Causes of two patterns of lichen zonation on cobbles in the Negev Desert, Israel

Two patterns of lichen zonation on cobbles were observed in the Negev Desert, Israel: growth on the top of the cobble and growth on the cobble margins. The causes of the different patterns are examined. Microscopic examinations did not reveal any textural or structural differences between the top and the margins of each cobble, excluding the possibility that mineralogical differences are responsible for the patterns observed. Field measurements have, however, shown a significant difference in the quantities of average daily dew, with the top of the cobbles receiving approximately double the quantity received at the margins. These results were consistent on all cobbles examined, loose as well as partially embedded cobbles. In addition, field observations following rain noted a greater moistness at the margins of partially embedded cobbles. This phenomenon, also observed following sprinkling experiments, was explained by water capillary rise. Whilst the top dwelling pattern may be explained by the significantly larger quantities of dew on the top of the cobble, the margin-dwelling pattern may be explained by greater moistness at the cobble-soil interface due to water capillary rise. Providing that the threshold moisture duration is met, lichen colonization may be expected. An estimated annual daylight moisture duration of approximately 260-320 h is thought to be necessary for lichen growth in the Negev. The findings may assist in evaluating weathering rates and paleoclimate reconstruction. (C) 2002 The British Lichen Society.