deciduous [ dee-si-joo-us ] adjective: not evergreen, used in botany to describe trees, shrubs, and lianas (vines) that shed all their leaves in one season
Deciduous trees fall mostly in the category of broadleaf trees, though there are a few conifers, such as larches and bald cypresses, which also lose their leaves in fall. The word comes from the Latin decidere, to fall off, and applies to trees that lose their leaves regularly in the fall season rather than describing leaf loss caused by drought or disease.
Deciduous forests appear in temperate climate zones with about 30 to 60 inches of precipitation distributed throughout the year. The three primary deciduous forest biomes are in eastern North America, Western Eurasia, and Northeastern Asia. Although much of the original temperate broadleaf deciduous forest in eastern North America has been replaced with second growth trees, it is nevertheless the most intact and varied in tree species as well as the flora and fauna they support. Deciduous forests are vulnerable to clearcutting because their processes produce a particularly humus-rich soil over time, making them desirable to populations wanting more agricultural land. Major and repeated clear-cuts have in many cases replaced the original varied forests of Europe and Asia with monocultures or at least far less tree diversity, resulting in a similar decline in diversity of other plants and animals in those areas.
Temperate climate zones tend to have a growing season of approximately six months. Trees leaf out in spring as days get longer and the weather warms. Most broadleaf trees require high levels of nutrients, which they bind into their leaves. In the fall, as days shorten and temperatures cool, the nutrient transporting systems of the trees slow down, causing a drought-like effect on the leaves, and reducing chlorophyll production. This allows other pigments to show, giving deciduous forests their often attractive autumn colors. The combination of less light and lower temperatures eventually causes the leaves to fall off and the trees to go dormant for winter.
In spring, warmer weather hastens decay of the thick layer of leaves dropped in fall, allowing their nutrients to return to the soil just in time to feed the new leaf growth, bloom, and eventual fruits and seeds of the season. The organically rich soil formed under broadleaf trees is called brown soil, distinct from the sands and more compacted red and yellow clays of other biomes.
Temperate broadleaf deciduous forests have five recognized layers: the tall stratum trees that grow to 60–100 feet, the small tree or sapling layer, the shrubs, the herbaceous layer of perennial forbs, and the ground layer of lichens, clubmosses, and true mosses, some of which also grow on tree trunks. In addition there can be lianas, or woody vines, climbing the trees.
In the Mid-Atlantic region, the tall trees include oaks, tulip poplar, maples, sweetgum, hickorys, and beech among others; many deciduous forests include evergreens such as pines, eastern hemlock, and eastern redcedar as well, though as a lesser percentage of total tree mass than the broadleafs. The montane spruce-fir-hardwood forests grow at the highest elevations and have a greater percentage of conifers and fewer deciduous trees such as striped and mountain maples, yellow birch, and mountain-ash. (As of 2020, Virginia had the largest known mountain-ash (Sorbus americana) in the country, growing at an elevation of 5338 feet.)
Understory Trees, Shrubs, and Vines
The smaller tree layer can include Amelanchier arborea (downy serviceberry), Asmina triloba (pawpaw), Cercis canadensis (eastern redbud), Cornus florida (flowering dogwood), and Ostrya virginiana (hop-hornbeam, ironwood). Shrubs such as Hamamelis virginiana (witch hazel), Ilex verticillata (winterberry), and Lindera benzoin (spicebush) appear in the shrub layer. Wisteria frutescens (American wisteria) is a native woody vine that can climb into the deciduous forest, as does Toxicodendron radicans (poison ivy), which can be either a vine or a woody shrub.
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