The influence of tree density on the grass layer of three semi-arid savanna types of southern Africa

The influence of bush density on the grass component of the herbaceous layer was investigated for three savanna veld types (Molopo Thornveld, Mixed Vaalbos Thornveld, and the Eastern Grass and Bushveld) in the Northern Cape and North-West Province of South Africa. Relationships between bush density and the density, species composition and phytomass production of the grass component were determined over a four-year period. Bush encroachment had an adverse influence on the density and production of the grass component, but not on the species composition. Fluctuations in seasonal rainfall appeared to be the most important factor governing changes in the species composition of the grass component. Bush enchroachment (2 500 tree equivalents ha^?l) decreased the potential grazing capacity by as much as 331%, 149% and 58% in the Molopo Thornveld, the Mixed Vaalbos Thornveld and the Eastern Grass Bushveld, respectively, In comparison to sites with tree densities of less than 400 tree equivalents ha^?1. In the encroached condition grazing capacities of 15, 13 and 10 ha LSU^?1 are recommended for the Molopo Thornveld, the Mixed Vaalbos Thornveld and the Eastern Grass and Bushveld, respectively. The implementation of bush control measures in all problem areas is proposed in order to restore the productivity of these areas in the long term.     

The effects of topo-edaphic variables and cultivation on the woody vegetation of Weenen Nature Reserve,KwaZulu-Natal

The soil types and woody vegetation of Weenen Nature Reserve (WNR), KwaZulu-Natal, were described. Mispah, overlying shale, and Shortlands, overlying dolerite were the most common of 18 the soil forms encountered. Six vegetation types were identified by TWINS PAN which differed in slope, aspect, land-use history, species richness and tree density. Vegetation types ranged from species-rich, dense 'Coddia rudis-Dombeya cymosa Closed Woodland' overlying dolerite and occurring on steep north-east facing slopes that had never been cultivated, to previously cultivated, species-poor 'Acacia sieberiana Open Woodland' on flat topography, overlying shale. Ordination analyses further described the main associated floristic-environmental gradient from steep slopes supporting a high diversity of broad leaved species through to species-poor, flatter areas with an abundance of microphyllous plants. A second main gradient described was from uncultivated sites on steep slopes of usually dolerite-derived soils through to flat, previously cultivated land on shallow soils overlying shale. Topo-edaphic variables and previous cultivation, which were confounded, were therefore the main determinants of floristic variation in WNR, whereas fire had no influence. Half of the dominant woody species exhibited a reverse-J size structure, indicating relatively constant population change. A number of species had a preponderance of individuals in the smallest (0.5–1.5m height) size class, owing possibly to recruitment during the exceptional wet season of 1995/6, and/or the inability of individuals to escape the 'fire-trap'. Several, mostly Acacia, species appear to have recruited intermittently. The vegetation changes of this system will have an affect on browse availability. The woody vegetation of WNR appears to be changing in response to successional processes and management influences.     

Quantified range condition assessment of open Camelthorn savanna along a degradation gradient

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.     

Reforming communal rangeland policy in southern Africa: challenges,dilemmas and opportunities

In the savanna rangelands of southern Africa, the debate about land reform tends to be about the redistribution of formerly freehold ranches and fencing-off the rangeland commons into ranches for better-off African farmers. The position of those who favour privatisation has been strengthened by the belief that the only environmentally sound way to manage the range is to subdivide it into private ranches because traditional open-range pastoralism is environmentally destructive. This point of view is at variance with an ever-increasing body of research. Insufficient attention has been given to the sustainable nature of communal rangeland in savanna areas and to the needs of the poor who depend upon them. Following decades of policies that further reduced and fragmented the commonage, governments have the challenging task of developing and implementing policies for those whose livelihoods depend on more secure and sustainable access to the remaining communal rangelands.     

