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...