The role of lichens in the cycling of metals in the Negev Desert

Abstract

The epilithic fruticose lichen Ramalina maciformis (Del.) Bory which grows in the Central Negev Highlands of Israel is grazed by the black dwarf goat of the Bedouins. In order to study the role of R. maciformis in the cycling of metals in the Negev Desert, epilithic thalli growing on flintstones on western aspects of a hill were detached, put in fiberglass window-screen litterbags, and secured on the soil surface of western, southern, and northern aspects of the hill. Simultaneously, flintstones carrying lichen thalli were transferred from west-facing slopes of the hill to adjacent south-facing slopes, where epilithic thalli of R. maciformis were observed to grow in very small quantities. The concentration of the heavy metals were determined in the original lichen material at the beginning of the experiment, and in in situ and relocated thalli after a period of 14 months. The concentrations of Fe, Zn, Mn, Cu, and Pb in the upper soil layer on the different aspects were determined at the end of the experimental period. A comparison of the concentration of heavy metals in epilithic thalli of R. maciformis examined in the same study site a decade ago, and of present findings, reveals a regional increase of airborne Mn and Zn derived from local traffic. The concentration of heavy metals in thalli detached from stones and kept in litterbags was higher than in epilithic thalli. Thalli detached from stones and exposed on northern aspects of the hill contained less heavy metals than those kept on south-facing slopes. It is suggested that detached thalli derive heavy metals from the upper soil layer in addition to elements derived from airborne dust. The decreased concentration of Fe and Mn in epilithic thalli in the experiment period is explained by the enhanced number of washing and freezing actions in the two unusually cold and wet winters of 1990/1991 and 1991/1992. It is suggested that extreme climatic fluctuations regulate the accumulation of minerals in in situ epilithic thalli of desert lichens.