The Mediterranean Garden
Surviving Summer Drought
Long summer drought. Months without rain. Water rationing. These are phrases that are familiar to Californians, and recently water issues have received a great deal of attention in the media. An understanding of the role of water in natural communities will help you appreciate the remarkable capacity for native plants to survive for months without rain.
In the Garden it is helpful to compare the extreme drought adaptations of the Succulent Garden plants with the less obvious drought adaptations of the Fern Canyon plants. Succulent Garden plants exhibit small, leathery, thick leaves that may be white-colored, waxy, or hairy. The Fern Canyon plants have larger, softer leaves. Notice the leaf orientation of desert plants compared with canyon plants.
In the closed-cone pine forest, many plants have broad, horizontal leaves. Patches of redwood sorrel (Oxalis oregana) have horizontal leaves in the shade, but when in direct sunlight, the leaflets collapse. After the plant is in the shade again, the leaflets spread out horizontally as water pressure in the plant becomes normal.
California is characterized by a Mediterranean climate; cool, moist winters and dry summers. Mediterranean climates are typical of areas between 30 and 35 degrees latitude on the west coasts of continents. Mediterranean climates are found in California, the Mediterranean region, South Africa, Chile, and small parts of Australia. Plants that are native to these areas have adaptations that enable them to survive the long summer drought.
Some remarkably adapted plants are native to arid regions. These include:
- Drought evaders germinate with fall and winter rains, and grow, flower, and set seeds by the end of spring. Thus, they pass the dry season living as small embryos in seeds, but respiring very slowly and losing almost no water.
- Summer dormant herbaceous perennials "go underground." They pass the dry season as dormant bulbs, rhizomes, or enlarged roots and put up shoots only after sufficient rainfall. Many native bulbs (mariposa lily - Calochortus, Brodiaea), ferns (California polypody - Polypodium californicum, goldenback fern - Pentagramma triangularis), and other plants (johnny jump-ups - Viola pedunculata) are summer dormant.
- Many shrubs and trees are summer deciduous. The California buckeye (Aesculus californicus) loses its leaves by late summer and several common shrubs (coastal sagebrush - Artemisia californica, black sage - Salvia mellifera) lose most of their leaves. In desert areas, some trees and shrubs (ocotillo - Foquieria splendens and palo verde - Parkinsonia florida) leaf out only after rain; they have green stems that are capable of photosynthesis year-round.
The following adaptations are critical to the survival of plants that keep their leaves in summer. One or more of these characteristics enable these remarkable plants to maintain green leaves capable of photosynthesis, even after six or more months without rain.
Vertical leaf orientation, like that of manzanita (Arctostaphylos) reduces the surface area of the leaf exposed to the sun during the hottest part of the day.
Waxy coatings, like those found on the coast live oak (Quercus agrifolia), reduce water loss through the epidermis.
Light colored coatings or hairs on leaves reflect light and reduce leaf temperature (like the difference between a white and a black T-shirt on a sunny day).
Hair on the undersides of leaves traps a boundary layer of still, relatively moist air.
Leaf rolling or thickening reduces leaf surface area exposed to the sun. The leaves of black sage (Salvia mellifera), roll under drought stress, exposing white (reflective) under-surfaces.
Thickening of leaves reduces the surface area to volume ratio of the leaf; thus water is not lost as easily as in leaves with a high surface to volume ratio. The thickened cells also reduce mechanical injury due to wilting.
Tiny or absent leaves reduce the amount of material exposed to dry air and hot sun. For example, cacti carry on photosynthesis in succulent, green stems, instead of in leaves.
Stomata, openings through which water vapor escapes, are located on the undersides of leaves away from direct sunlight.