<p>Plants often grow in communities closely surrounded by neighbouring plants. Plants can actively and intensely compete for resources and also anticipate competition by sensing environmental cues from the presence and identity of neighbours. Moreover, it’s been proposed that the evolution of both increased and decreased competitive ability may serve as a mechanism for invasiveness. However, still little is known about how plants integrate competitive responses when sensing multiples cues of competition and which individual competitive traits respond to the identity of competitors. In addition, whether and why the evolution of competitive traits may contribute to the ability of introduced species to become invasive is also poorly understood.</p> <p>Here I present a body of work that examined the competitive responses of a native and an invasive plant species to cues of competition and the identity of neighbours. I also examined how experimental manipulation of pot volume, to control belowground resources, affects plant growth and allocation. In one study I tested the competitive responses of the North American native, <em>Impatiens pallida</em>, to cues signalling the presence of neighbours above and belowground simultaneously in competitive environments composed of either siblings or strangers. I demonstrate that<em> I. pallida</em> can recognize siblings and shows more aggressive competitive behaviours towards strangers than kin.</p> <p>In two other studies, I compared the competitive responses of the invasive and native ecotypes of <em>Alliaria petiolata</em> to changes in density, as well as to the presence and identity of neighbours. I found that invasive ecotypes produced less competitive phenotypes especially under high density. Moreover, I found that invasive ecotypes performed better when sharing rooting space with neighbours that were siblings.</p> <p>Taken together, these results demonstrate the ability of these plant species to respond to the identity of neighbours and provide strong evidence in support of the evolution of reduced competitive ability hypothesis in invasive plant species potentially mediated by the action of kin selection in invasive ecotypes.</p> / Doctor of Philosophy (PhD)

Les plantes envahissantes menacent la biodiversité ainsi que les activités humaines. Afin de les maîtriser, la pulvérisation d'herbicides est une méthode fréquemment employée en Amérique du Nord. Cette approche ne fait pas toujours consensus et est même parfois interdite ou restreinte, ce qui justifie le recours à d'autres options. Les alternatives peuvent toutefois s'avérer rares, comporter d'importantes limitations ou sont peu documentées. Cette étude vise à tester l’efficacité de méthodes permettant de maîtriser et de prévenir les invasions de roseau commun (Phragmites australis), l'une des plantes envahissantes les plus problématiques sur le continent nord-américain, tout en limitant au minimum l'utilisation d'herbicides. Le potentiel de quatre méthodes de lutte aux petites populations de roseau bien établies a d'abord été évalué : l’excavation avec enfouissement sur place, le bâchage, l’excavation avec enfouissement sur place combinée au bâchage, et la fauche répétée. Les résultats ont montré que l'excavation avec enfouissement sur place, avec ou sans bâchage, a entraîné une élimination presque totale des populations visées, ce qui est comparable ou supérieur à l'effet généralement obtenu avec la pulvérisation d'herbicide. Le bâchage avec des toiles opaques, maintenues pendant un an, a pour sa part entraîné une maîtrise partielle du roseau, suggérant qu'une application prolongée serait nécessaire pour l'éradication de la plante. La fauche répétée, exécutée à raison de cinq fauches par été pendant deux ans, a fourni une efficacité mitigée. Les résultats suggèrent néanmoins que la fauche pendant plusieurs années contribue à affaiblir la plante, ce qui pourrait aider à son confinement. Une méthode additionnelle a été expérimentée afin de traiter les tiges éparses de roseau tout en limitant les risques d'effets hors cibles, soit le badigeonnage manuel d’herbicide. Suite à ces tests, les résultats ont montré une diminution importante de la densité des tiges, ce qui suggère que la méthode est efficace afin d'éliminer les repousses après un traitement initial, et pourrait également être employée sur de jeunes populations