An evergreen is a plant that 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. There are numerous kinds of evergreen plants, both trees and shrubs, including most species of conifers. Although evergreen trees lose their leaves as the leaves age, this loss is gradual throughout the year, not seasonal, and the leaves are subsequently replaced. The longevity of the leaves on evergreens ranges from a few months to several decades.
Most tropical rainforest plants are evergreens, whereas species found 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 and there is a predominance of conifers because few evergreen broadleaf plants cannot tolerate severe cold below about minus 26 degrees centigrade.
Whereas deciduous plants require a large input of energy and nutrients when they grow new leaves in the spring, evergreen trees conserve energy and nutrients by growing new foliage continuously throughout the year. This can be an advantage in regions where sunlight or other nutrients are limited, and it gives an evergreen a greater chance of surviving a spring with limited sunlight and/or rainfall than a deciduous plant. Evergreens also fertilize themselves with their nutrient-rich leaf litter, which also acts as mulch to protect the roots, and their foliage serves as year-round insulation that reduces damage to the branches and trunk from the sun and frost.