Tree

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Common ash (Fraxinus excelsior),
a broad-leaved tree
European larch (Larix decidua),
a coniferous tree

A tree is a perennial woody plant. It typically has many secondary branches supported clear of the ground by a single, self-supporting main stem or trunk. This 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. The branches divide into smaller branches and shoots. These typically bear leaves, which contain chlorophyll. Chlorophyll converts light energy into chemical energy by photosynthesis, providing the food needed by the tree for its growth and development. Flowers and fruit may also be present.

Trees tend to be long-lived,[1] some reaching several thousand years old. The tallest known specimen on Earth is 115.6 m (379 ft) and they have a theoretical maximum height of 130m (426 ft).[2] Trees have been in existence on the Earth for 370 million years and are found growing worldwide wherever the climate permits.[citation needed] Trees are not a taxonomic group but are a number of plant species that have independently evolved a woody trunk and branches as a way to tower above other plants and make full use of the sunlight.

Trees are an important component of the natural landscape.[clarification needed] They play a significant part in reducing erosion and moderating the climate. They remove carbon dioxide from the atmosphere and store large quantities of carbon in their tissues. Trees and forests provide a habitat for many species of animals and plants. Tropical rainforests are one of the most biodiverse habitats in the world. Trees provide shade and shelter, timber for construction, fuel for cooking and heating, and fruit for food as well as having many other uses. In parts of the world, forests are shrinking as trees are cleared to increase the amount of land available for agriculture. Because of their longevity and usefulness, trees have always been revered and they play a role in many of the world's mythologies.

Contents

Overview

Trees are an evolutionary adaptation to competition for space, by growing taller trees are able to compete better for sunlight.[3] They have modified structures that allow them to grow taller and spread out their foliage, such as thicker stems that are composed of specialized cells that add structural strength and durability. They are long-lived perennial plants that can increase their size each year by producing woody stems. They differ from shrubs, which are also woody plants, by usually growing larger and having a single main stem;[4] but the distinction between a small tree and a large shrub is not always clear,[5] made more confusing by the fact that trees may be reduced in size under harsher environmental conditions such as on mountains and subarctic areas. 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 estimated 100,000 species, the number of trees worldwide might total twenty-five percent of all living plant species.[6] Their greatest number grow in tropical regions and many of these areas have not yet been fully surveyed by botanists, making tree diversity and ranges poorly known.[7]

Trees exist in two different groups of vascular or higher plants, the gymnosperms and the angiosperms. Both groups are seed plants. The gymnosperms include conifers, cycads, ginkgophytes and gnetales. They are characterised by having naked seeds that are not enclosed, at the time of fertilization the ovules lie uncovered on scales or on the ends of branches. The angiosperms or flowering plants are characterised by having ovals enclosed in ovaries which develop into some type of fruit and the seeds store nutrients after they are fertilized as endosperm surrounding the embryo.[8] Angiosperm trees are also known as broad-leaved trees. Most angiosperm trees are dicotyledons, so named because the seeds contain two cotyledons or seed leaves.

Wood gives structural strength to a tree stem which is used to support 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. 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.

Trees are either evergreen, having foliage that persists and remains green throughout the year,[9] or deciduous, shedding their leaves at the end of the growing season and then having a dormant period without foliage.[10] Most conifers are evergreens but larches (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.[4] The crown is a name for the upper part[clarification needed] of a tree including the branches and leaves [11] and the uppermost layer in a forest, formed by the crowns of the trees, is known as the canopy.[12] A sapling is a young tree.[13]

Tree-like plants include some palms which are not trees but herbaceous[14][15] monocots that do not undergo secondary growth and never produce wood,[16][17][17][18] and hence do not meet the definition of tree used in this article. In many tree-like palms, the terminal bud on the main stem is the only one to develop so they have tall, unbranched trunks with spirally arranged large leaves. Some of the tree ferns, order Cyatheales, have tree-like growth forms, growing up to 20 metres (66 ft) but they are structurally very different from other trees[clarification needed] Their trunks are composed of rhizomes which grow vertically and which are covered by numerous adventitious roots.[19]

Distribution

In places where the climate is suitable, trees are the climax vegetation.[citation needed] In regions with a cool temperate climate, conifers tend to predominate, whereas in regions with tropical or warm temperate climates, broad-leaved trees are more common[citation needed].[20] Shade tolerance in young trees varies between species, and may determine the pattern of forest succession. For example, pines are particularly intolerant of shade[citation needed] while many broad-leaved tree saplings in forests grow tall and thin but survive; even if pines dominate woodland at an early stage in its succession, they may eventually be suppressed by broad-leaved species.[clarification needed][21]

More than half the species of terrestrial plants and animals on the Earth are thought to live in tropical rainforests even though these occupy just five percent of the land area.[22] In tropical regions with a monsoon climate, where a drier part of the year alternates with a wet period when the rains arrive, different species of broad-leaved trees dominate the forest, some of them being deciduous.[23]

