Plant spinescence in arid southern Africa: does moisture mediate selection by mammals?

Summary The prediction that spinescence in plants increases with aridity, soil fertility and mammalian herbivory was examined at regional and local scales in southern Africa. Spinescence tended to increase with aridity. Within arid areas, vegetation of moist, nutrient-rich habitats was more spinescent than that of the surrounding dry plains. Spinescence in plants of drainage lines and pans in arid southern Africa occurs in a wide range of genera and appears to have been selected by the effect of large mammals which concentrate on these moist patches. It is concluded that spinescence may be selected by breakage as well as herbivory, and that in arid areas moisture may be important in mediating mammalian selection of spinescence.     

Growth,flowering and recruitment of shrubs in grazed and in protected rangeland in the arid Karoo,South Africa

A three-year study involving 1300 marked shrubs of three species differing in palatability to sheep provided evidence that changes in the demographic structure of shrub populations in arid rangelands are a function of seed availability. Sheep reduced canopy size and flowering of their preferred forage species but had no effect on survivorship of seedlings or established plants during the study. Although annual turnover in all populations was low (<6%), abundant recent recruitment of non-forage species was observed on degraded rangeland. Natality of all three species was positively correlated with flower abundance. Mortality was correlated with natality because seedlings had a low probability of surviving their first year. Recruitment of both forage and non-forage species, appeared to be inhibited by competition and seedlings seldom survived near adult plants of the same growth form.     

Vegetation and condition of arid rangeland ecosystem in Central Saudi Arabia

Saudi Arabia rangeland ecosystems have undergone intense processes of degradation for many decades because of extreme climate and human activities such as overgrazing and socioeconomic changes. In this study, Hail and Qassim Regions of Saudi Arabia covering an area about 79610.73 km² were selected to study the rangeland vegetation and condition. Haloxylon salicornicum was the most dominant species, covering more than 56% of the total area. The second prominent community was Acacia-Lycium shawii, which covers about 21% of total area. It was found that about 65% of vegetation in the surveyed area is in good or very good condition compared with about 31% in poor or deteriorated condition. Effective measures such as determination of carrying capacities and development of grazing systems have to be implemented to ensure resources sustainably.     

Herbaceous vegetation change in variable rangeland environments: The relative contribution of grazing and climatic variability

Abstract. A 44‐yr record of herbaceous vegetation change was analysed for three contrasting grazing regimes within a semi‐arid savanna to evaluate the relative contribution of confined livestock grazing and climatic variability as agents of vegetation change. Grazing intensity had a significant, directional effect on the relative composition of short‐ and mid‐grass response groups; their composition was significantly correlated with time since the grazing regimes were established. Interannual precipitation was not significantly correlated with response group composition. However, interannual precipitation was significantly correlated with total plant basal area while time since imposition of grazing regimes was not, but both interannual precipitation and time since the grazing regimes were established were significantly correlated with total plant density. Vegetation change was reversible even though the herbaceous community had been maintained in an altered state for ca. 60 yr by intensive livestock grazing. However, ca. 25 yr were required for the mid‐grass response group to recover following the elimination of grazing and recovery occurred intermittently. The increase in mid‐grass composition was associated with a significant decrease in total plant density and an increase in mean individual plant basal area. Therefore, we failed to reject the hypotheses based on the proportional change in relative response group composition with grazing intensity and the distinct effects of grazing and climatic variability on response group composition, total basal area and plant density. Long‐term vegetation change indicates that grazing intensity established the long‐term directional change in response group composition, but that episodic climate events defined the short‐term rate and trajectory of this change and determines the upper limit on total basal area. The occurrence of both directional and non‐directional vegetation responses were largely a function of (1) the unique responses of the various community attributes monitored and (2) the distinct temporal responses of these community attributes to grazing and climatic variation. This interpretation supports previous conclusions that individual ecosystems may exist in equilibrial and non‐equilibrial states at various temporal and spatial scales.     

