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A tree is a perennial woody plant. It is sometimes defined as a woody plant that attains diameter of 10 cm (30 cm girth) or more at breast height (130 cm above ground). However, there's no set agreement regarding minimum size, the term generally applies to plants that grow to at least 5-6 meters (15-20 ft) high at maturity and having secondary branches supported on a main stem or stems, called a trunk. Most trees exhibit clear apical dominance, though this isn't always the case. Compared with most other plants, trees are long-lived, some of them getting to be several thousand years old and growing to up to 115 meters (375 ft) high.
Trees are an important component of the natural landscape because of their prevention of erosion and the provision of a specific weather-sheltered ecosystem in and under their foliage. 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. They are also 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).
Classification
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're 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. A forest of great age is called old growth forest or ancient woodland (in the UK). A young tree is called a sapling.
Morphology
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 (for example, palms), grow by addition of new material inwards.
As an exogenous tree grows, it creates growth rings. Also known as annual rings, each set of light/dark rings is equivalent to one year of growth in a small number of mostly temperate species. For the vast majority of tree species growth rings don't correspond to annual intervals and several may be laid down in a single year, or there may be intervals of several years between additions while in some tropical regions with constant year-round climate growth is continuous and distinct rings are not formed at all. In temperate climates, these rings are commonly visible because of changes in the rate of growth with temperature variation over an annual cycle. In temperate species 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, a practice is known as the science of dendrochronology. Very few, if any, tropical trees can be accurately aged in this manner. 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.
Not 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 don't 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's 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 wouldn't technically be 'trees', but one shouldn't confuse reference to the form of a species with the size or shape of individual specimens. A spruce seedling doesn't fit the definition of a tree, but all spruces are trees.
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,, and even, are now largely disregarded as unreliable, fantasy or outright fraud. The following are now accepted as the top five tallest reliably measured species:
- Coast Redwood Sequoia sempervirens: 115.55 m (379.1 ft), Redwood National Park, California, United States
- Coast Douglas-fir Pseudotsuga menziesii:, Brummit Creek, Coos County, Oregon, United States
- Australian Mountain-ash Eucalyptus regnans:, Styx Valley, Tasmania, Australia
- Sitka Spruce Picea sitchensis:, Prairie Creek Redwoods State Park, California, United States
- Giant Sequoia Sequoiadendron giganteum:, Redwood Mountain Grove, Kings Canyon National Park, California, United States
More about the coast redwood that's the tallest tree in the world.
Stoutest trees
The girth ( circumference) of a tree is much easier to measure than the height, as it's 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' ( gbh); this is defined differently in different situations, with most foresters measuring girth at above ground, while ornamental tree measurers usually measure at above ground; in most cases this makes little difference to the measured girth. In the United States, measurements are mostly done at a height of, while in the United Kingdom this is mostly done at . 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 don't exceed 10-11 meter (33–36 ft) diameter.
An addition problem lies in cases where multiple trunks (whether from an individual tree or multiple trees) grow together. A good example of the difficulty of such cases is the Árbol del Tule, an 11.42 meter (37.5 ft) diameter Montezuma Cypress Taxodium mucronatum in Oaxaca, Mexico. Whether or not it's comprised of multiple trunks has long been a source of controversy. It is now known to be genetically a single organism (Dorado et al, 1996), but whether it's a single trunk or simply fused trunks/branches isn't known (External Link). The Sacred Fig is another notorious example of this. A single tree can have hundreds of trunks, leading to diameter measurements of hundreds of meters (External Link).