Tropical regions with a drier savanna climate have insufficient rainfall to support dense forests; the canopy is not closed and plenty of sunshine reaches the ground which is covered with grass and scrub. Acacia and baobab are well adapted to living in such areas.[24]

In cool temperate parts of the world, particularly in the northern hemisphere, deciduous broad-leaved trees tend to be replaced by conifers. The long cold winter is unsuitable for plant growth and trees must grow rapidly in the short summer season when the temperature rises and the days are long. A wide range of pines, firs and spruces grow here. Light is very limited under their dense cover and there may be little plant life on the forest floor although fungi may abound.[25] What deciduous trees there are tend to be dwarf willows and birches. Similar woodland is found on mountains where the altitude causes the average temperature to be lower thus reducing the length of the growing season. The tree line is lower on north facing slopes than on south facing ones[citation needed]<>.[26]

Parts and function

A young red pine (Pinus resinosa) with spread of roots visible

Roots

The roots of a tree serve to anchor it to the ground and gather water and nutrients to transfer to all parts of the tree, and for reproduction defense, survival, energy storage and many, many other purposes. The first root produced by a newly germinated seedling is a taproot which goes straight downwards. Within a few weeks lateral roots branch out of the side of this and grow horizontally through the upper layers of the soil. In most trees, the tap root eventually withers away and the wide-spreading laterals remain. Near the tip of the finer roots are single cell root hairs. These are in immediate contact with the soil particles and can absorb water and nutrients such as nitrogen[citation needed] and potassium in solution. The root system spreads out about twice as far as the tree's canopy extends[citation needed] with most of the roots found in the top 2 ft (0.6 m) of soil.[citation needed] The roots require oxygen to respire and only a few species such as the mangrove and the pond cypress (Taxodium ascendens) can live in permanently waterlogged soil.[27]

Banyan (Ficus benghalensis) tree with pillar roots

Mangroves have several special adaptations of their root systems to cope with their marine habitat. The red mangrove (Rhizophora mangle), has prop roots that loop out of the side of both trunk and branches and then descend vertically into the mud. These help stabilize the tree in the shifting sediment.[28] The black mangrove (Avicennia germinans) lives in drier locations but the mud is poorly oxygenated so these mangroves have pneumatophores, roots which project up through the surface of the soil like a breathing tube, allowing gaseous exchange to take place.[28] The upriver orange mangrove (Bruguiera sexangula) has "kneed" roots which hump up out of the water and have lenticels or breathing holes.[28]

The Indian banyan tree (Ficus benghalensis) has aerial or pillar roots that drop down to the ground from the trunk and branches, either singly or in groups. These help provide water and nutrients and make the tree more stable.[29] The strangler fig takes this a stage further.[clarification needed] It starts life as an epiphyte high in the tree canopy[citation needed] and drops aerial roots down to the forest floor. These intertwine and thicken and if, as sometimes happens, the host tree eventually dies, the roots can take over the role of trunk for the fig.[29]

Buttress roots of the kapok tree (Ceiba pentandra)

Many large trees have buttress roots which flare out from the lower part of the trunk. These brace the tree rather like angle brackets and provide stability, reducing sway in high winds. They are particularly prevalent in tropical rainforests where the soil is poor and the roots are close to the surface. They may start several yards (metres) up the trunk and spread out widely on the surface of the ground. Some send out vertical sinker roots well away from the trunk. The roots to leeward resist compression while those to windward provide tension. They provide about six times more anchorage than the lateral roots of similar sized unbuttressed trees.[30]

In the soil, the roots encounter the hyphae of fungi. Many of these are known as mycorrhiza and form a mutualistic relationship with the tree roots. Some are specific to a single tree species, which will not flourish in the absence of its mycorrhizal associate. Others are generalists and associate with many species. The tree acquires minerals such as phosphorus from the fungus while it obtains the carbohydrate products of photosynthesis from the tree.[31] The hyphae of the fungus can link different trees and a network is formed, transferring nutrients from one place to another. The fungus promotes growth of the roots and helps protect the trees against predators and pathogens. It can also limit damage done to a tree by pollution as the fungus accumulate heavy metals within its tissues.[32] Fossil evidence shows that roots have been associated with mycorrhizal fungi since the early Paleozoic, four hundred million years ago, when the first vascular plants colonised dry land.[33]

Some trees such as the alders (Alnus spp.) have a symbiotic relationship with Frankia sp,, a filamentous bacterium that can fix nitrogen from the air, converting it into ammonia. They have actinorhizal root nodules on their roots in which the bacteria live. This process enables the tree to live in low nitrogen habitats where they would otherwise be unable to thrive.[34] Researchers have discovered that certain plant hormones called cytokinins initiate root nodule formation and that this process is closely related to the mechanisms involved in mycorrhizal association. [35]