Explaining grass-nutrient patterns in a savanna rangeland of southern Africa

Aim The search for possible factors influencing the spatial variation of grass quality is an important step towards understanding the distribution of herbivores, as well as a step towards identifying crucial areas for conservation and restoration. A number of studies have shown that grass quality at a regional scale is influenced by climatic variables. At a local scale, site factors and their interaction are considered important. In this study, we aimed at examining environmental correlates of grass quality at a local scale. The study also sought to establish if biotic factors interact significantly with abiotic factors in influencing a variation in grass quality. Location The study area is located in the Kruger National Park of South Africa. The study area stretches from west (22°49′ S and 31°01′ E) to east, (22°44′ S and 31°22′ E) covering an area of about 25 × 6 km in the far northern region of the Kruger National Park. Methods We collected environmental data such as soil texture, percentage grass cover and biomass as well as grass samples for chemical analysis from specific locations in the study area. In addition, a digital elevation model (DEM) with a resolution of 5 m was used to derive elevation, slope and aspect using a geographic information system (GIS), which were related to grass quality. We used correlation analysis and anova to relate environmental variables to grass quality. Multivariate analysis techniques were used to simultaneously analyse and explore the complex interactions between variables. Results and conclusions Our results indicate that there is a significant relationship between grass quality parameters and site‐specific factors such as slope, altitude, percentage grass cover, aspect and soil texture. Relatively, percentage grass cover and soil texture were more critical in explaining a variation in grass quality. Plant characteristics such as species type interact significantly with slope, altitude and geology in influencing nutrient distribution. The results of this study may give a better insight on foliar nutrient distribution patterns at a landscape scale in savanna rangelands. Furthermore, the results of this study may help in the selection of ancillary information, which could be used in conjunction with other data such as remotely sensed data to map grass quality – an important step towards understanding the distribution and feeding patterns of wildlife. However, we acknowledge that this study is based on one seasonal snapshot, therefore some slightly different findings may be obtained during other times of the year. Nevertheless, the study has revealed that under the conditions experienced during the study period, nutrient distribution varies with varying biotic and abiotic factors.     

Variability in glacial and Holocene marine pollen records offshore from west southern Africa

The distribution of pollen in marine sediments is used to record vegetation changes over the past 30,000 years on the adjacent continent. A transect of marine pollen sequences from the mouth of the river Congo (∼5°S) to Walvis Bay and Lüderitz (∼25°S) shows vegetation changes in Congo, Angola and Namibia from the last glacial period into the Holocene. The comparison of pollen records from different latitudes provides information about the latitudinal shift of open forest and savannahs (Poaceae pollen), the extension of lowland forest (rain forest pollen) and Afromontane forest (Podocarpus pollen), and the position of the desert fringe (pollen of Caryophyllaceae, Chenopodiaceae and Amaranthaceae). High Cyperaceae pollen percentages in sediments from the last glacial period off the mouth of the river Congo suggest the presence of open swamps rather than savannah vegetation in the Congo Basin. Pollen from Restionaceae in combination with Stoebe-type pollen (probably from Elytropappus) indicates a possible northwards extension of winter rain vegetation during the last glacial period. The record of Rhizophora (mangrove) pollen is linked to erosion of the continental shelf and sea-level rise. Pollen influx is highest off river mouths (10–2000 grains year⁻¹ cm⁻²), close to the coast (300–6000 grains year⁻¹ cm⁻²), but is an order of magnitude lower at sites situated far from the continent (<10 grains year⁻¹ cm⁻²).     

Plant species richness and composition a long livestock grazing intensity gradients in a Namaqualand (South Africa) protected area

The study described changes in floristic and vegetation structure in relation to livestock grazing intensity in a conservation area in the Succulent Karoo, South Africa. Grazing by goats and sheep is allowed in the Richtersveld National Park (a contractual National Park. which is also an area of high floristic richness and endemism. We used goat faecal pellet density, degree of trampling and percentage bare-ground at distances from the stock posts as surrogates for a gradient in grazing pressure. A stock post is the place where farmers keep, in most cases in an enclosure called a ‘kraal’, their animals at night and to which they return every evening after the day’s herding. Twenty-seven stock posts were located in the Richtersveld National Park; nine stock posts on flats, foot– slopes and mountain each. We measured plant species richness and diversity, and mean percentage cover of the various plant growth forms (including the number of species falling into each growth form category. in each of the five 10 m . 10 m plots (each 200 m apart. demarcated along a transect of one kilometre length from the centre of each stock post. The results showed that distance from the stock post does reflect grazing intensity use because densities in faecal pellets rapidly declined with increasing distances away from the stock post for all habitats studied. Faecal density was positively correlated with stocking density. Plant species richness and diversity was at a minimum near stock posts. Plants able to endure the effects of heavy grazing occurred near stock posts where declines in palatable plant species, assumingly sensitive to heavy grazing and trampling, were recorded. Grazing increased vegetation patchiness up to 800 m from the stock post for all the habitats. The degree to which this change in species composition occurred did not depend on stocking densities, suggesting that both grazing and landscape variability were responsible for vegetation changes in rangelands of that area of the Succulent Karoo biome.     

