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1.
Keletso Mopipi Winston S. W. Trollope Peter F. Scogings 《African Journal of Ecology》2009,47(4):680-687
The effects of irrigation, nitrogen fertilization, grass competition and clipping were investigated for one growing season at the research farm of the University of Fort Hare in the Eastern Cape Province of South Africa. The aim of the experiment was to assess the short‐term performance of Acacia karroo seedlings under different environmental conditions and the implications of such factors on the long‐term recruitment of plant species in savanna rangelands. There were no significant treatment effects on the survival of A. karroo seedlings. Using stem length and basal diameter as growth parameters, it was observed that irrigation enhanced both variables, while nitrogen fertilization did not have any significant effects. Clipping, grass competition and their interaction greatly suppressed the growth of the seedlings. Clipping increased the mean stem length when they were irrigated and fertilized. Control and fertilized plants had the highest stem length in the absence of grass competition, while grass competition combined with clipping resulted in the lowest stem length in both irrigated and nonirrigated plants. It was concluded that in the presence of grass competition, controlled browsing could be a viable solution to the problem of bush encroachment in savanna rangelands. 相似文献
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Aims Woody plant encroachments in arid and semiarid ecosystems are widely reported but the physiological mechanisms still need to be further revealed. In the current study, we aim to determine whether differences in leaf physiological traits help explain grassland susceptibility to woody plant encroachment and whether distinctive physiological adaptations allow some shrub species to invade grasslands.Methods We compared physiological traits (photosynthesis, leaf water status, pigment compositions and leaf antioxidant capacities) of six species representing three functional groups: woody encroachers (Prosopis velutina, Larrea tridentata), woody non-encroachers (Acacia greggii, Lycium fremontii) and C4 grasses (Bouteloua curtipendula, Bouteloua barbata) which are naturally growing in a botanical garden in University of Arizona, USA.Important findings We infer that P. velutina (encroacher) but not A. greggii or L. fremontii (non-encroachers) is encroaching in grasslands because the former species has higher water and light utilization efficiencies (instantaneous water use efficiency, instantaneous light use efficiency, and Fv/Fm). The extremely high carotenoid and total antioxidant capacity in its leaves appears to help the shrub L. tridentata (encroacher) survive high ambient oxidative damage caused by both drought and high light stresses in this grassland. The two C4 grass species, B. curtipendula and B. barbata, grow well in the arid ecosystem but may be susceptible to disturbances. 相似文献
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Clélia Sirami Colleen Seymour Guy Midgley Phoebe Barnard 《Diversity & distributions》2009,15(6):948-957
Aim Evidence is accumulating of a general increase in woody cover of many savanna regions of the world. Little is known about the consequences of this widespread and fundamental ecosystem structural shift on biodiversity.
Location South Africa.
Methods We assessed the potential response of bird species to shrub encroachment in a South African savanna by censusing bird species in five habitats along a gradient of increasing shrub cover, from grassland/open woodland to shrubland dominated by various shrub species. We also explored historical bird species population trends across southern Africa during the second half of the 20th century to determine if any quantifiable shifts had occurred that support an ongoing impact of shrub encroachment at the regional scale.
Results At the local scale, species richness peaked at intermediate levels of shrub cover. Bird species composition showed high turnover along the gradient, suggesting that widespread shrub encroachment is likely to lead to the loss of certain species with a concomitant decline in bird species richness at the landscape scale. Finally, savanna bird species responded to changes in vegetation structure rather than vegetation species composition: bird assemblages were very similar in shrublands dominated by Acacia mellifera and those dominated by Tarchonanthus camphoratus .
Main conclusions Shrub encroachment might have a bigger impact on bird diversity in grassland than in open woodland, regardless of the shrub species. Species recorded in our study area were associated with historical population changes at the scale of southern Africa suggesting that shrub encroachment could be one of the main drivers of bird population dynamics in southern African savannas. If current trends continue, the persistence of several southern African bird species associated with open savanna might be jeopardized regionally. 相似文献
Location South Africa.
