Abstract: Arid ecosystems cover about 40% of Earth's land area and are home to over two billion people, yet they remain vulnerable to climate change and human actions. Using numerical simulations, and data from Mediterranean ecosystems in Spain, Morocco and Greece, Kefi et al. show that patch-size distribution of vegetation follows a power law. As grazing pressure increases, patch size deviates from the power law close to the transition to desert conditions. So patch-size distribution may be a useful early warning of desertification. The cover shows an arid landscape (top) in the El Planeron nature reserve in Belchite, Spain, and the lower panels show degradation in this landscape. In a separate paper, Scanlon et al. use satellite imagery to show that the size distribution of tree clusters in the Kalahari basin also follows a scale-free power law. This can be explained by positive feedback associated with preferential environments near existing trees. In News & Views Ricard Sole discusses both papers. COVER IMAGE Sonia & Michael Kefi/ Yolanda Pueyo/ Santiago Begueria Portugues This paper describes and models the effect of grazing on vegetation patchiness in three arid Mediterranean ecosystems. The patch size distribution of the vegetation in these ecosystems follows a power law, which can be explained by invoking local positive interactions among plants. Deviations from power laws occur when grazing pressure is high, and may be a harbinger of imminent desertification. Humans and climate affect ecosystems and their services1, which may involve continuous and discontinuous transitions from one stable state to another2. Discontinuous transitions are abrupt, irreversible and among the most catastrophic changes of ecosystems identified1. For terrestrial ecosystems, it has been hypothesized that vegetation patchiness could be used as a signature of imminent transitions3,4. Here, we analyse how vegetation patchiness changes in arid ecosystems with different grazing pressures, using both field data and a modelling approach. In the modelling approach, we extrapolated our analysis to even higher grazing pressures to investigate the vegetation patchiness when desertification is imminent. In three arid Mediterranean ecosystems in Spain, Greece and Morocco, we found that the patch-size distribution of the vegetation follows a power law. Using a stochastic cellular automaton model, we show that local positive interactions among plants can explain such power-law distributions. Furthermore, with increasing grazing pressure, the field data revealed consistent deviations from power laws. Increased grazing pressure leads to similar deviations in the model. When grazing was further increased in the model, we found that these deviations always and only occurred close to transition to desert, independent of the type of transition, and regardless of the vegetation cover. Therefore, we propose that patch-size distributions may be a warning signal for the onset of desertification.

Abstract: Arid ecosystems cover about 40% of Earth's land area and are home to over two billion people, yet they remain vulnerable to climate change and human actions. Using numerical simulations, and data from Mediterranean ecosystems in Spain, Morocco and Greece, Kefi et al. show that patch-size distribution of vegetation follows a power law. As grazing pressure increases, patch size deviates from the power law close to the transition to desert conditions. So patch-size distribution may be a useful early warning of desertification. The cover shows an arid landscape (top) in the El Planeron nature reserve in Belchite, Spain, and the lower panels show degradation in this landscape. In a separate paper, Scanlon et al. use satellite imagery to show that the size distribution of tree clusters in the Kalahari basin also follows a scale-free power law. This can be explained by positive feedback associated with preferential environments near existing trees. In News & Views Ricard Sole discusses both papers. COVER IMAGE Sonia & Michael Kefi/ Yolanda Pueyo/ Santiago Begueria Portugues It is shown that clusters of tree canopies within Kalahari landscape in southern Africa lack characteristic size, with the size distributions following power laws. Model results indicate that this apparent self-organized behaviour can be explained by positive feedbacks that operate in this water-limited ecosystem as a result of preferential environments formed within the vicinity of existing trees. The concept of local-scale interactions driving large-scale pattern formation has been supported by numerical simulations, which have demonstrated that simple rules of interaction are capable of reproducing patterns observed in nature1,2. These models of self-organization suggest that characteristic patterns should exist across a broad range of environmental conditions provided that local interactions do indeed dominate the development of community structure. Readily available observations that could be used to support these theoretical expectations, however, have lacked sufficient spatial extent or the necessary diversity of environmental conditions to confirm the model predictions. We use high-resolution satellite imagery to document the prevalence of self-organized vegetation patterns across a regional rainfall gradient in southern Africa, where percent tree cover ranges from 65% to 4%. Through the application of a cellular automata model, we find that the observed power-law distributions of tree canopy cluster sizes can arise from the interacting effects of global-scale resource constraints (that is, water availability) and local-scale facilitation. Positive local feedbacks result in power-law distributions without entailing threshold behaviour commonly associated with criticality. Our observations provide a framework for integrating a diverse suite of previous studies that have addressed either mean wet season rainfall or landscape-scale soil moisture variability as controls on the structural dynamics of arid and semi-arid ecosystems.