Trunk

Beech (Fagus sylvatica) trunk in autumn

The main purpose of the trunk and branches of a tree is to maximise the amount of light that falls on the leaves.[citation needed] By overtopping other plants and shading them out, trees become the terminal vegetation cover on all but the most infertile and inhospitable land.[citation needed] A secondary purpose is to transport water and nutrients from the roots to the aerial parts of the tree and to distribute the food produced by the leaves to all other parts including the roots.[36]

In the case of Angiosperms and Gymnosperms, the outermost layer of the trunk is the bark and is mostly composed of dead cells. It provides a thick, waterproof covering to the living inner tissue. It protects the trunk against the elements, disease, animal attack and fire. It is perforated by a large number of fine breathing pores called lenticels through which oxygen diffuses. Bark is continually replaced by a living layer of cells called the cork cambium. The London plane (Platanus × acerifolia) periodically sheds its bark in large flakes. Similarly, the bark of the silver birch (Betula pendula) peels off in strips. As the tree's girth expands, newer layers of bark are larger in circumference, and the older layers develop fissures in many species. Different species of tree can sometimes be distinguished by the characteristics of their bark.[citation needed] In some trees such as the pine (Pinus spp.,) the bark exudes sticky resin which deters attackers whereas in rubber trees (Hevea brasiliensis) it is a milky latex that oozes out. The quinine bark tree (Cinchona officinalis) contains bitter substances to make the bark unpalatable.[36] Tree-like plants[clarification needed] in the Pteridophyta, Arecales, Cycadophyta and Poales such as the tree ferns, palms, cycads and bamboos have no true bark but all have an outer protective covering of some form.[37]

Although the bark functions as a protective barrier, it is itself attacked by boring insects such as beetles. These lay their eggs in crevices and the larvae chew their way through the cellulose tissues leaving a gallery of tunnels. This may allow fungal spores to gain admittance and attack the tree. Dutch elm disease is caused by a fungus (Ophiostoma sp.) carried from one elm tree to another by various beetles. The tree reacts to the growth of the fungus by blocking off the xylem tissue carrying sap upwards and the branch above, and eventually the whole tree, is deprived of nourishment and dies. In Britain in the 1990s, 25 million elm trees were killed by this disease.[38]

A section of yew (Taxus baccata) showing 27 annual growth rings, pale sapwood and dark heartwood

The innermost layer of bark is known as the phloem and this is involved in the transport of the sap containing the sugars made by photosynthesis to other parts of the tree. It is a soft spongy layer of living cells, some of which are arranged end to end to form tubes. These are supported by parenchyma cells which provide padding and include fibres for strengthening the tissue.[39] Inside the phloem is a layer of undifferentiated cells one cell thick called the vascular cambium layer. This is where all the growth in girth of the tree takes place.[citation needed] The cells are continually dividing, creating phloem cells on the outside and wood cells known as xylem on the inside.[40]

The newly created xylem is the sapwood. It is composed of water-conducting cells and associated cells which are often living, and is usually pale in colour. It transports water and minerals from the roots to the upper parts of the tree. The oldest, inner part of the sapwood is progressively converted into heartwood as new sapwood is formed at the cambium. The conductive cells of the heartwood are blocked in some species, and the surrounding cells are more often dead. Heartwood is usually darker in colour than the sapwood. It is the dense central core of the trunk giving it rigidity. Three quarters of the dry mass of the xylem is cellulose, a polysaccharide, and most of the remainder is lignin, a complex polymer. The concentric circles seen when a transverse cut is made across the trunk of many tree species are the annual growth rings.[citation needed] There are a number of streaks called rays running at right angles to these.[citation needed] They are plates of living cells transporting materials between the phloem, sapwood, and heartwood. If the tree is damaged and water and fungal spores come in contact with the heartwood, decay may occur and the heartwood will rot.[citation needed] This happens in many older trees which become hollow as a result but may still stand upright for many years.[41]

Buds and growth

Dormant magnolia (Magnolia sp.) bud
Bursting horse-chestnut (Aesculus hippocastanum) bud

Trees do not usually grow continuously throughout the year but mostly have spurts of active expansion followed by periods of rest. This pattern of growth is related to the climatic conditions, growth normally ceasing when conditions are either too cold or too dry. In readiness for the inactive period, trees form buds to protect the meristem, the zone of active growth. Before the period of dormancy, the last few leaves produced at the tip of a twig form scales. These are thick, small and closely wrapped and enclose the growing point in a waterproof sheath. Inside this bud there is a rudimentary stalk and neatly folded miniature leaves, ready to expand when the next growing season arrives. Buds also form in the axils of the leaves ready to produce new side shoots. A few trees, such as the eucalyptus, have "naked buds" with no protective scales and some conifers, such as the Lawson's cypress, have no buds but instead have little pockets of meristem concealed among the scale-like leaves.[42]

