyptus regnans: 97.0 m (318.2 ft.), Styx Valley, Tasmania (Tasmanian Giant Trees)
  4. Sitka Spruce Picea sitchensis: 96.7 m (317.3 ft.), Prairie Creek Redwoods State Park, California (Gymnosperm Database)
  5. Giant Sequoia Sequoiadendron giganteum: 94.9 m (307.1 ft.), Redwood Mountain Grove, Kings Canyon National Park, California (Gymnosperm Database)
 
A view of a tree from below; this may exaggerate apparent height

Stoutest trees

The girth (circumference) of a tree is – or at least should be – much easier to measure than the height, as it is a simple matter of stretching a tape round the trunk, and pulling it taut to find the circumference. Despite this, UK tree author Alan Mitchell made the following comment about measurements of yew trees in the British Isles:

"The aberrations of past measurements of yews are beyond belief. For example, the tree at Tisbury has a well-defined, clean, if irregular bole at least 1.5 m long. It has been found to have a girth which has dilated and shrunk in the following way: 11.28 m (1834 Loudon), 9.3 m (1892 Lowe), 10.67 m (1903 Elwes and Henry), 9.0 m (1924 E. Swanton), 9.45 m (1959 Mitchell) .... Earlier measurements have therefore been omitted".

As a general standard, tree girth is taken at 'breast height'; this is defined differently in different situations, with most foresters measuring girth at 1.3 m above ground, while ornamental tree measurers usually measure at 1.5 m above ground; in most cases this makes little difference to the measured girth. On sloping ground, the "above ground" reference point is usually taken as the highest point on the ground touching the trunk, but some use the average between the highest and lowest points of ground. Some of the inflated old measurements may have been taken at ground level. Some past exaggerated measurements also result from measuring the complete next-to-bark measurement, pushing the tape in and out over every crevice and buttress.

Modern trends are to cite the tree's diameter rather than the circumference; this is obtained by dividing the measured circumference by π; it assumes the trunk is circular in cross-section (an oval or irregular cross-section would result in a mean diameter slightly greater than the assumed circle). This is cited as dbh (diameter at breast height) in tree literature.

A further problem with measuring baobabs Adansonia is that these trees store large amounts of water in the very soft wood in their trunks. This leads to marked variation in their girth over the year, swelling to a maximum at the end of the rainy season, minimum at the end of the dry season. Although baobabs have some of the highest girth measurements of any trees, no accurate measurements are currently available, but probably do not exceed 10-11 m diameter.

The stoutest species in diameter, excluding baobabs, are:

  1. Montezuma Cypress Taxodium mucronatum: 11.42 m, Árbol del Tule, Santa Maria del Tule, Oaxaca, Mexico (A. F. Mitchell, International Dendrology Society Year Book 1983: 93, 1984).
  2. Giant Sequoia Sequoiadendron giganteum: 8.85 m, General Grant tree, Grant Grove, California (Gymnosperm Database)
  3. Coast Redwood Sequoia sempervirens: 7.44 m, Prairie Creek Redwoods State Park, California (Gymnosperm Database)

Largest trees

The largest trees in total volume are those which are both tall and of large diameter, and in particular, which hold a large diameter high up the trunk. Measurement is very complex, particularly if branch volume is to be included as well as the trunk volume, so measurements have only been made for a small number of trees, and generally only for the trunk. No attempt has ever been made to include root volume.

The top four species measured so far are (Gymnosperm Database):

  1. Giant Sequoia Sequoiadendron giganteum: 1489 m³, General Sherman
  2. Coast Redwood Sequoia sempervirens: 1045 m³, Del Norte Titan tree
  3. Western Redcedar Thuja plicata: 500 m³, Quinault Lake Redcedar
  4. Kauri Agathis australis: 400 m³, Tane Mahuta tree (total volume, including branches, 516.7 m³)

However, the Alerce Fitzroya cupressoides, as yet un-measured, may well slot in at third or fourth place, and Montezuma Cypress Taxodium mucronatum is also likely to be high in the list. The largest angiosperm tree is an Australian Mountain-ash, the 'El Grande' tree of 439 m³ in Tasmania (Tasmanian Giant Trees).

Oldest trees

The oldest trees are determined by growth rings, which can be seen if the tree is cut down or in cores taken from the edge to the center of the tree. Accurate determination is only possible for trees which produce growth rings, generally those which occur in seasonal climates; trees in uniform non-seasonal tropical climates grow continuously and do not have distinct growth rings. It is also only possible for trees which are solid to the center of the tree; many very old trees become hollow as the dead heartwood decays away. For some of these species, age estimates have been made on the basis of extrapolating current growth rates, but the results are usually little better than guesswork or wild speculation. White (1998)[2] proposes a method of estimating the age of large and veteran trees in the United Kingdom through the correlation between a tree's stem diameter, growth character and age.

The verified oldest measured ages are (Gymnosperm Database):

  1. Great Basin Bristlecone Pine Pinus longaeva: 4844 years
  2. Alerce Fitzroya cupressoides: 3622 years
  3. Giant Sequoia Sequoiadendron giganteum: 3266 years
  4. Huon-pine Lagarostrobos franklinii: 2500 years
  5. Rocky Mountains Bristlecone Pine Pinus aristata: 2435 years

Other species suspected of reaching exceptional age include European Yew Taxus baccata (probably over 3000 years) and Western Redcedar Thuja plicata.

The oldest verified age for an angiosperm tree is 2293 years for the Sri Maha Bodhi Sacred Fig (Ficus religiosa) planted in 288 BC at Anuradhapura, Sri Lanka; this is also the oldest human-planted tree with a known planting date.

Trees in culture

Main article: Tree (mythology)

The tree has been a popular cultural symbol in most of the world. Common icons are the World tree, for instance Yggdrasil, and the tree of life. The tree often represents nature or the environment itself. 

A tree can also symbolise investment, and is featured in insignia of some banks, for instance Swedbank.

Major tree genera

Flowering plants (Magnoliophyta; angiosperms)

Dicotyledons (Magnoliopsida; broadleaf or hardwood trees)

 
Yellow maple in fall.

Monocotyledons (Liliopsida)

 
Coconut Palm, a monocotyledonous tree.

Conifers (Pinophyta; softwood trees)

Ginkgos (Ginkgophyta)

Cycads (Cycadophyta)

Ferns (Pterophyta)

See also

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A moss-covered tree near Orosí, Costa Rica.

External links

Bibliography

  • Pakenham, T. (2002). Remarkable Trees of the World. ISBN 0-297-84300-1
  • Pakenham, T. (1996). Meetings with Remarkable Trees. ISBN 0-297-83255-7
  • Tudge, C. (2005). The Secret Life of Trees. How They Live and Why They Matter. Allen Lane. London. ISBN 0-713-99698-6

References

  1. ↑ Mitchell, A. (1978). Trees of Britain & Northern Europe. Harper Collins. London. ISBN 0-00-219213-6
  2. ↑ White, J. (1990). Estimating the Age of Large and Veteran Trees in Britain. Forestry Commission. Edinburgh.
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