The delineation of plant communities in relatively homogenous grassland

A relatively homogenous area of grassland was sampled by means of 2204 systematically place ½m² quadrats for the presence or absence of all vascular plants this data was processed by the method of normal association analysis which provided an hierarchical subdivision of the vegetation. This subdivision was considered to be too fragmentary. More meaningful vegetation units were thus obtained by terminating subdivision at a higher level, resulting in a reasonable accurate delineation of the plant communities. The method, with adaptations in field technique suited to the particular vegetation under investigation, is considered a valuable technique in ecological study.     

Effect of management on rangeland phytomass,cover and condition in two biomes in South Africa

In rangelands, grazing management is a main driver of rangeland condition. Due to masking effects of seasonal climate fluctuations, little is known about (dis)similarity of management effects on rangeland condition and forage provision across major dryland biomes. Taking a macro-ecological perspective, we analysed if management effects differed between South Africa’s central grassland and Kalahari savanna biomes. We recorded proxies of forage provision (phytomass, vegetation cover and their ratio) over five seasons, annual rainfall to account for seasonal climate fluctuations, and rangeland condition (through relative abundances of increaser and decreaser species). Regarding forage provision, we found effects of management for the savanna, where, irrespective of rainfall, rotational grazing management resulted in higher phytomass and phytomass–cover ratios than management with continuous grazing. In the grassland, however, this difference was only discernible for phytomass–cover ratio in two years with above-average antecedent rainfall. This suggests that management effects are biome-dependent and that modulating effects of annual rainfall are stronger in the grassland. In either biome, management effects on the dominance of increaser and decreaser species were negligible, i.e. rangeland condition did not differ across management types in either biome. We conclude that investigations on management effects should account for interactions with biome and rainfall.     

Effect of Prosopis (mesquite) invasion and clearing on vegetation cover in semi-arid Nama Karoo rangeland,South Africa

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.     

Impact of Prosopis (mesquite) invasion and clearing on vegetation species composition and diversity in semi-arid Nama-Karoo rangeland,South Africa

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.     

Patch grazing of Tall Grassveld by cattle after a spring burn

Patterns of herbage removal by rotationally stocked cattle (at a density of 4.85AUE ha^-1) on initially structurally homogenous Tall Grassveld were examined over a single season. Three unreplicated camps were stocked early (29 days), at the recommended '10-15cm' sward height (44 days), and late (80 days) after a spring burn. Temporal change in the spatial distribution of herbage in each treatment was described by intensive measurement of compressed sward height (CSH) with a disc pasture meter and analysed with parametric and spatial statistics. Grazing pressure at the start of the grazing was highest in the early graze, intermediate at the recommended, and least in the late graze treatment because of variation in initial herbage mass related to period of deferment after the burn. The early graze paddock was most spatially uniform and the late graze paddock the most uneven and patchy before grazing. Such initial conditions significantly affected subsequent herbage removal patterns: the greater the amount and spatial variability of herbage accumulated before grazing, the greater the chance for patches to develop through area-selective grazing. The early graze paddock was utilised non-selectively during the first two grazing periods but similar to other treatments, became patchier with time. Spatial pattern, a mosaic of short, intermediate and tall patches, peaked in late summer and was most pronounced in the late graze treatment. Intensive, early stocking after a burn is recommended for minimising intraseasonal development of patches and efficient use of high-quality forage available after a burn.     

The effect of prolonged heavy grazing pressure on the regrowth of two perennial grass species in a semi-arid communal rangeland

We tested the hypothesis that initial above-ground regrowth of defoliated grass tufts would be reduced in sites which have been exposed to prolonged heavy grazing. The regrowth of Themeda triandra and Urochloa mosambicensis tufts was compared between three heavily grazed sites close to a village and three relatively lightly grazed sites far from a village, in a semi-arid communal rangeland in the South African lowveld. A clipping experiment was done in the field using T. triandra and U. mosambicensis tufts. Regrowth was lower (P≤0.01) in the heavy grazing sites for both species. This was more a function of tiller production rate, in terms of numbers of tillers, than of tiller extension rate. Rate of regrowth was not strongly affected by tuft size.     

