Evergreen
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[[Image:Abies alba 02.jpg|right|300px|thumb|A [[Silver Fir]] shoot showing three successive years of retained leaves]]
In [[botany]], an '''[[evergreen]]''' plant is a plant having [[leaf|leaves]] all year round. This contrasts with [[deciduous]] plants, which completely lose their [[foliage]] for part of the year.
Leaf persistence in [[evergreen]] plants may vary from a few months (with new leaves constantly being grown and old ones shed), to several decades (over thirty years in [[Great Basin Bristlecone Pine]] ''Pinus longaeva'' <ref>Ewers, F. W. & Schmid, R. (1981). Longevity of needle fascicles of ''Pinus longaeva'' (Bristlecone Pine) and other North American pines. ''[[Oecologia]]'' 51: 107–115.</ref>).
[[Image:Liveoak 8231.JPG|[[right]]|thumb|A [[Southern live oak]] in winter]]
One additional special case exists in ''[[Welwitschia]]'', an [[African]] [[gymnosperm]] plant which produces only two [[leaves]], which grow continuously throughout the plant's life but gradually wear away at the apex, giving about 20–40 years' persistence of [[leaf]] tissue.
There are many different types of evergreens, both trees and shrubs, including most species of: [[Pinophyta|conifers]] (e.g. [[Pinus classification|white]]/[[scots pine|scots]]/[[jack pine]], [[Thuja plicata|red cedar]], [[blue spruce]]), [[holly]], [[Tsuga|hemlock]], 'ancient' gymnosperms like [[cycad]]s, [[rainforest]] trees and [[Eucalypt]]s.
===Reasons for being evergreen or deciduous===
Deciduous trees shed their leaves usually as an adaptation to a cold season or a dry season. Most [[tropical rainforest]] plants are evergreens, replacing their leaves gradually throughout the year as the leaves age and fall, whereas species growing in seasonally arid climates may be either evergreen or deciduous. Most warm [[temperate climate]] plants are also evergreen. In cool temperate climates, fewer plants are evergreen, with a predominance of conifers, as few evergreen [[flowering plant|broadleaf]] plants can [[Psychrophile|tolerate severe cold]] below about -30 °C.
In areas where there is a reason for being deciduous (e.g. a cold season or dry season), being evergreen is usually an adaptation to low nutrient levels. Deciduous trees lose nutrients whenever they lose their leaves, and they must replenish these nutrients from the soil to build new leaves. When few nutrients are available, evergreen plants have an advantage. In warmer areas, species such as some [[pine]]s and [[Cupressaceae|cypresses]] grow on poor soils and disturbed ground. In ''[[Rhododendron]]'', a genus with many broadleaf evergreens, several species grow in mature forests but are usually found on highly acidic soil where the nutrients are less available to plants. In [[taiga]] or [[boreal]] forests, it is too cold for the organic matter in the soil to decay rapidly, so the nutrients in the soil are less easily available to plants, thus favouring evergreens.
In temperate climates, evergreens can reinforce their own survival; evergreen leaf and needle litter has a higher carbon-nitrogen ratio than deciduous leaf litter, contributing to a higher soil acidity and lower soil nitrogen content. These conditions favour the growth of more evergreens and make it more difficult for deciduous plants to persist. In addition, the shelter provided by existing evergreen plants can make it easier for other evergreen plants to survive cold and/or drought.<ref>Aerts, R. (1995). The advantages of being evergreen. ''Trends in Ecology & Evolution'' 10 (10): 402–407.</ref><ref>Matyssek, R. (1986) Carbon, water and nitrogen relations in evergreen and deciduous conifers. ''Tree Physiology'' 2: 177–187.</ref><ref>Sobrado, M. A. (1991) Cost-Benefit Relationships in Deciduous and Evergreen Leaves of Tropical Dry Forest Species. ''Functional Ecology'' 5 (5): 608–616.</ref>
==Idiomatic use==
Owing to the botanical meaning, the idiomatic term "evergreen" refers to something that perpetually renews itself, or otherwise remains steady and constant (doesn't suddenly halt or "die off", as leaves on a deciduous tree). An ''evergreen market'', for example, is one where there is a constant, renewed demand for an item or items, as opposed to a market which is expected to saturate eventually.
==References==
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== See also ==
* [[Deciduous]]<noinclude></noinclude>
* [[Semi-deciduous]]
[[Category:Plants]]
[[ar:مستديمة الخضرة]]
[[ca:Perennifoli]]
[[da:Stedsegrøn]]
[[de:Immergrüne Pflanzen]]
[[es:Perennifolio]]
[[eo:Ĉiamverdo]]
[[fr:Sempervirent]]
[[gl:Perennifolio]]
[[is:Sígræn jurt]]
[[it:Pianta sempreverde]]
[[he:ירוק עד]]
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[[ja:常緑植物]]
[[no:Eviggrønn]]
[[pt:Folha persistente]]
[[ru:Вечнозелёные растения]]
[[simple:Evergreen]]
[[sv:Städsegrön]]
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[[vec:Senprevert]]
[[zh:常绿植物]]