Acacia Karroo Research Articles

Effects of drought acclimation on the mechanical properties of Ochroma pyramidale, Betula pendula and Acacia karroo tree seedling stems

Drought acclimation in woody plants tends to be associated with the development of denser vascular tissue. As wood density may be correlated with the mechanical properties of the tissue, we hypothesized that seedlings subjected to drought would develop stems that had a higher density as well as stiffer and stronger tissue. We tested our hypothesis on establishing trees of the species Betula pendula, Acacia karroo and Ochoma pyramidale. The seedlings were grown for 1 year under well-watered as compared with cyclically droughted conditions. The modulus of elasticity in bending, yield stress, density and pith- and bark fractions of the stems at different heights were measured. In all three species, individuals grown under drought conditions had on average stiffer and stronger stems than the well-watered individuals, though non-significantly so in A. karroo. These differences were associated with a higher density in O. Pyramidale but not in the other species. In B. pendula, the differences in mechanical properties appear to be associated with other factors, e.g. modifications of the micro fibril angle in the S2 cell wall layer. Our results indicate that drought acclimation in seedlings affects the mechanical properties of the stem but that the causal acclimational processes may vary between species.

Activity of South African medicinal plants against Listeria monocytogenes biofilms, and isolation of active compounds from Acacia karroo

In South Africa, the antimicrobial activity of many indigenous plants has been investigated. In general, studies have focused on planktonic bacteria, with less attention given to bacterial biofilms. Many organisms, however, including the opportunistic pathogen Listeria monocytogenes occur more frequently as biofilms. The aim of this study was to identify and select plants that exhibit the best antilisterial activity, isolate the bioactive compounds and determine their effect on the architecture of listerial biofilms. The ethyl acetate and chloroform extracts of thirteen plants were investigated for antilisterial activity. The ethyl acetate extract of Acacia karroo and Plectranthus ecklonii showed the best antilisterial activity, exhibiting a minimum inhibitory concentration (MIC) of 3.1 mg/ml and 0.5 mg/ml, respectively. These were further selected for the identification of bioactive compounds. Column chromatographic purification of the ethyl acetate extracts of the leaves of A. karroo led to the isolation of three known pure compounds, namely epicatechin ( 1), β-sitosterol ( 2) and epigallocatechin ( 3). Confocal scanning laser microscopy (CSLM) showed that the biomass of the listerial biofilm was reduced when the isolated compounds were added. The aggregation of cells that were exposed to β-sitosterol and epigallocatechin was reduced from 25 μm as observed in untreated cells to < 10 μm in length.

Population differentiation in a purported ring species, Acacia karroo (Mimosoideae)

Acacia karroo Hayne (Mimosoideae; Fabaceae) is a highly polymorphic species, ranging in height from 1 m to more than 30 m, and with enormous variation in the architecture of adults. Some populations of A. karroo with different morphologies are situated less than 20 km apart. This species has been considered to be a ring species on the basis of allozyme variation. I wished to determine whether this was supported by sapling morphology, and by chemical and physical defences to herbivory. I raised four phenotypes from the restricted area of Zululand (South Africa) in a common garden with controls, and with water and nutrient supplementation. I found that each of the four phenotypes maintained their differences in spite of nutrient and water supplementation. There was no significant genotype by environment interaction. I also found that the coastal population was significantly larger than another phenotype that grows just 12 km inland from it, suggesting that there might be local adaptation of these genotypes to particular soil types. I confirm that A. karroo maintains morphological differentiation even when there is substantial alteration of water and nutrient availability. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104, 748–755.

