Spatial and temporal changes in the concentration of K, Na, Mg and Ca in 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

Abstract

Our study centered on the epilithic hot-desert lichen Ramalina maciformis, which is very common in the Sinai and Negev deserts. The study had 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 it with the rate of accumulation/release of these elements into/from lichen thalli detached from lithic substrates and kept on the soil surface, and (2) to determine under laboratory conditions, electrolyte release from lichen thalli under extreme temperature and moisture conditions such as may occur in the Negev Desert. The hypothesis guiding our research 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 the accumulated minerals to avoid an excess of mineral burden on living thalli. We postulated that nutrient elements may differ in their accumulation rate in in situ thalli and in their release from decomposing 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 thalli of Ramalina maciformis were collected and examined after 14 months 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 wet and cold winters of the experimental period. Other thalli were detached from flintstones on west-facing slopes of a hill and kept in litterbags fixed on the soil surface on west-, north- and south-facing slopes of the hill; those exposed on south-facing slopes contained less K than those exposed on western and northern aspects of the hill. Detached thalli generally contained more Mg and Ca than epilithic thalli, suggesting that these elements originate from the upper soil layer. A seasonal variation in the electric conductivity of water in which thalli were soaked was observed, indicating either electrolyte leakage or uptake 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 that of epilithic thalli picked simultaneously on the same aspects, suggesting a loss of K from detached thalli of 1% per month. Thalli detached from flintstones, kept in litterbags and fixed on southern aspects of the hill contained 23% less K than epilithic thalli transferred with their natural substrate from western to southern aspects of the hill, suggesting an average K-leakage of 1.6% per month from detached thalli on south-facing slopes.