Dissertations / Theses on the topic 'Invasive plants – Australia'

Invasive plants pose serious threats to economic, social and environmental interests throughout the world. Developing strategies for their management requires a range of information that is often impractical to collect from ground based surveys. In other cases, such as retrospective analyses of historical invasion rates and patterns, data is rarely, if ever, available from such surveys. Instead, historical archives of remotely sensed imagery provides one of the only existing records, and are used in this research to determine invasion rates and reconstruct invasion patterns of a ca 70 year old exotic mesquite population (Leguminoseae: Prosopis spp.) in the Pilbara Region of Western Australia, thereby helping to identify ways to reduce spread and infill. A model was then developed using this, and other, information to predict which parts of the Pilbara are most a risk. This information can assist in identifying areas requiring the most vigilant intervention and pre-emptive measures. Precise information of the location and areal extent of an invasive species is also crucial for land managers and policy makers for crafting management strategies aimed at control, confinement or eradication of some or all of the population. Therefore, the third component of this research was to develop and test high spectral and spatial resolution airborne imagery as a potential monitoring tool for tracking changes at various intervals and quantifying the effectiveness of management strategies adopted. To this end, high spatial resolution digital multispectral imagery (4 channels, 1 m spatial resolution) and hyperspectral imagery (126 channels, 3 m spatial resolution) was acquired and compared for its potential for distinguishing mesquite from coexisting species and land covers.These three modules of research are summarised hereafter. To examine the rates and patterns of mesquite invasion through space and time, canopies were extracted from a temporal series of panchromatic aerial photography over an area of 450 ha using unsupervised classification. Non-mesquite trees and shrubs were not discernible from mesquite using this imagery (or technique) and so were masked out using an image acquired prior to invasion. The accuracy of the mesquite extractions were corroborated in the field and found to be high (R2 = 0.98, P<0.001); however, accuracy varied between classes (R2 = 0.55 to 0.95). Additional sampling may be required in some of the wider class intervals, particularly the moderate density class (30 to 90%) as sampling frequency was poor within the range of 60 to 90%. This is a direct result of there being relatively few quadrats available to be randomly selected in this class. That is, quadrats with between 60-90% cover were only evident in 4% of the test area. A more robust approach would, therefore, be to split this class into two (e.g. 30-60% and 60-90%) and select an additional 15 quadrats in the 60-90% range. The resolution of the imagery (1.4 m) precluded mapping shrubs smaller than 3 m2. Rates and patterns were compared to mesquite invasions in its native range.It was determined that: (i) the shift from grass to mesquite domination had been rapid, with rates of increase in canopy cover comparable to invasive populations where it is native; (ii) rate of patch recruitment was high in all land types (stony flats, red-loamy soils and the riparian zone), but patch expansion and coalescence primarily occurred over the riparian zone and redloamy soils; (iii) mesquite had been spread by sheep and macropods and the recent switch to cattle is likely to exacerbate spread as it is a far more effective dispersal vector; and (iv) early successional patterns, such as high patch initiation followed by coalescence of existing stands are similar to where mesquite is native, but patch mortality did not occur. A knowledge based model was used to predict which parts of the Pilbara region are most at risk. Several limitations of models often employed in predicting suitability ranges of invasive plants were identified and include: (i) an inability to incorporate the notion that within a suitability range there is likely to be a scale of favourability; (ii) an inability to assign greater importance to evidence that is likely to have more importance in defining the areas suitable for invasion; and (iii) an inability to control the level of conservatism in the final results. These three shortcomings were mitigated through the use of: (i) fuzzy membership functions to derive a range of favourability from poor to best; (ii) pairwise comparison to derive higher weights for layers perceived to be more important and vice versa; and (iii) the use of ordered weighted averaging to directly control the level of conservatism (or risk) inherent in the models produced.Based on the outcomes of the historical reconstruction of spatial rates and patterns, data sources included land types, land use, and the derivation of a steady state wetness index from spot height data. Model outputs were evaluated using two methods: the area under the curves (AUC) produced from relative operating characteristic (ROC) plots and by the maximum Kappa procedure. Both techniques agreed that the model most representative of the validation data was the one assuming the most risk. To create a Boolean output representing areas suitable/not suitable for invasion, optimal cut-points were derived using the point closest to the top left hand corner of the ROC plot and by the maximum Kappa method. Both methods obtained identical cut-points, but it is argued that the coefficient produced by the maximum Kappa method is more easily interpreted. The highest AUC was found to be 0.87 and, based on the maximum Kappa method, can be described as good to very good agreement with the validation records used. Digital multispectral imagery (DMSI), acquired in the visible and near infrared portions of the spectrum (3 visible bands, 1 near infrared) with a spatial resolution of 1 m and hyperspectral imagery (126 bands, 3 m spatial resolution) was acquired to assess the potential of developing a reliable and repeatable mapping tool to facilitate the monitoring of spread and the effects of control efforts. Woody vegetation was extracted from the images using unsupervised classification and grouped into patches based on contiguity. Various statistics (e.g. maximum, minimum, median, mean, standard deviation, majority and variety) were assigned to these patches to garner more information for species separation.These statistics were explored for their ability to separate mesquite from coexisting species using Tukey’s Honestly Significantly Different (HSD) test and, to reduce redundancy, followed by linear discriminant analysis. Two approaches were taken to select the patch statistics offering the best discrimination. The first approach selected patch statistics that best discriminated all species (named “overall separation”). This was compared to a second approach, which selected the best patch statistics that separated each species from mesquite on a pairwise basis (named “pairwise separation”). The statistics offering the best discrimination were used as input in an Artificial Neural Network (ANN) to assign class labels. An incremental cover evaluation, whereby producer’s accuracy was computed from mesquite patches grouped into various size-classes, showed that identification of mesquite patches smaller than 36 m2 was relatively low (43-51%) regardless of the method used for choosing between the patch statistics or image type. Accuracy improved for patches >36 m2 (66-94%) with both approaches and image types. However, both approaches used on the hyperspectral imagery were more reliable at capturing patches >36 m2 than the DMSI using either approach. The lowest omission and commission rates were obtained using pairwise separation on the hyperspectral imagery, which was significantly more accurate than DMSI using an overall separation approach (Z=2.78, P<0.05), but no significant differences were found between pairwise separation used on either media.Consequently, all methods and imagery types, except for DMSI processed using overall separation, are capable of accurately mapping mesquite patches >36 m2. However, hyperspectral imagery processed using pairwise separation appears to be superior, even though not statistically different to hyperspectral imagery processed using overall separation or DMSI processed using pairwise separation at the 95% confidence level. Mapping smaller patches may require the use of very high spatial resolution imagery, such as that achievable from unmanned airborne vehicles, coupled with a hyperspectral instrument. Alternatively, management may continue to rely on visual airborne surveys flown at low altitude and speed, which have proven to be capable at mapping small and isolated mesquite shrubs in the study area used in this research.