Historical and recent land-use impacts on the vegetation of Bathurst, a municipal commonage in the Eastern Cape, South Africa

We assessed past and present vegetation patterns in relation to land use on a municipal commonage in South Africa. We asked specifically whether the reassignment of the commonage for the use of historically disadvantaged town residents after 1994 has impacted negatively on the vegetation of the commonage. Analysis of land cover change using aerial photography time series revealed that the most significant human impacts on the Bathurst commonage occurred prior to 1942 due to heavy and uncontrolled communal land use. Vegetation change since then has been characterised by a recovery of thicket vegetation and an encroachment of woody species into grassland. Additional field sampling of the species composition and age class distributions of trees and shrubs in thicket and broken thicket vegetation showed these units to be of high veld condition status. Thornveld and old lands have both been encroached upon by woody species, particularly by Acacia karroo, and possessed the lowest veld condition scores. There is little evidence, however, that recent land-use activities have impacted negatively on commonage vegetation. We discuss these results in terms of the vegetation's potential to provide for the diverse needs of commonage users, and the implications for commonage management and policy.

Fatty acid composition of beef from Nguni steers supplemented with Acacia karroo leaf-meal

The objective of this study was to determine fatty acid composition of beef from Nguni cattle supplemented with Acacia karroo leaf-meal. Thirty 19-month-old steers were randomly assigned to A. karroo leaf-meal (AK), sunflower cake (SF) and the control with no supplement (CN) diets. The m. longissimus thoracis et lumborum was sampled for analyses. Highest α-linolenic acid and docosapentaenoic acid, and lowest n−6/n−3 ratios were recorded in beef from steers that received AK diet (P<0.05). Myristic acid and palmitic acid proportions were lower (P<0.05) while n−3 PUFA proportions were higher (P<0.05) in beef from steers that were given AK and CN diets than those on the SF diet (P<0.05). It was concluded that the inclusion of A. karroo in cattle diets can improve fatty acid composition of beef from a human health perspective.

Stress reactivity and its relationship to beef quality in Nguni steers supplemented with Acacia karroo leaves

The objective of this study was to determine the levels of catecholamines and their relationship to beef quality in Nguni steers fed on Acacia karroo leaves. A total of 30 19-month-old steers were randomly assigned to A. karroo leaves (AK), sunflower cake (SF) and the control with no supplement (CN) diets. The AK and SF diets provided the steers with an additional 150 g of protein per day for 60 days. Catecholamine levels were determined from urine samples collected from each steer before and after slaughter. The Musculus longissimus thoracis et lumborum was sampled for selected meat quality measurements. Nguni steers on the CN diet had higher (P < 0.05) concentrations of post-mortem urinary norepinephrine and dopamine compared with those that received the AK and SF diets. Norepinephrine was negatively linearly related (P < 0.05) to the Warner–Bratzler shear force value of meat aged for 21 days and cooking loss of meat aged for 2 days (CL2) in steers that were given the SF diet. Meat pH and drip loss values were inversely related (P < 0.05) to epinephrine concentration in steers that received the AK diet. Dopamine concentration was negatively linearly related (P < 0.05) to water holding capacity and CL2 for steers on the CN diet. For steers on the CN diet, lightness (L*) values increased (P < 0.05) with increase in dopamine concentration. It was concluded that stress responsiveness and its relationship to certain beef quality attributes could be positively manipulated by supplementation with A. karroo leaves.

Defoliation depletes the carbohydrate reserves of resprouting Acacia saplings in an African savanna

Over the past century there has been a global trend towards tree expansion and densification in rangelands and savannas. This phenomenon is commonly referred to as bush encroachment. In South Africa Acacia karroo is one of the key species responsible for bush encroachment. It has been suggested that the combination of fire and browsing might limit bush encroachment by A. karroo more effectively than either browsing or fire alone. We hypothesized that these repeated disturbances progressively deplete root carbohydrates and compromise resprouting ability. This was tested by burning and then manually defoliating A. karroo once a month for 1 year. Manual defoliation did not inhibit the rapid shoot elongation after topkill of A. karroo saplings. During this initial phase, the growth of the new shoots of A. karroo was dependent more on mobilised root reserves than on photoassimilates from the new shoots. Frequent manual defoliation of resprouting A. karroo saplings prevented the replenishment of starch reserves. We suggest a mechanism for how the interaction of browsing and fire can suppress and perhaps reverse bush encroachment in African savannas. Saplings that have reduced starch reserves at the end of dry season due to browsing will struggle to resprout if they are burnt. Even if they do not die, they will be less able to escape fire damage in the next fire than if they had been able to resprout unimpeded.

