Plants in water-controlled ecosystems: active role in hydrologic processesand response to water stress - IV. Discussion of real cases

Three water-controlled ecosystems are studied here using the stochastic des cription of soil moisture dynamics and vegetation water stress proposed in Part II (F. Laio, A. Porporato, L. Ridolfi, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 707-723) and Part III (A. Porporato. F. Laio, L. Ridolf i, I. Rodriguez-Iturbe, Adv. Water Res. 24 (7) (2001) 725-744) of this seri es of papers. In the savanna of Nylsvley (South Africa) the very diverse ph ysiological characteristics of the existing plants give rise to different s trategies of soil moisture exploitation. Notwithstanding these differences, the vegetation water stress for all the species turns out to be very simil ar, suggesting that coexistence might be attained also through differentiat ion of water use. The case of the savanna of Southern Texas points out how rooting depth and interannual rainfall variability can impact soil moisture dynamics and vegetation water stress. Because of the different responses t o water stress of trees and grasses, external climatic forcing could be at the origin of the dynamic equilibrium allowing coexistence in this ecosyste m. Finally, the analysis of a short grass steppe in Colorado provides an in teresting example of the so-called inverse texture effect, whereby preferen tial conditions for vegetation are dependent on soil texture and rainfall. Sites which are more favorable during wet conditions may become less suitab le to the same vegetation type during drier years. Such an effect is import ant to explain the predominance of existing species, as well as to investig ate their reproductive strategies. (C) 2001 Elsevier Science Ltd. All right s reserved.