Browsing by Author "Kirkman, Kevin Peter."

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Above- and belowground competition in Savanna systems.
(2008) Payne, Michelle Jennifer.; Kirkman, Kevin Peter.
The structure and composition of savanna vegetation is influenced by resource availability and disturbance. Grasses, a major component of savannas, influence this resource availability by competing directly with trees for light, water and soil nutrient resources. The direct causes of bush encroachment are not always apparent, but are commonly ascribed to overgrazing and consequent decreased grass competition. The interaction, both above and belowground, between tree and grass seedlings and the surrounding grass sward is dependant on many factors, such as soil depth, seedling species and sward composition. These factors, as well as the presence or absence of defoliation, in the form of grazing or fire dictate whether the system will remain in a transition state as savanna or move towards a stable woodland state. The major competitive effects experienced by the tree seedlings were dependant on grass species and nutrient level. A. nilotica was affected by aboveground competition while A. karroo was affected by belowground competition. E. capensis caused the greatest decrease in A. karroo plant biomass. Both E. capensis and H. hirta had large competitive effects on the aboveground biomass of A. nilotica, while S. africanus had the greatest effect on belowground biomass. Increasing nutrient availability resulted in an increase in the competitive effect exerted on A. karroo, while little to no change was seen in the competitive effect exerted on A. nilotica. Soil depth constrained plant size in both tree species. The intensity of belowground interactions on tree biomass was unaffected by soil depth, while aboveground competition had a significant effect on shallow soils. Belowground competition was also of greater importance than aboveground competition in dictating tree seedling height. Grass seedlings growing on all three soil depths differed in mean mass, with E. racemosa having the least mass and T. triandra having the greatest. Simulated grazing by cutting the surrounding sward resulted in biomass increases in all three grass species. Changes in savanna composition and structure are thus likely to be influenced by initial species composition and soil depth and soil nutrient composition. While grazing creates niches for grass seedling establishment, heavy grazing has been observed to increase grass seedling mortality. Encroachment is thus more likely to occur on intensively grazed shallow and deep soils than on medium depth soils. This highlights the importance of ensuring the grass sward remains vigorous by resting and monitoring stocking rates to ensure veld is not over-utilized. It is then possible to maintain some form of tree-grass coexistence at a level where available grazing is not compromised.
Aspects of the conservation of oribi (Ourebia ourebi) in KwaZulu-Natal.
(2006) Grey, Rebecca Victoria.; Downs, Colleen Thelma.; Kirkman, Kevin Peter.
The oribi Ourebia ourebi is probably South Africa's most endangered antelope. As a specialist grazer, it is extremely susceptible to habitat loss and the transformation of habitat by development. Another major threat to this species is illegal hunting. Although protected and listed as an endangered species in South Africa, illegal poaching is widespread and a major contributor to decreasing oribi populations. This study investigated methods of increasing oribi populations by using translocations and reintroductions to boost oribi numbers and by addressing overhunting. Captive breeding has been used as a conservation tool as a useful way of keeping individuals of a species in captivity as a backup for declining wild populations. In addition, most captive breeding programmes are aimed at eventually being able to reintroduce certain captive-bred individuals back into the wild to supplement wild populations. This can be a very costly exercise and often results in failure. However, captive breeding is a good way to educate the public and create awareness for the species and its threats. Captive breeding of oribi has only been attempted a few times in South Africa, with varied results. A private breeding programme in Wartburg, KwaZulu-Natal was quite successful with the breeding of oribi. A reintroduction programme for these captive-bred oribi was monitored using radio telemetry to assess the efficacy of such a programme for the oribi. As with many reintroductions of other species, this one was not successful and resulted in many mortalities. However, many variables have been identified that contributed to the failure of this programme and they can be adapted to increase the chances that captive breeding and reintroduction be a viable conservation tool for oribi. Besides captive breeding and reintroductions, the translocation of wild animals can also be effective in sustaining wild populations. Translocations usually appreciate a higher rate of success than reintroductions. A translocation of wild oribi was attempted in this study. This involved the translocation of four males and eleven females and a year-long monitoring programme. This translocation proved to be extremely successful in establishing a sustainable wild population of oribi with few mortalities and several births. However, such translocations can only be attempted when there is suitable habitat and high security from poaching. One source of oribi for the translocation part of the study was from a housing estate that had a high density population of oribi. The existence of such a thriving population of animals on what is often a controversial type of development led to a case study investigation. Housing developments are increasing in size and in numbers in South Africa, and are rarely held accountable for the destruction caused to local habitat or wildlife. This case study used this housing estate as an example of development and conservation cooperating and enjoying the success of a thriving population of endangered oribi. Many variables have been identified that contribute to the success of this venture and that could be used as a requisite for planned housing developments in the future. In particular, clustering of houses to leave open wild areas that are managed ecologically. Finally, the issue of illegal hunting was identified as a very serious threat to oribi conservation. Using surveys, residents of rural settlements and landowners were probed about this issue so that a demographic profile of hunters could be created. This profile could then be used to make recommendations on ways to slow the spread of illegal hunting as well as educate hunters and conservation laws. The results showed that many rural people hunt on a regular basis and most hunt with dogs. It also III showed that there is a high level of ignorance amongst these people on the laws concerning conservation and wildlife species. It was concluded from this study that captive breeding and reintroduction of oribi might be a way to enhance wild populations, but might be more useful in creating public awareness. Translocation, on the other hand, was extremely successful as a way of saving doomed populations and augmenting stable ones. However, it requires suitable and protected habitat. Using housing estates as havens for endangered species is an option but only if the right legislation is passed and cooperation demanded with large portions of land remaining undeveloped. Addressing illegal hunting is the most important, and possibly the most difficult hurdle for oribi conservation besides habitat destruction. Finally, recommendations for oribi conservation and management were made based on the results from this study.
Aspects of the ecology of grass seedlings used for revegetation of degraded land.
(2010) Ellis, Meghan Jane.; Kirkman, Kevin Peter.; Morris, Craig Duncan.
As restoration ecology has matured as a science there has been increased interest in the relationship between species diversity and landscape health. Degraded landscapes tend to be resource poor, which limits species diversity as only species which are capable of growing and reproducing in these resource limiting environments can inhabit the area. Additionally, the established species are strong competitors for resources and will exclude, by way of inter-specific competition, weaker competitor species attempting to invade the degraded area. Several studies have demonstrated that with increased species diversity the overall productivity and functionality of the grassland increases. Seedling development and competitive interactions between grass seedlings has a significant impact on the final community structure and species diversity. It is for this reason that aspects of the ecology of grass seedlings were investigated. The growth and competitiveness of Chloris gayana, Cynodon dactylon, Digitaria eriantha, Eragrostis curvula and E. tef seedlings were determined under three environmental stimuli, namely nitrogen availability, light availability and exposure to plant-derived smoke (in the form of smoke-infused water). The primary conclusion from the competition experiments was that the species can be split into superior and inferior competitors at the seedling stage. Chloris gayana, E. curvula and E. tef were the most competitive seedlings as they had the largest negative effect on the growth of other species (high nitrogen Relative Interactive Index (RII) = -0.449, -0.203 and -0.379 respectively) and they were least affected by competition (high nitrogen RII = -0.251, -0.168 and -0.248 respectively). The calculated RII indicates the strength of the competitive interactions, the more negative the RII the stronger the competitive interaction. Nutrient availability had limited effect on the competitive hierarchy of the tested species. Chloris gayana seedlings, however, increased in competitiveness with an increase in available nutrients. In other words, there was a decreased negative response to competition in a high nutrient environment (high nitrogen RII -0.251, no nitrogen RII -0.605). When D. eriantha was grown under varying shade, nutrient and competition levels it was evident that the primary stress factor was light deficiency (p<0.001), and nutrient availability had no affect on seedling growth (p=0.069). Smoke-infused water had no consistent affect on the germination success or the seedling’s root and shoot vigour for the five grasses. These results indicate that the introduction of a “2-phase” or “multi-phase” restoration plan may be beneficial for the development of species diverse rehabilitated grasslands. Manipulating the time and space that the different species are planted, or the distribution of nutrient concentration over the area, may increase the survivorship of all the species that are introduced to a restoration site.
An assessment of the effect of season of grazing, stocking rate and rainfall on the dynamics of an arid rangeland on the west coast of South Africa.
(2005) West, James Alexander.; Kirkman, Kevin Peter.