Abstract: The Standardized Precipitation Index (SPI) is now widely used throughout the world in both a research and an operational mode. For arid climates, or those with a distinct dry season where zero values are common, the SPI at short time scales is lower bounded, referring to non-normally distributed in this study. In these cases, the SPI is always greater than a certain value and fails to indicate a drought occurrence. The nationwide statistics based on our study suggest that the non-normality rates are closely related to local precipitation climates. In the eastern United States, SPI values at short time scales can be used in drought/flood monitoring and research in any season, while in the western United States, because of its distinct seasonal precipitation distribution, the appropriate usage and interpretation of this index becomes complicated. This would also be the case for all arid climates. From a mathematical point of view, the non-normally distributed SPI is caused by a high probability of no-rain cases represented in the mixed distribution that is employed in the SPI construction. From a statistical point of view, the 2-parameter gamma model used to estimate the precipitation probability density function and the limited sample size in dry areas and times would also reduce the confidence of the SPI values. On the basis of the results identified within this study, we recommend that the SPI user be cautious when applying short-time-scale SPIs in arid climatic regimes, and interpret the SPI values appropriately. In dry climates, the user should focus on the duration of the drought rather than on just its severity. It is also worth noting that the SPI results from a statistical product of the input data. This character makes it difficult to link the SPI data to the physical functioning of the Earth system. Copyright © 2006 Royal Meteorological Society.

Abstract: [1] Soil moisture is the environmental variable synthesizing the effect of climate, soil, and vegetation on the dynamics of water-limited ecosystems. Unlike abiotic factors (e.g., soil texture and rainfall regime), the control exerted by vegetation composition and structure on soil moisture variability remains poorly understood. A number of field studies in dryland landscapes have found higher soil water contents in vegetated soil patches than in adjacent bare soil, providing a convincing explanation for the observed preferential establishment of grasses and seedlings beneath tree canopies. Thus, because water is the limiting factor for vegetation in arid and semiarid ecosystems, a positive feedback could exist between soil moisture and woody vegetation dynamics. It is still unclear how the strength of such a feedback would change under different long-term rainfall regimes. To this end, we report some field observations from savanna ecosystems located along the south-north rainfall gradient in the Kalahari, where the presence of relatively uniform sandy soils limits the effects of covarying factors. The data available from our field study suggest that the contrast between the soil moisture in the canopy and intercanopy space increases (with wetter soils under the canopy) with increasing levels of aridity. We hypothesize that this contrast may lead to a positive feedback and explore the implications of such a feedback in a minimalistic model. We found that when the feedback is relatively strong, the system may exhibit two stable states corresponding to conditions with and without tree canopy cover. In this case, even small changes in environmental variables may lead to rapid and largely irreversible shifts to a state with no tree canopy cover. Our data suggest that the tendency of the system to exhibit two (alternative) stable states becomes stronger in the more arid regions. Thus, at the desert margins, vegetation is more likely to be prone to discontinuous and abrupt state changes.