Invasive plants pose serious threats to economic, social and environmental interests throughout the world. Developing strategies for their management requires a range of information that is often impractical to collect from ground based surveys. In other cases, such as retrospective analyses of historical invasion rates and patterns, data is rarely, if ever, available from such surveys. Instead, historical archives of remotely sensed imagery provides one of the only existing records, and are used in this research to determine invasion rates and reconstruct invasion patterns of a ca 70 year old exotic mesquite population (Leguminoseae: Prosopis spp.) in the Pilbara Region of Western Australia, thereby helping to identify ways to reduce spread and infill. A model was then developed using this, and other, information to predict which parts of the Pilbara are most a risk. This information can assist in identifying areas requiring the most vigilant intervention and pre-emptive measures. Precise information of the location and areal extent of an invasive species is also crucial for land managers and policy makers for crafting management strategies aimed at control, confinement or eradication of some or all of the population. Therefore, the third component of this research was to develop and test high spectral and spatial resolution airborne imagery as a potential monitoring tool for tracking changes at various intervals and quantifying the effectiveness of management strategies adopted. To this end, high spatial resolution digital multispectral imagery (4 channels, 1 m spatial resolution) and hyperspectral imagery (126 channels, 3 m spatial resolution) was acquired and compared for its potential for distinguishing mesquite from coexisting species and land covers.
These three modules of research are summarised hereafter. To examine the rates and patterns of mesquite invasion through space and time, canopies were extracted from a temporal series of panchromatic aerial photography over an area of 450 ha using unsupervised classification. Non-mesquite trees and shrubs were not discernible from mesquite using this imagery (or technique) and so were masked out using an image acquired prior to invasion. The accuracy of the mesquite extractions were corroborated in the field and found to be high (R2 = 0.98, P<0.001); however, accuracy varied between classes (R2 = 0.55 to 0.95). Additional sampling may be required in some of the wider class intervals, particularly the moderate density class (30 to 90%) as sampling frequency was poor within the range of 60 to 90%. This is a direct result of there being relatively few quadrats available to be randomly selected in this class. That is, quadrats with between 60-90% cover were only evident in 4% of the test area. A more robust approach would, therefore, be to split this class into two (e.g. 30-60% and 60-90%) and select an additional 15 quadrats in the 60-90% range. The resolution of the imagery (1.4 m) precluded mapping shrubs smaller than 3 m2. Rates and patterns were compared to mesquite invasions in its native range.
It was determined that: (i) the shift from grass to mesquite domination had been rapid, with rates of increase in canopy cover comparable to invasive populations where it is native; (ii) rate of patch recruitment was high in all land types (stony flats, red-loamy soils and the riparian zone), but patch expansion and coalescence primarily occurred over the riparian zone and redloamy soils; (iii) mesquite had been spread by sheep and macropods and the recent switch to cattle is likely to exacerbate spread as it is a far more effective dispersal vector; and (iv) early successional patterns, such as high patch initiation followed by coalescence of existing stands are similar to where mesquite is native, but patch mortality did not occur. A knowledge based model was used to predict which parts of the Pilbara region are most at risk. Several limitations of models often employed in predicting suitability ranges of invasive plants were identified and include: (i) an inability to incorporate the notion that within a suitability range there is likely to be a scale of favourability; (ii) an inability to assign greater importance to evidence that is likely to have more importance in defining the areas suitable for invasion; and (iii) an inability to control the level of conservatism in the final results. These three shortcomings were mitigated through the use of: (i) fuzzy membership functions to derive a range of favourability from poor to best; (ii) pairwise comparison to derive higher weights for layers perceived to be more important and vice versa; and (iii) the use of ordered weighted averaging to directly control the level of conservatism (or risk) inherent in the models produced.
Based on the outcomes of the historical reconstruction of spatial rates and patterns, data sources included land types, land use, and the derivation of a steady state wetness index from spot height data. Model outputs were evaluated using two methods: the area under the curves (AUC) produced from relative operating characteristic (ROC) plots and by the maximum Kappa procedure. Both techniques agreed that the model most representative of the validation data was the one assuming the most risk. To create a Boolean output representing areas suitable/not suitable for invasion, optimal cut-points were derived using the point closest to the top left hand corner of the ROC plot and by the maximum Kappa method. Both methods obtained identical cut-points, but it is argued that the coefficient produced by the maximum Kappa method is more easily interpreted. The highest AUC was found to be 0.87 and, based on the maximum Kappa method, can be described as good to very good agreement with the validation records used. Digital multispectral imagery (DMSI), acquired in the visible and near infrared portions of the spectrum (3 visible bands, 1 near infrared) with a spatial resolution of 1 m and hyperspectral imagery (126 bands, 3 m spatial resolution) was acquired to assess the potential of developing a reliable and repeatable mapping tool to facilitate the monitoring of spread and the effects of control efforts. Woody vegetation was extracted from the images using unsupervised classification and grouped into patches based on contiguity. Various statistics (e.g. maximum, minimum, median, mean, standard deviation, majority and variety) were assigned to these patches to garner more information for species separation.
These statistics were explored for their ability to separate mesquite from coexisting species using Tukey’s Honestly Significantly Different (HSD) test and, to reduce redundancy, followed by linear discriminant analysis. Two approaches were taken to select the patch statistics offering the best discrimination. The first approach selected patch statistics that best discriminated all species (named “overall separation”). This was compared to a second approach, which selected the best patch statistics that separated each species from mesquite on a pairwise basis (named “pairwise separation”). The statistics offering the best discrimination were used as input in an Artificial Neural Network (ANN) to assign class labels. An incremental cover evaluation, whereby producer’s accuracy was computed from mesquite patches grouped into various size-classes, showed that identification of mesquite patches smaller than 36 m2 was relatively low (43-51%) regardless of the method used for choosing between the patch statistics or image type. Accuracy improved for patches >36 m2 (66-94%) with both approaches and image types. However, both approaches used on the hyperspectral imagery were more reliable at capturing patches >36 m2 than the DMSI using either approach. The lowest omission and commission rates were obtained using pairwise separation on the hyperspectral imagery, which was significantly more accurate than DMSI using an overall separation approach (Z=2.78, P<0.05), but no significant differences were found between pairwise separation used on either media.
Consequently, all methods and imagery types, except for DMSI processed using overall separation, are capable of accurately mapping mesquite patches >36 m2. However, hyperspectral imagery processed using pairwise separation appears to be superior, even though not statistically different to hyperspectral imagery processed using overall separation or DMSI processed using pairwise separation at the 95% confidence level. Mapping smaller patches may require the use of very high spatial resolution imagery, such as that achievable from unmanned airborne vehicles, coupled with a hyperspectral instrument. Alternatively, management may continue to rely on visual airborne surveys flown at low altitude and speed, which have proven to be capable at mapping small and isolated mesquite shrubs in the study area used in this research.

