Microbial populations within hypersaline lakes often exhibit high activities of photosynthesis, dissimilatory sulphate reduction and other processes and, thus, can have profound impacts on biogeochemical cycles of carbon, nitrogen, sulphur and other important elements within arid lands. To further understand these types of ecosystems, the physicochemical and biological properties of Sidi Ameur and Himalatt Salt Lakes in the Algerian Sahara were examined and compared. Both lakes were relatively neutral in pH (7.2 to 7.4) and high in salt, at 12% and 20 % (w/v) salinity for Himalatt and Sidi Ameur Lakes, respectively, with dominant ions of sodium and chloride. The community compositions of microbes from all three domains (Bacteria, Archaea and Eukarya) were surveyed through the use of 16S and 18S ribosomal gene amplification and clone library clustering using amplified ribosomal DNA restriction analysis (ARDRA) in conjunction with DNA sequencing and analysis. A high level of microbial diversity, particularly among the bacteria of the Himalatt Salt Lake and archaea of Sidi Ameur Lake, was found within these environments. Representatives from all known halophilic bacterial phyla as well as 6 different genera of halophilic archaea were identified. Moreover, several apparently novel phylotypes among both archaea and bacteria were revealed.
Semi-arid shrublands of southern California, including chaparral and coastal sage, are found in widely varying elevation and microclimatic regimes and are subjected to disturbance such as fire and atmospheric N deposition that have the capacity to alter soil and litter C and N storage. Here we present a case study where soil and litter C and N were measured over 19 months in post-fire chaparral and mature coastal sage stands to assess whether differences in soil and litter C and N between these diverse shrublands could be attributed to differences in elevation, stand age, rainfall, and/or estimated N deposition exposure. Our results indicate that atmospheric N deposition exposure, either alone or in conjunction with other environmental variables (elevation, rainfall, and/or stand age), was the most frequent predictor of the spatial pattern in the soil and litter N and C variables observed. These results are consistent with those reported for high-elevation coniferous forests arrayed along an N deposition gradient in southern California, suggesting that N deposition may affect the soil N and C storage of semiarid shrublands and woodlands in a qualitatively similar manner.
The effects of elevated atmospheric carbon dioxide [CO(2)] on microbial communities in arid rhizosphere soils beneath Larrea tridentata were examined. Roots of Larrea were harvested from plots fumigated with elevated or ambient levels of [CO(2)] using Free-Air CO(2) Enrichment (FACE) technology. Twelve bacterial and fungal rRNA gene libraries were constructed, sequenced and categorized into operational taxonomical units (OTUs). There was a significant decrease in OTUs within the Firmicutes (bacteria) in elevated [CO(2)], and increase in Basiomycota (fungi) in rhizosphere soils of plots exposed to ambient [CO(2)]. Phylogenetic analyses indicated that OTUs belonged to a wide range of bacterial and fungal taxa. To further study changes in bacterial communities, Quantitative Polymerase Chain Reaction (QPCR) was used to quantify populations of bacteria in rhizosphere soil. The concentration of total bacteria 16S rDNA was similar in conditions of enriched and ambient [CO(2)]. However, QPCR of Gram-positive microorganisms showed a 43% decrease in the population in elevated [CO(2)]. The decrease in representation of Gram positives and the similar values for total bacterial DNA suggest that the representation of other bacterial taxa was promoted by elevated [CO(2)]. These results indicate that elevated [CO(2)] changes structure and representation of microorganisms associated with roots of desert plants.
High levels of atmospheric nitrogen (N) deposition to southern California chaparral shrublands may interact with fire to affect biomass production and plant species composition during secondary succession. To determine the potential interactions between post fire recovery and N deposition we compared rates of aboveground net primary production (ANPP), shrub growth, and the relative abundance of Adenostoma fasciculatum, other sub-dominant shrubs, and herbaceous species of three chaparral stands exposed to different levels of atmospheric N deposition over the first 3 years of post-fire succession. Our data suggest that rates of ANPP (gdw m(-2) month(-1)) and aboveground N storage (gN m(-2) month(-1)) for these chaparral stands were not related to N deposition even though sites exposed to high levels of N deposition had significantly higher rates of shrub growth (gdw plant(-1) month(-1)) and N uptake (gN plant(-1) month(-1)). However, high-N stands were composed of larger shrubs with a lower density, and this trade-off between shrub size and density may explain the low correlation between N deposition and post-fire ANPP. Differences in relative plant species abundance between sites were significantly correlated with N deposition exposure, where stands exposed to high N deposition had a lower relative abundance of A. fasciculatum and a higher relative abundance of other shrub and herbaceous species. While many factors can affect rates and patterns of post-fire recovery, these results suggest that chronic exposure to N deposition may significantly alter plant growth and species composition in successional chaparral stands.
The concentration of Se and the activity of the enzyme glutathione peroxidase (GSH-Px) were investigated in the whole blood of camels. Variable concentrations of whole blood Se (0.025–0.043 μg/ml) and GSH-Px activity (6.32–18.64 EU/ml) were demonstrated in both sexes, with females having significantly higher (P<0.001) blood Se concentration and GSH-Px activity than males. A highly significant correlation (r=0.88, P<0.001) has been demonstrated between whole blood Se concentration and the activity of the enzyme GSH-Px.
Interrelated, biotic (flora and fauna) and abiotic (pedogenesis and hydrology) processes were examined at four sites (30, and approximately 1000–3000, 7000–12 000, and 125 000 years before present) in the northern Mojave Desert. Data collected at each included floral and faunal surveys; soil texture, structure, and morphology; and soil hydraulic properties. Separate measurements were made in shrub undercanopy and intercanopy microsites. At all sites, shrubs made up greater than 86 percent of total perennial cover, being least on the youngest site (4 percent) and most on the 7000–12 000-year-old site (31 percent). In the intercanopy, winter annual density was highest on the 1000- to 3000-year-old site (249 plants/m2) and lowest on the oldest site (4 plants/m2). Faunal activity, measured by burrow density, was highest on the 1000–3000- and 7000–12 000-year-old sites (0.21 burrows/m2) and density was twice as high in the undercanopy versus the intercanopy. Burrow density was lower at the two oldest sites, although density was not statistically greater in the undercanopy than intercanopy. At the older sites, the soil water balance was increasingly controlled by Av horizons in intercanopy soils in which saturated hydraulic conductivity (Ksat) decreased 95 percent from the youngest to the oldest site. No significant reduction in Ksat in undercanopy soils was observed. Decreases in the intercanopy sites correlated with decreases in annual plant density and bioturbation, suggesting these processes are interrelated with surface age.