The ecology of sandy beaches in southern Africa

The physical features, macrofauna and meiofauna of four exposed sandy beaches along the southern coast of South Africa were quantitatively investigated. All beaches had medium to fine sands with relatively poor to moderate macrofauna and very rich meiofauna. The results are compared with the present knowledge of the southern African coastline. Sandy beaches types around the southern African coastline are summarized according to geomorphology and wave action and three Zoogeographie provinces are recognized. The macrofauna is dominated by crustaceans, mostly scavengers, on the warmer east coast and by molluscs, mostly filterfeeders, on the temperate south coast. Relationships between diversity and abundance of macrofauna, beach slope and particle size are analysed in detail. Intertidal zonation of macrofauna is described and a zonation scheme based on crustaceans proposed. Relationships between meiofauna composition and particle size are described as well as intertidal distribution patterns of the meiofauna. The role of surf circulation patterns and macrofaunal food sources in determining beach trophic structure is emphasized.     

Seasonal diet composition and forage selectivity of Boer goats in a semi-arid gypsophilous grassland

Botanical and chemical compositions of Boer goat diets, determined with repeated collection of forage samples taken from the goat’s mouth, were studied in a gypsophilous grassland during four seasons of the year. Ten pluriparous goats were used to collect selected forage species. Shrubs were a minor dietary component throughout the year (<7.5%). Goats selected more (p?<?0.01) grass during autumn and winter (34.2–37.8%) than during spring (21.2%) and summer (29.0%). During all seasons goat diets were dominated by forbs (>59%). Diets were lowest (p?<?0.05) in percentage of crude protein during spring, summer and autumn (13.2?±?3.5, 13.2?±?2.4 and 14.2?±?3.9, respectively) than winter (17.5?±?2.9) with levels above the recommended quantities for goats throughout the year. In vitro dry matter digestibility of forages selected by goats did not differ between seasons (range 47.4–53.3%). It was concluded that in this particular ecosystem goats preferred forb species over grasses across all seasons. In addition, goats maintained relatively stable diet quality by forage-class mixing of diets.     

我国南方林间草地的作用及其放牧利用技术

我国南方6 500多万hm2草山草坡中,有相当大一部分是林间草地,合理利用林间草地,对促进我国南方节粮型草地畜牧业的发展将起到积极的促进作用.但由于放牧中一些关键的控制技术未能很好地解决,而使林间草地未能得到充分的开发利用.通过论述林间草地利用的重要性,分析林间草地放牧利用的优势,提出了林间草地放牧利用的技术措施.     

草原的生态系统服务:III.价值和意义

连续综述了中国草原/草地生态系统服务价值的构成,草地生态系统服务的总价值,各类型草地的生态系统服务的价值。论证了草原/草地为中国提供了最大的生态系统服务价值,草地生态系统服务的生态价值远大于经济价值。论述了草地生态系统服务价值评估具有下列重要意义: 1. 指明了草地生态系统服务的具体项目及其不可代替性; 2. 确定了草地生态资本的价值; 3. 促进了环境成本核算和实现绿色GDP。文章在最后还讨论了草原/草地生态系统服务价值评估中存在的问题。     

Fire and the dynamics of a semi-arid grassland: influence on soil characteristics

