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The effect of defoliation environment on primary growth allocation and secondary tiller recruitment of two bunchgrasses
Abstract
Compositional shifts of bunchgrass species in the Southern Tall Grassveld in response to different combinations of non-selective mowing and burning appear to reflect differences in growth strategy among species, but there are few studies of South African grasses designed to test this. A descriptive study was undertaken both to observe how different species allocated resources in response to different defoliation treatments imposed at the start of spring, as it was thought that these responses might be critical for an understanding of tiller dynamics and compositional change in bunch grasslands. The study recorded the reproductive growth response of two spring-flowering species, Themeda triandra and Eragrostis curvula, subjected to three defoliation treatments: no defoliation (No Mow), defoliation during the previous summer (Summer Mow), and defoliation in early spring (Early Spring Mow). Plant growth response was separated into primary growth, measured as individual organ extension on individual phytomers, and secondary growth, measured as the number of secondary tillers recruited per live primary tiller. Phytomer growth depended on treatment and species. For either species, organ extension was greater for the No Mow treatment than for either of the two defoliated treatments. Tillers of E. curvula were substantially larger and grew faster than T. triandra tillers. In addition, apical meristems of E. curvula remained close to the ground until flowering. These combined differences are consistent with E. curvula being able to outcompete T. triandra in less-frequently defoliated swards. Secondary tiller recruitment was significantly related to defoliation treatment and species. Tiller recruitment was substantially lower on the No Mow treatment than for the two defoliation treatments, and tiller recruitment was greater for T. triandra plants than for E. curvula plants. Accumulated blade length was negatively related to tiller recruitment. Results suggest that tiller recruitment was limited by nutrient demand of primary structures, a consequence of which is that T. triandra will recruit more tillers than E. curvula when defoliation opens up space, accounting for T. triandra's dominance on more regularly-defoliated treatments.
... As a result of (often) confounded experimental designs, it is perhaps not surprising that some studies have found no influence of clipping on tiller numbers (provided that height of clipping was higher than that of the apical meristems) , Rethman 1971, Edroma 1985, Oesterheld and McNaughton 1988, Danckwerts 1989, Hodgkinson et al. 1989, Dube and Gwarazimba 2000, whereas other studies found that clipping could lead to an increase in tiller number , Rethman 1971, Edroma 1985, Oesterheld and McNaughton 1988, Hodgkinson et al. 1989, Dube and Gwarazimba 2000, Tomlinson and O'Connor 2005, although in some cases it depended on the severity and frequency of clipping , Rethman 1971, Edroma 1985, Oesterheld and McNaughton 1988, Hodgkinson et al. 1989, Dube and Gwarazimba 2000. Other studies found that clipping reduced tiller number (Edroma 1985, McNaughton 1992, Daphne 1992, Morris and Tainton 1993. ...
... observed a decline in culm tillers but an increase in basal tillers when defoliated but by the following season there was no difference between defoliated and undefoliated treatments. In addition, Hodgkinson et al. (1989) and Tomlinson and O'Connor (2005) observed that clipped tillers were slower to regrow than unclipped tillers. It has long been recognised that, regardless of the clipping regime, defoliation results in the production of lateral basal tillers (Stuart-Hill and Mentis 1982, McNaughton 1992, believed to be due to the release of the lateral buds from apical meristem dominance, allowing new secondary tillers to grow , Drewes and Tainton 1981, Richards et al. 1988). ...
Themeda triandra is a perennial tussock grass endemic to Africa, Australia and Asia. Within these regions it is found
across a broad range of climates, geological substrates and ecosystems. Because it is widespread across these
areas it has great economic and ecological value, as it is a relatively palatable species across most of its range. It is
of critical importance in supporting local populations of both native and introduced herbivores, and is thus central to
wildlife and livestock production, and consequently rural livelihoods. It is an important climax or subclimax species
that is well adapted to fire, a common element of many areas where it is found. Inappropriate grazing management,
however, can result in a decline of Themeda, as it is not well adapted to an uninterrupted, selective grazing
regime. A decline in abundance of Themeda in a grassland is usually coupled to a decline in grazing value, species
richness, cover and ecosystem function. In spite of its significant ecological and economic importance, there has
been no attempt to review and synthesise the considerable body of research undertaken on this grass. Our aim is to
summarise and synthesis work previously undertaken and identify areas where further research is required.
