Masters Degrees (Grassland and Rangeland Science)

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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 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.
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.
A comparative classification of the sourish-mixed bushveld on the farm Roodeplaat (293 JR) using quadrat and point methods.
(1995) Panagos, Michael David.; Westfall, Ribert Howard.; Zacharias, Peter John Kenneth.; Ellis, Roger.
An area and a point-based technique were used together at each of the same 75 sampling sites (stands), on a Sourish-Mixed Bushveld farm, to collect data for the classification and mapping of the vegetation. Both sets of data were synthesized using the same computer program package and the efficacy of the resulting classifications as well as the efficiency of the two field sampling techniques was compared. Following this, a continuous 7 752 point (1 m apart) transect was carried out, traversing the farm, in order to determine the optimum scales at which to sample Sourish-Mixed Bushveld so as to increase classification efficacy and improve community boundary recognition. The results indicated that (1) the arbitrarily chosen sampling scale of 1:8 000 was too large for "farm-scale" studies; (2) the area-based method proved to be satisfactory in that the classification and vegetation map produced with this method were verified spatially and environmentally; (3) the point-based method was deficient as a classificatory and mapping tool at large scales, since too few species were recorded with this method to make any sense of the classification and mapping of the vegetation was not possible; (4) less time per species was spent using the area-based method but because more species per stand were recorded with this method, the point-based method was quicker per stand; (5) the area-based method was easier to use in dense vegetation and irregular terrain; and (6) the optimum sampling scales for Sourish-Mixed Bushveld, as indicated by the synthesis of the continuous transect data, are about 1:12 000, 1:50 000 and 1:250 000.
A comparison of in-field techniques for estimating the feed intake of young boer goats on a Leucaena leucocephala/grass hay diet.
(1997) Letty, Brigid Aileen.; Zacharias, Peter John Kenneth.; Morris, Craig Duncan.
Two methods of estimating the intake of a 25% leucaena : 75% grass hay diet by young male Boer goats were assessed, (a) The purine derivative technique which uses the urinary excretion of purine derivatives (expressed relative to creatinine concentration in the same sample) as an index of feed intake, and (b) the conventional marker method, utilizing chromic oxide (Cr₂O₃) contained in gelatin capsules and dosed twice daily, as the marker. Following a prerun the two techniques were compared in three runs of an indoor experiment. In each run 10 goats were randomly allocated to five feeding levels (500 to 1100 g fodder d ¯¹ on air dried basis). A preliminary and an adaptation period during which goats were dosed with the Cr₂O₃ and fed their daily feed allowance, was followed by a 4 day collection period during which spot samples of urine were collected and analysed for allantoin and creatinine (allantoin being used instead of total PDs) and faecal samples were collected for chromium analysis and percentage dry matter determination. For the first two runs, two grab samples per day for each goat were bulked and analysed for chromium content. For the last run, the total daily faecal collection was subsampled and analysed for chromium. Work was done in metabolic crates to determine the effect of time of collection on the ratio of allantoin : creatinine (A/C) in spot urine samples and it was found to non-significant (P>0.05). Linear regressions of: (a) feed intake expressed per unit metabolic mass (g.d ¯¹.LW ¯°∙⁷⁵ (I_mmass)) against A/C ratio; (b) faecal output (g d ¯¹) against feed intake (g d ¯¹); and (c) faecal chromium concentration (mg kg ¯¹) against faecal output (g d ¯¹) were fitted to the data. During the prerun, only regression (a) was fitted and was non-significant (P>0.05), showing no trend at all. For the first true run, the regression of I_mmass against A/C ratio was significant and the correlation was high (P≤ 0.001, R² [A] 0.715, n = 10) but for the second and third runs, the correlations only became significant when the apparent outliers were discarded from the data. (Run 2: P≤ 