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1.
The Nama-Karoo is the largest of the three biomes that comprise the semi-arid Karoo-Namib ecoregion of southern Africa. Large tracts of Nama-Karoo rangeland have been invaded by alien leguminous trees of the genus Prosopis. We assessed the impact of Prosopis invasion and clearing on vegetation cover in heavily grazed Nama-Karoo rangeland on two sheep farms in the Beaufort West district of the Western Cape province of South Africa. Our results suggest that, below critical threshold cover levels, Prosopis trees do not affect indigenous vegetation and add to overall rangeland vegetation cover. However, once Prosopis cover exceeds critical thresholds, the trees begin to negatively impact on indigenous vegetation (especially grasses) and overall vegetation cover. In addition, our data suggest that vegetation cover recovers after Prosopis clearing, being temporarily higher than pre-invasion levels possibly as a result of enhanced post-invasion soil fertility. Lastly, our results suggest that Prosopis trees reduce rangeland erosion potential at low cover and increase it after they exceed critical threshold cover. Information about thresholds of Prosopis impact can be used to direct clearing efforts for maximum benefit when resources are limited.  相似文献   

2.
Following a field campaign to determine the species composition, canopy cover, aboveground annual production and leaf area index (LAI) of the semi-arid savanna of north-western Namibia, we present a production model that can be used by graziers to determine the livestock carrying capacity. The model predicts the annual aboveground net primary production (ANPP) from regression equations of canopy cover by annual production fraction for plant functional classes. We tested the output of the model against another fully independent net primary production (NPP) model, namely the MODIS NPP product. The mean MODIS NPP for the 29 sites was 343 ± 22?kg dry matter (DM) ha?1 y?1 as opposed to 285 ± 142?kg DM ha?1 y?1 for the fAP model that used the regression method (p < 0.01). As a proof of concept, these landscape-scale ANPP values are used to calculate a recommended livestock carrying capacity for the Ehirovipuka Communal Conservancy, a 1 980 km2 communal area with both wildlife and livestock populations. In addition, we also provide details of a field method for predicting landscape-scale LAI from line transect data. This approach can be used to ground reference the LAI values generated from the MODIS LAI product.  相似文献   

3.
The Nama-Karoo biome occupies 28% of South Africa’s land area. Alien leguminous trees of the genus Prosopis have invaded large tracts of Nama-Karoo rangeland. We evaluated the impact of Prosopis invasion and clearing on vegetation species composition and diversity (alien and indigenous species richness and cover) in Nama-Karoo rangeland on two sheep farms in the Beaufort West district of the Western Cape province of South Africa. Our results suggest that Prosopis invasion and clearing can significantly change Nama-Karoo rangeland species composition. Invasion and clearing appear to have no effect on alien species richness. Invasion, however, increases alien species cover, while clearing restores it to pre-invasion levels. In contrast, invasion reduces indigenous species richness while clearing restores it to pre-invasion levels. Invasion appears to have no effect on indigenous species cover. This lack of effect appears to be the serendipitous result of a site-specific trade-off between a decline in cover of grasses that are negatively affected by Prosopis invasion and a concomitant increase in the cover of positively affected grasses. Clearing increases indigenous species cover to above pre-invasion levels. The higher than usual indigenous species cover after clearing could be a transient legacy of Prosopis soil nutrient enrichment.  相似文献   