A grazing trial investigating the effect of season of grazing and stocking rate initiated at the Nortier Experimental Farm in 1988 provided an opportunity to assess the response of the veld to both grazing and environmental influences in an arid environment. The trial allowed an assessment of the relative influence of internal (equilibrium) and external (non-equilibrium) forces on the dynamics of an arid rangeland. This study involved the analysis of a nine year data set stretching from 1988 to 1996 and served to provide evidence supporting the existence of an equilibrium/non-equilibrium continuum in rangeland dynamics. The most significant implication of this result is that rangeland systems should not be classified as either equilibrial or non-equilibrial, but rather according to a continuum extending between equilibrium and non-equilibrium poles. The exact position of any system on this continuum is a function of the relative influence of internal and external forces on its species dynamics. The dynamics of the veld at the Nortier Experimental Farm showed significant response to both grazing and environmental variables suggesting conformity to both equilibrial and non-equilibrial paradigms. Both ordination and analysis of variance highlighted the importance of rainfall particularly in the fluctuations of the predominant grass species, Ehrharta calycina, which increased in abundance with rainfall. Partial ordination enabled the assessment of species variation following the removal of variation associated with rainfalL Partial ordinations revealed the gradual, directional movement of samples through multivariate space in response to grazing treatments. Individual plant species were also shown to be responding to grazing, the extent of which was influenced by season of grazing and stocking rate. Both the partial ordinations and the ANOVA showed Melothria sp., Tetragonia fruiticosa and Hermannia scordifolia as increasing and Ruschia caroli as decreasing in absolute abundance in response to grazing. Season of grazing was shown to significantly influence the abundance of H. scordifolia over time. The 'shrublherb complex', which constitutes the 'key resource' at the Nortier Experimental Farm displayed an increase in absolute abundance over the duration of the trial. This increase in absolute abundance was accompanied by an increase in the relative abundance of the palatable component of this resource. The application of medium to heavy stocking rates during spring, summer and autumn and low stocking rates during winter resulted in elevated absolute abundances of palatable plants. Furthermore, low stocking rates, when averaged across all season of grazing treatments, resulted in a significantly higher absolute abundance of unpalatable plants. These findings provide the basis for the development of management principles for the Strandveld Vegetation Type. The application of medium to heavy stocking rates within a rotational grazing system, as recommended by the literature dealing with grazing systems in the Karoo, is supported by the results of the Nortier grazing trial. Medium to heavy stocking rates should be applied during spring, summer and autumn and low stocking rates during the winter months. Furthermore, it is recommended that rests of between 12 and 14 months should be afforded to portions of the veld periodically due to the variability in growth, flowering and fruiting times ofdifferent plants in the Karoo.
Backing the burn: carbon sequestration in South african mesic grasslands through sustainable fire management.
(2024) Nicolay, Robyn Elizabeth.; Kirkman, Kevin Peter.; Tedder, Michelle Jennifer.; Mkhize, Ntuthuko Raphael.
Emphasising the ancient origins of the grassland biome in South Africa, much research has supported the role of grasslands in biodiversity, ecosystem services, and economic benefits for local populations. Despite this, the grassland biome faces challenges. It is highly transformed, poorly conserved, and urgently in need of preservation and restoration. Fire-adapted mesic grasslands are distinct ecologically from arid-adapted climatic climax grassland communities in that they are adapted to frequent defoliation, necessitating burning to maintain productivity and biodiversity. To fully understand the role of grasslands in the global climate change arena, it is necessary to quantify the impacts of grassland management, including fire and livestock grazing, on carbon exchange (source versus sink). Research in this thesis looks particularly at management with prescribed fire as a form of defoliation in mesic grasslands. To do this, I quantified soil total carbon stocks, fractionated stable pyrogenic carbon stocks, and soil carbon sequestration rates in various fire regime treatments at the Ukulinga Grassland Fire Experiment (UGFE), Pietermaritzburg. Additionally, I examined the impact of fire frequency on grazed grassland at Wakefield research farm, quantifying carbon stocks and release while exploring potential mechanisms behind observed patterns in this grazed system. Lastly, I monitored Eddy Covariate carbon flux data over a four year period at research catchment six, Cathedral Peak, KwaZulu-Natal, to understand seasonal and interannual flux within mesic grassland and observe patterns in source versus sink dynamics in these ecosystems. Research has emphasised the necessity of frequent fires to maintain grassy biomes and sustain the role of biotic and abiotic factors in this biome, through biochemical soil alteration in the form of ash deposition, and the alteration of above and below ground biomass. The complexity of managing this grassland is emphasized by the need to balance the impact of herbivory, prescribed fires, and the nature of biomass accumulation in these biomes, all of which influence carbon cycling. Differences were observed between different prescribed burning regimes. Substantial differences in soil organic carbon (SOC) and total nitrogen (TotN) stocks at different soil depths were observed, with the highest stocks observed in the top 5 and 10 cm of soil across all treatments. Annual winter and spring burns exhibit the highest SOC stocks and wider C:N ratios. Triennial burns display the lowest sequestration rates in the top 0 to 5 cm of soil, with negative rates within the 5 to 10 cm horizon. Over a period of 20 years, SOC sequestration increased in a 70-year-old experiment with no signs of stabilization within the 0 to 5 cm soil horizon, but SOC loss is noted below 5 cm in areas burnt triennially. Increased fire frequency in grassland also caused a reduction in the stable fraction of black carbon (BC), and contrastingly - increased levels of BC quantified in grassland burnt infrequently or excluded from fire. This pattern may be due to reduced alkaline ash deposition and subsequently greater soil acid saturation, suggested to result in increased pyrogenic carbon particulate size and reduced breakdown of this carbon in the soil. When considering the inclusion of livestock and grazing into grassland managed with prescribed fire, findings showed no significant differences in SOC and TotN levels between annually burned grasslands and those excluded from fire. Grassland managed with annual burning showed greater soil respiration rates compared to unburned sites, indicating greater soil microbial activity and root turnover. Annual burning and heavy grazing were both associated with reduced aboveground biomass accumulation compared to the adjoining unburned grassland. Additionally, annually burnt grasslands exhibit reduced aboveground biomass lignin and fibre percentage relative to adjacent unburned areas. Findings highlighted that increased fire frequency in grazed grassland influences livestock grazing behaviours through improvement of forage palatability and available biomass, contributing to greater belowground carbon turnover. Considering the mechanisms governing carbon dynamics in fire-dependent grassland, four years of flux data showed that following rainfall events, increased soil water content is linked to a rapid rise in soil respiration, aligning with heightened biological and photosynthetic activities during warmer growing seasons. These processes determine the rate and variability of grassland uptake and release of CO2. The findings support evidence that mesic grasslands managed with regular long term prescribed fires consistently act as carbon sinks, absorbing more carbon than they emit over periods exceeding 70 years. Findings from this research advocate for management practices utilizing a frequent burn regime, suggesting that such practices maintain persistent carbon sinks in South African mesic grasslands. This approach enhances the resilience and capacity of mesic grasslands to act as effective and consistent carbon sinks, even in the face of potential future climate change impacts. The evidence from these studies shows that prescribed fire during late winter / spring in South African mesic grasslands should enhance carbon sequestration and the role of these grasslands as a carbon sink.
Browse : quantity and nutritive value of evergreen and deciduous tree species in semi-arid Southern African savannas.
(2012) Penderis, Caryn Anne.; Kirkman, Kevin Peter.