Diet selection by Nguni goats in the Zululand Thornveld

The aim was to determine diet selection of goats grazing/browsing in Zululand Thornveld. An experiment was done in spring (November 2007) and autumn (March/April 2008) at Owen Sitole College of Agriculture, KwaZulu-Natal. Diet selection was estimated using direct observation of animals and an indirect plant-based method. The browse species observed in the plant-based methods were:Gymnosporia senegalensis (Gs), Acacia nilotica (An), Acacia karroo (Ak), Scutia myrtina (Sm) and Dichrostachys cinerea (Dc). Selection was determined by observing at regular intervals 40 marked branches on 10 plants of each browse species. Direct observation of diet selection was done by observing two goats in a base herd of 24 castrated males when they were allowed to forage together in a paddock of 1 ha. Observations were made every minute for three hours in the morning and 1.5 hours in the afternoon. The relative abundance of the browse species in the veld was determined and used to estimate the selection index (utilization relative to availability). In both seasons, the three most selected species according to the plant-based observations were Sm, Ak and Dc. Consistently, An experienced moderate defoliation and Gs the least. However, on the basis of the selection index the species followed the order: An>Dc>Ak>Sm>Gs in spring and An>Sm>Dc>Ak>Gs in autumn. The selection index was negatively correlated to all fibre attributes although the correlation attained significance only for NDF, ADF and lignin but not for tannins and cellulose. Both methods did not rank species in the same order, thus suggesting the need for further methodological refinement.

Ethno-ecological evidence for Hydnora abyssinica occurring in Johannesburg and Durban traditional medicine markets

Large quantities of plants are traded annually in South Africa's traditional medicine or ‘ muthi’ markets. A resource in high demand in the Faraday (Johannesburg) and Warwick (Durban) markets is uMavumbuka, a root holoparasite usually identified as either Hydnora africana Thunb. or Sarcophyte sanguinea Sparrm. subsp. sanguinea. However, rhizomes regularly observed in Faraday between 1994 and 2008 did not resemble either species, thereby suggesting that a third, and undocumented, species was being harvested. This was confirmed when the rhizomes were identified as H. abyssinica A.Br. by an American parasitic plant expert. An ethno-ecological study was initiated to verify its occurrence in selected muthi markets. The study further aimed to investigate the distribution of H. abyssinica through trader interviews, host species localities and some previously misidentified herbarium specimens. The study revealed that H. abyssinica was the only uMavumbuka species present in Faraday and Warwick in 2009. Furthermore, the rhizomes were being harvested in KwaZulu-Natal—an area not previously known to be part of its distribution range. Re-evaluated herbarium vouchers and recent photographs taken in the Kruger National Park have confirmed that H. abyssinica occurs in KwaZulu-Natal, Limpopo, Mpumalanga, Gauteng and Swaziland and hence eastern southern Africa. Fragments of Acacia xanthophloea Benth. roots were identified on 93% of the samples that had host roots attached, and we suspect that H. abyssinica follows the distribution of A. xanthophloea in suitable habitats north from KwaZulu-Natal and adjoining the South African border with Swaziland and Mozambique. Acacia karroo Hayne and A. grandicornuta Gerstner have also been positively identified as host species in South Africa from herbarium records. Plant harvesters in the markets cited the common names of several other species that uMavumbuka “grows under” that may be identified as hosts to H. abyssinica in the future. The collection of specimens in areas identified by the harvesters and in areas of suitable habitat is important to verify the occurrence, distribution and habitat of H. abyssinica in eastern southern Africa.

