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1.
Groundwater is a vital water resource in the arid to semi-arid southwestern United States. Accurate accounting of inflows to and outflows from the groundwater system is necessary to effectively manage this shared resource, including the important outflow component of groundwater discharge by vegetation. A simple method for estimating basin-scale groundwater discharge by vegetation is presented that uses remote sensing data from satellites, geographic information systems (GIS) land cover and stream location information, and a regression equation developed within the Southern Arizona study area relating the Enhanced Vegetation Index from the MODIS sensors on the Terra satellite to measured evapotranspiration. Results computed for 16-day composited satellite passes over the study area during the 2000 through 2007 time period demonstrate a sinusoidal pattern of annual groundwater discharge by vegetation with median values ranging from around 0.3 mm per day in the cooler winter months to around 1.5 mm per day during summer. Maximum estimated annual volume of groundwater discharge by vegetation was between 1.4 and 1.9 billion m3 per year with an annual average of 1.6 billion m3. A simplified accounting of the contribution of precipitation to vegetation greenness was developed whereby monthly precipitation data were subtracted from computed vegetation discharge values, resulting in estimates of minimum groundwater discharge by vegetation. Basin-scale estimates of minimum and maximum groundwater discharge by vegetation produced by this simple method are useful bounding values for groundwater budgets and groundwater flow models, and the method may be applicable to other areas with similar vegetation types.  相似文献   

2.
Rangeland degradation is a widespread problem throughout sub Saharan Africa and its restoration is a challenge for the management of many semi-arid areas. This study assessed the effectiveness of exclosures that have been protected from livestock from 5 to 15 years in restoring vegetation in northern Ethiopia. The species composition and diversity of herbaceous and woody plants were higher in the exclosures than in the grazed areas. Species richness responded positively to an increase in herbaceous productivity. The mean aboveground biomass measured inside the exclosures was more than twice that of the adjacent grazed areas and more biomass was produced from the young than the old exclosures. Stem height, canopy height, canopy cover, and browsing capacity of woody species were higher in the exclosures than in the grazed areas. Our study shows that degraded semi-arid vegetation is able to recover in a relatively short time when protected. Extended protection, beyond 8–15 years, reduces herbaceous species diversity and in one of the sites also the herbaceous biomass. Therefore, we suggest a slight shift in management where exclosures protected for longer periods may be moderately used by livestock.  相似文献   

3.
Communal rangelands provide diverse ecosystem services to millions of pastoralists and agro-pastoralists. Resettling destitute communities into hitherto uninhabited communal rangelands and forests, a common practice throughout Sub-Saharan Africa, is a threat to the sustainable use of range and forest land resources. In order to understand the effect of resettlement on a semi-arid woodland in northwestern Ethiopia, satellite imagery of 23 resettlement villages taken over a period of fourteen years, and woody vegetation floristic data for three old resettlements, three new resettlements, two refugee camps and one protected area were analyzed using ANOVA and canonical correspondence analysis (CCA). The normalized differential vegetation index (NDVI) and canopy cover around all village types decreased with disturbance gradients, while the magnitude of change varied according to the type of settlement. Limited canopy cover was observed in refugee camps and new resettlements, compared to old resettlements and protected areas. Woody vegetation height class showed a J shaped distribution in all sites except refugee camps (RC), indicating a decline in vegetation. CCA showed that variables like site type, altitude and disturbance gradient significantly affected the diversity of woody species at the different sites. Comparison of individual species responses to disturbances indicated that low fodder value invaders like Dichrostachys cinerea, and many Acacia species were increasing in proportion and coverage at the expense of some multipurpose species including Tamarindus indica, Diospyros mespiliformis, and Pterocarpus lucens. In the absence of regulated vegetation use, resettlements result in a decline in overall vegetation cover and a shift in floristic diversity in favor of invasive species.  相似文献   