Browse selection, intake, utilisation, palatability, quality and production are tightly linked and need to be considered together in trying to improve our understanding of browsing dynamics and the interactions between browsers and vegetation. Such an understanding is necessary in order to re-evaluate determinations of browser carrying capacities and evaluating actual and potential impacts of browsing animals on vegetation composition and diversity. Browser carrying capacity is determined by both the quantity and the nutritive value of forage. The measurement of browse quantity and nutritive value and the matching of browse supply to browser demand are central to sustainable utilisation and the monitoring of vegetation health. South African savannas are poorly studied with respect to tree canopy growth and browse production making it difficult to quantify the available browse biomass on which browsing capacity estimations are based, and consequently difficult to estimate levels of browsing that are sustainable. This study addressed these issues by investigating browse dynamics, broadly aiming to (1) explore factors affecting browse production, biomass and nutritive value; (2) develop models to assess and monitor these parameters across seasons and properties; (3) use the resultant models in improving our understanding of how to determine browser carrying capacities. More specifically, our study sought to examine the effects of plant physiognomy, forage nutritive value, canopy stratum, defoliation, temperature, rainfall and soil nutrient status on the browse production of evergreen (Carissa bispinosa, Euclea divinorum, Gymnosporia senegalensis), semi-deciduous (Spirostachys africana, Ziziphus mucronata) and deciduous (Acacia nilotica, Dichrostachys cinerea) savanna tree species from June 2003 – June 2005 in three sites along the northern Zululand coastline of KwaZulu-Natal. Available browse biomass, during the dry season, of four key savanna tree species (A. nilotica, E. divinorum, G. senegalensis, and S. africana) was estimated through the development of allometric regression equations. Non-linear regression was used to investigate the relation between the leaf dry mass (LDM) and canopy volume (CVol) of each of the four tree species. Exponential regression (y = a + brlnx) of the natural logarithm of CVol data provided the most accurate and precise description of the tree CVol – LDM relation. A study was undertaken to determine which factors may influence browse production in a southern African savanna. Regression tree models for the browse production identified that the dominant factors influencing browse production were CVol (m3), season, species and height to the lowest leaves of the tree canopy (HL) (m). The length of the growing season had a marked effect on the production potential of savanna tree species, suggesting that improved conditions for growth, i.e. greater rainfall, soil moisture content and improved soil nutrient availability result in a longer period of rapid sustained growth. Species was identified as an important contributing factor to differences in browse production rates, suggesting the need for the development of species or species group models. Mean annual browse production of evergreen trees was greater than that of deciduous and semi-deciduous trees. Mean quarterly (three monthly) browse production was highest, for all trees, during the wet season, with the greatest difference between wet and dry season production being observed in deciduous forms. Evergreen forms showed continuous growth over the whole study, with enhanced growth over the wet season. Deciduous forms, on the other hand, concentrated growth in spurts, when environmental conditions became favourable, with most production occurring during a short growing season. Browse nutritive value was found to be greatest during the wet season, when growth and photosynthesis are at their greatest. Further, browse nutritive value was greatest in deciduous species. Evergreen trees were found to have greater acid detergent fibre (ADF) concentrations than both the deciduous and semi-deciduous trees. By contrast, crude protein (CP) concentrations were greater in semi-deciduous and deciduous species than in evergreen species. The daily CP requirements for maintenance for an adult impala (45 kg) were met by all species over all three study areas and all seasons. Daily CP requirements for growth and lactation, however, were only ever met by deciduous and semi-deciduous species, though this result was not consistent over study areas and seasons. Predictive models for the production of browse on deciduous, semi-deciduous and evergreen trees in northern Zululand were developed using multivariate adaptive regression spline functions. The best predictors of growing season browse production in all three tree guilds (defined here as a group of trees having a characteristic mode of living) were primarily measurable tree dimensions, while the prevailing environmental conditions had little impact. Differences in the production, nutritive value and available browse biomass between the different tree forms and seasons have a profound effect on the determination of browser carrying capacities and need to be incorporated into any game or conservation management plan.
Burning wetlands: the influence of fire on wetland vegetation structure and composition.
(2013) Luvuno, Linda.; Kirkman, Kevin Peter.; Kotze, Donovan Charles.; Morris, Craig Duncan.
Water is a very important component of the natural world and human survival but water sources (river systems and wetlands) are becoming increasingly degraded and less functional. In particular the increase of woody C3 species into wetlands is a cause for concern, as they invade wetlands which are predominantly herbaceous. Woody species use more water than herbaceous species and this impacts wetland function. In moister savannahs and grasslands woody species are influenced significantly by fire, and fire is consequently used widely as a means of reducing woody plant density. However, in wetlands there is uncertainty about the effectiveness of fire in combating woody plant encroachment and the general impact of fire. The Kwambonambi wetlands of South Africa have been recently experiencing an invasion by woody species which are both indigenous and alien. This area was historically herbaceous and experienced frequent natural fire but is now largely under timber plantation and thus fire has been mainly excluded. This has led to a continual increase of woody species into the wetland and has seen a change from mainly herbaceous to a matrix of fern, herbaceous grasses and sedges and an invasion of swamp forest species such as Macaranga capensis. This has now affected ecosystem functions and changed fire behaviour in these wetlands. A search through the literature has revealed a lack of studies which investigate the influence of fire on wetland structure and composition. This ambiguity highlights the need for more focused research that will influence management decisions. In order to develop meaningful management strategies, there needs to be a good understanding of the problem and the underlying processes contributing to the degradation and loss of the system you are trying to manage, in this case it is wetlands. This study investigates wetland changes and losses at a small spatial and temporal scale for informing management on the best use of fire on wetlands. A temporal study (a change detection analysis) reveals that the main drivers of the vegetation structure in this landscape are the land use/land cover change in the form of large scale plantation forestry coupled with fire suppression. 92.4% of the landscape has been altered with the greatest degree of change in this landscape accounted for through the change from grassland and herbaceous wetland (1519ha and 524ha loss respectively) to timber plantation and the spread of indigenous forest indicated by an increase of 70% and 11% increase respectively. The large scale plantation forestry in the landscape has led to the drying of the landscape (which affects the hydrology of the wetlands) and therefore reduces the levels of soil saturation. Simultaneously, plantation forests are fire suppression areas to avoid tree loss. These factors, together with the disturbance of converting wetlands into plantation forest and clear felling (which occurred to 7%/155ha of the wetlands in the study site), have allowed forest species such as the fern Staenoclina tenuifolia and Macaranga capensis to invade the wetland areas. Over time, the combination of fire suppression, disturbance and drying encourages the establishment of woody seedlings, turning wetlands into swamp forests/woodlands. This regime shift is more evident in wetlands which were once converted into plantation forest with insufficient woody plant species control to accompany the withdrawal of plantation. The few wetlands which have maintained their herbaceous structure and function are those maintained with fire as a management strategy. A burn experiment shows that fire does have a significant negative effect on tree density in these wetlands-especially previous disturbed wetlands. The recommendation from this study is to remove the forest species out of the wetlands and reintroduce fire (biennial burns) into the management of these wetlands. A better relationship between the forest managers and researchers is recommended to continually co-adapt to any changes occurring in these wetlands.
Cattle and veld interactions at the Armoedsvlakte Research Station.
(2011) Le Roux, Gustav Nic.; Du Toit, Justin Christopher Okes.; Kirkman, Kevin Peter.