... As a result of (often) confounded experimental designs, it is perhaps not surprising that some studies have found no influence of clipping on tiller numbers (provided that height of clipping was higher than that of the apical meristems) , Rethman 1971, Edroma 1985, Oesterheld and McNaughton 1988, Danckwerts 1989, Hodgkinson et al. 1989, Dube and Gwarazimba 2000, whereas other studies found that clipping could lead to an increase in tiller number , Rethman 1971, Edroma 1985, Oesterheld and McNaughton 1988, Hodgkinson et al. 1989, Dube and Gwarazimba 2000, Tomlinson and O'Connor 2005, although in some cases it depended on the severity and frequency of clipping , Rethman 1971, Edroma 1985, Oesterheld and McNaughton 1988, Hodgkinson et al. 1989, Dube and Gwarazimba 2000. Other studies found that clipping reduced tiller number (Edroma 1985, McNaughton 1992, Daphne 1992, Morris and Tainton 1993. ...
... observed a decline in culm tillers but an increase in basal tillers when defoliated but by the following season there was no difference between defoliated and undefoliated treatments. In addition, Hodgkinson et al. (1989) and Tomlinson and O'Connor (2005) observed that clipped tillers were slower to regrow than unclipped tillers. It has long been recognised that, regardless of the clipping regime, defoliation results in the production of lateral basal tillers (Stuart-Hill and Mentis 1982, McNaughton 1992, believed to be due to the release of the lateral buds from apical meristem dominance, allowing new secondary tillers to grow , Drewes and Tainton 1981, Richards et al. 1988). ...
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... As a result of (often) confounded experimental designs, it is perhaps not surprising that some studies have found no influence of clipping on tiller numbers (provided that height of clipping was higher than that of the apical meristems) , Rethman 1971, Edroma 1985, Oesterheld and McNaughton 1988, Danckwerts 1989, Hodgkinson et al. 1989, Dube and Gwarazimba 2000, whereas other studies found that clipping could lead to an increase in tiller number , Rethman 1971, Edroma 1985, Oesterheld and McNaughton 1988, Hodgkinson et al. 1989, Dube and Gwarazimba 2000, Tomlinson and O'Connor 2005, although in some cases it depended on the severity and frequency of clipping , Rethman 1971, Edroma 1985, Oesterheld and McNaughton 1988, Hodgkinson et al. 1989, Dube and Gwarazimba 2000. Other studies found that clipping reduced tiller number (Edroma 1985, McNaughton 1992, Daphne 1992, Morris and Tainton 1993. ...
... observed a decline in culm tillers but an increase in basal tillers when defoliated but by the following season there was no difference between defoliated and undefoliated treatments. In addition, Hodgkinson et al. (1989) and Tomlinson and O'Connor (2005) observed that clipped tillers were slower to regrow than unclipped tillers. It has long been recognised that, regardless of the clipping regime, defoliation results in the production of lateral basal tillers (Stuart-Hill and Mentis 1982, McNaughton 1992, believed to be due to the release of the lateral buds from apical meristem dominance, allowing new secondary tillers to grow , Drewes and Tainton 1981, Richards et al. 1988). ...
Themeda triandra is a perennial tussock grass endemic to Africa, Australia and Asia. Within these regions it is found
across a broad range of climates, geological substrates and ecosystems. Because it is widespread across these
areas it has great economic and ecological value, as it is a relatively palatable species across most of its range. It is
of critical importance in supporting local populations of both native and introduced herbivores, and is thus central to
wildlife and livestock production, and consequently rural livelihoods. It is an important climax or subclimax species
that is well adapted to fire, a common element of many areas where it is found. Inappropriate grazing management,
however, can result in a decline of Themeda, as it is not well adapted to an uninterrupted, selective grazing
regime. A decline in abundance of Themeda in a grassland is usually coupled to a decline in grazing value, species
richness, cover and ecosystem function. In spite of its significant ecological and economic importance, there has
been no attempt to review and synthesise the considerable body of research undertaken on this grass. Our aim is to
summarise and synthesis work previously undertaken and identify areas where further research is required.
... As a result of (often) confounded experimental designs, it is perhaps not surprising that some studies have found no influence of clipping on tiller numbers (provided that height of clipping was higher than that of the apical meristems) , Rethman 1971, Edroma 1985, Oesterheld and McNaughton 1988, Danckwerts 1989, Hodgkinson et al. 1989, Dube and Gwarazimba 2000, whereas other studies found that clipping could lead to an increase in tiller number , Rethman 1971, Edroma 1985, Oesterheld and McNaughton 1988, Hodgkinson et al. 1989, Dube and Gwarazimba 2000, Tomlinson and O'Connor 2005, although in some cases it depended on the severity and frequency of clipping , Rethman 1971, Edroma 1985, Oesterheld and McNaughton 1988, Hodgkinson et al. 1989, Dube and Gwarazimba 2000. Other studies found that clipping reduced tiller number (Edroma 1985, McNaughton 1992, Daphne 1992, Morris and Tainton 1993. ...
