The effect of fire on germination in five common veld grasses

Abstract

The effect of fire on the germination of Themeda triandra, Heteropogon contortus, Panicum maximum, Aristida junciformis and Spowbolus africanus was investigated. Seeds exposed directly to the fire were destroyed in all species. The germination of seeds buried (7 to 9 mm) when exposed to fire was depressed in T. triandra (P ≤ 0,01), stimulated in H. contortus (P ≤ 0,05) and unaffected (P > 0,05) in the other species.     

Shoot and root biomass allocation and competitive hierarchies of four South African grass species on light,soil resources and cutting gradients

We examined the competitive ability and response to light (shaded or unshaded), nutrients (50% Hoaglands solution or tap water), water (frequent or infrequent watering) and cutting (cut or uncut) of four grass species that differ in their traits, habitat preferences and responses to management practices in KwaZulu-Natal. All species produced the greatest biomass in high-nutrient high-water treatments. Short to medium-height grass species, such as Themeda triandra and Aristida junciformis, produced nearly double the biomass of taller species such as Hyparrhenia hirta and Eragrostis curvula in the low-nutrient treatments, with the reverse being true in the high-nutrient treatments. Themeda triandra and A. junciformis produced the greatest root biomass both in the low- and high-nutrient treatments. The shorter species produced relatively greater root than shoot mass, with the reverse being true of the taller species. Tall species did not produce relatively less, when cut, or relatively more, when shaded, than the shorter species. However, A. junciformis and E. curvula were more shade-tolerant than the other species, suggesting that their increase in abundance with infrequent fire is due to the shading by litter in unburnt grassland. The competitive hierarchy among species reversed between low- and high-nutrient treatments, with A. junciformis and H. hirta having the greatest competitive effect on T. triandra in low- and high-nutrient treatments, respectively. These results contribute to an understanding of species distributions on fertility and burning gradients.     

Adaptation of a decreaser and an increaser grass species to defoliation in semi‐arid grassveld

Abstract

Grasses have developed through natural selection to deter, escape and tolerate herbivory, and to escape and tolerate fire. In the semi‐arid grassveld of the Eastern Cape, the species Themeda triandra and Sporobolus fimbriatus have been classified as Decreaser and Increaser II plants respectively. Both species have well‐developed escape and tolerance mechanisms, although T. triandra readily tolerates defoliation only during the dormant season, while S. fimbriatus is adapted to tolerating defoliation throughout the year. Furthermore, T. triandra has only moderately developed deterrents to herbivory, in contrast to S. fimbriatus, where deterrents are well developed. It follows that the Decreaser species T. triandra is adapted to relatively frequent fire and no more than modest herbivory, while the Increaser II species, S. fimbriatus, is adapted to heavy herbivore usage, but infrequent fire.     

Effects of selective defoliation and height of defoliation on tiller dynamics and herbage yield of Themeda triandra in a semi-arid rangeland in Zimbabwe

Understanding the impacts of intensity and selection of grazing on the performance of grasses is important in the management of grazing areas. It is especially important in semi-arid environments where, apart from moisture, the levels at which grasses are utilized has a major influence on their persistence in the environment. The effects of selective defoliation and height of defoliation (5 cm and 10 cm stubble heights) on the performance of the grass, Themeda triandra, were investigated in a field experiment for two growing seasons. Performance was measured as tiller production, rate of production, tiller mortality, herbage yield and quality. Tiller production was greater (81 tillers per plant) under non-selective defoliation than under selective defoliation (49 tillers per plant) in the 1995/1996 season. Tiller mortality was higher (66.45%) under heavy selective defoliation than under nonselective defoliation (21.98%). Herbage yield, apart from the control treatment, was high (13.6 g per plant) under light non-selective defoliation. Heavy selective defoliation reduced the nutrient levels (e.g. levels of soluble carbohydrates under heavy selective defoliation were 6 g kg^?1 glucose compared to 20 g kg^?1 glucose under light non-selective defoliation).     