4.
Woody plant effects on grass production at specific points in some rangeland savannas may be a function of numerous surrounding woody plants with lateral roots that extend into those patches of grass. This study determined the effects of increasing zones of honey mesquite (Prosopis glandulosa Torr.) influence on the production of three perennial grass types (C4 shortgrasses, C3 midgrasses, and C4 midgrasses) at specific points in gaps between mesquite trees in each of five years. Mesquite canopy cover was determined by geospatial analysis of aerial images for progressively increasing zones (0–5, 0–10, 0–15, and 0–20 m radius) surrounding each grass production point. The woody cover/grass production relationships were mostly linear for C4 shortgrasses and C3 midgrasses, and mostly a declining exponential curve for C4 midgrasses in all canopy zones, indicating that C4 midgrasses were most sensitive to increasing mesquite cover, especially at covers >30%. The relationship between mesquite cover and C4 shortgrass production was strongest (i.e., highest r2) when the smallest woody cover zones (0–5 and 0–10 m) were included. In contrast, the relationship between cover and C4 midgrass production was strongest when the largest zones (0–15 and 0–20 m) were included. These differences were attributed to an inability of C4 midgrasses to persist in smaller intercanopy gaps resulting from increases in mesquite density and infilling. Annual precipitation and C3 annual grass invasions played a large role in determining the woody cover/grass production relationship for each grass type. This study illustrates the complexity involved in quantifying woody cover/grass production relationships in savanna ecosystems. Maintaining productive stands of C4 midgrasses may be facilitated by maintaining woody cover below 30% threshold levels and possibly by limiting grazing during episodic high rainfall events in midsummer when this grass type becomes somewhat decoupled from woody cover effects.  相似文献   

5.
A study was undertaken in Kenya's southern savanna rangelands to determine the seasonal effect of Balanites glabra canopy cover on aboveground grass biomass, grass species composition, soil organic matter and soil moisture content. The study was conducted during the period June to December 1999 in order to capture both the dry and wet season effects. The grass biomass in the sub-canopy zone (2-4m from tree trunk) was found to be significantly higher than in the mid-canopy (0-2m from tree trunk) and open grassland zones (4-6m from tree trunk) during the dry season. However, the difference between the sub-canopy and the open grassland was not significant during the wet season, implying that the role of a tree canopy in enhancing grass biomass is greater during the dry than the wet season. Variations in percent grass species composition from the mid-canopy to the adjacent open grassland were observed, indicating that while B. glabra canopy cover favours certain grass species, other species find the micro-climatic conditions under the canopy unfavourable. Soil organic matter in the mid-canopy zone was significantly higher than in the sub-canopy and adjacent open zones during both dry and wet season. Although the sub-canopy zone exhibited significantly higher soil moisture content than the mid-canopy and open grassland zones during the dry season, the difference between the sub-canopy and the adjacent open grassland during the wet season was not significant, suggesting that the tree canopy influence on soil moisture is more pronounced in the dry than the wet season.  相似文献   

6.
Abstract

The investigation was concerned with the impact of an Acacia karroo tree on the productivity of the surrounding herbaceous layer.

It was established that a consistent pattern of grass production occurs around isolated A. karroo trees. This is characterized by high yields under and immediately south of the tree canopy, and low yields immediately to the north of the canopy. The former is attributed to favourable influences by the tree (e.g. shade and tree leaf litter), whereas the latter is probably a result of reduced water input associated with physical redistribution of rainfall by the tree and competition from the tree for soil water.

It is argued that the net effect of the favourable or unfavourable influences of A. karroo on grass production is dependent on tree density. This explains why grass production is greater where there are a few A. karoo trees than where there are no trees and why grass production declines as tree density increases beyond a critical level.  相似文献   

7.
草地生产力遥感动力模拟模型   总被引：3，自引：0，他引：3  
黄敬峰  李军 《草地学报》2005,13(Z1):10-14
根据植物光合作用机理和太阳辐射在植冠层中的传输方程,推导光合作用产物与叶面积指数关系的解析式和叶面积指数与植被指数关系的解析式,通过复合函数求导得到光合作用产物与植被指数关系的解析式,建立草地生产力遥感动力模拟模型.利用1992-1994年的产草量资料和对应时相的NOAA/AVHRR资料建立草地生产力光谱监测模型和卫星遥感监测模型,验证结果表明,用卫星资料可以准确监测草地生产力.  相似文献   