Thesis (MSc)--Stellenbosch University, 2012.
ENGLISH ABSTRACT: My master’s research combines aspects of pollination biology and invasion biology to explore the invasive predictors and impacts of a globally invasive group of plants – Australian acacias. Specifically, I investigate the role of reproductive biology in their invasion success (Chapter 2), the impacts of a specific species, Acacia saligna, on native plant-pollinator communities in South Africa (Chapter 3), and if the impact of A. saligna on native plant species can be predicted, based on floral trait similarity (Chapter 4). Australian acacias possess many floral traits implicated in plant invasion success, including extensive, dense floral displays, prolific seed production, long-lived seed banks, and propensity for vegetative reproduction. In Chapter 2 of my thesis, co-authors and I investigate if such reproductive traits influence whether or not a species becomes invasive once it is introduced by qualitatively and quantitatively analyzing literature and data collected from published and unpublished data. We found that invasive species reach reproductive maturity earlier and are more commonly able to resprout. Our findings have important implications for management of existing Acacia invasions as well as the assessment and prevention of further introductions of Acacia species leading to invasion. Impacts of invasive plant species on native plant visitation and consequent reproduction are often negative and have important implications for future ecosystem health. Acacia saligna is a problematic woody invasive shrub in many Mediterranean regions, and in South Africa, it invades fynbos vegetation, which boasts one of the highest plant diversities per area in the world as well as many specialized pollination mutualisms. It blooms during the flowering peak of most native species and forms dense, showy floral displays, thus the possibility of pollinator-mediated interactions with co-flowering native species is high. In Chapter 3, I assess the impact of flowering A. saligna on insect visitation to co-flowering native species by conducting flower observations at both an invaded and uninvaded (control) site. One of the native species most-visited by native honeybees, Roepera fulva, had high flower visitor overlap with A. saligna and suffered significantly lower visitation from all insects and from bees when A. saligna was present than at the control site. The native honeybee appears to be the most important visitor to A. saligna as it was the most frequent and mobile. Due to its foraging efficiency and dominance in pollinator communities, Apis mellifera subsp. capensis could be an important mediator of the negative effect of A. saligna on co-flowering natives, especially those frequently visited by honeybees. The use of floral traits as predictors of a species’ effect on co-flowering plants via pollination is commonly used in pollination biology. In the fourth chapter of my thesis, I apply the predictive principle of floral traits on flower visitation to see if floral traits can predict the impact of an invasive plant species on co-flowering native species, and if so, which floral traits are the most important. Following the same flower observation protocol as Chapter 3, I assess two measures of invasion impact on native flowers – change in visitation rate to native plant species between invaded and uninvaded sites and flower visitor overlap between A. saligna and native species – and test for a correlation with floral trait similarity of native species to and A. saligna. Similarity of categorical traits and all traits combined (categorical and continuous) were significantly positively correlated with flower visitor overlap, indicating that native species with categorical traits similar to A. saligna were more likely to share flower visitors with A. saligna. Floral symmetry and shape similarity were the most important categorical traits in driving flower visitor overlap. Findings suggest an important link between categorical floral traits and ability to predict invasive plant impact on native flower-insect interactions; however, more comprehensive studies are required for conclusive results.
AFRIKAANSE OPSOMMING: My Meestersgraad se navorsing kombineer aspekte van bestuiwingsbiologie en indringerbiologie om ondersoek in te stel na die indringing-voorspelbaarheid en die impak van Australiese Akasias, 'n wêreldwye indringende groep van plante. Ek het spesifiek gekyk na die rol van voortplantingsbiologie in hul indringing sukses (Hoofstuk 2), die impak van 'n spesifieke spesie, Acacia saligna, op die inheemse plant-bestuiwer gemeenskappe in Suid-Afrika (Hoofstuk 3), en of die impak van Acacia saligna op inheemse plantspesies voorspel kan word, gebaseer is op die ooreenkomste in blomeienskappe (Hoofstuk 4). Australiese Akasias het baie voortplantingseienskappe wat geassisioeer word met plantindringing sukses, insluitend ‘n uitgebreide, digte blommedrag, uitermatige hoë saadproduksie, langlewende saadbank, en die geneigdheid vir vegetatiewe voortplanting. In hoofstuk 2 van my tesis, ondersoek ek en my medeskrywers of sulke voortplantingseienskappe beïnvloed of 'n spesie ‘n indringer raak, deur 'n ontleding van die literatuur en data wat versamel is uit gepubliseerde en ongepubliseerde data. Ons het gevind dat indringerspesies geslagsrypheid vroeër bereik en meer algemeen in staat is om weer uit te loop. Ons bevindinge het belangrike implikasies vir die bestuur van die bestaande Akasiastande sowel as die evaluering en die voorkoming van verdere vestiging van Akasia spesies wat lei tot indringers. Impakte van uitheemse plantspesies op inheemse plant-besoeke deur bestuiwers, en gevolglike voortplanting is dikwels negatief en het belangrike implikasies vir die toekomstige ekosisteem gesondheid. Acacia saligna is 'n problematiese houtagtige indringende struik in baie Mediterreense streke, en in Suid-Afrika. Dit dring in die Fynbos in, wat spog met een van die wêreld se hoogste plant diversiteit per oppervlak, sowel as baie gespesialiseerde bestuiwing-mutualismes. Dit blom gedurende die bloeiseisoen van die meeste inheemse spesies en vorm digte, pronkerige blomuitstallings, dus is die moontlikheid hoog vir bestuiwer-gemedieerde interaksies met inheemse spesies wat saam blom. In Hoofstuk 3, het ek die impak van Acacia saligna, op insek besoeke aan saamblommende inheemse spesies bepaal, deur waarnemings op beide ingedringde en natuurlike studie gebiede. Een van die inheemse spesies, Roepera fulva, wat die meeste besoek is deur inheemse heuningbye, het 'n hoë blom besoeker oorvleueling met Acacia saligna en het aansienlik minder besoeke van alle insekte en van bye gekry toe Acacia saligna teenwoordig was as by die natuurlike studie gebied. Die inheemse heuningby, Apis mellifera subsp. capensis, blyk die mees belangrikste besoeker aan Acacia saligna te wees aangesien dit die mees gereeldste en beweeglikste was. As gevolg van sy oorheersing in bestuiwer gemeenskappe, is die by 'n belangrike faktor van die negatiewe uitwerking van Acacia saligna op die inheemse plante wat dieselfde tyd blom, veral dié wat gereeld besoek word deur heuningbye. Die gebruik van blomeienskappe as voorspellers van 'n spesie se effek op saamblommende plante deur middel van bestuiwing, word algemeen gebruik in bestuiwingsbiologie. In die vierde hoofstuk van my tesis, het ek die voorspellende beginsel van die blomeienskappe op blombesoeking toegepas, om te sien of blomeienskappe die impak van 'n uitheemse plantspesie kan voorspel op saamblommende inheemse spesies, en indien wel, watter blomeienskappe die belangrikste is. Deur dieselfde blom waarneming protokol as in Hoofstuk 3 te gebruik, het ek twee maatstawwe van indringings impakte op inheemse blomme vergelyk - verandering in besoek gereeldheid aan inheemse plante tussen ingedringde en natuurlike studie gebiede en blom besoeker oorvleueling tussen die Acacia saligna en inheemse spesies – en die toets vir 'n korrelasie van blomeienskap ooreenkomste in inheemse spesies en Acacia saligna. Soortgelykheid van kategoriese eienskappe en al die eienskappe gekombineer (kategoriese en deurlopende) is beduidend positief gekorreleerd met blombesoeker oorvleueling, wat aandui dat inheemse spesies met kategoriese eienskappe soortgelyk aan Acacia saligna meer geneig was om blom besoekers te deel met Acacia saligna. Blom simmetrie en vorm ooreenkoms was die belangrikste kategoriese eienskappe wat ly tot blombesoeker oorvleueling. Bevindinge dui op 'n belangrike skakel tussen die die kategoriese blomeienskappe en die vermoë om indringerplant impakte op inheemse blom-insek interaksies te voorspel, alhoewel, meer omvattende studies nog nodig is.
DST-NRF Centre of Excellence for Invasion Biology and the Department of Botany and Zoology at Stellenbosch University for research funding

Resumo parte 1: A invasão biológica é uma das principais ameaças à biodiversidade em todo o mundo. Entretanto, apesar dos trópicos concentrarem a maior parcela desta diversidade, ainda é pouco e esparso o conhecimento científico disponível sobre plantas invasoras nos ecossistemas tropicais. Este capítulo tem como objetivo comparar a quantidade de trabalhos publicados sobre invasões biológicas por plantas terrestres em ambientes tropicais e não tropicais, assim como a abordagem seguida nesses artigos. Foram lidos, selecionados e revisados os artigos publicados no intervalo de 1995 a 2004, em oito revistas científicas de alto índice de impacto. Ao todo, foram encontrados 239 artigos sobre plantas invasoras, sendo que a maioria deles aborda ecossistemas da América do Norte e Europa. Este é um reflexo do desequilíbrio na produção científica atual, que não dá a devida atenção aos ambientes com a maior biodiversidade, sendo que esta recebe vários tipos de ameaças à sua conservação. Resumo parte 2: As palmeiras da espécie Archontophoenix cf. cunninghamiana são invasoras em um pequeno fragmento florestal na região metropolitana da cidade de São Paulo (SP). Este trabalho investigou a abundância das sementes dessa espécie na chuva e no banco de sementes nesse local. Foram coletadas cinqüenta amostras de solo e de sementes da chuva de sementes em doze coletas mensais. Os resultados indicam que a dispersão de sementes dessa espécie invasora é efetiva na mata e que há a possibilidade dessa espécie formar um banco de sementes sazonal. Portanto, o sucesso da invasão poderia ser explicado pela capacidade de dispersão dessa espécie na mata, assim como por outras características da espécie e pela baixa resistência à invasão da comunidade neste fragmento.
Biological invasion is one of the worst threats to the world biodiversity. The actual extent of the impact of biological invasions in the tropics is still unknown. The aim of this chapter is to survey the literature published on plant invasions, comparing the material concerning the tropics with that regarding the rest of the world. 239 articles published in 8 ecological journals over the past ten years were reviewed. Most of them focused on temperate ecosystems, in North America and Europe. There is a weak effort on research concerning plant invasions in tropical ecosystems. Much more research is needed to evaluate the real scenario in the countries that contain the highest biodiversity of the world. The exotic palm Archontophoenix cf. cunninghamiana invaded a small forest fragment in São Paulo (São Paulo, Brazil). The abundance of this species in the seed rain and seed bank were investigated. The results indicate that this species may form a seasonal seed bank in the soil and that the seed dispersal is effective in the community. The success of the invasion by this exotic palm can be attributed to its high dispersal ability and to the community high invasibility due to the isolation of the forest fragment.