Towards a density measure for assessments of arid plant populations – examples from the southern Namib Desert

Plant species in arid environments are often widely spaced making population assessments difficult. In order to minimise observer bias we tested two repeatable simple methods during red data assessments in the southern Namib Desert. Both the transect‐based and the quadrat‐based method resulted in over‐estimates of the true population size. However, because of case of application, the transect method warrants further development.     

The role of domestic herbivores in endozoochorous plant dispersal in the arid Knersvlakte,South Africa

Herbivores can act as dispersal vectors by purposely or accidentally ingesting ripe fruits, and thus endozoochory is one determining factor for plant distribution patterns. The objective of our study was to investigate to what extent plants of major taxonomic groups of the Knersvlakte (Succulent Karoo, South Africa) are endozoochorously dispersed. On three different farms in the central Knersvlakte, dung of domestic herbivores was collected and analysed by the seedling-emergence method. The resulting species composition was compared to the standing vegetation of thirty-four 1000-m² plots each recorded on one hundred 400-cm² subplots. Our results show that domestic livestock facilitated the dispersal of taxa characteristic of the Knersvlakte, in particular Aizoaceae. Among the taxa of this family, the local endemic dwarf shrub Drosanthemum schoenlandianum emerged with the highest frequency in dung (14.5% of all seedlings). For the Asteraceae, which are frequent in the standing vegetation of the Knersvlakte, however, endozoochorous dispersal by livestock was only of minor importance. Conservation planning should consider these dispersal patterns on behalf of future population dynamics. The complete exclusion of livestock might change current processes and thus alter vegetation patterns.     

Alien plant assemblages near roads in arid and semi-arid South Africa

Herbaceous and woody alien plants visible from a moving vehicle were recorded along 1 km roadside transects at 5 km intervals over a distance of 5869 km in the semi-arid and arid Fynbos, Succulent Karoo, Nama Karoo and Arid Savanna (Kalahari) biomes in South Africa. Each 1 km transect was classified by biome and vegetation type, mean annual rainfall, rainfall seasonality, soil surface type and landuse adjoining the roadside. Although travelling speed affected the range and frequency of plant species observed, the method was repeatable at a speed of 100 km h^?1. Alien plants occurred in 98% of 119 Fynbos, 81% of 204 Succulent Karoo, 72% of 661 Nama Karoo, 47% of 171 Arid Savanna and 100% of seventeen Grassland transects. Alien species richness per site was correlated with mean annual rainfall, but in all regions, sites adjacent to cultivation had significantly more alien species than sites adjoining rangeland. The alien plant assemblage of the arid winter-rainfall Succulent Karoo included species from mesic winter-rainfall lowland Fynbos and the arid Nama Karoo receives unseasonal rainfall. The frequencies of Prosopis spp., Atriplex spp. and Opuntia ficus-indica were not significantly greater near cultivation, and these perennial plants, all of which are dispersed by indigenous and domestic animals, can invade natural rangeland in arid and semi-arid southern Africa.     

Land use,rangeland degradation and ecological changes in the southern Kalahari,Botswana

Dual‐scale analyses assessing farm‐scale patterns of ecological change and landscape‐scale patterns of change in vegetation cover and animal distribution are presented from ecological transect studies away from waterpoints, regional remotely sensed analysis of vegetation cover and animal numbers across the southern Kalahari, Botswana. Bush encroachment is prevalent in semi‐arid sites where Acacia mellifera Benth. is widespread in communal areas and private ranches, showing that land tenure changes over the last 40 years have not avoided rangeland degradation. Herbaceous cover is dominated in intensively grazed areas by the annual grass Schmidtia kalahariensis Stent and in moderately grazed areas by the perennial grass Eragrostis lehmanniana Nees. Nutritious perennial grass species including Eragrostis pallens Hack. Ex Schinz remain prevalent in Wildlife Management Areas. Other ecological changes include the invasion of the exotic Prosopis glandulosa Torr. and dense stands of Rhigozum trichotomum Kuntze. in the arid southwest. Regional patterns of wildlife species show that the expansion of cattleposts and fenced ranches has led to large areas of low wildlife conservation value even in areas where cattle production is not practiced. Findings show the need for integrated landscape‐scale planning of land use if the ecological value and biodiversity of the southern Kalahari is to be retained.     

