Дисертації з теми "Riparian tree"

In arid southern Africa, many rivers are ephemeral, and surface flow occurs for less than 10% of the year. Floods, which occur erratically, recharge groundwater resources on which riverine vegetation depends. Trees and shrubs are the most common life forms along these river systems, because surface flows are too erratic for aquatic organisms to flourish. There is concern that alterations to the current water regime of ephemeral rivers will affect the distribution of riparian vegetation as trees provide valuable fodder, shade, firewood and construction material in otherwise inhospitable surroundings. Climate change, population growth and development needs all place growing pressure on these water-limited ecosystems. Careful catchment management is essential to meet human water needs without jeopardising the ecosystems. This is easier said than done however, as ephemeral river ecology and functioning are hardly understood. Their unpredictable hydrology, strong groundwater association, often remote location and the presence of large terrestrial life forms make ephemeral rivers difficult to understand and current âEnvironmental water allocationâ methods for wetter systems are unsuitable. Alternative methods are being developed, but to date they still have a strong emphasis on aquatic life forms and riparian vegetation is still insufficiently addressed. This study provides an ecophysiological perspective on the water sources and drought tolerance levels of four key riparian tree species in Southern Africa. The aim is to contribute to an improved understanding of the interrelationships between river flow, groundwater and use of water by trees. Stable Hydrogen and Oxygen isotope results show that the three key species along the Kuiseb River (Acacia erioloba, Faidherbia albida and Tamarix usneoides) do not take up any water from the regularly occurring fog events. Instead, they depend on a seasonally fluctuating mix of shallow and deep soil moisture as well as groundwater. All these water sources rely on regular recharge from floods. It suggests that reductions in flood frequency and especially in magnitude and duration will reduce groundwater recharge and affect species productivity and survival. It was investigated why A. erioloba, growing in the presence of the alien Prosopis glandulosa, show high mortality rates. The study used stable 2H, 18O and 13C isotopes, xylem pressure potentials and percentage canopy dieback to conclude that within the riparian zone, the two species depend on the same water sources and that indigenous A. erioloba are 2 significantly more water stressed when growing alongside P. glandulosa. Further inland A. erioloba is entirely groundwater dependent while P. glandulosa seasonally switches between water sources. This resource partitioning is advantageous for A. erioloba, which does not display the same levels of water stress as in the riparian zone. An investigation into the physiological structure, including wood density, xylem vessel diameters and xylem vulnerability to cavitation for all four species showed that A. erioloba was structurally the most drought tolerant. This finding corresponded well with its high survival rate in the 1980s Namib drought. In the Kalahari, however, it is still outcompeted by P. glandulosa, suggesting that the invasive species has a competitive edge over A. erioloba that goes beyond xylem structure. P. glandulosa and T. usneoides are similar in structure. F. albida is structurally the most vulnerable of the species, supporting the high mortality rates observed in the 1980âs. Water sourcing using stable Hydrogen and Oxygen isotopes, xylem structure and phenology were compared for three A. erioloba stands with access to different groundwater depths (4 m, 21 m and 56 m). The aim was to determine whether A. erioloba physiology changed proportionally to groundwater depth. Results show that A. erioloba structure and phenology do respond proportionally to groundwater depth, suggesting that A. erioloba can reach deep groundwater, but that this comes at a cost of increased water stress and reduced vitality. Ttrees depending on deep groundwater may thus be closer to a water-induced survival threshold than those growing with access to shallower groundwater. Catchment managers should carefully weigh up the benefits of new water developments relative to potential losses of this ecologically and economically important species. As the final objective and synthesis this study aimed to evaluate if any of the studied species could be used as a bioindicator for tree health in relation to water availability. T. usneoides was previously observed to be less tolerant to drought conditions than F. albida and A. erioloba, while this study has shown F. albida to be structurally the most vulnerable. Hence no bioindicator could be indentified and it is concluded that the determination of drought tolerance and the identification of one or more bioindicator species for future monitoring is a complex matter that needs to include more structural studies and field documentation during drought. 3 Ultimately the use of indicator species and investigations into water sourcing as well as drought tolerance studies should inform catchment management and be included in future assessments of environmental water requirements (EWR) methods and river health. This study concludes that several of the methods applied in this study (shoot growth measurements, percentage canopy dieback, wood density, δ2H, δ18O and δ13C stable isotopes, xylem pressure potentials and xylem vessel diameters) are worth applying in in a EWR in conjunction with groundwater and flood (volume, duration and height) measurements. This will require a shift from a traditionally aquatic outlook to one that is more inclusive of terrestrial ecology. More interdisciplinary cooperation and lateral thinking between aquatic and terrestrial ecologists is crucial, so that the frequency, magnitude, predictability and duration of floods and associated groundwater recharge are assessed in terms of water needs of woody riparian species as well as the associated terrestrial fauna.