Water-use efficiency (WUE) as reflected in the leaf carbon stable isotope ratio was compared between co-occurring indigenous fynbos proteoids and invasive Australian hakeas H. sericea, H. gibbosa and H. suaveolens. At the driest site, H. suaveolens was slightly more WUE than several co-occurring proteoids; there was no significant difference between hakeas and proteas at the other sites. Transpiration rates of shoots and of whole trees were compared between Hakea sericea and Protea repens growing on Stellenboschberg northeast of Cape Town. Both measurements showed no real difference between the species and it is concluded that differences in water relations are not responsible for the highly competitive growth rates of hakeas in fynbos. It is hypothesised that hakeas may be able to vegetatively outcompete proteoids as a consequence of monopodial architecture and some ability to prevent shade-limitation of photosynthesis. A rough estimate of water loss due to transpiration and interception by H. sericea stands indicates that this species may have a significant effect on catchment water loss relative to open-canopy proteoid fynbos. This effect would be due not to transpiration rates of individual trees but to consistently high densities of mature hakea stands.

Thesis (MScConEcol)--Stellenbosch University, 2012.
ENGLISH ABSTRACT: Riparian ecotones, transitional areas between upland terrestrial communities and aquatic ecosystems, are very dynamic and complex ecosystems with intrinsic ecological properties differing in spatial structure, function and temporal dynamics. Riparian habitats along rivers of the Mediterranean south-western Cape are sensitive to environmental change and particularly vulnerable to invasion by invasive alien plants (IAPs), especially nitrogen-fixing Acacia spp., and yet relatively little work has focused on how riparian ecosystems in this region respond to such stressors. The important roles that intact riparian vegetation play in maintaining ecosystem integrity and services have been increasingly highlighted as we acknowledge the degradation of these habitats. While the Working for Water (WfW) programme has been shown to be very successful in eradicating IAPs in riparian zones in the short-term, the extent to which riparian ecosystems recover following alien clearing activities remains poorly understood. The results presented in this study addressed several different aspects of riparian structure and function and acts as a steppingstone for guiding future research and management in riparian zones by adding to the evaluation of the success of clearing initiatives and restoration thereof. The aim of this study was to assess plant functional type (PFT) cover, soil physical and chemical properties, and selected biogeochemical processes in natural, Acacia- invaded and cleared riparian ecotones and associated non-riparian upland fynbos. Fieldwork was performed in mountain and foothill sections of six perennial river systems within the south-western Cape. Eleven sites of three categories were chosen: four natural sites (uninvaded); four moderate to highly invaded sites (predominantly A. mearnsii); and three cleared sites (a formerly invaded site that had been cleared more than 7 years prior to the study). Within each site, four to five replicate plots were established along each of three geomorphological zones (wet bank, dry bank, and upland fynbos). Seasonal soil samples were collected for a period of one year. Results from this study showed that PFT cover and composition, soil physical and chemical properties and rates of nitrogen (N) and phosphorus (P) mineralization differed amongst invasion status, between geomorphological zones and across seasons. Regarding most soil physical and chemical properties and indices N and P cycling, river floodplains (dry banks) were very similar to terrestrial uplands. Acacia spp. changes soil properties and affects plant functional attributes by i) enriching the system with N; ii) enhancing litter inputs; iii) altering soil physical properties; iv) changing the composition and reducing the cover of PFT; and v) enhancing P mineralization rates. Although measured soil physical and chemical properties and N and P mineralization rates were reduced to levels that were similar to or resembled the situation at natural areas, available inorganic N remained two times higher after more than seven years of clearance. Furthermore, cleared areas were characterized by sparse woody cover and a high cover of alien grasses. Correlations between soil silt and clay content and several soil properties measured in this and other studies indicates important linkages between soil texture and resource availability. Clearing Acacia spp. may initiate restoration of invaded riparian ecosystems, but changes in ecosystem function (e.g. elevated soil N availability) as a result of invasion may necessitate active restoration following the removal of the alien species. Active restoration under such conditions would be required to facilitate the restoration of cleared riparian communities. However, we still lack the mechanistic understanding around fynbos riparian recovery after clearing, as the success of restoration may depend on complex interaction and feedback cycles between plants and their physical environment. A greater comprehensive understanding of fynbos riparian ecological processes will not only improve the effectiveness of restoration initiatives by integrating science and management, but also advance the field of riparian ecology.
AFRIKAANSE OPSOMMING: Rivier oewerwal-areas, oorgang gebiede tussen aangrensende terrestriële gemeenskappe en akwatiese ekosisteme, is baie dinamiese en komplekse ekosisteme met intrinsieke ekologiese eienskappe wat verskil in struktuur, funksie (bv. biogeochemie siklusse) en temporale dinamika. Oewerhabitatte langs riviere van die Mediterreense suid-wes Kaap is sensitief vir omgewingsveranderinge en kwesbaar vir indringing deur uitheemse plante (bekend as “invasive alien plants” (IAPs)), veral stikstof-fiksering Acacia spp., en relatief min werk het nog gefokus op hoe ekosisteme in die streek reageer op sulke veranderinge in die omgewing. Die belangrike rol wat gesonde oewerwal plantegroei speel in die handhawing van ekosisteemdienste- en integriteit, is al hoe meer uitgelig soos ons die agteruitgang van hierdie habitat in ag neem. Terwyl die Werk vir Water (WvW)-program al dat baie suksesvol was in die uitwissing van IAPs in oewersones in die kort termyn, is die mate waarin oewer-ekostelsels herstel na skoonmaakaksies swak verstaan. Fynbos oewerwal-areas is grootliks ingeneem deur houtagtige IAPs, veral stikstof fiksering Acacia spp. (soos Acacia mearnsii). Die resultate wat in hierdie studie aangebied is, het verskillende aspekte van oewer- struktuur en funksie aangespreek en dien as middel vir toekomstige navorsing en bestuur van oewerwal ekosisteme deur by te dra tot die evaluering van die sukses van skoonmaak inisiatiewe en die herstelproses daarvan. Die doel van hierdie projek was om die moontlikhede vir herstel van fynbos owerwal-ekostelsels te evalueer deur middel van verskeie grond- fisiese en chemiese eienskappe; plant funksionele groep dekking (genoem ‘plant functional types’ (PFT)); en geselekteerde grond biogeochemie prosesse in natuurlike, Acacia- aangetaste, en skoongemaakte rivierstelsels en nabygeleë terrestriese areas te vergelyk. Veldwerk is gedoen in bergstroom en voetheuwel rivierseksies van ses standhoudende rivierstelsels in Suid-wes Kaap, Suid Afrika. Van uit hierdie geselekteerde rivierstelsels is elf studie areas van drie kategorieë (of indringing status) gekies: vier natuurlike areas (nie aangetas); vier gematig- tot hoogs aangetaste areas (hoofsaaklik A. meanrsii); en drie skoongemaakte areas (rivieroewers wat meer as sewe jaar van te vore skoongemaak is). Binne elke studie area was vier tot vyf soortgelyke persele gevestig by elke van drie breë geomorfologiese sones: naamlik nat-, droë en hoogliggende terrestriese fynbos. Seisoenale grondmonsters vir 'n tydperk van een jaar is geneem. Resultate van hierdie studie het getoon dat PFT dekking en samestelling, grond fisiese- en chemiese eienskappe en N-mineralisasie en suur fosfatase aktiwiteit verskil tussen indringing status, geomorfologiese sones en oor seisoene. Ten opsigte van meeste grond fisiese en chemiese eienskappe en indekse van stikstof (N) en fosfor (P) siklusse kom die rivier vogregimes (droë oewersones) baie ooreen met die terrestriële gebiede. Aan die anderkant is die natbanksones gekenmerk deur grondeienskappe wat baie verskil van die ander twee geomorfologiese gebiede. Die gegewens ondersteun die hipotese dat indringing deur Acacia spp. verskeie grondeienskappe verander en plante se funksionele kenmerke beïnvloed deur i) die sisteem met voedingstowwe te verryk (veral N); ii) verhoog die toevoeging van plantmateriaal; iii) verander grond fisiese eienskappe; iv) verander die samestelling en verminder die dekking van PFT; v) en verhoog P biogeochemie. Hoewel grond fisiese -en chemiese eienskappe, en indekse van N en P mineralisasie verminder is tot vlakke wat soortgelyk aan natuurlike areas, het beskikbare anorganiese N twee keer hoër gebly by skoongemaakte gebiede. Nietemin, voorheen skoongemaakte gebiede is weer-binnegeval deur eksotiese grasse en die regenerasie of hertelling van inheemse fynbos gemeenskappe is taamlik beperk, veral houtagtige oewer struike en bome. Korrelasies tussen grond slik-en klei-inhoud en verskeie grondeienskappe gemeet in hierdie en ander studies dui op belangrike skakeling tussen die grondtekstuur en voedingstof beskikbaarheid. Die opruiming van Acacia spp. mag as aansporing dien vir die herstellingsproses van rivieroewerstelsels, maar veranderinge in die funksie van ekosisteme (bv. verhoogte grond N beskikbaarheid), as gevolg van indringing, mag aktiewe herstel noodsaak nadat die indringer spesies verwyder is. Aktiewe herstel onder sulke omstandighede sal verwag word om die herstel van skoongemaak oewer gemeenskappe te fasiliteer. Ons het wel egter nog 'n gebrek aan die meganistiese begrip in verband met die herstel van fynbos oewerwal areas na opruimings-inisiatiewe, sedert die sukses van herstel kan afhang van komplekse interaksie en terugvoer siklusse tussen die plante en hul fisiese omgewing. ʼn Meer omvattende begrip van fynbos rivieroewer ekologiese prosesse sal nie net die doeltreffendheid van opruimings-inisiatiewe deur die integrasie van wetenskaplike navorsing en bestuur verbeter nie, maar ook vooraf die gebied van rivieroewer-ekologie.