8.
Shrub encroachment into grasslands is a worldwide phenomenon with no signs of abating and numerous ecological consequences. In South Texas, honey mesquite (Prosopis glandulosa Torr.) and huisache (Vachellia farnesiana [L.] Wight & Arn.) are two shrubs encroaching into coastal prairies, reducing cover of the dominant native grass, gulf cordgrass (Spartina spartinae [Trin.] Merr. ex Hitchc.), and decreasing habitat for the endangered Aplomado Falcon (Falco femoralis), which requires grasslands or savannas for survival. To determine the best management approach for deterring shrub encroachment and restoring native grasslands, the US Fish and Wildlife Service used several shrub removal techniques within coastal prairies of the Bahía Grande Wetland Complex of the Laguna Atascosa National Wildlife Refuge, a core site for Aplomado Falcon reintroductions. Here, we assess native grass recovery over a 2-yr period in response to these shrub removal methods (mechanical plus prescribed fire and/or herbicide treatments) and degree of shrub encroachment before treatment. In general, areas with high levels of shrub encroachment before treatment had the highest amount of bare ground and lowest grass cover immediately following an initial mechanical treatment; this legacy effect persisted throughout the study irrespective of shrub removal treatment. Regardless of degree of shrub encroachment before treatment, grasses in areas treated with either mechanical or mechanical followed by herbicide methods recovered the slowest, likely due to residual woody material that hindered seed germination. Herbicide treatment following mechanical removal or mechanical removal plus fire effectively hindered shrub regrowth. Overall, mechanical treatment followed by prescribed fire and then herbicide application most effectively promoted grass recovery while hindering shrub regrowth. These findings suggest that grassland recovery following shrub encroachment into South Texas coastal prairies may be promoted through the application of shrub removal methods that combine mechanical, fire, and herbicide treatments.  相似文献   

9.
Soil seed banks provide not only a historical record of vegetation composition but also the potential for post-disturbance revegetation. Pechuel-Loeschea leubnitziae (wild sage) is a multistemmed, aromatic shrub, occurring in Namibia, Botswana, Swaziland, Zimbabwe and the northern regions of South Africa, that colonises disturbed regions on sandy, alkaline soils. The influence of differing levels of P. leubnitziae cover on soil seed-bank characteristics and aboveground herbaceous species composition was determined by examining aboveground species composition, recorded in the field, and soil seed-bank species composition, determined by recording germinations from topsoil samples collected at each of 104 sites in the southern Okavango Delta. No correlation was observed between aboveground and soil seed-bank composition (p = 0.209). Cover of P. leubnitziae did not affect soil seed-bank species composition (p = 0.31) or the grass:non-grass ratio within the seed bank (p = 0.308). It did, however, affect aboveground grass composition (p = 0.018), with increasing P. leubnitziae cover associated with increasing abundance of poor-quality, shade-tolerant grass species. This confirms the observations that encroachment of P. leubnitziae may be associated with a decrease in veld quality and carrying capacity, and highlights the necessity for further study to facilitate a better understanding of the species for management purposes.  相似文献   

10.
以C为驱动的WOFOST作物生长模型是基于作物生理生态过程,综合考虑了CO2、土壤、气候等因素对产量的胁迫作用,因此,对WOFOST模型参数进行本地化和优化便可实现时间连续且高精度的草地生物量监测。为探讨WOFOST参数敏感性分析结果在不同草地类型覆盖区表现出的不确定性问题,在天祝藏族自治县不同草地覆盖区选择了4个站点,利用气象数据、草地实测数据及土壤数据,基于扩展傅里叶幅度敏感性检验法（EFAST）研究潜在水平（指保证营养元素和水分为最佳供应,草地地上生物量仅由辐射、温度和作物特性决定）和水分限制水平（假设营养元素的供给仍然是最佳的, 但需考虑土壤有效水分对蒸发和草地生物量的影响）下WOFOST模型在不同草地类型覆盖区的全局敏感性参数和优化模型模拟精度。结果表明潜在生产水平下草地地上生物量（AGB）的敏感参数有比叶面积（SLATB）、单叶片CO2的初始光能利用率（EFFTB）、最大光合速率（AMAXTB）、根相对维持呼吸速率（RMR）、总干物质占根和叶的比例（FRTB和FLTB）,水分限制条件下的敏感参数有SLATB、AMAXTB、RMR和FLTB。不同生产水平下叶面积指数（LAI）的敏感参数一致,从出苗到出苗后60 d主要受到SLATB、FLTB和FRTB的影响,出苗后60~200 d的敏感性参数为FLTB、FRTB、SLATB和漫射可见光的消光系数（KDIFTB）,LAI开始下降后受到KDIFTB的敏感性增强。其中,山地草甸AGB的模拟值与观测值模拟精度最高,R2=0.94、RMSE=11.71 g·m-2,高寒草甸模拟精度最低,R2=0.83、RMSE=32.68 g·m-2。温性荒漠草原LAI的模拟值与观测值模拟精度最高,R2=0.96、RMSE=0.02,温性草原模拟精度最低,R2=0.66、RMSE=0.38。敏感性分析方法在WOFOST模型中的应用减少了人为主观因素的影响,极大地缩短了调参时间,对获取时间连续的草地生长监测方法选择提供参考。  相似文献   