Methods We assessed the potential response of bird species to shrub encroachment in a South African savanna by censusing bird species in five habitats along a gradient of increasing shrub cover, from grassland/open woodland to shrubland dominated by various shrub species. We also explored historical bird species population trends across southern Africa during the second half of the 20th century to determine if any quantifiable shifts had occurred that support an ongoing impact of shrub encroachment at the regional scale.
Results At the local scale, species richness peaked at intermediate levels of shrub cover. Bird species composition showed high turnover along the gradient, suggesting that widespread shrub encroachment is likely to lead to the loss of certain species with a concomitant decline in bird species richness at the landscape scale. Finally, savanna bird species responded to changes in vegetation structure rather than vegetation species composition: bird assemblages were very similar in shrublands dominated by Acacia mellifera and those dominated by Tarchonanthus camphoratus .
Main conclusions Shrub encroachment might have a bigger impact on bird diversity in grassland than in open woodland, regardless of the shrub species. Species recorded in our study area were associated with historical population changes at the scale of southern Africa suggesting that shrub encroachment could be one of the main drivers of bird population dynamics in southern African savannas. If current trends continue, the persistence of several southern African bird species associated with open savanna might be jeopardized regionally. 相似文献
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Encroachment of woody plants has been among the major threats to the livelihoods of Borana pastoralists and their ecosystem. An approach that integrated vegetation survey and pastoralists’ perception was followed to study the impacts of encroachment of woody plants in the Borana lowlands, Ethiopia. Density of woody species was determined in 192 plots of 500 m2. Canopy cover of woody plants was estimated in 123 quadrates of 400 m2. Pastoralists’ perception was assessed through group discussions and a semi‐structured questionnaire. Results showed that plant density was 3014 woody plants ha?1. Cover of woody plants was 52%, indicating an increasing trend from ≤40% cover reported in the early 1990s. It was concluded that the increase of woody plants density and cover has crossed the critical threshold and has entered into the encroached condition. Principal components analysis (PCA) and redundancy analysis (RDA) also showed that woody plants were negatively correlated with herbaceous biomass. Commiphora africana, Acacia melliphera, A. drepanolobium, A. brevispica and Lannea rivae were among the dominant encroachers. RDA revealed that soil nutrients were positively correlated with woody plants density and cover. The pastoralists perceived that encroachment of woody plants had decreased the production of their grazingland. A ban on fire was perceived as the major factor that caused encroachment of woody plants. Re‐utilization of fire and strengthening of traditional rangeland management strategies are recommended. 相似文献
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This study determined the effects of land-use practice had on the rate and extent of bush encroachment in a mesic savanna in KwaZulu-Natal, South Africa. Changes in woody cover were measured for 1 km2 sites in areas under communal, commercial and conservation land-use systems for the period between 1937 and 2000. Land users from each area were interviewed to gain the histories of each area and to determine how the changes in woody cover had impacted them and whether anything was being done to counteract the spread of trees and shrubs on their land. Bush encroachment occurred across all three of the land-use types in the 67-year period between 1937 and 2000. The results showed that land-use practice had enormous impacts on the process of bush encroachment. The communal site showed a decrease in grass (21%) and tree (5%) cover and an increase in shrub cover (13%). At the commercial site, there was a considerable decrease in grass cover (46%) and moderate increase in shrub cover (10%) and a massive increase in tree cover (36%). The area under conservation showed a substantial decrease in grass cover (47%), a slight decrease in shrub cover (19%) and a massive increase in tree cover (66%). The perceived causes of these changes were fairly similar amongst the different land users. The changes were mostly not perceived to be a problem for the communal land users. The main advantages mentioned were increased woody resources for building and firewood and increased browse availability. The commercial and conservation land users perceived the changes to have significant negative connotations including the loss of grazing land and biodiversity and secondary invasion of encroached areas by alien plant species. Despite these perceptions, very little has been done to combat bush encroachment in the commercial and conservation land use systems. 相似文献
