For your Trivial Pursuits an evergreen plant has its leaves through all seasons. The opposite is a deciduous plant that loses all of its foliage during winter or dry season.
There are numerous variations of evergreen plants, both trees and shrubs. Evergreens are 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 are included and most angiosperms from the frost-free climates such as eucalyptus and rainforest trees. As with everything in life, there is an exception and that is an African gymnosperm plant that produces only two leaves that 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 ranges from a few months to several decades.
What are the reasons for being evergreen or deciduous?
The key factors that make a plant either an evergreen or deciduous is temperature, moisture, and soil nutrients. 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 broad-leaf 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.
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 favor 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 one thing in common, almost all have the same diseases and pests. Again an exception, long-term air pollution, ash and toxic substances in the air are more injurious for evergreen plants than deciduous plants.
Owing to the botanical meaning, the term "evergreen" can refer metaphorically to something that is continuously renewed or is self-renewing.
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