... observed a decline in culm tillers but an increase in basal tillers when defoliated but by the following season there was no difference between defoliated and undefoliated treatments. In addition, Hodgkinson et al. (1989) and Tomlinson and O'Connor (2005) observed that clipped tillers were slower to regrow than unclipped tillers. It has long been recognised that, regardless of the clipping regime, defoliation results in the production of lateral basal tillers (Stuart-Hill and Mentis 1982, McNaughton 1992, believed to be due to the release of the lateral buds from apical meristem dominance, allowing new secondary tillers to grow , Drewes and Tainton 1981, Richards et al. 1988). ...
... Groundlayer plants in fire-prone ecosystems exhibit diverse morphologies, all adapted to protect their meristems and meristem banks by positioning them below the fire zone during the fire (Tomlinson and Archibald, In press). Among these, caespitose grasses or bunchgrasses dominate, forming tight clumps with meristems located near the ground, except during reproductive stages (Tomlinson and O'Connor, 2005;Ellsworth and Kauffman, 2010). Perennial herbs and juvenile trees rely on subterranean structures, such as tubers, taproots and rhizomes, to store water, nutrients, and carbohydrates, facilitating regrowth of unipolar stems following fires (Higgins et al., 2000;Maurin et al., 2014;Bond and Keane, 2017;Pausas et al., 2018), often with large leaf surfaces to support rapid carbohydrate accrual (Tomlinson et al., 2013). ...
... As the desirable plants become weaker and die off, the number of roots in the upper layer of the soil decreases. Defoliation removes plant biomass, which changes the light regime in a plant stand (Tomlinson and O"Connor, 2005) and this result in low photosynthetic rate of plants, which in turn reduces rangeland productivity. The bare areas between grasses become larger as the grass species are exhausted, causing a decline in the effective use of rainfall in the area. ...
Rangelands are lands on which the indigenous vegetation is predominantly grasses, grass-like plants, forbs, or shrubs and are managed as a natural ecosystem. They include grasslands, savannas, shrublands, deserts, tundras, marshes, and meadows. Globally, rangelands comprise the largest land use, estimated to cover about 25% of Earth's land surface. This makes them an essential resource for both maintaining environmental services like biodiversity conservation and as a source of livelihood, especially for rural communities. Rangelands are used primarily as a source of feed for livestock. They, however, provide other secondary resources such as firewood, wild foods, medicinal plants, and water. Land degradation is the major challenge in the rangelands of the earth. Rangeland degradation is occurring as a result of no grazing management plans, removal of vegetation for fuel wood and no clear authority of rangeland ownership. The major indicators of rangelands degradation are shift in species composition, loss of range biodiversity, reduction in biomass production, less plant cover, low small ruminant productivity, and soil erosion. Major changes in rangeland surface morphology and soil characteristics have a drastic effect on the primary productivity of the rangeland ecosystem, and in turn on livestock production. The condition of the grazing area is influenced principally by herbivore species, densities and landscape structure. Population densities of grazing animals and intensity of their foraging can determine some rangeland dynamics. It determines whether herbivore increases nutrient cycling and plant productivity or affects plant communities by driving changes in successional pathways decreasing nutrient cycling, and influencing biodiversity of those communities. Management of rangeland degradation can be divided into preventative and restoration measures. Answers to preventative measures can often be found within the causes of land degradation. In view of the massive scale of land degradation, restoration is of significant importance to land owners.
... Mass per tiller was approximately constant across all treatments, suggesting that secondary tillers are produced only once resources in excess of the requirements of existing tillers are available (Tomlinson and O'Connor 2004). The rate of tiller recruitment, ranging from approximately 7.5 to 27 tillers per plant over a five-month period, is considerably higher than that of T. triandra in natural, undefoliated grasslands (1.07 tillers per primary tiller) found by Tomlinson and O'Connor (2005) over a period of three months. Fertilisation (low or high) with N increased the shoot:root mass ratio (leafiness) slightly from approximately 1:1 to 1:1.34. ...