clairsemées. L'effet d'un ensemencement préventif de plantes herbacées sur l'établissement de semis de roseau a également été étudié, suite à des traitements sur de vastes parcelles de sol nu. Les résultats suggèrent que la méthode est efficace afin de limiter la propagation du roseau par semences et qu'un suivi périodique suite à l'intervention serait suffisant afin de maintenir l'effet préventif. / Invasive plants threaten biodiversity as well as human activities and in North America, attempts to control them often rely on the spraying of herbicides. This approach does not always achieve consensus and is sometimes forbidden or restricted, encouraging managers to consider other options. Alternatives can however prove to be rare, involve serious limitations or be poorly documented. This study aims at testing the efficiency of methods for the control and prevention of invasions by common reed (Phragmites australis), one of the most problematic invasive plants on the North American continent, all while reducing herbicide use to a minimum. The potential of four methods for the control of small, well established stands of common reed has first been evaluated: excavation and on-site burial, covering with heavy-duty tarps, excavation and on-site burial combined with covering with heavy-duty tarps, and repeated cutting. Results have shown that excavation with on-site burial, followed or not by covering with heavy-duty tarps, led to an almost complete elimination of the populations targeted, which is comparable or superior to the effect typically obtained with herbicide spraying. Covering common reed with opaque heavy-duty tarps, held in place for one year, lead to partial control of the plant, suggesting that holding the tarps in place for a longer period would be necessary to foresee the eradication. Cutting, repeated five times per summer for two years, gave mixed results. The results suggest, however, that cutting during multiple years contributes to weakening the plant, which could help to its confinement. An additional method, the hand-wicking of common reed stems with herbicide, has been tested to treat scattered stems while limiting off-target effects. Following these tests, the results have shown an important reduction of the density of reed stems, which suggests that the method is effective to control common reed regrowth after an initial treatment, and could be used on young stands. The effect of seeding herbaceous plants on the establishment of common reed seedlings has also been evaluated, following the sowing of vast plots of bare soil. The results suggest that the method is an efficient way of limiting the propagation of common reed through seed dispersal. Results also suggest that periodic follow-ups to eliminate seedlings would be sufficient to maintain the preventive effect.

Competition between different species (interspecific competition) is an important factor to consider when estimating population trends, geographic distributions, and management options of species. Many historically common vascular plant species found in agricultural environments have been negatively affected by changes in community composition and in turn, changes in competition pressures. Even so, the relative importance of plant competition in an ecological context is still unclear. In this study I examine the competitive ability of the meadow plant cornflower (Centaurea cyanus L.) when grown together with common oat (Avena sativa) and common poppy (Papaver rhoeas), during a seven-week long growth chamber experiment. Seeds were sown in pots in four different setups; 1) C. cyanus control, 2) C. cyanus + P. rhoeas, 3) C. cyanus + A.sativa, 4) All three species. Six different growth parameters were measured (aboveground dry-weight, belowground dry-weight, root length, leaf area, number of leaves and above/belowground dry-weight ratio). I found that growth rates of C. cyanus were significantly inhibited according to all six growth parameters when C. cyanus competed solely with A. sativa. Competition from P. rhoeas had an insignificant effect on C. cyanus growth in five out of six growth parameters. Finally, I discuss the possibility that historically common meadow plants have declined in abundance in part because of weak competitive abilities, and that rare vascular plant species are negatively affected by growing in proximity with cereal crops.

