In botany, the term evergreen refers to a tree, shrub, or other plant having foliage that persists throughout the year. This terminology includes temperate and Arctic-zone cold-tolerant species whose foliage remains throughout the winter and into the next growing season the following summer, and tropical and semi-tropical plants whose foliage remains for more than one annual cycle. Evergreen contrasts with deciduous, which refers to plants that completely lose their foliage for part of the year.
Leaf persistence in evergreen plants may vary from a few months (following a constant process of new leaves being grown and old ones shed) to several decades (over thirty years in Great Basin bristlecone pine Pinus longaeva) (Ewers and Schmid 1981).
There are many different types of evergreens, including trees, shrubs, and vines; species of conifers, flowering plants, and cycads; and plants with broadleaf, needlelike, scalelike, or other leaf types. Evergreens include most species of conifers (for example, white/scots/jack pine, red cedar, blue spruce), as well as such plants as holly, palms, gymnosperms like cycads, rainforest trees, and eucalypts. Retention of leaves even during the winter confers certain advantages to evergreen trees, but for humans in cold climates it also adds to the diversity and beauty of nature, providing green foliage in stark contrast to the snow and leafless trees.
Owing to the botanical meaning, the idiomatic term "evergreen" has come to refer to something that perpetually renews itself, or otherwise remains steady and constant (does not suddenly halt or "die off," as leaves on a deciduous tree.) In business, for example, an evergreen market is one where there is a constant, renewed demand for an item or items. In folk customs, a high proportion of plants utilized are evergreen, either because of symbolic meanings—such as representing the unconquered life-force or immortality—or because of the practicality of being available in all seasons (Simpson and Roud 2000).
In botany and horticulture, deciduous plants, including trees, shrubs and herbaceous perennials, are those that lose all of their leaves for part of the year. This process of leaf loss is called abscission. In some cases, leaf loss coincides with winter—namely in temperate or polar climates. In other areas of the world, including tropical, subtropical and arid regions, plants may lose their leaves during the dry season or during other seasonal variations in rainfall. The deciduous characteristic occurs widely among woody plants. Deciduous trees include maple, many oaks, elm, aspen, and birch, among others, as well as a number of coniferous genera, such as larch and Metasequoia. Deciduous shrubs include honeysuckle, poison oak, and many others. Most temperate woody vines are also deciduous, including grapes, poison ivy, virginia creeper, wisteria, and so forth. However, there are no deciduous species among tree-like monocotyledonous plants, such as palms and yucca.
The converse of deciduous is evergreen, in which green foliage is persistent year round. In the Glossary of Botanical Terms used in the OpenKey Project of University of North Carolina at Chapel Hill and the University of Illinois at Urbana-Champaign, evergreen is defined as "bearing green leaves through the winter and into the next growing season" and "persisting two or more growing seasons" (Seiberling 2005). Although the term evergreen often is used as if synonymous with the cone-bearing conifers (division Pinophyta), particularly those with needle-like leaves, evergreen includes all types of plants, with many broad-leafed flowering plants having foliage that persists throughout the year in tropical and semi-tropical areas, while some conifers, such as the larch (genus Larix) with its needle-like leaves, are not evergreen, but deciduous.
The persistence of leaves in evergreen plants varies from a few months, with new leaves constantly replacing old ones that are shed, to leaves lasting over thirty years in the Great Basin bristlecone pine, Pinus longaeva (Ewers and Schmid 1981). However, very few species show leaf persistence of more than five years. 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 about 20–40 years' persistence of leaf tissue.
There also are botanical categories intermediate between deciduous and evergreen. Semi-deciduous plants lose their old foliage as new growth begins; that is, they lose their foliage for a very short period, when old leaves fall off and new foliage growth is starting. This phenomenon occurs in tropical and sub-tropical woody species, for example in Mimosa bimucronata. Semi-deciduous may also describe mild-weather trees that lose leaves in a manner similar to deciduous trees in an especially cold autumn. Semi-evergreen may also describe plants that lose their leaves before the next growing season but retain some during winter or during dry periods (Weber 2001). Seiberling (2005) defines semi-evergreen (also tardily deciduous or winter deciduous) as "bearing green leaves into or through the winter, but dropping them by the beginning of the next growing season." Some trees, including a few oak species, retain desiccated leaves on the tree through winter; these dry persistent leaves are called marcescent leaves and are dropped in the spring as new growth begins.
The characteristic of evergreen versus deciduous is useful in plant identification. For instance, in parts of Southern California and the American Southeast, deciduous and evergreen oak species may grow side by side.
Deciduous trees shed their leaves usually as an adaptation to a cold season or a 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 needle-leaf 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 (for example, 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, and they must replenish these nutrients from the soil to build new leaves. When few nutrients are available, evergreen plants have an advantage. 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.
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 other evergreen plants to survive cold and/or drought (Aerts 1995; Matyssek 1986; Sobrado 1991).
Plants with deciduous foliage have both advantages and disadvantages compared to plants with evergreen foliage. Since deciduous plants lose their leaves to conserve water or to better survive winter weather conditions, they must regrow new foliage during the next suitable growing season; this uses more resources, which evergreens do not need to expend. Evergreens in comparison suffer greater water loss during the winter and they also can experience greater predation pressure, especially when small. Losing leaves in winter may reduce damage from insects; repairing leaves and keeping them functional may be more costly than just losing and regrowing them (Labandeira et al. 1994).
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