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Themeda triandra Forssk. is a common perennial grass in southern African grasslands that tolerates low nutrient soils. Establishment of the species into degraded or transformed areas is difficult, but there have been some advances with clonal propagation using tillers. To generate propagules (tillers), tiller production of plants must be maximised. This research focused on the growth response of T. triandra to fertilisation with nitrogen (N), phosphorus (P), and potassium (K) each at three levels in a fully factorial pot experiment. Fertilisation increased growth. The number of tillers and aerial shoot mass were controlled by an N × P interaction, and total mass and root mass were controlled by N and by P, with N having a larger effect. Mass per tiller was not related to treatment. Shoot:root ratio and root mass per tiller were controlled by N. Potassium had no effect on any response. For maximum tiller production of T. triandra, plants should be fertilised with high levels of N and P.
... In grassland landscapes, large herbivores reduce the cover and biomass of plants (Rambo and Faeth 1999;McEvoy et al. 2006;Olofsson 2006;Johnson and Cushman 2007). However, some grass species have the ability to rapidly invest resources into secondary tiller growth following defoliation (Tomlinson and O'Connor 2005). Despite such phenological adaptations, the persistent removal of aboveground biomass can increase the presence of bare ground in grasslands (Dorrough et al. 2004;McEvoy et al. 2006). ...
... Consequently, phytomer recruitment interval will lengthen or shorten in response to the blade length of the associated phytomer. Therefore, phytomer recruitment rate depends on species properties such as blade growth rates and blade length (Tomlinson and O'Connor, 2005 ) and on environmental factors such as temperature and soil water pressure (Hsiao and Acevedo, 1974; Watts, 1974; Nemoto et al., 1995). The TILLERTREE model uses this consecutive phytomer co-ordination property to control phytomer recruitment and expansion. ...
Interactions between structural architecture and resource allocation affect the ability of plants to utilise environmental resources. Architecture defines the structural layout and relationships of organs and other structural units at different hierarchical levels in plants. Resource allocation determines how competing structural units are awarded resources at different levels of hierarchy. Functional-structural plant models combine architecture and resource allocation as interacting components of plant growth and functioning. Existing functional-structural plant models concentrate on growth of unitary trees and therefore, lack sufficient structural definition to simulate growth of clonal plants. On the other hand, simulation models designed to consider clonal growth rarely attempt to simulate clonal architecture at a more detailed level than individual ramets. This paper introduces a functional-structural type model, TILLERTREE, which integrates the architectural growth of bunchgrasses with resource capture and allocation of nitrogen and carbohydrate. Resource allocation is implemented using a procedural algorithm based on object hierarchy and pri-ority, and not mechanistically. The model is used to illustrate that growth of bunchgrass clones is regulated by patterns of resource allocation between competing units at low levels of hierarchy, by considering the effect of resource rules controlling secondary tiller recruit-ment on clonal growth. Simulations are conducted using a chosen model C 4 bunchgrass species Themeda triandra.
... In grassland landscapes, large herbivores reduce the cover and biomass of plants (Rambo and Faeth 1999;McEvoy et al. 2006;Olofsson 2006;Johnson and Cushman 2007). However, some grass species have the ability to rapidly invest resources into secondary tiller growth following defoliation (Tomlinson and O'Connor 2005). Despite such phenological adaptations, the persistent removal of aboveground biomass can increase the presence of bare ground in grasslands (Dorrough et al. 2004;McEvoy et al. 2006). ...
The impact of elephants (Loxodonta africana) on woody plants is well known. Elephants can be regarded as drivers of ecosystem functioning by, for example, decreasing woody plant litter accumulation through defoliation. However, their influence within grassland landscapes is, by comparison, very poorly understood. We assessed the influence of elephants on grassland functionality at three separate sites (1, high elephant density, long occupation time; 2, low elephant density, short occupation time; 3, no elephants) in the Addo Elephant National Park, Eastern Cape Province, South Africa. Using landscape function analysis (LFA) we described the landscape organisation of each site, and, using visual surrogates, calculated indices of landscape stability, infiltration and nutrient cycling. The number of vegetation patches/10m of transects surveyed was higher at sites where elephant density was high (3elephants/km2) and where elephants had been present for a long time (>70 years). However, patch size was significantly smaller when elephant density/time of occupation increased, and the proportion of bare soil was higher where elephant density and occupation time were highest. In addition, stability, infiltration and nutrient cycling indices at a site scale were significantly lower where elephants were present at high densities and after a long occupation time. However, bare soil stability was not greatly affected by elephant grazing pressure, implying that a 'threshold of potential concern' has not yet arisen. We conclude that the functioning of this grassland landscape is significantly altered when elephants are present. These conclusions highlight the importance of management factors such as containment and the provision of artificial water points which may be compromising the functionality of these landscapes. We recommend ongoing assessments to inform future management decisions.