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Bond WJ Midgley GF 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2012,367(1588):601-612
Savannahs are a mixture of trees and grasses often occurring as alternate states to closed forests. Savannah fires are frequent where grass productivity is high in the wet season. Fires help maintain grassy vegetation where the climate is suitable for woodlands or forests. Saplings in savannahs are particularly vulnerable to topkill of above-ground biomass. Larger trees are more fire-resistant and suffer little damage when burnt. Recruitment to large mature tree size classes depends on sapling growth rates to fire-resistant sizes and the time between fires. Carbon dioxide (CO(2)) can influence the growth rate of juvenile plants, thereby affecting tree recruitment and the conversion of open savannahs to woodlands. Trees have increased in many savannahs throughout the world, whereas some humid savannahs are being invaded by forests. CO(2) has been implicated in this woody increase but attribution to global drivers has been controversial where changes in grazing and fire have also occurred. We report on diverse tests of the magnitude of CO(2) effects on both ancient and modern ecosystems with a particular focus on African savannahs. Large increases in trees of mesic savannahs in the region cannot easily be explained by land use change but are consistent with experimental and simulation studies of CO(2) effects. Changes in arid savannahs seem less obviously linked to CO(2) effects and may be driven more by overgrazing. Large-scale shifts in the tree-grass balance in the past and the future need to be better understood. They not only have major impacts on the ecology of grassy ecosystems but also on Earth-atmosphere linkages and the global carbon cycle in ways that are still being discovered. 相似文献
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Hasan Yusuf Anna C. Treydte Sebsebe Demissew Zerihun Woldu 《African Journal of Ecology》2011,49(4):397-409
Ecological survey was executed to assess woody species encroachment into the grassland plain of Nechisar National Park (NNP). Forty‐one woody species were recorded. Dichrostachys cinerea Wight & Arn., Acacia mellifera (Vahl) Benth., Acacia nilotica (L) Willd., Acacia senegal (L.) Willd., Acacia seyal Del. and Acacia tortilis (Forssk.) Hayne were among the major encroaching woody species. The majority of the woody species were found to be highly aggregated in their pattern of distribution, while only few species showed some degree of randomness. The mean woody species density was ca. 1995 woody plants ha?1. Mean cover of woody, grass, unpalatable forbs and total herbaceous species were 31%, 58%, 68% and 121%, respectively. The woody species density and cover, unpalatable forbs and bare land cover were significantly higher in the highly grazed and fire‐suppressed part of the grassland plain. Pearson correlation coefficient matrix indicated that woody species cover and density were negatively correlated with total herbaceous and grass cover. The high woody, unpalatable forbs and bare land cover indicated the progressively increasing perennial grass species diversity deterioration in the grass plain of the Park. Decline in the grassland condition, unless reversed, will jeopardize the biological diversity as well as the aesthetic value of the NNP. 相似文献
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灌丛是中国山地和干旱地区的常见植被类型,分布范围广泛、生态功能多样、经济价值可观。然而,自然资源、生态环境、国民经济等相关部门多将灌丛拆分归属于森林或草原,其重要性未能充分体现。学术界有关灌丛资源的概念、内涵未有明确统一共识,基于资源属性特点的灌丛资源分区分类体系尚未建立。草原灌丛化的发生范围、速度及其生态影响不够明确,严重影响灌丛资源的高效保护、科学管理与可持续利用。这已然成为亟需凝聚共识和深入探究的理论技术难点与实践问题。建议在科学界定灌丛资源的概念内涵、建立健全灌丛资源分类体系基础上,深入发展和应用灌丛植被多基遥感和人工智能识别技术,科学掌握不同区域灌丛变化特别是草原灌丛化的过程及原因,全面系统地评估灌丛化的生态经济影响,优化分区分类灌丛管理利用策略,促进灌丛资源可持续管理和利用。 相似文献
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F. Patrick Graz 《Ecohydrology》2008,1(4):340-348
