Transient Soil Moisture Dynamics and Climate Change in Mediterranean Ecosystems

Plants in Mediterranean ecosystems have developed different strategies to cope with transient soil-moisture dynamics induced by the markedly out of phase seasonal behavior of rainfall and temperature. Deep-rooted plants use the soil moisture stored in the wet winter (extensive users), while shallower rooted plants exploit both the wet season storage and the more sporadic growing season rainfall (intensive users). Using stochastic models of soil moisture dynamics, we present an analytical and numerical description of the probabilistic structure of the soil-moisture storage at the beginning of the growing season in relation to the dynamics of the wet season, and then study its evolution during the subsequent growing season. Special attention is devoted to plant water stress as a function of the rooting depth and the soil moisture storage at the beginning of the growing season. The existence of an optimal rooting depth for Mediterranean climates and its dependence on future hydro-climatic scenarios are discussed with reference to a test case in Sicily, Italy. Our analyses suggest that the forecasted decrease in rainfall for the Mediterranean regions might lead to a considerable increase of plant water stress and favor vegetation with shallower root systems.