When growing conditions improve, such as the arrival of warmer weather and the longer days associated with spring in temperate regions, growth starts again. A surge of sap rises through the tree[citation needed] and the buds start to grow. The expanding shoot pushes its way out, shedding the scales in the process. These leave behind scars on the surface of the twig. By counting the number of these scars, it is possible to establish the age of a twig.[citation needed][clarification needed] The whole year's growth may take place in just a few weeks. The new stem is unlignified at first and may be green and downy. The leaves grow either alternately on each side of the stem or in opposite pairs, depending on the species.[citation needed] Palm trees[clarification needed] have their leaves spirally arranged on an unbranched trunk.[42] In some tree species in temperate climates, a second spurt of growth, a Lammas growth may occur which is believed to be a strategy to compensate for loss of early foliage to insect predators.[43]

Primary growth is the term used to describe the elongation of the stems and roots. Secondary growth takes place in the older twigs, branches, trunk and larger roots.[citation needed] This consists of a progressive thickening and strengthening of the tissues as the outer layer of the epidermis is converted into bark and the cambium layer creates new phloem and xylem cells. The bark is inelastic and becomes fissured and wrinkled as the girth of the tree expands.[citation needed][44] Sooner or later[which?] the growth of a tree slows down and stops and it gets no taller. Primary growth is limited to the production of leaves with hardly any elongation of the twigs.[citation needed] As long as the crown remains in balance with the roots[clarification needed] the tree should remain healthy but its ability to defend itself against fungal attack is diminished. If damage occurs to the tree and fungal spores gain admission to the interior, decay will occur[citation needed] and the tree may in time become hollow.[45]

Leaves

Buds, leaves, flowers and fruit of oak (Quercus robur)
Buds, leaves and reproductive structures of white fir (Abies alba)

The branches and twigs of a tree are spread out in such a way as to maximise the amount of light falling on the leaves[citation needed] Tree are much taller than herbaceous or shrubby plants and ensuring the upper-most leaves are supplied with water originating in the root system requires that water is drawn up through the xylem from the roots by the suction produced as it evaporates from the leaves.[clarification needed] If insufficient water is available, as may happen in times of drought, the leaves droop,[citation needed] and if this state continues they will die.[46]

The leaves of trees come in a wide range of shapes and sizes which have evolved in response to environmental pressures including climate and predation. They can be broad or needle-like, simple or compound, lobed or entire, smooth or hairy, delicate or tough, deciduous or evergreen. The needles of coniferous trees are compact but are structurally similar to those of broad-leaved trees. They lose less water through transpiration[citation needed] and are adapted for life in environments where resources are low or water is scarce. Frozen ground may limit water availability and conifers are often found in colder places at higher altitudes and higher latitudes than broad leaved trees. In many cases, their branches hang down at an angle to the trunk which decreases the likelihood of them breaking when weighed down by snow.[clarification needed] Where resources permit, the higher leaf nitrogen concentrations in broad-leaved species[citation needed] enables them to out-compete conifers.[47] Broad leaved trees in temperate regions have a different[clarification needed] strategy for dealing with winter weather. When the days get shorter and the temperature begins to decrease, the leaves no longer makes new chlorophyll and the red and yellow pigments already present in the blades become apparent.[48] Synthesis in the leaf of a plant hormone called auxin also ceases. This causes the cells at the junction of the petiole and the twig to weaken and sooner or later the joint breaks and the leaf floats to the ground. In tropical and subtropical regions, many trees keep their leaves all year round. Individual leaves may fall intermittently and be replaced by new growth but most leaves remain intact for some time. Other tropical species and those in arid regions may shed all their leaves annually at a particular time of year.[citation needed] Often this will coincide with the onset of the dry season or some other climatic event.[clarification needed] Many deciduous trees flower before the new leaves emerge.[49]

Reproduction

Form, leaves and reproductive structures of queen sago Cycas circinalis

Tree forms are found in a wide range of plants and their reproductive strategies are substantially the same as shrub or herbaceous plant forms.

Many trees are wind pollinated which may be an evolutionary adaptation to take advantage of increased wind speeds high above the ground, particularly in the case of those that produce pollen before the leaves emerge.[50] A vast quantity of pollen is produced because of the low likelihood of any particular grain landing on an appropriate female flower. Wind-pollinated flowers of broad-leaved trees are characterised by a lack of showy parts, no scent and a copious production of pollen, often with separate male and female flowers, or separate male and female trees. The male flowers may be high up in the tree, often in the form of dangling catkins. The female flowers may be lower down the tree. The pollen of pine trees contains air sacs which give it buoyancy and it has been known to travel as far as 800 kilometres (500 mi).[51] Tree pollen can cause allergies. A prime example would be hay fever, which can be caused by pollen.[52]

Seeds

Seeds are the primary way that trees reproduce and their seeds vary greatly in size and shape. Some of the largest seeds come from trees, but the largest tree, Sequoiadendron giganteum, produces one of the smallest tree seeds.[53] The great diversity in tree fruits and seeds reflects the many different ways that tree species have evolved to disperse their offspring.