The short-term impact of fire burning with the wind (head fire) and against the wind (back fire) on soil characteristics (soil-water content, soil compaction, soil temperature, organic matter, pH, exchangeable Ca, Mg, K, Na and extractable P) were determined in a semi-arid grassland. This study was conducted over two growing seasons (2000/01 and 2001/02) after only one burning treatment in August 2000. The back fire moved approximately 7.5 times slower than the head fire, leading to the greatest change in hydrological characteristics. The decline in plant cover due to the fire resulted in a considerable increase in soil compaction, soil temperature and a decrease in organic material in turn leading to a lowered soil-water content. Much of the influence of these variables had not been completely reduced two growing seasons after the fire. Burning reduced the soil-water content (0mm to 300mm soil depth) after one month and one year by 13.4% and 31% respectively. Temporary wilting for all grass species occurred more rapidly with burning. During March of the first year, the greatest soil temperature difference (50mm depth) occurred between burnt and unburnt grassland, with the highest soil temperatures recorded in January. The highest temperature on the surface of the soil for each year after burning also occurred during January. Over the first growing season burning significantly reduced organic carbon (C) and total nitrogen (N) content and extractable phosphate, but increased the pH and concentration of exchangeable Ca, Mg, Na and K in the soil, over the first 0–50mm layer. There were no differences in the soil properties of head and back fires, except for organic material that was further lowered by the back fire. With depth, organic matter and K decreased significantly and Ca increased between surface (0mm to 50mm) and lower (50mm to 100mm) layers due to burning. Thus, an unplanned event can increase the intensity and frequency of seasonal droughts in the ecologically sensitive semi-arid grassland areas leading to increase risk management in the short-term.     

Statistical power of commonly used point methods in grassland monitoring

It has become almost universally accepted that the management of conservation areas and game ranches be subject to pre-determined sets of objectives and goals. Key to management decision making are the results of vegetation monitoring protocols that are designed to indicate changes in vegetation status. These changes are frequently measured as a proportional species composition and measure of basal cover and based on samples of 100-200 points in specific fixed sites. Minor changes of the magnitude of the order of single percentage points often form the basis of a management decision relating to herbivore stocking rates. These values are often quoted sans confidence limits or any other measure of dispersion. This paper investigates the statistical power of within stand replicates of three commonly encountered point methods, replicated within the same stand on Suikerbosrand Nature Reserve, Gauteng Province, and shows unacceptably high Type II error probabilities.     

赤峰市草地现状及生态治理对策

沙漠化的严重后果已不容忽视,我国是世界上沙漠化危害严重的国家之一.赤峰市位于我国最典型的土地沙漠化区域,过度放牧使草地沙化加速,是形成沙漠化的主要人为因素之一.从可持续发展的角度,对赤峰市的草地情况进行了分析,提出了治理荒漠化草地及防止草地退化的一些建议.     

我国南方地区天然牧地营养价值和生产能力评价及其开发潜力

在我国南方地区,西部天然牧地的营养价值与东部比较接近;生产力后者较高;生产量前者显著高.中高山地带天然牧地的营养价值高于丘陵低山地带,但生产力和生物生产量低于后者;营养生产量,西部以前者高,东部以后者高.与北方牧区比较,南方地区天然牧地的生产力较高,营养价值和生产量较低.从开发潜力看,南方地区西部高海拔地带以人工牧地较大;东部以农林用地生产饲用植物的较大.     

Aboveground biomass production of a semi-arid southern African savanna: towards a new model

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.     

The phyllophorid holothurians of southern Africa with the erection of a new genus

The southern African dendrochirotid holothurians of the family Phyllophoridae (sensu Pawson & Fell, 1965) are revised and a new genus Thyonina is erected to accommodate Thyone articulata Vaney, 1908, which possesses only slender spectacle-shaped rods as body wall deposits. In addition, Thyone proceracorona Cherbonnier, 1952 and T. turrisolida Cherbonnier, 1954, are declared junior subjective synonyms of T. aurea (Quoy & Gaimard, 1833); Havelockia imperfecta Cherbonnier, 1970 is transferred to the genus Thyone, and T. venusta Selenka, 1868 (synonym T. okeni Bell, 1884) is recorded for the first time from southern Africa. These changes bring the total number of phyllophorid holothurians now known from southern Africa to five genera and 11 species. All species are keyed, briefly described and/or discussed, and their local distributions mapped.