Abstract: Based on the data from China’s second national soil survey and field observations in northwest China, we estimated soil organic carbon (SOC) storage in China and investigated its spatial and vertical distribution. China’s SOC storage in a depth of 1 meter was estimated as 69.1 Pg (1015 g), with an average density of 7.8 kg m−2. About 48% of the storage was concentrated in the top 30 cm. The SOC density decreased from the southeast to the northwest, and increased from arid to semi-humid zone in northern China and from tropical to cold-temperate zone in the eastern part of the country. The vertical distribution of SOC differed in various climatic zones and biomes; SOC distributed deeper in arid climate and water-limited biomes than in humid climate. An analysis of general linear model suggested that climate, vegetation, and soil texture significantly influenced spatial pattern of SOC, explaining 78.2% of the total variance, and that climate and vegetation interpreted 78.9% of the total variance in the vertical SOC distribution.

Abstract: This paper analyses monthly differences in drought impact on vegetation activity in a semi-arid region in the north-east of the Iberian Peninsula between 1987 and 2000. The study determines spatial differences in the effects of drought on the natural vegetation and agricultural crops by means of the joint use of vegetation indexes derived from AVHRR images, a drought index (standardized precipitation index), and Geographic Information Systems. The results show that the effect of drought on vegetation varies noticeably between areas, a pattern that is determined mainly by the location of land-cover types. The influence also varies each month and, in general, is higher during the spring and summer. Aridity and vegetation characteristics similarly account, in part, for spatial differences in the impact of drought on vegetation. In general, the most arid areas, where vegetation cover and activity are low, are those in which the interannual variability of vegetation activity is more determined by the drought occurrence. In assessing drought impact, this analysis takes into account the effects of drought on the vegetation and also considers spatial and seasonal differences. The results should be useful for the management of natural vegetation and crops and for the development of better drought mitigation strategies.

Abstract: Dedication.- List of Figures.- List of Tables.- Preface.- SECTION I: Introduction on arid zone soil: Arid zone soils: Nature and properties. SECTION II: Biogeochemistry of trace elements in arid environments: Trace element distribution in arid zone soils.- Solution chemistry of trace elements in arid zone soils.- Selective sequential dissolution for trace elements in arid zone soils.- Binding and distribution of trace elements among solid-phase components in arid zone soils.- Transfer fluxes of trace elements in arid zone soils - a case study: Israeli arid soils.- Bioavailability of trace elements in arid zone soils.- Trace element pollution in arid zone soils.- Global perspectives of anthropogenic interferences in the natural trace element distribution: Industrial age inputs of trace elements into the pedosphere.- References.- Index.

Abstract: Plant leaf wax carbon isotopes provide a record of C 3 versus C 4 vegetation, a sensitive indicator of aridity, from the southeast African tropics since the Last Glacial Maximum. Wet and arid phases in southeast Africa were in phase with conditions in the global tropics from 23 to 11 ka, but at the start of the Holocene these relationships ended and an antiphase relationship prevailed. The abrupt switch from in phase to out of phase conditions may partially be attributed to a southward displacement of the Intertropical Convergence Zone (ITCZ) during the last glacial. Southward displacements of the ITCZ are also linked to arid conditions in southeast Africa during the Younger Dryas and the Little Ice Age.

Abstract: The effects of land-use changes on the runoff process in the midstream plain of this arid inland river basin are a key factor in the rational allocation of water resources to the middle and lower reaches. The question is whether and by how much increasingly heavy land use impacts the hydrological processes in such an arid inland river basin. The catchment of the Heihe River, one of the largest inland rivers in the arid region of northwest China, was chosen to investigate the hydrological responses to land-use change. Flow duration curves were used to detect trends and variations in runoff between the upper and lower reaches. Relationships among precipitation, upstream runoff, and hydrological variables were identified to distinguish the effects of climatic changes and upstream runoff changes on middle and downstream runoff processes. The quantitative relation between midstream cultivated land use and various parameters of downstream runoff processes were analysed using the four periods of land-use data since 1956. The Volterra numerical function relation of the hydrological non-linear system response was utilized to develop a multifactor hydrological response simulation model based on the three factors of precipitation, upstream runoff, and cultivated land area. The results showed that, since 1967, the medium- and high-coverage natural grassland area in the midstream region has decreased by 80.1%, and the downstream runoff has declined by 27.32% due to the continuous expansion of the cultivated land area. The contribution of cultivated land expansion to the impact on the annual total runoff is 14-31%. on the annual, spring and winter base flow it is 44-75%, and on spring and winter discharge it is 23-64%. Once the water conservation plan dominated by land-use structural adjustments is implemented over the next 5 years, the mean annual discharge in the lower reach could increase by 8.98% and the spring discharge by 26.28%. This will significantly alleviate the imbalance between water supply and demand in both its quantity and temporal distribution in the middle and lower reaches. Copyright (C) 2006 John Wiley & Sons, Ltd.