Aspects of the seed ecology of Acacia karroo in the Eastern Cape, South Africa

Seed ecology may play a role in the widespread increase of Acacia karroo in savanna and grassland. Accordingly, fecundity, predation, mortality by fire, dispersal by livestock, and seed longevity were studied. Seed production per tree (P of pod production, pods per tree) was positively related to tree height, negatively related to tree density, and was greater for sites receiving run-on. Predispersal predation by bruchids, cerambycids, and wasps depleted seed production by 18%. The ant Messor capensis effectively eliminated surface seed except during the early dry season following seed shed. Rodent predation of seed was minimal. Fires >100 °C killed most exposed seed but a thin layer of soil increased survival and maintained germinability. A greater proportion of ingested seeds passed through cattle (57%) and sheep (50%) than through goats (15%). 50% of seeds were recovered within 48 h. During passage, soft seeds were digested, some hard seeds were softened, but most were egested as hard seeds; germinability was not increased. Cattle were confirmed as dispersal agents. Using experimental seed banks, it was shown that buried or surface-exposed seeds did not persist for longer than a year, but 24% of surface-shaded seeds persisted for two years. Most seeds germinated but most buried seeds died before emergence. Seed persistence in deep shade should facilitate invasion into grassland. Seed banks of. A. karroo did not exceed 18 seeds m−2 because of the above-described effects. Seed availability (fecundity, dispersal by cattle, seed longevity) is concluded to contribute to the invasiveness of A. karroo, but use of fire may offer partial control of seed availability.

Seasonal variation in time spent foraging by indigenous goat genotypes in a semi-arid rangeland in South Africa

The feeding behaviour of Xhosa lop-eared (XLE), Nguni (NGN) and Nguni × Boer crossbred (NBC) goat genotypes was compared in a False Thornveld rangeland by recording time spent on browsing, grazing and other non-foraging activities in four seasons; cool-dry (June to August); hot-dry (September to October), hot-wet (November to February) and post-rainy (March to May). The False Thornveld rangeland consists of diverse plant species dominated by Acacia karroo, Grewia occidentalis, Scurtia myrtina Themeda triandra, Panicum maximum, Digitaria eriantha, Eragrostis spp., Cynodon dactylon and Pennisetum clandestinum, and receives an annual rainfall of not more than 490 mm. Four observers monitored 8 goats per genotype at 15-minute intervals for 4 h (0800–1200 h) in the morning and 4 h (1200–1600 h) in the afternoon. Genotype, season and time of day had an effect on the feeding behaviour of goats on the rangeland (P < 0.05). Xhosa lop-eared goats spent more time browsing (P < 0.05) in the cool-dry and hot-wet seasons compared to NGN and NBC goats. Nguni goats spent more time browsing (P < 0.05) in the hot-dry season. Time spent grazing varied among the goat genotypes with the NBC cross spending more time grazing (P < 0.05) in hot-dry, cool-dry and post-rainy seasons. Generally, season and time of day had no effect on time spent on non-foraging activities for the three goat genotypes. It was concluded that NBC goats spent more time grazing compared to other genotypes across all seasons. Conversely, XLE and NGN goat genotypes spent more time feeding on browse species across seasons. Xhosa lop-eared and NGN goat genotypes are thus adapted to the surrounding environment for they may utilise resources more efficiently.

Rivers Through Time: Historical Changes in the Riparian Vegetation of the Semi-Arid, Winter Rainfall Region of South Africa in Response to Climate and Land Use

This paper examines how the riparian vegetation of perennial and ephemeral rivers systems in the semi-arid, winter rainfall region of South Africa has changed over time. Using an environmental history approach we assess the extent of change in plant cover at 32 sites using repeat photographs that cover a time span of 36-113years. The results indicate that in the majority of sites there has been a significant increase in cover of riparian vegetation in both the channel beds and adjacent floodplain environments. The most important species to have increased in cover across the region is Acacia karroo. We interpret the findings in the context of historical changes in climate and land use practices. Damage to riparian vegetation caused by mega-herbivores probably ceased sometime during the early 19th century as did scouring events related to large floods that occurred at regular intervals from the 15th to early 20th centuries. Extensive cutting of riparian vegetation for charcoal and firewood has also declined over the last 150years. Changes in the grazing history as well as increased abstraction and dam building along perennial rivers in the region also account for some of the changes observed in riparian vegetation during the second half of the 20th century. Predictions of climate change related to global warming anticipate increased drought events with the subsequent loss of species and habitats in the study area. The evidence presented here suggests that an awareness of the region's historical ecology should be considered more carefully in the modelling and formulation of future climate change predictions as well as in the understanding of climate change impacts over time frames of decades and centuries.

