In botany, an evergreen is a plant that has leaves throughout the year, 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., pine, hemlock, blue spruce, red cedar, and white/scots/jack pine), but not all (e.g., larch)
live oak, holly, and "ancient" gymnosperms 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 persistence in evergreen plants varies from a few months 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/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, with a predominance of conifers, as few evergreen broadleaf plants can tolerate severe cold below about −30 °C (−22 °F).
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.