0.001, R² [A] 0.824, n = 8; Run 3: P≤ 0.05, R² [A] 0.430, n = 9). It was concluded that the relation between I_mmass and A/C ratio is not well enough defined to be used for predictive purposes. When regression (b) was investigated, all the runs produced significant results (P≤0.001, P≤0.01, P≤ 0.001 for runs 1,2 and 3 respectively) however the correlations were not as high as expected (R² [A] being 0.714, 0.565 and 0.863 respectively). For the regression of faecal Cr concentration against faecal output (regression c), all runs showed significant relations (P< 0.001, P< 0.0001, P:s 0.001 for runs 1,2 and 3 respectively) and the correlations were high (R²[A] being 0.836, 0.837 and 0.912 respectively). The data from the three runs were pooled and single equations established for regressions (b) and (c) to allow for the prediction of intake from faecal chromium concentration. Faecal output = feed intake * 0.448 + 19.341 (P≤ 0.001, r 0.853, R² [A] 0.718, SE 25.664, n - 30) Faecal chromium concentration = faecal output * -241.547 + 1.315E+05 (Ps 0.001, r 0.904, R² [A] 0.811, SE 5603.788, n = 30). In vitro figures were determined for a range of leucaena : hay mixes but no apparent trend was found between percentage leucaena in the mix and the digestibility of the mix. These results compared favourably with in vivo results obtained for a 25% leucaena : 75% hay mix. Neither technique proved entirely satisfactory, but the external marker method was found to be more effective than the purine derivative technique. More work is required especially with respect to the latter method.
Determinants of grass production and composition in the Kruger National Park.
(2003) Zambatis, Nicholas.; Zacharias, Peter John Kenneth.; Morris, Craig Duncan.; Biggs, Harry Cawood.
The dynamics and complexities of climate-soil-vegetation relations in the Kruger National Park are poorly known. Although primary production and composition of the grass layer are very important components of the Park's ecosystem, equally little is known about the determinants of these parameters. A better understanding of these processes and relations will be of value to the management of this Park, as well as providing a better insight into these complex dynamics. A study was consequently undertaken covering a 14-year period to identify the most important determinants of above-ground grass production and composition. At the core of the study is the soil water balance. The use of evapotranspiration data in a study of this nature is however not absolutely essential, provided a variety of rainfall parameters are used, though it has the important advantage of providing a much more detailed and more complete insight into the relations of the grass sward with its environment. Stepwise and tree regression procedures were used to identify the important factors. It is concluded that rainfall in its various forms is the primary determinant of grass production, standing crop, and composition, the latter either as perennials or Decreasers. Secondary determinants, in varying degrees of importance, are the thickness and base status of the A horizon, distance to permanent drinking water, and competition by woody plants. Herbivore utilization is insignificant or at most, plays a relatively minor role. Herbivores appear to exert a negative influence on Decreaser abundance only when soil moisture stress exceeds a threshold level. When this is exceeded, relatively low herbivore densities are apparently sufficient to reduce Decreaser abundance. The definitions of Decreasers and Increasers consequently require revision to take into account the overriding influence of environmental factors, particularly those of soil moisture stress. The calibration of the disc pasture meter was re-evaluated. The relation between mean disc height and standing crop is non-linear. Up to a mean disc pasture meter height of 260 mm, the correlation between this parameter and above-ground standing crop is very strong (r2 = 0.95; P<0.0005). Beyond this height, the correlation is very poor (r2 =0.09; P<0.0005), apparently being strongly influenced by the structure of the grass plant, with tall grasses, or grasses with highly lignified culms resulting in a weaker correlation.
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.
The effect of burning frequency on invertebrate and indigenous flowering forb diversity in a Drakensberg grassland ecosystem.
(2006) Arnott, Wendy Lynn.; Hamer, Michelle Luane.