Effect of long-term exclusion of fire and herbivory on the soils and vegetation of sour grassland

The effect of long-term exclusion of fire and herbivory, as mediated by aspect, on soils and the structure, composition, and diversity of vegetation was investigated in Döhne Sourveld (34 years), Piet-Retief Sourveld (45 years) and Southern Tall Grassveld (49 years). Exclusion had no effect on the soil chemistry of the surface 15 cm of soil in Dohne and Piet Retief Sourveld. Exclusion promoted the ingress of woody elements, including heath-like species, but the extent of ingress was related to water availability (rainfall, aspect), proximity of propagules, duration of protection, and accidental fires. Alien invasive woody species, in particular Acacia mearnsii established at all sites. Woody species were mostly bird- or wind dispersed, some had no known nearby population, although some Acacia species were herbivore-dispersed. The wettest grassland, Piet Retief Sourveld on a southern aspect, was transformed to a shrubland matrix (mainly Helichrysum splendidum) with tree clumps of forest precursor species (Rhus dentata, Halleria lucida, Myrsine africana, Buddleja salviifolia, Leucosidea sericea) and A. mearnsii. A species-rich grassland on the northern aspect had become dominated by tall Hyparrhenia rufa, with scattered shrubs of L. sericea and Helichrysum rogulosum. The southern aspect exclosure in Döhne Sourveld had developed an A. mearnsii - B. salviifolia savanna, with Cymbopogon excavatus, Themeda triandra and Tristachya leucothrix as common grasses. The northern aspect exclosure on Döhne Sourveld maintained a grassland character, although heath-like elements (Stoebe vulgaris, Erica aspalanthifolia) established. Protection of the Southern Tall Grassveld promoted an open savanna of exotic and indigenous Acacia and broadleaf species, an increase in life forms including the succulent Aloe maculata, and the transformation of a T. triandra - T. leucothrix sward to an Aristida junciformis sward. Ferns benefitted from exclusion of fire and herbivory. Fire and herbivory rather than soil properties would seem the primary determinants of the structure of sour grasslands.     

Communal farmers’ ecological knowledge and perceptions of grasses in the central Eastern Cape province,South Africa: similarities with field studies and scientific knowledge and their implications

The local ecological knowledge (LEK) and perceptions of grasses were investigated in six semi-arid communal areas in the central Eastern Cape province, South Africa, and compared with field studies and scientific knowledge. Of the total of 21 grasses identified in the field survey, communal farmers had vernacular names only for 11 species. Respondents rated Digitaria eriantha and Pennisetum clandestinum to have the highest (p < 0.05) grazing value. Respondents’ ratings of Eragrostis obtusa and Sporobolus africanus as having similar (moderate) grazing values to Themeda triandra is against the established knowledge. Respondents’ ratings of T. triandra and S. africanus as having higher (p = 0.01) ecological value than the other grasses concur with the scientific knowledge, whereas their nomination of E. obtusa and D. eriantha as providing less ecological benefits than Aristida conjesta disagrees with some ecologists. The study hinted at the complementarity and discrepancies between LEK and scientific knowledge that need ratification. The respondents’ lack of detailed knowledge is suggestive of a gradual eroding of LEK among the local communities that will adversely impact the local people’s ability to implement sustainable rangeland utilisation practice. The current study also recognised farmers’ familiarity as well as training needs that would expound their knowledge and management of grasses.     

Plant and sward response to patch grazing in the Highland Sourveld

Abstract

Selective grazing by livestock inevitably results in patch grazing. This study was initiated to determine the effects of patch grazing on (1) the vigour of a single grass species, Themeda triandra, and of the sward, and (2) sward species composition. The generally held idea that a full season's rest followed by an early spring burn would prevent preferential grazing of patch grazed areas which had developed in the seasons before the rest was also tested. The vigour of 71 triandra was estimated from etiolated growth of marked tufts while sward vigour was indexed by above‐ground herbage production (AGHP). Etiolated growth of T. triandra and AGHP of the sward within patches were negatively affected by three seasons of grazing, but a full season's rest appeared sufficient to restore both the etiolated growth of T. triandra and the AGHP of the sward to a level similar to that in the non‐patches. A full season's rest followed by spring burning did not, however, prevent preferential grazing of grazed patches which had developed in the seasons prior to the rest. Species composition within patches (characterised by Increaser II species) differed significantly from the species composition of non‐patches (characterised by Increaser I species). Patch grazing may therefore initiate the rangeland degradation process in Highland Sourveld and patch grazing may be the focus from which rangeland degradation proceeds.     