A long-term grazing trial was started in 1977 at Armoedsvlakte Research Station, about 10km west of Vryburg, in Tarchonanthus veld of the Ghaap’s Plateau, which is a variation of the Kalahari Thornveld veld type. The main aim of this study was to use the extensive veld condition and animal production data set to investigate the effects and interactions of stocking rate, grazing system applied and seasonal rainfall on veld condition and cattle production. The grazing trial has changed three times since its inception resulting in three different phases. The main changes in veld condition during phase one (1977-1991) was due to density independent effects (e.g. seasonal rainfall) and not density dependent effects (e.g. stocking rate). A major change occurred in 1985 following a multiple year drought. The drought resulted in adverse changes in species composition, basal cover and residual biomass of all treatments. The system did not recover from the drought during phase one, despite well above mean seasonal rainfall for a number of years after the drought. During phase two (1992-1999) and phase three (2000 to present) completely different vegetation dynamics occurred than what was experienced during phase one. Density dependent effects (e.g. stocking rate) were more important in explaining variation in veld condition during these two phases. High stocking rates resulted in adverse changes in species composition, poor basal cover and a low residual biomass production. It is however important to note that seasonal rainfall did explain a significant additional amount of variation in veld condition. This suggests that a continuum of non-equilibrium and equilibrium vegetation dynamics occurred in these two phases. The residual biomass and seasonal rainfall model for phase one indicate completely different results for the gain per animal data. In the seasonal rainfall model, stocking rate does not have a significant effect on gain per animal, but seasonal rainfall and the interaction of stocking rate with seasonal rainfall explains most of the variation in gain per animal. This suggest a continuum of non-equilibrium and equilibrium dynamics and that animal production is more sensitive to seasonal rainfall than to stocking rate, although the significant interaction of stocking rate with seasonal rainfall suggest that the seasonal rainfall effect on animal production is dependant on stocking rate. The residual biomass model however indicates that stocking rate is more important than rainfall in explaining variation in the mass gains per animal. The stocking rate effect on gain per animal was significant and indicated that as stocking rate increased, that gain per animal decreases. Seasonal rainfall and the interaction of stocking rate with seasonal rainfall had no significant effect on gain per animal. The amount of variation explained by the seasonal rainfall model was larger than the residual biomass model and this indicates that rainfall explains more variation in gain per animal, than residual biomass does. This possibly indicates that non-equilibrium effects are stronger than the equilibrium effects, but it is important to notice that stocking rate had a significant effect in some cases. The gain per hectare models (seasonal rainfall and residual biomass) for phase one indicates that stocking rate has a significant effect on gain per hectare. Increasing stocking rates resulted in higher gain per hectare, which suggests that the turning point of the typical “Jones and Sandland model” has not been reached and this might be due to light stocking rates applied during the duration of phase one. The seasonal rainfall model however has significant effects of seasonal rainfall and interactions of stocking rate with seasonal rainfall on gain per hectare. This suggests that the effect of stocking rate is dependent on seasonal rainfall and that seasonal rainfall explain an additional amount of variation in gain per hectare. In general, it appreared that the optimal stocking rate for animal production was higher than those applied during the duration of the trial, but this is due to lower than planned actual stocking rates applied during all three phases of the trial. It is very difficult to determine a generic optimal stocking rate for different rainfall volumes and it is recommended that the actual stocking rate for different ecological zones be determined based on rainfall, biomass, species compos[i]tion, basal cover and available browse and not just on the provisional recommendations. The type of grazing system applied did not show any statistically significant effects on both gain per animal and gain per hectare for the animal production data during phase one. This result is interesting and contradictive to most of the scientific literature where some authors concluded from their studies that rotational grazing systems produce higher animal production than continuous grazing systems, whereas others researchers state that continuous grazing systems produce higher animal production than rotational grazing systems. In phase two both the residual biomass and seasonal rainfall models for phase two did not show any significant effects and interactions of stocking rate, seasonal rainfall level and/or residual biomass on both gain per animal and gain per hectare. Both the residual biomass and seasonal rainfall models for phase three did not show any significant effects and interactions of stocking rate, seasonal rainfall level and/or residual biomass on animal gains per animal. The seasonal rainfall model did not show any any significant effects and interactions of stocking rate, seasonal rainfall level and/or residual biomass on animal gains per hectare. However, the residual biomass model indicated that stocking rate had a significant effect on gain per hectare and the production closely followed the Jones and Sandland (1974) model as at low stocking rates, gain per hectare increases at a rapid rate, but as stocking rates increases to high stocking rates, the rate of increase in gain per hectare declines, until it eventually reaches a turning point, where after gain per hectare declines with increasing stocking rates. Stocking rate only had a significant effect on the condition score of cows during phase two and phase three, as high stocking rates resulted in poor animal condition in both phases. No significant effects and interactions of stocking rate and seasonal rainfall were indicated on calving percentage, weaning percentage, conception rates and percentage of desirable meat produced during phase two.
The coastal grasslands of Maputaland, South Africa : effects of fire and grazing on vegetation structure, diversity, and composition.
(2007) Dalton, Brian Patrick Alexander.; Kirkman, Kevin Peter.
A series of trials and investigations were implemented to address concerns surrounding the dynamics of the fire-climax wooded/edaphic grasslands within the iSimangaliso Wetland Park, northern KwaZulu-Natal, South Africa. The research problem surrounded inadequate historical evaluations of changes in vegetation structure, grasslands progressing to a woody dominated composition, and increases in Helichrysum kraussii (Curry bush). These were addressed as follows: Firstly, the recovery of vegetation in response to different periods of fire exclusion in different communities along a topographical gradient of a coastal dune area, was assessed over a two year period. Secondly, the regeneration after wildfire of the persistent, stress tolerant shrub H. kraussii, was studied on different catenal positions with differing fire exclusion periods and with and without defoliation of surrounding plant biomass in the coastal edaphic grasslands north of Manzengwenya, South Africa. Thirdly, aerial photography from 1937, 1975, and 2000 was georectified, digitised and analysed using a Geographic Information System to examine broad vegetation changes in response to different management regimes for a site on the Eastern Shores of Lake St Lucia and a site within the Tewate Wilderness Area. In the absence of fire, the coastal edaphic grasslands progressed to a closed canopied scrub forest within six years. An increase in fire exclusion period resulted in a decrease in species abundance, an increase in woody height, and a decrease in plant density. Richness increased initially but declined marginally with increased fire exclusion period. Higher lying east and west facing sites had a better veld condition index compared with bottom sites and had an increased response (vigour) to defoliation but were far more likely to succeed through to woody scrub forest. Woody plant biomass vigour was greater for west facing sites. Ordination of species composition across sites in response to fire exclusion and catenal position revealed greater similarities within exclusion periods than between. Bottom sites were more similar with similarity decreasing for east and west facing sites. Fire exclusion resulted in an initial increase in woody species and a subsequent increase in herbaceous species. iii Growth response of H. kraussii was unaffected by catenal position and fire exclusion period, whereas defoliation of surrounding grass tended to increase in size (P<0.05). Density and height for this species however increased with increasing fire exclusion. An increase in soil moisture negatively affected H. kraussii growth indicating susceptibility to high water tables. The number of other woody species establishing beneath H. kraussii may be due to changes in the transmission of light through the canopy where an increase in canopy diameter resulted in an increase of photosynthetically active radiation at the soil surface. The effects of fire on landscape change were investigated for the Eastern Shores and Tewate Wilderness Area, iSimangaliso Wetland Park, South Africa using aerial photography. Changes to historical disturbance regimes largely through active exclusion of fire resulted in the majority of the higher lying coastal grasslands changing to savanna scrub or closed canopied forest within 63 years on the Eastern Shores. The degree of fragmentation of these grasslands was greatly reduced within the Tewate Wilderness Area where disturbance regimes included greater frequencies of fire. Hygrophilous grasslands remained largely unaffected by woody encroachment but did not preclude woody species establishment indicating possible susceptibility during long drier periods. Frequent fires result in the maintained distribution of the higher grasslands. This vegetation type is a system which becomes resilient in response to fire, whereas in the absence of fire readily progresses to Dune Forest. The coastal grasslands above the high water table are therefore highly unstable and transformed easily in the absence of regular disturbance. It would appear that a threshold of approximately six years exists, after which substantial management intervention may be required to reverse the succession back to grassland. The growth of H. kraussii was unaffected by fire and remained persistent irrespective of fire exclusion period. An ability to attain size (height and canopy diameter) was limited with increased soil moisture but density was reduced through regular burning. Frequent fires are necessary to reduce density of H. kraussii and reduce the competitive advantage gained with age.
The competitive effect of vetiver grass on native grasses and implications for grassland rehabilitation in South Africa.
(2018) Dlamini, Lindokuhle Xolani.; Kirkman, Kevin Peter.; Tedder, Michelle Jennifer.
Astract available in PDF file.
Determinants of community composition and diversity in KwaZulu-Natal mesic grasslands : evidence from long-term field experiments and pot and plot competition experiments.
(2003) Fynn, Richard Warwick Sinclair.; Kirkman, Kevin Peter.