4.
The effect of grazing was studied on vegetation structure, herbaceous biomass, basal and bare ground covers, together with soil nutrient concentrations in two locations in an Ethiopian semi-arid savanna. The lightly grazed sites had significantly higher herbaceous diversity, total abundance, basal cover and aboveground biomass, and a lower percentage of bare ground compared with the heavy grazed sites. Grazing pressure had no effect on the density and number of woody species as well as on the proportion of encroaching woody species. The light grazing sites had higher organic carbon, phosphorus and exchangeable bases, and therefore a higher pH and higher electrical conductance, indicating an improved soil nutrient status compared with heavy grazing sites, mainly attributed to the higher basal cover and standing biomass at light grazed sites, and the export of nutrients through grazing and dung collection from the heavily grazed sites. There were significantly higher soil nutrients, species diversity, aboveground biomass and basal cover in the light grazing sites compared with heavy grazing sites. We concluded that changes in herbaceous vegetation, standing biomass and soil compositions are caused by interactions between grazing, soil and vegetation, and these interactions determine the transitions of semi-arid savannas.  相似文献   

5.
The open coastal heathlands in Norway have changed character in recent decades, mainly because traditional practices such as burning, cutting and grazing have been abandoned or reduced. In a project aimed at increasing our knowledge of woodland regeneration in oceanic boreal heathlands, an area on the island of Kalv?ya in the archipelago of Vikna on the extreme outer coast of central Norway has been studied. Kalv?ya has calcareous bedrock; rich heath, rich fens and low-herb woodland cover 20% of the island. Large parts are paludified, and wet heath and mire vegetation cover 60%. The island has not been inhabited, but it was used for summer farming in the latter part of the 19th century and later for stock grazing. The woodland and scrub were mapped using aerial photographs from 1961 and 1981, and by fieldwork in 2001. Vegetation changes were analyzed using GIS techniques. Woodland and scrub increased from 3.0% in 1961, to 4.3% in 1981, to 11.6% in 2001. Betula pubescens is the commonest woody species. The oldest trees are 150 to 175 years old, growing in the most heavily wooded area, far away from the summer farm. In addition to human impact (summer farm, grazing pressure), the geology, terrain (degree of exposure), and climatic factors are important for woodland colonisation and distribution.  相似文献   

6.
The colonization and development of biological soil crusts (BSCs) are rarely discussed when investigating vegetation restoration with difference arrangement and structure of anthropogenically damaged areas in semi-arid regions. The present study analyzes the relationships among coverage, height and density of woody vegetation and coverage and thickness of BSCs on the surface mine dumpsite in Heidaigou, China. Results showed that PR (Prunus sibirica L.), PT (Pinus tabulaeformis Carr.) and PPr (P. tabulaeformis Carr., P. sibirica L.) types had the highest coverage of total BSCs, which were 76.8%, 75.9% and 78.9%, respectively and PR showed the thickest BSCs of 4.41 mm. There was a significant correlation between coverage and thickness of BSCs and coverage and height of woody vegetation as a unimodal curve. Our findings suggest that a single woody plant species and low level coverage and height (no more than 30% and 300 cm, respectively) of woody plants may be able to create suitable conditions for facilitating BSCs restoration on the surface of mine dumpsites. The effects of vegetation arrangement and structure on BSCs colonization and development should be considered in reconstructing and managing woody vegetation in disturbed environments, such as surface mine dumpsites in semi-arid areas.  相似文献   

7.
Recent remote sensing studies have documented a greening trend in the semi-arid Sahel and Sudan zones of West Africa since the early 1980s, which challenges the mainstream paradigm of irreversible land degradation in this region. What the greening trend means on the ground, however, has not yet been explored. This research focuses on a region in central Senegal to examine changes in woody vegetation abundance and composition in selected sites by means of a botanical inventory of woody vegetation species, repeat photography, and perceptions of local land users. Despite the greening, an impoverishment of the woody vegetation cover was observed in the studied sites, indicated by an overall reduction in woody species richness, a loss of large trees, an increasing dominance of shrubs, and a shift towards more arid-tolerant, Sahelian species since 1983. Thus, interpretation of the satellite-derived greening trend as an improvement or recovery is not always justified. The case of central Senegal represents only one of several possible pathways of greening throughout the region, all of which result in similar satellite-derived greening signals.  相似文献   

