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A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance.[1] A minimum height specification at maturity is cited by some authors, varying from 3 m[2] to 6 m;[3] some authors set a minimum of 10 cm trunk diameter (30 cm girth).[4] Woody plants that do not meet these definitions by having multiple stems and/or small size are usually called shrubs, although many trees such as Mallee do not meet such definitions. Compared with most other plants, trees are long-lived, some reaching several thousand years old and growing to up to 115 m (379 ft) high.[5]

Trees are an important component of the natural landscape because of their prevention of erosion and the provision of a weather-sheltered ecosystem in and under their foliage. They also play an important role in producing oxygen and reducing carbon dioxide in the atmosphere, as well as moderating ground temperatures. They are also elements in landscaping and agriculture, both for their aesthetic appeal and their orchard crops (such as apples). Wood from trees is a building material, as well as a primary energy source in many developing countries. Trees also play a role in many of the world’s mythologies (see trees in mythology).[6]

Classification

A tree is a plant form that occurs in many different orders and families of plants. Most species of trees today are flowering plants (Angiosperms) and conifers. Trees show a variety of growth forms, leaf type and shape, bark characteristics and reproductive organs. For the listing of examples of well-known trees and how they are classified, see List of tree genera.

The tree form has evolved separately in unrelated classes of plants, in response to similar environmental challenges, making it a classic example of parallel evolution. With an estimate of 100,000 tree species, the number of tree species worldwide might total 25 percent of all living plant species.[7] The majority of tree species grow in tropical regions of the world and many of these areas have not been surveyed yet by botanists, making species diversity and ranges poorly understood.[8] The earliest tree-like organisms were tree ferns, horsetails and lycophytes, which grew in forests in the Carboniferous period, however these were plants were not trees, since they lacked woody tissue. Trees evolved in the Triassic period, with conifers, ginkgos, cycads and other gymnosperms appeared producing woody tissue, and were subsequently followed by tree-form flowering plants in the Cretaceous period.

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 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 grouped into exogenous and endogenous trees according to the way in which their stem diameter increases. Exogenous trees, which comprise the great majority of trees (all conifers, and almost all broadleaf trees), grow by the addition of new wood outwards, immediately under the bark. Endogenous trees, mainly in the monocotyledons (e.g., aloes and dragon trees), grow by addition of new material as discrete bundles within the existing trunk tissue.

Trees can be identified to genus or species by a combination of the tree’s shape, and the characteristics of its bark, leaves, flowers, and fruit. The leaves may be either deciduous or evergreen.[2][3]

As an exogenous tree grows, it creates growth rings as new wood is laid down concentrically over the old wood. In species growing in areas with seasonal climate changes, wood growth produced at different times of the year may be visible as alternating light and dark, or soft and hard, rings of wood.[3] In temperate climates, and tropical climates with a single wet-dry season alternation, the growth rings are annual, each pair of light and dark rings being one year of growth; these are known as annual rings. In areas with two wet and dry seasons each year, there may be two pairs of light and dark rings each year; and in some (mainly semi-desert regions with irregular rainfall), there may be a new growth ring with each rainfall.[9] In tropical rainforest regions, with constant year-round climate, growth is continuous and the growth rings are not visible nor is there a change in the wood texture. In species with annual rings, 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 known as the science of dendrochronology. Very few tropical trees can be accurately dated in this manner. Age determination in this manner 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. However, while ground nutrients are essential to a tree’s growth the majority of its biomass – over 90 percent – comes from carbon dioxide absorbed from the atmosphere (see photosynthesis).[10] 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, the saguaro cactus of North America has no functional leaves. 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 precise 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.

Record breaking trees

The world’s champion trees can be rated on height, trunk diameter or girth, total size, and age.

Tallest trees

The heights of the tallest trees in the world have been the subject of considerable dispute and much exaggeration. Modern verified measurements with laser rangefinders, other measuring devices, or with tape drop measurements made by tree climbers (such as those carried out by canopy researchers or members of groups like the U.S. Eastern Native Tree Society), have shown that some older measuring methods and measurements are often unreliable, sometimes producing exaggerations of 5% to 15% or more above the real height. Historical claims of trees growing to 130 m (430 ft), and even 150 m (490 ft), are now largely disregarded as unreliable, and attributed to human error. Historical records of fallen trees measured prostrate on the ground are considered to be somewhat more reliable. The following are now accepted as the top nine tallest reliably measured species (taken only currently standing specimens):