Increases in woody plant densities in savanna grazing lands worldwide have resulted in a decline in the grazing capacity of these rangelands. At present, the actual cause of the problem is unknown although a vast body of literature deals with various aspects relevant to the issue. It is generally assumed, however, that the changes in the tree:grass ratio are a response to changes in soil‐water development brought about by rangeland utilization. These utilization patterns differ from those under which the savanna system evolved. While changes in physiognomy are very prominent, these are generally preceded by changes in grass species assemblage. The development of species composition has been used as an indicator of rangeland condition for many years; the increase or decrease of individual species is considered in this context. Thus far, however, the shift from perennial to annual grasses has not been evaluated for its effect on soil moisture development, and subsequent implications for woody plant establishment and growth. The review presented here consolidates the existing information in order to provide a basis for understanding the woody weed encroachment problem. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
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Shrub encroachment has been a worldwide phenomenon, but the interactions between the surface hydrological cycle and the shrub-encroached landscape at different hillslope positions remain poorly investigated. The present study was undertaken to explore the interactions between the water infiltration patterns affected by shrub encroachment. At four slope positions of a hillslope encroached by Caragana microphylla Lam, soil water content and temperature were continuously measured at 10-min intervals at four or five depths in both the presence (shrub patches) and absence (grass matrix) of shrubs. Then, the infiltration of rainfall and meltwater was estimated based on analyses of the above data. The rainfall infiltration ratios (IRs) of grass matrix were as high as 0.78 ± 0.08, except at the lower site, which had a value of only 0.47. The IRs of shrub patches increased from 0.38 at the top site to 0.77 at the lower site. In snowy years, snow and ice were blown upward by wind and accumulated in the shrub patches and their leeward areas, which resulted in higher water input for the shrub patches than for the grass matrix. Shrub encroachment changed the microtopography and soil properties of the hillslope and further affected the surface hydrological processes. The feedbacks between shrub encroachment and water infiltration varied among sites of the hillslope, which may in turn have affected the development of shrub patches. 相似文献
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William J. Platt Alex K. Entrup Emily K. Babl Cody Coryell‐Turpin Viet Dao James A. Hebert Claire D. LaBarbera Jené F. L. Noto Similoluwa O. Ogundare Lucas K. Stamper Nabin Timilsina 《Restoration Ecology》2015,23(6):909-917
Shrub encroachment occurring worldwide in savannas and grasslands has commonly been hypothesized to result from anthropogenically altered environments. Two disturbance‐based approaches to restoration have involved: (1) application of selective herbicides to reduce density/cover of shrubs; (2) reinstatement of natural fire regimes to generate environmental conditions favoring herbaceous species. We studied short‐term responses of native shrubs, vines, and grasses in a Louisiana pine savanna to herbicides followed by a prescribed fire and fire alone. Plots established in the summer, 2013, were hand‐sprayed in the fall with Imazapyr and Triclopyr, Triclopyr alone, or no herbicide, then prescribed burned the following spring. Numbers of species of shrubs and vines at scales of 1 and 100 m2, numbers of stems and regrowth of stems produced by six common species of shrubs, and the number of flowering culms of perennial C4 grasses were assessed postfire in 2014. Compared with fire alone, herbicides followed by fire resulted in (1) small reductions in species richness of shrubs and no effects on vines, (2) fewer stems comprising shrub genets, but similar postfire regrowth of resprouting shrub stems, and (3) fewer flowering culms of C4 grasses. Underground storage organs of savanna shrubs and vines survived both aboveground disturbances. Thus, single applications of herbicides followed by fires reduced, but did not reverse shrub encroachment and negatively affected grasses. Because effects of herbicides overrode those of prescribed fires, these disturbances did not act synergistically, suggesting that reinstating natural fire regimes should be a priority in restoration of savannas and grasslands. 相似文献