A hypothesis for understanding the stability of northern Chihuahuan Desert landscapes is that the distribution of soil resources changes from spatially homogeneous in arid grasslands to spatially heterogeneous in invading shrublands. Since radioactive fallout 137Cesium (137Cs) was deposited uniformly across the landscape during the 1950s and 1960s and was quickly adsorbed to soil particles, any redistribution of 137Cs across the landscape would be due to soil redistribution or instability at either plant-interspaces or on a landscape scale. The concentration of 137Cs in soils collected from different vegetation communities (black grama grass, tarbush, tobosa grass, and mesquite) at the USDA-ARS Jornada Experimental Range in the Northern Chihuahuan Desert in New Mexico was determined. At the black grama grass and tobosa grass sites, 137Cs was uniformly distributed at the plant interspace scale. At the mesquite sites, 137Cs was concentrated in the dune area under mesquite shrubs with little to no 137Cs in the interdune areas. 137Cs data support the hypothesis that significant soil redistribution has occurred at dune sites created by invading mesquite. In the arid grassland-shrub sites with black grama grass, tobosa grass, and tarbush the 137Cs data support the hypothesis of spatially homogeneous distribution of soil resources. High concentrations of 137Cs in the biological soil crusts (0–5 mm) at the tarbush sites indicate that biological soil crusts can contribute to the stability of these sites.
The world fallout of caesium-137 (137Cs) associated with nuclear weapons testing during the 1950s and 1960s has provided a valuable man-made tracer for studies of soil erosion and sediment delivery. But relatively few researchers have used it to estimate wind erosion. In this paper, the137Cs technique is introduced into the study of wind erosion and its modern processes in the Qinghai-Tibet Plateau. Two137Cs reference inventories of 982·11 and 2376·04 Bq m−2was established preliminarily, which distribute in the south and mid-north parts of the study area respectively. By analysing the patterns of137Cs depth profiles from sampling sites, the aeolian processes of erosion and deposition along nearly 40 years has been revealed, i.e. the shrub coppice dunes(S1) and semi-fixed dunefields (S3) had experienced the alternation of erosion and deposition, while the grasslands (S4, S6 and S7) and dry farmlands (S5) suffered erosion only. By using the137Cs model, the average wind erosion rates for shrub coppice dune (S1), semi-fixed dunefields (S3), dry farmlands (S5) and grasslands (S4, S6 and S7) were estimated to be 84·14, 69·43, 30·68 and 21·84 tha−1a−1respectively, and for the whole Plateau, averaging 47·59 ha−1a−1which can be regarded as the medium erosion standard. These results derived from137Cs for the first time have significant implications for the further research of wind erosion and desertification control in the Qinghai-Tibet Plateau.
A model to simulate 137Cs profiles in soils during the time in which they are being eroded is proposed. The model uses one parameter to characterize the cesium transference in the soil and another to express the erosion rate. To test the model, 137Cs profiles of stable and eroded soils were collected at sampling sites located on semi-arid and temperate slopes in the Central Ebro basin, Spain. The 137Cs profiles, corresponding to uncultivated soils with natural vegetation cover, were simulated using this model. The 137Cs inventories and profiles calculated with the model are very similar to those measured experimentally, and thus it is possible to calculate soil erosion rates in physiographically diverse Mediterranean environments.
An ongoing project monitors modern dust accumulation in the arid southwestern United States to (1) determine the rate and composition of dust inputs to soils and (2) relate dust accumulation to weather patterns to help predict the effects of climate change on dust production and accumulation. The 16-year records of 35 dust-trap sites in the eastern Mojave Desert and southern Great Basin reveal how generation and accumulation of dust, including the silt-clay, carbonate, and soluble-salt fractions, is affected by the amount and seasonal distribution of rainfall and the behavior of different source types (alluvium, dry playas, and wet playas).
Fat sand rats (Psammomys obesus) are strictly herbivorous and live in densely complex burrows in the desert of North Africa and the Middle East. Little is known however about the effect of their foraging and burrowing activities on surface morphology and plant community attributes. This study evaluated such effects by comparing burrow and mound surface morphology, canopy of the main host chenopod shrub Anabasis articulata, vegetation cover, and plant abundance and species richness on and off both active and abandoned colonies in the semi-stabilized sand dunes of the Northern Sinai. In general, active burrow systems were characterized by reduced A. articulata canopy area, and more soil disturbance, with higher and larger burrow mounds dominated by bare ground, dung, and dead and fresh litter. The abandonment of mounds for five years has resulted in significant increases of plant cover, canopy height, abundance and species richness. Vegetation structure and plant species assemblages differed between mound and non-mound patches of both active and abandoned sites. The results suggest that fat sand rats can have significant direct and indirect, short-term and long-term effects on vegetation dynamics and structure through their mound building and foraging.
Precipitation varied substantially in the Mojave Desert through the 20th century in a manner broadly similar to the other warm North American deserts. Episodes of drought and prolonged dry conditions (1893–1904, ca. 1942–1975, and 1999-present) alternated with relatively wet periods (1905–ca. 1941 and ca. 1976–1998), probably because of global-scale climate fluctuations. These are the El Niño-Southern Oscillation that affects interannual climate and the Pacific Decadal Oscillation that evidently causes decadal-scale variability such as prolonged dry and wet episodes. Studies done in the late 20th century demonstrate that precipitation fluctuations affected populations of perennial vegetation, annuals, and small herbivores. Landscape rephotography reveals that several species, particularly creosote bush, increased in size and density during the ca. 1976–1998 wet period. A brief, intense drought from 1989 to 1991 and the ongoing drought caused widespread mortality of certain species; for example, chenopods and perennial grasses suffered up to 100% mortality. Drought pruning, the shedding of above-ground biomass to reduce carbon allocation, increased substantially during drought. Overall, drought had the greatest influence on the Mojave Desert ecosystem.
