Tree

For other uses, see Tree (disambiguation).

Common ash (Fraxinus excelsior),
a broad-leaved tree

European larch (Larix decidua),
a coniferous tree

A tree is a perennial woody plant. It most often 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. 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 which can convert light energy to chemical energy by photosynthesis and provide the food needed by the tree for its growth and development. Also sometimes present are the flowers and fruit which provide the seeds needed for the tree to reproduce.

Trees tend to be long-lived, some reaching several thousand years old and growing up to 115 m (379 ft) tall. Trees have been in existence on the Earth for 370 million years and are found growing worldwide wherever the climate permits. 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. They play a significant part in reducing erosion and moderating the climate. They remove carbon dioxide from the atmosphere and store up 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.

Types and terminology

A tree is a tall perennial plant with a woody stem or trunk. It differs from a shrub in that it is usually larger and has a single main stem whereas a shrub usually has several stems. There are two principal groups of vascular or higher plants, the gymnosperms and the angiosperms. The gymnosperms include conifers, cycads, ginkgophytes and gnetales. They are characterised by having naked seeds that are not contained in ovaries. The angiosperms are the flowering plants and these are characterised by having seeds formed in ovaries which have endosperm surrounding the embryo. Most angiosperm trees are dicotyledons, so named because the seeds contain two cotyledons or seed leaves, but one important group, the palms, is monocotyledonous, having a single cotyledon. In many 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. Other palms have many stems. Some of the tree ferns, order Cyatheales, have tree-like growth forms, growing up to 20 metres (66 ft) in tropical forests but they are structurally very different from other trees. Their trunks are composed of rhizomes which grow vertically and which are covered by numerous adventitious roots.

Trees are either evergreen, having foliage that persists and remains green throughout the year, or deciduous, shedding their leaves at the end of the growing season and then having a dormant period without foliage. 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. The crown is a name for the upper part of a tree including the branches and leaves and the uppermost layer in a forest, formed by the crowns of the trees, is known as the canopy. A sapling is a young tree. The wood of conifers is known as softwood while that of broad-leaved trees is hardwood.

A small wooded area, usually with no undergrowth, is called a grove and a small wood or thicket of trees and bushes is called a coppice or copse. A large area of land covered with trees and undergrowth is called woodland or forest. An area of woodland composed primarily of trees established by planting or artificial seeding is known as a plantation and an area of land planted with fruit or nut trees is an orchard. 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. A forest of great age is called ancient woodland and a forest in its natural state, before being explored or exploited by man is a virgin forest.

Parts and function

The ground has eroded away around the roots of this young red pine (Pinus resinosa)

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. 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 and potassium in solution. The root system spreads out about twice as far as the tree's canopy extends with most of the roots found in the top 2 ft (0.6 m) of soil. The roots need to breathe and only a few species such as the mangrove and the pond cypress (Taxodium ascendens) can live in permanently waterlogged soil.

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. 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. The upriver orange mangrove (Bruguiera sexangula) has "kneed" roots which hump up out of the water and have lenticels or breathing holes.

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. The strangler fig takes this a stage further. It starts life as an epiphyte high in the tree canopy 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.

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.

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. 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. 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.

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. 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.

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. By overtopping other plants and shading them out, trees become the terminal vegetation cover on all but the most infertile and inhospitable land. 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.

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. These tend to get blocked in polluted environments and the London plane (Platanus × acerifolia) gets round this by periodically shedding its bark in large flakes. Similarly, the bark of the silver birch (Betula pendula) peels off in strips. As the tree's girth expands, fissures appear in the bark and new corky growth appears at the base of the split. Different species of tree can be recognised from each other by the characteristics of their bark. 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.

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.

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 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. Inside the phloem is a layer of undifferentiated cells one cell thick called the vascular cambium layer. This is where all the growth of the tree takes place. The cells are constantly dividing, creating phloem cells on the outside and wood cells known as xylem on the inside.

The newly created xylem is the sapwood. It is composed of living cells and is usually pale in colour. It transports water and minerals from the roots to the upper parts of the tree. The greater the size of the tree's canopy and the more vigorously it is growing, the wider the sapwood needs to be. The oldest, inner part of the sapwood is progressively converted into heartwood as new sapwood is formed at the cambium. The heartwood consists of dead cells and is usually darker in colour than the sapwood. It is the dense central core of the trunk giving it rigidity and serving as a store of food reserves. 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 are the annual growth rings. There are a number of dark streaks called rays running at right angles to these. They are plates of living cells transporting materials between the sapwood and the heartwood. If the tree is damaged and water and fungal spores come in contact with the heartwood, decay will occur and the heartwood will rot. This happens in many older trees which become hollow as a result but may still stand upright for many years.

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.

