When leafout is completed (marked by the transition from bright green spring leaves to dark green summer ones) the chlorophyll level in the leaves remains stable until cool temperatures arrive in autumn. Leaf drop or abscission involves complex physiological signals and changes within plants. However, for most temperate regions it takes place in late September through early November and in subtropical climates such as the southern US, it may be November into December. In sub-Arctic climates such as Alaska, leaves begin turning colors as early as August. They will be among the earliest trees to lose their leaves in fall. The exact conditions required will vary with the species, but generally more cold-tolerant genera such as Salix will leave out earlier and lose their leaves later, while genera such as Fraxinus and Juglans can only grow in warm, frost-free conditions so they need at least 13 hours of daylight and air temperatures of around 70☏ (21☌) to leaf out. Spring leafout and fall leaf drop are triggered by a combination of daylight and air temperatures. This strategy is not without risks, as the flowers can be damaged by frost or, in dry season regions, result in water stress on the plant. The absence of leaves improves wind transmission of pollen for wind-pollinated plants and increases the visibility of the flowers to insects in insect-pollinated plants. Many deciduous plants flower during the period when they are leafless, as this increases the effectiveness of pollination. Like a number of other deciduous plants, Forsythia flowers during the leafless season. Other plants are semi-evergreen and lose their leaves before the next growing season, retaining some during winter or dry periods. Plants that are intermediate may be called semi-deciduous they lose old foliage as new growth begins. The converse of deciduous is evergreen, where foliage is shed on a different schedule from deciduous plants, therefore appearing to remain green year round because not all the leaves are shed at the same time. In other parts of the world, including tropical, subtropical, and arid regions, plants lose their leaves during the dry season or other seasons, depending on variations in rainfall. In some cases leaf loss coincides with winter-namely in temperate or polar climates. 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. "Deciduous" has a similar meaning when referring to animal parts, such as deciduous antlers in deer, deciduous teeth (baby teeth) in some mammals (including humans) or decidua, the uterine lining that sheds off after birth. In plants, it is the result of natural processes. Generally, the term "deciduous" means "the dropping of a part that is no longer needed or useful" and the "falling away after its purpose is finished". The antonym of deciduous in the botanical sense is evergreen. ə s/) means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn to the shedding of petals, after flowering and to the shedding of ripe fruit. In the fields of horticulture and botany, the term deciduous ( / d ɪ ˈ s ɪ dʒ u. Three deciduous forests shown in summer, autumn and winter.
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