8.
荒漠河岸植被的受损过程与受损机理分析   总被引：9，自引：0，他引：9  
以塔里木河多年平均地下水位数据为依据,将地下水位划分为6个环境梯度,各梯度上6次重复采集植被样地数据。从物种多样性、植被盖度与群落类型等几方面分析了植被的受损过程,以及导致此过程的受损机理。结果表明：(1) 草本植物丰富度受损发生在地下水埋深大于4 m,而木本植物丰富度受损发生在地下水埋深大于8 m。(2) 植被盖度减少始于草本植物盖度受损,与群落多样性受损的临界地下水位相同,发生在地下水埋深大于4 m;在地下水埋深大于6 m之后,植被盖度不断减少则是由木本植物盖度的减少所引起。(3) 群落类型受损体现在芦苇群落和胡杨林群落的结构与类型变化上,芦苇群落的衰退演变出现了优势种的更替,而胡杨林群落中优势种的优势地位没有变化。(4) 此受损过程是由荒漠河岸生态系统脆弱的生态基质和外界干扰共同作用的结果,起因于人口的增加、需求的增长。植被退化是人类干扰作用于植被赖以生存的环境主导因子所致。在生态受损过程中,植物功能型差异与所承受的干扰强度差异对植被退化的程度有一定影响。  相似文献   

9.
Nebkhas are often extensively distributed in arid and semi-arid areas, and play an important role in the stabilization of ecological environment in these areas. This paper reviews the following aspects, including formation and succession, spatial distribution, shape and surface airflow characteristics, balance between erosion and deposition, ecological and physiological characteristics of vegetation, response to precipitation and groundwater, soil properties as well as the protection measures. We found that nebkhas were studied without differentiation of succession periods, and shape characteristics, surface airflow, soil properties as well as vegetation dynamics in different succession periods were not fully understood, which made it difficult to explain the succession dynamics of nebkhas and its affecting factors. Previous studies of nebkhas were over-emphasized for its role as an indicator of land degradation, while its ecological functions in degraded ecosystems were neglected, which was unfavorable for the ecological restorations in arid and semi-arid areas. Future studies should pay more attention to the variation of vegetation, soil as well as hydrological process in the succession of nebkhas and its interaction between different influencing factors. In addition, positive role of nebkhas in degraded ecosystems in arid and semi-arid areas should be fully discussed. According to the data got from the above, effective protection measures of nebkhas should be explored.  相似文献   

10.
Nebkhas are often extensively distributed in arid and semi-arid areas, and play an important role in the stabilization of ecological environment in these areas. This paper reviews the following aspects, including formation and succession, spatial distribution, shape and surface airflow characteristics, balance between erosion and deposition, ecological and physiological characteristics of vegetation, response to precipitation and groundwater, soil properties as well as the protection measures. We found that nebkhas were studied without differentiation of succession periods, and shape characteristics, surface airflow, soil properties as well as vegetation dynamics in different succession periods were not fully understood, which made it difficult to explain the succession dynamics of nebkhas and its affecting factors. Previous studies of nebkhas were over-emphasized for its role as an indicator of land degradation, while its ecological functions in degraded ecosystems were neglected, which was unfavorable for the ecological restorations in arid and semi-arid areas. Future studies should pay more attention to the variation of vegetation, soil as well as hydrological process in the succession of nebkhas and its interaction between different influencing factors. In addition, positive role of nebkhas in degraded ecosystems in arid and semi-arid areas should be fully discussed. According to the data got from the above, effective protection measures of nebkhas should be explored.  相似文献   