A predictive understanding of plant community response to various environmental influences (e.g. type, timing and frequency of disturbance, site productivity, fertilization, etc.) is a general goal of plant ecology. This study sought to further understanding of mesic grassland dynamics in KwaZulu-Natal using long-term field experiments (> 50 years) and short-term pot and plot competition experiments. The specific objectives were to: 1) examine the effects of long-term burning of grassland on soil organic matter content because of its potential impact on nitrogen cycling and community composition, 2) examine patterns of community composition and species richness in response to different type, timing and frequency of disturbance (burning and mowing) in a long-term grassland burning and mowing experiment and to different type and amounts of fertilizer application in a long-term grassland fertilization experiment, 3) develop hypotheses concerning the response of different species to disturbance and fertilization, and test these hypotheses using pot and plot competition experiments, and 4) provide a general synthesis of the results of the various field, pot and plot experiments that may be used to develop a predictive theoretical framework for mesic grassland dynamics. Total soil nitrogen was lowest in sites burnt annually, intermediate in sites burnt triennially and highest in sites protected from disturbance and sites mown annually in the dormant-period (spring or winter). Winter burning reduced soil organic carbon and total soil nitrogen more than spring burning. Mineralizable nitrogen was reduced by burning. The different effects of type, timing and frequency of disturbance on total soil nitrogen appeared to be an important determinant of community composition and species richness. Short-grass species (Themeda triandra, Eragrostis capensis, Heteropogon contortus, Diheteropogon amplectens and Eragrostis racemosa) were most abundant in annually burnt sites (especially winter burnt sites), whereas medium and tall-grass species (Eragrostis curvula, Cymbopogon spp., Hyparrhenia hirta and Aristida junciformis) were most abundant in triennially burnt sites, sites protected from disturbance and sites mown annually in the dormant-period, all of which had higher total soil nitrogen than annually or biennially burnt sites. Species richness and short-grass species declined with increasing levels of nitrogen fertilization in the fertilizer experiment and declined with increasing productivity and nitrogen availability in both the fertilizer and burning and mowing experiments. Thus, it was hypothesized that the type, timing and frequency of disturbance resulted in different compositional states through different effects on soil resources (especially nitrogen), which affected the competitive balance between short and tall species. The hypothesis that composition was determined by disturbance-mediated soil nitrogen availability was supported by competition experiments, which revealed that shortgrass species were most competitive in low-nutrient/low-productivity treatments and tall-grass species most competitive in high-nutrient/high-productivity treatments. The fertilizer experiment and a competition experiment revealed that tall broad-leaved species were most competitive in sites of highest productivity, fertilized with both nitrogen and phosphorus, whereas tall narrow-leaved species were most competitive in sites of intermediate productivity, fertilized with nitrogen only. It was hypothesized that summer mowing increased the abundance of short-grass species and decreased the abundance of tall-grass species in the burning and mowing experiment by increasing the competitive ability of short-grass relative to tall-grass species, rather than the tall-grass species being less tolerant of mowing. A competition experiment revealed that tall-grass species (Hyparrhenia hirta and Panicum maximum) were as tolerant of cutting as a short-grass species (Themeda triandra). Themeda triandra was shown to become extremely competitive in cutting treatments, reducing the biomass of most other species relative to their monoculture biomass, showing that its dominance of mown sites in the burning and mowing experiment was a result of its superior competitive ability rather than greater tolerance of mowing. However, many tall erect herbaceous dicots appeared to be intolerant of summer mowing, probably because their meristems are aerial and easily removed by mowing, whereas short creeping herbaceous dicots were increased by summer mowing probably because their meristems were below the mowing height. Further, these short species would be vulnerable to shading in unmown sites. Thus, species with basal meristems (hemicryptophytes) or meristems near the soil surface (geophytes) appear to be more tolerant of mowing than species with aerial meristems (phanerophytes), but the tradeoff is that a low meristem height renders them vulnerable to shading in unmown sites. Very high litter accumulation in the sites protected from disturbance appeared to have a direct influence on community composition and species richness. Species that dominated these sites (e.g. Tristachya leucothrix & Aristida junciformis) initiated tillers below-ground and had sharp erect shoots that appeared to be an adaptation for penetrating litter. Species that initiate tillers below-ground are probably less vulnerable to the effects of shading by litter because their tiller initiation is not dependent on high light availability. The fact that Aristida junciformis was shown to have very low competitive ability in two competition experiments, suggests its dominance of protected sites was through tolerance of high litter levels rather than competitive exclusion of other species. Low grass species richness in these sites was probably a result of an inability of many species to tolerate these high litter levels. This study has revealed that inherent site productivity and its interaction with the effect of disturbance on soil resources and litter levels is a major determinant of community composition and species richness. The effect of type, timing and frequency of disturbance on soil nitrogen was able to account for the principal changes in community composition. Thus, the influence of disturbance on soil nitrogen is a unifying principle in plant ecological theory that enables greater understanding of disturbance-composition relationships. However, intolerance of certain forms of disturbance (e.g. mowing) by species with aerial meristems, or intolerance of accumulating litter in the absence of disturbance by species without sharp erect shoots, may also have important influences on composition. In addition, this study has revealed that plant traits (height, leaf width, position of tiller initiation, shoot morphology and position of meristems) were well correlated with the various effects of disturbance and fertilization on community composition, indicating that a plant trait-productivity-disturbance framework has great potential for understanding and predicting species response to disturbance and multiple limiting nutrients.
Drivers of vegetation change in the eastern Karoo.
(2019) Du Toit, Justin Christopher Okes.; Kirkman, Kevin Peter.
The Nama-Karoo Biome occupies much of the western central region of South Africa and transitions into the Grassland Biome along its eastern boundary along a gradual ecotone. The area is characterised by hot summers and long, frosty winters, relatively low rainfall, peaking in mid- to late-summer, with high inter-annual variability, and botanically by a co-existence of grasses and dwarf shrubs, with grass abundance positively related to average annual rainfall that increases from west to east. Biome shifts in response to changes in rainfall pattern and grazing have been suggested but never directly examined. Major drivers of botanical composition are rainfall and grazing by livestock. Fire is rare, occurring sporadically if high rainfall allows for good grass growth. This thesis focused on understanding the influence of rainfall, grazing, low temperatures, and fire on botanical composition at Grootfontein, a site in the ecotone between the Nama-Karoo and Grassland Biomes that is home to grazing trials up to 85 years old. The following specific questions were addressed: Question 1: Over the long term, has Grootfontein shown patterns of rainfall cyclicity or experienced directional change, and how might these have influenced composition and productivity? Using data from 1888 to 2012, cyclicity in rainfall was evident for periods of approximately 20 and 60 years. Rainfall has also consistently increased since the mid-1970s, and this increase corresponds with a general pattern of increased grassiness in the eastern Karoo. Question 2: How do rainfall and grazing, alone and in interaction, influence vegetation composition in the eastern Karoo? Compositional data from the 1960s and 2010s from various treatments at two sites at Grootfontein (Camp 6 and Seligman grazing trials) show a shift from dwarf-shrub- to grass-dominated vegetation, consistent with the increased rainfall over that time. An influence of grazing, both present and historical, was evident but secondary to the effect of rainfall. In some cases, there has been a shift to grassland. Data from the Camp 6 and Seligman grazing trials from the 1940s to the 1960s further indicate a combined influence of season of grazing and of rainfall. High-intensity summer-only grazing by livestock largely extirpates grasses and allows shrubs to thrive, while summer grazing in the form of rotational grazing or continuous grazing allow for a balance of grasses and dwarf-shrubs. Severe declines in both grasses and shrubs occurred apparently in response to drought, though the exact conditions required to cause such mortality remain unclear. Plant cover data from 2008 to 2015 from the Boesmanskop grazing trial showed that consecutive years of exceptionally high rainfall increased plant cover to nearly 100%, and increased the abundance of grasses. Competitive exclusion of dwarf-shrubs by grasses was not evident. Question 3: What have been the trends in minimum temperatures, frost, and potential growth season at Grootfontein, how might these have influenced botanical composition and productivity, and is there evidence of increasing temperatures consistent with global warming? This is addressed using minimum-temperature data from 1916 to 2014. Minimum temperatures were lower than are usually reported. Variability in minimum temperatures was high, including a cooling from the 1910s to the 1950s and a warming from the 1950s to the 2010s. The length of the growing season (last frost to first frost of the subsequent season) varied considerably, and may have the potential to influence botanical composition. Question 4: What is the influence of fire in Karoo vegetation? Based on the effects of a single fire on Grootfontein, fire killed some species while most species resprouted. Grasses appeared unaffected in terms of survival, several species of dwarf shrub (notably Eriocephalus ericoides and Ruschia intricata) were killed, and will need to re-establish by seed (termed nonsprouters), while most dwarf shrub species resprouted. This resprouter/ nonsprouter dichotomy was found to be evident at a range of other fire sites in the Karoo. Heavy grazing appeared to strongly impede the recovery of burnt veld, maintaining it as a sparse grassland dominated by annual species and occasional unpalatable shrubs. Should the grassiness of the Karoo continue to increase, then fire may become more frequent thereby maintaining a grassland state. The findings allow for greater understanding of interactions among rainfall, grazing, and fire in eastern Karoo ecosystems, and these are discussed in the context of an existing state-andtransition model of eastern Karoo vegetation dynamics. The importance of long-term rainfall trends, rather than short-term variability, are highlighted. Long-term increases in rainfall will likely induce a biome shift to grassland, concomitant with a drastic reduction in dwarf-shrubs. This will likely alter both long-term carrying capacity for livestock and the type of animals that may be optimally stocked. Increased grassiness will result in the increased likelihood of fire, and if post-fire grass fuel loads remain above a critical level, a fire/grass feedback loop may be initiated whereby dwarf-shrubs are largely eliminated owing to their slow rates of growth or re-establishment. Introduction of infrequent fire will likely result in resprouter-dominated vegetation proliferating. It is demonstrated that the resilience of Karoo veld may be higher than previously thought, with severe grazing, droughts, and fire not pushing veld beyond a threshold into a state of denudation. Thus the prospects of conserving Karoo landscapes despite historical management remain high. Some key future research efforts needed to improve our understanding of Karoo ecology include the life-histories of dwarf-shrubs, the conditions of drought and herbivory under which grasses and dwarf-shrubs die, and how and when perennial dwarf-shrubs and grasses regenerate. Based on historical trends, the continued existence of long-term research trials, such as those at Grootfontein, may be under threat and should receive attention.
