In botany, an evergreen is a plant that has leaves in all four seasons, always green. This contrasts with deciduous plants, which completely lose their foliage during the winter or dry season. There are many different kinds of evergreen plants, both trees and shrubs. Evergreens include:
- most species of conifers (e.g., hemlock, blue spruce, red cedar, and white/scots/jack pine)
- live oak, holly, and "ancient" gymnosperms such as cycads
- most angiosperms from frost-free climates, such as eucalypts and rainforest trees
The Latin binomial term sempervirens (literally, "always green") refers to the evergreen nature of the plant, for instance:
- Acer sempervirens (a maple)
- Cupressus sempervirens (a cypress)
- Lonicera sempervirens (a honeysuckle)
- Sequoia sempervirens (a sequoia)
- Ulmus parvifolia 'Sempervirens' (an elm)
An additional special case exists in Welwitschia, an African gymnosperm plant that produces only two leaves which grow continuously throughout the plant's life but gradually wear away at the apex. Welwitschia can live for over 1,000 years.
Leaf persistence in evergreen plants varies from a few months (with new leaves constantly being grown as old ones are shed) to several decades (over thirty years in the Great Basin Bristlecone Pine).
Reasons for being evergreen or deciduous
Deciduous trees shed their leaves usually as an adaptation to a cold or dry season. Evergreen trees do lose leaves, but not all at the same time the way that deciduous trees do. Different trees shed their leaves at different times, so the forest as a whole looks green. 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, with a predominance of conifers, as few evergreen broadleaf plants can 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. In warmer areas, species such as some pines and 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 younger evergreen plants to survive cold and/or drought.
Evergreen plants and deciduous plants have almost all the same diseases and pests, but long-term air pollution, ash and toxic substances in the air are more injurious for evergreen plants than deciduous plants (for example spruce Picea abies in European cities).
Owing to the botanical meaning, the term "evergreen" can refer metaphorically to something that is continuously renewed or is self-renewing. One example of metaphorical use of the expression is the term "Evergreen content" used to describe perennial articles or guides about topics that do not change frequently.
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- Aerts, R. (1995). "The advantages of being evergreen". Trends in Ecology & Evolution 10 (10): 402–407.
- Matyssek, R. (1986) "Carbon, water and nitrogen relations in evergreen and deciduous conifers". Tree Physiology 2: 177–187.
- Sobrado, M. A. (1991) "Cost-Benefit Relationships in Deciduous and Evergreen Leaves of Tropical Dry Forest Species". Functional Ecology 5 (5): 608–616.
- Gomes, Diego. (2011).  What is evergreen content