Plants in water-controlled ecosystems: active role in hydrologic processesand response to water stress - III. Vegetation water stress

The reduction of soil moisture content during droughts lowers the plant wat er potential and decreases transpiration; this in turn causes a reduction o f cell turgor and relative water content which brings about a sequence of d amages of increasing seriousness. A review of the literature on plant physi ology and water stress shows that vegetation water stress can be assumed to start at the soil moisture level corresponding to incipient stomatal closu re and reach a maximum intensity at the wilting point. The mean crossing pr operties of these soil moisture levels crucial for water stress are derived analytically for the stochastic model of soil moisture dynamics described in Part II (F. Laio, A. Porporato, L. Ridolfi, I. Rodriguez-Iturbe. Adv. Wa ter Res. 24 (7) (2001) 707-723). These properties are then used to propose a measure of vegetation water stress which combines the mean intensity, dur ation, and frequency of periods of soil water deficit. The characteristics of vegetation water stress are then studied under different climatic condit ions, showing how the interplay between plant, soil, and environment can le ad to optimal conditions for vegetation. (C) 2001 Elsevier Science Ltd. All rights reserved.