Dry woodland and savanna vegetation dynamics in the Eastern Okavango Delta, Botswana.
(2012) Tedder, Michelle Jennifer.; Kirkman, Kevin Peter.; Bonyongo, Mpaphi Casper.; Morris, Craig Duncan.; Trollope, Winston Smuts Watts.
The Okavango Delta is an extremely dynamic system with variable vegetation comprised of permanent swamps, seasonal swamps, dry islands, floodplains and dry grassland, savanna and woodland. The system is largely driven by the interaction between fire and the annual flood, which filters down from the Okavango River catchments in Angola. While extensive research has been conducted on the flood-driven vegetation little is known about the dry woodland and savanna regions bordering these flood-driven habitats. A taxonomic classification of woody species composition resulted in eleven vegetation types. These data were then reanalyzed in terms of woody species morphology allowing these eleven vegetation types to be grouped into four functional response groups in order to provide a platform for improving the understanding of how dry woodland and savannas interact with the environment. These four groups were the savanna group mixed thornveld and the three woodland groups; mixed broadleaf woodland, shrub mopane woodland and tall mopane woodland. Burning in mixed thornveld and mixed broadleaf woodland was found to decrease woody species density and grass fuel loads and could be used for grazing management to remove unpalatable growth and improve grass species composition, while burning in shrub mopane woodland and mixed mopane woodland merely decreased the woody understory and is not recommended. Utilization dominated by grazing livestock resulted in overutilization of the grass sward leading to bush encroachment in both mixed thornveld and shrub mopane woodland, while utilization by goats alone resulted in underutilization of the grass sward and a dominance of herbaceous annuals. Livestock utilization had no effect on the occurrence of Pecheul-loeschea leubnitziae, a shrubby pioneer previously thought to be an indicator of overgrazing, however extensive P. leubnitziae cover was associated with a sward dominated by shade-tolerant grasses with low forage quality. Shrub mopane woodland and tall mopane woodland appear to be more stable vegetation states than mixed broadleaf woodland and mixed thornveld being less vulnerable to colonization by pioneer species and alteration as a result of utilization or environmental factors. For this reason management and monitoring of mixed thornveld and mixed broadleaf woodland is essential to prevent vegetation degradation and to ensure optimal forage availability for both livestock and wildlife.
Ecological effects of Festuca costata on the Drakensberg grasslands.
(2018) Mwambilwa, Kabemba.; Tsvuura, Zivanai.; Kirkman, Kevin Peter.
Fire and herbivory are important in determining the vegetation structure in savannas and grasslands. In addition, anthropogenic activities have significantly contributed to alterations in the relative abundance and distribution of plant species in many grasslands through increased nitrogen deposition and elevated concentration of carbon dioxide in the atmosphere. Using a field experiment, the study explored the effect of neighbouring plants, defoliation, and fire on the ecological performance of a major C3 grass, Festuca costata, whose range is expanding in Afro-temperate grasslands of the Drakensberg Mountains in South Africa. The experiment had three factors: fire at two levels (burned, unburned), defoliation by clipping 50mm above-ground surface, at four levels (no defoliation, defoliation of focal plant, defoliation of neighbouring plant within a 300 mm radius, and defoliation of both the focal and the nearest neighbouring grass within a 300 mm radius from the focal plant). The third factor was plant neighbourhood types at three levels (the focal plant growing alone, focal plant growing with other F. costata plants, focal plant growing with any other grass species) for a total of 24 treatment combinations with five replicates. Using a pot experiment under garden conditions, the study evaluated the influence of soil nutrient amendment with 140 KgHaˉ¹ limestone ammonium nitrate in the performance of F. costata. The variables of interest in both experiments were; above-ground biomass production, tuft circumference, number of tillers and specific leaf area of the focal plant. Generalised Linear Models in SPSS were used to analyse the data from both experiments. The results showed that burning had a significant negative effect on above-ground biomass production, number of tillers and tuft circumference of the focal plant (P < 0.001), with the unburned grassland showing greater mean values for all parameters than the burned grassland. Defoliation had significant negative effects on above-ground biomass production, number of tillers, tuft circumference while it showed strong positive influence on specific leaf area of the focal plant. Neighbourhood types were significantly negative for above-ground biomass production, number of tillers and tuft size of the focal plants (P< 0.001 in all cases). Nutrient addition reduced the number of tillers of F. costata and tuft size and showed no significant effects on above-ground biomass production and specific leaf area. Defoliation significantly increased SLA, above-ground biomass production, number of tillers and tuft circumference were not affected.. Plant neighbourhood types showed significant negative influences on tuft circumference. Defoliation accounted for the most variations in reduced tuft size, above-ground biomass and number of tillers, however, it resulted in increased specific leaf area. These results suggest that defoliation by herbivores and use of fire may be complementary tools for managing high altitude grasslands under climate change. The findings of both the field and pot experiment suggest that F. costata may not be tolerant to selective herbivory. In addition, the study shows that the increase in nitrogen availability due to anthropogenic activity may have a negative influence on the performance of F. costata.
The effect of application of nitrogen, phosphorus, potassium and sulphur fertilisers to a perennial ryegrass sward on yield, quality and apparent intake by dairy cows.
(2010) Findlay, Nicola Jean.; Du Toit, Justin Christopher Okes.; Kirkman, Kevin Peter.
Perennial ryegrass is an intensive, temperate pasture grass that responds well to applied fertiliser. The purpose of this project was to study the effects of fertiliser on the productivity and quality of perennial ryegrass in KwaZulu-Natal and how this impacts on animal intake. It was hypothesised that over-application of fertiliser to a perennial ryegrass pasture would negatively affect the quality of the herbage for grazing by dairy cattle and that intake would be affected. Thus the project aimed to assess the effects of applied fertiliser on yield, quality and intake of an established perennial ryegrass pasture. The trial consisted of a set of six separate experiments. Each experiment focused on the interaction between two of the major nutrient elements nitrogen (N), phosphorus (P), potassium (K) and sulphur (S). The experiments (NxP, NxK, NxS, PxK, PxS and KxS) were managed separately to avoid possible transfer of nutrients during grazing, which would result in the contamination of treatments. Each factor had three levels (low, medium and high), giving a total of nine treatments per experiment. Each of the experiments was replicated three times in a randomised block design. Increased fertiliser N application rates increased perennial ryegrass yield with a pattern of diminishing return, where split applications above 40 kg N ha-1 produced smaller increases in yield when compared with the response at lower applications of N. Applied P, K and S did not affect yield, suggesting that even the lowest application levels were sufficient to not limit production. Nitrogen application affected apparent intake, but it is suggested that this is due to the yield effect rather than a direct effect of N on apparent intake. The application of P, K and S did not affect apparent intake. Results from this study showed that the quality of perennial ryegrass herbage, especially in terms of feed value to dairy cows, can be significantly affected by applied fertiliser. The extent of the response was affected by sampling date (i.e. time of year) and this must be taken into account when planning a fertiliser management strategy. This is particularly so with respect to N fertiliser recommendations. Crude protein (CP) content of herbage increased with increasing levels of applied N and the extent of the response was influenced by season. P, K and S did not affect CP concentration in herbage, except in the PxK experiment where increased levels of K lowered herbage CP. Applied N considerably increased the concentration of non-protein nitrogen (NPN) in perennial ryegrass herbage. P and S did not affect NPN levels, whereas applied K decreased NPN content in the iv NxK and PxK experiments. Non-structural carbohydrate (NSC) content of herbage was decreased by applied N but was unaffected by applications of P, K and S. Neither neutral detergent fibre (NDF) nor acid detergent fibre (ADF) was affected by applied fertiliser. In this study herbage P declined and herbage Ca increased with increasing levels of applied N. The addition of fertiliser K resulted in lower herbage Ca values. There was no herbage S response to applied fertiliser in this study. Classification and regression tree (CART) analysis identified the primary determinant of apparent intake in experiments containing N as a factor as the amount of material available to be grazed and that NSC, NPN and ADF are also determinants of apparent intake. Cows do not regulate diet choice within the short-term time frame of a meal. Thus intake is determined by short-term needs rather than by meeting long-term nutrient requirements. Fibre creates physical fill within the rumen, thus restricting intake. High NPN content is associated with high nitrate values. The reduction in intake of herbage with high nitrate content may be due to reduced palatability or to a negative feedback system limiting the intake of nitrate and ammonium. Increased NSC content is associated with increased intake, possibly through the effect of sugar on herbage palatability.