Most extant Gymnosperms produce seeds in cones.[citation needed][54][18] The single extant species of Ginkgophyta (Ginkgo biloba) has small fleshy seeds produced at the ends of short branches on female trees,[55] and Gnetum, a tropical and subtropical group of gymnosperms produce seeds at the tip of a shoot axis.[56] The seeds of conifers, the largest group of Gymnosperms, are enclosed in a cone and most species have seeds that are light and papery that can be blown considerable distances once free from the cone.[57] Sometimes the seed remains in the cone for years waiting for a trigger event to liberate it. Fire stimulates release and germination of seeds of the jack pine, and also enriches the forest floor with wood ash and removes competing vegetation.[58] Similarly, a number of Angiosperms including Acacia cyclops and Acacia mangium have seeds that germinate better after exposure to high temperatures.[59]

Wind dispersed seed of elm (Ulmus), ash (Fraxinus) and sycamore (Acer pseudoplatanus)

Angiosperm tree produce seeds in a wide variety of fruits, some of them include acorn, nut, berrie, pome, drupe, samaras, hesperdium, capsule and legume.[60]

For a tree seeding to grow into an adult tree it needs light and space, if seeds only fell straight to the ground, competition among the concentrated saplings and the shade of the parent would likely prevent it from flourishing. Many seeds such as birch are small and have papery wings to aid dispersal by the wind. Ash trees and maples have larger seeds with blade shaped wings which spiral down to the ground when released. The kapok tree has cottony threads to catch the breeze.[61] The flame tree does not rely on fire but shoots its seeds through the air when the two sides of its long pods crack apart explosively on drying.[61] The miniature cone-like catkins of Alder trees produce seeds that contain small droplets of oil that help disperse the seeds on the surface of water. Mangroves often grow in water and some species have propagules, which are buoyant fruits with seeds that start germinating before becoming detached from the parent tree.[62][63] These float on the water and may become lodged on emerging mudbanks and successfully take root.[61] Other seeds, such as apple pips and plum stones, have fleshy receptacles and smaller fruits like hawthorns have seeds enclosed in edible tissue; birds and animals[clarification needed] eat the fruits and the seeds are either discarded or are consumed and pass through the gut to be deposited in the animal's droppings well away from the parent tree. In some cases, germination is improved by being processed in this way.[64] Nuts and other large seeds are gathered by animals that hide in caches[citation needed] any not immediately consumed.[65] Many of these caches are never revisited, the nut-casing softens with rain and frost and the seed germinates in the spring.[66] Pine cones may be hoarded in a similar way by red squirrels, and grizzly bears raiding the caches may also help to disperse the seed.[67]

Evolutionary history

Palms and cycads as they might have appeared in the middle Tertiary

The earliest tree-like organisms were tree ferns, horsetails and lycophytes, which grew in forests in the Carboniferous period. The first tree may have been Wattieza, fossils of which have been found in New York State in 2007 dating back to the Middle Devonian (about 385 million years ago). Prior to this discovery, Archaeopteris was the earliest known tree.[68] Both of these reproduced by spores rather than seeds and are considered to be links between ferns and the gymnosperms which evolved in the Triassic period. The gymnosperms include conifers, cycads, gnetales and ginkgos and these may have appeared as a result of a whole genome duplication event which took place about 319 million years ago.[69] Ginkgophyta was once a widespread diverse group [70] of which the only survivor is the maidenhair tree Ginkgo biloba. This is considered to be a living fossil because it is virtually unchanged from the fossilised specimens found in Triassic deposits.[71]

During the Mesozoic (245 to 65 million years ago) the conifers flourished and became adapted to live in all the major terrestrial habitats. Subsequently the tree forms of flowering plants evolved during the Cretaceous period. These began to dominate the conifers during the Tertiary era (65 to 2 million years ago) when forests covered the globe. When the climate cooled 1.5 million years ago and the first of four ice ages occurred, the forests retreated as the ice advanced. In the interglacials, trees recolonised the land[clarification needed] only to be driven back again at the start of the next ice age.[72]

Tree ecology

Trees are an important part of the terrestrial ecosystem, and provide a habitat for a community of arboreal animals and plants.[73] Epiphytic plants such as ferns, orchids and some species of parasitic plants such as mistletoe hang from mossy and lichen-clad branches,[citation needed] providing their own micro-habitats. Leaves, flowers and fruits are seasonally available. On the ground underneath trees there is shade, shelter, undergrowth, leaf litter, fallen branches and decaying wood.[clarification needed] Trees stabilise the soil, prevent rapid run-off of rain water, help prevent desertification, have a role in climate control and help in the maintenance of biodiversity and ecosystem balance.[74]

Many species of tree support their own specialised invertebrates. In their natural habitats, 284 different species of insect have been found on the English oak (Quercus robur) [75] and 306 species of invertebrate on the Tasmanian oak (Eucalyptus obliqua).[76] Non-native tree species provide a less biodiverse community, for example in the United Kingdom the sycamore (Acer pseudoplatanus), which originates from southern Europe, has few associated invertebrate species, though its base rich bark[clarification needed] does support a wide range of lichens, bryophytes and epiphytes.[77]