  1. Coast Redwood (Sequoia sempervirens): 115.56 m (379.1 ft), Hyperion, Redwood National Park, California, United States[11]
  2. Australian Mountain-ash (Eucalyptus regnans): 99.6 m (327 ft), south of Hobart, Tasmania, Australia[12]
  3. Coast Douglas-fir (Pseudotsuga menziesii): 99.4 m (326 ft), Brummit Creek, Coos County, Oregon, United States[13]
  4. Sitka Spruce (Picea sitchensis): 96.7 m (317 ft), Prairie Creek Redwoods State Park, California, United States[14]
  5. Giant Sequoia (Sequoiadendron giganteum): 94.9 m (311 ft), Redwood Mountain Grove, Kings Canyon National Park, California, United States[15]
  6. Tasmanian Blue Gum (Eucalyptus globulus): 90.7 m (298 ft), Tasmania, Australia[16]
  7. Manna Gum (Eucalyptus viminalis): 89 m (292 ft), Evercreech Forest Reserve, Tasmania, Australia[16]
  8. Shorea faguetiana: 88.3 m (290 ft) Tawau Hills National Park, in Sabah on the island of Borneo[17]
  9. Alpine Ash (Eucalyptus delegatensis): 87.9 m (288 ft), Tasmania, Australia[16]

Stoutest trees

The girth of a tree is usually 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:

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 that 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 cited as dbh (diameter at breast height) in tree and forestry literature.[3][18] Breast height is defined differently in different situations, with most forestry measurements taking girth at 1.3 m above ground,[18] while those who measure ornamental trees usually measure at 1.5 m above ground;[3] 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.[3][18] 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.[19]

Modern trends are to cite the tree’s diameter rather than the circumference. Diameter of the tree is calculated by finding the medium diameter of the trunk, in most cases obtained by dividing the measured circumference by π; this assumes the trunk is mostly circular in cross-section (an oval or irregular cross-section would result in a mean diameter slightly greater than the assumed circle). Accurately measuring circumference or diameter is difficult in species with the large buttresses that are especially characteristic in many species of rainforest trees. Simple measurement of circumference of such trees can be misleading when the circumference includes much empty space between buttresses.

One 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 (though not more than about 2.5%[20]), swelling to a maximum at the end of the rainy season, minimum at the end of the dry season.

The stoutest living single-trunk species in diameter are:

  1. Montezuma Cypress Taxodium mucronatum: 11.62 m (38.1 ft), Árbol del Tule, Santa Maria del Tule, Oaxaca, Mexico.[21] Note though that this diameter includes buttressing; the actual idealised diameter of the area of its wood is 9.38 m (30.8 ft).[21]
  2. Giant Sequoia Sequoiadendron giganteum: 8.85 m (29.0 ft), General Grant tree, General Grant Grove, California, United States[22]
  3. Coast Redwood Sequoia sempervirens: 7.9 m (26 ft), Lost Monarch Jedediah Smith Redwoods State Park, California, United States.
  4. Australian Oak Eucalyptus obliqua: 6.72 m (22.0 ft)
  5. Australian Mountain-ash Eucalyptus regnans: 6.52 m (21.4 ft), Big Foot
  6. Western Redcedar Thuja plicata: 5.99 m (19.7 ft), Kalaloch Cedar, Olympic National Park
  7. Sitka Spruce Picea sitchensis: 5.39 m (17.7 ft), Quinalt Lake Spruce, Olympic National Park
  8. Kauri Agathis australis (Te Matua Ngahere): 5.33 m (17.5 ft).[23]
  9. Alerce Fitzroya cupressoides: 5.0 m (16.4 ft)

An additional problem lies in instances where multiple trunks (whether from an individual tree or multiple trees) grow together. The Sacred Fig is a notable example of this, forming additional “trunks” by growing adventitious roots down from the branches, which then thicken up when the root reaches the ground to form new trunks; a single Sacred Fig tree can have hundreds of such trunks.[1] The multi-stemmed Hundred Horse Chestnut was known to have a circumference of 57.9 m (190 ft) when it was measured in 1780.

Largest trees

The largest trees in total volume are both tall and large in diameter and, in particular, 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. Measuring standards vary.

The top ten species measured so far are*:

  1. Giant Sequoia Sequoiadendron giganteum: 1,487 m³ (52,508 cu ft), General Sherman[24]
  2. Coast Redwood Sequoia sempervirens: 1,203 m³ (42,500 cu ft), Lost Monarch[25]
  3. Montezuma Cypress Taxodium mucronatum: 750 m³ (25,000 cu ft), Árbol del Tule[26]
  4. Kauri Agathis australis (Tāne Mahuta): 516 m³ [27]
  5. Western Redcedar Thuja plicata: 500 m³ (17,650 cu ft ), Quinault Lake Redcedar[24]
  6. Tasmanian Blue Gum Eucalyptus globulus: 368 m³ (13,000 cu ft), Rullah Longatyle (Strong Girl, also Grieving Giant) [16]
  7. Australian Mountain-ash Eucalyptus regnans: 360 m³ (12,714 cu ft), Arve Big Tree[16]
  8. Coast Douglas-fir Pseudotsuga menziesii 349 m³ (12,320 cu ft) Red Creek Tree
  9. Sitka Spruce Picea sitchensis 337 m³ (11,920 cu ft) Queets Spruce[citation needed]
  10. Australian Oak Eucalyptus obliqua: 337 m³ (11,920 cu ft) Gothmog[16]
  11. Alpine Ash Eucalyptus delegatensis: 286 m³ (10,100 cu ft), located in Styx River Valley[16]

(*)This list does not take into account now dead specimens.

