Ashrub orbush is a small to medium-sizedperennialwoody plant. Unlikeherbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished fromtrees by their multiplestems and shorterheight, less than 6–10 m (20–33 ft) tall.[1][2] Small shrubs, less than 2 m (6.6 ft) tall are sometimes termed assubshrubs.
Some[who?] define a shrub as less than 6 m (20 ft) and a tree as over 6 m tall. Others[who?] use 10 m (33 ft) as the cutoff point for classification.[2] Many trees do not reach this mature height because of hostile, less than ideal growing conditions, and resemble shrub-sized plants. Others in such species have the potential to grow taller in ideal conditions. Some only last about five years in good conditions. Others, usually larger and more woody, live beyond 70 years. On average, they die after eight years.[3]
Shrubland is the natural landscape dominated by various shrubs; there are many distinct types around the world, includingfynbos,maquis,shrub-steppe,shrub swamp andmoorland. In gardens and parks, an area largely dedicated to shrubs (now somewhat less fashionable than a century ago) is called ashrubbery, shrub border or shrub garden. There are many gardencultivars of shrubs, bred for flowering, for examplerhododendrons, and sometimes even leaf colour or shape.
Apart from the severalberry-bearing shrub species (using the culinary rather than botanical definition), few are eaten directly, and they are generally too small for muchtimber use unlike trees. Those that are used include several perfumed species such aslavender androse, and a wide range of plants with medicinal uses.Tea andcoffee are on the tree-shrub boundary;[4] they are normally harvested from shrub-sized plants, but these would be large enough to become small trees if left to grow instead.
Shrubs are perennial woody plants, and therefore have persistent woodystems above ground (compare with succulent stems ofherbaceous plants). Usually, shrubs are distinguished from trees by their height and multiple stems. Some shrubs aredeciduous (e.g.hawthorn) and othersevergreen (e.g.holly).[2]Ancient Greek philosopherTheophrastus divided the plant world into trees, shrubs and herbs.[5] Small, low shrubs, generally less than 2 m (6.6 ft) tall, such aslavender,periwinkle and most small garden varieties ofrose, are often termed assubshrubs.[6][7]
Most definitions characterize shrubs as possessing multiple stems with no main trunk below. This is because the stems have branched below ground level. There are exceptions to this, with some shrubs having main trunks, but these tend to be very short and divide into multiple stems close to ground level without a reasonable length beforehand. Many trees can grow in multiple stemmed forms also while being tall enough to be trees, such asoak orash.[2]
Ecologists and evolutionary biologists have paid little attention to why there are two major growth forms of woody plants; shrubs and trees, both occurring on large land area, and often together. While the time when the first shrubs appeared on the Earth is difficult to establish, hypotheses explaining the adaptive value of having multiple stems, compared to a single one in trees, exist. One model is based on the advantages of having many stems, with larger total stem area for photosynthesis (on, and under bark), larger area of cambium and phloem, and larger area for sprouting from stems, compared to a (small) tree with same initial woody volume.[8] This model could successfully predict faster shrub growh than tree growth at early stages. Other advantages for shrubs (compared to trees) include for instance extra stems in case one die, and faster seed production. It is not surprising that at least angiosperm trees, if cut or broken, revert to shrub growth form for fast growth (due to their capacity of sprouting at the base, and thus becoming shrub-like for some time).
Many shrubs, such asSalix species, have good dispersal ability (seeds disperse by air) and many shrubs are also able to grow in more extreme habitats, such as alpine areas, where trees are less common. The last decades have seen 'shrubification' in the arctic, with marked increase of shrublands. One reason is increased temperature, with consequences for plant community structure and carbon balance.[9]
An area ofcultivated shrubs in apark or agarden is known as ashrubbery.[10] When clipped astopiary, suitable species or varieties of shrubs develop densefoliage and many small leafybranches growing close together.[11] Many shrubs respond well to renewalpruning, in which hard cutting back to a "stool", removes everything but vital parts of the plant, resulting in long new stems known as "canes".[12] Other shrubs respond better to selective pruning to dead or unhealthy, or otherwise unattractive parts to reveal their structure and character.[13]
Inbotany andecology, a shrub is more specifically used to describe the particularphysical canopy structure orplant life-form of woody plants which are less than 8 metres (26 ft) high and usually multiple stems arising at or near the surface of the ground. For example, a descriptive system widely adopted inAustralia is based on structural characteristics based on life-form, plus the height and amount of foliage cover of the tallest layer ordominantspecies.[15]
For shrubs that are 2–8 metres (6.6–26.2 ft) high, the following structural forms are categorized:
dense foliage cover (70–100%) —closed-shrubs
mid-dense foliage cover (30–70%) —open-shrubs
sparse foliage cover (10–30%) —tall shrubland
very sparse foliage cover (<10%) —tall open shrubland
For shrubs less than 2 metres (6.6 ft) high, the following structural forms are categorized:
