Evergreen Shrubs

Abstract

Evergreen shrubs that obtain all of their water supply from the unsaturated zone are true drought-resistant plants in that they retain a hydrated canopy during predictable dry seasons. In most instances, these plants also maintain at least a partial leaf canopy during climatic drought, i.e., through extended periods of anomalously dry weather. Evergreens have been classified into three broad adaptation types (Gupta 1979): (1) sclerophyllous shrubs; (2) leafless shrubs with green stems; and (3) xerohalophytes with succulent leaves. The “strategies” employed by most desert evergreens include aboveground xeromorphy and the development of a belowground lateral/tap root system. Xeromorphy entails sclerophyllous leaf structure, tight control of cuticular and stomatal transpiration, high tolerance of heat and water stress, and high water-use efficiency (Oppenheimer 1960). Many evergreens have a well-developed lateral/tap root system in which surface laterals can extend well beyond the canopy of the plant and efficiently utilize near-surface soil moisture derived from more frequent rain events, while a central taproot can extend to deeper depths to exploit soil moisture reserves derived from major rainfall events during times of extended drought (Drew 1979).