A general paradigm in semi-arid and arid systems is that woody plants with dimorphic root systems will exhibit preferential use of deeper soil water because it represents a more stable source of water than short duration pulses of shallow soil moisture derived from summer rainfall. However, whether this holds across all woody species and whether use of deeper soil water interacts with use of shallow soil water is not determined for many species in different ecosystems. Understanding the amount of water plants derive from groundwater and shallow soil water is critically important to accurate calculations of local and regional water balance. The focus of this research was to determine if dominant woody species in semi-arid riparian ecosystems used shallow soil water and how depth to groundwater and defoliation might affect root proliferation and water uptake. This research found that the functional grouping "phreatophytes" encompasses a variety of responses to environmental variation. Stable isotopic analyses determined that Prosopis velutina Woot. (Velvet mesquite) and Populus fremontii Wats. (Fremont cottonwood) used shallow soil water derived from summer rainfall, and the proportion of shallow soil water was higher at sites with greater depth to groundwater. In contrast Salix gooddingii Ball (Goodding willow) did not use shallow soil water at any location regardless of depth to groundwater. Field experiments using defoliation treatments, to limit carbon assimilation and reduce plant photosynthate pools, confirmed that Prosopis velutina exhibited flexible response in water uptake patterns in response to defoliation. Defoliation, which presumably reduced available photosynthate, increased the reliance of this species on shallow soil water; contrary to predictions that woody species should maintain extensive deep root systems to buffer themselves from seasonal drought. Greenhouse experiments with Populus fremontii and Prosopis velutina also indicated changes in belowground biomass of fine roots, which were associated with changes in water-source use for Populus fremontii, but not for Prosopis velutina. These results imply that in terms of predicting plant response to changes in future climates, or modeling fluxes of water from the soil to the atmosphere that are largely controlled by plant transpiration, intra- and interspecific variability will need to be considered.

Includes abstract.
Includes bibliographical references (leaves 113-134).
Riparian zones are dynamic, as a result of varying levels of disturbance from natural flooding regimes, and this makes them particularly susceptible habitats to invasion by alien plants. In South Africa, particularly the Fynbos Biome, closed-stand invasions by alien Acacia and Eucalyptus species have been able to develop within riparian areas. Their impacts on water resources and biodiversity have been countered by manual clearing in order to protect the valuable ecosystem services provided by intact riparian zones, as well as the biodiversity of indigenous communities. The Working for Water programme is tasked with the important role of controlling invasive alien plants with an assumption that indigenous vegetation will recover naturally. Current management objectives are to reduce above ground biomass of invasive alien plants by labour intensive means, after which indigenous vegetation is usually left to recover without further intervention. However, it is unclear to what extent natural recovery can be achieved. The main aims of this study were to ascertain the nature of riparian vegetation recovery, as well as determine which clearing treatment was most successful in promoting recovery. This was achieved by focusing on: 1) the recovery of species composition and biodiversity, 2) recovery of vegetation structure (assumed to be a surrogate for ecosystem function) and 3) whether a particular clearing treatment best promoted indigenous riparian vegetation recovery. Reference sites (control), as determined by Prins et aI., (2004), were compared to alien impacted sites in order to analyse variation among vegetation variables. Three initial clearing treatments were identified, namely: Fell Only (trees are felled and slash left on site), Fell & Remove (slash is removed from the riparian zone) and Fell & Bum (the slash is left for six months to a year before it is burnt).

Tree-ring formation of eastern white-cedar (Thuja occidentalis L.) at a boreal lake in northwestern Quebec, Canada, was monitored using manual band dendrometers to (i) retrace cambial activity phases, (ii) evaluate the effects of flooding on radial growth, and (iii) analyze the relationships with meteorological factors. The daily circumferential activity of four trees at each of two sites, a riparian and an upland site, was recorded during the growing season of 1996, a year with an extreme spring flood. First cambium cell divisions occurred near June 9, followed by a distinct and sustained upward trend in the stem basal area until mid-July that reflected the earlywood formation. The strongly synchronous circumferential activity at both sites suggests no adverse flooding effect on growth of the riparian trees, which is explained by the rapid retreat of the water just before growth initiation in early June. The following month until mid-August was characterized by strong short-term fluctuations caused by alternating drought and rain periods and a slight downward trend of the basal area for six of the eight banded white-cedars. The dendrometers of two trees, the closest to the lake, showed a slight upward trend probably reflecting latewood formation. Pearson correlation with meteorological data indicated that precipitation was positively related to the daily changes in basal area of all trees except during the period of earlywood formation, which probably resulted from the high soil moisture after spring snow-melting. Mean and minimum air humidity were positively related and maximum temperature negatively related to the daily variations in stem circumference during the whole monitoring period, emphasizing the importance of the internal water status on stem size.