11.
Grasslands are important to domestic and wild animals. Migratory shorebirds are important components of coastal rangeland ecosystems. Buff-breasted Sandpiper (BBSA, Calidris subruficollis) and American golden-plover (AMGP, Pluvialis dominica) are two insectivorous, migrant shorebirds that rely on livestock-grazed grasslands in the Southern Cone of South America during their nonbreeding season, as well as on migration in North America. We studied habitat selection of these species and contrasted their needs with livestock requirements needed to develop recommendations for grazing management that benefit wildlife and livestock production. Short grass height was positively related to BBSA and AMGP abundance, with ideal grass heights from 2 to 5 cm. However, maximum livestock production is associated with grass height over 6 cm. The amount of forest cover, which is used to provide shade to livestock, was negatively related to the occurrence of both shorebird species, likely due to higher risks of predation. Grassland improvement did not affect BBSA but negatively affected AMGP abundance. Short grass habitat was selected by both shorebird species in spite of the higher arthropod biomass in taller grasslands, suggesting that other factors besides food abundance, such as the ability to detect prey and predators, are driving habitat selection. To enhance shorebird (and other wildlife) conservation and livestock production, we recommend managers adjust grazing intensity so that grass height is > 6 cm from mid-February to September, when the Nearctic migrant shorebirds are absent, and from 2 to 5 cm from October to early February when shorebirds are present. These austral summer adjustments should be restricted to paddocks with low forest cover so that livestock production in paddocks with high forest cover remains maximized. All adjustments should be evaluated by each farmer to ensure adequate economic returns are met.  相似文献   

12.
The recovery of cleared Colophospermum mopane thicket in the eastern Transvaal lowveld, to a level where its effect on grass production is similar to that in the pre‐cleared state, occurs within fourteen years. Based on certain assumptions, the ‘effective duration’ of the increase in grass production after clearing is about half of the recovery period. A model describing the recovery of basal area with time since clearing is presented. Simulations show that the recovery period is shortened by higher rainfall and lengthened by drought. It is hardly affected by the degree of clearing mortality typically achieved by the use of hormonal herbicides, but is extended to twenty‐two years if 90% mortality is achieved. Complete clearing mortality and a seedling establishment rate of four seedlings/ha/yearresults in recovery within forty years.  相似文献   

13.
Measurements were taken at 20 different sites in a relatively homogenous area of open Camelthorn savanna in eastern Namibia. Ordination of grass species composition did not provide a good degradation gradient, due to domination by different species of annual grasses at many of the sites. However a centred PCA ordination of perennial grass species indicated a degradation gradient. This gradient was used to allocate a range condition score to each site, which was then correlated with other measurements. The range condition score did not correlate well with mulch cover, basal cover, proportion of bare soil, above-ground grass biomass, proportion of moribund grass, proportion of seedlings amongst the perennial grasses, density of woody plants lower than O.5m and canopy cover of browsable woody plants over 0.5m tall. A weak correlation (r2 = 0.68, p<0.0001, n = 20) was obtained through a power trend line with woody canopy cover of only the bush thickening species. A better correlation (r2 = 0.90, p<0.0001, n = 20) was obtained with mean distance from sampling point to the nearest perennial grass. This may therefore be an appropriate indicator to include in range condition score for such types of savanna.  相似文献   

