In botany, an evergreen plant is a plant that has leaves in all seasons. 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:
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, giving 20–40 years' persistence of leaf tissue.
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. 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 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.
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