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1.
Vegetation community structure is a key indicator of long-term vegetation change in semi-arid ecosystems. This study uses ground-based spectral measurements and a canopy reflectance model to investigate the potential of Earth observation data to characterize variations in vegetation structure along a grazing gradient in the eastern Kalahari, Botswana. Despite differences in the bidirectional reflectances of soil, plant litter, bush and grass canopies towards the end of a dry season, it is unlikely that Earth observation data can be used to estimate vegetation structure at this time. This is due to an ambiguity in the relationship between spectral reflectance and vegetation structure caused primarily by the limited dimensionality of reflectance data. Variations in canopy architecture cause differences in the level of inter-canopy shadowing but the net effect—lower reflectance with an increase in bush cover—parallels the darkening effects of any vegetation cover on relatively bright sandy soils in semi-arid environments. These results highlight the continued need for long-term ground-based ecological monitoring in conjunction with satellite-based monitoring of changes in vegetation cover.  相似文献   

2.
Namibian rangelands are encroached with Acacia mellifera, partially resulting from a poor understanding of vegetation dynamics. A conceptual state-and-transition model of vegetation dynamics in the semiarid Highland savanna in central Namibia, emphasising bush thickening by A. mellifera, is described. Two main states, a grassy and a bushy state, are identified. These are further subdivided, and 11 transitions are identified. The key transition initiating a change from grassy to bushy state can be termed a “leap” (an occasional, infrequent mass recruitment event) following a long “sleep” (no or little change in A. mellifera density). It is rare because it requires three consecutive years of above-average rainfall for seedling establishment. Fire, coinciding with seedling establishment, can interrupt it, while a low biomass grass sward facilitates it. The phenology and physiology of the encroaching species, seed predation and sapling herbivory influence this transition. The model proposes opportunistic management interventions, particularly the use of fire, to minimise the risk of further landscape-scale transitions to a bushy state. It highlights areas where understanding of vegetation dynamics is lacking and recommends crucial research foci. Conceptual models of bush-thickening processes need to account for differences in climate and phenological details of encroaching species.  相似文献   

3.
Species-specific allometric models were developed to predict aboveground biomass (AGB) of eight woody species in the Borana rangelands, Ethiopia. The 23 equations developed (8 species; three biomass components: total aboveground, stem and branches) fit the data well to predict total AGB and by components for each of the species (r2 > 0.70; p < 0.001). The AGB of tree shaped species (e.g., Acacia bussei and Acacia etabaica) were significantly predicted from a single predictor (circumference of the stem at ankle height), with a high coefficient of determination (r2 > 0.95; p < 0.001). In contrast, the AGB of bushy shrubs (e.g., Acacia oerfota) was more effectively predicted by using the canopy volume (r2 = 0.84; p < 0.001). Shrubs with a tall stem and an umbrella-like canopy structure (e.g., Acacia mellifera) were most accurately predicted by a combination of both circumference of the stem at ankle height and canopy volume (r2 = 0.95; p < 0.001). Hence, our species-specific allometric models could accurately estimate their woody aboveground biomass in a semi-arid savanna ecosystem of southern Ethiopia. These equations will help in future carbon-trade discussions in times of climate change and CO2 emission concerns and mitigation strategies.  相似文献   

4.
The impact of Prosopis species invasion in the Turkwel riverine forest in Kenya was investigated under three contrasting: Acacia, Prosopis and Mixed species (Acacia and Prosopis) canopies. Variation amongst canopies was assessed through soil nutrients and physical properties, tree characteristics and canopy closure. Invasion impact was evaluated by comparing herbaceous species cover and diversity, and occurrence of indigenous tree seedlings. Soil characteristics under Prosopis and Mixed species canopies were similar except in pH and calcium content, and had lower silt and carbon contents than soil under Acacia canopy. Tree density was higher under Prosopis intermediate under Mixed and lower under Acacia canopies. Prosopis trees had lower diameters than Acacia tortilis trees. Diameter classes' distribution in Mixed species canopy revealed invasion of Prosopis into mature A. tortilis stands. Herbaceous species cover and diversity were negatively correlated to Prosopis tree density; thus explaining the lower herbaceous species cover and diversity under Prosopis than under Acacia and Mixed species canopies. The study suggests a gradual conversion of herbaceous rich A. tortilis woodland to herbaceous poor Prosopis species woodland or thickets, through indiscriminate Prosopis invasion.  相似文献   

