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Hyper-arid rangeland vegetation is typically dominated by large woody species which are often overlooked in herbivory studies. Long-term responses of tall shrub populations to herbivory change are poorly understood in the Arabian Peninsula. Population and size of 1559 individuals from four shrub species were assessed over an 11-year period under two herbivory regimes, one in which domestic livestock (camels) were replaced by semi-wild ungulates (oryx and gazelles) before, and the other during, the study period. Each shrub species exhibited a different response to the change in herbivory. Populations of Calotropis procera decreased dramatically. Populations of both Calligonum polygonoides and Lycium shawii increased through sexual reproduction, but the spatial distribution of recruits indicated different modes of seed dispersal. Average lifespans were estimated at 22 and 20 years respectively. The persistence strategy of Leptadenia pyrotechnica was similar to tree species of this habitat in that vegetative regrowth was prioritized over recruitment, and average lifespan was estimated at 95 years. Shrub responses to changes in ungulate management are therefore species-specific. Response of individual plant size was faster than response of population size, which was limited by slow sexual recruitment (L. pyrotechnica) or localized seed dispersal (C. polygonoides).
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Most range management decisions are based on quantitative measurements of production and utilization of key range plant species. A study was conducted to estimate biomass production and utilization of three perennial grasses using dimension analysis and height–weight profiles. Plant height, basal diameter and canopy diameter were highly correlated with biomass ofLasiurus sindicus. Plant biomass ofPennisetum divisumandStipagrostis drariiwas correlated to basal and canopy diameters. The three variables were evaluated for predicting biomass of the three grass species using best subset regression approach. From a practical point of view, basal and canopy diameters would be good predictors ofL. sindicusandS. drariibiomass. ForP. divisum, all three dimension measurements are needed for satisfactory results. Height–weight profiles were developed for each species to assess utilization based on estimation of percentage of height removed in relation to percentage weight removed. Although good correlation was found between height and weight, it is likely that light grazing will be underestimated. However, this would not limit the use of height–weight profiles as defoliation thresholds are never approached under long-term light grazing.
Article
Ecosystems protected from heavy grazing impacts, such as national parks and refuges, are generally considered to sustain higher plant species diversity and better ecosystem composition and structure compared to heavily grazed areas. To evaluate the impact of livestock grazing, we sampled vegetation characteristics from two areas having different grazing intensity levels. The first site has high protection from grazing and is located inside the Bou Hedma National Park in Southern Tunisia. The second site has a low protection from grazing and is situated within an open area located immediately outside the park boundary where human populations and their livestock have unrestricted access to ecosystem resources. Total plant cover, density, perennial species cover and their contribution were compared between the two grazing level sites. Results show that considerable positive effects occur in the areas protected from grazing. As compared to the overgrazed (open) sites. Several species known for their high palatability, such as Cenchrus ciliaris L., Salvia aegyptiaca L., Echiochilon fruticosum Desf. and Helianthemum sessiliflorum Desf., are more abundant inside the park than outside. These results are very important for managers to apply this technique as a tool for increasing the resilience of arid ecosystems, qualified very vulnerable to climate change.
Article
1. Leptadenia pyrotechnica (Forsk.) Decne. grows in valleys of the Eastern Desert in Egypt where the climatic and edaphic drought is very severe. 2. The transpiration rate is low in winter months and rises by the onset of summer. The rise in transpiration rate is not comparable with that of the evaporation. 3. The maximum transpiration rate is generally attained earlier than the maximum of the evaporating factors, specially in summer. The transpiration curves show a rapid decrease after this maximum despite the continuous rise of the evaporating power of the atmosphere. This points to an effective stomatal regulation. 4. The osmotic pressure ofLeptadenia is relatively low. The osmotic pressure values show narrow daily and seasonal fluctuations. 5. Leptadenia pyrotechnica is one of the most drought resistant plants. Its xeromorphic leafless habit implies reduction in the transpiring surface, while its deep extensive roots and low transpiration rate ensure a favourable water balance. The mechanisms of drought resistance in this plant were discussed.