Between 1749–1730 a widespread occurrence of fungus-caused heart rot affected western juniper in the interior Pacific Northwest, U.S.A. In this study 12 Juniperus occidentalis spp. occidentalis chronologies collected from various sites throughout the western juniper range were examined. The presence of heart rot precluded dating many of these samples prior to the 1730s. Subsequently, sample size decline in these chronologies was examined and it was found that a substantial decrease in chronology sample size was present in nine chronologies, with no other periods exhibiting a decline of equal magnitude. Since heart rot requires open wounds to establish, and the major heart rot episode was confined to a short period, we suspect that a regional–scale climatic event such as a windstorm or icestorm was the initiating cause of this outbreak.
The habitat choice and population structure of Heterogamisca chopardi were studied in a middle altitude desert of Saudi Arabia (30 km east of Taif) during June 1992. Two populations were compared, located, respectively, in a 3-year-old protected area and in a grazed area. Cockroaches live in the sand beneath shrubs. They preferred shrub species according to their cushion form, possibly depending on grazing pressure. Population densities ranged between 0·25 and 0·42 individuals per m2. Most individuals in June were middle-instar nymphs, with few first-instar nymphs and old females. Oothecae were frequently parasitized by flies (Bombyliidae). In comparison with the protected area, the grazed area showed higher density, the presence of younger middle-instar nymphs and of old females, higher fecundity, larger oothecae, a sex ratio less biased towards females, and probably lower nymphal survival.
A critical check-list of novel fungal taxa having original localities in the arid Middle East region and introduced between 1940 and 2000 is presented. The list considers 246 taxonomic units at the species and subspecies levels. The taxa are grouped according to major Classes of Fungi. For each taxon the reference, country of origin and the nature of the original substratum are provided. The nomenclatural status of invalid names with an indication of the relevant article of the International Code of Botanical Nomenclature is specified. The changes in taxonomic position are also reported with the relevant reference.All major taxonomic groups are represented in this body of novel taxa but in varying proportion. Anamorphic fungi and the Ascomycetes (95 and 85 units, respectively) comprise 70.7% of taxa considered. The Basidiomycetes have a comparatively limited relative weight (37 units). As expected, the Oomycetes and the Zygomycetes are underrepresented with, respectively, 12 and 8 units, but they stand ahead of the three remaining groups, viz. Chytridiomycetes, Agonomycetes and Myxomycetes.The contribution also includes a selection of major recent titles on the biodiversity of the fungi in this region. Few of these publications have a regional character but most relate to a specific country of the area. The check-list and the accompanying bibliography is intended to stimulate the taxonomic research on the mycobiota of this particular arid region.
Temporal changes in the stabilization process along the coastal dunes of Israel were assessed using a series of 23 aerial photographs taken over the period 1944–1999. The stabilization rate was then quantified using a specially developed method for the calculation of sand dune movement and by the calibration of the gray-scale images into vegetation cover maps. An episodic reactivation of the dunes during the 1970s was identified and examined with respect to various physical and human factors. The Mediterranean climate along the coast of Israel and the low wind energy are favorable for the stabilization of sand dunes in the absence of human activity. It was shown that military maneuvers and recreational traffic (pedestrians and off-road vehicles) increased fragmentation but did not lead to the reactivation of the dunes. Based on archival records and interviews with officials who have worked in the area over the past 25 years, it was concluded that the apparent changes in the stabilization process should be attributed to changes in the grazing and vegetation cutting practices of Bedouin farmers along the coast of Israel between the late 1960s and the late 1970s, concomitant with the changing policies of successive Israeli governments.
In the early 1980s, the situation on the northern part of the Central Plateau of Burkina Faso was characterized by expanding cultivation on lands marginal to agriculture, declining rainfall, low and declining cereal yields, disappearing and impoverishing vegetation, falling ground-water levels and strong outmigration. This crisis situation provoked two reactions. Farmers, as well as technicians working for non-governmental organizations, started to experiment in improving soil and water conservation (SWC) techniques. When these experiments proved successful, donor agencies rapidly designed SWC projects based on simple, effective techniques acceptable to farmers. A study looked at the impact of SWC investments in nine villages and identified a number of major impacts, including: significant increases in millet and sorghum yields since the mid-1980s, cultivated fields treated with SWC techniques have more trees than 10–15 years ago, but the vegetation on most of the non-cultivated areas continues to degrade, greater availability of forage for livestock, increased investment in livestock by men and women and a beginning change in livestock management from extensive to semi-intensive methods, improved soil fertility management by farmers, locally rising ground-water tables, a decrease in outmigration and a significant reduction in rural poverty. Finally, data are presented on the evolution of land use in three villages between 1968 and 2002.
Vegetated wetlands at Owens Dry Lake, California, are embedded in a large arid region. The wetlands initially established when Owens Lake desiccated in the early 1900s, and largely exist below the historic shoreline, due to ground-water discharge. MSS satellite imagery was used to measure the coverage of wetlands on five dates from 1977 through 1992. Coverage from vegetated wetlands both declined and increased during the study period, and appeared to have an inverse relationship with annual runoff in the hydrologic watershed. Also, soil and/or water chemistry appear to interact with ground-water elevation to affect the wetland's spatial dynamics.
Fire has been historically infrequent in the Mojave Desert, and its increased prevalence caused by the invasion of non-native annual grasses is a major concern for land managers there. The most dramatic changes have occurred in middle elevation shrublands dominated by creosotebush (Larrea tridentata), Joshua tree (Yucca brevifolia), and/or blackbrush (Coleogyne ramossissima), where most of the fires occurred between 1980 and 2004. This zone is more susceptible than other areas of the Mojave Desert to increased fire size following years of high rainfall. Increases in fire size are likely related to the flush of non-native annual grasses, Bromus rubens in particular, that produces continuous fuelbeds following years of high rainfall. This dynamic also has occurred to some degree at lower elevations, but the background cover of native perennial fuels there is already very low, muting the effects of the ephemeral fuels. At elevations above the middle elevation shrublands, fire size does not vary with rainfall, indicating that native woody fuels dictate fire regimes. These results suggest that an invasive plant/fire regime cycle is currently establishing in the middle and possibly the low elevation shrublands of the Mojave Desert, but not at higher elevations.