5.
Silvopasture, the planting of suitable tree species in pastures, can improve the sustainability and productivity especially in dry rangelands. While recent studies provide information on the effects of different tree compositions on biological productivity, water and soil protection, additional parameters such as fodder production by the trees or the impact of silvopasture on the grazing potential themselves have rarely been addressed. We determined fodder production in Acacia victoriae woodland and savanna by measuring annual vegetation and tree biomass growth. We developed mathematical models for calculating the vegetative biomass available for grazing. In order to get accurate estimation for the grazing capacity, we differentiated between the grazing and the browsing fodder availability, and adapted the model to the most abundant grazers in the Negev, goats and sheep. Grazing capacity for sheep and goats was twice bigger in A. victoriae woodland than in the adjacent sustainably managed shrubland and four times higher than in degraded shrubland. The mathematical model presented in this paper can be applied in order to plan and realize high yielding and sustainable silvopasture in arid environments resistant to degradation and desertification while providing adequate fodder reserves during drought years.  相似文献   

6.
塔里木河荒漠植被光谱可分性模拟   总被引：3，自引：2，他引：1  
以塔里木河典型植被为研究对象,分析胡杨、芦苇叶片及柽柳冠层的可分性,并计算背景的影响。首先用ASD光谱仪测新鲜叶片光谱,找出光谱特征点;然后模拟EO-1高光谱数据和TM多光谱数据;最后植被与土壤光谱按比例混合,分析背景的影响。以上三步分别计算植被指数（VI）。结果显示:叶片光谱特征位置430 nm、670 nm、750 nm附近,黄边斜率和红外平台平均高度,1 080~1 280 nm、1 430~1 650 nm能够区分塔里木河流域3个主要植被类型。模拟的EO-1波谱保持了控制波形的10个特征,TM 只有绿反射峰和红吸收谷、近红外1个反射峰3个特征,大部分特征都消失了。植被指数显示(R680-R500)/R750、(R680-R550)/R705、R1430+\:+R1650、D712/D688能够区分3类,且指数值差异较大,为绿峰、红谷和近红外波峰的组合;模拟的EO-1数据(R680-R500)/R750、(R680-R550)/R705、R1430+\:+R1650能分别区分植被,TM多波谱数据不能有效区分植被。  相似文献   

7.
A perennial forage grass, Agropyron desertorum was sown under the canopies of four shrub species or in open areas to test for facilitation during seedling establishment in an arid rangeland in Karnakh, Northeast Iran. Height, area and volume of shrubs were measured. Microclimate conditions and seedling establishment were assessed three times within two consecutive growing seasons. Near surface light intensity and air temperature were lower under shrubs, which led to initially higher soil moisture and grass establishment under the canopy of some shrub species. The leguminous shrub (Astragalus gossypinus) showed facilitation during moderate stress (summer 2009), but shifted to a negative effect during the severe drought (summer 2010). Competition, possibly for light, reduced the establishment of Agropyron seedlings under the cushion-like shrub (Acantholimon prostegium). Salsola arbusculiformis and Artemisia kopetdaghensis respectively showed a neutral and a facilitation effect in the first season, but a combined effect of allelopathy and drought led to the high mortality of Agropyron seedlings under their canopy. In conclusion in this arid rangeland, shrubs may facilitate establishment of understory plants under moderate drought stress, and for non-resource factors (light and temperature), but these positive effects are suppressed due to competition under severe drought conditions.  相似文献   

8.
Yabello牧场是埃塞俄比亚博拉纳的一个半干旱地区的牧场,目前面临着草场退化的严重挑战。植被覆盖的变化、过度放牧和季节性变化极大地影响了Yabello牧场的牧草组成和生物量。本文评估了植被覆盖、放牧和季节对Yabello牧场的牧草组成和生物量的影响。首先采用1 m×1 m的随机样方进行实验,根据植被覆盖类型和放牧变化选择样点,并对季节影响进行评估。使用SAS统计软件和MicrosoftExcel分析牧草组成、牧草高度和质量数据。本研究总共记录了26种草种,其中Chloris roxburghiana, Chrysopogon aucheri和Chrysopogon aucheri草种均表现出最高的平均单种覆盖高度和生物量产量。因此,建议将这些草种用于研究区域退化草地的恢复。研究结果还表明,植被覆盖类型、放牧和季节变化是决定牧草种类组成、牧草高度和生物量产量的关键因素。最后,研究结论还认为控制灌木丛植被并平衡放牧水平的可持续管理对于该地区的可持续牧草生产和生物多样性保护至关重要。  相似文献   