Trees can play a role in the development of an ecosystem,[clarification needed] for example in mangrove swamps the roots of the mangrove trees reduce the speed of flow of tidal currents and hence trap water-borne sediment, leading over time to a reduction in water depth and the creation of suitable conditions for further mangrove colonisation. Thus mangrove swamps tend to extend seawards in suitable locations.[78] Mangrove swamps also provide an effective buffer against the more damaging effects of cyclones and tsunamis.[79]

Uses

Silviculture is the practice of controlling the establishment, growth, composition, health, and quality of forests, which are areas that have a high density of trees. Cultivated trees are planted and tended by humans, usually because they provide food (fruits or nuts), ornamental beauty, or some type of wood product that benefits people. A small wooded area, usually with no undergrowth, is called a grove [80] and a small wood or thicket of trees and bushes is called a coppice or copse.[81] A large area of land covered with trees and undergrowth is called woodland or forest.[82] An area of woodland composed primarily of trees established by planting or artificial seeding is known as a plantation [83] and an area of land planted with fruit or nut trees is an orchard.[84] Several biotopes are defined largely by the trees that inhabit them, examples being rainforest and taiga. A landscape of trees scattered or spaced across grassland is called a savanna.[85] A forest of great age is called ancient woodland[citation needed][86] and a forest in its natural state, before being explored or exploited by man is a virgin forest.[citation needed][87] Trees have conservation value and add interest to the landscape. They can be planted as isolated specimens in hedgerows or as shelter belts. They provide shade for people and animals. They can be planted in grand avenues in parkland or alongside roads in town and country.

Food

Forest honey
Sugar maple (Acer saccharum) being tapped

Trees are the source of many of the world's best known fleshy fruits. Apples, pears, plums, cherries and citrus are all grown commercially in temperate climates and a wide range of edible fruits are found in the tropics. Other commercially important fruit include dates, coconuts and other nuts, figs and olives. Palm oil is obtained from the fruits of the oil palm (Elaeis guineensis). The fruits of the cocoa tree (Theobroma cacao) are used to make cocoa and chocolate and the berries of coffee trees, (Coffea arabica) and (Coffea canephora), are processed to extract the coffee beans. In many rural areas of the world, fruit is gathered from forest trees for consumption.[88]

Many trees have flowers rich in nectar which are attractive to bees. The production of forest honey is an important industry in rural areas of the developing world where it is undertaken by small-scale beekeepers using traditional methods.[89] The flowers of the elder (Sambucus) are used to make elderflower cordial and petals of the plum (Prunus spp.) and the rose (Rosa spp.) can be candied.[90]

The leaves of trees are widely gathered as fodder for livestock and some can be eaten by humans but they tend to be high in tannins which makes them bitter. Leaves of the curry tree (Murraya koenigii) and kaffir lime (Citrus × hystrix) are eaten in the east[where?] [91] and those of the European bay tree (Laurus nobilis) and the California bay tree (Umbellularia californica) are used for flavouring food.[92] Camellia sinensis, the source of tea, is a small tree but seldom reaches its full height, being heavily pruned to make picking the leaves easier.[93]

In temperate climates there is a sudden movement of sap at the end of the winter as trees prepare to burst into growth. In North America, the sap of the sugar maple (Acer saccharum) is most often used in the production of a sweet liquid, maple syrup. About 90% of the sap is water, the remaining 10% being a mixture of various sugars and certain minerals.[94] The sap is harvested by drilling holes in the trunks of the trees and collecting the liquid that flows out of the inserted spigots. It is piped to a sugarhouse where it is heated to concentrate it and improve its flavour. One litre of maple syrup is obtained from every forty litres of sap and has a sugar content of exactly 66%.[94] A similar process happens in northern Europe when the spring rise in the sap of the silver birch (Betula pendula) is tapped and collected. This is either drunk fresh or is fermented into an alcoholic drink. In Alaska, the sap of the sweet birch (Betula lenta) is similarly collected and converted into birch syrup with a sugar content of 67%. Sweet birch sap is more dilute than maple sap and one hundred litres are required to make one litre of birch syrup.[95]

Various parts of trees are used as spices. These include cinnamon, made from the bark of the cinnamon tree (Cinnamomum zeylanicum) and all spice, the dried small fruits of the pimento tree (Pimenta dioica). Nutmeg is a seed found in the fleshy fruit of the nutmeg tree (Myristica fragrans) and cloves are the unopened flower buds of the clove tree (Syzygium aromaticum). Sassafras oil is an important flavouring obtained from distilling bark from the roots of (Sassafras albidum).[92]

Fuel

Selling firewood at a market

Wood has traditionally been used for fuel, especially in rural areas. In less developed nations it may be the only fuel available and collecting firewood is often a time consuming task as it becomes necessary to travel further and further afield in the search for fuel.[96] It is often burned inefficiently on an open fire. In more developed countries other fuels are available and burning wood is a choice rather than a necessity. Modern wood-burning stoves are very fuel efficient and new products such as wood pellets are available to burn.[97]

