Arnon, A.I., Department of Agronomy and Natural Resources, Institute of Plant Sciences, Volcani Center, Bet Dagan 50250, Israel
Ungar, E.D., Department of Agronomy and Natural Resources, Institute of Plant Sciences, Volcani Center, Bet Dagan 50250, Israel
Svoray, T., Department of Geography and Environmental Development, Ben-Gurion University of the Negev, Be'er Sheva 84105, Israel
Shachak, M., Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sede Boqer 84990, Israel
Blankman, J., Department of Agronomy and Natural Resources, Institute of Plant Sciences, Volcani Center, Bet Dagan 50250, Israel
Perevolotsky, A., Department of Agronomy and Natural Resources, Institute of Plant Sciences, Volcani Center, Bet Dagan 50250, Israel, Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sede Boqer 84990, Israel

Ecosystems of mixed woody and herbaceous vegetation are under increasing pressure and threat from human activity and global climate changes. Many processes that shape these ecosystems remain poorly understood despite their large geographic extent, the services they provide, and their importance for wildlife and livestock. Some of these processes occur simultaneously on small and large scales; therefore their study requires methodologies that combine high spatial resolution with large spatial extent. In this study, we explored the phenomenon of rings - "circlets" - of relatively dense herbaceous biomass that seem to occur around patches of Sarcopoterium spinosum in the semiarid northern Negev. We developed a novel, non-destructive method to estimate herbaceous biomass at a high spatial resolution, over an area of 1500 m2. Steps in the study process included: low-altitude aerial photography, image rectification, delineation of shrub patches, computation of herbaceous biomass in the intershrub area, and analysis of herbaceous biomass as a function of distance from the nearest shrub. Our results confirmed the existence of circlets, and we estimated their width to be approximately 10 cm. Herbaceous biomass at the peak of the green season was approximately 40% greater in the circlet than in the remainder of the intershrub area. Circlets are probably an important feature of the ecosystem; since they covered ca. 20% of the intershrub area, their contribution to primary (herbaceous) production at the ecosystem level, and, in turn, to secondary production, is substantial. We discuss possible mechanisms in the creation of circlets, as well as the possible implications of circlets for range management and improvement. © 2007 Science From Israel / LPPLtd.