The stoutest single-trunk species in diameter, excluding baobabs, are:
Kauri Agatha australis: 16.41 metres (54 feet) Te Matua Ngahere, Waipoua Forest of Northland Region, New Zealand
Jequitibá Cariniana excelsa Casar.: (http://www.cr.nps.gov/history/online_books/science/hartesveldt/chap2.htm)
Giant Sequoia Sequoiadendron giganteum:, General Grant tree, Grant Grove, California, United States
Coast Redwood Sequoia sempervirens:, Prairie Creek Redwoods State Park, California, United States
Kauri Agathis australis: , Mercury Bay, New Zealand
Eucalyptus obliqua: , Reynolds Falls Nature Reserve, Tasmania, Australia (E. regnans has similarly large diameters)
Western Redcedar: Thuja plicata,, Lake Quinault, Washington (Van Pelt, 2001)
Charles Darwin reported finding Fitzroya cupressoides with trunk circumferences of up to 40 meters (130 ft)(External Link), implying a diameter of about 12 meters (40 ft), but this may be an anomaly as the largest known measurements are about 4.5 meters (15 ft).(External Link)
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:
Giant Sequoia Sequoiadendron giganteum: 1,489 m³ (55,040 cu ft), General Sherman
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 don't 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) 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):
Great Basin Bristlecone Pine Pinus longaeva: 4844 years
Alerce Fitzroya cupressoides: 3622 years
Giant Sequoia Sequoiadendron giganteum: 3266 years
Huon-pine Lagarostrobos franklinii: 2500 years
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
The tree has been and still is 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 symbolize investment for humans, and is featured in insignia of some banks, for instance Swedbank.
Major tree genera
Flowering plants (Magnoliophyta; angiosperms)
Dicotyledons (Magnoliopsida; broadleaf or hardwood trees)
Altingiaceae (Sweetgum family)
Anacardiaceae (Cashew family)
Annonaceae (Custard apple family)
Apocynaceae (Dogbane family)
Aquifoliaceae (Holly family)
Araliaceae (Ivy family)
Betulaceae (Birch family)
Bignoniaceae (family)
Cactaceae (Cactus family)
Cannabaceae (Cannabis family)
Cornaceae (Dogwood family)
Dipterocarpaceae family
- Garjan Dipterocarpus species
- Sal Shorea species
Ebenaceae (Persimmon family)
Ericaceae (Heath family)
Eucommiaceae (Eucommia family)
Fabaceae (Pea family)
Fagaceae (Beech family )
Fouquieriaceae (Boojum family)
Hamamelidaceae (Witch-hazel family)
Juglandaceae (Walnut family)
Lauraceae (Laurel family)
Lecythidaceae (Paradise nut family)
Lythraceae Loosestrife family
Magnoliaceae (Magnolia family)
Malvaceae (Mallow family; including Tiliaceae and Bombacaceae)
Meliaceae (Mahogany family)
Moraceae (Mulberry family)
Myristicaceae (Nutmeg family)
Myrtaceae (Myrtle family)
Nyssaceae (Tupelo family; sometimes included in Cornaceae)
Oleaceae (Olive family)
- Olive, Olea europaea
- Ash, Fraxinus species
Paulowniaceae (Paulownia family)
Platanaceae (Plane family)
Rhizophoraceae (Mangrove family)
- Red Mangrove, Rhizophora mangle
Rosaceae (Rose family)
- Rowans, Whitebeams, Service Trees Sorbus species
- Hawthorn, Crataegus species
- Pear, Pyrus species
- Apple, Malus species
- Almond, Prunus dulcis
- Peach, Prunus persica
- Apricot, Prunus armeniaca
- Plum, Prunus domestica
- Cherry, Prunus species
Rubiaceae (Bedstraw family)
Rutaceae (Rue family)
Salicaceae (Willow family)
Sapindaceae (including Aceraceae, Hippocastanaceae) (Soapberry family)
Sapotaceae (Sapodilla family)
Simaroubaceae family
Theaceae (Camellia family)
Thymelaeaceae (Thymelaea family)
- Ramin, Gonystylus species
Ulmaceae (Elm family)
Verbenaceae family
Monocotyledons (Liliopsida)
Agavaceae (Agave family)
Arecaceae (Palmae) (Palm family)
Poaceae (grass family)
- Bamboos Poaceae subfamily Bambusoideae
Note that banana 'trees' are not actually trees; they're not woody nor is the stalk perennial.
Conifers (Pinophyta; softwood trees)
Araucariaceae (Araucaria family)
Cupressaceae (Cypress family)
Pinaceae (Pine family)
Podocarpaceae (Yellowwood family)
Sciadopityaceae,
Taxaceae (Yew family)
Ginkgos (Ginkgophyta)
Ginkgoaceae (Ginkgo family)
Cycads (Cycadophyta)
Cycadaceae family
- Ngathu cycad, Cycas angulata
Zamiaceae family
- Wunu cycad, Lepidozamia hopei
Ferns (Pterophyta)
Cyatheaceae and Dicksoniaceae families
Fossil Trees
Wattieza, the oldest known tree
External results
Click here for more details on Tree
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