Questions
How consistent are assembly rules for grassland communities from different regions in response to management? We compared vegetation response using long‐term experiments in subtropical South Africa with published temperate Park Grass (U.K.) and Konza Prairie (U.S.A.) results.
Location
Ukulinga, Pietermaritzburg, South Africa.
Methods
Long‐term experiments in the South African site included a long‐term fertilizer experiment (VFE) that were compared with Park Grass and a burning and mowing experiment (BME) that were compared with Konza Prairie.
Results
We found consistent vegetation patterns of this subtropical grassland and the temperate grasslands at Park Grass and Konza Prairie. In the comparison of the burning and mowing experiments, species richness in the BME at Ukulinga declined across mowing treatments but did not change between control and intermediate burns as found at Konza Prairie. There was dominance by a few species in the subtropical grassland and the two temperate grasslands under high productivity and frequent burns. Annual burns resulted in lower species richness at both subtropical Ukulinga and temperate Konza Prairie. At both temperate Park Grass and Ukulinga, few species dominated in highly fertilized plots, both sites showed a negative effect of nitrogen amount, and there was a positive effect of lime on species richness.
Conclusions
Consistent results from long‐term series of subtropical and temperate grasslands suggest that there are similar community assembly rules regardless of different climatic conditions and different species pools. A possible underlying mechanism for community assembly at Ukulinga and Konza Prairie for both fertilization, burning and mowing may be shade tolerance. At Ukulinga and Park Grass, changes in species richness (negative effects of increasing nitrogen amount and positive effects of lime) were remarkably similar. Most of the common grass species in South Africa showed evidence of competition, supporting niche‐based assembly rules.
The annual replacement of tillers of Agropyron desertorum (Fisch. ex Link) Schult., a grazing-tolerant, Eurasian tussock grass, was examined in the field following cattle grazing. Heavy grazing before internode (culm) elongation seldom affected tiller replacement. Heavy grazing during or after internode elongation, which elevates apical meristems, increased overwinter mortality of fall-produced tillers and reduced the number and heights of these replacement tillers. Unexpectedly, tussocks grazed twice within the spring growing season tended to have lower overwinter tiller mortality, greater tiller replacement, and larger replacement tillers than tussocks grazed only once in late spring. These responses of twice-grazed tussocks, however, were still less than those of ungrazed tussocks or tussocks grazed moderately in early spring. The presence of ungrazed tillers on partially grazed tussoks did not increase the replacement of associated grazed tillers relative to tillers on uniformly grazed plants. This result indicates that resource sharing among tillers, if present, is short-lived or ecologically unimportant in this species. Although A. desertorum is considered grazing-tolerant, tiller replacement on heavily grazed tussocks, particularly those grazed during or after internode elongation when apical meristems were removed, was usually inadequate for tussock maintenance. These observations at the tiller (ramet) level of organization in individual tussocks (genet) may explain the often noted reduction in stand (population) longevity with consistent heavy grazing.
South African grasslands support a rich flora that is attracting growing conservation interest. Fire has long been used to manage grasslands for livestock production. However, there is very little information on the effects of fire on forb diversity to help guide conservation management. We studied plant diversity at scales of 1 and 100 m2 in three long-term burning experiments in mesic, montane and semi-arid grasslands, respectively, to explore forb responses to different fire regimes. Though the dominant grasses were strongly influenced by season and frequency of fire, forb diversity showed no consistent trends. Ordination results showed that forb composition varied less with fire treatment than with local site conditions. Forbs in all three grasslands seem remarkably resilient to fire. However, all three sites showed large compositional changes if fires were excluded for about 10 or more years, with the replacement of many species by a suite of, mostly, woody species. Patterns of beta-diversity sampled in the montane grassland, showed somewhat different patterns, with species turnover increasing with inter-fire interval. Our results indicate that most forb species tolerate a wider range of season and frequency of fires than the dominant grasses. However, to accommodate those species with low tolerance of frequent fires, parts of the landscape will require less frequent fires.
A simulation model of perennial graminoid growth was developed that united morphometric traits with physiological processes: a shoot number submodel simulated vegetative production; a carbon submodel simulated assimilation and allocation; a nitrogen submodel simulated uptake and allocation. Growth of the coexisting shoot populations on a single plant were simulated simultaneously and growth was partitioned into four tissue types, based on morphometric measurements: blade, sheath, stem, and flower. Tissue was classified by age and canopy height, resulting in similar classifications of carbon and nitrogen. The processes of shoot growth, tillering, root growth, photosynthesis, nitrogen uptake, and translocation were interrelated through nitrogen/carbon ratios and plant reserve carbohydrate status.Simulation results indicated how morphology may influence ecological properties and interact with physiology to influence plant fitness. Simulated plant growth based upon the properties of Serengeti, East African graminoids suggested that an increased understanding of both primary production and plant adaptation could result from increased attention to the interactions among: (a) physiological processes; (b) demographic processes associated with populations of shoots on a single plant; and (c) morphometric traits that determine plant form.
