Microclimate and ecophysiology of biocrusts


Dew formation and activity of biological soil 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.

Veste, M., Heusinkveld, B.G., Berkowicz, S.M., Breckle, S.-W., Littmann, T., Jacobs, A.F.G:  Ecological Studies 200, 305-318. 2008. (more..)

Dewfall and its geo-ecological implication for biological surface crusts in desert sand dunes
(north-western Negev, Israel)

Dew is an important water source for biological soil crusts and lichens in arid and semi-arid ecosystems. These crusts influencing the ecosystem processes resulting in a patchy ecotope and vegetation distribution. Microclimatic boundary conditions for nocturnal wetting were determinate. Maximum activity of crust is reached a few hours later after dewfall starts when cumulative dewfall exceeds 0.1 mm at dew point temperature differences around 0 K. Different microclimatic approaches were applied to estimated dewfall amounts in sand dunes of the north-western Negev. The annual dewfall amounts obtained from the zeroplane model was 26 mm a-1 and from the load cell 33 mm a-1, whereas other models mostly overestimated the dew amounts. However, spatial differences in shading after sunrise could explain the crust pattern in the sand dunes.

Veste, M., Littmann, T., Journal of Arid Land Studies 16(3), 139-147. 2006. (more…)

Microclimatic boundary conditions for activity of soil lichen crusts in sand dunes of the north-western Negev desert, Israel 

Photosynthetic activity of soil crust lichens was thoroughly investigated. Its interrelations with microclimatic boundary conditions was measured during two field experiments in the central part of the sand dune field in the north-western Negev Desert. After nocturnal rainfall the lichens were active well until noon when they dried out finally. However, over most of the year dewfall seems to be the primary controlling factor for activation as in other lichen communities. The microclimatic conditions for activity were determined in detail. It was found that after sunset terrestrial radiation leads to a progressive development of a stable air layer above ground accompanied by decreasing temperatures and wind speed. Well before midnight dewpoint temperature differences drop below 1.0 K  and leaf wetness sensors indicate the formation of dew. It is exactly in this situation when lichen activity starts. Maximum activity, however, is reached a few hours later when cumulative dewfall exceeds 0.1 mm at dewpoint temperature differences around 0 K. In nights with advective labilization and subsequent dewfall evaporation, no lichen activity was observed. Even a heavy foggy night did not lead to any activity at the soil surface.

Veste, M., Littmann, T., Friedrich, H., Breckle, S.-W., Flora 196 (6), 465-476, 2001. (more…)

Fulgensia fulgens Negev, Israel

Biological soil crusts with Fulgensia fulgens und Collema tenax in the Haluza sand dunes, Negev, Israel

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