11.
Nebkhas are often extensively distributed in arid and semi-arid areas,and play an important role in the stabilization of ecological environment in these areas.This paper reviews the following aspects,including formation and succession,spatial distribution,shape and surface airflow characteristics,balance between erosion and deposition,ecological and physiological characteristics of vegetation,response to precipitation and groundwater,soil properties as well as the protection measures.We found that nebkhas were studied without differentiation of succession periods,and shape characteristics,surface airflow,soil properties as well as vegetation dynamics in different succession periods were not fully understood,which made it difficult to explain the succession dynamics of nebkhas and its affecting factors.Previous studies of nebkhas were over-emphasized for its role as an indicator of land degradation,while its ecological functions in degraded ecosystems were neglected,which was unfavorable for the ecological restorations in arid and semi-arid areas.Future studies should pay more attention to the variation of vegetation,soil as well as hydrological process in the succession of nebkhas and its interaction between different influencing factors.In addition,positive role of nebkhas in degraded ecosystems in arid and semi-arid areas should be fully discussed.According to the data got from the above,effective protection measures of nebkhas should be explored.  相似文献   

12.
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13.
Roads extend throughout savannas, yet few studies have quantified their effects on adjoining woody vegetation structure. Airborne LiDAR imagery collected over 168 experimental fire plots in the Kruger National Park, all bounded by graded firebreak roads, provided an opportunity to study if, and to what extent, roads influence woody vegetation structure under different rainfall, geologic and fire conditions. In 91.7% of the plots, woody canopy cover was higher on the edges of roads compared to areas farther away. The increase was most pronounced within 5 m of the road edge, but was detectable 10–15 m from the edge. On average, the area within 15 m from the road had approximately 6% and 2% higher woody vegetation cover than areas further than 15 m from the edge on wetter granitic and drier basaltic savanna landscapes, respectively. Increased edge effects on woody cover were observed even in fire exclusion plots, suggesting that non-fire processes, likely altered hydrological regimes, may be the underlying reason for woody encroachment. This study illustrates that roads cause selective woody plant thickening in savannas, even in areas without road edge management, and therefore careful consideration should be paid on how road edges are managed and when roads are planned.  相似文献   

14.
Display-site selection by houbara bustards was studied in Mori, Xinjiang, China during the breeding seasons from April to June 2000. Most of the habitats chosen for displaying were short sub-shrubby and open areas close to high shrub patches. The displaying males clearly prefer low covered areas and avoid densely covered and high vegetation sites. The vegetation density and number of plant species at display sites were significantly lower from that at randomly selected sites. The average distance to the closest shrub patch was significantly shorter at display sites than at random sites. Plant species richness, vegetative density, vegetative cover and distance to the shrub patches are possibly the most important factors that determined the display-sites selection of houbara bustard.  相似文献   

15.
地下水埋深对半干旱区典型植物群落土壤酶活性的影响   总被引：1，自引：0，他引：1  
土壤酶活性是反映土壤功能的关键指标,尤其在受到水分限制的半干旱区,土壤水分驱动的土壤酶活性生态功能的变化可以改变土壤养分周转并影响土壤碳质量。地下水埋深对半干旱区典型植物群落土壤酶活性的影响机制尚不明晰。以半干旱区科尔沁沙质草地两种典型植物群落（白草Pennisetum centrasiaticum和差巴嘎蒿Artemisia halodendron）为研究对象,开展地下水埋深模拟试验,地下水埋深分别为0.5、1.0、2.0 m。分析不同土层的土壤理化性质和土壤酶活性,探讨不同地下水位埋深深度下不同植被类型土壤酶活性的变化特征。结果表明：4种土壤水解酶（酸性磷酸酶（AP）、葡萄糖苷酶（βG）、乙酰氨基葡萄糖苷酶（NAG）和亮氨酸氨基肽酶（LAP））和2种氧化还原酶（过氧化氢酶（CL）和多酚氧化酶（POX））活性均受地下水位埋深和植被类型的影响。随着地下水埋深的增加,白草和差巴嘎蒿群落内土壤酶活性分别呈现不显著性和显著性的变化规律。同时,在各处理中土壤酶活性均随土层深度的增加而减小。地下水埋深对科尔沁沙质草地白草与差巴嘎蒿群落的土壤水解酶和氧化还原酶活性产生不同的影响。未来在半干旱区进行植被恢复时,建议考虑不同植物群落对地下水位变化的适应对策的差异,以更好地恢复半干旱区植物群落的地上地下生态系统功能。  相似文献   

