Deciduous and Evergreen Desert Trees

Among desert and desert-adapted tree species, the typically distinct line between evergreen and deciduous trees can get a bit fuzzy. Evergreens can be defined as any plant (tree or shrub) that retains a full compliment of leaves the year round. In reality evergreen trees are losing leaves and growing new ones constantly. Deciduous trees lose all their leaves on a seasonal basis, usually in the fall and winter months. For deciduous trees, leaf shed is controlled by the plant's genetics and biochemical machinery which uses environmental cues, like day length (the number of hours of sunlight) to "measure" the time of year. When days shorten, as winter approaches, the tree physiology shifts from producing new leaves to dropping leaves.

As part of their adaptation to arid conditions many desert species can shed leaves in response to adverse environmental conditions or retain leaves over an extended period if conditions are conducive to growth. Water loss through plant leaves (trans-piration) is essential for photosynthesis, the movement of water from the roots to the shoots of plants and to regulate the temperature of the leaf surface. Plants lose the majority of their water through the leaves, so regulating this loss is critical to the survival of desert species. To survive in desert climates, desert adapted species can drop their leaves in response to drought, extreme temperature (high and low), the onset of seasonal new spring growth, as well as changes in day length.

Desert tree species may fall into one or more of the following categories: Evergreens, like many of the Australian Acacias, that hold nearly a full canopy of leaves all year long; Day-length Deciduous, like Honey Mesquites, that lose all their leaves during the fall and winter; Temperature Deciduous, like Thornless Mesquite, where the extent of leaf shed is dependent on winter temperatures (as temperatures grow progressively colder, more and more leaves are shed, but in mild winters or within warmer microclimates fewer leaves are shed). These are sometimes called Semi-Deciduous; Drought and High Temperature Deciduous are self-explanatory terms, but both are temporary conditions and most desert species re-grow new leaves rapidly when favorable environmental conditions return. In response to drought or sudden extreme high temperature, desert trees shed leaves rapidly as a means of conserving precious water. Conversely, warm, mild fall and winter days present opportunities for Temperature Deciduous desert trees to extend their growing season. In response to these conditions, many leaves may remain attached with some growth continuing into the fall months. Some leaves may remain active until the following spring. In native desert settings this ability to extend the growing season has significant advantages.

In the urban landscape, where water and nutrients can be applied more frequently, desert species may fail to "harden off"  as winter approaches. Any new, tender growth can be severely damaged by sudden freezing temperatures. There are environmental and horticultural differences between the native desert growing conditions and those found in the urban landscape. For example a tree may grow along a stream in it’s native habitat, or in a dry wash that only runs after a heavy rain, yet in the urban landscape the tree may be situated close to a building, walkway or raised, bermed area. Some species, then, may be evergreen in native settings and deciduous or semi-deciduous in urban landscapes, and vice versa. A given desert tree species may exhibit one or more of these deciduous habits depending on the local weather conditions or microclimatic factors.

Typical Leaf Shed Characteristics of Trees Grown by Arid Zone Trees

The information listed here is a combination of data from the horticultural/botanical literature and our staff's experience with these species when grown in the desert southwest of the United States.

a) A given desert tree species may exhibit one or more leaf shed habit depending on the local weather conditions, microclimate or other factors.

b) Semi-Deciduous is intended to cover trees that lose some of their leaves in response to drought, temperature extremes (high or low), or that retain leaves in selected microclimates. Some trees may be semi-deciduous or deciduous for the first few years following transplanting, but are evergreen or less deciduous once established