The KwaZulu-Natal Drakensberg, South Africa, is predominantly a grassland ecosystem maintained by fire. The effect of the current burning regime on invertebrate and flowering forb diversity in this ecosystem is poorly understood. The overall aim ofthis study was to contribute towards the development of an effective burning regime for the KwaZulu-Natal Drakensberg that will conserve invertebrates and indigenous forbs, two major components of biodiversity. The objectives were to examine the effect of fire and fire frequency on flowering forb and invertebrate species diversity, to determine whether fire frequency, time since last burn or locality were influencing species composition, and to identify potential biodiversity indicators that reflect overall species richness for use in monitoring of invertebrates and forbs. Sampling took place in March, September and November of 2002 at Giants Castle Game Reserve. Invertebrates were sampled using sweep netting and targeted netting along transects, yellow pan traps and soil quadrats. Invertebrate taxa sampled were ants (Formicidae), butterflies (Lepidoptera), grasshoppers (Orthoptera), leafboppers (Cicadellidae), bees (Apoidea), bee flies (Bombyliidae), hover flies (Syrphidae), robber flies (Asilidae), spiders (Araneae), earthworms (Oligochaeta) and millipedes (Diploda). These were identified to species level with the assistance of taxon experts. Flowering forbs were sampled using five replicates of five by five metre quadrats randomly placed in each site. Overall flowering forb and invertebrate species diversity was higher in grasslands that were burnt for two consecutive years in 2001 and 2002 than in grasslands that were not burnt during those two years. Frequently (annual) and intermediately (biennial) burnt grasslands had significantly higher invertebrate and flowering forb diversity than infrequently (five years without burning) burnt grasslands. This, together with the fact that grasslands burnt during the year of sampling had higher species richness than grasslands burnt two and five years previously suggests that invertebrates and forbs are generally resilient to fire and many forb species appear to be stimulated by fire. However, each burn frequency had its own suite of unique flowering forb and invertebrate species. Invertebrate communities were influenced mostly by locality and the length of time past since the last fire and flowering forb communities were influenced mostly by the length oftime past since the last fire. Fire frequency had the least influence on both invertebrate and forb communities. Ecological succession occurred after each fire in the invertebrate communities but forb communities appear to need more than five years without fire for ecological succession to occur. The findings of this study therefore suggest that using a combination of three fire frequencies would result in patches of grassland in various stages of ecological succession, and would conserve species unique to each burning frequency, and would therefore conserve maximum diversity. Flowering forb species richness and certain invertebrate taxa (ants, leafboppers, spiders and bees) have the potential to act as indicators of overall invertebrate species richness for use in monitoring programmes.
Effect of elevated temperatures on growth and defence of Vachellia 1 sieberiana seedlings grown with or without grass.
(2020) Ncisana, Lusanda.; Scogings, Peter Frank.; Mkhize, Ntuthuko Raphael.
Warming is suggested to increase globally in the next few decades. Warming, soil nutrients, 2 water, CO2 and sunlight are the most important environmental factors for plant life. Elevated 3 temperatures can highly impact plant metabolism, photosynthesis and consequently woody 4 plant performance and fecundity. An increase in minimum temperatures will possibly result in 5 the transformation of grasslands to savannas. The effects of rising temperatures on growth and 6 defence of woody plant seedlings and trade-offs between growth and physical defence 7 particularly thorn length of woody plants that potentially invade grasslands have been studied 8 utilising open-top warming chambers (OTCs). Furthermore, it is important to study the effect 9 of warming on woody plant growth and defence when competing with grass. Field experiments 10 were conducted in three different seasons to determine the effect of warming on growth and 11 defence of Vachellia sieberiana seedlings growing with or without grass, and the effect of 12 warming on trade-offs between growth and defence of V. sieberiana seedlings growing with or 13 without grass. It was predicted that elevated temperatures would increase growth and thorn 14 length of V. sieberiana seedlings growing (1) with or without grass in the dormant season and 15 early growing season, but (2) in the later part of the growing season only when grass cover is 16 absent. It was also predicted that warming, with or without grass cover increases seedling 17 growth, but elevated temperatures would decrease investment in defences