Effect of fire frequency on herbivore distribution and behaviour in the Kruger National Park, South Africa.
(2012) Chamane, Sindiso Charlotte.; Kirkman, Kevin Peter.; Hagenah, Nicole.; Smith, Melinda.
Fire plays an important role in structuring and maintaining savanna grassland ecosystems. Although regular fires are a characteristic feature of savannas, the effects of fire frequency on these systems are less well known, particularly with respect to how frequency of fire influences large herbivore distribution and behaviour. The expectation is that large herbivores should be attracted to frequently burned sites as a consequence of changes in forage quality and quantity, and/or vegetation structure and composition. The former could be driven by alterations in soil nutrients, such as N and P. Alterations in vegetation also could be important in determining risk of predation. For example, an increase in woody vegetation could decrease predator visibility making large herbivores more vulnerable to predation. The objectives of this study were to investigate the effects of long-term alterations in fire frequency on herbivore distribution and behaviour, as well as the mechanisms (soil nutrients, vegetation structure and composition, and forage quality and quantity) potentially driving the distribution of large herbivores. To address these objectives, I conducted large herbivore surveys on a bi-weekly basis from 2009-2010 in a series of plots in the Experimental Burn Plots (EBPs) burnt at different frequencies (annual, triennial and unburnt) over the last five decades at three study sites in the Kruger National Park, South Africa. Surveys also were conducted on new plots that were established adjacent to the long-term plots. These new plots have a fire return interval of 4 years which is similar to the triennially burned plots of the EBPs. They were established in the landscape adjacent to the EBPs to assess whether the responses of herbivores to fire observed in the EBPs reflected was at landscape level. The distribution of all large herbivore species combined and of grazers (e.g. zebra) or browsers (e.g. kudu) only were not affected by fire frequency. In contrast, the abundance of mixed-feeders, such as impala, was significantly higher in the unburnt (control) and annually burned plots than the triennially burned plots. Although season did not have a significant impact on the distribution of browsers and mixed-feeders, overall more grazers were recorded across all burn treatments in the dry season compared to the wet season. Similar patterns of herbivore distribution were observed between the new plots and the triennially burned EBP plots, suggesting that responses observed to the long-term fire frequency treatments reflects herbivore responses at the landscape level. The long-term fire frequency treatments significantly affected soil nutrients (N, organic C, P, and K were significantly lower with annual burning), vegetation structure (abundance of woody plants were greater in unburned plots), and forage quantity (unburned plots had higher biomass) but not quality. More frequent fires improved visibility by reducing tree height and density and herbaceous biomass, thereby potentially reducing predation risk, when compared to less frequent burning. As a result, herbivores selected sites with more frequent fires. The behaviour of the herbivore species investigated was predominantly influenced by seasonal-induced changes to their environment rather than fire frequency. In the wet season irrespective of the burning treatment visibility was low due to high rainfall that increases plant biomass, whereas in the dry season visibility was improved because there is little to no rainfall. This potential alteration in predation risk likely resulted in herbivores being more vigilant in the wet season than the dry season. Overall, results from this study suggest that the combination of fire frequency and season drive herbivore distribution and behaviour by altering mainly the vegetation structure which can influence predation risk.
The effect of high-density, short-duration stocking on soils and vegetation of mesic grassland in South Africa.
(2018) Chamane, Sindiso Charlotte.; Kirkman, Kevin Peter.
High-density, short-duration stocking (HDG) is currently gaining popularity amongst farmers in South African mesic grasslands but little is known about its potential impact on soil properties and plant species composition, particularly the forbs (herbaceous dicotyledonous and non-graminoid monocotyledon) which contribute more to plant diversity than grasses. Under HDG, animals are stocked at higher stocking rates and densities than conventional grazing systems and burning is discouraged. This study used a fence-line contrast approach to compare the long-term impact of “real world” HDG systems with rotational grazing systems at a lower stocking density (LDG) on soils and vegetation composition including forb growth habits at two study sites, Kokstad and Cedarville. An experimental trial was set up at Ukulinga Research Farm to determine the short-term effects of HDG compared with no grazing on plant species composition and demography of the selected perennial forbs. Another field experiment was used to determine the response of three mesic grassland perennial forb species (Afroaster hispida, Gerbera ambigua and Hypoxis hemerocallidea) to intense defoliation and interspecific competition with a grass species (Themeda triandra). Soils were more compacted under HDG but soil chemical properties did not differ between HDG and LDG at both Kokstad and Cedarville. There was a low percentage cover of desirable palatable grasses and high forb species turnover under HDG at Kokstad and low grass and forb species responses at Cedarville. There was high litter accumulation under HDG over the long- and short-term period. High litter accumulation reduces irradiance for plants, and may lead to lower basal cover. The intense grazing and trampling due to the higher stocking rate and stocking density under HDG resulted in less erect forb growth habits and more prostrate growth habits at Kokstad. A study of demography revealed that HDG threatened future populations of the grazing-sensitive species Afroaster hispida, Agathisanthemum chlorophyllum and Gerbera ambigua through increased mortality or reduction in the recruitment of large from small individuals. Intense defoliation altered the competitive response of A. hispida, it had a high competitive response when undefoliated but when defoliated its competitive response was reduced. Gerbera ambigua and H. hemerocallidea were not affected by the interaction between defoliation and competition. Findings from this study has shown that HDG potentially has a negative impact on soil health and vegetation composition of South African mesic grassland.
The effects of burning and mowing on microclimate and soil resources and implications for species change in the southern tall grassveld of KwaZulu-Natal.
(2004) Ghebrehiwot, Habteab Mesghina.; Kirkman, Kevin Peter.
Promotion of a predictive understanding of plant community response to various forms, frequencies and seasons of disturbance, either through the direct physical effect on biota and or indirect effect on plants, through modification of microclimate and soil attributes is currently a major goal in plant ecology. In particular, the effect of disturbance on altering the ratio between available light and nutrients and their resultant effect on growth, shoot/root allocation, and thus community composition has gained considerable recognition in connection with the mechanisms of plant succession under a popular heading "the resource ratio hypothesis of plant succession". Contemporary and long-term (>50 years) burning and mowing experiments in KwaZulu-Natal (KZN) provide important sites for investigation that in the mesic grasslands of KZN, community composition changes in response to the frequency, time and type of disturbances such as burning, mowing and veld fertilization. However, the relationship between disturbance-resource-plant traits and their interactive role in species change is virtually unknown. This study sought to improve understanding of mesic grassland dynamics in. KZN, using short-term pot and plot experiments. The principal objectives were: 1) by subjecting plants to different levels of resources viz. light, nutrients, water and cutting to determine the relative above and below-ground growth performances (biomass allocation) of species from contrasting habitat preference in KZN, which implies their relative competitive ability for limiting resources and tolerance to cutting, 2) by using a short-term (one-season period) burning and mowing experiment to determine the effect of different veld management practices on microclimate and availability of soil resources and their subsequent effect on plant growth performances, 3) testing the relative shade tolerance of representative species from contrasting habitat preferences, 4) by combining the outcomes from these experiments, to provide a general synthesis concerning species' response to disturbance/resource which further signifies species change. The hypothesis that competitive ability as a function of biomass allocation is fertility dependent was supported by a pot experiment. In low nutrient treatments short grass species that predominate infertile soils in KZN viz. Aristida funcifarmis and Themeda triandra attained double the shoot biomass, more than double root biomass, initiated more tillers and re-grew better (after cutting) than those inherently tall species that predominate fertile sites viz. Eragrostis curvula and Hyparrhenia hirta. In contrast, in high nutrient treatments, tall species attained far higher shoot biomass and grew taller in height. Interestingly, short species had a smaller shoot: root ratio than tall species, consistent with the prediction of the resource ratio hypothesis. However, no evidence was obtained suggesting that tall species were more shade tolerant than short species. A field-based shade experiment rather showed that, those species that initiate tillers below-ground viz. Aristida junciformis, Eragrostis curvula and Tristachya leucothrix were more shade tolerant than those species that initiate tillers above-ground viz. Hyparrhenia hirta and Themeda triandra. On the other hand, the effect of disturbance/resource relationship in influencing the growth (biomass production, growth rate, and basal circumference) of contrasting species was examined by conducting a short-term (one season) burning and mowing experiment. The effect of disturbance, its form and frequency through its effect on light and soil moisture was able to account for a substantial difference in species vigour, which can potentially impact community composition. Short species (Themeda triandra and Tristachya leucothrix) showed their highest biomass production and higher basal circumference enlargement in burnt summer mown sites, whereas medium to tall species (Aristida junciformis, Eragrostis curvula and Hyparrhenia hirta) were less tolerant to summer mowing. Aristidajunciformis and Eragrostis curvula appeared to be more vigorous (both in terms of above-ground biomass production and growth rate) in burnt but not mown and control treatment respectively. High biomass and litter accumulation on sites protected from disturbance appeared to have a large influence on species vigour. Species such as Aristida junciformis, Eragrostis curvula and Tristachya leucothrix had high tolerance to litter accumulation while in contrast Hyparrhenia hirta and Themeda triandra were more vulnerable. In total this study has revealed that the association of some short species e.g. Themeda triandra with the reccurrence of disturbance is mainly due to increases in light availability and lowered dominance from tall species in frequently disturbed sites rather than nutrient related. However, this study has revealed that there are some indications whereby the notion that the inverse relationship between available light and nitrogen are important driving variables in species change is an important working theory in the mesic grassveld of KZN.