Grasses occupy a greater area of the world's land surface than any other plant family, occurring in almost every terrestrial environment and providing a vital source of food for humans and animals. This volume presents the most recent information on their population biology, bringing together contributions from researchers studying both applied and fundamental aspects of this important group of plants. Demographic, physiological, ecological and molecular approaches to understanding grass populations are considered in relation to reproduction and to aspects of life history patterns such as dispersal, germination, seedling establishment, population dynamics and reproduction. Other areas covered include the role of genetic variation and phenotypic plasticity in shaping life history traits, the impact of biotic factors, and the ecology of specific species in major grass-dominated ecosystems in Africa, Australia and Japan.
Disturbance may be an important determinant of plant community composition and diversity owing to its effects on competitive interactions, resources, dominance and vigour. The effect of type, timing and frequency of disturbance on grass and forb species richness was examined using data from a long-term (> 50 yr) grassland burning and mowing experiment in KwaZulu-Natal, South Africa. Grass species richness declined considerably (> 50%) in the absence of disturbance, whereas forb species richness was unaffected. Annual burning in sites not mown in summer tended to increase grass species richness relative to triennial burning (22% increase) with the reverse being true in sites mown twice in summer (37% decline). Forb species richness declined by 25% in sites mown twice in summer relative to sites mown in early summer only. Disturbance was necessary to achieve maximum grass species richness presumably by removing litter and increasing the availability of light. The interaction of time of mowing in summer (early versus late) and time of burning during the dormant period (spring versus winter) had the most dramatic effect on species richness. Time of burning had no effect on richness in sites mown in early summer, but winter burning resulted in a dramatic decline (27-42%) in richness in sites mown in late summer. This effect may be related to possible greater soil desiccation with this combination of disturbances.
Clones of Kyllinga nervosa from a semiarid location in the Serengeti region of Tanzania were propagated vegetatively and grown at 12-h photoperiods, 30oC maximum daytime temperature and 15o minimum nighttime temperature. Clipping height, clipping frequency, nitrogen concentration of the nutrient medium, and watering frequency were varied in an unbalanced factorial experiment. Plants were harvested after 5 mo. Yield to grazers was controlled by N supply, yield to producers was controlled by water, and yield to decomposers was controlled by clipping height and N. Plant growth was stimulated by defoliation. Plants overcompensated for amounts removed at moderate defoliation and high N, undercompensated at severe defoliation, and just compensated under other conditions. Therefore, increments of energy flow to grazers can be additive in ecosystem budgets due to compensatory growth of affected plants. The most important mechanisms accounting for compensatory growth were activation of plant meristems and stimulated rates of leaf elongation in defoliated plants. Allocation of residual plant biomass to various organs was virtually independent of environment, indicating genetic control of physiological processes tending to balance allocation over a broad environmental range.-from Authors
The more grazing-tolerant Agropyron desertorum consistently produced more regrowth in the absence of photosynthesis than A. spicatum but severe preclipping treatment (which has been shown to reduce carbohydrate reserves by >40%) did not significantly reduce etiolated regrowth in either species. Meristematic limitations appear to be the dominant control on the amount of etiolated regrowth produced. These limiations also appear to be of prime importance in determining regrowth in the light and for grazing tolerance of plants. -from Authors
This investigation, conducted over a three‐year period, emphasized the strong effect of nitrogen fertilization on both hay and seed production. The optimum level of nitrogen, for conditions on the Eastern Highveld being 100–150 kg N/ha applied in spring, followed by subsequent dressings dependent on rainfall. Although the influence of spring burning and stubble height on hay and seed production was not as consistent, both had a strong influence under certain conditions and with more research into their action could become important considerations in management strategies for Eragrostis curvula.
. Despite intensive research effort, the role of growth substances in the life of the intact growing plant is far from clear. Two reasons are suggested which may account for the lack of progress. The first is the failure to recognize the unique aspects of plant development. These which are expressed by the regenerative, organizational and developmental plasticity of the meristem probably result from the possession of growth substances. The second is the concept of growth substances as hormones. This represents the main conceptual thrust of research and is considered critically, starting with the historical system, the coleoptile and dealing with other major growth substance systems in turn. It is concluded that a hormonal concept which includes control by changes in growth substance concentration fails to explain the developmental phenomena under examination. A role for growth substances as integrating agents is suggested and the notion of quantitative tissue sensitivity variation is developed to explain the major growth patterns of developing shoots.