Abstract: Preface: H. S. Wheater, S. Sorooshian and K. D. Sharma 1. Modelling hydrological processes in arid and semi-arid areas - an introduction H. Wheater 2. Global precipitation estimation from satellite imagery using artificial neural networks S. Sorooshian, K.-L. Hsu, B. Imam and Y. Hong 3. Modelling semi-arid and arid hydrology and water resources - the southern Africa experience D. A. Hughes 4. Use of the IHACRES rainfall-runoff model in arid and semi-arid regions B. F. W. Croke and A. J. Jakeman 5. KINEROS2 and the AGWA modelling framework D. J. Semmens, D. C. Goodrich, C. L. Unkrich, R. E. Smith, D. A. Woolhiser and S. N. Miller 6. A distributed spatial sediment delivery model for arid regions K. D. Sharma 7. The Modular Modeling System (MMS): a toolbox for water and environmental resources management G. H. Leavesley, S. L. Markstrom, R. J. Viger and L. E. Hay 8. Calibration, uncertainty and regional analysis of conceptual rainfall-runoff models H. Wheater, T. Wagener and N. McIntyre 9. Real-time flow forecasting P. C. Young 10. Real-time flood forecasting - Indian experience R. D. Singh 11. Groundwater modeling in hard-rock terrain in semi-arid areas: experience from India S. Ahmed, J.-C. Marechal, E. Ledoux and G. de Marsily Appendix.

Abstract: South African agriculture has always been ideologically contested, because of its relationship with controversial land ownership issues. This book takes the question of farm workers' fortunes beyond the land debate, to consider their current and future livelihoods. The book argues that the question of farm workers needs to be understood as part of a broader spectrum of economic and social questions. Where should farm workers live? Should rural-urban migration be encouraged? What kind of job prospects can be fostered? How can their participation in the rural and peri-urban economy be promoted? Do farm workers need land, or jobs, or municipal services? Who should provide support to this neglected segment of society? A valuable study of past policy failures and future policy options, the book is primarily aimed at policy-makers and students of rural development. It is explicitly aimed at promoting new approaches, synergies and partnerships amongst stakeholders, including government, commercial farmers, agricultural co-operatives, municipalities, training agencies, and farm worker trade unions. The book also o ers suggestions that transcend the South African rural experience. It can therefore serve as a case study for students and practitioners of rural development elsewhere in the developing world.

Abstract: Drylands are regarded as highly sensitive to climatic changes. A positive relationship between rainfall and environmental factors (water availability for plants, productivity, species diversity, etc.) is often assumed for areas with an annual rainfall of 100–300 mm. This assumption disregards the fact that a climatic change in arid and semi-arid areas is not limited to climatic factors. This change is often accompanied by a parallel change in surface properties. The alteration of surface properties may have opposite effects on the water regime and ecosystem characteristics. Data on rainfall, runoff, soil moisture regime, and vegetation cover were collected at five monitoring sites in a sandy area along the Israeli–Egyptian border, where average annual rainfall varies from 86 to 160 mm. Data obtained show a decrease in water availability for perennial plants with increasing annual rainfall, clearly expressed by a lower survival of perennial plants in the wetter area. Results obtained cast doubt of the prevailing idea regarding the positive relationship between average annual rainfall and ecosystem characteristics. The findings are attributed to the decisive role played by the non-uniform properties of the topsoil biological crusts along the rainfall gradient considered. Under wetter conditions a thick topsoil biological crust develops. This crust is able to absorb large rain amounts, limiting thus the depth of rainwater infiltration and water availability for the perennial vegetation. A better water regime was found in the drier area, where a thin crust allows deeper water infiltration and water concentration by surface runoff. The process described may be regarded as a desertification process with increasing annual rainfall.