Remotely sensed measurements from NOAA-AVHRR expressed as normalized difference vegetation index (NDVI) have generated a 23-year time series appropriate for long-term studies of Sahel region. The close coupling between Sahelian rainfall and the growth of vegetation has made it possible to utilize NDVI data as proxy for the land surface response to precipitation variability. Examination of this time series reveals two periods; (a) 1982–1993 marked by below average NDVI and persistence of drought with a signature large-scale drought during the 1982–1985 period; and (b) 1994–2003, marked by a trend towards ‘wetter’ conditions with region-wide above normal NDVI conditions with maxima in 1994 and 1999. These patterns agree with recent region-wide trends in Sahel rainfall. However taken in the context of long-term Sahelian climate history, these conditions are still far below the wetter conditions that prevailed in the region from 1930 to 1965. These trend patterns can therefore only be considered to be a gradual recovery from extreme drought conditions that peaked during the 1983–1985 period. Systematic studies of changes on the landscape using high spatial resolution satellite data sets such as those from LANDSAT, SPOT and MODIS will provide a detailed spatial quantification and description of the recovery patterns at local scale.
Water balances for the Colorado River mainstem complex, the Cienega de Santa Clara, and El Indio wetlands were calculated for the Colorado River delta in Mexico for the period 1992–1998. Discharge for the mainstem complex was disaggregated into flood and non-flood years, reflecting the marked variability of mainstem discharge at the Southerly International Boundary (SIB) delimiting the United States and Mexico. In non-flood years, agricultural and municipal returns to the mainstem below SIB contributed 180% of mainstem discharge at SIB, but may not be sufficient to generate the floodstage discharge required by native riparian vegetation.
With the transition to democracy in 1994, South Africa was faced with an enormous challenge in redressing the highly unequal and racialized pattern of land rights inherited from the colonial and apartheid past. In Namaqualand, a history of land dispossession and racial segregation presented the new government with a complex set of problems, which led to a series of distinct policy responses within the context of the wider national land reform programme. Land reform in Namaqualand aims to impact positively on local people's access to land, improve livelihood opportunities and develop the local economy. Unique features of the land reform process in Namaqualand include the reform of tenure in the former Coloured Rural Areas, the prominent role played by local municipalities and the heavy reliance on municipal commonage as a form of landholding. This study provides and overview of the process of land reform in Namaqualand since 1994, considering the three elements of tenure reform, land redistribution and restitution of historical land rights. It concludes that, while considerable progress has been made in provision of additional land to historically disadvantaged communities, obstacles remain in the area of post-transfer support to new and emerging farmers.
Degradation in semi-arid Karoo rangelands has been ascribed to over-utilization by livestock and variations in rainfall regime. The understanding of vegetation dynamics in confined plant–herbivore systems is hampered by the difficulty in uncoupling biotic and abiotic determinants of vegetation change, and a paucity of long-term studies. Vegetation change in permanent fenced and open plots in Karoo National Park was monitored over 10 years, largely falling within a high rainfall phase. Herbivore pressure more than tripled during this period with notable increases in the larger ungulates and ostrich. No clear correlation could be established between rainfall and vegetation parameters. Rangeland condition improved and species richness increased over time. Annual grass cover decreased and perennial grass cover increased with time following a change in land use from small-stock farming to conservation and reintroduction of wild ungulates. Changes were more rapid in exclosures than in areas exposed to herbivory. Rangeland condition was closely correlated with canopy spread cover. To differentiate rainfall-induced fluctuations from directional changes in vegetation dynamics caused by herbivory, monitoring needs to be conducted for extended periods that include various rainfall cycles. The value of vegetation monitoring would be greater if additional data were collected to measure persistence of uncommon species.
An intense storm, including strong winds, a dust storm, ‘blood rain’ and heavy rains affected the Canary Archipelago between 5 and 10 January 1999, producing damage valued at 156 million euros. The present paper analyses the weather conditions and sedimentological features of the dust. The resulting data provide a possible explanation of palaeoclimatic conditions essential for the stabilization of sand dunes in the eastern parts of the Canary Islands.
The aim of this study was to identify trends in plant diversity, and examine their predictive relationships with environmental variables (water stress, soil texture and grazing intensity) and vegetation characteristics (total vegetation and individual species cover), along a 340 km transect through an area of hyper-arid desert in southern Egypt. The major part of the transect is located within the boundaries of the Wadi Allaqi Man and Biosphere (MAB) Reserve. In total 55 plant species were recorded from the transect. A TWINSPAN classification of the data identified four of the seven main vegetation types, previously identified as present in the area by Springuel et al. (1997). Sites dominated by Vegetation type I occurred exclusively in the lowest stretch of the Wadi Allaqi system, most heavily influenced by flood and seepage from Lake Nasser, and characterized by dense stands of Tamarix nilotica (Ehrenb.) Bunge and Glinus lotoides L. Vegetation type IIIb was represented at sites higher up the wadi system, characterized by sparser vegetation dominated by Acacia ehrenbergiana Hayne, Acacia tortilis subsp. raddiana (Savi) Brenan and Aerva javanica (Burm. f.) Juss.ex Schult. Vegetation type IIIa occurred in the middle region of Wadi Allaqi and was characterized by Salsola imbricata Forssk. andCullen plicatum (Delile) C. H. Stirt. Vegetation type IV, the most species rich, was limited to upstream sites of Wadis Allaqi and Eiqat. Plant β -diversity (as species turnover between individual sites along the transect, assessed using Whittaker's βwindex) showed a rising trend of plant species turnover from sites low in the wadi system to sites located in the upper part of the wadi. Plant α -diversity within these vegetation types (as species richness R: number of species present per 500 m2unit area) was linearly correlated with plant diversity measured using Simpson's index (D). R was reasonably well predicted from three variables: plant cover, soil moisture and grazing intensity (following a third order polynomial model). This model predicted R in the original data set with 52·9% success (r=0·73, p<0·001). The study suggested that vegetation cover estimates, in association with simple estimates of intensity of habitat stress (water availability) and disturbance (grazing intensity), can provide a rapid indication of the biodiversity support capacity of desert habitats.
