Combretum Apiculatum Research Articles

Deciduous sapling responses to season and large herbivores in a semi‐arid African savanna

AbstractImpacts of large herbivores (>5 kg) on woody plants in African savannas are potentially most severe among plants shorter than 1.6 m. It is well established that severe browsing leads to longer shoots, yet prevents saplings from recruiting into adult size‐classes in African savannas. Increased shoot length, indicating faster shoot growth, is often associated with reduced concentrations of tannins and increased nutrient concentrations, suggesting carbon limitation. We hypothesized that, on average, large herbivores suppress stem height or circumference, but increase shoot length. We also hypothesized that if there were concomitant positive effects on nutrients, or negative effects on tannin concentrations, they would be greatest early in the wet season. We sampled saplings of four deciduous woody species (Acacia grandicornuta, Dichrostachys cinerea, Combretum apiculatum and Grewia flavescens) at different stages of the wet season in a large‐scale, long‐term herbivore exclusion experiment in Kruger National Park, South Africa. Plant height, shoot length and stem circumference were generally not adversely affected by large herbivores, suggesting C limitation is rarely present among deciduous saplings in semi‐arid African savannas, allowing them to tolerate browsing. Time since first rainfall emerged as a predominant factor consistently affecting nutrient and tannin concentrations, rather than large herbivores. Nitrogen and phosphorus generally decreased (by 20–50%), while condensed tannin concentration increased (150–350%) during the wet season, except for one species. We postulate that A. grandicornuta is less prone than other species to accumulating tannins during the wet season because of high investment of C in spines. Although nutrient and tannin concentrations were generally not affected by large herbivores, species‐specific responses were evident very early in the wet season, which is when herbivore populations are most likely to be affected by differential forage quality among plants.

Isolation of antioxidant constituents from Combretum apiculatum subsp. apiculatum

Species of the family Combretaceae are used extensively in traditional medicine against inflammation and infections, and although antibacterial activity has been reported in non-polar extracts, further rationale for the widespread use of the Combretaceae is expected to exist. Methanol extracts of leaves of ten different Combretum species were evaluated for antioxidant activity by spraying TLC chromatograms of each leaf extract with 2,2-diphenyl-1-picrylhydrazyl (DPPH). Compounds with antioxidant activity were detected by bleaching of the purple DPPH colour. Leaf extracts of Combretum apiculatum subsp. apiculatum had the most antioxidant compounds. This species was consequently selected for phytochemical investigation. A DPPH assay-directed fractionation of the leaf extracts of C. apiculatum led to the isolation of four antioxidant compounds from the ethyl acetate and butanol soluble fractions. The structures of the compounds were determined by spectroscopic analyses (1H-NMR, 13C-NMR and MS) and identified as: cardamonin (1), pinocembrin (2), quercetrin (3) and kaempferol (4). In a quantitative antioxidant assay, the more polar fractions (ethyl acetate and butanol) obtained by solvent–solvent fractionation had the highest antioxidant activity among the solvent fractions obtained from C. apiculatum, with EC50 values of 3.91±0.02 and 2.44±0.02μg/ml respectively. Of the four isolated compounds, quercetrin (4) and kaempferol (3) had the strongest antioxidant activity, with EC50 values of 11.81±85 and 47.36±0.03μM respectively. Cardamonin (1) and pinocembrin (2) did not demonstrate strong activity. L-ascorbic acid was used as standard antioxidant agent (EC50=13.37±0.20μM or 2.35μg/ml). The cytotoxicity of cardamonin and pinocembrin was evaluated on Vero kidney cells using the MTT (3-(4,5-dimethylthiazol)-2,5-diphenyl tetrazolium bromide) assay with berberine as positive control. At concentrations higher than 50μg/ml of cardamonin or pinocembrin, the cells were not viable. Cardamonin was more toxic (LC50=1.97μg/ml) than pinocembrin (LC50=29.47μg/ml) and even the positive control, berberine (LC50=12.35μg/ml).