Ecological filtering by a dominant herb selects for shade tolerance in the tree seedling community of coastal dune forest

The regeneration niche is commonly partitioned along a gradient from shade-tolerant to shade-intolerant species to explain plant community assembly in forests. We examined the shade tolerance of tree seedlings in a subtropical coastal forest to determine whether the ecological filtering effect of a dominant, synchronously monocarpic herb (Isoglossa woodii) selects for species at either end of the light response continuum during the herb's vegetative and reproductive phases. Photosynthetic characteristics of seedlings of 20 common tree species and the herb were measured. Seedlings were grown in the greenhouse at 12-14% irradiance, and their light compensation points measured using an open-flow gas exchange system. The light compensation points for the tree species were low, falling within a narrow range from 2.1 ± 0.8 μmol m(-2) s(-1) in Celtis africana to 6.4 ± 0.7 μmol m(-2) s(-1) in Allophylus natalensis, indicating general shade tolerance, consistent with a high and narrow range of apparent quantum yield among species (0.078 ± 0.002 mol CO(2) mol(-1) photon). Rates of dark respiration were significantly lower in a generalist pioneer species (Acacia karroo) than in a forest pioneer (C. africana), or in late successional phase forest species. We argue that the general shade tolerance, and phenotypic clustering of shade tolerance, in many tree species from several families in this system, is a result of ecological filtering by the prevailing low light levels beneath the I. woodii understorey, which excludes most light-demanding species from the seedling community.

Causes of arrested succession in coastal dune forest

Dispersal or recruitment limitation may arrest succession after disturbance. In north-eastern South Africa the Acacia karroo successional pathway is used to facilitate coastal forest recovery after strip-mining. However, although A. karroo establishes naturally, it forms monospecific stands, arresting forest succession for decades. This casts doubt on the efficacy of this restoration pathway. We investigated the causes of arrested succession. The seed and seedling banks of A. karroo stands and of forest at Cape Vidal, and three A. karroo stands (7–27 years old) on rehabilitated strip-mined dunes at nearby Richards Bay were examined. The establishment and growth of seedlings at Cape Vidal were also considered. The seed bank was larger and more diverse in forest, but the seedling bank was larger in Acacia stands. At Richards Bay, the size of the seed bank increased and the seedling bank decreased with Acacia stand age. Excluding mammalian herbivores in Acacia stands at Cape Vidal resulted in greater species richness and survival of naturally established seedlings, as well as two experimentally planted species. Neither seed dispersal nor seedling establishment limited recruitment of tree species in Acacia stands. Herbivory arrested forest succession by causing the differential mortality of seedlings. In contrast, at Richards Bay where there were few mammalian herbivores, the advanced regeneration in A. karroo stands converged on the diversity of nearby forests 29 years after restoration. Controlling herbivore access and seeding Acacia stands with forest species are site-specific options for preventing arrested succession when using the A. karroo successional pathway.

Is the lack of leguminous savanna trees in grasslands of South Africa related to nutritional constraints?

As with many grasslands globally, the Highveld grasslands of South Africa are tree-less, despite having a climate that can support tree growth. Models predict that fire maintains these grasslands. The question arises as to why fire-tolerant savanna trees do not survive in these ecosystems? Savanna tree survival in mesic areas is restricted by demographic bottlenecks, specifically limitations to sapling-escape from fire. It was hypothesised that ancient highly leached soils from grassland areas would prevent saplings from growing fast enough to escape the fire-trap. Growth rates of savanna tree seedlings (Acacia karroo Hayne and Acacia sieberiana Burtt Davy) were measured in a common garden experiment using soils from ten sites collected along a savanna-grassland continuum. Soils from grassland sites were relatively nutrient-poor compared to those from savannas with lower pH, and associated cations. A. sieberiana growth rates responded to pH and these nutrients, whereas A. karroo growth was less strongly linked to specific nutrients. Even so, both species accumulated more biomass when grown in soils from savanna sites compared to grassland sites. An exception was a low elevation low nutrient savanna site that resulted in poor growth, yet sustains high tree biomass in situ. Differences between growth in grassland and savanna soils were small. They may contribute to, but are unlikely to explain, the treeless nature of these grasslands.