Despite the extreme aridity of the region, there is good evidence that sheep were being herded in small numbers in Namaqualand up to 2000 years ago. Archaeological research at Spoegrivier Cave on the Atlantic coast confirms the presence of sheep at 2100 years ago. Increasing aridity in Namaqualand around 1300 years ago lead to the abandonment of the cave. Jakkalsberg on the banks of the Orange River in the Richtersveld was occupied around 1300 BP by people who were herding sheep rather than hunting game. People with sheep and goats were herding their livestock at Bethelsklip, in central Namaqualand, between 800 and 360 BP. The archaeological evidence clearly supports a long tradition of herding in the region. The question is whether these early herder groups are the ancestors of the Namaqua Khoekhoen pastoralists who occupied Namaqualand during historical times?Historical records from the 17 to 20th centuries testify to the seasonal transhumance of the Namaqua with cattle, sheep and goats between the Kamiesberg Mountains and the plains. On occasions pastoralist groups also moved to the interior plateau of Bushmanland to make use of the seasonal grasses after summer thunderstorms. Traditional land-use practices have a long history in the region and a study of these land-use patterns may assist in interpreting the archaeological record and in developing a sustainable land-use policy for the region.
Understanding the dynamics of semi-arid rangelands is a prerequisite for their proper management and long-term enclosure experiments are an important tool to investigate grazing impact. Hein [2006. The impacts of grazing and rainfall variability on the dynamics of a Sahelian rangeland. Journal of Arid Environments 64, 488–504] presents findings from a 10 year enclosure experiment in the semi-arid Ferlo, Senegal. His main conclusion is that current high-grazing pressure (0.15–0.20 TLU/h) negatively affects rain use efficiency and productivity especially in dry years because differences between treatments are significant for the whole period as well as for dry years only.A re-analysis under the framework of non-equilibrium theory of rangeland science leads to an alternative interpretation of the data. The vegetation on the more intensively grazed site possesses a remarkable resilience after the drought of 1983 and 1984. Standing crop recovers fast and for 2 years is even higher on the “high grazing” treatment than on the less intensively grazed treatment. Statistical analysis confirms this: a general linear model for standing crop against effective precipitation and grazing treatment finds a significant contribution of precipitation only (p<0.0001). Thus, vegetation dynamics in the semi-arid Ferlo largely follows a non-equilibrium dynamic as it is rather driven by precipitation dynamics than by grazing. This also leads to different policy implications: droughts reduce livestock density and thus are important to allow the vegetation to rest for 1 or 2 years.
During the United Nations General Assembly's 58th Ordinary Session in 2003, a decision was adopted declaring 2006 the International Year of Deserts and Desertification (IYDD). This paper critically reviews this International Year. It draws on the key outputs from IYDD events from across the globe to highlight the challenges and ways forward in both combating desertification and implementing the United Nations Convention to Combat Desertification (UNCCD). The paper considers what the IYDD outputs mean for the current and historical controversies surrounding the desertification issue and presents an overall evaluation of the successes of IYDD for the different stakeholder groups within the desertification regime. It is concluded that while the International Year can be considered to have met the United Nations's four objectives: to address the long-term oriented implementation of the UNCCD; raise awareness of the implications of desertification; facilitate networking with all stakeholders; and disseminate information relating to the UNCCD, the real challenge lies in moving the IYDD outcomes away from the conferences, meetings and networks that contributed to their generation, towards a more concrete, tangible effort to conserve deserts and effectively monitor and control desertification and land degradation on the ground.
This study represents the vegetation history of the last 3500 years and conducts an analysis of the climatic fluctuations on a 75 km long transect on the western Dead Sea shore. Palynological and sedimentological data are available from six cores near Mount Sedom, Ein Boqueq, and Ein Gedi and from outcrops near Ze'elim and Ein Feshkha. The comparison of the pollen data with the lake levels shows synchronous trends. During the Middle Bronze Age, Iron Age and Hellenistic to Byzantine Period the high lake level of the Dead Sea signals an increase in precipitation. Contemporaneously, values of cultivated plants indicate an increase in agriculture. Lake level is low during the Late Bronze Age, within the Iron Age and at the end of the Byzantine period, indicating dry periods when all pds show a decrease of cultivated plants. Forest regeneration led by drought-resistant pines is observed in all pollen diagrams (pds) following the agricultural decline in the Byzantine period and, in the pds near Ein Boqeq, Ze'elim and Ein Feshkha, during the late Iron Age. The modern vegetation gradient is reflected in the palaeo-records: a stronger expansion of Mediterranean vegetation and cultivated plants in the northern sites is recognisable.
A 0.6-m-long horizontal core from a stalagmite in the entrance area of Wonderwerk Cave in South Africa, a National Heritage Site, has provided a 35 ka record of environmental change. A pollen sequence from the stalagmite, and two animal dung deposits, is longer and more detailed for the end of the Pleistocene than previous palynological reports from the southern Kalahari region. This pollen record closely matches information from spring peats at Wonderkrater in Northern Province indicating that speleothem pollen can provide reliable paleovegetation data. The δ18O and δ13C records resemble those from Cold Air Cave in Northern Province and parallel variations in Greenland Ice Sheet GISP2 paleotemperature. This indicates that past climate changes in southern Africa were linked to changes in global atmospheric and oceanic circulation patterns possibly triggered by conditions in the North Atlantic region. The Wonderwerk data suggest wetter conditions at ca. 33 ka, from 23 to 17 ka, and from 4 ka to present. Conditions were drier 17–13 ka, when microstromatolitic carbonate was deposited on the flank of the stalagmite, and possibly also during depositional hiatuses from 33 to 23 ka and 13 to 4 ka.