Thesis (MScFor) (Forest and Wood Science)--University of Stellenbosch, 2003. / Planting of upperstorey trees along boundaries has been introduced in KabaleUganda with good reception from local farmers. Trees have been planted along agricultural fields, but both Alnus acuminata and Grew/lea robusta out-compete food crops. Managing competition between trees and crops for water, light, and nutrients to the benefit of farmers is a determinant of successful agroforestry. The scarcity and fragmentation of farmland coupled with the hilly nature of Kabale, highlights the need to address the question of tree-crop competition for resources if the technology of on-farm tree planting is to be widely disseminated and adopted in its different guises. Five-year old trees of A acuminata and G. robusta were subjected to treatments of pollarding, or a combination of pollarding and one side root pruning and compared with unpruned controls. The objectives were to assess their potential in reducing competition with food crops and providing firewood to farmers as well as their effects on tree growth. Pollarding has many benefits to farmers because it provides firewood and stakes for climbing beans, it reduces competition for resources between trees and crops and enables continued tree planting on-farm. Continued on-farm tree planting alleviates problems associated with limited land and contributes to environmental resilience. To ensure this, effect of pollarding and root pruning of upperstorey boundary trees of A acuminata and G. robusta was tested on 12 farmers' fields in Kabale. Food crops (beans and maize) grown in the sequence beans-maize-beans, grew very well at less than 50 em from trees that had been pollarded and root pruned one side. In general, pooled data from 12 sites over 5 m away from trees indicated that a combination of pollarding and root pruning increased bean yield by 240% and maize by 154%, while pollarding alone increased bean yield by 181% and maize yield was increased by 123% in comparison to non-pruned trees. However, pollarding and root pruning treatments reduced tree growth rates.Notable was more competition with crops by A. acuminata than by G. robusta. This was attributed to differences in root architecture, diameter at breast height (dbh) sizes, crown spread and crown density between the two species. Five-year-old A. acuminata had bigger dbh (12.40 cm), wider crown spread (6 m) and a dense crown, while G. robusta had dbh 10.82 em, 3 m crown spread and a light crown. A. acuminata also had more branches per tree (34) compared to G. robusta with only 25. These factors influence water uptake, light penetration through the canopy and transpiration rates, and thus affect tree-food crop competition. It is concluded that pollarding and root pruning have a great potential to reduce tree-crop competition, thereby paving the way for continued on-farm tree planting. The effect of pollarding on timber quality, moisture seepage into timber through the cut surface, if any, and the extent of its damage are areas for further research. The rate of root recovery is also to be followed closely to determine an appropriate frequency for cutting back of roots to recommend to farmers how often they need to prune their trees. It is also suggested that a thorough study be conducted on the amount of water uptake from the soil by each of the species Alnus acuminata and Grevillea robusta. This will help further explain the differences in competition between the two species.

Thesis (MScFor)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: Increases in woody plant density ('bush encroachment') reduce livestock production and biodiversity. By convention, soil moisture, soil nutrients, fire and herbivory are regarded as the principal factors governing the tree-grass ratio of savannas. An experiment with a completely-crossed design was employed to investigate woody seedling (Acacia me/lifera) recruitment near Kimberley, Northern Cape, South Africa, after fire and under conditions of maximum-recorded rainfall, nitrogen addition and grazing. The field experiment was repeated as a garden experiment to determine if the two experiments yield comparable results. Tree germination in the field was extremely low, probably due to below-average natural rainfall in plots that only received natural rain, and insufficient watering frequency in irrigated plots. As a result of low germination, none of the treatments (rain/nitrogen/fire/grazing) had a significant effect on tree recruitment in the field experiment. The duration of the experiment (2000/2001 growing season) was insufficient for the treatments to affect grass composition, although the high rainfall treatment and grazing exclusion significantly improved grass cover and height. The garden experiment showed that frequent watering, no nitrogen addition and grass clipping significantly enhanced tree germination and survival (termed 'recruitment'). There were also significant interactions among rain, nitrogen and grazing