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Andrew L. Skowno Mark W. Thompson Jens Hiestermann Brad Ripley Adam G. West William J. Bond 《Global Change Biology》2017,23(6):2358-2369
Increases in woody plant cover in savanna grassland environments have been reported on globally for over 50 years and are generally perceived as a threat to rangeland productivity and biodiversity. Despite this, few attempts have been made to estimate the extent of woodland increase at a national scale, principally due to technical constraints such as availability of appropriate remote sensing products. In this study, we aimed to measure the extent to which woodlands have replaced grasslands in South Africa's grassy biomes. We use multiseason Landsat data in conjunction with satellite L‐band radar backscatter data to estimate the extent of woodlands and grasslands in 1990 and 2013. The method employed allows for a unique, nationwide measurement of transitions between grassland and woodland classes in recent decades. We estimate that during the 23‐year study period, woodlands have replaced grasslands over ~57 000 km2 and conversely that grasslands have replaced woodlands over ~30 000 km2, a net increase in the extent of woodland of ~27 000 km2 and an annual increase of 0.22%. The changes varied markedly across the country; areas receiving over 500 mm mean annual precipitation showed higher rates of woodland expansion than regions receiving <500 mm (0.31% yr?1 and 0.11% yr?1, respectively). Protected areas with elephants showed clear loss of woodlands (?0.43% yr?1), while commercial rangelands and traditional rangelands showed increases in woodland extent (>0.19% yr?1). The woodland change map presented here provides a unique opportunity to test the numerous models of woody plant encroachment at a national/regional scale. 相似文献
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全球气候变化背景下,荒漠草原人工灌丛引入加速其灌丛化进程,对草原土壤水分产生重要影响。为了解宁夏东部荒漠草原灌丛引入过程中土壤水分动态及亏缺现状,选取了封育草地、放牧草地、不同年限(3a、12a、22a)和间距(40 m、6 m、2 m)灌丛柠条(Caragana korshinskii)地进行土壤水分测定,并利用土壤水分相对亏缺指数(compared soil water deficit index,CSWDI)、样地土壤水分相对亏缺指数(plot compared soil water deficit index,PCSWDI)对土壤水分亏缺进行定量分析。结果表明:灌丛引入过程中不同年限、间距灌丛地0—200 cm土层土壤含水量均显著低于封育草地与放牧地(P<0.05);各样地季节动态均表现为春季返潮、夏季消耗、秋季蓄积的季节规律,但不同年限、间距灌丛地表现为春季返潮微弱,土壤含水量仅为7.80%—10.90%,显著低于封育草地和放牧地(11.90%—16.09%);灌丛引入过程中各灌丛地0—100 cm有效储水量(-16.98—18.69 mm)均低于封育草地(34.67 ... 相似文献
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The phenomenon of woody plant thickening in grasslands has been observed globally and is likely to have widespread ecological consequences. It has been proposed that woody plant thickening is driven in part by rising atmospheric [CO2] enhancing the resprouting ability of woody plants relative to grasses so they respond more strongly after disturbances such as herbivory and fire. The aim of this study was to examine the CO2 effect on the resprouting ability of 16 co‐occurring temperate woody plant and grass species (eight species from each growth form). Plants were grown in a controlled glasshouse experiment under ambient (400 ppm) and elevated [CO2] (600 ppm) for 14 weeks after which their resprouting ability was measured. Root non‐structural carbohydrate (NSCmass) and nitrogen (Nmass) storage was used as proxies to measure the resprouting ability of woody plants while for the grasses it was measured directly. We found that both the woody plants (22% on average; P = 0.003) and grasses (20% on average; P = 0.003) produced more biomass under elevated [CO2]. Despite the woody plants not allocating additional carbon to belowground storage under elevated [CO2], they had significantly greater root NSCmass (23% on average; P = 0.007) due to increased root biomass production (8% on average; P = 0.007). In contrast, root Nmass of the woody plants did not differ between CO2 treatments (P = 0.373). Surprisingly, the resprouting ability of the grasses did not significantly differ between the CO2 treatments (P = 0.067). These results provide evidence that the differing resprouting response of woody plants and grasses under elevated [CO2] may be contributing to woody plant encroachment of grasslands worldwide. 相似文献
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