Dew formation and activity of biological crusts

Biological soil crusts are prominent in many drylands and can be found in diverse parts of the globe including the Atacama desert, Chile, the Namib desert, Namibia, the Succulent-Karoo desert, South Africa, and the Negev desert, Israel. Because precipitation can be negligible in deserts ¿ the Atacama desert being almost rain-free ¿ or restricted to infrequent rains during short rainfall seasons, atmospheric moisture in the form of dew and/or fog can be a major, regular supplier of water for cryptogams. To study in situ microclimatic boundary conditions of dew formation and/or influence on biological crust activity in a hot desert, a variety of intensive field experiments were conducted by the authors in the Haluza sand dune region, North- Western Negev desert. Microclimatic parameters such as the radiative energy budget, specific humidity, or difference between air temperature and dewpoint are needed to determine the onset and termination of lichen photosynthetic activity. In the present paper, the physiological activation of soil lichens was measured by chlorophyll fluorescence (as used by Schroeter et al. 1992; Leisner et al. 1997). For the biological sand crusts, general meteorological stations were established on a dune slope or along a transect, in addition to intensive field campaigns where a variety of meteorological sensors were operated in parallel with manual and automatic microlysimeter dew measurements of both physical and biological crusts. The purpose focused on acquiring detailed information on the dew formation and drying process and dew quantities that could condense overnight. Full details regarding the experiments and instrumentation may be found in Jacobs et al. (1999, 2000a), Veste et al. (2001), Heusinkveld et al. (2006) and Littmann and Veste (2006).