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We studied colonization of annual plants in small-scale disturbancesand undisturbed soil for four years in the northern Negev desert. The experiment consisted of 24 patches of 1 m2: eight undisturbed patches, eight 30 cm deep pits and eight 20 cm high mounds. Disturbance removed the seed bank from the pits and mounds. Rainfall wasaverage during 1992 and 1993 (163.5 and 157.0 mm), very low in 1994 (97.5 mm) and very high in 1995 (283.0 mm). For all ca 100 species inthe assemblage, we measured abundance, as the average local density in patches occupied by the species, and incidence, the proportion of patches occupied. Abundance and incidence were positively correlated in the four years of the study (R2 = 0.71, 0.56, 0.56 and 0.64). The relationship became steeper during the course of colonization. A minority of species colonized all patches rapidly and increased exponentially in abundance. These high incidence and abundance species responded little or not at all to variations in rainfall. Most ofthe species experienced frequent local extinctions and colonizationsin few patches. These low abundance and low to medium incidence species responded strongly to rainfall by fluctuations in abundance. Differences per species between abundance and/or incidence in pits, mounds and matrix were evident in many species along the entire relationship. These were mostly in favor of pits. We propose that the positionsand trajectories of the individual species in the abundance incidence phase plane signify constraints on population growth along the abundance axis, and on colonization and population persistence along the incidence axis. Since species with lower incidence and abundance respond strongly to rainfall variation, we conclude that their local populations and spatial distribution are to a large extent site-limited. High-abundance and high-incidence species have few or weak constraints and experience density-independent population growth, which is onlyseed-l [ABSTRACT FROM AUTHOR]

Describes snails as an unusual, important source of soil nitrogen in the Central Negev Highlands of Israel. Endolithic lichens as food; Nitrogen transfer to soil; Important contribution to higher plant production.

In arid and semiarid regions, aeolian particle input. is one of the major sources of soil nutrients. The particle input also improves thesoil depth and strongly influences the water-holding capacity of thesoil. The authors studied the airborne particle accumulation dynamics on the soil surface in the Northern Negev desert (Sayeret Shaked Park) in comparison to the Central Negev highland (Sede Boqer and Shivta). The measurements of the components of the aeolian deposition and accumulation at the study site of the Sayeret Shaked Park were carried out for 3 years from August 1992 to July 1995. The average soil accumulation for this period was 0.22 g m-2 d-1 (80.64 g m-2 year- 1). The minimal quantity during the research period was during July 4 1993 (1.35 g m-2 month- 1) and the maximal was in March 1993 (24.42 g m- 2 month-1). Significant differences were obtained during the whole study period regarding the slope directions of the watershed. The aeolian accumulation and the deposition on the north-facing slope was significantly lower (P < 0.05) than on the south- facing slope (average 5.09 g m-2 month- 1 to 6.97 gm-2 month- 1, respectively). The highest accumulation was obtained in the wadi (average 8.27 g m-2 month- 1). Almost all the granulometric analyses conducted on monthly collected samples for the study period showed significant amounts of clay-size (<16 mu m) and silt-size (between 16 to 50 mu m) particles. Chemical composition of the airborne particles showed that two major constituents, Si and Ca, dominate all the samples. Particle-size analysis of the aeolian particles accumulated during the study period at the Sayeret Shaked area showed a median diameter varying between 25 and 35 mu m. This leads to a layer of fine airborne particles or loess, as scattered silt or clay particles, of 14 mu m m-2 [ABSTRACT FROM AUTHOR]