Abstract: [1] Shrub encroachment, a widespread phenomenon in arid landscapes, creates “islands of fertility” in degraded systems as wind erosion removes nutrient-rich soil from intercanopy areas and deposits it in nearby shrub-vegetated patches. These islands of fertility generally are considered to be irreversible. Recently, fire has been observed to alter this pattern of resource heterogeneity through the redistribution of nutrients from the fertile islands of burnt shrubs to the surrounding bare soil areas. Despite the recognized relevance of both fires and wind erosion to the structure and function of arid ecosystems, the interactions between these two processes remains poorly understood. This study tests the hypothesis that fire-induced soil hydrophobicity developing in the soils beneath burned shrubs enhances soil erodibility by weakening the interparticle wet-bonding forces. To test this hypothesis, the effects of grass and shrub fires on changes in soil erodibility and on the intensity of fire-induced soil water repellency are compared at both the field and patch scales in heterogeneous arid landscapes. Higher water repellency was observed in conjunction with a stronger decrease in wind erosion threshold velocity around the shrubs than in grass-dominated patches affected by fire, while neither water repellency nor changes in threshold velocity was noticed in the bare soil interspaces. Thus, fires are found to induce soil hydrophobicity and to consequently enhance soil erodibility in shrub-vegetated islands of fertility. These processes create temporally dynamic islands of fertility and contribute to a decrease in resource heterogeneity in aridland ecosystems following fire.

Abstract: Ground-water recharge in the arid and semiarid southwestern United States results from the complex interplay of climate, geology, and vegetation across widely ranging spatial and temporal scales. Present-day recharge tends to be narrowly focused in time and space. Widespread water-table declines accompanied agricultural development during the twentieth century, demonstrating that sustainable ground-water supplies are not guaranteed when part of the extracted resource represents paleorecharge. Climatic controls on ground-water recharge range from seasonal cycles of summer monsoonal and winter frontal storms to multimillennial cycles of glacial and interglacial periods. Precipitation patterns reflect global-scale interactions among the oceans, atmosphere, and continents. Large-scale climatic influences associated with El Niño and Pacific Decadal Oscillations strongly but irregularly control weather in the study area, so that year-to-year variations in precipitation and ground-water recharge are large and difficult to predict. Proxy data indicate geologically recent periods of multidecadal droughts unlike any in the modern instrumental record. Anthropogenically induced climate change likely will reduce ground-water recharge through diminished snowpack at higher elevations, and perhaps through increased drought. Future changes in El Niño and monsoonal patterns, both crucial to precipitation in the study area, are highly uncertain in current models. Land-use modifications influence ground-water recharge directly through vegetation, irrigation, and impermeable area, and indirectly through climate change. High ranges bounding the study area—the San Bernadino Mountains and Sierra Nevada to the west, and the Wasatch and southern Colorado Rocky Mountains to the east—provide external geologic controls on groundwater recharge. Internal geologic controls stem from tectonic processes that led to numerous, variably connected alluvial-filled basins, exposure of extensive Paleozoic aquifers in mountainous recharge areas, and distinct modes of recharge in the Colorado Plateau and Basin and Range subregions.

Abstract: Question: Does ecosystem engineering by small mammals have a significant influence on vegetation patterns in the arid steppe vegetation of southern Mongolia? Location: Gobi Altay Mountains, southern Mongolia. Methods: We assessed the impact of the small lagomorph Ochotona pallasi on plant community composition, nutrient levels and biomass production in montane desert steppes. Data were derived from vegetation releves, harvests of above-ground standing crop and a bioassay, followed by analyses of soil and plant nutrient contents. Results: Although the local climate is arid with <150 mm annual precipitation, clear evidence of allogenic ecosystem engineering was found. Plant communities on burrows differed from those on undisturbed steppe in that they contained more species of annuals and dwarf shrubs, and a greater abundance of the important fodder grass Agropyron cristatum. Standing crop and nutrient concentrations were higher for plants growing on burrow soil. In situ measurements and a pot exper...