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 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. 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. Palm trees have their leaves spirally arranged on an unbranched trunk.

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. 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. Sooner or later 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. As long as the crown remains in balance with the roots 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 and the tree may in time become hollow.

Leaves

Buds, leaves, flowers and fruit of oak (Quercus robur)

Buds, leaves, flowers and fruit of white fir (Abies alba)

A tree's leaves contain the pigment chlorophyll which allows the leaf to absorb energy from light. This is used in a process called photosynthesis to convert carbon dioxide, obtained from the air, and water, drawn up from the roots, into glucose and oxygen. The tree's branches and twigs are spread out in such a way as to maximise the amount of light falling on the leaves. The petioles or leaf stems have hinged joints where they join the stem which allow the leaves to vibrate in the wind and increase the rate of gas exchange. Each leaf is several cells thick and is surrounded by a transparent epidermis which is covered by a waterproof cuticle which prevents unwanted water loss. There are openings in the epidermis called stomata through which gases can enter and leave. The openings are mostly on the underside of the leaf and the guard cells surrounding them can expand and contract to regulate the amount of water that is lost. Nevertheless, a large tree can lose 300 litres (66 imp gal; 79 US gal) a day during the summer through transpiration. Water is drawn up through the xylem from the roots by the suction produced as it evaporates from the leaves. If insufficient water is available, as may happen in times of drought, the leaves droop, and if this state continues they will die.

In the interior cells of the leaf, the chlorophyll is contained in organelles called chloroplasts. Here the reactions take place which turn carbon dioxide and water into glucose and oxygen. The different substances involved are moved between the cells in the leaf by osmosis. The underside of the leaf has veins which are part of the vascular system. In them are branches of the xylem bringing water and minerals up from the roots and branches of the phloem taking the finished products away from the leaf.

The leaves of trees come in a wide range of shapes and sizes which suit the habitat in which the tree grows. 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 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. Where resources permit, the higher leaf nitrogen concentrations in broad-leaved species enables them to out-compete conifers. Broad leaved trees in temperate regions have a different 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. 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. Often this will coincide with the onset of the dry season or some other climatic event. Many deciduous trees flower before the new leaves emerge.

Reproduction

Form, leaves and reproductive structures of queen sago Cycas circinalis

The sweet-scented flowers of the fony baobab (Adansonia rubrostipa) open at dusk and are pollinated by moths.

Gymnosperms have different reproductive structures to those of flowering plants and their life cycle involves an alternation of generations. In conifers, a mature tree is the sporophyte and bears reproductive organs called strobili or cones. Microspores develop in the male cones and megaspores in the female ones. There are two sporangia in each scale of the male cone in which meiosis occurs and a tetrad of spores is formed. Gametophyte development continues by mitosis and results in two prothallial cells, a tube cell and a generative cell. The sporangium breaks open and the immature gametophyte, the pollen grain, is carried away by the wind. In each scale of the female cone the two megasporangia undergo meiosis and four megaspores are formed. One of these develops into a multi-celled female gametophyte which takes up to a year to develop and produces two or three archegonia with egg cells. When a wind-borne pollen grain encounters a suitable female cone, the tube cell lengthens and enters the megasporangium through a small aperture, the micropyle. The generative cell eventually divides into two sperm cells which travel through the tube and one of these fertilises the egg to produce a zygote. The embryo develops between the scales of the cone into a naked seed which is shed after about two years. The cycads have a similar reproductive cycle but pollination is often performed by insects and the sperm in the pollen grain has flagella and is motile. The ginkgos are dioecious. Male trees bear catkin-like strobili and the microsporangia produce pollen grains that are dispersed by the wind. Female trees bear megasporangia at the end of twigs, each with two ovules. Typically one of these develops after fertilisation into a fruit with two layers of fleshy tissue surrounding a hard seed.

The flowers of angiosperm trees are similar to those of other flowering plants. They have two essential parts for reproduction; the male reproductive organs, the Androecium, which consists of stamens that produce pollen, and the female reproductive organs, the Gynoecium which includes the stigma that receives the pollen, and the ovaries that contain eggs. Many tree flowers are hermaphrodite, with both male and female parts in the same flower, but other tree species such as the hazel, beech and birch are monoecious, with separate male and female flowers on the same tree. Others such as the holly and yew are dioecious and have their male and female flowers on separate trees. This means that not all holly trees are capable of bearing berries, and a female tree will only do so if there is a male tree in the vicinity.