Growth responses of African savanna trees implicate atmospheric [CO2] as a driver of past and current changes in savanna tree cover

AbstractAtmospheric CO2 has more than doubled since the last glacial maximum (LGM) and could double again within this century, largely due to anthropogenic activity. It has been suggested that low [CO2] contributed to reduced tree cover in savanna and grassland biomes at LGM, and that increasing [CO2] over the last century promoted increases in woody plants in these ecosystems over the past few decades. Despite the implications of this idea for understanding global carbon cycle dynamics and key global role of the savanna biome, there are still very few experimental studies quantifying the effects of CO2 on tree growth and demography in savannas and grasslands. In this paper we present photosynthetic, growth and carbon allocation responses of African savanna trees (Acacia karroo and Acacia nilotica) and a C4 grass, Themeda triandra, exposed to a gradient of CO2 concentrations from 180 (typical of LGM) to 1000 µmol mol−1 in open‐top chambers in a glasshouse as a first empirical test of this idea. Photosynthesis, total stem length, total stem diameter, shoot dry weight and root dry weight of the acacias increased significantly across the CO2 gradient, saturating at higher CO2 concentrations. After clipping to simulate fire, plants showed an even greater response in total stem length, total stem diameter and shoot dry weight, signalling the importance of re‐sprouting following disturbances such as fire or herbivory in savanna systems. Root starch (per unit root mass and total root starch per plant) increased steeply along the CO2 gradient, explaining the re‐sprouting response. In contrast to the strong response of tree seedlings to the CO2 gradient, grass productivity showed little variation, even at low CO2 concentrations. These results suggest that CO2 has significant direct effects on tree recruitment in grassy ecosystems, influencing the ability of trees to recover from fire damage and herbivory. Fire and herbivore regimes that were effective in controlling tree increases in grassy ecosystems could thus be much less effective in a CO2‐rich world, but field‐based tests are needed to confirm this suggestion.

Growth of N2‐fixing African savanna Acacia species is constrained by below‐ground competition with grass

Summary 1. The coexistence of leguminous trees and C4 grasses in many African savannas forces competition between these two distinct plant life‐forms. The nature of tree–grass coexistence and the consequent competition for resources remains poorly understood. 2. We hypothesized that grass suppression of tree seedling growth was due to competition for N in plants not capable of fixing N2 and due to the costs of N2 fixation in those capable of nodulation. 3. Growth and N2 fixation of nodulating (Acacia karroo, Acacia nilotica, Acacia tortilis and Acacia nigrescens) and non‐nodulating (Acacia ataxacantha Acacia brevispica and Acacia schweinfurthii) Acacia spp. in response to soil characteristics and fertilizer variations in irrigated field experiments were compared. The extent of N2 fixation was determined from δ15N values. 4. Grass suppressed the growth of all species, but particularly of the non‐nodulating species. The N2‐fixing tree seedlings had high tissue N and high water‐use efficiencies (WUE, based on δ13C) relative to non‐nodulating legumes, making the N2‐fixing species strong competitors with C4 grasses, which have low tissue N contents and high WUE. The suppression of seedling growth by grass was smaller for plants grown in a soil with 66 mg kg−1 than in a soil with 9 mg kg−1 of Bray II P. The tissue N : P ratios (15.1) indicated that P‐limitation constrained growth of seedlings in soils with low Bray II P when grass was present, rather than the energetic costs of N2 fixation. 5. Synthesis. In Acacia savannas and in the absence of water limitation, competition for N is the main constraint imposed by grass on growth of Acacia seedlings and that the ability to fix N2 overcomes this limitation. However, the growth of N2 fixing Acacias may be additionally limited by grass competition for P.