Invasive plants can outcompete native plants, replacing diverse plant communities with monocultures, which can negatively impact the whole ecosystem. One invasive plant, Phragmites australis, has invaded wetlands across North America. In Utah’s Great Salt Lake, it has greatly reduced the area of native plants that are important habitat for migratory birds. Here we describe experiments that assess multiple treatments for Phragmites removal and evaluate the return of native plants after Phragmites management. The treatments were applied to Phragmites patches at two scales (small 1/4-acre plots and large 3-acre plots) and across multiple sites to evaluate how patch size and environmental differences can influence the plants that return after Phragmites removal. The treatments (applied over 3 years and monitored two more) compared two different herbicides (glyphosate and imazapyr) and different herbicide and mowing timings. The treatments evaluated in the large patch study were 1.) untreated control 2.) fall glyphosate, winter mow, 3.) summer imazapyr, winter mow, 4.) summer glyphosate, winter mow. The treatments evaluated in the small patch study included treatments 1-4 above plus 5.) summer mow, fall glyphosate, 6.) summer mow, then black plastic solarization. In the small patches, we also monitored the seeds in the soil to assess how Phragmites management treatments can change the densities of Phragmites and native seeds. Fall glyphosate treatments were superior for Phragmites cover reduction. After the initial treatment, summer herbicide and mow treatments reduced Phragmites seed production, while fall glyphosate did not. Phragmites seeds were plentiful in the soil but were reduced following three years of all herbicide treatments. Native plant recovery following Phragmites management was extremely variable across sites. Sites with high soil moisture had better Phragmites removal and more native plants. But when flooding was deep, native plants were rare. Native seed density in the soil did not change due to Phragmites management, but soil seed densities were different across sites, which influenced native plant recruitment. Phragmites was removed more effectively and native plants returned in greater numbers in small patches compared with large. This was because small patches were typically near established native plant communities, which likely provided more native plant seeds and had hydrology that was less disturbed by human activity. In sites where native plants do not return after Phragmites management, practitioners may need to try revegetation with native plant seeds to restore important native plant communities.

The recent and rapid expansion of Phragmites australis in North America provides an opportunity to relate dispersal patterns to landscape structure. Linear wetlands, such as roadside or agricultural ditches, may serve as corridors that facilitate dispersal at the landscape scale, but there is little quantitative information on patterns of invasion in these habitats. Remote sensing data, GIS, and field observations were used (1) to assess the feasibility of mapping populations of P. australis in linear wetlands and (2) to quantify and explain recent invasion patterns of P. australis in two periurban areas of southern Quebec. An accuracy assessment demonstrated the reliability of aerial photographs to map populations in linear habitats. The intrinsic growth rate observed is high compared to those reported in natural wetlands. Spatial analysis revealed that colonization events are not constrained by the network's configuration. Logistic regressions confirmed the effect of road corridors on the spread of P. australis.

Biological invasions have adverse economic, cultural, and ecological effects worldwide. Among the most impactful in North America is the invasion of Phragmites australis (Phragmites), a large-scale clonal grass that rapidly colonizes wetlands. Phragmites crowds out native plants and alters habitat for native fauna. In doing so, Phragmites also alters human access to water resources and has adverse economic effects, including decreasing property value, inhibiting recreational use, and limiting populations of game species. The efforts described in this dissertation are a component of a large, multidisciplinary effort to better understand the anthropogenic stressors to Chesapeake Bay, Maryland, at the land/water interface. Utah State University worked in collaboration with the Smithsonian Environmental Research Center and other academic and public organizations to address this problem from multiple directions. The diverse and extensive studies ranged from aquatic and avian faunal composition and submerged aquatic vegetation to our work on the invasive wetland grass, Phragmites. Having assessed the existing literature and its shortcomings, we conducted a large-scale, long-term study of the effects of Phragmites removal on the Bay. By removing Phragmites from plots in select sub-estuaries of Chesapeake Bay through herbicide spraying, leaving associated plots intact, and comparing both with native wetland conditions, we sought to better understand herbicidal management of Phragmites and the potential for wetland plant community recovery. Although sprayings decreased the relative cover, stem diameters, and stem densities of Phragmites, we found that herbicide treatment alone was not adequate to restore native plant communities or significantly affect seedbank composition. Our results demonstrate the resilience of Phragmites and call for a diverse range of control measures, including mowing, grazing, burning to expose the seedbank to germination, and—if economics allow—active revegetation to establish the desired plant community composition. This dissertation provides beneficial data for those who seek to manage Phragmites in wetland plant communities, but there is much work still to be done. The literature review, seedbank study, and community analysis included in this volume are components of a larger research program on Phragmites management. Future studies should, in particular, investigate revegetation and nutrient amelioration as means to recover pre-invasion vegetation.

Thesis (M.S.)--Kent State University, 2008.
Title from PDF t.p. (viewed Oct. 27, 2009). Advisor: Laura Leff. Keywords: Phragmites australis; Scirpus cyperinus; glyphosate; microbes; ergosterol; invertebrates. Includes bibliographical references (p. 47-59).

