In botany, an evergreen is a plant which has foliage that remains green and functional through more than one growing season. This contrasts with deciduous plants, which completely lose their foliage during the winter or dry season.
The Latin binomial term sempervirens, meaning "always green", refers to the evergreen nature of the plant, for instance:
Leaf longevity in evergreen plants varies from a few months to several decades (over thirty years in the Great Basin bristlecone pine[1]).
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.[2] They have larger leaf biomass per unit leaf area, and hence a lower specific leaf area. Construction costs do not differ between the groups.[citation needed] Evergreens have generally a larger fraction of total plant biomass present as leaves (LMF),[3] but they often have a lower rate of photosynthesis.
Deciduous trees shed their leaves usually as an adaptation to a cold or dry/wet season. Evergreen trees also 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.[citation needed] In cool temperate climates, fewer plants are evergreen. In such climates, there is a predominance of conifers because few evergreen broadleaf plants can tolerate severe cold below about −26 °C (−15 °F).[clarification needed][citation needed]
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 usually have hard leaves and have an excellent water economy due to scarce resources in the area in which they reside.[4] The excellent water economy within the evergreen species is due to high abundance when compared to deciduous species.[4] Whereas 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 favoring evergreens.