relative to growth of 18 V. sieberiana seedlings when grass cover is absent. To test these predictions, 120 seedlings 19 that were three weeks old were transplanted into 20 field plots. Ten plots were warmed, and the 20 other ten plots were not warmed. In both warmed and unwarmed plots the grass was either 21 cleared or not. OTCs raised air temperature by 1.0 to 2.5 °C. Stem length, plant height, stem 22 diameter, thorn length, and dry mass of shoot, leaf, and root were measured after 6 weeks for 23 the dormant season, and after 6 weeks and 12 weeks in the growing season for the first 24 experiments. For the second experiment, plant height, stem length, stem diameter and thorn 25 length were measured after 6 weeks. To determine effects of warming and grass cover on the 26 relationship between defence and growth, ratios of thorn length (a measure of defence) to plant 27 height, stem length and stem diameter (measures of growth) were calculated. The first 28 experiment revealed that warming increased growth of V. sieberiana seedlings in all seasons. 29 Defence was low in the presence of grass cover in the growing season, but not in the dormant 30 season. The second experiment revealed that warming and grass cover significantly and 31 independently reduced the thorn length: plant height ratio and thorn length: stem length ratio. There was a significant interaction effect between warming and grass cover on the thorn length: 1 stem diameter ratio. Lastly these results suggest that, with an increase of 1 – 2.5 °C regardless 2 of grass cover at the time of seedlings establishment, the rate of woody encroachment will 3 increase as temperature rises. The findings also suggest that there are trade-offs between the 4 growth of V. sieberiana seedlings and physical defence in terms of thorn length.
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.
Effect of perennial water on soil, vegetation and wild herbivore distribution in southeastern Zimbabwe.
(1999) Clegg, Sarah.; Goodman, Peter S.; O' Connor, Timothy Gordon.
The effects of artificially supplied perennial water on soil properties, vegetation dynamics and the distribution of large herbivores was investigated in southeastern Zimbabwe. Data collection took place between March 1997 and July 1998. Water points were situated primarily on three different soil types (clay-loam, sandy-clay-loam and sand), and in four different vegetation types (Hill communities, Colophospermum mopane veld, Acacia nigrescens woodland and Albizia petersiana woodland). One water point in C. mopane veld (Bandama) had been closed two years prior to data collection, while another, in the Hill community (Manyoka), had been introduced, two years prior to data collection. Changes in physical (infiltration) and chemical (organic carbon and nutrients) properties of soils around water points were largely restricted to within 100 m of water. Chemical enrichment of the soil occurred only at water points that had been in place for more than two years. Soil surface conditions were altered to distances beyond 100 m from water. Manyoka (the new water point) was an exception, with extreme changes limited to within 100 m of water. Herbaceous and woody species composition changed in response to distance from water with changes best described by asymptotic equations. Changes in species composition of the woody component appeared to be longer lasting than changes to the herbaceous component. Most perennial grass species declined close to water, but Urochloa mosambicensis increased close to water in areas outside of the Hills. Herbaceous species diversity was adversely affected by distance from water on sandy soils (Hill communities and A. petersiana woodland), but was largely unaffected on clay-loam (A. nigrescens woodland) and sandy-clay-loam soils (C. mopane veld). Woody species composition and density was altered out to 500 m from perennial water in Acacia nigrescens woodland on clay-loam soils. Results suggest that this vegetation type may be susceptible to bush encroachment close to water. Trends in woody canopy utilisation were generally similar to trends in woody species composition, and it is proposed that the former may be used to indicate future changes in the latter. Conversion of trees to shrubs was highest at Manyoka (the new water point) indicating that woody destruction by elephants is extreme during the initial years following water introduction. Large herbivore biomass was greatest close to water (< 1 km) during the dry season but not during the wet season. Herbivore species distributions appeared to be influenced by the position of perennial water, but since all range was within easy access of water, it is unlikely that animal distributions were constrained directly by the position of surface water. It is more likely that herbivores were spatially separated on the basis of habitat type.
Effect of season and type of fire on Colophospermum mopane woodland in the south-eastern lowveld of Zimbabwe.
(2000) Walters, Michael John.; O' Connor, Timothy Gordon.