The Huaynaputina eruption (1600 AD, Moquegua, S Peru) in the northern Atacama Desert denuded the Omate area of all vegetation and deposited deep pumice layers. Data on the flora, climate and soil characteristics of these slopes near Omate at 1600–2600 m a.s.l. are provided. Fifty-nine angiosperm species established themselves on the pumice slopes in the past ca. 400 years, with the bulk of the small and herbaceous species and several species new records for Peru. Three Omate sites were sampled in both a dry and a wet year and species numbers differed widely (14 versus 45 spp.). Among areas compared floristic composition is most similar to the Lomas de Tacna, and has less in common with geographically closer Lomas or Sierra formations. Nine species represent highly disjunct populations (200–>700 km) from their nearest known living populations in central Peru, Chile, or Argentina/Bolivia and appear to have reached the area via long-distance dispersal. Abiotic conditions may have played an important role in limiting the establishment of species from the neighboring vegetation. Four taxa on the pumice slopes show clear morphological differences to populations elsewhere, two of them may represent neoendemics of the Omate pumice, indicating rapid morphological divergence.
The present-day Sahara occupies an area of slightly over 8 million km2in Africa, between latitudes 16 and 32° N, circumscribed within the isohyet of 100 ± 50 mm mean annual rainfall. The hyperarid area alternately expanded and shrank on both sides of a seemingly narrow semi-permanent eremitic zone along the Tropic of Cancer during the course of the Quaternary epoch (1·7 Ma). The Cenozoic, Mesozoı̈c and Paleozoı̈c Sahara, in turn, has undergone drastic climatic changes as the African continent drifted northward from its Antartic position to reach its present latitudinal situation. But, seemingly the Sahara was never the large desert it now is, with the exception perhaps of the Upper Triassic Lower Liassic epochs. The Pleistocene and Holocene contrasting climate changes induced large variations in flora and fauna distribution, as well as in geomorphic processes.
The Mercur Gold Mine tailings pond experienced a dam failure more than 70 years ago, creating a denuded area that was the subject of this study. Arsenic levels in the surface soils ranged from 45 to 3237 mg As kg−1. Vascular plants, mosses, algae, and lichens all showed a threshold effect to elevated arsenic. Over 2100 mg As kg−1, chlorophyll a was consistently depressed. Over 1700 mg As kg−1, grasses, annuals and mosses were absent, and over 1400 mg As kg−1, cyanobacteria were notably affected. We conclude that the microbiotic crusts will not recover so long as the soil remains contaminated.
Sediment cores from the San Bernardino Ciénega near Douglas, Arizona and Agua Prieta, Sonora, Mexico were examined to reconstruct the fire history of this region and inform restoration efforts. A ∼8000 year vegetation and fire history record was reconstructed from these sediments using fossil pollen and charcoal. Results from the fire reconstruction show an increase in fire activity coincident with the onset of ENSO, and an increase in fire frequency during the Medieval Climate Anomaly. Preliminary pollen data show taxa that reflect winter-dominated precipitation (Ephedra and Artemisia) correspond to times of greater fire activity. These fire data shed light on the long-term history of fire in desert environments that may be helpful in understanding what fire regimes may be expected with global warming and also how to best incorporate fire into management plans.
Winterfat is a native shrub in the prairies of North America and seedbed conditions determine seedling establishment and population restoration. Thirteen seedbed treatments were created with 2–3 sowing dates and irrigation levels in the field over 2 years to quantify the effect of soil temperature and water potential on seedling emergence. Seedling emergence ranged from 3% to 69% of viable seeds in these field conditions. Higher seedbed temperatures associated with late sowing dates greatly reduced seedling emergence. An exponential relation of thermal time requirements between seed germination and seedling emergence was established. The constructed hydrothermal time model had variable accuracy in predicting field emergence, depending on soil temperature and soil water potential. The correlation between predicted and observed emergence can be as high as R2=0.79. Generally, the hydrothermal time model fits well for emergence when soil temperatures are 10–15 °C and soil water is >−0.5 MPa. These seedbed conditions occur in the spring when snow melts, favouring seedling emergence of winterfat. Results from this study provide practical information for restoration using winterfat.
Protection by fencing greatly increased both vegetation production and plant species diversity in the arid rangelands of Jordan. The botanical composition of the Al-Mowaqqar site changed continuously from 1986 to 1992 as new plant species emerged that had better chances of survival under protection. Improving the range by planting fodder shrubs and by water harvesting produced much more forage than that obtained by fencing alone.
Coronilla juncea is a leguminous shrub native to semi-arid regions of the Mediterranean Basin, particularly south-eastern Spain, and is browsed by livestock such as sheep and goats. We provide here information about this source of forage: allometric regression equations (phytovolume vs. total and forage phytomass), nutritive value and seed germination. The regression equations calculated were highly significant (r2= 0·86 for volume/total phytomass and 0·93 for volume/forage phytomass, p < 0·001). The values recorded for the in vitro digestibility of dry matter and organic matter were 65·93% and 62·13%, respectively. The optimum temperature for seed germination was 10°C. Scarification in goat rumen for 24 h increased the overall germination rate of fruit with pericarp although in sheep rumen overall germination decreased slightly. We suggest that this fodder shrub be included in programmes for land management and the rehabilitation of degraded semi-arid-to-arid ecosystems.
We detail the impact of abandoned traditional settlements (or bomas) on plant and nutrient succession in the Amboseli ecosystem, southern Kenya, over the course of a century. Plant and soil data were sampled on and around abandoned settlements. The term, ‘onsite’, refers to the area within the perimeter fence, ‘offsite’ to the area up to 200 m beyond the fence. Herbaceous standing biomass onsite increased in the course of succession to peak at twice offsite levels within two decades. Biomass remained elevated for six decades then dropped to the background levels at the limit of sampling distance. Plant species richness onsite increased rapidly in the course of succession, then stabilized on older bomas. Species composition changed throughout succession, with pioneer herbs and grasses giving way to boma-edge species and woody vegetation later in succession.Soil nutrients, including carbon, nitrogen, magnesium and phosphorus, were highly elevated on abandoned settlements. The various nutrients declined at different rates during the course of plant succession. Potassium, phosphorus and magnesium levels remained at twice offsite levels for over a century, creating islands of high fertility and high plant biomass in the savanna landscape. We conclude that the perturbation caused by shifting nomadic settlements creates localized nutrient and plant diversity hotspots in savanna ecosystems that remain distinct from the surrounding savanna for decades, possibly centuries.