[Truncated abstract] Mediterranean southwest Australia, a global biodiversity hotspot, has nutrient deficient soils, exacting climatic conditions and is species rich with 7380 native vascular plant species, of which 49% are endemic. The region is expected to experience one of the world's highest degrees of biodiversity loss and change in the coming decades, with introduced species presenting a major threat. Limited knowledge is available on the mechanisms of ecosystem change associated with invasion and fire in this biodiversity hotspot region. Banksia woodland, an iconic complex species-rich natural ecosystem is one of the major vegetation types of the coastal sandplain, extending from 15 to 90 km inland and 400 kms along the west coast. The following hypothesis was tested to explore the ecological impacts of invasion: Is invasion of Banksia woodland by the introduced species Ehrharta calycina and Pelargonium capitatum accompanied by an alteration in ecosystem properties and processes, whereby the degree of change is related to fire frequency and abundance of introduced species? Different vegetation conditions, i.e. Good Condition (GC), Medium Condition (MC), Poor Condition invaded by Ehrharta calycina (PCe) and Poor Condition invaded by Pelargonium capitatum (PCp) were utilized for field assessments. ... In the soil seed bank, species numbers and germinant density decreased significantly for native and seeder (fire sensitive) species between GC sites and invaded sites. Surprisingly 52% of germinants at GC sites were from introduced species, with much of the introduced soil seed bank being persistent. Native species were dominated by perennial shrubs, herbs and sedges, while introduced species were dominated by perennial and annual grasses and herbs. Invasion by introduced species, associated with frequency of fire, altered the ecosystem, thus disadvantaging native species and improving conditions for even greater invasion within the Banksia woodland. Significantly higher soil phosphorus P (total) and P (HCO3) were found at PCe and PCp sites compared to GC sites. Leaf nutrient concentrations of phosphorus were significantly higher, and potassium and copper significantly lower in PCe and PCp sites, with introduced species having significantly greater concentrations than native species (except Manganese). This study demonstrated the key role of phosphorus in the Banksia woodland, in contrast to other research which identified nitrogen as the major nutrient affected by invasion. Higher levels of soil and leaf phosphorus, loss of species diversity and function, changes in fire ecology and canopy cover and a limited native soil seed bank make restoration of a structural and functional Banksia woodland from the soil seed bank alone unlikely. Without management intervention, continuing future fire is likely to result in a transition of vegetation states from GC to MC and MC to PC. The knowledge gained from this study provides a better ecological understanding of the invasive process. This enhanced understanding will enable the development of adaptive management strategies to improve conservation practices within a biodiversity hotspot and reduce the impact of the key threatening process of invasion.

This thesis investigated how management affects plants, seedbanks, and rhizospheric bacterial communities in grasslands. Control strategies studied reduced invasive grass cover in desert grasslands even though the invader holds unique root-associations with bacteria. Seedbank stores of invaders were high in both desert and dairy-country grasslands suggesting control strategies should also reduce seedbanks.

Thesis (MSc)--Stellenbosch University, 2012.
ENGLISH ABSTRACT: This thesis investigates the invasiveness and current status of two Acacia species recently identified as invaders in South Africa in order to determine the feasibility of their eradication. Australian acacias are among South Africa’s worst invasive species and many have had widespread damaging impacts on native ecosystems. In addition, several Acacia species still exist as small isolated populations in the country and have been targeted for eradication in order to prevent potential widespread impacts. This work assesses Acacia implexa (Chapter 2) and Acacia stricta (Chapter 3) as potential eradication targets by quantifying the extent of their invasion in South Africa, assessing the risk they pose to the country and evaluating the feasibility of their eradication based on estimated costs of clearing. Results of formal risk assessments show that both A. implexa and A. stricta should be considered high risk species, and bioclimatic model predictions indicate that both species have large potential ranges in South Africa. Detailed population surveys found that A. implexa and A. stricta each occur at several distinct localities all in the Western Cape Province. Acacia implexa populations were found at three sites (Tokai, Wolseley and Stellenbosch) where they have densified by means of vegetative suckering allowing A. implexa to outcompete native vegetation. No evidence of large seed banks of A. implexa were found, however vigorous resprouting following damage makes the control of A. implexa difficult. Acacia stricta was found at nine localities all in the Knysna area of the Garden Route, where populations are spreading along disturbed roadsides in plantations. Acacia stricta produces large amounts of seeds and can accumulate large seed banks. Seed spread is most likely due to large-scale soil movement by road maintenance vehicles which can easily lead to the establishment of new populations. We therefore used a predictive risk mapping approach based on the association of A. stricta to roadsides and disturbed plantations to enable effective searching to detect all infestations of A. stricta. Based on the high risk of both species and the limited range sizes of the currently known populations, we recommend that A. implexa and A. stricta remain targets for eradication. Management strategies proposed for these species (Chapter 4) include clearing on an annual (in the case of A. stricta) or biannual (for A. implexa) basis to prevent seed production, and targeted awareness campaigns at a national scale to determine whether our current knowledge of the extents of A. implexa and A. stricta are accurate. This work has shown that detailed assessments of species at intermediate stages of invasion is an important initial step in an eradication attempt, and better understanding of species specific invasion characteristics can help to improve management and potentially increase the probability of success of eradication.
AFRIKAANSE OPSOMMING: Hierdie tesis ondersoek die invasieve en die huidige status van twee Acacia spesies onlangs geïdentifiseer as indringers in Suid-Afrika ten einde die lewensvatbaarheid van hul uitwissing om te bepaal. Australiese akasias is onder Suid-Afrika se ergste indringerspesies en baie het wydverspreide skadelike impak op die inheemse ekosisteme. Verder het verskeie Acacia spesies bestaan nog steeds as 'n klein geïsoleerde bevolkings in die land en wat geteiken is vir uitwissing in om moontlike grootskaalse impakte te voorkom. Hierdie werk beoordeel Acacia implexa (Hoofstuk 2) en Acacia stricta (Hoofstuk 3) as 'n moontlike uitwissing teikens deur die kwantifisering van die omvang van hul inval in Suid-Afrika, die beoordeling van die risiko wat hulle inhou vir die land en die evaluering van die haalbaarheid van hul uitwissing op grond van beraamde koste van die wiele ry. Resultate van formele risikobepalings toon dat beide die A. implexa en A. stricta moet oorweeg word om 'n hoë risiko spesies, en bioclimatic model voorspellings dui daarop dat beide spesies het 'n groot potensiaal bereik in Suid-Afrika. Uitgebreide bevolkings opname gevind dat A. implexa en A. stricta elk by verskeie afsonderlike plekke in die Wes-Kaap voorkom. Acacia implexa is op drie plekke (Tokai, Wolseley en Stellenbosch) gevind, waar hulle deur middel van vegetatiewe suier densified en inheemse plantegroei oorwin het. Geen bewyse van groot nageslag banke van A. implexa is gevind, maar in kragtige resprouting volgende skade maak die beheer A. implexa moeilik is. Die Acacia stricta is op nege plekke in die Knysna-omgewing van die Tuinroete, waar die bevolkings verspreiding langs die versteurde paaie in plantasies. Acacia stricta produseer groot hoeveelhede saad en kan versamel groot saadbanke. Saad versprei is waarskynlik te danke aan grootskaalse grond beweging deur die instandhouding van paaie voertuie wat kan lei tot die vestiging van nuwe bevolkings. Ons het dus 'n voorspellende risiko kartering benadering wat gebaseer is op die vereniging van A. stricta aan paaie en versteurde plantasies in staat te stel om doeltreffend te soek alle besmettings van A. stricta op te spoor. Gegrond op die hoë risiko van beide spesies en die beperkte reeks groottes van die bevolking wat tans bekend is, beveel ons aan dat A. implexa en A. stricta bly teikens vir uitwissing. Bestuurstrategieë vir hierdie spesies (Hoofstuk 4) voorgestel word, sluit in die skoonmaak op 'n jaarlikse (in die geval van A. stricta) of die halfjaarlikse (vir A. implexa) basis van die saad produksie, en geteikende bewusmakingsveldtogte om te voorkom dat 'n nasionale skaal om te bepaal of ons huidige kennis van die omvang van A. implexa en A. stricta akkuraat is. Hierdie werk het getoon dat uitgebreide aanslae van spesies op intermediêre fases van die inval is 'n belangrike eerste stap in 'n poging van die uitwissing, en 'n beter begrip van spesies spesifieke inval eienskappe kan jou help om te verbeter en potensieel verhoog die waarskynlikheid van sukses van die uitroeiing nie.