in their effects on tree recruitment. The effects of rain on tree recruitment were more pronounced under nitrogen supplementation and vice versa. Similarly, high rain and high nitrogen enhanced the effect of grazing on tree recruitment. It is inferred that above-average rainfall years with frequent rainfall events are required for mass tree recruitment. Tree seedlings can further benefit from space and resources which are made available through grass defoliation. Conversely, nitrogen enrichment improves the competitive ability of the fast-growing grasses relatively more than that of the N2-fixing tree component, thereby suppressing tree recruitment. In contrast to conventional wisdom that grazing alone causes encroachment, it is suggested that there are complex interactions between the abovementioned factors and 'triggering' events such as unusually high rainfall. Contrary to many claims that equilibrium models are inappropriate for explaining savanna dynamics, it was shown that consumer-resource theory has explanatory power for bush-grass dynamics of the savanna studied. The state-space approach that was used facilitated the understanding of savanna dynamics and enabled predictions about the system's response to perturbations. The applicability of consumer-resource theory to semi-arid nutrient-poor savannas confirmed the importance of resource competition in structuring natural systems. / AFRIKAANSE OPSOMMING: Toenames in die digtheid van houtagtige plante ('bosverdigting') verlaag veeproduksie en biodiversiteit. Volgens konvensie word grondvog, grondvoedingstowwe, vuur en beweiding beskou as die belangrikste faktore wat die boom-gras verhouding van savannas bepaal. 'n Volledig-gekruisde ("completely crossed") eksperiment is ontwerp om boomsaad-ontkieming (Acacia mellifera) naby Kimberley, Noord-Kaap, Suid- Afrika, te bestudeer na 'n brand en onder toestande van maksimum-aangetekende reënval, stikstof toevoeging en beweiding. Die veldeskperiment is herhaal as 'n tuineksperiment om vas te stel hoe resultate van die twee eksperimente vergelyk. Boomsaad-ontkieming was uiters laag in die veld, waarskynlik weens ondergemiddelde reënval in persele wat slegs natuurlike reënvalontvang het, en 'n te lae benattingsfrekwensie in die besproeide persele. As gevolg van lae ontkieming in die veldeksperiment, het geen behandeling (reënval/stikstof/brand/beweiding) boomsaad-ontkieming beduidend geaffekteer nie. Die duur van die eksperiment (2000/2001-groeiseisoen) was te kort vir die behandelings om grassamestelling te beïnvloed, alhoewel besproeiing en geen beweiding die grasbedekking en -hoogte betekenisvol verhoog het. Die tuineksperiment het getoon dat boomsaadontkieming en vroeë oorlewing (genoem 'vestiging') betekenisvol verhoog is deur gereëlde benatting, geen stikstof toevoeging en die sny van gras. Daar was ook beduidende interaksies tussen reën, stikstof en beweiding in hul invloed op boomvestiging. Die reënbehandeling het 'n groter uitwerking op boomvestiging gehad onder stikstoftoevoeging en vice versa. Hoë reënval en stikstoftoevoeging het ook die invloed van beweiding op boomvestiging versterk. Die afleiding is dat bogemiddelde reënvaljare met gereëlde reënbuie 'n vereiste is vir grootskaalse boomvestiging. Boomsaailinge kan verder voordeel trek uit die spasie en hulpbronne wat beskikbaar raak wanneer gras ontblaar word. Daarteenoor verhoog stikstofverryking die mededingendheid van die vinniggroeiende grasse meer as dié van die N2-bindende boomkomponent, met die gevolg dat boomvestiging onderdruk word. In kontras met die konvensionele veronderstelling dat beweiding opsigself bosverdigting veroorsaak, word voorgestel dat ingewikkelde interaksies plaasvind tussen die bogenoemde faktore en ander 'sneller-gebeurtenisse', soos buitengewoon hoë reënval. In teenstelling met baie aansprake dat ekwilibrium modelle ontoepaslik is om savanna-dinamika te verklaar, is getoon dat die verbruikers-hulpbron teorie ("consumer-resource theory") oor verklaringsvermoë beskik vir boom-gras dinamika van die savanna wat bestudeer is. Die staat-spasie ("state-space") benadering gebruik, het begrip van savanna-dinamika bevorder en voorspellings moontlik gemaak aangaande die sisteem se reaksie op versteurings. Die toepaslikheid van verbruikers-hulpbron teorie vir semi-ariede, nutriënt-arm savannas bevestig dat kompetisie vir hulpbronne 'n sentrale rol speel in die strukturering van ekologiese sisteme.

The study is focused on comparative ecology of three closely related species from Myosotis palustris group. Importance of various functional traits of individual species was evaluated experimentally in the greenhouse (germination experiments), in the pot experiment and in the reciprocal transplant field experiment. Germination, production of clonal structures and response to various environmental conditions (competition, moisture conditions, etc.) were studied. The experimental data are supplemented with an analysis of phytosociological relationships of the studied species.