A number of studies have begun to show the large impact that pastoralism has on African savanna ecosystems. Here we look at the impact of abandoned settlements on the distribution of the large ungulates of Amboseli, Kenya.Monthly dung counts show that all 9 species studied are attracted by settlements for up to a century after abandonment. Densities increased in the first two decades of vegetation succession, when plant biomass is the highest, then declines steadily to background levels as plant biomass. The pattern of herbivore use varied among species in the course of succession, with grazers dominant in early successional stages and browsers later stages, when shrubs and trees began to replace grasses. The densities of both grazers and browsers correlated positively with herbaceous and woody biomass respectively. We conclude that the additional biomass and higher quality forage produced by heavy dung deposits in livestock settlements are a major factor governing the distribution and movement of large herbivore in the savannas. The abandonment of seasonal migrations and construction of permanent settlements under way in African savannas is likely to restrict wildlife movements and reduce habitat patchiness and species richness of large ungulates.
Excessive ground-water use and saline intrusion to the aquifer led, in less than three decades, to an increase in abandoned agricultural fields at La Costa de Hermosillo, within the Sonoran Desert. Using a chronosequence from years since abandonment, patterns of field succession were developed. Contrary to most desert literature, species replacement was found, both in fields with and without saline intrusion. Seasonal photosynthetic capacity as well as water and nitrogen use efficiencies were different in dominant early and late successional plant species. These ecological findings provided a framework for a general explanation of species dominance and replacement within abandoned agricultural fields in the Sonoran Desert.
Data were collected simultaneously at different succession stages, and were analysed using the quantitative classification method (TWINSPAN) and the ordination technique (DCA). The succession series of plant communities from abandoned croplands of loess soils was as follows: Assoc. Ixeris chinensis var. versicolor+Setaria viridis → Assoc. Artemisia spp. → Assoc. Artemisia lavandulaefolia+Elymus dahuricus → Assoc. Elymus dahuricus+Poa annua+Artemisia spp. → Assoc. Hippophae rhamnoides → Assoc. Pinus tabulaeformis → Assoc. Quercus liaotungensis → Assoc. Larix principis-rupprechtii. This established a model of the natural recovery of vegetation on abandoned croplands in the Loess Plateau. The structure, composition and life-forms changed significantly during succession. Six indices of species diversity were used to analyse changes in the richness, evenness and heterogeneity of species during the succession process. As the succession progressed, the richness increased significantly, the evenness decreased slightly and the heterogeneity increased.
Soil and vegetation dynamics were examined along roads abandoned for 5, 10, 21, 31, 55 and 88 years in southern Nevada in an attempt to elucidate factors controlling desert succession. None of the measured soil or vegetation parameters varied significantly with road age. Differences were found, however, between soils and vegetation on roads vs. nearby controls, and soils differed between roads created by surface vehicular traffic and bulldozing. Studies of recovery following disturbance in deserts must take into account natural patterns of plant and soil heterogeneity and initial disturbance type.
Syrian qanat Romani (subterranean aqueducts) were studied as part of a broader project examining the role of qanats in a modern world. Fieldwork was designed to: (1) produce a comprehensive map showing the location and current status of all Syrian qanats; (2) determine the relationship between qanat sites, ancient settlements, and ecological zones; and (3) evaluate the degree to which modern water technology has displaced qanat systems and assess the impact of these changes on Syria's ground-water resources. A map showing the distribution and status of qanats across Syria presents a picture of widespread abandonment of recently desiccated qanats, implicating ground-water depletion and underscoring the need for coordinated and sustainable aquifer management in Syria.
To mitigate erosion on abandoned fields in semi-arid ecosystems, it is important to understand how vegetation and soil properties and patterns develop after land abandonment. Our objective was to investigate the development of spatial heterogeneity in vegetation and soil properties after land abandonment. We described the vegetation composition, collected soil samples and made detailed aerial photographs for two series of abandoned fields on marl and calcrete in Southeast Spain. The images were classified into bare and vegetated patches, and spatial metrics were calculated for each site. Our results showed that recovery of vegetation and change in soil properties after land abandonment are slow and take at least 40 years in such a semi-arid environment. Succession on calcrete appeared to be faster than on marl, probably because more water is available due to the higher rock fragment cover. Organic matter, aggregate stability and electrical conductivity were all significantly higher under vegetated patches. We found a clear linear relationship between vegetation cover and most spatial metrics, which offers the possibility of upscaling spotted vegetation patterns. The results of our integrated approach to study spatial heterogeneity in vegetation and soil properties can be used to improve predictions of runoff and erosion.
This study was conducted to evaluate the population of the arbuscular mycorrhizal fungi (AMF) in the semi-rid agro-ecosystem of North Jordan and to evaluate the effect of the biotic and abiotic factors on AMF population. Soil samples were collected from 12 locations where several soil samples were collected from each location. The collected soil samples were analysed for chemical and physical properties and spores were separated from the soil by the floating–adhesion technique and examined by the dissecting microscope. The AMF spores densities vary among locations and crops. The spore density was relatively low, with the highest observed under fruit trees and the lowest under a fallow system. Eight species were identified.Glomus mosseae was the most common species among samples, which was found in 85% of the soil samples collected.G. geosporum was found in 20%, while G. constrictum and G. clarum were found in 10% of the samples collected. G. caledomium was found in 8% of the samples. The lowest occurrence was observed for the G. monosporum and G. clariodum, which were found in only 5% of samples collected. Twenty percent of the samples collected contained Acaulospora spp. The spore density was not clearly affected neither by the host plants nor by the location, which suggest the absence of location or plant specificity for the AMF observed in soils of North Jordan. This may suggest that other factors could have an effect on mycorrhizal distribution. Therefore, biotic factors may be relatively less important than abiotic factors for establishing population patterns. The spore density of AMF had a very weak correlation with the soil pH and electrical conductivity (EC), and no significant correlation with the percentages of soil clay and silt particles. The organic matter (OM) and CaCO3 percentages were significantly positively correlated with AMF spore density. On the other hand, the AMF spore density was negatively related to the soil phosphorus. In conclusion, the G. mosseae was the most common AMF species and no host plant or geographic location specificity was observed, suggesting the population of AMF species was affected mainly by abiotic factors and cropping patterns.
