A perennial plant that seasonally loses its leaves (and/or stems) is deciduous. This loss is typically related to water stress; for example, green portions of the plant are shed during the dry season. Dry does not have to mean no (or low) precipitation . It simply means that little water is available for the plant. Winter in some areas brings heavy snows; unfortunately, the ground water is frozen and unavailable for the plant. Most of the water lost by a plant occurs because of transpiration (evaporation through its leaves). By dropping its leaves during the dry season, deciduous plants are able to avoid dehydration.
In colder climates, it is typically the reduction in both day length and temperature that cause the process to begin. In desert (or equatorial) regions, the winter (shortest days) is often the rainy season. Many plants in these areas do not develop leaves or aboveground portions until they have sufficient moisture. Loss of these portions is more closely associated with a decrease in moisture rather than a change in day length.
Whether caused by shorter day length, cooler temperatures, or decreased moisture availability, the process is the same. Environmental cues prompt the production of an important hormone, ethylene, which in turn induces the formation of an abscission zone. This zone is near the point of attachment of leaf (or stem) to the rest of the plant. The abscission zone is composed of two parts: the separation layer and the protection layer. The separation layer marks the area where the leaf will break away from the plant or abscise. The protection layer seals the wound left by the dropped leaf, protecting the plant from infection and moisture loss.
The formation of the abscission zone breaks many of the connections the leaf had with the plant. This causes a decrease in the amount of water available to the leaf, causing a consequent decrease in chlorophyll production. With the loss of chlorophyll, the other pigments in the leaf (anthocyanins and carotenoids) show through. The net result is the dramatic fall color changes seen in many deciduous forests.
Not all perennial plants are deciduous. Many are considered evergreen, since they retain their leaves throughout the year. Deciduous and evergreen plants can often be found in the exact same habitats growing next to one another. Each has its own adaptations to deal with seasonal water stress. For instance, some adaptations that evergreens have to deal with dry conditions include thick, waxy leaves, leaves with low surface area (conifer needles, for example), or water storage tissues (such as cacti).
There is a cost to dropping leaves, which is why some perennial plants are not deciduous. It can take a lot of nutrients from the soil to produce the number of leaves produced in an average growing season. In areas where nutrients are plentiful, an annual leaf drop is not a problem. However, in areas with nutrient poor soils, dropping leaves may be very costly for the plant. The advantages of decreased water stress and the detriments of a seasonal loss of nutrients must be balanced to insure an evolutionarily stable strategy.
see also Anthocyanins; Carotenoids; Chlorophyll; Deciduous Forests; Hormones; Leaves; Pigments; Trees.
de·cid·u·ous / diˈsijoōəs/ • adj. (of a tree or shrub) shedding its leaves annually. Often contrasted with evergreen. ∎ inf. (of a tree or shrub) broad-leaved. ∎ denoting the milk teeth of a mammal, which are shed after a time.DERIVATIVES: de·cid·u·ous·ly adv.de·cid·u·ous·ness n.