Soil nutrients are not responsible for arrested succession in disturbed coastal dune forest

Coastal dune forest succession frequently proceeds via the Acacia karroo pathway, but may become arrested. We examine whether soil fertility arrests forest succession in A. karroo stands in coastal dune forest in KwaZulu-Natal province, South Africa. We examined soil fertility of A. karroo stands, the adjacent forest, and forested dune slacks at Cape Vidal, and four rehabilitating A. karroo stands (13- to 28-yr-old) at Richards Bay. The effect of nitrogen supplementation on growth of three tree species (a forest pioneer, a late successional forest species, and A. karroo) was compared between A. karroo stands and adjacent dune forest at Cape Vidal. Soil fertility in A. karroo stands and the adjacent forest at Cape Vidal was similar and neither total nor readily mineralisable nitrogen were limiting in either habitat. At Richards Bay, where the dunes were previously strip-mined, total nitrogen accumulated rapidly (2.1–8.0 g N m−2 yr−1) and the oldest rehabilitating A. karroo stands (26–28 yr) had similar total nitrogen and other soil nutrient levels as stands twice their age at Cape Vidal. Seedling growth was unaffected by nitrogen supplementation. All species grew fastest in A. karroo stands demonstrating that soil nutrient levels in disturbed forest colonised by A. karroo are suitable for the growth of forest tree species. Soil fertility, including available nitrogen, is not limiting secondary succession at Cape Vidal, yet forest species are not replacing A. karroo stands at this site. Post-emergence factors, such as herbivory, are likely responsible for the arrested succession of forest in A. karroo stands.

Effects of moisture, nitrogen, grass competition and simulated browsing on the survival and growth of Acacia karroo seedlings

AbstractThe effects of irrigation, nitrogen fertilization, grass competition and clipping were investigated for one growing season at the research farm of the University of Fort Hare in the Eastern Cape Province of South Africa. The aim of the experiment was to assess the short‐term performance of Acacia karroo seedlings under different environmental conditions and the implications of such factors on the long‐term recruitment of plant species in savanna rangelands. There were no significant treatment effects on the survival of A. karroo seedlings. Using stem length and basal diameter as growth parameters, it was observed that irrigation enhanced both variables, while nitrogen fertilization did not have any significant effects. Clipping, grass competition and their interaction greatly suppressed the growth of the seedlings. Clipping increased the mean stem length when they were irrigated and fertilized. Control and fertilized plants had the highest stem length in the absence of grass competition, while grass competition combined with clipping resulted in the lowest stem length in both irrigated and nonirrigated plants. It was concluded that in the presence of grass competition, controlled browsing could be a viable solution to the problem of bush encroachment in savanna rangelands.

Meat quality of Nguni steers supplemented with Acacia karroo leaf-meal

The objective of the study was to determine the meat quality of Nguni steers supplemented with Acacia karroo leaf-meal. Thirty 19-month old steers were randomly assigned to A. karroo leaf-meal (AK), sunflower cake (SF) and a control (CN) diets. Steers on the AK and SF diets were given an additional 150 g of protein per day for 60 days. The steers were slaughtered at 21 months and the m.longissimus thoracis et lumborum was sampled for meat quality measurements. Steers that received SF diet had lighter-coloured (L*) (P<0.05) meat than those on AK and CN diets. The highest redness (a*) (17.3+/-0.29) values were recorded in steers supplemented with AK compared to those that received SF and CN diets. Diet had no effect on pH, drip loss, water holding capacity, sarcomere length, muscle bundle areas, myofibrillar fragment length, Warner-Bratzler shear force and cholesterol values. Steers supplemented with AK (22.4+/-0.08%) and SF (22.5+/-0.08%) had higher (P<0.05) meat protein content than those on the CN (20.2+/-0.08%) diet. Highest muscle fibre area (3472.6+/-119.51 microm(2)), fat content (1.2+/-0.11%) and cooking loss (25.2+/-0.73%) of meat aged for 2 days were recorded in steers given the SF diet compared to those on the AK and CN diets. Supplementing Nguni cattle with A. karroo leaf-meal produced beef of comparable quality to natural pasture alone but with a fresher appearance and higher protein content.