Charcoal can be made by slow pyrolysis of wood by heating it in the absence of air in a kiln. The carefully stacked branches, often oak, are burned with a very limited amount of air. The process of converting them into charcoal takes about fifteen hours. Charcoal is used as a fuel in barbecues and by blacksmiths and has many industrial and other uses.[98]

Wood smoke can be used to preserve food. In the hot smoking process the food is exposed to smoke and heat in a controlled environment. The food is ready to eat when the process is complete, having been tenderised and flavoured by the smoke it has absorbed. In the cold process, the temperature is not allowed to rise above 100 °F (38 °C). The flavour of the food is enhanced but raw food requires further cooking. If it is to be preserved, meat should be cured before cold smoking.[99]

Timber

Joinery and roof trusses made from softwood

Wood has been an important, easily available material for construction since humans started building shelters. Engineered wood products are available which bind the particles, fibres or veneers of wood together with adhesives to form composite materials. Plastics have taken over from wood for some traditional uses.[100]

Wood is used in the construction of buildings, bridges, trackways, piles, poles for power lines, masts for boats, pit props, railway sleepers, fencing, hurdles, shuttering for concrete, pipes, scaffolding and pallets. In housebuilding it is used in joinery, for making joists, roof trusses, roofing shingles, thatching, staircases, doors, window frames, floor boards, parquet flooring, panelling and cladding.[101]

Wood is used to construct carts, farm implements, boats, dugout canoes and in shipbuilding. It is used for making furniture, tool handles, boxes, ladders, musical instruments, bows, weapons, matches, clothes pegs, brooms, shoes, baskets, turnery, carving, toys, pencils, rollers, cogs, wooden screws, barrels, coffins, skittles, veneers, artificial limbs, oars, skis, wooden spoons, sports equipment and wooden balls.[101]

Wood is pulped for paper and used in the manufacture of cardboard and made into engineered wood products for use in construction such as fibreboard, hardboard, chipboard and plywood.[101] The wood of conifers is known as softwood while that of broad-leaved trees is hardwood.[102]

Bark

Recently stripped cork oak (Quercus suber)

Cork is produced from the thick bark of the cork oak (Quercus suber). It is harvested from the living trees about once every ten years in an environmentally sustainable industry.[103] More than half the world's cork comes from Portugal and is largely used to make stoppers for wine bottles.[104] Other uses include floor tiles, bulletin boards, balls, footwear, cigarette tips, packaging, insulation and joints in woodwind instruments.[104]

The bark of other varieties of oak has traditionally been used in Europe for the tanning of hides though bark from other species of tree has been used elsewhere. The active ingredient, tannin, is extracted and after various preliminary treatments, the skins are immersed in a series of vats containing solutions in increasing concentrations. The tannin causes the hide to become supple, less affected by water and more resistant to bacterial attack.[105]

At least a hundred and twenty drugs come from plant sources, many of them from the bark of trees.[106] Quinine originates from the cinchona tree (Cinchona) and was for a long time the remedy of choice for the treatment of malaria.[107] Aspirin was synthesized to replace the sodium salicylate derived from the bark of willow trees (Salix) which had unpleasant side effects.[108] The anti-cancer drug Paclitaxel is derived from taxol, a substance found in the bark of the Pacific yew (Taxus brevifolia).[109] Other tree based drugs come from the paw-paw (Carica papaya), the cassia (Cassia spp.), the cocoa tree (Theobroma cacao), the tree of life (Camptotheca acuminata) and the downy birch (Betula pubescens).[106]

The papery bark of the white birch tree (Betula papyrifera) was used extensively by Native Americans. Wigwams were covered by it and canoes were constructed from it. Other uses included food containers, hunting and fishing equipment, musical instruments, toys and sledges.[110] Nowadays, bark chips, a by-product of the timber industry, are used as a mulch and as a growing medium for epiphytic plants that need a soil-free compost.[111]

Other uses

Latex being collected from the rubber tree (Hevea brasiliensis)

Latex is a sticky defensive secretion that protects plants against herbivores. Many trees produce it when injured but the main source of the latex used to make natural rubber is the Pará rubber tree (Hevea brasiliensis). Originally used to create bouncy balls and for the waterproofing of cloth, natural rubber is now mainly used in tyres for which synthetic materials have proved less durable.[112] The latex exuded by the balatá tree (Manilkara bidentata) is used to make golf balls and is similar to gutta-percha, made from the latex of the "getah perca" tree Palaquium. This is also used as an insulator, particularly of undersea cables, and in dentistry, walking sticks and gun butts. It has now largely been replaced by synthetic materials.[113]

