Deciduous

Deciduous means "temporary" or "tending to fall off" (deriving from the Latin word decidere, to fall off) and is typically used in reference to trees or shrubs that lose their leaves seasonally. In a more specific sense deciduous means the dropping of a part that is no longer needed, or falling away after its use is finished. In plants it is the result of natural processes, in other fields the word has similar meaning, including deciduous antlers in deer or deciduous teeth in some mammals including human children.^[1]

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Botany

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, the leaf loss coincides with winter in temperate or polar climates, while others lose their leaves during the dry season in climates with seasonal variation in rainfall. The converse of deciduous is evergreen; plants that are intermediate may be called semi-deciduous. Some tree species like some Oaks have desiccated leaves that remain on the tree through winter, they are called marcescent leaves and they are dropped in the spring as new growth begins.

Many deciduous plants flower during the period when they are leafless, as this increases the effectiveness of pollination. The absence of leaves improves wind transmission of pollen in the case of wind-pollinated plants, and increases the visibility of the flowers to insects in insect-pollinated plants. This strategy is not without risks, as the flowers can be damaged by frost, or in dry season areas, result in water stress on the plant. Nevertheless, by losing leaves in the cold winter days, plants can reduce water loss since most of the water would appear as ice, and there is much less branch and trunk breakage from glaze ice storms when leafless.^[2]

Leaf drop or abscission involves complex physiological signals and changes within plants. The process of photosynthesis steadily degrades the supply of chlorophylls in foliage; plants normally replenish chlorophylls during the summer months. When days grow short and nights are cool, or when plants are drought stressed, deciduous trees decrease chlorophyll pigment production allowing other pigments present in the leaf to become apparent, resulting in fall color. These other pigments include carotenoids that are yellow, brown, and orange. Anthocyanin pigments produce reds and purple colors, though they are not always present in the leaves but are produced in the foliage in late summer when sugars are trapped in the leaves after the process of abscission begins. Parts of the world that have showy displays of bright fall colors are limited to locations where days become short and nights are cool. In other parts of the world the leaves of deciduous trees simply fall off without turning the bright colors produced from the accumulation of anthocyanin pigments.

The beginning of leaf drop starts when an abscission layer is formed between the leaf petiole and the stem. This layer is formed in the spring during active new growth of the leaf, it consists of layers of cells that can separate from each other. The cells are sensitive to a plant hormone called auxin that is produced by the leaf and other parts of the plant. When the auxin coming from the leaf is produced at a rate consistent with that of the auxin from the body of the plant, the cells of the abscission layer remain connected; in the fall or when under stress the auxin flow from the leaf decreases or stops triggering cellular elongation within the abscission layer. The elongation of these cells break the connection between the different cell layers, allowing the leaf to break away from the plant, it also forms a layer that seals the break so the plant does not lose sap.

A number of deciduous plants remove nitrogen and carbon from the foliage before they are shed and store them in the form of proteins in the vacuoles of parenchyma cells in the roots and the inner bark. In the spring these proteins are used as a nitrogen source during the growth of new leaves or flowers.^[3]

Plants with deciduous foliage compared to plants with evergreen foliage, have both advantages and disadvantages in growth and competition for space. Since deciduous plants lose their leaves to conserve water or to better survive winter weather conditions, they must regrow new foliage when the next growing season is suitable, this uses more resources which evergreens do not need to expend.

Deciduous trees

Deciduous trees include Maple, Oak (but not all species), Elm, Aspen, and Birch, among others, as well as a number of coniferous genera, such as Larch and Metasequoia. Periods of leaf fall often coincide with seasons: winter in the case of cool-climate plants or the dry-season in the case of tropical plants.^[4]

Regions

Deciduous forests can be found in sections of: North America, South America, Europe, Asia, Australia, and Africa (Madagascar). Forests with a majority of tree species that lose their foliage at the end of the typical growing season are called deciduous forests. These forests have distinctive ecosystems, understory growth, and soil dynamics.^[5]

Two distinctive types of deciduous forest are found growing around the world.

Temperate deciduous forest biomes are plant communities distributed in America, Asia and Europe. The have formed under climatic conditions which have great seasonable temperature variability with growth occurring during warm summers and leaf drop in fall and dormancy during cold winters. These seasonally distinctive communities have diverse life forms that are impacted greatly by the seasonality of their climate, mainly temperature and precipitation rates. These varying and regionally different ecological conditions produce distinctive forest plant communities in different regions.

  Tropical and semi tropical deciduous forest biomes have developed in response not to seasonal temperature variations but to seasonal rainfall patterns. During prolonged dry periods the foliage is dropped to conserve water and prevent death from drought. Leaf drop is not seasonally dependent as it is in temperate climates, and can occur any time of year and varies by region of the world. Even within a small local area there can be variations, with different sides of the same mountain showing great variations in leaf drop, as well as areas that have low water tables or along streams and rivers.^[6]

See also

• Evergreen
• Semi-evergreen
• Semi-deciduous
• evergreen-deciduous
• Madagascar dry deciduous forests
• Temperate deciduous forest

References