The impact of water regime on shrubs productivity in a semiarid hillslope
Semiarid ecosystems draw attention as they support the life of large communities but function under threats of degradation due to drought. Long drought periods may reduce productivity in semiarid ecosystems and cause irreversible desertification. Mathematical models were developed to predict desertification based on the assumption that shrub cover indicates water stress. However, local traits may mask this generalized relationship. Empirical support to this relationship is highly sought but is limited by field data cost. Using new models and remotely sensed data, we examined the effect of water regime on vegetation cover in the semiarid part of the Negev Desert, Israel. By combining numerical solution of the soil water equations with a 16-year realistic spatial and temporal database, distributions of soil moisture across a typical hillslope were simulated.
The results of this study demonstrate that patch size is not necessarily proportional to mean soil water content. Additional factors such as slope inclination, soil depth, rock cover, and other disturbances, such as grazing and fire, may play a role in the determination of patch size. In semiarid regions, the water contribution resulting from direct rainfall is generally lower than the water requirements of local vegetation. The results of this study show that runon emerges as a significant process providing additional component to the water budget of vegetated patches. Runon contribution should be accounted for to better explain and simulate observed total vegetation cover, or predict its evolution in response to changes in environmental conditions. The simulation accounting for runon at the hillslope scale explained the stabilization of vegetation cover during the last 20 years as reproduced from the analysis of current and historical airphotos. Both patch size distribution analysis and the runon additional water estimates emphasized the large effect that water availability has on shrub patterns in semiarid hillslopes. The results presented here suggest that shrub size and vegetation cover in semiarid ecosystems is limited by the combined effect of direct rainfall and runon, and is the outcome of the balance between runoff contributing areas as sources and vegetated patches as sinks. It also explains partly the difference between the smooth output of mathematical models based on synthetic data and the patterns observed in the field.