Transient soil‐moisture dynamics and climate change in Mediterranean ecosystems

Abstract

[1] 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 hydroclimatic 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 in plant water stress and favor vegetation with shallower root systems.

How do variations in the temporal distribution of rainfall events affect ecosystem fluxes in seasonally water-limited Northern Hemisphere shrublands and forests?

“…Plants with deeper roots may respond to infrequent heavy rainfall events rather differently and may not benefit from short-term changes in soil moisture in the upper soil layers in the way predicted by Knapp et al's model. In fact, the response of vegetation to rain events is likely to be highly dependent on plant life form, which is itself a function of climate and soil conditions (Sala and Lauenroth, 1985;Schenk and Jackson, 2002;Ogle and Reynolds, 2004;Viola et al, 2008). In dry climates, Porporato et al (2001) found that there is an intermediate rainfall frequency at which drought stress is minimum, and that this optimal frequency depends on plant and soil properties (see also Laio et al, 2001).…”

Parameterization of a bucket model for soil-vegetation-atmosphere modeling under seasonal climatic regimes

“…Am., 2008). The field capacity concept was originally introduced for irrigation scheduling purposes under a simplistic view of soil water movement in the SVA system (Veihmeyer and Hendrickson, 1927). It was soon used also by hydrologists who defined it as the maximum quantity of water that can be permanently retained in the soil against the pull of gravity (Horton, 1935).…”

Stochastic soil water balance under seasonal climates

“…Indeed, the full solution to the stochastic differential equation (2.1), p t (x), is available when constant parameters are assumed for this case [13]. However, because these assumptions do not place an upper bound on the value of x t , they also result in significant overestimation of x t when there is leakage or run-off.…”

“…However, because these assumptions do not place an upper bound on the value of x t , they also result in significant overestimation of x t when there is leakage or run-off. Other implications for soil moisture and plant water stress are extensively discussed in reference [13] under constant climate conditions for the growing season. The next three cases are newer improvements on equation (2.5) that approximate LQ t using various assumptions for p t (x).…”

“…In terms of potential evapotranspiration, tropical dry and monsoon climates exhibit high year-round net radiation with little fluctuation between the seasons owing to their presence in the lower latitudes, whereas Mediterranean climates, which are typically found on the western continental margins of the mid-latitudes, experience larger seasonal variations. Mediterranean climates are also marked by out-of-phase cycles of rainfall and potential evapotranspiration which manifest as hot, dry summers and cool, wet winters; this is particularly challenging for plant life, because high productivity is limited to the beginning of the summer season before soil moisture depletes [13]. It is rare in the study of these climates (and their coexisting ecosystems) to reference commonalities shared with other seasonally dry regions, perhaps because of their collectively expansive geographical extent and disparate meteorological drivers.…”