"July 2004."
Bibliography: leaves 120-136.
vii, 136 leaves : ill. (some col.), maps ; 30 cm.
Title page, contents and abstract only. The complete thesis in print form is available from the University Library.
Investigates the effects of soil moisture dynamics on the growth and interactions between invasive annual grasses and native perennial grasses in the mid-north of South Australia. At most sites annual grass abundance was positively correlated with rainfall, soil moisture after rainfall and higher soil productivity. Perennial grass abundance was negatively correlated with annual grass abundance and soil moisture after rainfall, and was weakly positively correlated with percentage summer rainfall, elevation, radiation, gravel and slope. Overall perennial grasses responded little to the environmental variables investigated, but strongly to annual grass abundance, while for annual grasses soil moisture was the driving variable.
Thesis (Ph.D.)--University of Adelaide, School of Earth and Environmental Sciences, Discipline of Environmental Biology, 2004

"July 2004."
Bibliography: leaves 120-136.
vii, 136 leaves : ill. (some col.), maps ; 30 cm.
Investigates the effects of soil moisture dynamics on the growth and interactions between invasive annual grasses and native perennial grasses in the mid-north of South Australia. At most sites annual grass abundance was positively correlated with rainfall, soil moisture after rainfall and higher soil productivity. Perennial grass abundance was negatively correlated with annual grass abundance and soil moisture after rainfall, and was weakly positively correlated with percentage summer rainfall, elevation, radiation, gravel and slope. Overall perennial grasses responded little to the environmental variables investigated, but strongly to annual grass abundance, while for annual grasses soil moisture was the driving variable.
Thesis (Ph.D.)--University of Adelaide, School of Earth and Environmental Sciences, Discipline of Environmental Biology, 2004

The effects of plant invasions on ecosystem structure and function are well studied but the pathways and mechanisms that underlie these effects remain poorly understood. In depth investigation of invasion mechanisms is vital to understanding why invasive plants impact only certain systems, and why only some invaders have disproportionately large impacts on the invaded community. There are many mechanisms such as lack of natural enemies or control mechanisms, the individual characteristics of the invader and invaded communities, direct and indirect resource competition, evolution or hybridisation, altered ecosystems processes, and allelopathy that may explain the invasion processes of plant species. Among these possible influences on invasion, allelopathy has received increased attention and study with the rise in understanding of its implications and potential disproportionate influence. However, identifying allelopathy and consequent phytotoxic effects as an important mechanism of plant invasion is a difficult task due to the potential for an individual plant to have many component chemicals with multiple modes of action, interactive effects, and synergistic interactions. For allelopathy to be implicated as a mechanism that facilitates invasion, multiple aspects of the plant species allelopathic properties must be examined. This research investigated allelopathy as a mechanism of the invasion process in Phragmites australis by a series of ecologically realistic experiments in the laboratory, greenhouse and field.

Dissertação de mestrado em Ecologia (Ecologia Aplicada), apresentada ao Departamento de Ciências da Vida da Faculdade de Ciências e Tecnologia da Universidade de Coimbra.
Two Australian acacias considered invasive in Portugal, Acacia dealbata and Acacia melanoxylon, were examined for plant-soil feedbacks in parallel greenhouse experiments. Feedback was evaluated by examining biomass variation in plants grown in unsterilized and sterilized soils from different areas. Soils were collected from areas of native vegetation, areas invaded by A.dealbata and areas disturbed by wildfire. To explore the causes of feedback, several soil-related factors were measured: pH, nutrients, arbuscular mycorrhizal root colonization and nodulation by rhizobes. Results indicate the occurrence of biotic resistance of some native communities and facilitation of acacia invasion by disturbance by wildfire. Invasion and disturbance also enhanced acacia nodulation and mycorrhization. The most important predictors of plant growth were high soil ammonium content, and low organic matter content. Mycorrhization and nodulation were important for A.dealbata and A. melanoxylon, respectively. The importance of belowground mutualists on acacia growth was confirmed by lower plant growth in sterilized soils.
As relações de feedback planta-solo de duas espécies de acacias Australianas consideradas invasoras em Portugal, Acacia dealbata e Acacia melanoxylon, foram estudadas em experiências paralelas. O feedback foi examinado através da variação de biomassa em plantas crescidas em solos não esterilizados e esterilizados provenientes de diferentes áreas. Os solos foram colhidos em áreas de vegetação nativa, áreas invadidas por A. dealbata e áreas perturbadas por incêndios. De modo a explorar as causas de feedback, foram medidos vários factores relacionados com o solo: pH, nutrientes, colonização de raízes por fungos micorrízicos arbusculares e nodulação por rizóbios. Os resultados indicam a existência de fenómenos de resistência biótica por parte de algumas comunidades nativas e de facilitação da invasão por acácias através de perturbação pelo fogo. Ainda, os fenómenos de invasão e perturbação aumentaram a nodulação e a micorrização. Teores do solo elevados em amónio e reduzidos em matéria orgânica foram encontrados como sendo os factores mais importantes para o crescimento das plantas. A micorrização e a nodulação foram também relevantes para A. dealbata e A. melanoxylon, respectivamente. A importância dos mutualistas do solo para o crescimento das acácias foi confirmada pelo menor crescimento das plantas nos solos esterilizados.

Les plantes envahissantes sont considérées comme l'une des plus grandes menaces sur les espèces, mais leurs impacts sur les amphibiens demeurent peu connus. L’objectif de ce projet est de déterminer l’effet de l’établissement du roseau commun (Phragmites australis) sur la répartition des amphibiens et sur leur développement larvaire. Il est présumé que cette plante s'accapare l'espace et les ressources disponibles en produisant une importante biomasse, qu’elle peut modifier l’hydrologie des marais et la structure des communautés d’amphibiens. J’ai évalué les facteurs influençant la répartition des amphibiens selon les caractéristiques des étangs et du paysage dans 50 sites envahis ou non à divers degrés. Des expériences ont également été menées afin d’étudier les répercussions de trois densités de roseau sur des têtards de la Grenouille des bois (Lithobates sylvaticus) et sur la qualité de son habitat. Mes résultats suggèrent que le roseau à forte densité ralentit le développement larvaire de la grenouille des bois et influence les assemblages phytoplanctoniques. Cependant, il n’y a aucune relation entre la densité de la plante et la survie, la morphologie des têtards et les caractéristiques biotiques et abiotiques de l’eau. Dans notre aire d’étude, le paysage autour des étangs a une plus grande influence sur la répartition des amphibiens que l'établissement du roseau. Toutefois, la probabilité d’assèchement est plus élevée lorsque la plante est établie en grande quantité, ce qui, si l’envahissement s’intensifie, aura un effet néfaste sur la survie des têtards et mettra en péril la persistance des populations.
Invasive plants are considered one of the greatest threats to species, but their impact on amphibians is still poorly understood. The objective of this project is to determine the effect of the establishment of common reed (Phragmites australis) on amphibian distribution and larval development. It is thought that this plant monopolizes space and resources by producing a large biomass, and may alter wetland hydrology and amphibian community structure. I evaluated the factors influencing amphibian distribution according to the characteristics of ponds and the surrounding landscape in 50 sites invaded or not by reeds to varying degrees. Experiments were also conducted to study the impacts of three reed densities on wood frog tadpoles (Lithobates sylvaticus) and the quality of their habitat. My results suggest that high reed density slows wood frog larval development and influences phytoplankton assemblages. However, there is no relationship between, plant density and survival, tadpole morphology and water biotic and abiotic characteristic. In our study area, the landscape surrounding ponds has a greater influence on amphibian distribution than does reed establishment. However, the desiccation probability is higher when the plant is established in high quantities, which, if the invasion intensifies, will have an adverse effect on tadpole survival and therefore population persistence.