Abstract: Australia's large desert floodplains are among the world's most hydrologically variable wetlands and vegetation in these habitats changes dramatically over time in response to flooding and drought. Annual forb and grass communities in these desert floodplains rely on large, diverse soil seed banks as critical sources of propagules for recruitment. I investigated the effects of flooding on seedling emergence from the soil seed bank of the Cooper Creek floodplain in arid central Australia. My objective was to determine the effects of differences in both short-term flooding characteristics as well as the longer-term flood history of sediments on the composition of plant communities establishing from the soil seed bank. I conducted a greenhouse experiment in which sediments collected from high (inundated at least once every 1–5 yrs), medium (inundated approximately once in every 5–10 yrs), and low (inundated less than once a decade) flood frequency zones were subjected to different durations of subme...

Abstract: In arid lands, where competition for water is acute, trees should be planted only when and where necessary and possible. Arid lands are among the world’s most fragile ecosystems, made more so by periodic droughts and increasing overexploitation of meagre resources. Arid and semi-arid lands cover around one-third of the world’s land area and are inhabited by about one billion people, a large proportion of whom are among the poorest in the world. Forests, trees and grasses are essential constituents of arid zone ecosystems and contribute to maintaining suitable conditions for agriculture, rangeland and human livelihoods. In providing goods (especially fuelwood and non-wood products) and environmental services to the rural poor and in contributing to the diversification of their household sources of income, forests and trees in arid zones boost poverty alleviation strategies and reduce food insecurity. Roughly 6 percent of the world’s forest area (about 230 million hectares) is located in arid lands (FAO, 2001). Trees outside forests (scattered in the landscape, in arable lands, in grazing lands, in savannahs and steppes, in barren lands and in urban areas) have a vital role in arid lands, although it is difficult to assess their extent. Availability of water – surface water, groundwater and air moisture – is usually the main factor limiting natural distribution of trees in arid lands, along with climate (rainfall, temperatures, wind) and soil quality. Each tree species is adapted to certain conditions and is located in its “niche”. When optimal conditions are widely distributed, forests or shrubs may cover large areas. More often, limited by water scarcity, vegetation is concentrated where runoff can accumulate or where groundwater is accessible. This leads to the uneven distribution of trees and bushes, for example in striped bush (fragmented bush stands), riparian forests, the deepest channels of valleys (thalwegs) and oases, and isolated in the landscape.

Abstract: Five sites were selected in the semi-arid northern Yilgarn Craton (Jaguar VHMS Cu-Zn-Ag, Moolart Well Au, Rumour Au, Gossan Hill VHMS Cu-Zn-Au and McGrath North Au deposits) to test the use of vegetation as an exploration sampling medium in areas of transported overburden. A variety of vegetation samples (litter, roots, bark, branch wood and phyllodes) were collected from Mulga ( Acacia aneura ) plants. In addition, the soil from 10–20 cm below surface was sampled and analysed following total, partial and selective digests to investigate any potential chemical signature of bedrock mineralization at surface. At all sites, selective extractions of the There is also an enrichment in metals and an expression of the bedrock mineralization with good anomaly contrast in the uppermost soil horizon (0–4 cm) at Jaguar and this may indicate some mixing of litter and soil by bioturbation, and fixing on organic material, and Fe and/or Mn oxides. This contrasts with greater depths (10–20 cm) where no significant metal enrichment appears to have occurred, suggesting that most contained metals were fixed near-surface and do not percolate deeper into the profile. This is possibly due to slow decomposition of litter in arid terrains, with continuous loss of soil and fine litter particles by wind erosion and bush fires. Our data support the hypothesis that vegetation plays an important role in bringing metals to surface in areas with a semi-arid to arid climate and a low water-table.