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The hyper-arid climate of Ouargla area (Northern Sahara of Algeria) is important in the field of the paleoenvironmental reconstitution. This paper investigates the importance of palaeolake sediments as significant terrestrial archives for studying Quaternary climate changes. Through morphological and mainly micromorphological investigations of the studied sites (Sabkha of Ouargla and palaeolake of Mellala), we noted the fluctuation of climate between arid and more humid conditions during Quaternary. Some pedofeatures have recorded these environmental changes. The studied calcareous crusting has a lacustrine origin when precipitations were more important. The clay coatings around the quartz grains indicate an old origin and different geochemical conditions that led to their dissolution. Lenticular gypsum crystals were formed from groundwater saturated with Ca2+andions. Calcite pseudomorphs after lenticular gypsum crystals indicate changes in local climatic parameters. Gypsum crusts can be explained by formation under wet climatic conditions in a lagoonal environment. Black and gray layers indicate evolution of organic matter at a wetter period. This paper highlights an unknown palaeolake which can be useful for the comprehension of the ecological past of the Sahara. The comparison between this palaeolake and other Saharan palaeolakes has allowed us to assign it to a probable active period between 10000 and 4000 years B.P. [ABSTRACT FROM PUBLISHER]

Jebel Marra, a volcanic complex in western Sudan, is an important site of early settled agriculture, with high plant diversity, supported by orographic rainfall. Vegetation types were examined in relation to habitat, altitude, soils, and land management. In 52 sites, 274 species, predominantly Fabaceae and Poaceae, were recorded (with 17 new records). Sites were clustered using TWINSPAN, and Detrended Correspondence Analysis (DCA), generating eight vegetation types, six new since the 1970s. Changes in the dominant species show that the savanna has become sparser, with annuals displacing perennials. Soils were mostly "sandy clay loams," so vegetation types did not directly correspond to soil types, although Type VIII vegetation grew in soils with a higher clay content. The sand fraction that predominated in soils of all other vegetation types was, mainly (87%) "soft" or aeolian sand. Canonical Correspondence Analysis (CCA) separated communities and species along a first axis, associated with finer soil textures, higher Fe, and lower elevations. The second axis was positively associated with elevated phosphorus, and negatively with sandy loams. CCA showed that rainfall alone was less pertinent than soil texture, which determines plant-available water capacity. The region's vegetation was not uniformly diverse; instead, a mosaic of patches of diverse terrain, associated with different vegetation "types," collectively generates a diverse flora. Besides climate change, overgrazing and increasing human pressures due to conflict, local population growth and an influx of refugees place these (already stressed) plant resources at risk. Our survey provides a baseline to track changes and develop adaptive management strategies. [ABSTRACT FROM AUTHOR]

The study aimed at determining the root dynamics of cactus pear following a soil-water gradient and to characterize some root types of the speciesO. ficus-indica(L.) Miller (cultivar Morado–green cladode) andO. robustaWendl. (cultivar Monterey–blue cladode). One-year-old plants, planted in pots (210 mm diameter and 550 mm deep soil) and grown in the greenhouse at day/night temperatures of 25–30/15–18°C, were investigated. The water treatments applied were 0–25%, 25–50%, 50–75%, and 75–100% depletion of plant available soil water. The impact of water stress was quandtified in terms of root and cladode mass, root length, water-use efficiency (WUE: dry weight gain for roots and/or cladodes per unit water used–evapotranspiration), and water needed to fill up a cladode. Root mass decreased and root length increased significantly for both species with water stress. The root mass and length ofO. ficus-indicawere for all water treatments higher (p <0.05) than that ofO. robusta. By contrast, the root length/root mass ratio was higher4 forO. robustathanO. ficus-indica. Roots ofO. ficus-indicaandO. robusta, on average for all water treatments, comprised only 8.6% and 8.9% of the total plant biomass (dry weight), respectively. Root WUE decreased significantly (p <0.05) with water stress for both species. A water application of only 12 to 14 mm is needed to fill up the cladodes of the twoOpuntiaspecies with water stress. Given water shortages on arid lands and climate change, more development attention needs to be given toOpuntiaspecies. [ABSTRACT FROM PUBLISHER]

Fluctuating climatic patterns are increasing the frequency and severity of drought, a concern for native plant communities on grazed semi-arid rangelands. Vegetation successional models have focused on the impact of management and have failed to quantify the effects of extreme drought. From 2001 to 2011, plant community composition was sampled on ecological sites in a semi-arid rangeland managed with conservative grazing and frequent fire since 1937. Ordination and classification were used to assess the interactive effects of ecological site and extreme drought on plant species composition, holding all other external drivers constant. Deeper soil clay loam sites had 4x greater beta diversity than shallower and rockier low stony hill sites, an indication of greater species turnover and instability in response to extreme drought. Cumulative effects of drought years explained similarity between sites and species composition. Response to extreme drought varied by species; no response (Bouteloua curtipendula), decreased (Nassella leucotricha), and increased (Bouteloua rigidisetaandEriochloa sericea). Annual C3 plant responses were explained by short-term drought and perennial C3 and C4 plant responses were explained by long-term drought. Clay loam sites had maximum species richness and diversity values during neutral periods with quadratic declines associated with climatic extremes (dry or wet) compared to the more xeric sites which had minimum species richness and diversity during neutral periods with quadratic increases during climatic extremes. The interaction between site and drought, holding all other external drivers constant, can enhance our understanding of plant community dynamics and secondary plant succession of degraded semi-arid rangelands. [ABSTRACT FROM PUBLISHER]

The climate model, United Kingdom Meteorological Organization model (UKMO) and multivariate statistics, principal component analysis (PCA) and hierarchical cluster analysis (HCA) were employed to determine the climate diversity and agroclimatic indicators in future climate change. Monthly weather data from 1968 to 2000 at 36 weather stations in Iran were used to generate climate change scenarios for years 2025 and 2050. The UKMO model predicted a temperature rise of 2.7°C and a rainfall decrease of 12% by 2050. By 2050, length of the growth period is predicted to increase by 16 days, length of the dry period will increase by 22 days because of a delay in the first freezing day and an advance in the last freezing day, and the subsequent increase in temperature and decrease in rainfall. Cluster analysis of weather station data shows that 10 currently defined agroenvironment zones will be reduced to 8 by 2025 and to 7 by 2050. Climate change will decrease geographic differences in temperature and precipitation in Iran, and precipitation will be increasingly a determining indicator in the future. [ABSTRACT FROM AUTHOR]

Desertization is defined as the irreversible extension of desert land forms and landscapes to areas where they did not occur in a recent past. Such extension does not result from a would-be increasing aridity as there is no evidence of any global long-term trends in rainfall in arid lands since the period of instrumental record (ca. 150 years). The 0.5° C of global temperature increase over the past 100 years remains well within the range of historical fluctuations. The creation of man-made deserts is actually the result of long-lasting mismanagement of natural resources under the pressure of excessive human and livestock populations. Such land abuse may drive ecosystems beyond their resilience potential for recovery. The causes and mechanisms of arid land degradation pertain to direct and indirect activities, processes, and phenomena, which are not necessarily linked to aridity or drought. Causes, severity, and extent of desertization are quantified from various sources. The potential for biological recovery is analyzed and the ways and means to achieve rehabilitation assessed. The conclusion addresses the issue of the future of desertization under the additional threat of an increased aridity that might occur assuming a possible global temperature increase of the planet, triggered by atmospheric pollution. It tries to quantify the possible increase of aridity under various scenarios. [ABSTRACT FROM AUTHOR]