Cassava (Manihot esculenta Grantz) is one of the most important foods for man and for animals, especially in poor and emergent countries. Maize (Zea mays L.) is the third cereal more produced worldwide. These two crops are often grown in small farms of Rio Grande do Sul. The objectives this study were to estimate the phyllochron and to identify a morphological indicator based on the accumulated number of leaves (NF) on the main stem for the beginning of starch accumulation (IAA) in de fibrous roots of cassava grown in several planting dates, and to quantify growth, development and yield of cassava and maize in different plant spacing in monocropping and intercropping, considering two sowing dates of maize. Two field experiments were carried out in Santa Maria, RS, Brazil during 2005/2006. One experimental was with cassava variety RS 13, with four planting dates (28/09/2005; 11/10/2005; 16/11/2005 and 27/12/2005) and planted in 12 l pots, buried in the soil and spaced 0.8 m. Daily degree days (STd, °C day) were calculated starting at emergency. IAA was determined on the plants when one root reached a diameter of 0.01 m. NF on the date of IAA and the main sterm final leaf number (NFF) up to the first sympodial braches were determined. The phyllochron varied from 20.3 to 29.5 °C day leaf-1 with larger values in the two later planting dates. The NFF varied among the planting dates, increasing as planting was delayed. The NF at IAA was similar for the four planting dates and occurred when NF = 20,9 (±1,0)leaves. The other experiment was carried using the cassava variety RS 13 that was planted on 27/09/2005 and the maize variety BRS Missões sown on two dates: when 50% of the cassava plants were emerged and when the cassava presented five visible leaves. Twelve treatments were used characterizing different plants spacing in the monocropping and intercropping. Several growth, development and yield parameters of both crops were analyzed. Thermal time (Tb=14°C for cassava and Tb=10°C for maize) of some developmental phases and the phyllochron were calculated. Growth and development were not affected by plant spacing in both monocropping and intercropping. The greatest land use efficiency was obtained with intercropping cassava and maize, with cassava planted in a double row spacing of 1.6 x 0.5 x 0.6 m and a single row of maize spaced 0.8 x 0.21 m or two rows of maize spaced 0.4 x 0.42 m between the double row of cassava and maize sowed at cassava emergence. / A mandioca é um dos mais importantes alimentos para o homem e para os animais, especialmente em países pobres e emergentes. O milho (Zea mays L.) é o 3º cereal mais produzido no mundo. Na pequena propriedade familiar do Rio Grande do Sul, é freqüente o cultivo de mandioca e milho. Os objetivos dessa dissertação foram estimar o filocrono e identificar um indicador morfológico baseado no número de folhas acumuladas (NF) na haste principal para o início de acumulação de amido (IAA) em mandioca cultivada em diferentes épocas e quantificar o crescimento, o desenvolvimento e o rendimento da mandioca e do milho em diferentes arranjos de plantas em cultivo solteiro e consorciado considerando-se duas épocas de semeadura do milho. Dois experimentos foram conduzidos na área experimental do Departamento de Fitotecnia da Universidade Federal de Santa Maria, RS, em 2005/2006. Um experimento utilizou a mandioca variedade RS 13 em quatro épocas de plantio (28/09/2005; 11/10/2005; 16/11/2005 e 27/12/2005), plantados em baldes de 12 litros, enterrados no espaçamento com 2,0 x 0,8 m. A soma térmica diária (STd, °C dia) foi calculada a partir da emergência. O IAA foi determinado nas plantas quando uma raiz atingia um diâmetro de 0,01 m. O NF na data do IAA e o número final de folhas (NFF) até o primeiro simpódio também foram determinados. O filocrono variou de 20,3 a 29,5 oC dia folha-1 e seus valores foram maiores quanto mais tardio foi o plantio. O NFF variou entre as épocas de plantio, aumentando com o atraso do plantio. O NF no IAA foi similar nas quatro épocas de plantio e ocorreu quando em média o NF= 20,9 (±1,0) folhas. Outro experimento foi conduzido com as culturas de mandioca variedade RS 13 que foi plantada em 27/09/2005 e o milho variedade BRS Missões que foi semeado em duas épocas: quando 50% das plantas de mandioca estavam emergidas e quando a mandioca apresentava cinco folhas visíveis. Doze tratamentos foram usados caracterizando diferentes arranjos de plantas nas culturas solteiras e consorciados. Várias variáveis de crescimento, desenvolvimento e componentes do rendimento das duas culturas foram analisadas. Foi calculada a soma térmica (Tb=14°C para mandioca e Tb=10°C para milho) de algumas fases e determinado o filocrono. O crescimento e o desenvolvimento das duas culturas não foram afetados pelo arranjo de plantas, tanto em cultivo solteiro como em consórcio. O maior uso eficiente da terra (UET) foi obtido no consórcio em que a mandioca foi cultivada em fileiras duplas de 1,6 x 0,5 x 0,6 m e milho em uma fileira de 0,8 x 0,21 m ou em duas fileiras de 0,4 x 0,42 m entre as fileiras duplas de mandioca, sendo o milho semeado na emergência da mandioca.