Les marais filtrants artificiels sont des écosystèmes recréés par l’homme dans le but d’optimiser l’épuration des eaux usées. Lors de la sélection d’espèces végétales pour la mise en place de ces marais filtrants, l’utilisation d’une polyculture ainsi que d’espèces indigènes non invasives est de plus en plus recommandée. Néanmoins, la plupart des marais filtrants existants sont des monocultures utilisant des plantes envahissantes, probablement à cause du manque d’évidences scientifiques sur les avantages de la diversité végétale et de la performance des espèces locales. Ainsi, les questions de recherche autour desquelles s’oriente ma thèse sont: Les polycultures présentent-elles un potentiel épuratoire aussi ou plus grand que les monocultures, et une espèce indigène est-elle aussi efficace et performante qu’une espèce exotique envahissante dans des marais filtrants ? Trois expériences ont été conduites afin de répondre à ces questions. J’ai d’abord testé l’influence de la richesse végétale sur l’élimination des polluants en deux dispositifs expérimentaux: 1) comparant deux espèces de plantes émergentes en monoculture ou combinées séquentiellement, et 2) évaluant la performance de quatre espèces flottantes plantées en monoculture par rapport à des associations de deux (avec toutes les combinaisons possibles) et de quatre espèces. Une troisième expérience a été réalisée afin de comparer l’efficacité épuratoire de l’haplotype européen envahissant du roseau commun (Phragmites australis) et de la sous-espèce locale non-invasive (P. australis subsp. americanus). La composition en espèces végétales a produit un effet notable sur la performance des marais filtrants. La comparaison des performances en mono- et en polyculture n’a pas permis de démontrer clairement les avantages de la diversité végétale pour l’élimination des polluants dans les marais filtrants. Toutefois, les marais filtrants plantés avec une combinaison d’espèces étaient aussi efficaces que les monocultures des espèces les plus performantes. La comparaison entre les deux sous-espèces de P. australis indiquent que la sous-espèce indigène pourrait remplacer le roseau exotique envahissant, évitant ainsi les potentiels risques environnementaux sans toutefois compromettre l’efficacité du traitement. Les résultats prometteurs de la sous-espèce indigène de P. australis doivent encore être testés dans des expériences à grande échelle avant d’utiliser largement cette espèce dans les marais filtrants. Nos résultats suggèrent que, dans des conditions où la performance des macrophytes disponibles est inconnue ou ne peut être déterminée, l’utilisation d’une combinaison d’espèces présente les meilleures chances d’accomplir le plus haut niveau possible d’élimination de polluants. De plus, même si la diversité végétale ne présente pas un avantage mesurable en termes d’efficacité épuratoire, celle-ci améliore la résilience des marais filtrants et leur résistance aux stress et aux maladies.
Treatment wetlands (TWs) are complex engineered ecosystems designed to remove pollutants from wastewater. In selecting plant species for TWs, the use of polycultures as well as the choice of local non-invasive species is increasingly recommended. However, to date, the majority of TWs have been planted in monoculture using invasive species, probably because the performance of native species and the possible advantages associated with plant diversity have not been clearly demonstrated. The research questions of my thesis are: Are polycultures as good as or better than monocultures for pollutant removal, and could a native species be as efficient as an exotic invasive species in TWs? Three experiments were performed to answer this question. The influence of plant richness on pollutant removal was tested in two mesocosm experiments: 1) comparing two emergent plant species in monoculture and planted in sequence, and 2) evaluating four free-floating species, each in monoculture versus pairs of species (all possible combinations) and the four species planted together. A third experiment was carried out to compare the removal efficiency of the invasive European haplotype of common reed (Phragmites australis) and the native non-invasive subspecies (P. australis subsp. americanus). Plant species composition influenced the performance of TWs. The comparison of the performance of monocultures versus polycultures showed that, overall, TWs planted with a combination of species were as efficient as monocultures of the best performing species. Comparing the performance of native and invasive P. australis strongly suggests that the native could replace the invasive species in TWs, thereby avoiding possible environmental risks without compromising treatment efficiency. The promising results of the native P. australis subspecies need to be further tested in full-scale experiments in order to encourage the use of this species in TWs. The results obtained comparing plant species combinations suggest that under circumstances in which the performance of available macrophytes is unknown or cannot be determined, using a mixture of species offers the best chances of achieving the highest possible level of pollutant removal. Furthermore, it has been suggested that plant diversity contributes to TWs in terms of their resilience to stress and disease.

The investigations reported in this thesis examined the relationships between biomass reduction (burning and mowing), soil disturbance, soil nutrient levels (N, P and K) and weed invasion in native grassland remnants in Victoria, Australia. The study was carried out at three native grassland remnant sites, Derrimut grassland reserve, Laverton grassland reserve and Victoria University of Technology, St. Albans campus grassland reserve on the westem edge of Melboume, Australia. These grassland remnants belong to the Western Basalt Plains grassland community which is listed as threatened in Victoria, and which forms part of the lowland temperate grasslands which once extended across south eastern Australia. A major threat to the survival of these communities is invasion by a variety of weeds. Weed invasion is often markedly enhanced by disturbance of the soil due to ploughing and overgrazing. It is known that some type of biomass reduction (burning, mowing or grazing) is required to prevent the dominant native perennial tussock grasses from over shadowing the diversity of forbs that grow between the tussocks, but the effects of various biomass reduction methods on weediness, and the underlying causes of weediness, have not been studied in any depth. The outcomes of the investigation are currently being applied to the management and rehabilitation of lowland grassland remnants. Most previous studies of lowland grassland rehabilitation and management have assumed that the key determinant of competition between native and exotic plant species in lowland grasslands is above-ground cover. The results reported in this thesis clearly show that below-ground competition for nutrients is also important in determining the extent of exotic and native plant growth in intact and disturbed areas. The successful establishment of native species during revegetation, with low levels of weed growth, will depend on the extent to which below ground growth of natives is achieved to lower available nutrient levels quickly, and prevent large scale growth of exotics. The management of lowland grassland remnants with low levels of flatweeds and annual grasses will likewise depend on the maintenance of a healthy sward of native vegetation which can keep available nutrients in the soil at low levels, especially when above-ground cover is removed by biomass reduction actions such as burning, mowing and grazing.

The exotic invasive Chilean needle grass Nassella. neesiana (Trin. & Rupr.) Barkworth has been recognised as a major threat to biodiversity in the endangered natural grasslands of south-eastern Australia. Research was undertaken in Canberra and Melbourne grasslands to examine mechanisms by which N. neesiana invades native grasslands, the impacts of invasion on vascular plant and invertebrate biodiversity and mechanisms of impact. Time series analysis of historical aerial photographs demonstrated that invasion was absent or very slow when infestations abutted healthy grassland, but grasslands in poor condition experienced linear infestation expansion rates >5 m per year. A field experiment demonstrated that major disturbance (death of the native grasses) was required for N. neesiana recruitment from panicle seeds and that intact grassland was not invaded, even with high propagule pressure. Gaps of c. 1 m (as opposed to 10-30 cm) were required for establishment. Pin transect sampling demonstrated that increased senescence of Themeda triandra swards correlated with greater invasion. Nassella neesiana was found to deplete soil water in spring compared to T. triandra, a likely mechanism of biodiversity impact. Areas occupied by N. neesiana in three disparate grasslands were found to have significantly reduced native vascular plant diversity (species m-2). Diversity decreased with increasing size of the N. neesiana patch. Forbs were the most affected group but one or more dominant grasses were absent at the smallest patch sizes. Exotic plant diversity was similar inside and outside patches. Analysis of sweep net samples determined that invertebrate populations and species richness were significantly reduced in N. neesiana grassland, although numerous native insect species consume the plant. Much of the loss of diversity in invaded areas probably precedes invasion and is caused by anthropogenic disturbances including T. triandra senescence dieback, mowing and major soil disturbance. Native grasslands in good condition are resistant to invasion.

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.