Plant species dynamics in the Southern Tall Grassveld under grazing,resting and fire

An analysis of temporal changes in botanical composition in a long‐term grazing trial indicates that species dynamics in the Southern Tall Grassveld of Natal are determined by the specific combination of grazing, mowing and fire impacts. Species composition of a grazing systems trial was recorded at intervals during 16 years, and in the 14 years following the removal of herbivores, during which time the experimental area was burnt periodically. Site trajectories in ordination space facilitated the assessment of the nature, magnitude and rate of species composition change under various combinations of impacts. Under rotational grazing and mowing, botanical change was minimal, both during the grazing and the subsequent rest and fire phases of the trial. It is suggested that the interruption of continuous grazing at a high stocking rate by a seasonal rest (rotational resting) promoted the invasion of the sward by Aristida junciformis. This also occurred in the continuously‐grazed treatment at a high stocking rate when stock were removed from the treatment and periodic burning was introduced. It appears that swards dominated by A. junciformis remain stable under a rest and burning regime.     

Forage production of some grass species in the false Thornveld of the Eastern Cape

Abstract

The most commonly occurring grass species in the False Thornveld (Acocks, 1953; veld type no. 21) were clipped before and after grazing on veld in good and poor condition at different times of the year. Herbage yield per species was expressed per unit basal tuft area, and the difference in yield before and after grazing was considered to reflect the production of available forage. Themeda triandra produced more forage than any other species at all times of the year. Sporobolus fimbriatus generally produced the second highest amount of available forage. Cymbopogon plurinodis, despite having the second highest herbage yield before grazing, was not grazed to a significant extent. On the basis of the results, Themeda triandra and Sporobolus fimbriatus were identified as key grass species in the False Thornveld of the Eastern Cape.     

Patterns of plant selection by grazing cattle in two savanna grasslands: A plant's eye view

The amount of grazing experienced by individual tufts of the most abundant perennial grasses of two Transvaal savannas was monitored at regular intervals for two growing seasons. In general, Setaria incrassata and Themeda triandra were heavily grazed, Bothriochloa insculpla and Aristida bipartita were lightly grazed, and Heteropogon conlortus and Digitaria eriantha were intermediate. Tufts with a greater amount of moribund material were less heavily grazed. The smallest tufts of all species were the most lightly grazed, although the most heavily‐grazed tufts were on occasion intermediate in size rather than being the largest. A tuft had an increased likelihood of being heavily grazed if it had previously been heavily grazed, both within a growing season and for two successive growing seasons. A spatial pattern of grazing was evident, as individual species were less heavily grazed at different locations within the paddocks. The influence of species identity on the amount a tuft was grazed was not absolute but rather quantitative and conditional on the level of other variables. Tuft size and the amount a tuft had been previously grazed generally had a greater influence on the amount a tuft was grazed during an individual grazing event than species identity, location or moribund material. Species selection is possibly, in part, a consequence of selection for plant structure which optimizes intake.     

Translocation of radioactive paraquat in some veld grasses

In two pot experiments radioactive paraquat was applied to certain important veld grasses (Experiment I — Aristida junciformis, Themeda triandra, Elyonurus argenteus, Andropogon filifolius, Eragrostis cur‐vula ; Experiment II — A. junciformis, E. argenteus) to determine the extent of translocation at a young stage of growth and with treatment at different times of the day. In the first experiment movement occurred in all grasses along the length of the treated leaf, while in E. curvula and A. filifolius the herbicide was translocated to other leaves also. The second experiment indicated the importance of application of paraquat at. 1700 hours for maximum translocation.     

Evaluation of the grazing potential of grass species in Tarchonanthus veld of the northern cape 1

Abstract

The grazing value of the seven most prominent grass species in Tarchonanthus‐limeveld was investigated from 1977 to 1981.