9.
Elephant grass (Pennisetum purpureum Schum.) is a new fast-growing alternative forage crop. However, salinity is a major concern for its production in the arid south-western United States. This study was conducted in the arid Imperial Valley of Southern California to evaluate salt tolerance of elephant grass. Salinity treatments were created in field plots irrigated with water possessing an electrical conductivity (ECiw) of 1·5, 5, 10, 15, 20, and 25 dS m−1, respectively. Canopy spectral reflectance, temperature, plant height, leaf area index (LAI), chlorophyll-SPAD meter readings, and dry weights were measured over time. Results indicated that canopy reflectance in the near-infrared spectral region was reduced incrementally with increasing levels of salt stress. Canopy temperature increased with increasing salinity, especially at longer times after salinity treatment. Plant height and LAI were reduced with increasing salinity. Biomass accumulation was reduced incrementally with increasing salinity. About 50% yield reduction was found when ECiw increased from 5 to 25 dS m−1. The study shows that elephant grass is sensitive to salt stress, and relatively low salinity must be maintained to achieve a high rate of growth and biomass production.  相似文献   

10.
Abiotic environmental factors have a major impact on the distribution and performance of plant species. In order to assess two major species–environmental relations in Sudano-Sahelian Acacia woodlands, we tested the relationship of soil and climate variables on plant diversity as well as on species responses.The indicator species values clustered in five vegetation units characterized by three to ten diagnostic species with woody species richness means varying from three to seven species per 0.09 ha. The NMS ordination explained 65% of the variation in species composition and revealed that soil properties, annual precipitations and temperature range structured the diversity of Acacia communities. Along the annual precipitations gradient, the response of Acacia polyacantha and Acacia hockii showed maxima in the wettest zone of our study area (more than 850 mm/year) whilst Acacia laeta showed a maximum response in the driest zone (below 500 mm/year). The unimodal response of A. hockii, Acacia gourmaensis and Acacia seyal to the soil available water gradient spanned their central borders, respectively from 13 to 18% (optimum 16%), 11–20% (optimum 15%) and 4–12% (optimum 7.5%).The response of Acacia communities and species to soil and climate gradients, makes them performant afforestation species in specific habitats of the Sudano-Sahelian zone.  相似文献   

11.
Both aboveground and belowground processes play important roles in tree-grass interactions in savannas. Little consideration has been given to within-site heterogeneity in the strengths of co-occurring canopy and root effects of trees on grasses in savanna communities. Here, we attempted to correlate the spatial pattern of grass morphological traits with the strengths of canopy and root effects. The results from a spatial analysis suggested that the grass traits had lower variability within the operating domain of the root effect than within that of the canopy effect in sub-canopy areas; in contrast, the operating domain of the root effect presented higher variability of grass traits than that of the canopy effect in inter-canopy areas. Combined with root investigations on vertical distribution patterns, these results suggested that the root effect appeared to outweigh the canopy effect in the sub-canopy areas, where apparent vertical root separation between trees and grasses was shown; while the canopy effect could outweigh the root effect in the inter-canopy areas, where root separation was not observed. This study could provide correlative information on the relative importance of canopy and root effects, and has some useful implications on within-site heterogeneity in terms of aboveground and belowground components in savannas.  相似文献   

12.
Monitoring cattle inflows and outflows over time is a good indicator of herd productivity. In this study, ninety households were monitored for a year to determine the effect of season, rangeland type and herd size on births, purchases, sales, deaths, off-take, cattle production potential (CPP) and cattle production efficiency (CPE) in the sweet and sour rangelands of South Africa. Most births were recorded in large herds in the sour rangelands during the hot-wet season (P < 0.05). Farmers with large herd sizes purchased significantly more cattle compared to those with small herd sizes. Cattle sales were significantly high in the large herds in the cool-dry season. Mortality was significantly high in the hot-wet season in the sour rangeland and was mainly caused by gall-sickness (24%) and redwater (20%). The highest mortalities were observed in cows (45%). Households owning small herds in the sweet rangeland had the lowest CPP in the cool-dry season (P < 0.05). The CPE was significantly high in the households owning large herds in the sweet rangeland. It was concluded that, although the CPP was low in the sweet rangeland, households in these areas had higher CPE compared to those in the sour rangelands.  相似文献   