The research conducted within this thesis aims to advance our understanding of plant-water interactions, specifically the identification of the source(s) of water available for tree growth within riparian zones. Through highly resolved tree ring isotopic analyses, it is possible to deconvolve the δ18O signature of water utilised by a tree during the growing season and relate this to dynamic hydroclimatic processes. In doing so, this methodology allows for the reconstruction of the seasonal evolution of tree source water availability for the historical period. The utility of this method is initially demonstrated for the determination of the isotopic signature of water utilised by individuals of co-occuring riparian tree species over ten consecutive years. It is found that species-specific seasonal source water variability is indicative of tree rooting depth and the hydroclimatic partitioning of water to each of the floodplain hydrological reservoirs. The technique is then developed in to a methodological 'toolkit', comprised of tool groups representing differing levels of data availability. The aim is to enable continued progress in this research area, even under conditions of sub-optimal data availability, whilst also making the suite of methods accessible to other researchers and resource managers. Finally, the hydroclimatic expression of seasonal tree source water avaialbility is characterised across a climate gradient utilising over 800 δ18O cellulose measurements. The results show that the seasonal progression of tree water availability responds to near- and far-field hydroclimatic processes, but this is mediated by species' functional type and site lithology. Where near-field processes dominate water availability, there is a potential for increased vulnerability to localised drought conditions and species-specific competition for water resources. Retrodictions of seasonal water source use may prove useful in forest restoration and rehabilitation efforts, assessment of forest vulnerability to future hydroclimatic regimes and for improving agroforestry and sustainable water management practices.

Though land conversion and flow alteration have heavily impacted the Sacramento River riparian ecosystem, restoration opportunities still exist in the hydrogeomorphically active Middle Reach. This study of riparian forest succession focused on six dominant riparian tree species to explore relative establishment timing and the potential impacts of altered flow regimes. We utilized tree inventory data and increment cores collected from riparian forest stands to establish a temporal chronosequence of floodplain surfaces and associated tree ages and colonization timing. Tree age calculations incorporated raw ring counts and sampling error simulations. Results were then used to construct species-specific, diameterage models and predict age distributions for all inventoried trees. Cottonwood’s colonization window was longer than expected (up to 95 years after floodplain creation), whereas box elder and walnuts established on floodplains <50 years old. This study lays the groundwork for future research into the health and development of the Middle Reach riparian forest.

Riparian corridors are often farmed or grazed because they are typically productive and offer a low cost water supply for livestock. These areas are also critical in terms of water quality, soil retention, and aquatic and terrestrial wildlife habitat. Conservation of riparian areas can be conflicting due to loss of productive land for other uses. Agroforestry riparian buffers planted with native fruit and nut trees could help integrate conservation and production in this valuable space by giving landowners the opportunity to provide beneficial environmental services, while enhancing their opportunities for revenue. In order to realize potential use of this type of system, it is important to understand how to communicate benefits and improve prediction of adoption. With this in mind, we conducted a mail survey of creek side owners within three Virginia subwatersheds to 1) create typologies of underlying groups of landowners in order to develop relevant communication strategies for native fruit and nut tree ripairan buffers and 2) test the Unified Theory of Acceptance and Use of Technology (UTAUT) singularly and with additional agroforestry concepts to predict intention to adopt native fruit and nut tree ripairan buffers. Three landowner types were identified within the larger population of creek owners and exhibit differences that merit tailored communication strategies. Both models predict a noteworthy amount of intention to plant native fruit and nut tree riparian buffers. However, when agroforestry measures were added they explained a notable portion of variance.
Master of Science

