Evergreen

A Silver Fir shoot showing three successive years of retained leaves

In botany, the term evergreen refers to a tree, shrub, or other plant having foliage that persists throughout the year. This terminology includes cold-tolerant species in temperate and Arctic zones 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 (with new leaves constantly being grown and old ones shed), to several decades (over thirty years in Great Basin bristlecone pine Pinus longaeva (Ewers and Schmid 1981). However, very few species show leaf persistence of over 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 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 (e.g. white/scots/jack pine, red cedar, blue spruce), as well as such plants as holly, palms, gymnosperms like cycads, rainforest trees, and eucalypts.

Owing to the botanical meaning, the idiomatic term "evergreen" refers to something that perpetually renews itself, or otherwise remains steady and constant (doesn't suddenly halt or "die off", as leaves on a deciduous tree.) An evergreen market, for example, is one where there is a constant, renewed demand for an item or items, as opposed to a market which is expected to eventually saturate.

Overview

File:Liveoak 8231.JPG

A Southern live oak in winter

The two basic categories of plants delineated in terms of duration of leaves are deciduous and evergreen.

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 is called abscission. In some cases leaf loss coincides with winter—namely in temperate or polar climates—while in other areas of the world, plants lose their leaves during the dry season or during other seasonal variations in rainfall, including tropical, subtropical and arid regions of the world. The deciduous characteristic has developed repeatedly among woody plants. 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, and 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, where 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).

There also are botanical categories intermediate between deciduous and evergreen. Semi-deciduous are those plants that 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 are those 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 as "bearing green leaves into or through the winter, but dropping them by the beginning of the next growing season; tardily deciduous or winter deciduous." Some trees, including a few oak species have desiccated leaves that remain on the tree through winter; these dry persistent leaves are called marcescent leaves and are dropped in the spring as new growth begins.

Although the term evergreen often is used as if synonymous with the cone-bearing conifers (division Pinophyta), it 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 are not evergreen, but deciduous, such as larch (genus Larix).

The characteristic of evergreen versus deciduous is useful in plant identification. For intance, in parts of Southern California and the American Southeast, deciduous and evergreen oak species may grow side by side.

Reasons for being evergreen or deciduous

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.

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, 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 suffer greater water lose 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 1994).

References

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• Aerts, R. 1995. The advantages of being evergreen. Trends in Ecology & Evolution 10(10): 402–407.

• Ewers, F. W., and R. Schmid. 1981. Longevity of needle fascicles of Pinus longaeva (Bristlecone pine) and other North American pines. Oecologia 51: 107–115..

• Labandeira, C. C., D. L. Dilcher, D.R. Davis, and D. L. Wagner. 1994. Ninety-seven million years of angiosperm-insect association: paleobiological insights into the meaning of coevolution. Proceedings of the National Academy of Sciences of the United States of America 91(25): 12278-12282. Retrieved May 23, 2008.

• Matyssek, R. 1986. Carbon, water and nitrogen relations in evergreen and deciduous conifers. Tree Physiology 2: 177–187.

• Seiberling, S. M. 2005. Glossary of botanical terms used in the OpenKey Project. The Illinois - North Carolina Collaborative Environment for Botanical Resources, a project of the University of Illinois at Urbana Champaign and the University of North Carolina at Chapel Hill. Retrieved May 23, 2008.

• Sobrado, M. A. 1991. Cost-benefit relationships in deciduous and evergreen leaves of tropical dry forest species. Functional Ecology 5(5): 608–616.

• Weber, W. 2001. African Rain Forest Ecology and Conservation: An Interdisciplinary Perspective. New Haven: Yale University Press. ISBN 0300084331.

See also

• Deciduous
• Semi-deciduous

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