Additions to the rust fungi of South Africa

This paper presents new species, combinations, national reports and host records for the South African rust fungi (Uredinales/Pucciniales). Endophyllum mpenjatiense on cf. Hibiscus sp. (Malvaceae), Phakopsora combretorum (anamorph Uredo combreticola) on the new host Combretum apiculatum (Combretaceae) and Uredo sekhukhunensis on Ziziphus mucronata (Rhamnaceae) are described as new species. Dietelia cardiospermi and E. metalasiae are proposed as new combinations to replace Aecidium cardiospermi on Cardiospermum halicacabum (Sapindaceae) and A. metalasiae on Metalasia spp. (Asteraceae), respectively. Four species are new records for South Africa: Crossopsora antidesmae-dioicae on Antidesma venosum (Euphorbiaceae), Phakopsora ziziphi-vulgaris on Z. mucronata, and Uromyces cypericola and Puccinia subcoronata, both on a new host, Cyperus albostriatus (Cyperaceae). The record of P. subcoronata is the first one from outside the New World. Puccinia scirpi is reported as a possible addition to the South African rust fungi. New host records and observations are presented for Pucciniastrum agrimoniae that is recorded on two new host genera and species, Cliffortia odorata and Leucosidea sericea (Rosaceae), Uromyces cypericola whose urediniospores are described for the first time, Phakopsora stratosa in that spermogonia and Uredo-like aecia were discovered, and for Sphaerophragmium dalbergiae in that characters of the urediniospores are re-evaluated. A lectotype is selected for Aecidium garckeanum and spermogonia are reported for this rust for the first time. The rust fungi of Ehrharta (Poaceae) are discussed and critically evaluated in the light of spore morphology and host species.

Differential phenolic profiles in six African savanna woody species in relation to antiherbivore defense

Low molecular weight phenolics are suggested to have a role in mediating diet selection in mammalian herbivores. However, very little is known about low molecular weight phenolic profiles of African savanna woody species. We determined low molecular weight phenolic profiles of six woody species with different life history, morphological and functional traits. We investigated interspecific phytochemical variation between species and found that: (1) related Acacia species were chemically dissimilar; (2) similarity percentage analysis revealed that Acacia grandicornuta was most dissimilar from other species and that the evergreen and unpalatable Euclea divinorum had a qualitatively similar chemical profile to the deciduous and palatable Acacia exuvialis and Combretum apiculatum; (3) C. apiculatum had the highest chemical diversity; (4) relative to spineless plants, spinescent plants contained significantly less HPLC phenolics and condensed tannins; and (5) the major quantitative difference between the evergreen and unpalatable E. divinorum and other species was its high myricitrin concentration.

Improving Discrimination of Savanna Tree Species Through a Multiple-Endmember Spectral Angle Mapper Approach: Canopy-Level Analysis

Differences in within-species phenology and structure are controlled by genetic variation, as well as topography, edaphic properties, and climatic variables across the landscape, and present important challenges to species differentiation with remote sensing. The objectives of this paper are as follows: 1) to evaluate the classification performance of a multiple-endmember spectral angle mapper (SAM) classification approach in discriminating ten common African savanna tree species and 2) to compare the results with the traditional SAM classifier based on a single endmember per species. The canopy spectral reflectance of the tree species ( Acacia nigrescens, Combretum apiculatum , Combretum imberbe, Dichrostachys cinerea, Euclea natalensis, Gymnosporia buxifolia, Lonchocarpus capassa, Pterocarpus rotundifolius, Sclerocarya birrea, and Terminalia sericea) was extracted from airborne hyperspectral imagery that was acquired using the Carnegie Airborne Observatory system over Kruger National Park, South Africa, in May 2008. This study highlights three important phenomena: 1) Intraspecies spectral variability affected the discrimination of savanna tree species with the SAM classifier; 2) the effect of intraspecies spectral variability was minimized by adopting the multiple-endmember approach, e.g., the multiple-endmember approach produced a higher overall accuracy (mean of 54.5% for 20 bootstrapped replicates) when compared to the traditional SAM (mean overall accuracy = 20.5%); and 3) targeted band selection improved the classification of savanna tree species (the mean overall percent accuracy is 57% for 20 bootstrapped replicates). Higher overall classification accuracies were observed for evergreen trees than for deciduous trees.