Wind pollination occurs in conifers and in many broad-leaved trees. These trees often have separate male and female flowers to avoid self-pollination. Their flowers are characterised by a lack of showy parts, no scent and a copious production of pollen. 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. A vast quantity of pollen is produced because of the low likelihood of any particular grain landing on a female flower. 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). Tree pollen can cause allergies such as hay fever and tends to be the main cause of this in the spring. Palynology is the study of the pollen grains that were deposited in the past in lake beds and peat bogs. Examination of these and identification of the predominant species of tree from which the pollen grains came gives an idea of what the climate was like at the time. With the help of radiocarbon dating, absolute dates can be obtained and such things as the advance and retreat of ice sheets can be deduced.

Insect pollination is a method commonly used by trees. The flowers have brightly coloured petals, produce plenty of nectar and are often sweetly scented to attract insects. The pollen grains of these flowers tend to be large and somewhat sticky. To avoid self-pollination, the stamens and stigma may not mature simultaneously. Bees like a landing platform and seek out yellow and blue flowers and those with ultra-violet guidelines. Butterfly pollinated flowers tend to have large flat heads with the nectar hidden in deep recesses for them to probe with their long proboscises. Butterflies are attracted to red flowers while moths prefer whites and pale colours that show up well at night.

In the tropics and southern temperate zones, birds are important pollinators. They often have long, narrow, curved beaks which they insert into trumpet-shaped flowers. Birds cannot smell so the flowers pollinated by them tend to have no perfume. They are often red however, a colour to which birds' eyes are particularly sensitive. Hummingbirds, sunbirds, honeyeaters, honeycreepers, white-eyes and flowerpeckers all feed on nectar and pollinate the flowers at the same time. The flowers of baobab trees are large, pale-coloured, perfumed and nectar-rich and are pollinated by moths and bats.

Seeds

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

For a seedling tree to develop it needs light and space. If tree seeds fell straight to the ground there would be intense competition among the saplings growing in the shade of their parent and none would be likely to flourish. Different trees have developed varying strategies for distributing their seed further afield. 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. The seeds of conifers are enclosed in a cone but the seed itself is light and papery and can be blown considerable distances. Sometimes it 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, enriching the forest floor with wood ash and removing competing vegetation. Similarly, acacia seed germinates better after exposure to heat. 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.

Alder trees grow beside rivers and have seed enclosed in miniature cones. The seeds contain small droplets of oil and can be dispersed by water, floating downstream until grounded in a backwater. Coconut palms have sprung up on islands after the fruit have floated there across the sea. Mangroves grow in the water and have propagules, buoyant seeds which start germinating before becoming detached from the parent tree. These float on the water and may become lodged on emerging mudbanks and successfully take root.

Other seeds, such as apple pips and plum stones, have fleshy receptacles and smaller fruits like the haw have their seeds enclosed in edible tissue. Birds and animals eat these fruits and the seeds are either discarded or 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. Nuts with tough outer casings are gathered by animals that hide any not required for immediate consumption. Many of these caches are never revisited, the nut-casing softens with rain and frost and the seed germinates in the spring. 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.

Distribution

Trees are found throughout the world wherever the climate permits. Which trees grow in which locations depends primarily on the amount of rain that falls, its distribution throughout the year and the temperature range. In the coldest parts of the world, particularly in the northern hemisphere, conifers tend to predominate but in the hotter and wetter climate of the tropics, broad-leaved trees are more common. In many places where the climate is suitable, trees are the climax vegetation. In an area of vacant land, perhaps left untended by agriculture, plants soon appear. In the first year these are mostly rapid growing annuals to be followed in subsequent years by biennials, perennials and small shrubs. Tree seedlings appear and grow up through the undergrowth. As these develop into saplings and mature trees, less light penetrates to ground level. Shade tolerance in young tree of different species varies and may determine the pattern of forest succession. Pines are particularly intolerant to shade while many broad-leaved tree saplings in forests grow tall and thin but survive. If a gap in the canopy appears as a result of a large tree falling, these young trees are ready to grow rapidly and make use of the light. Even if pines dominate woodland at an early stage in its succession, they may be over-topped and eventually suppressed by broad-leaved species.

In the tropics the sun is nearly overhead and the temperature is high. Thunderstorms occur most afternoons as the heat of the sun causes warm moist air to rise and form clouds. Trees grow well all year round under these conditions. They are mostly evergreen, broad-leaved trees which support a rich, biodiverse community. The canopy is about 30 metres (98 ft) above the forest floor with a few even taller trees emerging above it. Beneath this are understorey layers of smaller trees, palms and branches strung with lianas, orchids, ferns, bromeliads and bryophytes. There are few shrubs and plants on the forest floor because little light filters through to ground level. More than half the species of plants and animals on the Earth are thought to live in tropical rainforests even though these occupy just five percent of terrestrial land area. Further from the equator there is usually a drier part of the year alternating with a wet period when the monsoon rains arrive. Different species of broad-leaved trees dominate the forest here, some of them being deciduous.

