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botany Botany, also called , plant biology or phytology, is the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowledge in th ...

botany
, an evergreen is a plant which has foliage that remains green and functional through more than one growing season. This also pertains to plants that retain their foliage only in warm climates, and contrasts with deciduous plants, which completely lose their foliage during the winter or dry season.


Evergreen species

There are many different kinds of evergreen plants, both
tree
tree
s and
shrub A shrub (or bush, but this is more of a gardening term) is a small- to medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be deciduous or evergreen. They are distingu ...
s. Evergreens include: *Most species of conifers (e.g.,
pine A pine is any Pinophyta, conifer in the genus ''Pinus'' () of the family (biology), family Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. The Plant List compiled by the Royal Botanic Gardens, Kew and Missouri Botanical Garden a ...
,
hemlock Hemlock may refer to: Plants *Several poisonous plants in the family Apiaceae **''Cicuta'' (water hemlock) **''Conium'', four species, of which ''maculatum'' is the only endemic outside of southern Africa; in history given to poison and execute p ...
, blue spruce, and red cedar), but not all (e.g.,
larch Larches are conifers in the genus ''Larix'', of the family Pinaceae (subfamily Laricoideae). Growing from tall, they are native to much of the cooler temperate northern hemisphere, on lowlands in the north and high on mountains further south. La ...
) * Live oak, holly, and "ancient"
gymnosperm The gymnosperms, also known as Acrogymnospermae, are a group of seed-producing plants that includes conifers, cycads, '' Ginkgo'', and gnetophytes. The term "gymnosperm" comes from the composite word in el, γυμνόσπερμος ( el, γ ...
s such as cycads *Most angiosperms from frost-free climates, such as eucalypts and rainforest trees * Clubmosses and relatives The Latin binomial term '' sempervirens'', meaning "always green", refers to the evergreen nature of the plant, for instance :''Cupressus sempervirens'' (a cypress) :''Lonicera sempervirens'' (a honeysuckle) :''Sequoia sempervirens'' (a sequoia) Leaf longevity in evergreen plants varies from a few months to several decades (over thirty years in the Pinus longaeva, Great Basin Bristlecone Pine).


Evergreen families

Sciadopitys, Japanese umbrella pine is unique in that it has its own family of which it is the only species.


Differences between evergreen and deciduous species

Evergreen and deciduous species vary in a range of morphological and physiological characters. Generally, broad-leaved evergreen species have thicker leaves than deciduous species, with a larger volume of parenchyma and air spaces per unit leaf area. They have larger leaf biomass per unit leaf area, and hence a lower specific leaf area. Construction costs do not differ between the groups. Evergreens have generally a larger fraction of total plant biomass present as leaves (LMF), but they often have a lower rate of photosynthesis.


Reasons for evergreen or deciduous

Image:Live oak Georgetown.jpg, A Quercus virginiana, southern live oak in South Carolina during winter Deciduous trees shed their leaves usually as an adaptation to a cold or dry/wet season. Evergreen trees do lose leaves, but each tree loses its leaves gradually and not all at once. Most tropical rainforest plants are considered to be 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. In this climate, there is a predominance of conifers because few evergreen flowering plant, broadleaf plants can Psychrophile, tolerate severe cold below about . In areas where there is a reason for being deciduous, e.g. a cold season or dry season, evergreen plants are usually an adaptation of low nutrient levels. Additionally, they are usually sclerophyllous,(meaning hard-leaved) and have an excellent water economy due to scarce resources in the area in which they reside. The excellent water economy within the evergreen species is due to high abundance when compared to deciduous species. Whereas, deciduous trees lose nutrients whenever they lose their leaves. In warmer areas, species such as some
pine A pine is any Pinophyta, conifer in the genus ''Pinus'' () of the family (biology), family Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. The Plant List compiled by the Royal Botanic Gardens, Kew and Missouri Botanical Garden a ...
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 Soil organic matter, organic matter in the soil to decay rapidly, so the nutrients in the soil are less easily available to plants, thus favoring 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. This is the case with Mediterranean evergreen seedlings, which have unique C and N storages that allow stored resources to determine fast growth within the species, limiting competition and bolstering survival. These conditions favor 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 younger evergreen plants to survive cold and/or drought.


Threats to the Evergreen

A major threat to evergreen trees is the fungus ''Rhizosphaera kalkhoffii,'' which causes the commonly green needles to appear brown or purple in colour. This disease is called needle cast disease, and is widespread in Minnesota and other US states. This fungi spreads through splashing water to adjacent trees and needles. It has been found that needle infection occur at an average temperature of 25 degrees Celsius. Some particular trees are in fact resistant to the disease, such as Plant Norway (''P. abies'') or white spruce (''P. glauca''), while others such as Colorado blue spruce (''Picea pungens'') are extremely at risk of contracting the disease. Preventative efforts to protect the evergreens include fungicide, mulch at the base of the tree and increased sprinkler control to prevent the water contamination. Within recent years, this detrimental disease has entered into Canada and provides challenges for commercial conifer tree growers (such as Christmas trees), and extensive research was conducted into possible fungicides for this disease with no clear cut option being presented. Therefore the future research regarding evergreen longevity is highly linked to additional counter measures to ''Rhizosphaera kalkhoffii'' by looking at other fungicide potions.


See also

*Semi-deciduous (semi-evergreen)


References


External links

* {{Cite Americana, wstitle=Evergreens, author=Helen Ingersoll , short=x Plants Botany