Influence of fire frequency on <i>Colophospermum mopane</i> and <i>Combretum apiculatum</i> woodland structure and composition in northern Gonarezhou National Park, Zimbabwe

We investigated the long-term effects of fire frequency on Colophospermum mopane and Combretum apiculatum woodland structure and composition in northern Gonarezhou National Park (GNP), Zimbabwe. Fire frequency was categorised as high (every 1–2 years), medium (every 3–4 years) and low (every 5–6 years). The following variables were measured or recorded: plant height, species name, canopy depth and diameter, basal circumference, number of stems per plant, plant status (dead or alive) and number of woody plants in a plot. There was a positive correlation (r = 0.55, P = 0.0007) between annual area burnt (total from January to December) and annual rainfall (average over two rain stations per rain year, July to June) between 1972 and 2005. A total of 64 woody species were recorded from C. mopane and C. apiculatum woodlands. Mean plant height increased from 4.5 to 8.2 meters in C. mopane woodland and from 4.5 to 5.1 meters in C. apiculatum woodland in areas subjected to high and low fire frequencies. In C. mopane woodland, low fire frequency was characterised by a significantly low density of woody plants (P < 0.001), however, with a significantly high mean basal area (P < 0.001). Fire frequency had no significant effect on species diversity (P > 0.05). Our results suggest that C. mopane and C. apiculatum woodlands are in a state of structural transformation. Fire frequency effects, however, appear to be woodland specific. Fire management strategies in GNP should take into consideration annual rainfall and the different vegetation types.Conservation implication: This study provides valuable information on fire frequency effects on woody vegetation in northern GNP, which can be used in fire management programmes for the park. The positive relationship between annual rainfall and annual area burnt emphasises the need for wildlife managers to consider annual rainfall in fire management.

Leaf green‐up in a semi‐arid African savanna ‐separating tree and grass responses to environmental cues

AbstractQuestion: Can satellite time series be used to identify tree and grass green‐up dates in a semi‐arid savanna system, and are there predictable environmental cues for green‐up for each life form?Location: Acacia nigrescens /Combretum apiculatum savanna, Kruger National Park, South Africa (25° S, 31° E).Methods: Remotely‐sensed data from the MODIS sensor were used to provide a five year record of greenness (NDVI) between 2000 and 2005. The seasonal and inter‐annual patterns of leaf display of trees and grasses were described, using additional ecological information to separate the greening signal of each life form from the satellite time series. Linking this data to daily meteorological and soil moisture data allowed the cues responsible for leaf flush in trees and grasses to be identified and a predictive model of savanna leaf‐out was developed. This was tested on a 22‐year NDVI dataset from the Advanced Very High Resolution Radiometer.A day length cue for tree green‐up predicted 86% of the green‐ups with an accuracy better than one month. A soil moisture and day length cue for grass green‐up predicted 73% of the green‐ups with an accuracy better than a month, and 82% within 45 days. This accuracy could be improved if the temporal resolution of the satellite data was shortened from the current two weeks.Conclusions: The data show that at a landscape scale savanna trees have a less variable phenological cycle (within and between years) than grasses. Realistic biophysical models of savanna systems need to take this into account. Using climatic data to predict these dynamics is a feasible approach.

An ecological and conservation assessment of the fauna of Bombyliidae (Diptera) occurring in the Mkhuze, Phinda and False Bay reserves, KwaZulu-Natal, South Africa

The savanna is an important biome covering a large area of southern Africa ; however its invertebrate fauna are very poorly documented. A variety of invertebrate taxa was sampled within three reserves in this biome, all located in the Maputaland centre of endemism in KwaZulu-Natal, South Africa. Bombyliidae (Diptera) were a significant and easily visible element of the fauna sampled. This paper assesses the fauna of this group, by examining habitat associations and comparing the fauna sampled with data from other regions ('Mfongosi', KwaZulu-Natal ; Shimba Hills, coastal Kenya) ; checklists of species from 'Mfongosi' and Shimba Hills are provided. Species richness for the sampling area was estimated using the survey data, and sampling strategy was evaluated by assessing sampling methods and effort. Twenty two species were sampled during the wet season (November to March), of which 6 are new records for KwaZulu-Natal. These are listed, along with relevant distribution and phenological data. Bombomyia bicolorata (Bezzi, 1924) is newly synonymised with Bombomyia bombiformis (Bezzi, 1924). Adult bombyliid species were most strongly associated with broad-leaf vegetational habitats (19 of 22 species). Some 68% of species occurred in Terminalia sericea woodland, 41% in Acacia nigrescens low open woodland, 32% in Combretum apiculatum mountain low thicket, and fewer species in other vegetation types. Active capture using nets along transects was the most effective method used, sampling all 22 species. Species richness estimates for the study region varied from 27 to 33 species, suggesting that 67-73% of the estimated number of species were sampled. Detailed biological studies are required to determine the factors limiting species to particular habitats, particularly adults feeding on flowers and the discovery of the hosts of larval stages. Most species sampled are widespread with no local endemism, and are therefore not a priority for specieslevel conservation within the study region. We emphasise their important functional role as pollinators. Many of the species are restricted to the east African coastal vegetation mosaic. Similar systematic sampling of inland localities is urgently required to confirm this biogeographical restriction. Given their widespread distribution patterns, well resolved taxonomy, and ease of sampling, bombyliids may provide an ideal group for studying large-scale biogeographical affinities and patterns across Africa.