The majority of the vegetation types occurring on Malilangwe Estate, in the south-eastern lowveld of Zimbabwe, are dominated by Colophospermum mopane (mopane). Over the past 30-50 years the stand density of these mopane vegetation types has increased, and an investigation was undertaken to determine the effect of season of burning and type of fire on mopane woodlands. From this study the following was ascertained: 1) A single predictive equation cannot be used over all seasons to estimate standing crop (fuel load) using the standard disc pasture meter procedure. The calibration equations developed using this procedure accounted for between 39 and 72% of the variation in standing crop, illustrating the high variation in basal cover of the grass sward, as well as the variation between months. Although the revised procedure, developed for areas with low basal cover, accounts for a lot more of the variation in standing crop, this procedure was not used to estimate standing crop over the study period because the calibration equation covered a number of vegetation types, and was not specific to the mopane woodlands. 2) Standing crop tracks effective rainfall (monthly rainfall divided by monthly pan evaporation) closely, with a lag period of less than one month. Standing crop can be estimated using a predictive equation that utilizes effective rainfall from the previous month. There is a positive relationship between peak standing crop and rainfall. A predictive equation was developed to estimate peak standing crop, using annual rainfall. Standing crop declines through the dry season as effective rainfall decreases, and this 'decrease function' allows for the estimation of the standing crop for a particular month, after peak standing crop is reached. 3) Two leaf quantification equations were developed for mopane trees in the south-eastern lowveld of Zimbabwe, one for coppicing and for non-coppicing individuals. These allow for the estimation of leaf dry mass from measured canopy volume. 4) There was no significant difference between the fire intensities attained for the three seasons of burning. Over all seasons, head fires were significantly more intense than back fires. 5) Percentage topkill after late dry season burns was significantly higher than topkill after early dry season burns. There was no significant difference between mid and late dry season burns, and head fires led to significantly more topkill than back fires. Plants < 150 cm experienced significantly more topkill (80 %) than did individuals > 150 cm (44%). 6) Fire per se led to an increase in stand density over all seasons and types of fire, but this change was not significant. Fire did not influence the nett recruitment of new individuals. Height class one (0-50 cm) and three (151-350 cm) were impacted most by fire. This reflects a change in tree structure, with an increase in the amount of leaf material in height class three, and a subsequent decrease in the amount of material in height class one. 7) The effect of season of burning on the change in tree height was significant, whereas the effect of type of fire was not significant. All treatments, except early dry season back fires, led to a reduction in tree height, whereas trees in the no burn areas increased in height. 8) Burning in any season, and implementing either type of fire, led to an increase in the number of stems. Mid dry season burns led to the highest increase in number of stems. However, the more intense the fire the smaller the increase in number of stems. 9) All three seasons of burning (head and back fires) led to a significant decrease in maximum canopy diameter per tree, while the maximum canopy diameter of trees in the no burn areas increased. Mid dry season burns resulted in the greatest decrease in canopy diameter. 10) The effect of burning on the change in leaf dry mass per tree was highly significant. All three seasons of burning led to a decrease in leaf dry mass, while there was no difference between head and back fires. Leaf dry mass in the control areas increased however. High fire intensities led to the greatest decrease in leaf dry mass, late dry season head fires having the greatest decrease. This study suggests that mopane plants face a constraint due to fire and/or browsing, and a tradeoff occurs between canopy volume, canopy diameter, canopy area; and number of stems. Fire leads to an increase in the number of stems through coppicing, while canopy volume and leaf dry mass decreases. This decrease is either (i) a tradeoff in response to increasing stem number, or (ii) a reduction in canopy because additional leaves on the new stems contribute to photosynthesis. The most important response to season of burning is the altered phenophase (phenological stage) of the plant. Early dry season burns cause the trees to be leafless during the early dry season (when unburnt trees are carrying full leaf), and then to be in leaf at the end of the dry season (when unburnt trees are leafless). It would appear that fire disturbance initiates leaf senescence after burning, and then leaf expansion earlier than normal i.e the whole leaf senescence/growth process is brought forward. Trees in late dry season burn areas remain leafless at the start of the rains, while trees in unburnt areas are carrying leaf. Being leafless these trees do not photosynthesize during this time, and it is proposed that the grass sward is advantaged by the reduced competition from the tree component. The consequences of these two changes in phenophase could not be addressed in this study, but are pertinent questions that must be answered if mopane woodland dynamics are to be more fully understood. Management recommendations for (1) the removal of unacceptable moribund grass material, or (2) the reduction of encroachment by woody species on Malilangwe Estate are also given. In an attempt to combat the increase in stand density of mopane it is recommended that high intensity head fires be implemented, when standing crop (fuel load) is sufficient and climatic conditions are conducive to maintaining high intensity fires. These should be carried out at the end of the dry season, before the onset of the rains.