The frequency of utilization of Chrysopogon serrulatus was exceptionally high (93%), while Sporobolus fimbriatus, Themeda triandra and Digitaria seriata were intermediate (50–60%) followed by Cymbopogon plurinodis (34%). Eragrotis lehmanniana and Fingerhuthia africana were poorly grazed (12 to 16%). The utilization of species was influenced by stocking rate. Statistical differences between species indicated a relatively high biomass production (600 to 740 g DM/m² per year) for E. lehmanniana, C. plurinodis and T. triandra, while F. africana, S. fimbriatus and D. seriata were intermediate (320 to 420 g DM/m² per year). E. lehmanniana, C. plurinodis and T. triandra produced the highest above‐ground phytomass, but had a lower crude protein content (2,9 to 7,4%) and digestibility (46,1 to 61,3%) than the other four species (3,8 to 9,7% crude protein and 50,5 to 68,4% digestibility).     

Grass defoliation affecting survival and growth of seedlings of Acacia karroo,an encroaching species in southwestern Zimbabwe

Abstract

Two experiments were conducted, one in the field and the other in the greenhouse, to investigate the effects of the intensity and frequency of grass defoliation on the survival and growth of Acacia karroo seedlings. In the greenhouse, seedlings growing with heavily clipped grasses had higher biomass production than those competing with moderately clipped grasses. Root/shoot ratios were higher in treatments with undipped grasses. There was a negative relationship between grass root production and A. karroo biomass production. The field experiment was carried out in two paddocks, one previously heavily‐grazed and the other lightly‐grazed. Grazing in both paddocks was simulated by artificial defoliation. Generally more A. karroo seedlings emerged under lightly defoliated treatments. Clipping frequency had a strong effect (P=0.066) on the survival of emerged seedlings during the wet season. There were no differences in survival rate at the end of the dry season. Though grass defoliation was shown to enhance seedling growth under controlled conditions, no evidence was found to suggest that seedling establishment during the first year is influenced by the intensity of grass defoliation.     

The influence of tiller age and time of year on the growth of Themeda tri andra and Sporobolus fimbriatus in semi‐arid grassveld

Abstract

Sequential growth measurements were made on successive populations of Themeda triandra and Sporobolus fimbriatus tillers initiated at different times of the year. The independent influences of tiller age and time of year on tiller growth were isolated by deriving growth indices for tillers of the same age at different times of the year and for tillers of all ages at each time of the year respectively.

Isolating the effect of plant age on growth showed that T. triandra is most productive under a regime of long periods of undisturbed growth, while S. fimbriatus is better suited to frequent interruptions in growth. Isolating the effect of time of year shows that both species are particularly sensitive to perturbations in spring. However, the photosynthetic efficiency of T. triandra is strongly influenced by time of year while S. fimbriatus seems to be a more opportunistic assimilator of sunlight.     

Elevation of shoot‐apices of two ecotypes of Themeda triandra on the Transvaal Highveld

In a series of investigations from 1965 to 1970 on the Transvaal Highveld it was found that trie elevation of shoot apices of two ecotypes of Themeda triandra was affected by season, ecotype and treatment. The absence of defoliation increased the susceptibility of the ecotypes to defoliation, whereas the shorter the defoliation the greater the subsequent resistance to grazing. The most important finding was that general application of earlier research findings from Natal was not possible in this case.     

The effect of leaf age on the photosynthetic rate of themeda triandra forsk 1

Abstract

The apparent photosynthesis of progressively older leaves of Themeda triandra was recorded in a controlled environment by means of gas exchange measurements. Photosynthetic rate increased with increasing leaf age and was maximal in the third fully expanded leaf. Thereafter, photosynthesis dropped sharply, but the photosynthetic rate of the oldest green leaf was still 55 % of that of the most efficient leaf. The implication is that old green leaves can probably still play an important role in initiating regrowth after defoliation of younger leaves.     