Between 20° and 35° of latitude the climate gets drier and savanna is the predominate landscape. This rolling grassland tends to have scrubby small trees or isolated large ones. Acacia and baobab are well adapted to living in such areas with limited rainfall. The canopy is not closed and plenty of sunshine reaches the ground which is covered with grass and scrub.

In the temperate zone, up to about 50° of latitude, the temperature tends to be moderate in summer and cold in winter. Where rainfall is adequate for them, the naturally occurring species are broad-leaved deciduous trees. Plenty of light reaches the forest floor during much of the year and there is usually a dense cover of brambles and other small shrubs and plants which mostly flower in the spring. In drier areas such as heathland, pines or other conifers may occur but not usually in dense stands.

Further north again in Europe, Asia and North America is the realm of the coniferous forest (there are no similar land masses in the southern hemisphere). The long cold winter is unsuitable for plant growth and the 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. 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.

Evolutionary history

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

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. 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. 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. 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. Ginkgophyta was once a widespread diverse group 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.

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 only to be driven back again at the start of the next ice age.

Tree ecology

Trees are an important part of the terrestrial ecosystem. They supply oxygen into the atmosphere that helps sustain organisms living in distant parts of the planet, a commensal relationship that benefits other species at no cost or harm to the trees. Because of their size, they greatly enlarge the volume of space available in which animals and plants may live. They provide a habitat for a community of animals and plants that are quite different from the ground-based ones. There is safety among the branches from terrestrial predators, a place for birds to feed, monkeys to climb, tree frogs to reproduce and bees to nest. Ferns, orchids and mistletoe hang from the mossy and lichen-clad branches providing their own microhabitats. Leaves, flowers and fruits are seasonally available. On the ground underneath trees there is shade, dappled light, shelter, undergrowth, leaf litter, fallen branches and decaying timber. Trees play a part in soil and water conservation, stabilize 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.

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) and 306 species of invertebrate on the Tasmanian oak (Eucalyptus obliqua). The planting of non-native species of tree provides a much less biodiverse community. The sycamore (Acer pseudoplatanus) originates from southern Europe and in the United Kingdom has few associated invertebrate species though its base rich bark does support a wide range of lichens, bryophytes and epiphytes.

Uses

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. They produce oxygen and sequester carbon, removing carbon dioxide from the air and incorporating it into their tissues. When they die the carbon is released slowly back into the atmosphere as they gradually decay. The tropical rainforests have been called the "lungs of the planet" and when they are cleared and burned, vast quantities of carbon dioxide get returned to the atmosphere. Planted in towns, they remove pollution from the air and moderate the climate round buildings. They can improve people's health and sense of well being.

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. 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.

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. 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.

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 and those of the European bay tree (Laurus nobilis) and the California bay tree (Umbellularia californica) are used for flavouring food.

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. 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%. 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.

Sap from palm trees such as the date (Phoenix dactylifera) contains a high proportion of sucrose. When tapped and collected, this liquor ferments naturally and in twenty four hours produces a 5% v/v alcoholic drink known as palm wine or toddy.

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).

Fuel

Main article: Wood 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. 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.

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.

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.

Timber

Main article: Wood

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.

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.

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.

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.

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. More than half the world's cork comes from Portugal and is largely used to make stoppers for wine bottles. Other uses include floor tiles, bulletin boards, balls, footwear, cigarette tips, packaging, insulation and joints in woodwind instruments.

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.

At least a hundred and twenty drugs come from plant sources, many of them from the bark of trees. Quinine originates from the cinchona tree (Cinchona) and was for a long time the remedy of choice for the treatment of malaria. Aspirin was synthesized to replace the sodium salicylate derived from the bark of willow trees (Salix) which had unpleasant side effects. The anti-cancer drug Paclitaxel is derived from taxol, a substance found in the bark of the Pacific yew (Taxus brevifolia ). 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).

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. 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.

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. 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.

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.

Apart from a low quality timber from the trunk, products from palm trees have many other uses. The fibres are used to make rope, the whole leaves are used for thatching and partitioning, their midribs for construction, crate-making and furniture and the leaflets are plaited and used to make baskets and matting. The camphor tree (Cinnamomum camphora) produces an essential oil and the eucalyptus tree (Eucalyptus globulus) is the main source of eucalyptus oil which is used in medicine, as a fragrance and in industry.

Mythology

Main article: Tree worship

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. 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.

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.

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. 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. 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). 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.

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.

Record trees

Main article: Record-breaking 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. The tallest known broad-leaved tree is a swamp gum (Eucalyptus regnans) growing in Tasmania with a height of 97 metres (318 ft). 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. It is thought likely that other bristlecone pines exceed 5,000 years of age. 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.

See also

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