Effect of stocking rate and rainfall on rangeland dynamics and cattle performance in a semi-arid savanna, KwaZulu-Natal.
(1998) Fynn, Richard Warwick Sinclair.; O'Connor, Timothy Gordon.
Considerable understanding of the functioning of semi-arid systems is still needed to enable range managers to formulate management policies, with a degree of confidence. Long term data sets that encompass a wide range of interactions between the various major components of a semi-arid system (vegetation, herbivory, animal performance, landscape and rainfall), are unfortunately rare but essential to provide sufficient depth of data to adequately test various hypotheses about rangeland dynamics. This study comprises an analysis of a ten year data set derived from two cattle stocking rate trials in the semi-arid savanna of KwaZulu/Natal. Statistical analysis revealed that the most pronounced and rapid compositional change was due to rainfall, but that stocking rates between 0.156 and 0.313 AU ha ¯¹ had an important effect as well. Sites on steeper slopes with heavy stocking rates, exhibited the greatest amount of compositional change between 1986 and 1996 (40 Euclidean points in heavy stocking rate treatments on slopes vs 21-24 Euclidean points in heavy stocking rate treatments on flatter land, or 11-24 Euclidean points in low stocking rate treatments). Heavy stocking rates in conjunction with low rainfall tended to cause decreases in densely tufted perennial grasses and increases in annuals and weakly tufted perennials. Multiple regression analysis revealed that seasonal peak grass production (measured as disc height) declined between 1986 and 1996 only at those sites on steeper slopes with heavy stocking rates. The camps that declined in productivity also underwent the greatest degree of compositional change. The decline in grass productivity in certain high stocking rate camps did not translate into a decline in cattle performance. Depending on rainfall, cattle gained on a seasonal basis between 112 and 241 kg at low stocking rates, 82 and 225 kg at medium stocking rates and 84 and 217 kg at high stocking rates Rainfall, compared with stocking rate, accounted for the greatest amount of variance in seasonal peak grass production and cattle performance. Cattle performance had a strong curvilinear response to rainfall, which also proved to be a better predictor of cattle performance than grass biomass. There were no clear trends in soil physical and chemical characteristics between low and high stocking rates that could provide convincing evidence that loss of soil nutrients was an important mechanism of range degradation. The total standing crop of plant nitrogen but not of phosphorus tended to decline at high stocking rates. Plant nutrient and van Soest analyses suggested that forage quality was higher at heavy stocking rates. The results of this study generally supported traditional concepts of rangeland dynamics with regard to rainfall and grazing effects on compositional change and seasonal grass production. The results were important in being able to show quantitatively that heavy stocking rates result in a decline in grass production and that this effect is dependent on an interaction between stocking rate and landscape position or slope, and that there is a link between a decline in seasonal grass production and compositional change. The results also highlighted areas for future research that would be useful for furthering our understanding of various aspects of rangeland dynamics and mechanisms of degradation.
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.
Evaluation of improved Brachiaria grasses in low rainfall and aluminium toxicity prone areas of Rwanda.
(2010) Mutimura, Mupenzi.; Everson, Theresa Mary.
Abstract available in the print copy.
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.

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