14.
王秀梅  董建军 《草地学报》2020,28(6):1711-1718
本研究基于广义线性模型（generalized linear model,GLM）,利用中分辨率成像光谱仪（moderate resolution imaging spectroradiometer,MODIS）植被指数产品和野外实测的地上生物量数据（aboveground biomass,AGB）,分析了2009-2015年主要生长季5-10月内蒙古荒漠草原及典型草原AGB动态变化及气候因子的关系。结果表明：NDVI幂函数模型最佳模拟内蒙荒漠草原AGB,RVI对数函数模型最优模拟内蒙典型草原AGB;七年间荒漠草原平均地上总生物量为6.51 Tg,典型草原平均地上总生物量为36.94 Tg;2009-2015年主要生长季内蒙荒漠草原及典型草原AGB变化趋势呈现单峰曲线,生物量高峰出现在每年7,8月份,低值出现在每年10月份;AGB空间分布5-10月,以8月为节点随着季相变化南北交替推进;最后采用面板分析方法解释2类草原的限制因子,结果表明荒漠草原降水贡献较大,典型草原气温贡献较大,因此水热动态假说对该格局解释最佳。  相似文献   

15.
《African Zoology》2013,48(2):273-278
We studied tree agama (Acanthocercus a. atricollis) habitat use in the Magaliesberg mountain range in northern South Africa using sightings of marked individuals, and in a few cases, radio-telemetry. Acanthocercus a. atricollis preferentially selected thorn trees (46%; Acacia karroo), followed by common sugarbush (10%; Protea caffra) and dead trees (9%). The type of tree selected was unrelated to lizard age class or sex. Multivariate analysis failed to show any age class or sex effects for specific tree physical characteristics, but did reveal a preference for trees with greater diameter, canopy cover and incidence of parasitic plants. We suggest that more ‘complex’ trees may enhance crypsis, facilitate escape from predation, or provide a foraging advantage. Acanthocercus a. atricollis selected night-time perches higher than those used during the day, possibly further offsetting predation risk.  相似文献   

16.
Common broomweed (Amphiachyris dracunculoides [DC] Nutt. Ex Rydb.) is an annual forb that occurs throughout the southern Great Plains, USA. During years of abundant growth, broomweed is problematic because it can reduce grass production and interfere with livestock foraging. In contrast, the canopy structure of broomweed may provide habitat cover for wildlife, including the northern bobwhite quail (Colinus virginianus Linnaeus). During an extreme outbreak of broomweed in north Texas in 2007, we observed apparent differences in broomweed individual plant growth characteristics in mesquite (Prosopis glandulosa Torr.) woodland areas versus areas that had recently been cleared of mesquite. Our objective was to document differences at the individual plant and population levels. Individual plant mass, canopy diameter, and basal stem diameter were much greater in the cleared treatment than the mesquite woodland. In contrast, plant height was greater in the woodland than in the cleared treatment. Population variables of stand-level production, percentage canopy cover, plant density, and visual obstruction were not different between treatments. Total perennial grass production was greater in the cleared than the woodland treatment, because of the negative effect of mesquite on grass production. Variations in broomweed growth characteristics may have implications regarding livestock foraging and wildlife habitat.  相似文献   

17.
Grazing by large mammals alters vegetation physiognomy, consequently changing habitat suitability for small mammal communities. We investigated the response of terrestrial small mammals to grazing by wild and domesticated ungulates at the boundary of a protected area (Telperion Nature Reserve) and surrounding cattle ranches in Mpumalanga, South Africa over two seasons. Fifteen paired grids were set on either side of the boundary fence at which small mammals were trapped in Sherman live traps placed flat on the ground. A total of 11 760 trap nights resulted in the capture of 187 animals belonging to 14 species (11 rodents, two shrews and one elephant shrew). The small mammal communities in grasslands grazed by domesticated or wild ungulates were similar in abundance, species richness, diversity and demographic parameters, likely due to the fact that vegetation structure of the two grazing systems was also similar. We used generalised linear models to show that rock and grass cover were plausible predictors of small mammal abundance in this system. Rock cover showed a positive relationship with small mammal abundance whilst grass cover showed a negative relationship. Our observations suggest that at the scale of our study and with the current stocking densities, wild and domesticated ungulates have similar impacts on the small mammal community.  相似文献   