Effect of continuous grazing in the dohne sourveld on species composition and basal cover

Abstract

In a grazing trial conducted at the Dohne Agricultural Research Station, natural grassland was grazed continuously for 36 years with cattle. In a second investigation conducted on similar grassland, paddocks were grazed or rested, in all combinations of the four seasons of the year, over a ten year period. Changes in botanical composition and cover of the sward were monitored at regular intervals. The results of these investigations indicate that under a regime of non‐selective grazing the composition of Dohne Sourveld is remarkably stable. After 36 years of continuous grazing the species composition of the sward was largely maintained. Floristically Dohne Sourveld is dominated by a small number of grass species of which Andropogon appendiculatus, Elionurus muticus, Heteropogon contortus, Sporobolus africanus, Themeda triandra and Tristachya leucothrix are the most important. These species react differently to under‐utilization or increasingly severe defoliation, enabling them to be classified into decreaser or increaser categories. Severe defoliation resulted in a marked decrease in basal cover but only small changes in relative species composition. As the intensity of defoliation increases, decrease in grass cover is associated with an increase of forbs, such as Senecio retrorsus.     

The dynamics of Themeda Triandra tillers in relation to burning in the natal Drakensberg

Abstract

Marked tillers of Themeda triandra were sampled regularly over a two year period in Highland Sourveld burnt annually in winter, biennially in spring and after a single summer and early winter burning treatment. Parameters recorded were height of shoot apex, tiller mass, number of new lateral tillers and time of flowering. Monitoring of marked tillers before and after each burn enabled the life history of populations of tillers to be followed.

There were no significant differences in survivorship between tillers in the annual winter and biennial spring burn treatments but winter burning stimulated the development of a greater number of daughters per parent. Both populations exhibited a constant death rate over the two year period. Only 8% of the marked T. triandra tillers survived the single summer burning treatment.     

Measuring the impact of indigenous herbivores: reply to Kerley andLandman

Pteronia paniculata is an indigenous, unpalatable shrub that invades mismanaged Karooveld, resulting in degraded rangelands with low species diversity and grazing potential. We conducted a series of trials in the Succulent Karoo Randteveld near Barrydale to determine if the uniform defoliation of P. paniculata dominated vegetation at two heights (0.05 m and 0.20 m above ground level) using a brush-cutter and, in one trial, application of a second cut will improve the plant species composition, productivity and grazing capacity of the veld. Brush-cutting treatments and the uncut control all resulted in a change in species composition towards greater species diversity and more palatable species and an average increase of 540 kg ha^?1 (28%) in above-ground biomass over four years. It appears that there was a pervasive improvement in species composition associated with a general decline in the cover and abundance of P. paniculata over the time-scale of the present study that was not influenced by the defoliation treatments, except for the 1996-cut treatment where the cover of P. paniculata increased. The absence of propagules of palatable species in the soil seed bank and competition from P. paniculata (a long-lived, perennial shrub) are assumed to be among the main reasons for the lack of response of the vegetation community to the defoliation treatments. Brush-cutting (in the absence of reseeding), aimed at reducing the dominance of unpalatable karoo shrubs, was more costly but not significantly better than long-term resting in improving veld composition or forage production.     

Production of irrigated Trifolium repens L. under a cutting regime in the subtropical region of Gauteng,South Africa

Abstract

The productivity of Trifolium repens L. cv. Ladino, under irrigation in the subtropical region of Gauteng, South Africa, is unknown. The performance of T. repens was determined over five years (1981/1982 to 1985/1986), at two defoliation frequencies, four levels of nitrogen (0, 400, 800 and 1 600 kg N ha^?1 ‐ total applied over five years), and either the removal or recycling of harvested material. Frequent, compared to infrequent defoliation, resulted in higher dry matter (DM) yields from the second year onwards. Dry matter yields were positively affected by the recycling of material, compared to no recycling. A mean DM yield of 191 ha^?1 in the first year and 12 t ha^?1 in the final year, was obtained. The highest DM yields occurred during October for all years and lowest DM yields from January to March. The first and final year showed similar peaks (late October) and troughs (March till April). Crude protein (CP) content of the herbage was influenced more by frequency of defoliation than recycling of material, where the highest CP content in any year was obtained from frequent defoliation. Nitrogen fertiliser applied up to a total of 1 600 kg N ha^?1 over five years had no effect on either DM yield or crude protein content. This study has shown that T. repens could be a productive pasture for at least five years in the subtropical region of Gauteng, South Africa. Productivity and quality of T. repens were enhanced by a defoliation interval of less than five weeks and when soil high fertility was maintained by recycled plant material.