Abiotic conditions and interactions among plants are important factors determining plant geographical distributions. With the aim of analyzing the distribution of a perennial grass species in a semiarid mountain range, we experimentally tested above- and belowground effects of shrubs on the grass along a gradient of aridity. We measured growth, physiology and reproductive output of Stipa tenacissima tussocks surrounded by intact or manipulated neighbour shrubs. Performance of Stipa improved under mesic conditions, but the net effect of shrubs on Stipa did not change along the gradient. Belowground, we did not find significant effects, suggesting belowground resource partitioning between tussocks and shrubs. Aboveground net effects of shrubs on Stipa were generally positive and independent from abiotic conditions. The improved performance of Stipa at the mesic end of gradient suggests that its absolute physiological optimum lies out of its realized range. Overall, the arid end of the geographical distribution of Stipa in this mountain range may be determined by environmental severity. The coincidence of the upper limit of Stipa with the lower tree limit suggests that the more mesic end of Stipa distribution may depend on other factors; most likely competition with trees, which, unlike shrubs, could be excluding Stipa tenacissima from forests.
Tylosema esculentum (Burch) A. Schreib is an underutilized drought-tolerant legume, native to the arid and semi-arid regions of southern Africa, which produces protein- and oil-rich seed and tubers with potential for use as human food and animal fodder. Laboratory experiments were conducted, in order to investigate the nature of the characteristic leaflet movements of this species and the potential effects of potassium on them. The rhythm of leaflet movements was maintained in constant dark conditions and at several temperature regimes and it is also synchronized with the light cycles. Substantially, this study confirms the endogenous nature of T. esculentum leaflet movements, since they have all the main criteria of circadian rhythms. The crucial role of potassium on these movements and consequently on the total water economy of the species is also indicated, as long as there was an undoubted absence of leaflet movements in the potassium-deficient plants. This effect has to be taken seriously into account for any future attempts of the cultivation of the plant in arid and semi-arid environments.
Preservation of biodiversity can be at odds with preservation of sociodiversity, i.e., human groups with different cultures, social organization and economic activities coexisting within a region. We analyzed this problem in the Cushamen Reserve, a pastoralist Mapuche aboriginal settlement in Patagonia, Argentina. We found that the current stocking rate of domestic herbivores is twice the rangeland carrying capacity, and this overstocking has resulted in a 20–30% reduction in plant cover, productivity, floristic richness and pastoral value compared to similar sites located on neighboring capitalist farms. The potential economic income of a generic Cushamen farm (625 ha) under the current stocking rate (0.27 sheep units∙ha−1) and productive parameters (wool and number of lambs produced per sheep each year and the number of sheep that died or were discarded per year) is above the family poverty line, but this potential may be only rarely achieved due to interannual variability of productive parameters. However, under the carrying capacity, only an improbable combination of 0.84 lambs∙sheep−1 yr−1, 5.5 kg wool∙sheep−1 yr−1 and a 16% rate of annual sheep discarding would equal the present potential income. Active policies aimed at increasing carrying capacity of the Cushamen Reserve are needed to preserve both bio- and sociodiversity.
The composition and spatial pattern of weeds was analysed in an arable-soil seed bank after the harvest of wheat. Four hundred cores were collected from a 10×10 m grid in April 1999. Three indices of pattern detection (variance/mean ratio, Lloyd's index of mean crowding, and Morisita's index of aggregation) and Moran's I statistic of spatial autocorrelation were computed. The above-ground vegetation of the corresponding area was sampled by 100 randomly placed quadrats in March and September 1999 and variance/mean ratio of all the species was calculated. The seed bank of the field was dominated by annuals including Convovulus arvensis, Amaranthus viridis, Anagallis arvensis, Chenopodium album and Conyza canadensis. In all 27 species were recorded from the seed bank. For most of the species with a mean higher than 0.05 per core, the seed pattern (with two exceptions) was aggregated as indicated by various indices of pattern detection. Among the common species, only Amaranthus viridis exhibited a random pattern. Aggregated pattern of seeds was generally due to seeds settling in the neighbourhood of the mother plant and the clumped distribution of plant species in the overlying vegetation. Suggestions have been made regarding the improvement of the precision of the soil seed bank sample, in view of the aggregated distribution of seeds in soil. Seed bank showed considerable qualitative similarity with the above-ground vegetation, which is presumably the consequence of recurrent disturbances in the arable field.
Nitrogen amendments (0, 25, 50 and 100 kg NH4NO3/ha) were used to study the responses of primary production, microbial biomass and nematode population in desert soil. The study was conducted in the Israeli Negev Desert, a region characterized by low and randomly distributed rainfall. Over a 1-year study period, nitrogen amendments resulted in a significant (p<0·01) increase in soil microbial biomass. Soil microbial biomass also increased concomitantly with the increase in soil organic matter. The number of free-living nematodes in the soil increased with the increase in soil moisture, ranging from 43,000 individuals per square metre at the end of the summer to 351,000 individuals per square metre during the rainy season. No significant correlation was found between the nitrogen treatments and the nematode population, whereas a significant positive correlation was found between the nitrogen amendments and the above-ground biomass (r2=0·94, p<0·03). The nitrate proportion of the total soluble nitrogen in the soil also increased with the increase in soil moisture. This study provides baseline data for nitrogen amendments on soil microbial status, as well as insights into the importance of nitrogen in fertility in arid environments.