16.
This study answers the following research questions: 1) What are the change trajectories of woody vegetation elements at the landscape level? 2) What are the differences in change trajectories amongst the various categories of forest, non-forest and reclamation woody vegetation? 3) How do the change trajectories differ in mining and non-mining landscapes? The study area, measuring 209.6 km2, is located in the north-western part of the Czech Republic and may be broken down into 76.8 km2 of mining landscape and 132.8 km2 of non-mining landscape. Brown coal mining began in this region during the second half of the 18th century and led to the radical transformation of the landscape, including woodlands, during the second half of the 20th century. The source data for this study was obtained from the original stable cadastre maps (1842) and the landscape field mapping performed in 2010. The various woody vegetation elements (forest, non-forest, and reclamation woody plants) and land use/cover (LULC) categories were identified. The GIS symmetrical difference tool was subsequently used to perform an overlay analysis for the individual woody vegetation elements in order to study the change trajectories and to obtain information about the woodlands that have remained unchanged (continuous), the ones that have disappeared (extinct), and the ones that have newly appeared in the landscape (recent). In the case of the non-mining landscape, the total proportion of woodlands has increased (from 17 to 32%), but there has been a decline in the overall volume of forest woody plants found in these areas (from 93 to 74%). As far as the mining landscape is concerned, there has also been an increase in the area covered by woodlands (from 10 to 20%), however, the proportion of forest woody plants has decreased to a much greater extent (from 90 to 31%). From the perspective of extinct woody vegetation, 23.3% of all types of woodlands in the mining landscape may be classified as such, as compared to 10.8% in the non-mining landscape. The primary causes of this decline are mining activities and newly built-up areas. More continuous woody vegetation may be found in the non-mining landscape (42.1%) as compared to the mining landscape (15.4%). Recent woody vegetation, which has primarily replaced grasslands and partially arable land, prevails in both the mining (61.3%) as well as the non-mining (47.1%) landscapes. Different categories of woodlands (forest, non-forest, and reclamation woody vegetation elements) exhibit various change dynamics due to their different structure and the functions they serve. At the most basic level, there has been an overall increase in the occurrence of woodlands in the studied areas. However, once GIS spatial analysis is applied it is possible to see more complex processes in the development of woodland areas as characterised by gains and losses, and it is possible to identify mining and agricultural extensification as the two most significant factors behind the historical changes. Mining leads to a direct decrease in the area of woodlands; conversely, the spontaneous succession of vegetation resulting from agricultural extensification and forest reclamation facilitates woodland recovery. Forest reclamation and reforestation are essential on order to ensure the time continuity of woodlands in both types of landscape, i.e. mining and non-mining. The study presented in this paper proves that it is relevant to analyse the changes occurring in different woodland categories separately. The same methodology may be applied when studying the change dynamics of other important landscape elements, such as wood pastures and wetlands.  相似文献   

17.
黑河下游绿洲NDVI对地下水位变化的响应研究   总被引：3，自引：3，他引：0  
本研究选取黑河下游核心绿洲区作为研究区域,通过对绿洲区植被变化和地下水位的长期监测,分析黑河下游绿洲植被与地下水位的关系;通过表面分析以及空间分析方法,探讨地下水位变化对绿洲植被的分布格局的影响,揭示绿洲植被变化对地下水位的响应。结果表明：额济纳地区2001年归一化植被指数(NDVI)值整体上小于1990年NDVI值,植被生长状况较1990年差;2009年河道附近绿洲区局部NDVI值增加,但大部分NDVI值仍小于1990年。相较2001年,2009年西河中上段河道附近植被有明显改善,西河下段植被仍呈现退化趋势,这主要与东西河分水措施有直接关系;东河上段河道附近绿洲呈现明显好转,东河中段变化不大,而东河下段植被面积略微增加,东居延海周围植被明显好于2001年。通过比较1990年、2001年和2009年地下水位与NDVI值的关系发现,地下水较为适宜的水位仍保持在2~4 m之间,地下水位下降与植被退化或者植被盖度降低有直接关系。  相似文献   