Growth, chemical responses and herbivory after simulated leaf browsing in Combretum apiculatum

Vegetative and chemical responses to simulated leaf browsing during the growth season, and their subsequent effect on herbivory, were studied on Combretum apiculatum Sonder (Combretaceae) in Botswana. Treatments (50% and 100% leaf and shoot apex removal) were performed just before the shoot growth curve levelled out, and responses recorded 3 months later, just before leaf fall. Compared to controls, defoliation treatments, removing apical dominance, reduced growth in tree height and increased shoot mortality, although the production of lateral shoots increased. At the end of the trial, there was no difference in total length of annual shoots between treatment groups. Significant refoliation occurred only after 100% defoliation. Refoliated leaves were smaller and the 100% defoliated trees had a lower final leaf biomass. Total leaf biomass production was, however, equal for all treatment groups. Refoliated leaves contained higher levels of N, lower levels of acid-detergent fibre (ADF) and total phenolics, and showed a trend towards lower levels of condensed tannins, compared to leaves on control trees. Such chemical changes may be due to either carbon stress or to younger physiological age of new leaves. In spite of the observed potential increase in food quality, we found no evidence of increased levels of insect or ungulate herbivory on refoliated leaves, which, at least for insect herbivory, may be explained by the reduction in temporal availability of leaves. We conclude that the single severe defoliation was not detrimental to C. apiculatum in the short-term, although the resource loss and induced compensatory growth may produce negative effects during subsequent growth seasons.

The diet of kudus in a mopane dominated area, South Africa

The composition of the plant species eaten by kudu (Tragelaphus strepsiceros) determines the diet quality, which impacts on kudu condition and mortality levels. The yearround diet composition of kudus in the Limpopo Province, a mopane (Colophospermum mopane) dominated area, was determined by faecal analysis. The most important dietary plant species were Colophospermum mopane, Grewia bicolor, Terminalia prunioides, Tinnea rhodesiana, Boscia albitrunca and Combretum apiculatum, with C. mopane comprising on average 39.2 % of diet per month. Small amounts of herbs, grasses and seeds made up the remaining part of the diet. The contribution of C. mopane in the diet was negatively correlated with precipitation. Colophospermum mopane was consumed, irrespective of its high condensed tannin load (5.2–9.8 % DW) for the majority of the months. No seasonally significant differences were detected for modelled kudu diet crude protein, tannin or phenol concentrations. Colophospermum mopane showed significant seasonal differences with lowest values of protein, tannin and phenols in the late wet season. Surprisingly, crude protein concentrations were positively correlated with high levels of tannins and phenols for C. mopane. The diet of kudus comprised of significantly more species during the wet season compared to the dry season. Diet diversification, instead of protein maximization, seems a potential tool to satisfy protein requirements while reducing potential toxic effects associated with a high intake of secondary compounds. A significant positive correlation was therefore detected between the tannin concentration of C. mopane leaves and the number of plant species in the diet.

