Trees
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Trees are not a taxonomic group but include a variety of plant species that have independently evolved a woody trunk and branches as a way to tower above other plants to compete for sunlight. Trees tend to be long-lived, some reaching several thousand years old. In looser definitions, the taller palms , tree ferns , bananas and bamboos are also trees. Trees have been in existence for million years. It is estimated that there are just over 3 trillion mature trees in the world.
A tree typically has many secondary branches supported clear of the ground by the trunk. This trunk typically 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.
Phylogenetic classifications
Below the ground, the roots branch and spread out widely; they serve to anchor the tree and extract moisture and nutrients from the soil. Above ground, the branches divide into smaller branches and shoots. The shoots typically bear leaves, which capture light energy and convert it into sugars by photosynthesis , providing the food for the tree's growth and development.
Trees usually reproduce using seeds. Flowers and fruit may be present, but some trees, such as conifers, instead have pollen cones and seed cones. Palms, bananas, and bamboos also produce seeds, but tree ferns produce spores instead. Trees play a significant role 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 among 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, with sacred groves in various cultures, and they play a role in many of the world's mythologies. Although "tree" is a term of common parlance, there is no universally recognised precise definition of what a tree is, either botanically or in common language. A commonly applied narrower definition is that a tree has a woody trunk formed by secondary growth , meaning that the trunk thickens each year by growing outwards, in addition to the primary upwards growth from the growing tip. Certain monocots may be considered trees under a slightly looser definition; [8] while the Joshua tree , bamboos and palms do not have secondary growth and never produce true wood with growth rings, [9] [10] they may produce "pseudo-wood" by lignifying cells formed by primary growth.
Aside from structural definitions, trees are commonly defined by use, for instance as those plants which yield lumber. The tree growth habit is an evolutionary adaptation found in different groups of plants: They differ from shrubs , which have a similar growth form, by usually growing larger and having a single main stem; [5] but there is no consistent distinction between a tree and a shrub, [17] 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 60,, species, the number of trees worldwide might total twenty-five per cent of all living plant species. The majority of tree species are angiosperms. There are about species of gymnosperm trees, [21] including conifers , cycads , ginkgophytes and gnetales ; they produce seeds which are not enclosed in fruits, but in open structures such as pine cones , and many have tough waxy leaves, such as pine needles.
There are also some trees among the old lineages of flowering plants called basal angiosperms or paleodicots ; these include Amborella , Magnolia , nutmeg and avocado, [23] while trees such as bamboo, palms and bananas are monocots. Wood gives structural strength to the trunk of most types of tree; this supports 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. Trees, as relatively tall plants, need to draw water up the stem through the xylem from the roots by the suction produced as water evaporates from the leaves.
If insufficient water is available the leaves will die. 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, [26] or deciduous , shedding their leaves at the end of the growing season and then having a dormant period without foliage. Many tall palms are herbaceous [31] monocots; these do not undergo secondary growth and never produce wood.
Why are trees so important?
The number of trees in the world, according to a estimate, is 3. The estimate is about eight times higher than previous estimates, and is based on tree densities measured on over , plots. It remains subject to a wide margin of error, not least because the samples are mainly from Europe and North America. The estimate suggests that about 15 billion trees are cut down annually and about 5 billion are planted. In suitable environments, such as the Daintree Rainforest in Queensland , or the mixed podocarp and broadleaf forest of Ulva Island, New Zealand , forest is the more-or-less stable climatic climax community at the end of a plant succession, where open areas such as grassland are colonised by taller plants, which in turn give way to trees that eventually form a forest canopy.
In cool temperate regions, conifers often predominate; a widely distributed climax community in the far north of the northern hemisphere is moist taiga or northern coniferous forest also called boreal forest. Light is very limited under their dense cover and there may be little plant life on the forest floor, although fungi may abound. Where rainfall is relatively evenly spread across the seasons in temperate regions, temperate broadleaf and mixed forest typified by species like oak, beech, birch and maple is found.
In tropical regions with a monsoon or monsoon-like climate, where a drier part of the year alternates with a wet period as in the Amazon rainforest , different species of broad-leaved trees dominate the forest, some of them being deciduous. Acacia and baobab are well adapted to living in such areas. 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. They are also used for reproduction, defence, survival, energy storage and many other purposes.
The radicle or embryonic root is the first part of a seedling to emerge from the seed during the process of germination. This develops into 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 taproot 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 potassium in solution.
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. 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.
12 Trees That Will Ruin Your Yard
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 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. Some trees such as the alders Alnus species have a symbiotic relationship with Frankia species, 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. It has been demonstrated that some trees are interconnected through their root system, forming a colony. The interconnections are made by the inosculation process, a kind of natural grafting or welding of vegetal tissues. The tests to demonstrate this networking are performed by injecting chemicals, sometimes radioactive , into a tree, and then checking for its presence in neighbouring trees.
The roots are, generally, an underground part of the tree, but some tree species have evolved roots that are aerial. The common purposes for aerial roots may be of two kinds, to contribute to the mechanical stability of the tree, and to obtain oxygen from air. An instance of mechanical stability enhancement is the red mangrove that develops prop roots that loop out of the trunk and branches and descend vertically into the mud. 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.