13.
Shrubs in semi-arid ecosystems promote micro-environmental variation in a variety of soil properties and site characteristics. However, little is known regarding post-fire seedling performance and its association with environmental variation in former shrub canopy and interspace microsites. We compared post-fire seeding success and various soil properties important for seedling establishment between shrub canopy and interspace microsites in Wyoming big sagebrush (Artemisia tridentata) plant communities in southeast Oregon, U.S.A. We burned 5, 20 × 20 m sites and established paired canopy and interspace micro-transects seeded with bluebunch wheatgrass (Pseudoroegneria spicata, 193 seeds/m) or crested wheatgrass (Agropyron cristatum, 177 seeds/m). At one year post-fire, seedling density was 69% higher (p = 0.012) for crested wheatgrass (compared to bluebunch wheatgrass) and 75% higher (p = 0.019) for interspace microsites (compared to canopy). However, tiller and leaf area production were over twice as high (p < 0.05) in canopy microsites. Soil color and soil temperature, explained 19–32% of variation in seedling performance metrics. Shrub effects on seeding success are complex and interact with abiotic disturbances, but patterns of increased seedling performance in canopy microsites and their relationships to soil variables may suggest tactics for increasing success of restoration practices.  相似文献   

14.
水分胁迫条件下棉花生理变化及其高光谱响应分析   总被引：19，自引：4，他引：15  
利用ASD地物光谱仪,测定水分胁迫条件下棉花不同生育时期内叶片的光谱反射率,应用微分技术处理棉花的反射光谱,并结合棉花叶面积指数(LAI)、叶绿素(a b)含量(Chlt)、叶片全氮(TN)含量等生物参数进行分析,研究棉花水分胁迫情况下的高光谱特征,结果表明,一阶微分光谱720nm波段的数值与LAI的正相关(R=0．7656);750nm处一阶微分值与叶绿素含量呈显著正相关关系(R=0．7774);微分光谱690nm～740nm数值积分面积与TN含量呈正相关(R=0．7669),采用比值反射率对反射光谱1300nm～1500nm波段范围内最小值与棉花叶片的含水量作相关分析,达到极显著水平(R^2=0．8298),验证了一阶微分光谱数据与棉花的生理参数有很好的相关性,可见光和近红外波段光谱反射率能够反映出棉花生长发育的动态特征;证明了棉花的花铃期是高光谱遥感对棉花长势和生理参数定量诊断的最佳时期。本研究通过建立一系列线性光谱模型对棉花生理参数进行估测,为基于高光谱数据的棉花生长模型和棉花长势的遥感监测提供了理论依据。  相似文献   

15.
内蒙伊盟牧场利用与沙漠化及其防治   总被引：1，自引：0，他引：1  
黄兆华 《中国沙漠》1981,1(1):21-32
本世纪七十年代以来, 沙漠化问题在世界范围内引起了广泛的注意。沙漠化对社会生产和人类的生活与生产是一个严重的威胁。内蒙古自治区伊克昭盟是一个沙漠化比较严重的地区, 迄一九七七年, 全盟流沙面积达27, 666平方公里, 比解放初扩大1.3倍。伊盟牧业在国民经济中比重很大, 牧区约占全盟面积72%。在沙漠化成因中, 牧场的利用不合理是一个主要的方面。因此, 研究牧场利用与土地沙漠化的关系, 总结牧民合理利用牧场的经验, 并提出防治沙漠化的途径, 对于解决这个地区的沙漠化问题, 是十分重要的。  相似文献   

16.
The study was conducted to assess the status and trends of soil erosion and relate the perceptions of farmers on cattle productivity and botanical indicators to measured ecological conditions of rangelands in three altitude regions of southwest Ethiopia. A total of 342 farmers were interviewed. In addition, the ecological condition of rangelands was assessed. Severe soil erosion, ranked as the primary restriction to free-ranging livestock, occurred predominantly in the lower altitude region (LAR) (P < 0.05). More farmers in LAR witnessed an inadequacy of palatable plant biomass, grazable pasture as well as increased gully formation and expansion, which are strong indicators of soil erosion (P < 0.001). In addition to a decrease in grass cover and productivity of cattle, botanical composition, species richness and grazing capacity of herbaceous plants, less fodder trees and shrubs were observed (P < 0.05). There was a corresponding increase in the percentage of bare ground and soil erosion status along the degradation gradients (P < 0.05). The reported shift in botanical composition, and especially encroachment of invading plant species, can be attributed to soil erosion (P < 0.001). The results suggest that erosion is associated with reduced availability of feed resources and is related to altitude variation.  相似文献   

