Evergreen
In botany, an evergreen plant is a plant that retains its leaves all year round, with each leaf persisting for more than 12 months. This contrasts with deciduous plants, which completely lose all their foliage for part of the year, becoming bare and leafless.
Leaf persistence in evergreen plants may vary from only just over a year (shedding the old leaves very soon after the new leaves appear), up to a maximum of 45 years in Great Basin Bristlecone Pine Pinus longaeva (Ewers & Schmid 1981). However, very few species show leaf persistence of over 5 years.
One additional special case exists in Welwitschia, an African gymnosperm plant which 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.
Reasons for being evergreen
In warm tropical regions, most rainforest plants are evergreen, 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 -25 °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, even though their leaves and needles must be able to withstand cold and/or drought, and are thus less efficient at photosynthesis. 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 other evergreen plants to survive cold and/or drought.
References
- Aerts, R. (1995). The advantages of being evergreen. Trends in Ecology & Evolution 10 (10): 402-407.
- Ewers, F. W. & Schmid, R. (1981). Longevity of needle fascicles of Pinus longaeva (Bristlecone Pine) and other North American pines. Oecologia 51: 107-115.
- Matyssek, R. (1986) Carbon, water and nitrogen relations in evergreen and deciduous conifers. Tree Physiology 2: 177–187.
- Sobrado, M. A. (1991) Cost-Benefit Relationships in Deciduous and Evergreen Leaves of Tropical Dry Forest Species. Functional Ecology 5 (5): 608-616.
See also
- Evergreen game, a famous chess game