Effects of climate and management on biomass, species composition and phenology and their regenerative responses in a mesic grassland.
(2024) Ojo, Theresa Abosede.; Tedder, Michelle Jennifer.; Kirkman, Kevin Peter.
Increasing temperatures, attributed mainly to increased greenhouse gas emissions, are a leading cause of climate change. Evidence shows that changing climatic conditions significantly affect terrestrial ecosystem structure and function. Fire, grazing, and recurrent drought are some of the more common disturbances that impact many grasslands. Two ongoing experiments (Ukulinga Grassland Fire Experiment (UGFE) and Drought-Net experiment at the Ukulinga Research Farm, University of KwaZulu-Natal, Pietermaritzburg, KwaZulu-Natal) were used to investigate the impacts of these disturbances on grassland community dynamics. This research assessed the effects of rainfall variation and warming and management factors (defoliation and fire) on phenology of grass species vis-a-vis their regenerative responses from bud banks, soil seed bank, and seed rain and aboveground species composition and biomass in the mesic grasslands of South Africa over two years. The Drought-Net experiment treatments include the combined effect of rainfall variation (drought (50%), ambient (100%), and wet (150%)) and temperature (warmed and ambient) manipulation. The warming treatment was applied using open-topped warming chambers, and the rainfall treatment was applied using rainout shelters to intercept 50% of the ambient rainfall. The intercepted rainfall was piped onto the neighboring plot to increase rainfall by 50%. In the first objective (Chapter 3), the combined effect of rainfall variation and temperature manipulation on the phenology of grass species vis-a-vis their regenerative responses from bud banks was conducted. Drought delayed the timing of budding and flowering and shortened the duration. There was no effect on the total number of buds per vegetative tiller per tuft compared to the reproductive tillers. Warming advanced the budding timing and extended the budding duration. This shows that the response of timing and duration of budding and flowering of these studied grass species was sensitive to the short-term effects of drought and warming. In the second objective (Chapter 4), the effect of burning frequency, winter defoliation type (fire/mow), summer defoliation with differing winter defoliation (defoliation time) applied over ~70 years and temperature manipulation on the phenology of grass species vis-à-vis their regenerative responses from bud banks was conducted. Across the two years of warming treatment application, the sensitivity and timing of species phenological responses and bud bank density relative to resource allocation to vegetative versus sexual reproduction were examined. Warming delayed the time of budding and flowering and increased duration slightly (p < 0.005). Annual winter mow delayed timing and extended duration of budding and flowering as compared to annual winter fire (p < 0.005). Summer defoliation frequency quickened the timing and shortened the duration of budding and flowering compared to winter defoliation frequency (p < 0.005). The flowering duration for Themeda triandra was shortened under annual burning and lengthened under the triennial burn plots (p < 0.005). In ambient plots, vegetative status had more buds per tiller and number of tillers per tuft compared to warmed plots. Annual winter defoliation by fire had the greatest number of tillers per tuft for Heteropogon contortus and the lowest recorded in the annual winter defoliation by mow plots. A greater number of buds per tiller was recorded under the summer defoliation plots as against the winter defoliation plots. Summer defoliation plots had a greater number of tillers per tuft for H. contortus than the winter defoliation plots. The results of this study indicate that the type, time, and frequency of defoliation and their interaction with the effect of warming have a notable influence on the budding and flowering periods of the species studied, ultimately impacting the length of their reproductive cycle and altering the ratio of vegetative to flowering reproductive status. The reason for the contrasting results where warming advanced the time of budding in chapter 3 and delayed in chapter 4 could be as a result of the insurance hypothesis whereby the variation of species responses to changes in environmental condition of an ecosystem can insure the system against reduction in its functioning. The third objective (Chapter 5), combined effect of management factors (defoliation and fire), and temperature manipulation on the species composition, soil seed bank, and seed rain in the Drought-Net experiment and UGFE. Plant community composition was examined to determine sensitivity to climate and management. On the Drought-Net experiment, the main and interacting effects of warming and the various rainfall regimes did not impact the soil seed bank and seed rain (p > 0.05). On the UGFE, the main effect of warming and its interactions with the burning and defoliation treatments had no significant effect on soil seed bank and seed rain (p > 0.05). The seed rain composition was only affected by defoliation frequency, while the main effects of fire and defoliation frequencies affected the soil seed bank (p < 0.05). The result of the study showed that the responses of the seedlings that germinated from the seed bank and seed rain were species-specific towards the effects of the management factors. The lack of effect of drought and warming shows that mesic grasslands are resilient to fluctuations in moisture and temperature regimes in the short term. The fourth objective (Chapter 6), examined the effect of different rainfall regimes and temperature manipulation on the species composition and biomass of a mesic grassland in the Drought-Net experiment. The results showed that the main effect of rainfall variation and their interactions with warming had an impact on the grass species composition, with just the main effect of warming being observed in the following year. No impact on the forb composition was seen in either year. While there was no evidence of an interaction impact, the main effects of rainfall variation and warming on biomass production were significant. Drought and warming reduced biomass production. This demonstrated how quickly drought reduced the grass community composition, which resulted in changes in productivity. Objective five (Chapter 7), this study examined combined effect of warming and management factors (defoliation and fire) on the aboveground species composition and biomass at UGFE. Plant community composition was examined to determine sensitivity to the impact of long-term burning and defoliation frequency under ambient and warmed conditions and whether shifts in plant community composition resulted in changes in productivity. The study showed that the main effect of warming, burning, and defoliation regimes resulted in a shift in plant community composition, which altered productivity. The overall conclusion of this study is that under different defoliation frequencies, types and timing, the species-specific seasonal patterns of species composition, phenological development and bud bank production were strongly influenced by the resultant effect of these disturbances on the availability of soil moisture and light intensity. This resulted in a reduced productivity of the grass community in our mesic grassland.
Factors affecting the seasonal variation of veld quality in South Africa.
(1988) Kirkman, Kevin Peter.; Tainton, Neil M.
This project was initiated to investigate the factors affecting the seasonal variation of veld quality in South Africa, with specific objectives as follows: (1) to establish what factors might play a role in seasonal variation of plant quality in South African grassveld; {2) to provide a more objective definition of the terms sweetveld and sourveld than already exists; (3) to determine an objective and quantitative method of measuring or indexing the degree of sweetness or sourness of a representative species of both sweetveld and sourveld using Themeda triandra as the reference species and (4) to establish which factors, if any, may be manipulated to improve veld quality. The results indicate that cellulase dry matter digestibility, neutral detergent fibre, nitrogen status and phosphorus levels were the plant factors most important in indicating veld quality. Climate and soil fertility were found to have no consistent relationship with veld quality. Thus any definition of sweetveld and sourveld will have to be based on the winter quality of veld and not on the climate or soil fertility. The winter quality of veld appears to be a function of the seasonal quality patterns of all species present and not only a reflection of the winter quality of '.L_ triandra. As the seasonal quality patterns are likely to vary between species, management will influence the degree of sourness in the long term by influencing species composition and in the short term by affecting the ontogeny of the plants present i.e. the amount of preferred material left on palatable and unpalatable species available for winter grazing. The seasonal quality pattern and inherent winter quality of '.L_ triandra has been shown to vary considerably from area to area. As each species is expected to show unique quality patterns, the sweetveld/sourveld situation becomes extremely complex. There are no obvious factors (except management) that can be manipulated to improve the winter quality of veld.