S. W. Breckle

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. 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).

Sensitivity of a Sandy Area to Climate Change Along a Rainfall Gradient at a Desert Fringe

Global climate change has become a strongly and frequently addressed issue in the last decades. The aspect is crucial in dry-land areas, which cover approximately one third of the globe’s total land area. The relationship between average annual rainfall and environmental variables has attracted the attention of many scientists. Climatologists use aridity indices to express relationships between climatic and environmental variables (Köppen 1931; Budyko 1974; Wallen 1967; Bailey 1979). These indices, based on purely climatic variables such as annual precipitation, temperature, evaporation and radiation, tend to imply that the acuteness of aridity is inversely related to annual precipitation. Although aware that soil water content depends on local soil type and precipitation regime, Walter (1939, 1960) asserted that at a larger, global scale, standing biomass is positively correlated to average annual rainfall. This approach is still followed by many researchers who assume a positive relationship between average annual rainfall and environmental variables such as water availability for plants, vegetation cover, productivity, species diversity, soil properties, human activity, and erosion rates for sub-humid to arid areas (Issar and Bruins 1983; Shmida 1985; Seely 1991; Lavee et al. 1991; Kutiel et al. 2000; Meron et al. 2004). This approach is certainly correct at the global scale, as well as for non-irrigated annual crops in dry-land areas. It is, however, questionable for arid and semi-arid areas, usually regarded as highly sensitive to climate change, especially for perennial plants. With decreasing annual rainfall, the number of rainstorms and storm rain amounts decrease. Under such conditions, water availability for plants may be highly dependant on the relationships between rainfall and surface properties which greatly influence the degree to which water will percolate or will be transformed into runoff, thereby significantly affecting the spatial redistribution of water resources. For example, it is well known that rocky hill slopes devoid of extensive soil and vegetation cover are characterized by extremely low infiltration rates, and quickly develop surface runoff. Due to the short duration of most individual rain showers, flow distances are short, resulting in water concentration and deep water percolation at nearby down-slope positions (Yair and Danin 1980; Yair 1983, 1994,

Automatische Detektion von Wurzelsystemen in Minirhizotron-Bildern

An der Wurzelforschung besteht aufgrund der vielfaltigen Wechselwirkungen zwischen Wurzelsystem und Boden ein groses Interesse. Fur aussagekraftige Untersuchungen ist die Analyse von grosen Bildmengen erforderlich. Die manuelle Auswertung ist zeitaufwendig und die Qualitat der Ergebnisse hangt sehr stark von der Erfahrung der auswertenden Person ab. Dieses fuhrt zu einer zunehmenden Automatisierung der Bildauswertung. Jedoch enthalten die Bilder komplexe und zufallig verzweigte Wurzelsysteme (s. Abb. 2). Die Detektion der Wurzelregionen ist besonders schwierig, da der Hintergrund sehr stark variiert und der Kontrast zwischen Boden und Wurzel oft sehr gering ist. Alle existierenden Verfahren zur automatischen Auswertung dieser Bilder sind regionenbasierte Ansatze, die davon ausgehen, das die Grauwerte des Wurzelsystems sich gut von den Grauwerten des Hintergrundes trennen lassen. Diese Annahme ist aber im allgemeinen nicht erfullt. Aus diesem Grund verfolgen wir in dieser Arbeit einen konturorientierten Ansatz. Nach der Detektion von signifikanten Wurzelbereichen, erweitern wir diese mittels heuristischer Suche zur optimalen Kontur. Die Basis des Verfahrens bildet der A—Algorithmus. Bei der Suche wird die parallele Struktur der Wurzel ausgenutzt, d.h. die Informationen der rechten und der linken Kontur fliesen gemeinsam in die Bewertung ein.

Nitrogen Input Pathways into Sand Dunes: Biological Fixation and Atmospheric Nitrogen Deposition

In arid and semiarid regions, water availability is considered to be the controlling factor for the productivity and pattern of vegetation. The total biotic and abiotic N pool size of desert ecosystems is lower than in most other ecosystems (Skujins 1981). Several studies have found that even in arid lands, nutrients are critical for plant growth and successions (McLendon and Redente 1992). After good rainy years, nitrogen can become the limiting factor (Trumble and Woodroofe 1954) whereas added nitrogen increased productivity in several experiments in dry areas (Ettershank et al. 1978; Ludwig 1987). The main N input pathways into the ecosystems are atmospheric deposition in wet, dry and gaseous forms, and the biological fixation of atmospheric nitrogen N 2 . Biological fixation is carried out by free-living bacteria, Fabaceae–Rhizobium symbiosis and associative symbiontic free-living cyanobacteria, as well as by cyanobacteria in lichens. Another N source is by non-leguminous nitrogen-fixing species; particularly shrubs and trees play a major role in these ecosystems (Schulze et al. 1991; Valladares et al. 2002). In most drylands, the ‘biological soil crust’ influences ecosystem processes (West 1990; Belnap and Lange 2001; Veste et al. 2001a). Nitrogen-fixing cyanobacteria of the genera Nostoc, Microcoleus, Chroococcus and Calothrix are common in such soil crusts, and have been reported in places such as arid and semiarid regions of Australia, North America and the Negev (Lange et al. 1992; Zaady et al. 1998; Belnap 2001; Chap. 10, this volume) Soil lichens with cyanobacterial phytobionts are also able to fix nitrogen. The importance of biological N fixation by soil crusts has been emphasised by several authors (e.g. Shields et al. 1957; Rychert and Skujins 1974; West 1990; Evans and Ehleringer 1993; Zaady et al. 1998), although determining N fixation under field conditions has several methodological problems (West 1990). As a result, high variation has been revealed among different drylands investigated under simulated field conditions. An increase in total nitrogen has been observed beneath the soil lichens and crusts of the sand dunes of the north-western Negev (Veste et al. 2001a). Most of the information about their contribution to N input in different dry ecosystems results only from laboratory investigations or simple estimates based on crust development

Standing Biomass and its Modelling

The sand dunes of the north-western Negev are characterized by a small-scale vegetation pattern (cf. Chap. 8, this volume). Aim of this investigation is to distinguish the standing biomass in the major ecotopes resulting from long-term ecological processes controlling ecotope pattern. Furthermore, we will present a simple numerical approach for modelling the actual standing biomass distribution in the sand dune mosaic.