THE ROLE OF LICHENS IN THE CYCLING OF METALS IN THE NEGEV DESERT

The epilithic fruticose lichen Ramalina maciformis (Del.) Bory which g rows in the Central Negev Highlands of Israel is grazed by the black d warf goat of the Bedouins. In order to study the role of R. maciformis in the cycling of metals in the Negev Desert, epilithic thalli growin g on flintstones on western aspects of a hill were detached, put in fi berglass window-screen litterbags, and secured on the soil surface of western, southern, and northern aspects of the hill. Simultaneously, f lintstones carrying lichen thalli were transferred from west-facing sl opes of the hill to adjacent south-facing slopes, where epilithic thal li of R. maciformis were observed to grow in very small quantities. Th e concentration of the heavy metals were determined in the original li chen material at the beginning of the experiment, and in in situ and r elocated 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 w ere 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 st ones and kept in litterbags was higher than in epilithic thalli. Thall i detached from stones and exposed on northern aspects of the hill con tained less heavy metals than those kept on south-facing slopes. It is suggested that detached thalli derive heavy metals from the upper soi l layer in addition to elements derived from airborne dust. The decrea sed concentration of Fe and Mn in epilithic thalli in the experiment p eriod is explained by the enhanced number of washing and freezing acti ons in the two unusually cold and wet winters of 1990/1991 and 1991/19 92. It is suggested that extreme climatic fluctuations regulate the ac cumulation of minerals in in situ epilithic thalli of desert lichens.