By Bobbie Stephenson.
Adaptations to Aridity
We are having a glorious spring because the rains have been spaced to allow the germination of abundant annual wildflowers and blooming shrubs and trees. Native species have many adaptations to our arid climate and are able to survive years of drought such as we have recently experienced. These plants have evolved to adapt to our regional climate as it has changed over millions of years from subtropical to arid. Due to these adaptations, we cannot treat native species in our gardens the same way we treat most ornamental plants; in particular, we should not water them as much, especially in the summer.
Annual native plants avoid the extreme dryness of summer. They germinate during winter rains (a characteristic of California’s Mediterranean climate) and then complete their life cycles very quickly. The drought-resistant seeds stay dormant in the soil over the hot dry summer until the next year’s rains. But not all the seeds germinate in any given year; a seed bank is built up in the soil so that seeds are available for future years, even if we have one or more rainy seasons that don’t provide the annuals with enough water to sustain a full life cycle. An example is the native annual canchalagua, or California centaury (Zeltnera venusta).
Other plants handle, rather than avoid, the aridity. Plant stomata, the tiny epidermal pores that are most abundant in plant leaves, open to allow both carbon dioxide to enter the plant for photosynthesis, and water and oxygen to escape the plant. Although these processes are crucial to photosynthesis, losing too much water can be dangerous, and plants have found interesting methods of offsetting this loss. Many of our local coastal sage scrub species are drought-deciduous – they drop their leaves during the driest time of the year. In the desert, ocotillo (Fouquieria splendens) can sprout leaves quickly when it rains and drop leaves as soon as the weather dries, thereby reducing water loss. An ocotillo can do this repeatedly throughout a season. The desert shrub chuparosa (Justicia californica) reduces water loss by having few leaves and pale green stems that photosynthesize.
Of course, cacti are the masters of water conservation. Most species of cacti have lost true leaves, retaining only spines, which are highly modified leaves without stomata. The stomata on cacti open at night when it’s cooler rather than during the day. Cactus spines help shade the plant and reduce airflow close to it. In addition, cactus ribs can expand to store water in the plant when it rains and cacti have a waxy coating that helps seal in the moisture. Cacti have root systems that radiate away from the plant, just below the surface of the ground, ready to capture any water and conduct it to the plant’s stems. Further, within an hour of a rainfall, cactus roots sprout tiny rootlets to help absorb water quickly. These are just a few examples of how plants cope with their arid environments. Have you noticed these or other adaptations in your garden?