18.
In the semi-arid steppe rangelands of Central Turkey, Festuca valesiaca and Thymus sipyleus ssp rosulans have become the dominant species on degraded pastures. We hypothesized that decreases in species richness and abundance are correlated with increasing prevalence of these two species. Therefore, our objectives were to determine whether there are patterns in examined vegetation; how dominant species contribute to these patterns; and how patterns differ between grazed and ungrazed vegetation. We determined that protection from grazing increased species richness. Grazing significantly changed composition through decreasing total plant, forb, grass and F. valesiaca covers, while substantially increasing T. sipyleus cover. Topography, soil and grazing appear to impact the dominance of plant communities where F. valesiaca and T. sipyleus prevail. These two dominant species had a significant effect in shaping vegetation patterns. Based on regression analysis, alterations in species richness with changes in cover of forbs and shrubs were evident, and spatial heterogeneity of F. valesiaca and T. sipyleus indicated unstable vegetative patterns in heavily grazed pastures and successional changes in protected pastures. Our study results identify F. valesiaca and T. sipyleus as indicator species of vegetation suppression in condition assessments of degraded steppe rangelands.  相似文献   

19.
In arid and semi-arid areas, woody encroachment is the increase in density, cover, extent and/or biomass of woody plants. Woody encroachment is often associated with increased runoff and soil erosion. Hydrological and erosional responses of woody encroachment and of pastures established after management of encroachment in semi-arid Australia are not well understood. This study compared the hydrological and erosional responses across vegetation states comprising woody plant encroachment (>1200 stems ha−1), recently established pastures (<23 years of age), long-established pasture (50-100 years of age) and open woodland (<330 stems ha−1) in semi-arid eastern Australia. Responses were measured using rainfall simulation with intensity of 35 mm h−1 for 30 min applied on 1 -m2 plots. Runoff and sediment production did not differ significantly between vegetation states. Average runoff in woody encroachment was 9.0 mm h−1, followed by recent pasture (8.2 mm h−1), long-established pasture (5.9 mm h−1) and open woodland (4.2 mm h−1). Total sediment production in recent pasture was 11.6 g m−2, followed by woody encroachment (9.0 g m−2), long-established pasture (7.3 g m−2) and open woodland (4.3 g m−2). Runoff and sediment production were significantly lower at one pasture site (0.9 mm h−1 and 1.3 g m−2) where rotational grazing and minimum tillage had been implemented than in the adjacent paired woody encroachment site (10.3 mm h−1and 6.5 g m−2, respectively). This example of a pasture that had been managed to increase ground cover illustrated the effect of pasture management on reducing runoff and sediment production. Across all vegetation states, small scale runoff and sediment production were minimal or zero where total ground cover was 73% or higher.  相似文献   

20.
There is a growing requirement for techniques to assess land susceptibility to wind erosion, i.e. land erodibility, over large geographic areas (>104 km2). This requirement stems from a lack of wind erosion research between the field (101 km2) and regional scales, and a need to evaluate the performance of spatially explicit wind erosion models across these scales. This paper addresses this issue by presenting a methodology for monitoring land erodibility at the landscape scale (103 km2). First, we define criteria suitable for evaluating land erodibility based on empirical relationships between soil texture, vegetation cover, geomorphology, and wind erosion. The criteria were used to visually assess land erodibility over long distances (103 km) using vehicle-based transects run through the rangelands of western Queensland, Australia. Application of the data for testing the performance of a spatially explicit land erodibility model (AUSLEM) is then demonstrated by comparing the visual assessments of land erodibility with the model output. The model performed best in the west of the study area in the open rangelands. In regions with higher woody shrub and tree cover the model performance decreases. This highlights the need for research to better parameterise controls on erodibility in semi-arid landscapes consisting of forested and rangeland mosaics.  相似文献   

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