17.
We studied current recruitment failure of Dobera glabra, an important famine-food throughout the dryland parts of the Horn of Africa. The species has previously been found to have recalcitrant seeds, a strategy not common for plants in dryland areas. We experimentally tested (1) how germination capacity of D. glabra is affected by seed form, storage period and moisture under nursery conditions, and (2) how seed predation by vertebrate herbivore and seed exposure at germination site affect germination under field conditions in northern Afar rangelands, Ethiopia. D. glabra seeds that received either mulching or supplementary watering (3 days/week) had a higher germination success than controls. The seeds are probably desiccation sensitive since stored seeds had poor germination performance compared to fresh seeds. Seeds in the rangeland plots protected from herbivores had significantly higher germination success compared to open plots. Our findings demonstrate that seed predation and moisture limitation highly reduced the germinability of the recalcitrant D. glabra seeds, indicating that the species might have persisted due to past wetter periods and low herbivore density in dry environments.  相似文献   

18.
Early-warning indicators of rangeland health can be used to estimate the functional integrity of a site and may allow sustainable management of desert rangelands. The utility of several vegetation canopy-based indicators of rangeland health at 32 Great Basin rangeland locations was investigated. The indicators were originally developed in rangelands of the Chihuahuan Desert. Soil resources are lost through wind and water-driven erosion mainly from areas unprotected by plant canopies (i.e. bare soil). Study sites in Idaho had the smallest bare patches, followed by sites in Oregon. The more arid Great Basin Sagebrush Zone sites in Utah had the largest bare patches. Several vegetational indicators including percent cover by vegetation, percent cover by life-form, percent cover by sagebrush, and percent cover by resilient species were negatively related to mean bare patch size and are potential indicators of Great Basin rangeland condition. Plant community composition and the range of bare patch sizes were different at sites in the three locations in Idaho, Oregon and Utah. Therefore, expected indicator values are location specific and should not be extrapolated to other locations. The condition of study sites were often ranked differently by different indicators. Therefore, the condition of rangeland sites should be evaluated using several indicators.  相似文献   

19.
A savanna system is a natural ecosystem in which the competition between grass and woody vegetation in a semi-arid rangeland should be maintained for its sustainable development. Finding an optimal management plan for obtaining maximum economic profit from raising cattle without loss of sustainability of the savanna system during a planning period is a great challenge for rangeland managers. In this study, we formulate the sustainable development planning of the savanna system as an optimal control model, in which maximization of the stocking rate of cattle during the planning period is chosen as the objective while sustainable development requirements are achieved through the constraints represented by the desired final state of the system. Using Pontryagin's maximum principle, the model is transformed into a two-point boundary-value problem with nonlinear differential equations that is then solved using an iterative approach. An example with a specified desired final state in a savanna system without fire is used to demonstrate the performance of the model and the algorithm. Numerical experiments show that the planning strategy obtained from the optimal control model achieves maximum economic profit from raising cattle during the planning period and simultaneously improves the resilience of the system and maintains sustainable development of the rangeland. The outcomes demonstrate that resilience, sustainable development and economic profit are consistent concepts in optimal management strategy for rangeland management.  相似文献   

20.
We examined the impact of shrub-shrub interactions and soil type (rocky or sandy) on growth and spatial distribution of the two savanna shrub species Tarchonanthus camphoratus and Acacia mellifera. To explore plant interactions, we compared the size of juvenile and mature T. camphoratus shrubs between different locations (under the subcanopy of A. mellifera and in the open). Juvenile T. camphoratus shrubs had similar sizes regardless of location; however, in rocky soil, mature shrubs in the open were larger than those near A. mellifera, implying an inter-specific competitive effect of A. mellifera on T. camphoratus. Juvenile T. camphoratus shrubs grew faster in the sandy than in the rocky area. Furthermore, we used the Wiegand-Moloney O-ring statistics to explore the spatial distribution of T. camphoratus. T. camphoratus showed spatial aggregation, but in the rocky area T. camphoratus juveniles were positively associated with A. mellifera (indicating facilitation as the pattern-creating process), whereas in the sandy area they were positively associated with mature T. camphoratus shrubs (indicating seed dispersal as the pattern-creating process). T. camphoratus exhibited encroachment potential in the sandy area. We showed how spatial pattern analysis can help to explore processes determining woody plant spacing and recommend its further use.  相似文献   

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