Fine-Scale Spatial Heterogeneity of Resource Modulation in Semi-Arid “Islands of Fertility”

Abstract

In arid ecosystems, shrub patches are regarded as “islands of fertility,” enriched in resources that facilitate growth of herbaceous vegetation. It is often assumed that the modulation of resources and the effect that it has on herbaceous vegetation are homogeneous within each shrub patch. We examined the modulated resources in terms of their fine-scale spatial extent, level, and response to anthropogenic disturbances, that is, canopy removal and grazing. We defined three patch types—core of shrub patch, periphery of shrub patch, and intershrub patch—and recorded their levels of soil nutrients (organic matter, potassium, phosphorus, nitrate, and ammonium) and temperature regimes, with and without canopy removal, and/or grazing. Overall, our study found higher soil-nutrient contents and lower maximum temperatures at the core of the shrub patch than in the intershrub matrix. However, we also found that resource modulation was not spatially homogeneous within the shrub patch: soil nutrient levels were higher in the core than at the periphery. Anthropogenic disturbances did not affect soil nutrients within the 2-year time scale of our study. Our results emphasize the importance of examining the landscape at the spatial scale of the modulated resources, in order to develop patch management that increases productivity and diversity, and prevents desertification.

Keywords:

• ecosystem engineers
• islands of fertility
• patch mosaics
• Sarcopoterium spinosum
• stress gradient hypothesis

Acknowledgments

We thank Sonya Rozin for laboratory assistance and Eli Zaady for technical and scientific contributions. Primary funding sources included: the James S. McDonnell Foundation; grant 1077-03 from the Israel Science Foundation; the Eshkol Program of the Israel Ministry of Science & Technology; grants 277-0097 and 277-0267 from the Israel Rangelands Board and the Chief Scientist of the Israel Ministry of Agriculture and Rural Development. This is publication No. 759 of the Marco and Louise Mitrani Department of Desert Ecology.