Long-term population changes in the Giant Quiver Tree, Aloe pillansii in the Richtersveld, South Africa

With probably fewer than 3000 individuals alive in the biodiversity hotspot of the Succulent Karoo in southern Africa, populations of the endemic, Giant Quiver Tree, Aloe pillansii, are thought to be declining and thus threatened with extinction. Using repeat photography and field data we investigated the long-term changes in one population of A. pillansii at its type locality, the roughly 100 ha Cornell’s Kop in the Richtersveld, South Africa. There are currently 75 individuals alive at this site. Of these, 44% are <1 m in height (seedlings), 4% are 1–3 m (juveniles) and 52% are >3 m (adults). An analysis of 14 repeat photographs shows that since 1937 an average of 1.4% of the plants >3 m in height has died annually. At this rate all the remaining 39 plants on Cornell’s Kop in this size class will be dead in 71 years. The relative paucity of plants in the 1–3 m size classes could be explained by several factors including plant theft, animal damage and unfavourable recruitment conditions during the first 80 years of the 20th century. Annual growth rates decrease as plants age. Individuals <1 m in height grow at 42.5 mm yr⁻¹ while plants 1–3 m and those >3 m grow at 31.0 and 16.4 mm yr⁻¹ respectively. At 8 m, the tallest plant on Cornell’s Kop could be as old as 382 years and thus to maintain itself at this site, A. pillansii would only need to recruit relatively infrequently. The relatively high proportion of seedlings suggests that conditions have recently been favourable for recruitment at this site. Seedling ages, estimated from their heights, indicate that over 50% of the plants <1 m in height germinated 5–10 years ago. This is consistent with local rainfall records which show that rainfall was consistently above the long-term annual average of 75 mm during this period. However, the loss of six seedlings from the population in the last 5 years, probably due to grazing or theft, suggests that without intervention this species will not survive on Cornell’s Kop.     

Biogeography of prostrate-leaved geophytes in semi-arid South Africa: hypotheses on functionality

Nowhere is the species diversity of geophytes greater than in the five mediterranean-climate ecosystems of the world. Of these, the Cape mediterranean zone of South Africa is the most speciose. While the relative diversity and importance of geophytes of all of the other four mediterranean regions of the world drops off sharply as one moves into adjacent winter-rainfall desert regions, geophytes in the semi-arid to arid Succulent Karoo (including Namaqualand) remain a very important component of the flora, both in terms of abundance and diversity (comprising 13 to 29% of the regional floras in this region). Apart from species richness, there are also a number of interesting geophyte growth forms in this region. One unusual growth form is geophytes with flattened leaves that lie prostrate on the soil surface. At least eight families (Amaryllidaceae, Colchicaceae, Eriospermaceae, Geraniaceae, Hyacinthaceae, Iridaceae, Orchidaceae and Oxalidaceae) exhibit this growth form. While this growth form is relatively common in many geophyte lineages in the Succulent Karoo biome and the Cape mediterranean zone (Fynbos biome), and occurs infrequently through the summer-rainfall temperate regions of Africa, it is virtually absent in other regions worldwide. A null hypothesis is that the prostrate leaved trait is a neutral characteristic, however biogeographical data do not support this. A neutral trait would be unlikely to show such a clear pattern of distribution. Several alternative hypotheses on the adaptive significance of this growth form are discussed. These include: avoidance of herbivory, reduction in competition from neighbors, creation of a CO₂ enriched environment below the leaves, reduction of water loss around the roots, reduction of water loss through transpiration, precipitation of dew on the leaves and maintenance of optimal leaf temperatures for growth.     