Some tree species have developed root extensions that pop out of soil, in order to get oxygen, when it is not available in the soil because of excess water. These root extensions are called pneumatophores , and are present, among others, in black mangrove and pond cypress. The main purpose of the trunk is to raise the leaves above the ground, enabling the tree to overtop other plants and outcompete them for light. In the case of angiosperms and gymnosperms, the outermost layer of the trunk is the bark , mostly composed of dead cells of phellem cork. 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 or phellogen. 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. In some trees such as the pine Pinus species 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 species 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 s, 25 million elm trees were killed by this disease. 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. The cells are continually 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 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.
We must respect them and protect them for the future. This tree, which attains heights in excess of 90 metres and may have a trunk diameter of about 7. The evolution of discrete plant body parts with separate functions allowed plants to move onto the land and undergo an incredible adaptive radiation. Tree climbing is an "on rope" activity that puts together many different tricks and gear originally derived from rock climbing and caving. Wood from the tree species have long been used for furniture. They can be grouped informally, often surrounded by plantings of bulbs, laid out in stately avenues or used as specimen trees. As seen in the section above, trees are found among many plant families that also include shrubs and herbs, so that the concept of tree is not a phylogenetic one.
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. A transverse section through a tree trunk or a horizontal core will show concentric circles or lighter or darker wood - tree rings. These rings are the annual growth rings [64] There may also be rays running at right angles to growth rings. These are vascular rays which are thin sheets of living tissue permeating the wood.
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 climatic conditions; growth normally ceases 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. The expanding shoot pushes its way out, shedding the scales in the process. These leave behind scars on the surface of the 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 Arecaceae palms have their leaves spirally arranged on an unbranched trunk. Primary growth is the elongation of the stems and roots. Secondary growth 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. You are signed up.
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The parts of a tree are the roots , trunk s , branches , twigs and leaves. Tree stems are mainly made of support and transport tissues xylem and phloem. Wood consists of xylem cells, and bark is made of phloem and other tissues external to the vascular cambium. As a tree grows, it may produce growth rings as new wood is laid down around the old wood. In areas with seasonal climate, wood produced at different times of the year may alternate light and dark rings.
In temperate climates, and tropical climates with a single wet-dry season alternation, the growth rings are annual, each pair of light and dark rings being one year of growth. In areas with two wet and dry seasons each year, there may be two pairs of light and dark rings each year; and in some mainly semi-desert regions with irregular rainfall , there may be a new growth ring with each rainfall.
In tropical rainforest regions, with constant year-round climate, growth is continuous. Growth rings are not visible and there is no change in the wood texture. In species with annual rings, these rings can be counted to find the age of the tree. This way, wood taken from trees in the past can be dated, because the patterns of ring thickness are very distinctive. Very few tropical trees can be accurately dated in this manner.
The roots of a tree are generally down in earth, providing anchorage for the parts above ground, and taking in water and nutrients from the soil. Most trees need help from a fungus for better uptake of nutriens: Most of a tree's biomass comes from carbon dioxide absorbed from the atmosphere see photosynthesis.
Above ground, the trunk gives height to the leaf-bearing branches, competing with other plant species for sunlight. In many trees, the order of the branches makes exposure of the leaves to sunlight better. Not all trees have all the organs or parts as mentioned above. For example, most palm trees are not branched, the saguaro cactus of North America has no functional leaves, tree ferns do not produce bark, etc. Based on their general shape and size, all of these are nonetheless generally regarded as trees. Trees can vary very much. Even though that is true, no precise differentiation between shrubs and trees is possible.
Given their small size, bonsai plants would not technically be 'trees', but one should not confuse reference to the form of a species with the size or shape of individual specimens. A spruce seedling does not fit the definition of a tree, but all spruces are trees. A tree is a plant form that can be found in many different orders and families of plants. Trees show many growth forms , leaf type and shape, bark traits and organs. The tree form has changed separately in classes of plants that are not related, in response to similar problems for the tree.
With about , types of trees, the number of tree types in the whole world might be one fourth of all living plant types. The earliest trees were tree ferns , horsetails and lycophytes , which grew in forests in the Carboniferous period; tree ferns still survive, but the only surviving horsetails and lycophytes are not of tree form. Later, in the Triassic Period, conifers , ginkgos , cycads and other gymnosperms appeared, and subsequently flowering plants in the Cretaceous period. Most species of trees today are flowering plants Angiosperms and conifers.
A small group of trees growing together is called a grove or copse , and a landscape covered by a dense growth of trees is called a forest. Several biotopes are defined largely by the trees that inhabit them; examples are rainforest and taiga see ecozones. A landscape of trees scattered or spaced across grassland usually grazed or burned over periodically is called a savanna.
A forest of great age is called old growth forest or ancient woodland in the UK. A very young tree is called a sapling. The tallest trees in Australia are all eucalypts , of which there are more than species. The so-called 'mountain ash'. The stoutest living single-trunk species in diameter is the African baobab: Some trees develop multiple trunks whether from an individual tree or multiple trees which grow together.
The sacred fig is a notable example of this, forming additional 'trunks' by growing adventitious roots down from the branches, which then thicken up when the root reaches the ground to form new trunks; a single sacred fig tree can have hundreds of such trunks. The life-span of trees is determined by growth rings.