SPATIAL AND TEMPORAL CHANGES IN THE CONCENTRATION OF K, NA, MG AND CAIN EPILITHIC AND IN DECOMPOSING DETACHED THALLI OF THE LICHEN RAMALINA-MACIFORMIS AND ITS POTENTIAL ROLE IN THE CYCLING OF THESE ELEMENTS IN THE NEGEV DESERT

Our study centered on the epilithic hot-desert lichen Ramalina macifor mis, which is very common in the Sinai and Negev deserts. The study ha d two objectives: (1) to estimate the rate of accumulation of airborne nutrients such as K, Na, Mg and Ca in in situ thalli and to compare i t with the rate of accumulation/release of these elements into/from li chen thalli detached from lithic substrates and kept on the soil surfa ce, and (2) to determine under laboratory conditions, electrolyte rele ase from lichen thalli under extreme temperature and moisture conditio ns such as may occur in the Negev Desert. The hypothesis guiding our r esearch was that in situ thalli of the desert lichen would accumulate high amounts of airborne elements in the lifetime of these long living organisms but at the same time they should release a certain part of thr accumulated minerals to avoid an excess of mineral burden on livin g thalli. We postulated that nutrient elements mall differ in their ac cumulation rate in in situ thalli and in their release from decomposin g detached thalli, and presumed that the input/release of nutrients to /from lichen thalli would be influenced by extreme climatic conditions such as prevail in the study area. In the present study, epilithic th alli of Ramalina maciformis were collected and examined after 14 month s for their K, Na, Mg and Ca contents. The concentration of K in these thalli increased in this period, whereas concentrations of Na and Mg decreased. These decreases are associated with the two exceptionally w et and cold winters of the experimental period. Other thalli were deta ched from flintstones on west-facing slopes of a hill and kept in litt erbags fixed on the soil surface on west-, north- and south-facing slo pes of the hill; those exposed on south-facing slopes contained less K than those exposed on western and northern aspects of the hill. Detac hed thalli generally contained more Mg and Ca than epilithic thalli, s uggesting that these elements originate from the upper soil layer. A s easonal variation in the electric conductivity of water in which thall i were soaked was observed, indicating either electrolyte leakage or u ptake in most of the treated thalli. The concentration of K in thalli kept in litterbags on western aspects of the hill was 15% lower than t hat of epilithic thalli picked simultaneously on the same aspects, sug gesting a loss of K from detached thalli of 1% per month. Thalli detac hed from flintstones, kept in litterbags and fixed on southern aspects of the hill contained 23% loss K than epilithic thalli transferred wi th their natural substrate from western to southern aspects of the hil l, suggesting an average K-leakage of 1.6% per month from detached tha lli on south-facing slopes.