The abundance of invasive alien plants, with their numerous associated negative effects, has resulted in challenging and costly management requirements in South Africa. Management of invasives is further complicated when species have rapid inherent growth rates, atmospheric nitrogen fixing capabilities and large, persistent seed banks. Management strategies other than the usual mechanical, chemical, and biological (and combinations thereof) methods may be more effective. An alternative strategy is ’habitat management’, whereby management may be aimed at favouring native species. If the native species could compete favourably with, and possibly exclude, the invasive species, which is more likely when the species are ecologically similar, management may be more effective. This study investigated the competitive performance of two invasive species, Acacia mearnsii De Wild. and Acacia melanoxylon R.Br., and an indigenous ecological analogue, Virgilia divaricata Adamson, on the southern Cape Coast of South Africa. Both acacias are included among the five worst invasive species in the native forest and fynbos shrubland ecosystems. Performance was compared among the three species in terms of germination success (responses to five stimuli, namely chipping, hot water exposure, dry heat pulse, smoke, and control, incubated at 12 or 20°C), soil-stored seed bank densities and viability at different soil depths, and individual growth performance in seedling and sapling stages (and the effect of inter-specific competition on the latter). Virgilia divaricata performed comparably to the acacias in most aspects. Responses to germination stimuli at both temperature regimes were comparable among species, with chipping and hot water exposure resulting in the highest germination success. Acacia melanoxylon displayed poorer germination response under the cool temperature regime than the warm regime. Of the three species, A. mearnsii (7 596 seeds m-2) had the highest seed bank density, followed by V. divaricata (938 seeds m-2) and A. melanoxylon (274 seeds m-2). Seed viability was high (87-91%) in all three species. Neither seed density nor seed viability differed significantly between soil depths, and there were no clear differences in this aspect between species. In terms of growth during the seedling stage, V. divaricata outperformed A. mearnsii. In the sapling stage, growth was comparable between the three species and was not affected by the extent of competition from neighbouring saplings. Overall, the findings of this study suggest that the success of A. mearnsii as an invader is primarily attributable to its large seed banks. However, the good performance of V. divaricata seedlings and no apparent negative effect of competition on sapling growth from A. mearnsii and A. melanoxylon suggest that it may be used in the management of the acacias along with other management strategies. Due to the similarity observed between the three species, any management operation (such as fire) that would benefit V. divaricata recruitment would benefit the acacias equally (except for potentially reduced germination of A. melanoxylon in cooler seasons). Management should thus attempt to prevent the germination of Acacia seeds. Alternatively, dense localised Acacia seed banks may be reduced by repeat burning. On a small scale, areas where V. divaricata would occur naturally (or which are completely compromised ecologically) can be seeded with pre-treated (chipped or hot water exposure) V. divaricata seeds. If V. divaricata is given enough time to establish dense stands, subsequent germination and growth of the acacias should be suppressed and simultaneously promote forest re-establishment. To further decrease Acacia success, biocontrol (seed-boring weevils, flower-galling midges, and rust fungus) should be widely disseminated.

