The Baja California Elephant Tree (Pachycormus discolor) is a microphyllous (small-leaved),
Jon P. Rebman, Ph.D., Mary and Dallas Clark Curator of Botany
As noted in Part One of this article, published last month, many of our San Diego plants have evolved various physical and physiological adaptations to survive and even thrive in the dry conditions of our region.
One of the most common adaptations throughout the world’s dry areas is that of succulence in leaves and stems. Succulent plants have at least some tissues that are modified and capable of storing large amounts of water, making the plant part appear fleshy, succulent, or swollen. For instance, the fleshy leaves of agaves and aloes, the fattened stems of cacti, and the swollen trunks of elephant and Boojum trees are all obvious examples of xerophytic modifications. Succulent plants are sometimes even further differentiated from other plants with fleshy leaves or stems by having the ability to use stored water reserves in their tissues and subsequently tolerate long periods of aridity, although it is sometimes difficult to discern this functional difference.
Throughout the botanical kingdom, species in various plant families and genera have independently evolved the adaptation to succulence as a mechanism for conserving water and survival in arid environments. San Diego County has a relatively high diversity of native succulent plants. It is estimated that there is a total of 66 succulent taxa (64 species) in 26 genera and 14 different plant families in the county. The two plant families that have the most succulent members within the borders of San Diego County are Cactaceae (24 species) and Crassulaceae (12 species).
Other physical features such as hairs and spines shade the plant and decrease the surface temperature where transpiration occurs or where the plant is exposed directly to the environment. The rate of water evaporation doubles for every temperature increase of about 10° Celsius. Thus, it is important for the plant to keep its surfaces as cool as possible to conserve its water. For example, dense hair patches and spine clusters are located on the growing tips of most cacti such as seen in the California Barrel Cactus (Ferocactus cylindraceus). These structures not only provide shade for the plan’s surface, which reduces water loss, but they also create a “boundary layer” of humid air near the surface of the plant. This reduces water loss by creating a gradient of moisture from the stomata (minute openings in the leaf epidermis) to the arid environment surrounding them. Water is lost more slowly in a humid environment than in dry air. At the surface of the plant where the stomata are located, there is 100% humidity and water is being converted from a liquid to a vapor. In a typical desert environment, the relative humidity of the air is about 16%. Thus, by having a boundary layer of stable air over the stomata that is higher in humidity than that of the desert, a plant can greatly reduce its water loss.
Smaller leaves like those found on many desert trees and shrubs such as Blue Palo Verde (Cercidium floridum) and the Elephant Tree (Bursera microphylla) also reduce plant surface temperatures because there is not as much leaf surface to heat up under the intense desert sun. Many of these small-leaved species are also drought deciduous and lose their leaves during times of drought or when water is not readily available, reducing continued water loss through the leaf stomata.
In respect to physiology, some plants have evolved to live with reduced levels of photosynthesis and are very adept at utilizing low levels of carbon dioxide so that they do not have to expose themselves as much to the environment. These are called C4 plants and they are usually found in dry climates with high daytime temperatures and intense sunlight. Familiar examples of C4 plants include Corn (Zea mays), Bermuda Grass (Cynodon dactylon), and amaranth (Amaranthus spp.) species. Various succulent plants such as cacti, agaves, and dudleyas in our area have taken this adaptation one step further and are defined as CAM (Crassulacean Acid Metabolism) plants. These succulents open their stomata at night to take in carbon dioxide and convert it to an organic acid which is stored until the next day when light is available as the energy source to fuel photosynthesis. Then, the plants can break down the stored acids and use the carbon for making sugars. The advantage of this in the desert environment is that temperatures are much cooler at night and much less water is lost to the environment through transpiration, thus conserving their precious water reserves.
Since plants cannot run or hide from the sun or the heat of the day, they have necessarily evolved various processes and features for dealing with water loss in their own way. There are probably just about as many different strategies and adaptations for coping with water relations as there are plants on the face of the Earth. Water, or the lack thereof, has altered and shaped the plants around us, creating the beauty and diversity of plants species that we see in our region today.
Jon P. Rebman, Ph.D., is the Mary and Dallas Clark Curator of Botany at the San Diego Natural History Museum. Photos by Jon Rebman, Ph.D.
The Cedros Island Century Plant (Agave sebastiana) has succulent leaves covered by a thick, protective cuticle made of wax that gives it a gray coloration and helps it to reduce water loss.
SAN DIEGO NATURAL HISTORY: FIELD NOTES, October 2008