Differential photosynthetic responses of broad- and fine-leafed savanna trees to elevated temperatures

South African savannas are poorly studied with respect to leaf and canopy photosynthesis making it difficult to quantify their response to global warming and contribution to the overall global carbon budget. This study addressed these issues by examining the effects of different leaf temperatures (range 25-35°C) on the photosynthesis of broad-leafed (Combretum apiculatum, Dalbergia melanoxylon, Sclerocarya birrea) and fine-leafed (Acacia nilotica, Acacia nigrescens) savanna tree species during two summer dry spells in the Kruger National Park. Results showed that photosynthetic rates of fine-leafed tree forms were significantly higher than those of broad-leafed tree forms over the entire range of photosynthetic flux densities (range: 0–2 000µmol m− 2 s− 1 PFD) but not over the entire range of external (Ca) and internal (Ci) leaf CO2 concentrations (range: 0–120Pa Ci). The lower photosynthetic rate, carboxylation efficiency (Vcmax) and electron transport capacity (Jmax) of broad-leafed forms was not reflected in their leaf N, chlorophyll or carotenoid concentrations, but did correspond with a greater foliar accumulation of non-structural carbohydrates known to inhibit photosynthesis through several routes. Despite these differences, photosynthesis in both functional types was suppressed at leaf temperatures of 35°C, the magnitude of photosynthetic acclimation equivalent in broad- and fine-leafed forms. In both forms, the decline in photosynthetic rate corresponded with a concomitant reduction in stomatal conductance, a consequence of a steep leaf-to-air vapour pressure deficit gradient with increasing leaf temperature, leading to a decrease in carboxylation efficiency as indicated by reduced Ci:Ca ratios. It is concluded that anticipated increases in temperature due to global warming may negatively impact on primary production of especially broad-leafed savannas with potential consequences for the global carbon budget.

Links between terrain characteristics and forage patterns of elephants (Loxodonta africana) in northern Botswana

Spatial vegetation utilization of elephants was investigated within mixed woodland savanna along the Chobe River in northern Botswana in the dry season of 1998. Using multiple linear regression, accumulated stem breakage by elephants was predicted by a terrain index, distance to water, stand density, number of trees > 4 m tall, tree height, density of Combretum apiculatum, C. elaeagnoides, C. mossambicense and the density of other (accumulated) tree species. Within mixed woodland at 2–7 km distance from the river fine-grained terrain ruggedness was the most important factor contributing to 55% of observed differences in use by elephants, while distance to water and the density of C. apiculatum contributed an additional 20% and 4%, respectively to the multiple linear regression model. Stem breakage was, on average, almost twice as high in rugged terrain compared with flat terrain at similar distance to water within the same vegetation type. Rugged terrain had 2–3-fold higher proportion of plots with very high Combretum shrub densities. These results suggest that the terrain index may be useful in management, predicting the areas most sensitive to vegetation change in a woodland system with increasing elephant densities.

Vegetation changes during a 36‐year period in northern Chobe National Park, Botswana

AbstractChanges in vegetation cover in northern Chobe National Park (Botswana) were assessed using aerial photographs from 1962, 1985 and 1998, with subsequent ground proofing. In addition, cumulative browsing by elephants and the occurrence of fire scars were recorded on random vegetation sites within shrubland (n = 20) and mixed woodland (n = 20). Coverage of woodland vegetation decreased from 60% to 30% between 1962 and 1998, while shrubland vegetation increased from 5% to 33% during the same period. During the study period, woodland has gradually retreated away from the river front. While riparian forest covered a continuous area along the riverfront in 1962, only fragments were left in 1998. We found a significant decrease in browse use with increasing distance to the Chobe river for Combretum apiculatum, Combretum elaeagnoides, Combretum mossambicense and other woody plants combined (all P < 0.0001). The occurrence of fire (P < 0.0001) and basal area (P < 0.0001) were positively related to distance to the river. Elephant browsing occurred on >70% of available stems within 2 km from the river, while less than 20% of the trees had fire scars in the same zone. Beyond 7 km from the river, elephant browsing was reduced to >50% of available stems, while more than 50% of the trees had fire scars. The density of any of the shrubs was not related to distance to the river neither within shrubland (all P > 0.05) nor within mixed woodlands (all P > 0.05).