Thesis (MScAgric)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: African Acacia species are often major contributors to the progressive increase in the woody component of savannas, a phenomenon commonly referred to as bush encroachment. In Hluhluwe- Umfolozi Park, the numbers of adult Acacia nilotica (L.) Willd. Ex Del. trees per hectare far exceed (by III) that of A. karroo Hayne adults. The relative dominance is reversed in the juvenile stage with A. karroo (725 ha') outnumbering A. nilotica (225 ha-I) threefold outside closed woodlands. African acacias produce large quantities of seed and may have large soil-stored seed banks. They suffer pre-dispersal predation by bruchid beetles and may be either wind or animal dispersed. Once dispersed they are vulnerable to post-dispersal attack. This study tested several hypotheses regarding various aspects of seed ecology of A. karroo and A. nilotica. The null hypothesis that seed ecology does not contribute to the success of A. karroo over A. nilotica, was tested. Acacia karroo trees were smaller (mean basal diameter: 7.8 cm) than A. ni/otica trees (mean basal diameter: 18.5 cm) on average, but produced more seeds (A. karroo mean: 1628; A. nilotica mean: 992) for a given basal diameter size class. It was found that A. karroo showed less bruchid infestation (mean: 1.36-3.81%) than A. nilotica (mean: 14.67-86.70%) at all stages of pod development with a proportion of A. karroo seeds (7.1 %) being able to germinate after bruchid attack. Bruchid attack rendered A. ni/otica seeds unviable. There was no difference between the two species with regards to the soil-stored seed bank and the viability of seeds found in the soil. Acacia karroo showed higher germination levels (5.1%) and better establishment (4.9%) than A. nilotica (1.5% and 0.4% respectively). On average, there was no difference in germination levels between burnt and unbumt seeds, but there was a significant difference in germination of burnt seeds in both burnt (4.5%) and unbumt (2.5%) sites and unbumt seeds in both burnt (2.8%) and unbumt (4.9%) sites when considered separately. Post-dispersal predation of A. karroo seeds (21.8%) was higher than that of A. nilotica (12.7%). There was more rodent predation in tall grass areas (26.0%) than short grass (10.7%) or canopy areas (15.2%), and most seeds were lost from unprotected control groups. Rodent presence was a significant factor in unexplained seed disappearance. The ability of A. karroo to germinate easily and the low levels of beetle predation experienced by this species seemed to be its main advantage over A. nilotica as an encroaching species in Hluhluwe-Umfolozi Park. / AFRIKAANSE OPSOMMING: Die Acacia spesies van Afrika is dikwels belangrike bydraers tot die progressiewe toename in die houtkomponent van savannas. Hierdie verskynsel word algemeen na verwys as bosindringing. In die Hluhluwe-Umfolozi Park is die aantal volwasse Acacia nilotica (L.) Willd. Ex Del. bome per hektaar aansienlik meer (l l l meer) as die aantal volwasse A. karroo Hayne bome. In die jong stadium is die oorheersing omgekeerd, met driekeer soveel A. karroo (725 ha-I) as A. nilotica (225 ha-I) bome buite beboste gedeeltes. Afrika se Acacia spesies produseer groot hoeveelhede saad en kan oor aansienlike grond-gebergde saadbanke beskik. Voor verspreiding word die saad aan predasie deur bruchid-kewers blootgestel. Die saad kan óf deur wind óf diere versprei word en na verspreiding word dit ook aan predasie blootgestel. Hierdie studie het verskillende hipoteses rakende verskeie aspekte van die saadekologie van A. karroo en A. nilotica getoets. Die nulhipotese dat saadekologie nie tot die groter sukses van A. karroo teenoor A. nilotica bydrae nie, is getoets. Acacia karroo bome was oor die algemeen kleiner (gemid. basale omtrek: 7.8 cm) as A. nilotica (gemid. basale omtrek: 18.5 cm) bome maar het meer saad (A. karroo gemid.: 1628; A. nilotica gemid.: 992) per gegewe basale diameter grootte klas gelewer. Daar is geen verskil tussen die twee spesies rakende grondgebergde saadbanke en die lewensvatbaarheid van hierdie saad gevind nie. Acacia karroo het hoër vlakke van ontkieming (5.1%) en beter vestiging (4.9%) as A. nilotica (l.5% en .4% respektiewelik) getoon. Daar was oor die algemeen geen verskil in die ontkiemingsvlakke van gebrande en ongebrande sade nie, maar wel 'n beduidende verskil in die ontkieming van gebrande sade in beide gebrande (4.5%) en ongebrande (2.5%) areas en ongebrande sade in gebrande (2.8%) en ongebrande (4.9%) areas as dit afsonderlik geëvalueer is. Die predasie van A. karroo saad na verspreiding (21.8%) was hoër as dié van A. nilotica (12.7%). Daar was meer knaagdier-predasie in gebiede met lang gras (26.0%) as dié met kort gras (10.7%) of boomryke gedeeltes (15.2%). Die meeste saad is in onbeskermde kontrolegroepe verloor. Die teenwoordigheid van knaagdiere het 'n belangrike rol in die onverklaarde verdwyning van saad gespeel. Dit is gevind dat A. karroo se vermoë om maklik te ontkiem, asook die lae vlakke van insek skade aan die saad, die belangrikste voorsprong is wat dié spesie oor A. nilotica as 'n indringer in Hluhluwe-Umfolozi Park het.

Thesis (MSc)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: West Coast Renosterveld is one of the most threatened vegetation types in South Africa. Less than 5% of the original extent of this vegetation type remains, of which 80% is on private land. In addition to fragmentation, much of the vegetation has been ploughed for crop production and then abandoned and invaded by alien plants. Restoration of transformed areas may improve the conservation status of this vegetation type. Indigenous species do not return to abandoned agricultural fields for decades even if these are adjacent to natural areas since their return is limited either by seed dispersal or seedling establishment. The aim of this study was to examine the recovery of indigenous vegetation on abandoned fields. Renosterveld, as we know it today, is an asteraceous shrubland, dominated mainly by renosterbos (Eytropappus rhinocerotis), but might have been a grassland or a grassland-shrubland mosaic. Historical records indicate that species of large game were common in the Western Cape when the early settlers arrived, but most of these have since disappeared. It is thus impossible to reconstruct exactly the ecological processes and functioning of Renosterveld. The first part of the study was designed to examine the effects of grass competition, grazing by indigenous large herbivores, and interaction of these two factors on the establishment, growth and survival of transplanted Renosterveld seedlings on an abandoned agricultural field. Experimental transplanting of indigenous shrubs into an old field showed that most of the plants investigated competed for resources with lawn grasses on the field, and competition affected the seedlings throughout the experiment. Mortality was higher, and growth was reduced for seedlings exposed to grass competition. With the exception of wild olive (Olea europaea spp.africana), herbivory alone had no significant impact on the target species. Herbivory was at a low intensity (20 ha/large animal unit); higher grazing pressures might have given different results. No interaction between competition and herbivory was found for the species investigated; competition and grazing therefore seem to influence the seedlings independently. The second part of this study was conducted to examine the effects of different management strategies, viz: brush cutting, burning and herbicide application on plant species recruitment and community composition and to ascertain their applicability by farmers for large scale restoration of Renosterveld. My comparison of the different strategies for controlling annual alien grasses indicated that these did not differ significantly in their effects on species richness. Burning reduced shrub cover and increased overall species richness and diversity. Burning also reduced grass biomass, and increased recruitment of indigenous seedlings. The use of herbicide resolved the problem of grass biomass invasion and increased shrub species richness. The herbicide application did not appear to have long-term negative effects on the soil quality. Brush cutting did not remove grass biomass on the old field. Experimental re-seeding with an indigenous grass and shrub species into treated plots resulted in low