Resin is another plant exudate that may have a defensive purpose. It is a viscous liquid composed mainly of volatile terpenes and is produced mostly by coniferous trees. It is used in varnishes, for making small castings and in ten-pin bowling balls. When heated, the terpenes are driven off and the remaining product is called "rosin" and is used by stringed instrumentalists on their bows. Some resins contain essential oils and are used in incense and aromatherapy. Fossilized resin is known as amber and was mostly formed in the Cretaceous (145 to 65 million years ago) or more recently. The resin that oozed out of trees sometimes trapped insects or spiders and these are still visible in the interior of the amber.[114]

The camphor tree (Cinnamomum camphora) produces an essential oil [92] and the eucalyptus tree (Eucalyptus globulus) is the main source of eucalyptus oil which is used in medicine, as a fragrance and in industry.[115]

Care

Dead trees pose a safety risk, especially during high winds and severe storms, and removing dead trees involves a financial burden, whereas the presence of healthy trees can clean the air, increase property values, and reduce the temperature of the built environment and thereby reduce building cooling costs. During times of drought, trees can fall into water stress, which may cause a tree to become more susceptible to disease and insect problems, and ultimately may lead to a tree's death. Irrigating trees during dry periods can reduce the risk of water stress and death. Irrigation can be accomplished by use of a garden hose, soaker hose, sprinkler, or modified five-gallon bucket.[116]

Mythology

Yggdrasil, the World Ash (Norse)
Looking upwards from the base of the General Sherman Tree

Trees have been venerated since time immemorial. To the ancient Celts, certain trees held special significance as providing fuel, building materials, ornamental objects and weaponry. The mighty oak, the mystical yew and the mistletoe were their most sacred trees.[117] Other cultures have similarly revered trees, often linking the lives and fortunes of individuals to them or used them as oracles. In Greek mythology, dryads were believed to be shy nymphs who inhabited trees. The Tree of Life in Mesopotamia linked the dead with the living and was associated with the Earth Goddess who brought fertility. The birds that perched on its branches were thought to be the souls of the dead.[118]

The Oubangui people of west Africa plant a tree when a child is born. As the tree flourishes, so does the child but if the tree fails to thrive, the health of the child is considered at risk. When it flowers it is time for marriage. Gifts are left at the tree periodically and when the individual dies, their spirit lives on in the tree.[119]

Trees have their roots in the ground and their trunk and branches extended towards the sky. This concept is found in many of the world's religions as a World Tree which links the underworld and the earth and holds up the heavens. The best known example from European mythology is the ancient Norse Yggdrasil.[120] In India, Kalpavriksha is a wish-fulfilling tree that was one of nine jewels that emerged from the primitive ocean. Icons are placed beneath it to be worshipped, tree nymphs inhabit the branches and it grants favours to the devout who tie threads round the trunk.[121] Democracy started in North America when the Great Peacemaker formed the Iroquois Confederacy, inspiring the warriors of the original five American nations to bury their weapons under the Tree of Peace, an eastern white pine (Pinus strobus).[122] In the creation story in the Bible, the tree of life and the knowledge of good and evil was planted by God in the Garden of Eden.[123]

Sacred groves exist in China, India, Africa and elsewhere. They are places where the deities live and where all the living things are either sacred or are companions of the gods. Folklore lays down the supernatural penalties that will result if desecration takes place for example by the felling of trees. Because of their protected status, sacred groves may be the only relicts of ancient forest and have a biodiversity much greater than the surrounding area.[124] Some Ancient Indian tree deities, such as Puliyidaivalaiyamman, the Tamil deity of the tamarind tree, or Kadambariyamman, associated with the kadamba tree were seen as manifestations of a goddess who offers her blessings by giving fruits in abundance.[125]

Superlative trees

The tallest living tree is believed to be a coast redwood (Sequoia sempervirens) at Redwood National Park, California. It has been named Hyperion and is 115.66 metres (379.5 ft) tall.[126] The tallest known broad-leaved tree is a swamp gum (Eucalyptus regnans) growing in Tasmania with a height of 97 metres (318 ft).[127][128] The largest tree by volume is believed to be a giant sequoia (Sequoiadendron giganteum) known as the General Sherman Tree in the Sequoia National Park in Tulare County, California. Only the trunk is used in the calculation and the volume is estimated to be 1,487 m³ (52,508 cu ft). Also in California is the oldest living tree with a verified age. It is a Great Basin bristlecone pine (Pinus longaeva) called Methuselah growing in the White Mountains. It has been dated by drilling a core sample and counting the annual rings and was considered to be 4,844 years old in 2012.[129] It is thought likely that other bristlecone pines exceed 5,000 years of age.[129] A little further south, at Santa Maria del Tule, Oaxaca, Mexico, is the tree with the broadest trunk. It is a Montezuma cypress (Taxodium mucronatum) known as Árbol del Tule and its diameter at breast height is 11.62 m (38.1 ft) giving it a girth of 36.2 m (119 ft). The tree's trunk is far from round and the exact dimensions may be misleading as the circumference includes much empty space between the large buttress roots.[130]

See also

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