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 bark to the center of the tree. Accurate determination is only possible for trees that produce growth rings, generally those 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 that are solid to the center. Many very old trees become hollow as the dead heartwood decays. For some of these species, age estimates have been made on the basis of extrapolating current growth rates, but the results are usually largely speculation. White (1998)[28] 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:

  1. Great Basin Bristlecone Pine (Methuselah) Pinus longaeva: 4,844 years[29]
  2. Alerce Fitzroya cupressoides: 3,622 years[29]
  3. Giant Sequoia Sequoiadendron giganteum: 3,266 years[29]
  4. Sugi Cryptomeria japonica: 3,000 years[30]
  5. Huon-pine Lagarostrobos franklinii: 2,500 years[29]

Other species suspected of reaching exceptional age include European Yew Taxus baccata (probably over 2,000 years[31][32]) and Western Redcedar Thuja plicata. The oldest known European Yew is the Llangernyw Yew in the Churchyard of Llangernyw village in North Wales, which is estimated to be between 4,000 and 5,000 years old.

Damage

The two sources of tree damage are either biotic (from living sources) or abiotic (from non-living sources). Biotic sources include insects that bore into the tree, deer that rub bark off, and fungi.[33]

Abiotic sources include lightning, vehicles impacts, and construction activities. Construction activities can involve a number of damage sources, including grade changes that prevent aeration to roots, spills involving toxic chemicals such as cement or petroleum products, or severing of branches or roots.

Both damage sources can result in trees becoming dangerous, and the term “hazard trees” is commonly used by arborists, and industry groups such as power line operators. Hazard trees are trees that, due to disease or other factors, are more susceptible to falling in windstorms, or having parts of the tree fall.

Evaluating the danger a tree presents is based on a process called the Quantified Tree Risk Assessment.[34]

Assessment as to labeling a tree a hazard tree can be based on a field examination. Assessment as a result of construction activities that will damage a tree is based on three factors: severity, extent and duration. Severity relates usually to the degree of intrusion into the TPZ and resultant root loss. Extent is frequently a percentage of a factor such as canopy, roots or bark, and duration is normally based on time. Root severing is considered permanent in time.

Trees are similar to people. Both can withstand massive amounts of some types of damage and survive, but even small amounts of certain types of trauma can result in death. Arborists are very aware that established trees will not tolerate any appreciable disturbance of the root system.[35] However, lay people and construction professionals seldom recognise how easily a tree can be killed.

One reason for confusion about tree damage from construction involves the dormancy of trees during winter. Another factor is that trees may not show symptoms of damage until two years or longer after damage has occurred. For that reason, persons uneducated in arboricultural science may not correlate the actual cause and resultant effect.

Various organisations, such as the International Society of Arboriculture, the British Standards Institute and the Tree Industry Association (formerly the National Arborist Association), have long recognised the importance of construction activities that impact tree health. The impacts are important because they can result in monetary losses due to tree damage and resultant remediation or replacement costs, as well as violation of government ordinances or community or subdivision restrictions.

As a result, protocols for tree management prior to, during and after construction activities are well established, tested and refined. These basic steps are involved:

  • Review of the construction plans
  • Development of the related tree inventory
  • Application of standard construction tree management protocols
  • Assessment of potential for expected tree damages
  • Development of a tree protection plan (providing for pre-, concurrent, and post construction damage prevention and remediation steps)
  • Development of a tree protection plan
  • Development of a remediation plan
  • Implementation of tree protection zones (TPZ)
  • Assessment of construction tree damage, post-construction
  • Implementation of the remediation plan

 

Trees in culture

The tree has always been a cultural symbol. Common icons are the World tree, for instance Yggdrasil,[36] and the tree of life. The tree is often used to represent nature or the environment itself.

Tree value approximation (USA)

Studies have shown that trees contribute as much as 27% of the appraised land value in certain markets and cite the following[37] which can be extrapolated with care.

Basic tree values (varies by region)[38]
diameter
(inches)
value
(1985 US$)
10 $1,729
14 $3,388
18 $5,588
26 $11,682
30 $15,554

These most likely use diameter measured at breast height, 4.5 feet (140 cm) above ground, not the larger base diameter. A general model for any year and diameter is Value = 17.27939*(diameter^2)*1.022^(year-1985) assuming 2.2% inflation per year.[39] The right side of this equation is written to paste into Excel or Google to perform the calculation. Extrapolations from any model can vary widely, so value estimates for diameters larger than 30 inches (760 mm) may have to be capped so trees do not exceed 27% of total appraised land value.

Written on February 23rd, 2012 , Botany, Forestry Tags:

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