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Abstract: A mathematical model for plant communities in water-limited systems is introduced and applied to a mixed woody–herbaceous community. Two feedbacks between biomass and water are found to be of crucial importance for understanding woody–herbaceous interactions: water uptake by plants’ roots and increased water infiltration at vegetation patches. The former acts to increase interspecific competition while the latter favors facilitation. The net interspecific interaction is determined by the relative strength of the two feedbacks. The model is used to highlight new mechanisms of plant-interaction change by studying factors that tilt the balance between the two feedbacks. Factors addressed in this study include environmental stresses and patch dynamics of the woody species. The model is further used to study mechanisms of species-diversity change by taking into consideration tradeoffs in species traits and conditions giving rise to irregular patch patterns. [Copyright &y& Elsevier]

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.

Abstract: We studied the successional stages of biological soil crusts (BSCs) by using in-situ spectroscopic techniques during 6 years of recovery following scraping-sterilization and scraping-crumbling disturbances on north- and south-facing slopes and in plots with and without overland water runoff barriers. Two spectral indices, the Brightness Index (BI) and the Normalized Difference Vegetation Index (NDVI), were used as indicators for evaluating BSC succession, with special attention to differences between the north- and the south-facing slopes. We found that BSC succession could be expressed as linear regressions of the above-mentioned indicators during the experimental years for the different treatments. Both indicators were found to be significantly different in each of the experimental years: BI values decreased while NDVI values increased for each of the three treatments. Thus, the BI can serve as a good indicator during the early years following disturbance while the NDVI can be useful after crusts have become established. We conclude that spectral reflectance measurements of BSCs can be a useful monitoring technique for studying the regeneration of the soil surface without direct disturbance. [Copyright &y& Elsevier]

Abstract: Understanding the structure and dynamics of plant communities in water-limited systems often calls for the identification of ecosystem engineers—key species that modify the landscape, redistribute resources and facilitate the growth of other species. Shrubs are excellent examples; they self-organize to form patterns of mesic patches which provide habitats for herbaceous species. In this paper we present a mathematical model for studying ecosystem engineering by woody plant species in drylands. The model captures various feedbacks between biomass and water including water uptake by plants’ roots and increased water infiltration at vegetation patches. Both the uptake and the infiltration feedbacks act as mechanisms for vegetation pattern formation, but have opposite effects on the water resource; the former depletes the soil-water content under a vegetation patch, whereas the latter acts to increase it. Varying the relative strength of the two feedbacks we find a trade-off between the engineering capacity of a plant species and its resilience to disturbances. We further identify two basic soil-water distributions associated with engineering at the single patch level, hump-shaped and ring-shaped, and discuss the niches they form for herbaceous species. Finally, we study how pattern transitions at the landscape level feedback to the single patch level by affecting engineering strength. [Copyright &y& Elsevier]

This paper discusses several difficulties encountered in detecting and monitoring temporal changes in vegetation using multispectral imagery from airborne or spaceborne sensors. These difficulties are due to (1) temporal change in the vegetation state; (2) temporal change in the soil/rock signature; and (3) difficulty in discriminating vegetation from soil or rock background. The seasonal dynamics of soil and vegetation was investigated over two years on permanent sample plots in a natural fenced-off area in the semi-arid region (200 mm annual average rainfall) of the Northern Negev, Israel. Results show that temporal analysis of natural vegetation in semi-arid regions should take into account three ground features—perennials, annuals, and biological soil crusts; all having phenological cycles with the same basic elements—oscillation from null (or low) to full photosynthetic status. However, these cycles occur in successive periods throughout the year. The phenological cycle of perennial plants is related to the adaptation of desert plants to scarcity of water. Annuals are green only for a relatively short period during the wet season and turn into dry organic matter during the summer. The microphytic communities (lower plants) of the biological soil crusts are rapidly affected by moisture and turn green immediately after the first rain, in a timescale of minutes. In arid environments, where the higher plants are sparse, this type of plant has considerable importance in the overall production of the greenness signal. However, crust-covered areas are visually similar to bare soil throughout the dry period. This paper concludes that a priori knowledge of the phenological changes in desert plants (lower and higher) is valuable in the interpretation of remote sensing data of arid environments. It is shown that rainfall amount and regime are the keys for understanding the dynamic processes of the different ground features. Through polynomial fitting, simple functions describing the annual variations in the NDVI of the different cover types have been formulated and validated; showing the feasibility and viability of modelling the processes. Although fluctuations in the rainfall regime between years poses a problem to designing a unique model, it is believed that such a problem can be overcome with long-term observations. [ABSTRACT FROM AUTHOR]

Abstract: Understanding biotic influences on soil processes is a major research frontier made challenging by organismal diversity, variation in distribution, and variety of interactions. Nevertheless, two fundamental influences can be recognized: assimilation/dissimilation (uptake, metabolism, wastes, death) and physical ecosystem engineering (non-assimilatory/dissimilatory, organismally-induced, structurally-mediated changes in energy and materials in the abiotic environment). Because many organisms can engineer soils, predicting their effects is particularly challenging. Here we use Hans Jenny’s State Factor Equation as a flexible, integrative tool for understanding these effects. We distinguish organismal influences via engineering from those of assimilation/dissimilation, explicitly placing engineers into the equation as independent state factors. We then ask: What abiotic state factors does an engineer affect? What relationships among state factors does it change? How does this affect soil processes? Using examples from our work, we illustrate use of this conceptual framework for a physical process—soil erosion; a chemical process—desalination; and a biogeochemical process—denitrification. We show that the framework can be used to: Identify conditions for small or large engineering effects on soil processes; assess engineer impacts on multiple soil processes; compare effects of different engineers on a given soil process; and integrate effects of multiple engineers on a single soil process. [Copyright &y& Elsevier]