recruitment. My conclusion is that grass can reduce recruitment and growth of many indigenous shrub species. My recommendation for the restoration of old fields in West Coast Renosterveld is to apply herbicide to remove grass competition, and then, after the herbicide has degraded, to oversow the field with seeds of indigenous shrub and grass species of early successional stages to increase overall species diversity. / AFRIKAANSE OPSOMMING: Weskus Renosterveld is een van die mees bedreigde plantegroei soorte in Suid Afrika. Minder as 5% van die oorspronklike omvang van hierdie plantegroei tipe is oor, en dit hoofsaaklik (80%) op privaatlande. Saam met fragmentasie, is baie Renosterveld areas ook omgeploeg vir boerdery en dan net so gelos, met die gevolg dat uitheemse plante hierdie areas ingedring het. Restorasie of herstelling van sulke bewerkte lande kan dalk die bewaringsstatus van hierdie plantegroei tipe verbeter. Natuurlike vestiging van Renosterveld spesies op sulke ou bewerkte lande gebeur nie, selfs al is daar Renosterveld direk langs so ‘n ou veld. Die hervestiging van inheemse spesies is dus tot saadverspreiding or saailingbevestiging beperk. Die doel van hierdie studie is om die stadige terugkoms van inheemse plantegroei na verlate bewerkte velde te verduidelik. Renosterveld is ‘n struikveld waarin die renosterbos (Eytropappus rhinocerotis) domineer, maar kan ook ‘n grasveld of ‘n grasveld-struikland mengsel wees. Groot herbivore was algemeen in die Wes Kaap toe die eerste settelaars gearriveer het, maar intussen het omtrent al die groot wild spesies verdwyn. Dit is dus ontmoontlik om die ekologiese prosesse en funksionering van Renosterveld presies so te herstel. Die eerste deel van hierdie studie bestudeer die effek van graskompetisie, weiding deur inheemse groot herbivore, en die interaksie tussen hierdie twee faktore op die vestiging, groei en oorlewing van oorgeplante Renosterveld saailinge in ‘n verlate ou veld. Die eksperimentele oorplanting van inheemse struike in ‘n ou land het gewys dat die meeste van hierdie plante kompeteer vir hulpbronne met kweekgras wat op die ou veld groei. Kompetisie het die saailinge deur die hele eksperiment geaffekteer. ‘n Hoër mortaliteit en verminderede groei in saailinge wat aan gras kompetisie blootgestel was, is waargeneem. Met die uitsondering van Olea europaea spp. africana, het herbivorie alleen geen betekenisvolle impak op plant spesies gehad nie. Weidingsdruk was laag (20 ha/groot vee eenheid); ‘n groter weidingsdruk sou miskien ‘n ander uitkoms gehad het. Geen interaksie tussen kompetisie en herbivorie is waargeneem in die bestudeerde plantspesies nie. Dit wil dus voorkom of kompetisie en weiding die saailinge onafhanklik van mekaar beïnvloed. Die tweede deel van hierdie studie was onderneem om die effek van verskillende behandelings (kontrole, sny, brand en herbisied toediening) op plantspesie vestiging en samestelling te bestudeer asook om bestuurmetodes te toets was deur boere op groot skaal gebruik kan word om Renosterveld te herstel. In ‘n vergelyking van verskillende bestuur metodes (kontrole, besnoeiing, brand en herbisied) om eenjarige uitheemse gras te beheer, is gewys dat die behandlings nie betekenisvol van mekaar verskil in hulle effek op spesierykheid nie. Vuur het struikbedekking verminder en totale spesies rykheid en diversitiet verhoog. Die gebruik van ‘n herbisied het die probleem van grasindringing opgelos en het ook struikspesiesrykheid verhoog. Die herbisied het nie lang termyn negatiewe effekte op grond kwaliteit gehad nie. Sny het nie gras biomassa verlaag op die ou land nie. Eksperimentele plant van inheemse grasse en struike in die behandelde areas, het lae vestiging tot gevolg gehad. My algemene afleiding is dus dat gras die hervestinging en groei van baie inheemse struikspesies verminder. Ek stel voor dat herbisied gebruik moet word om gras kompetisie te verminder. Nadat herbisied residue in die grond afgebreek is, moet die ou land met inheemse struik en grasspesies, wat in vroeë suksessie stadiums is, beplant word om sodoende totale spesiediversiteit te verhoog en uiteindelik ou bewerkte lande in Weskus Renosterveld te herstel.

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