The environment and vegetation of the flux measurement site near Skukuza, Kruger National Park

The SAFARI-2000 intensive study site is located 13 km WSW of Skukuza. Detailed measurements of the exchanges of energy, water, carbon dioxide and other substances between the savanna and the atmosphere have been made there since April 2000. This paper provides basic information regarding the climate, soils and vegetation at the site. The site is located near the top of a gentle rise in an undulating granitic landscape. Most of the data were collected within a 300 m square centred on the flux tower situated at 25@01.184' S, 31@29.813' E and oriented true north. The tower stands exactly on the ecotone between a ridgetop broad-leafed Combretum savanna on sandy soil and a midslope fme-leafed Acacia savanna on clayey soil. The ecotone is marked by a 10 m wide band of sedges. The tree basal area within the sample square was 6.8 mVha (@ 1.0 standard error), the tree density 128 @ 16 plants/ha and the tree crown cover 24 @ 4 . Shrubs, defined as woody plants greater than 0.5 m but less than 2.5 m tall, contributed a further 7.6 crown cover. The basal area weighted mean height of the trees was 9 m, and the maximum height 13m. Nineteen woody plant species were recorded within the square, with 70 of the woody plant basal area dominated by Combretum apiculatum, Sclerocarya birrea and Acacia nigrescens. The rooted basal area of grasses was 7.1 @ 0.6 and in June 2000 the grass standing crop was 400 g DM m2.

An investigation on the biological activity of Combretum species

Leaf extracts of 20 Combretum species, many of which are used in southern African traditional medicine, were screened for anti-inflammatory, anthelmintic, anti-bilharzia (antischistosomal) and DNA-damaging activity. Significant activity in more than one bioassay was exhibited by Combretum apiculatum, Combretum hereroense, Combretum molle and Combretum mossambicense. Ethyl acetate extracts were generally most active, followed by acetone and then water extracts.

Plant responses and herbivory following simulated browsing and stem cutting of Combretum apiculatum

Abstract. Time and mode of herbivory on savanna trees and their subsequent responses are dependent on, among other things, earlier herbivory and fire. We used clipping (simulated browsing) and stem cutting (simulated heavy browsing and to some extent simulated fire) to evaluate such interactions. Study organisms were a deciduous, broad‐leaved tree species, Combretum apiculatum (Combretaceae), browsing large herbivores and leaf‐eating insects. The treatments were done in the late dry season before bud break. Late in the following wet season, we recorded plant responses to treatment and browsing. The treated trees, especially the cut ones, responded by producing larger and fewer annual shoots. Compared to control trees, there was a slight increase in shoot biomass of clipped trees and a strong reduction of cut ones. Leaf area increased in clipped trees, but decreased in cut ones. A marked increase in the number of browsed trees was recorded amongst treated trees. Number of bites, consumption and utilization also increased with severity of treatment. In contrast, insect herbivory was reduced on both clipped and cut trees. The observed patterns are discussed in relation to current ideas on plant ‐ herbivore interactions.

Characteristics and management potential of some indigenous firewood species in Malawi

Taking a case study from Malawi's Central Region, this paper identifies the locally important firewood species and discusses options for their management. The wood properties (moisture content and density) of 15 commonly used indigenous firewood species were determined and according to the fuel value index employed, the high grade firewoods were Combretum apiculatum, Pericopsis angolensis and C. molle. Supported by socio-economic, inventory and ecological data, the status (stocking and basic density) and coppice management of the preferred firewoods was then discussed. Whilst the high grade species had a low stocking in the woodland, their coppicing response was good, reaching a mean basal diameter of 4.4 cm and mean height of 3.3 m in 4 years. The paper proposes that these species be managed for domestic firewood on a coppice rotation of 5 years upwards and recommends that national policies for fuelwood production develop mechanisms for incorporating the rural woodland users into the management process.

The impact of an increasing elephant population on the woody vegetation in southern Sabi Sand Wildtuin, South Africa

In 1961, a fence was erected between privately owned Sabi Sand Wildtuin (SSW) and the Kruger National Park (KNP), which largely prevented elephants entering the SSW. In 1993, the fence was removed. This lead to a rapid influx of elephants into the SSW during the winter months, most of which move back into the KNP during the wet summer season. In 1993, the SSW elephant population was 1/1045 ha but increased to 1/305.8 ha in 1996. It more than doubled to 1/146 ha in 1998. This study was undertaken on the property Kingston, in southern SSW, to assess the impact of elephants on woody vegetation and determine why they show seasonal dietary preferences for specific tree parts. Vegetation utilisation was recorded on a five kilometer transect of vehicle track in 1996 and repeated in 1998. From the transect, species density was calculated for those trees impacted on. Trees that had been newly bark stripped were recorded in 1996 and 1998. Cambium samples were collected in summer and winter from eight tree species. Field observations of elephants impacting on woody vegetation augmented the data base. Transect analysis showed a strong correlation between tree utilisation and density. The most visual damage was of Combretum apiculatum, Acacia burkei, Pterocarpus rotundifolius and Grewia species. Tree damage increased by 73 from 1996 to 1998. Significantly higher levels of nitrogen, sodium and magnesium were found in the species most regularly bark stripped. Bull elephants were responsible for 94 of the trees seen uprooted. The results suggested that SSW can sustain the present elephant population, but further influx at the present rate of increase, will have a negative impact on the reserve.

