Chaparral Shrublands

Plant-Water Relations

Chaparral vegetation occupies areas with conditions that will not sustain forest growth but that are sufficient enough to allow grass or deep-rooted woody plants to thrive. Water is the major limiting factor for plant growth and the deep rooting habit of chaparral shrubs and deep weathering of parent material enables chaparral shrubs to survive periodic seasonal droughts. Both climate and vegetation in the chaparral type favor large ET losses. Because ET accounts for such a large proportion of precipitation, water yield might be considerably improved if transpiration can be reduced.

Climate, aspect, plant, and soil affect ET. Climatic factors include precipitation, solar radiation, temperature, humidity, and wind. Aspect of the site influences the local climate, particularly solar radiation and temperature. The plant influences transpiration by its crown, leaf, and root characteristics, and by its stomatal openings. Soil depth, texture, and permeability affect the supply and delivery of water to the plant roots. Since chaparral conversion involves changes in the plant cover, the factors of most importance are those associated with the plant and soil (Hibbert et al 1974).

The 2 rainy seasons in Arizona, combined with the broadleaved evergreen character of chaparral shrubs, present an interesting hydrologic situation (Hibbert et al 1974). Because the shrubs are foliated year-round, they can transpire whenever atmospheric and soil conditions are favorable. The soil is recharged in the winter when precipitation is heaviest. Some winter transpiration undoubtedly occurs, although the rate is low because of cool temperatures. Summer rains break the long drought which lasts from early spring to midsummer. During this prolonged dry period, atmospheric conditions are favorable for high ET losses, and the soil moisture reserve is depleted. Summer rains seldom are concentrated enough to recharge the soil, although the moisture is important for plant growth. Any water in excess of immediate plant demands is drawn on during the dry fall months when the ET potential is still high. Consequently, early winter rains are utilized to recharge the soil mantle that dried in the preceding months.

Water availability to vegetation depends first on the water supply, and then on the amount of storage and depth of rooting in the soil. When the soil is only a few feet deep, the type of vegetation should not appreciably influence water yield unless there is a difference in length of season of water use or in transpiration rate. For example, short-lived annuals would be expected to use less water than perennials. In deep, well-drained soils, the type of vegetation markedly affects water yield. Deep rooted shrubs, whose roots may occupy much of the soil mantle, can deplete the soil-water reserve to a greater extent than shallow-rooted grasses and forbs. Even the various shrubs species may have differ rooting characteristics that can influence their ability to utilize available soil water. Other factors also contribute. Interception of precipitation by shrubs is greater than by grass (Corbett and Crouse 1968); thus more water enters the soil under grass cover. Seasonal dormancy of grass, particularly during the winter, results in less water withdrawal, and greater recharge below the grass rooting depth. Still other factors such as albedo, leaf structure, and stomatal behavior may cause water use by grass to be less, but most of the water savings are believed to result from water being less available to the shallow roots (Hibbert et al 1974).