Seasonal changes in dietary quality were measured using steers fistulated at the oesophagus. From spring through to winter, crude protein (CP), in vitro digestibility (IVD) and leaf percentage in the diet declined significantly while percentage dead material increased significantly. Consumption of stem material increased gradually but declined in winter. Dietary CP and IVD were positively correlated with leaf percentage but negatively correlated with percentage dead material in the diet. The decline in dietary quality towards winter could be largely explained by changes in plant part consumption. It is recommended that protein supplementation of animals grazing veld be initiated in February in the area.
Marked populations of Themeda triandra tillers were sequentially sampled at fortnightly intervals under normal conditions and without moisture stress respectively, covering a period from tiller initiation through to senescence after flowering. Parameters recorded were height of shoot apex, tiller mass, leaf area, leaf number, secondary tiller development and premature tiller mortality. Tillers of irrigated plants had a significantly higher mass, more elevated shoot apices, and produced a greater total number of leaves than those of non‐irrigated plants. Fewer unstressed tillers senesced prematurely than those experiencing moisture stress.
Results are reported of a series of experiments designed to study the management of E. curvula for seed production in the Natal Region. The following are the main points which arose from this investigation: (a) approximately 90% of the seedheads present at harvest in December were produced by tillers which emerged in the previous autumn; (b) inflorescence emergence in the spring spanned an 18 to 20 day period and the later emerged heads, although smaller than the early heads, contributed almost 50% of the seedhead population; (c) both the time and rate of applying nitrogen influenced seed production to a considerable extent (maximum seed yield was obtained with an application of 120 kg N/ha in spring); (d) removal of over-wintered forage by burning in early spring had a marked effect on subsequent seed production, giving a yield of over 500 kg/ha of seed, and (e) defoliation at inflorescence initiation had no detrimental effect on seed yield when the stand was supplied with 180 kg N/ha in spring.
Tufts of Themeda triandra were subjected to different frequencies of defoliation during a single season under irrigated and non‐irrigated conditions. Frequent defoliation (> 5 times per year) severely reduced clipped yield on both moisture treatments. On irrigated treatments, root mass declined and shoot apex elevation was suppressed with increasing frequency of defoliation. Tiller densities and stubble mass were not affected by frequency of clipping. Severe drought experienced during the experimental period probably masked some of the responses to frequency of defoliation on non‐irrigated treatments.
A veld condition assessment was used to determine the effect of past burning treatments on the species composition of Highland Sourveld at Cathedral Peak. Veld condition scores in grassland protected from fire were significantly lower than where veld had been burnt or burnt and grazed at regular intervals. Frequent defoliation was found to maintain the grassland composition largely unchanged over a period of 30 years. Individual species were found to react strongly to defoliation frequency. Uittreksel Veldtoestand veranderings is gebruik om die invloed van vorige brandbehandelings op spesiesamestelling van Hoëlandsuurveld te Cathedral Peak te bestudeen Grassveld wat teen vuur beskerm is het ‘n betekenisvolle laer veldtoestand‐puntetelling gehad as dié wat gebrand is of gebmnd en met gereelde tussenposes bewei is. Dit is gevind dat herhaalde ontblaring die samestelling van grasveld grootliks onveranderd gelaat het oor ‘n tydperk van 30 jaar. Enkele soorte reageer merkbaar op frekwensie van ontblaring.
It has previously been proposed that the fundamental function of phytochrome in the natural environment is the perception of the relative proportions of red and far-red light, i.e. the red: far-red ratio. This paper re-evaluates this hypothesis, for vegetative green plants, in the light of recent findings. Essentially, three issues are considered: (a) the modulation of the response to red: far-red by fluence rate: (b) the anticipation of competition for light by perception of changes in red: far-red that precede actual shading: and (c) characteristics of phytochrome that may be important in the mechanism of photoperception (i.e. the accumulation of photoconversion intermediates, and the stability of Pfr). We conclude: (a) the red: far-red ratio provides a reliable signal of plant density, even before shading by neighbours occurs: (b) plants are able to perceive and respond to these signals, and that possible ambiguities due to low red: far-red at low solar angles may be avoided by modulation of the perception process by fluence-rate dependent mechanisms; (c) although direct experimental evidence does not yet exist, circumstantial evidence suggests that the perception of red: far-red may confer positive adaptive advantage; and (d) plants of certain species perceive and respond to fluence rate changes, mediated perhaps by a blue-light absorbing photoreceptor or by phytochrome, but that these responses do not necessarily lead to shade avoidance reactions and their ecological relevance is not fully understood.