MSc., Faculty of Science, University of the Witwatersrand, 2011 / ABSTRACT Observations of the changes in woody plant density in the Kruger National Park (KNP) over 58 years have shown an increase in large woody plant density on granite substrates, which is attributed to fire and herbivore density. Woody plants persist in areas with frequent fires, herbivory and drought by resprouting or protecting seeds in the ground. Soil seed banks, which are stores of seeds below ground or in leaf litter, provide 'insurance' for trees and allow populations to persist in unfavourable environments. No comprehensive studies have been conducted on soil seed bank ecology of Acacia species in the Kruger National Park, a research gap which this study aimed to fill. The spatial distribution and density of in situ soil seed banks for four Acacia species, A. grandicornuta, A. nilotica, A. senegal and A. tortilis was assessed in the Skukuza land system of the KNP, South Africa. In situ soil seed banks were quantified for eight mature trees per species during 2005/2006. Greenhouse and field seed burial trials were carried out for one year and 16 months respectively, between 2005 and 2007, to investigate the persistence of Acacia seeds over an extended period of time. Post-dispersal seed predation of Acacia seeds was investigated during July 2006 in six demarcated grids within 15 km of Skukuza. Overall soil seed bank density differed significantly among species, being highest for A. tortilis (19.5 ± 6.4 seeds m-2), followed by A. grandicornuta (12.1 ± 6.9 seeds m-2), A. nilotica (4.9 ±1.8 seeds m-2) and lowest for A. senegal (0.6 seeds ± 0.4 seeds m-2). Generally, seed bank density decreased with depth in the soil and distance from the centre of the tree canopy. Seed bank density increased significantly with a decrease in soil compaction for A. senegal only, while it was not related to over-storey canopy shading or herbaceous biomass for any of the species. No significant relationship was found between seed bank density and tree characteristics such as stem diameter, bark thickness or tree canopy area for any of the species. Viability of seeds from the seed bank decreased between species as follows: A. tortilis (77% of 142 seeds), A. nilotica (61% of 39 seeds), A. grandicornuta (58% of 87 seeds), and A. senegal (0% of 4 seeds). For all species with viable seeds, viability decreased with distance from the centre of the tree canopy. Bruchid beetle predation (assessed on 100 newly produced seeds) was low for all four species. Fifty seeds each of A. grandicornuta, A. senegal and A. tortilis and 100 A. nilotica seeds were destroyed by fire during the field seed burial trial, of which four hundred seeds/species were used. Of the remaining seeds, 15% of A. senegal, 19% of A. grandicornuta, 34% of A. nilotica and 66% of A. tortilis remained intact after 16 months in the field. Of these, 65% of A. tortilis, 27% of A. nilotica, 5% of A. grandicornuta and no A. senegal seeds were still viable. The percentage of remaining intact, viable seeds was highest under tree canopy cover and buried for A. tortilis (86%), A. nilotica (39%) and A. grandicornuta (6%), but the micro-site placement of seeds had a significant effect on viability for A. nilotica only (d.f. = l; χ2 = 7.5; P = 0.006). In the greenhouse seed burial trial (150 seeds/species/treatment), one percent of the total seed lot germinated, which was 2.9% of A. grandicornuta, 0.7% of A. senegal and 0.2% of both A. nilotica and A. tortilis. A. tortilis had the highest percentage of remaining intact, viable seeds (92.2%), followed by A. nilotica (58.3%), A. grandicornuta (57.6%) and A. senegal (0%). The number of remaining intact, viable seeds was highest when watered with the average rainfall (327 seeds), followed by the highest (314 seeds) and lowest rainfall (296 seeds). There was no association between rainfall treatments and the number of remaining intact, viable seeds for any of the species, except for A. grandicornuta where the number of remaining intact, viable seeds increased significantly with the average rainfall. Across six grids in the Skukuza land system, A. grandicornuta was the most dominant woody plant of six study species, followed by Dichrostachys cinerea, A. tortilis, A. nilotica, A. senegal and A. nigrescens. Woody plant density in grids varied between 226 plants ha-1 (Grid 3) to 1618 plants ha-1 (Grid 5), with a mean density of 862 ± 195 plants ha-1. Overall, woody plant species diversity was low (Shannon Wiener Index, 1.8 ± 2.8; Evenness Index, 0.7 ± 0.02; Simpson's Reciprocal Index, 4.5 ± 0.6). The dung of nine species of large herbivore was recorded across all six grids. Large herbivores favoured seeds of indehiscent (55 A. tortilis seeds and 11 A. nilotica seeds) over dehiscent pods (1 A. grandicornuta seed). Only 9% (five A. tortilis seeds and one A. grandicornuta seed) of the 67 seeds extracted from dung germinated after a six-week germination trial. Less than half the remaining ungerminated A. nilotica seeds (46%) and A. tortilis seeds (40%) tested viable. There was no correlation between the number of termitaria recorded and the number of Acacia trees growing on them (r = 0.07). Termite mounds occupied 0.0009 ± 0.0003 ha per grid matrix (0.8%). Only four rodent species were recorded across all six grids, Mastomys coucha (multimammate mouse), Rhabdomys pumilio (striped mouse), Aethomys chrysophilus (red veld rat) and Tatera leucogaster (highveld gerbil). Rodent species diversity was low (Shannon Wiener Index, 0.6 ± 0.2; Evenness Index, 0.6 ± 0.2; Simpson's Reciprocal Index, 1.9 ± 0.3). In the field cafeteria trial there was a significant difference in the percentage of seeds removed between seed species (P < 0.05; F = 2.8; d.f. = 3, 236). There was a significant difference in the percentage of seeds removed from trays placed under vegetation cover compared with trays placed in the open (P = 0.034). This study suggests that A. grandicornuta, A. nilotica and A. tortilis seeds form short-term persistent seed banks, while A. senegal seeds are transient and do not form seed banks. Seeds of several woody plants were ingested by large herbivores and selected by rodents. The relevance of soil seed banks to regeneration of Acacia trees needs to be evaluated by investigating whether these species rely more on seed production or resprouting for individual recruitment into tree populations. Once this issue is clarified the effect of certain factors on seed fate and consequently, recruitment of individuals into plant populations, can be more clearly understood. This will assist in managing and understanding these potentially encroaching species in the Kruger National Park, South Africa.

Please read the abstract in the section 00front of this document / Thesis (PhD (Botany))--University of Pretoria, 2007. / Plant Science / unrestricted

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.