Article
Calligonum azel, C. comosum and C. arich (Polygonaceae) are three dominant and economically important species widely distributed in active sand dunes in the southern desert of Tunisia, which differ in growth form and may co-occur under the same climatic constraints. The aims of this work were to compare their phenological and water potential patterns, as well as branch, flower and fruit production during three years at monthly intervals. Predawn (Ψpd) and midday (Ψmd) water potentials followed similar trends with a few exceptions. For all species, Ψmd reached values more negative than −2.5 MPa during the hot and dry summer, with extremes in July. Ψpd differed between the species, and C. comosum attained the most negative values (−1.71 MPa). Seasonal differences in Ψpd were less pronounced for Calligonum species established in dunes as compared to C. comosum from interdunes. Spatial and temporal variations in soil water content provoked changes in water potentials of these species. Diurnal amplitudes resulting from the difference between Ψmd and Ψpd were more pronounced during the dry season compared to the wet months. The phenological pattern of the three species showed a similar sequence of phenophases with growth activity occurring mostly between spring and summer. We found a species-dependent response to summer drought, as C. comosum ended all phenophases in June, while C. azel and C. arich extended their vegetative growth into the summer period of low precipitation. Water availability for the plants was better on the dune slopes. Differences were statistically significant among species, concerning green branch production and numbers of initially formed flowers and mature fruits. Our data suggest that changes in the studied parameters may depend on the rooting depth of the species.
Article
Camel grazing is recognized as a primary cause of ecological degradation in the UAE. A study of perennial plant species <1 m in height was conducted along a fence separating continuously camel grazed land from land in which camels had been replaced by oryx and gazelle species for 5 years (Al Maha). Vegetation regeneration in Al Maha in the absence of camels was considerable on all substrates (gravel, stable sand, and semi-stable sand) but was greatest on the gravel substratum, indicating that ecology in this habitat is most at risk. Observed regeneration was primarily through vegetative reproduction and growth of existing plants, showing that existing species can tolerate heavy grazing. Therefore, an equilibrium grazing model of continuous and reversible vegetation dynamics is most suitable for management of this ecological zone. Species richness was greater in Al Maha due to the greater number of plants, but biodiversity was unaffected. There was some evidence of localized dune stabilization within Al Maha due to increased vegetative cover. Further recovery of vegetation within Al Maha is discussed. This study highlights the need for reduced grazing pressure throughout the Dubai inland desert, and in particular on gravel substrata.
Article
Germination and survival of 11 perennial plant species were documented in an undisturbed shrub
Article
Water-limited hot environments are good examples of hyper-aridity. Trees are scarce in these environments but some manage to survive, such as the tree Moringa peregrina. Understanding how trees maintain viable populations in extremely arid environments may provide insight into the adaptive mechanisms by which trees cope with extremely arid weather conditions. This understanding is relevant to the current increasing aridity in several regions of the world. Seed germination experiments were conducted to assess variation in seed mass, seed germination, and seedling traits of Moringa peregrina plants and the correlations among these traits. A seed burial experiment was also designed to study the fate of M. peregrina seeds buried at two depths in the soil for two time periods. On average, seeds germinated in three days and seedling shoots grew 0.7 cm per day over three weeks. Larger seeds decreased germination time and increased seedling growth rates relative to smaller seeds. Seeds remained quiescent in the soil and germination was very high at both depths and burial times. The after-ripening time of Moringa peregrina seeds is short and seeds germinate quickly after imbibition. Plants of M. peregrina may increase in hyper-arid environments from seeds with larger mass, shorter germination times, and faster seedling growth rates. The results also illustrate the adjustment in allocation to seed biomass and correlations among seed and seedling traits that allows M. peregrina to be successful in coping with aridity in its environment.
Article
Species' life-history and population dynamics are strongly shaped by the longevity of individuals, but life span is one of the least accessible demographic traits, particularly in clonal plants. Continuous vegetative reproduction of genets enables persistence despite low or no sexual reproduction, affecting genet turnover rates and population stability. Therefore, the longevity of clonal plants is of considerable biological interest, but remains relatively poorly known. Here, we critically review the present knowledge on the longevity of clonal plants and discuss its importance for population persistence. Direct life-span measurements such as growth-ring analysis in woody plants are relatively easy to take, although, for many clonal plants, these methods are not adequate due to the variable growth pattern of ramets and difficult genet identification. Recently, indirect methods have been introduced in which genet size and annual shoot increments are used to estimate genet age. These methods, often based on molecular techniques, allow the investigation of genet size and age structure of whole populations, a crucial issue for understanding their viability and persistence. However, indirect estimates of clonal longevity are impeded because the process of ageing in clonal plants is still poorly understood and because their size and age are not always well correlated. Alternative estimators for genet life span such as somatic mutations have recently been suggested. Empirical knowledge on the longevity of clonal species has increased considerably in the last few years. Maximum age estimates are an indicator of population persistence, but are not sufficient to evaluate turnover rates and the ability of long-lived clonal plants to enhance community stability and ecosystem resilience. In order to understand the dynamics of populations it will be necessary to measure genet size and age structure, not only life spans of single individuals, and to use such data for modelling of genet dynamics.