This article continues the discussion on sensitivity prompted by the publication of an earlier paper by the author in 1981. It is indicated that appropriate measurements of sensitivity can help uncover the function of growth substances in development. The experimental constraints which are necessary for unambiguous measurements of sensitivity are outlined and it is shown that a specific sensitivity measurement, termed control strength, would most readily clarify function and help obviate controversy. Methods for measuring control strength which deal with the problem of compartmentalization are considered and it is suggested that stochastic variation in single cell biochemistry might provide a constraint on the accuracy of control strength determinations.
Supplemental far-red (FR) illumination of light-grown grass seedlings inhibits tiller production while enhancing leaf elongation. Although much is known about FR enhancement of internode elongation in dicots, relatively little research has been conducted to determine the effects of FR on monocot development. In growth chamber experiments, fibre optics were used to direct supplemental FR to elongating leaf blades, main stem bases and mature leaf blades of light-grown barley (Hordeum vulgare L.) seedlings. Our objective was to identify specific sites of perception for FR enhancement of leaf elongation and inhibition of tiller production, and to assess potential FR effects on tiller senescence. Far-red illumination of elongating leaves or of the main stem base reduced the total number of tillers per plant, primarily by reducing secondary and tertiary tiller production, and enhanced leaf elongation. However, leaf elongation was less sensitive to stem base treatments than to illumination of the elongating blade. Increased leaf length resulted from increased leaf elongation rate, while the duration of leaf elongation was unaffected. Exposure of mature leaf blades to FR had no effect on tillering or leaf elongation. None of the FR treatments led to tiller senescence. Localization of FR perception in vertically oriented tissues such as elongating blades and stem bases permits early detection of reflected light from neighbouring plants, allowing rapid response to impending competition.
Two perennial tussock grasses of savannas were compared in a glasshouse study to determine why they differed in their ability to withstand frequent, heavy grazing; Cenchrus ciliaris is tolerant and Themeda triandra is intolerant of heavy grazing. Frequent defoliation at weekly intervals for six weeks reduced shoot biomass production over a subsequent 42 day regrowth period compared with previously undefoliated plants (infrequent) in T. triandra, but not in C. ciliaris. Leaf area of T. triandra expanded rapidly following defoliation but high initial relative growth rates of shoots were not sustained after 14 days of regrowth because of reducing light utilising efficiency of leaves. Frequently defoliated plants were slower in rate of leaf area expansion and this was associated with reduced photosynthetic capacity of newly formed leaves, lower allocation of photosynthate to leaves but not lower tiller numbers. T. triandra appears well adapted to a regime where defoliation is sufficiently infrequent to allow carbon to be fixed to replace that used in initial leaf area expansion. In contrast, C. ciliaris is better adapted to frequent defoliation than is T. triandra, because horizontally orientated nodal tillers are produced below the defoliation level. This morphological adaptation resulted in a 10-fold higher leaf area remaining after defoliation compared with similarly defoliated T. triandra, which together with the maintenance of moderate levels of light utilising efficiency, contributed to the higher leaf area and shoot weight throughout the regrowth period.
Themeda triandra Forsk., a medium height perennial grass common in semi-arid Africa, was collected from Serengeti National Park in Tanzania, propagated vegetatively and grown in controlled environments simulating native conditions. The experiment demonstrated mechanisms that result in inability to withstand frequent defoliation, and thus how plant-herbivore interactions are affected by plant morpho-physiology. Clipping reduced most plant yield components. After two months leaf elongation rate was greater in clipped plants, but over the whole experiment aboveground yield was unaffected by clipping. Since clipping did not stimulate growth, residual leaf area was less in clipped plants. Tillering was not stimulated by clipping so there was no mechanism to increase leaf area below the clipping height. The resulting reduced leaf area accounted for reduced root and crown production by clipped plants.
Thesis (D.Phil.)--University of Natal, 1985.
Some effects of fire in the tall grassveld of Natal
- Rf Dillon
Dillon RF 1980. Some effects of fire in the tall grassveld of Natal.
MSc thesis, University of Natal, Pietermaritzburg, South Africa,
132pp.
Die invloed van brand en maai op onbeweide langgrasveld in Natal
- Le Roux
Le Roux CJG 1989. Die invloed van brand en maai op onbeweide
langgrasveld in Natal. Journal of the Grassland Society of
Southern Africa 9: 59–64.