Use of browses (Terminalia serecia, Combretum apiculatum or Euclea schimperi) as a supplement for growing Tswana goats.

Twenty yearling male castrated Tswana goats were weighed and randomly divided into four groups of 5 animals. All the animals were fed buffel grass hay (Cenchrus ciliaris) as a basal diet, while lucerne (Medicago sativa) was fed to the control group as supplement. The other three groups were fed either Combretum apiculatum, Terminalia serecia or Euclea schimperi as a supplement. The basal diet comprised 60% of the ration, while the lucerne or the browses made up the remaining 40%. Water was provided daily. Leftover water and feed were measured daily. The goats were weigted every 2 weeks, prior to the morning feed. The data collected were subjected to analysis of variance. The average daily dry matter intake of the lucerne, C. apiculatum, T. serecia and E. schimperi supplements was 702.17, 769.91, 776.68 and 752.50 g, respectively, and was not significantly different between treatments (p > 0.05). The average daily water intake was significantly different (p < 0.05), with the goats fed lucerne drinking significantly more water (1270 +/- 31.62 ml) than the goats on T. serecia (1150 +/- 103.2 ml) but not significantly more than those on C. apiculatum (1166 +/- 52.25 ml) or E. schimperi (1184 +/- 100.15 ml). The average daily body weight gain was higher for the goats on lucerne at 78 g, compared to 77 g, 67 g and 64 g, for the C. apiculatum, E. schimperi and T. serecia supplements, respectively, but these were not significantly different (p > 0.05). Differences in feed conversion ratio were not significant (p > 0.05), but the ratio was lower for the goats on lucerne at 9.00 compared to 10.41, 11.98, and 11.23 for C. apiculatum, T. serecia and E. schimperi, respectively. The study demonstrated that these browses can be used to replace lucerne as a supplement for Tswana goats fed on buffel grass hay as the basal diet.

Impact of sustained drought on a semi‐arid Colophospermum mopane savanna

Vegetation change on different substrate types was measured over 15 years in semi‐arid Mopani Veld. Baseline data were collected in the terminal year (1982) of a decade of above‐average rainfall, whilst the period of monitoring was characterised by below‐average rainfall, with severe drought years occurring in 1988/1989 and 1991/1992. Grazing pressure was, on average, alleviated over the period of recording so that changes reflect primarily the effect of rainfall. Long‐lived perennial grasses had declined markedly, annual and micro‐perennial grasses had increased markedly, and the proportion of forbs had increased. Despite 7–17 years since the replacement of livestock grazing with wildlife at lower stocking rates, there was no evidence of an improvement in sward or soil condition, owing ostensibly to the effect of drought. The extent of change in botanical composition and in sward condition was influenced by substrate type. The rate of growth of the woody component had been slow: the average per annum increase in the radius of trees was 0.67 mm. For the dominant species, Colophospermum mopane, only individuals of less than 1 m in height consistently showed an increase in size. The basal area of dead standing trees had increased threefold. Complete mortality or substantial topkill of shrub species, in particular of Grewia spp., Ximenia americana, and Combretum apiculatum, and of adults of the latter, had occurred owing to drought. Shrub mortality was most pronounced on colluvial soils and sandveld. All species exhibited partial dieback of the crown. Overall, shrub recruitment has been limited except for that of H. mopane on colluvial soils but not on other substrate types, which is therefore likely to increase further its dominance on this substrate at the expense of species richness. Grazing pressure on desirable perennial grasses and browsing pressure on palatable woody plants has probably increased because of their decline, even if animal abundance has not increased, which will have a negative effect on their re‐establishment. Acting in opposition, dieback of trees and shrubs has probably created a window of opportunity for the recovery of perennial grasses, but it would likely require an extended run of above‐average years to reverse the changes in herbaceous vegetation, for which the availability of seed of perennial grasses is likely to be a major constraint.