Article
Seventeen browse plants eaten by cattle in Umudike, Abia State of Nigeria were subjected to nutritional evaluation. The plants were classified into heavily, moderately and occasionally browsed species based on their frequency of browsing by cattle. The highest and the lowest crude protein (CP) values were obtained in Napoleona vogelii (25.55%) and Grewia pubescens (13.65%), respectively. Average CP was highest in the occasionally browsed species (18.62%) and lowest in heavily browsed plants (17.92%). The corresponding value for the moderately browsed plants was 18.35%. The calculated in vitro organic matter digestibility (IVOMD) was highest for the heavily browsed (51.32%) plants while the occasionally browsed species had the lowest value (48.69%). Generally, browses contained higher concentration of calcium than phosphorous. The average calcium content of the heavily browsed (1.57%) plants was higher than values obtained for either the moderately (1.38%) or the occasionally browsed plants (1.45%), Phosphorous concentration was however, slightly higher in the moderately browsed species (0.39%) than in either the heavily or the occasionally browsed (0.37%) plants. The digestible crude protein (DCP) followed similar pattern with CP and ranged from 9.17% in Grewia pubescens to 20.24% in Napoleona vogelii. The highest mean DCP was obtained from the occasionally browsed plants (13.78%). The acid detergent fibre (ADF) and neutral detergent fibre (NDF) values were 48.05, 55.62% for the heavily, 41.60, 48.21% for the moderately and 48.65, 46.75% for the occasionally browsed plants, respectively. CP and DCP content of browse did not necessarily influence browsing intensity or behaviour in cattle.
Article
We quantified seed dispersal in a guild of Sonoran Desert winter desert annuals at a protected natural field site in Tucson, Arizona, USA. Seed production was suppressed under shrub canopies, in the open areas between shrubs, or both by applying an herbicide prior to seed set in large, randomly assigned removal plots (10-30 m diameter). Seedlings were censused along transects crossing the reproductive suppression borders shortly after germination. Dispersal kernels were estimated for Pectocarya recurvata and Schismus barbatus from the change in seedling densities with distance from these borders via inverse modeling. Estimated dispersal distances were short, with most seeds traveling less than a meter. The adhesive seeds of P. recurvata went farther than the small S. barbatus seeds, which have no obvious dispersal adaptation. Seeds dispersed farther downslope than upslope and farther when dispersing into open areas than when dispersing into shrubs. Dispersal distances were short relative to the pattern of spatial heterogeneity created by the shrub and open space mosaic. This suggests that dispersal could contribute to local population buildup, possibly facilitating species coexistence. Overall, these results support the hypothesis that escape in time via delayed germination is likely to be more important for desert annuals than escape in space.
Phenotypic Variations in Communities of Calligonum comosum 459 L'Her (Polygonaceae) from Saudi Arabia. Desert Plants
  • W K Taia
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A study on the species selection of Calligonum and its forestation in the drift-sand area of Cele County
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Zhang, H. N. 1992. A study on the species selection of Calligonum and its forestation in the drift-sand area of Cele County. Arid Zone Research 9 (2):8-12.
United Arab Emirates: A new perspective
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Abed, I., and P. Hellyer. 2001. United Arab Emirates: A new perspective.Winnipeg, Canada: Trident Press Ltd.
Disturbances and dangers operating on desert vegetation in Saudi Arabia
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Rangeland ecology of the Arab Gulf Countries
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Boulos, L. 2009. Flora of Egypt checklist. Revised Annotated Edition. Dokki, Egypt: Al-Hadara Publishing.