18.
放牧活动不但影响草原生态过程,而且改变草原群落空间格局。为进行草地冠层连续变化的尺度对放牧强度响应的研究,基于无人机机载激光雷达和多光谱遥感获取亚米级的草原冠层数据,利用小波分析方法研究了不同放牧强度下草原冠层空间格局及其尺度特征,探究草原冠层高度(CH)、植被覆盖度(FVC)、归一化植被指数(NDVI)等不同冠层参数空间尺度特征对放牧强度的响应。结果表明:CH、FVC、NDVI的小波方差随放牧强度增大呈下降趋势,草原冠层空间结构整体趋于均匀;小波方差和系数实部分析显示,CH,FVC和NDVI的空间格局存在170~180 m、116~140 m等2个主特征尺度,不随放牧强度而变化,较为稳定;中度、重度放牧使植物群落在空间中产生了许多小斑块,植被盖度的空间格局在小尺度上的变化较大。  相似文献   

19.
There is limited information about the effects of cattle grazing to longer-term plant community composition and herbage production following fire in sagebrush steppe. This study evaluated vegetation response to cattle grazing over 7 yr (2007–2013) on burned Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis [Beetle & Young] Welsh) steppe in eastern Oregon. Treatments, replicated four times and applied in a randomized complete block design, included no grazing on burned (nonuse) and unburned (control) sagebrush steppe; and cattle grazing at low (low), moderate (moderate), and high (high) stocking on burned sagebrush steppe. Vegetation dynamics were evaluated by quantifying herbaceous (canopy and basal cover, density, production, reproductive shoot weight) and shrub (canopy cover, density) response variables. Aside from basal cover, herbaceous canopy cover, production, and reproduction were not different among low, moderate, and nonuse treatments. Perennial bunchgrass basal cover was about 25% lower in the low and moderate treatments than the nonuse. Production, reproductive stem weight, and perennial grass basal cover were greater in the low, moderate, and nonuse treatments than the control. The high treatment had lower perennial bunchgrass cover (canopy and basal) and production than other grazed and nonuse treatments. Bunchgrass density remained unchanged in the high treatment, not differing from other treatments, and reproductive effort was comparable to the other treatments, indicating these areas are potentially recoverable by reducing stocking. Cover and production of Bromus tectorum L. (cheatgrass) did not differ among the grazed and nonuse treatments, though all were greater than the control. Cover and density of A.t. spp. wyomingensis did not differ among the burned grazed and nonuse treatments and were less than the control. We concluded that light to moderate stocking rates are compatible to sustainable grazing of burned sagebrush steppe rangelands.  相似文献   

20.
North American grasslands make up less than 75% of their historic pre-European settlement area, and they continue to be converted to woodlands by woody plant encroachment. Conversion of grassland to woodland alters nutrient cycling, water use, and light penetration, which drives herbaceous plant community dynamics. Because studies examining this relationship among Juniperus species are limited largely to individual trees, we designed a study to examine the relationship between stand-level canopy cover of eastern redcedar (Juniperus virginiana L.) and the herbaceous plant community. We documented herbaceous plant species composition, abundance, and biomass within a North American tallgrass prairie invaded by eastern redcedar in which canopy cover of eastern redcedar ranged from 0% to 80%. Herbaceous species richness declined as a function of increased canopy cover of eastern redcedar and subsequent loss of open space, but this decrease in species richness closely followed a species–area model. Moreover, composition of C3 and C4 grasses and forbs did not change with increasing canopy cover. Herbaceous biomass, which declined with increasing canopy cover, varied most within those plots with intermediate canopy cover. While we found that species richness and biomass declined as canopy cover increased, the decline followed a species–area relationship and was without abrupt change typical of ecological thresholds. We recommend additional research with removal of eastern redcedar trees over a range of canopy cover to assess restoration potential along the encroachment gradient.  相似文献   

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