Diets and habitat preferences of three species of crocidurine shrews in arid southern Africa

The diets and habitat preferences of three species of crocidurine shrews, Crocidura cyanea, C. fuscomurina , and C. hirta (Insectivora: Soricidae), were studied in arid areas of Namibia in March and April, 1992. Simultaneous sampling of the prey ingested and available along foraging trails showed that, although the shrews are generalist insectivores that take most types of prey from the range available, each species selected Araneida, Chilopoda, Isoptera, and insect larvae but avoided Formicidae. The preferred prey were mostly soft-bodied with a relatively high ratio of body water to energy content, which may assist in maintenance of body water balance. The three species showed inconsistent preferences for rarer categories of prey such as Thysanura and Orthoptera, as well as for Coleoptera. Each species of shrew selected sites providing dense ground level vegetation and deep leaf litter, With the preference for cover being strongest in C. fuscomurina. Dense vegetation probably provides shelter from predators and high daytime temperatures; both C. fuscomurina and C. hirta used denser vegetation by day than by night. Each species foraged in leaf litter, perhaps increasing access to preferred types of prey. Preference for moist soil, especially in C. fuscomurina , probably also reflects increased access to preferred prey, although moist soil may also facilitate burrow construction. Because patterns of habitat and diet selection appear to be linked, future research on resource use in these shrews should be pursued most fruitfully in field experiments or captive laboratory trials.     

Climate variability and change over southern Africa: impacts and challenges

In this paper, the influence of climate variability and change on the environment was studied over southern Africa using ground-based and remotely sensed data. A time series analysis of rainfall and temperature anomalies indicated that there was a high rainfall and temperature variability in the region. The influence of global teleconnections on rainfall patterns over southern Africa showed that in some areas there was a spatial variation in their strength, increasing from west to east. Maps of NDVI, from 1982 to 2004, showed that changes in vegetation cover were more apparent during the dry season than during the wet season. The study also revealed that climate variability and change are linked to decreasing rainfall and hence, decreasing regional water resources and biodiversity and increasing environmental degradation. With the regional population expected increase, this depletion of resources poses the greatest regional environmental challenge to humankind.     

Effects of invasion of fire‐free arid shrublands by a fire‐promoting invasive alien grass (Pennisetum setaceum) in South Africa

Arid shrublands in the Karoo (South Africa) seldom accumulate sufficient combustible fuel to support fire. However, as a result of invasion by an alien perennial grass (Pennisetum setaceum), they could become flammable. This paper reports on an experiment to assess the effects of fire following invasion by P. setaceum. We established 10 plots (5 × 10 m) separated by 2.5 m, and added grass fuel to five plots (5 and 10 tons ha^?1 to alternate halves of the plot) leaving the remaining five plots as interspersed controls. Plots with fuel added were burnt, and fire behaviour was measured during the burns. Rates of fire spread were generally low (0.01–0.07 m s^?1) and did not differ significantly between burn treatments. Mean fireline intensities were higher in the high compared with the low fuel treatments (894 and 427 kW m^?1, respectively). We recorded plant species and their cover before and after burning on each of the plots. After 15 months of follow‐up monitoring in the burn plots, only two species, the dwarf shrub (Tripteris sinuata) and the perennial herb (Gazania krebsiana) resprouted. Most individuals of other species were killed and did not reseed during the 15‐month study. The mass of added fuel load (high or low) did not influence vegetation recovery rates after fire. Should future invasions by P. setaceum lead to similar fuel loads in these shrublands, inevitable fires could change the vegetation and may favour spread of the flammable grass. Our results have important implications for predicting the effects of invasive alien plants (especially grasses) on fire‐free ecosystems elsewhere. The predicted impacts of fire may alter species composition, ultimately affecting core natural resources that support the Karoo economy.     

Phenological description of natural vegetation in southern Africa using remotely‐sensed vegetation data

Abstract. Various attempts have been made to describe and map the vegetation of southern Africa with recent efforts having an increasingly ecologi cal context. Vegetation classification is usually based on vegetation physiognomy and floristic composition, but phenology is useful source of information which is rarely used, although it can contribute functional information on ecosystems. The objectives of this study were to identify a suite of variables derived from time‐series NDVI data that best describe the phenological phenomena of vegetation in southern Africa and, secondly, to assess a classification of pixels of the study area based on NDVI variables using a preexisting map of the biomes that was delimited on the basis of life forms and climate. A number of variables were derived from the satellite data for describing phenological phenomena, which were analysed by multivariate techniques to determine which variables best explained the variation in the satellite data. This set of variables was used to produce a phenological classification of the vegetation of southern Africa, the results of which are discussed in relation to their concordance with the existing biome boundaries.