Deciduous

In the fields of horticulture and botany, the term deciduous (/dɪˈsɪdʒuəs/) 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. Generally, the term deciduous means 'the dropping of a part that is no longer needed' and the 'falling away after its purpose is finished'. In plants, it is the result of natural processes. '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 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. 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. The converse of deciduous is evergreen, where foliage is shed on a different schedule from deciduous trees, therefore appearing to remain green year round. Plants that are intermediate may be called semi-deciduous; they lose old foliage as new growth begins. Other plants are semi-evergreen and lose their leaves before the next growing season, retaining some during winter or dry periods. 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 for 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 regions, result in water stress on the plant. Nevertheless, there is much less branch and trunk breakage from glaze ice storms when leafless, and plants can reduce water loss due to the reduction in availability of liquid water during cold winter days. The conditions that trigger leafout in spring can vary depending on the species or genera of plant. Most herbaceous perennials and some woody plants are triggered to leaf out by warming air or soil temperatures, for example birches (Betula) and willows (Salix) will attempt to send out flowers and/or leaves if there are a few days where ambient air temperatures exceed 50°F (10°C). This strategy is risky as a renewed burst of cold air may freeze off the new growth. Other woody plants such as oaks, walnuts, and hickories leaf out based on photoperiod, meaning they wait until the day length is long enough. These tend to be plants that have frost-intolerant foliage, so leafout is held off until late spring when the danger of frost has largely passed. Leaf drop in the fall months is based on photoperiod and varies by genera and species. Walnuts tend to drop their leaves early while some trees such as Norway Maple and willows have extremely late leaf drop, often in the middle of November. 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 autumn arrives and the days are shorter or when plants are drought-stressed, deciduous trees decrease chlorophyll pigment production, allowing other pigments present in the leaf to become apparent, resulting in non-green colored foliage. The brightest leaf colors are produced when days grow short and nights are cool, but remain above freezing. These other pigments include carotenoids that are yellow, brown, and orange. Anthocyanin pigments produce red and purple colors, though they are not always present in the leaves. Rather, they 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 autumn 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.