Within the active zone of rivers, riparian vegetation dynamics are controlled by ecological and hydro geomorphological processes and their interactions. Within Europe, understanding of natural riparian tree establishment has been limited by a lack of unregulated, natural river systems. This thesis examines key biotic and abiotic factors influencing the establishment of three riparian tree species, Populus nigra, Salix elaeagnos and Alnus incana, that grow within the active zone of the River Tagliamento, a morphologically-intact alpine/mediterranean river in Northeast Italy. Field observation, field experimentation and greenhouse experiments indicate: (i) Interspecific and propagule form variations in survival and growth response to hydrogeomorphic conditions: P. nigra cuttings performed best in low free-draining sites with a small proportion of fine sediment but which were not subject to prolonged inundation, while seedlings preferred fine sediments and higher elevations. Salix elaeagnos cuttings performed best at lower elevations, with a preference for mixed sediments, and proved tolerant to drought and inundation. S. elaeagnos seedlings grew best at higher elevations and in coarse/mixed substrates. Alnus incana cuttings only grew in fine sediments under stable water levels in greenhouse experiments; mortality was total in field experiments. Deposited trees of an species survived better at lower elevations. (ii) Differences in growth rates according to propagule type: cuttings grew twice as fast as seedlings in the first year but not in the second, and whole deposited trees produced far higher levels of shoot growth and biomass. (iii) Timing of deposition in the growing season and size of propagule also significantly affected performance. These results suggest that species and propagule form directly influence the rate of vegetation establishment within the active zone of rivers and that plant physiology alongside hydrogeomorphology is critical to riparian vegetation dynamics. These results are explored in the context of the development of river islands, an endangered but ecologically important type of landform once common within natural, braided European rivers.

The ecological value and functionality of riparian systems along ephemeral, intermittent, and perennial streams in the Southwest is well established. In Pinal County, Arizona the existing datasets available to environmental managers and governing bodies drastically underestimate the extent and presence of riparian zones. This study addresses the issue through the use of remote sensing land cover classification techniques. Landsat 8 data, topographic data, and high-resolution color infrared (CIR) imagery, and several derived vegetation indices are used to construct a classification and regression tree (CART) model. Using training data, the CART model is used for the identification and delineation of basic land cover classes across the County. Woody annual and perennial species are identified and associated to riparian zones using a valley bottom model (VBM) developed by the United States Department of Agriculture. The CART model (kappa value of 0.76) found that 929 square-miles of annual vegetation and 651 square-miles of perennial vegetation are present across Pinal County. Annual and perennial vegetation classifications are assessed for density using a 0.33 acre moving window. The density values for both classes are then used in conjunction to differentiate upland, xeroriparian, mesoriparian, and hydropriarian vegetation zones. Vegetation zones are clipped to regions where the VBM identifies valley bottom probability to be 62 percent or greater. The results generated provide a sufficiently comprehensive dataset that gives County managers and environmental professionals improved insight into the presence and distribution of important riparian habitats.

Stretches of forest along the Limpopo and Shashe Rivers have been classified as a unique forest type in the vegetation of South Africa and are considered as being "critically endangered" by the South African Biodiversity Institute. Roughly 400 hectares of this riverine forest area inside the western section of the Mapungubwe National Park (MNP), a UNESCO World Heritage site, were deforested and therefore degraded due to previous agricultural cultivation practices. Given the extent of forest degradation that has occurred, the restoration of this area by means of the re–vegetation of indigenous trees to its former composition is one of the objectives of the MNP's management plan. The successful establishment of tree seedlings, especially in semi–arid systems, is however presented with a wide range of constraints and limiting conditions, which often result in very high mortality rates during restoration projects. An experimental exclosure, as identified by South African National Parks (SANParks), was therefore fenced off inside the degraded old lands to act as a demonstration site for the restoration of indigenous trees. A pilot study conducted in 2006, involved the transplantation of selected indigenous tree species with the aim of evaluating suitable re–vegetation technologies. The research contained in this dissertation was also conducted inside the experimental exclosure, where recommendations derived from the pilot study were evaluated, including the assessment of new re–vegetation technologies to enhance the establishment of the indigenous trees. This study was therefore a follow–up project which involved both field– and greenhouse trials. Seedlings of the following species were either transplanted into the experimental exclosure (field trial) or cultivated inside a controlled environment in the greenhouse at the North–West University: Acacia xanthophloea Benth. (fever tree), Berchemia discolor (Klotzsch) Hemsl. (brown–ivory), Combretum imberbe Wawra (leadwood), Faidherbia albida (Delile) A. Chev. (ana tree), Philenoptera violacea (Klotzsch) Schrire (apple–leaf), Salvadora australis Schweick. (narrow–leaved mustard tree) and Xanthocercis zambesiaca (Baker) Dumaz–le–Grand (nyala tree). During the follow–up study the effects of various enhancement treatments were tested regarding the survival, growth and physiological performance of seedlings in both the field– and greenhouse trials. The enhancement treatments consisted of the addition of compost and indigenous arbuscular mycorrhizal fungi (AMF). In addition, seedlings transplanted during the pilot study, which did not include enhancement treatments, were also monitored for establishment and growth. The potential use of established Acacia tortilis Hayne trees to facilitate growth and establishment and to act as "nursing plants", was also assessed. In addition, various pre–sowing treatments were also applied to seeds of selected tree species in the greenhouse to assess the germination rate. The survivorship and growth of seedlings in both the field– and greenhouse trial were determined by using three growth parameters, namely "stem diameter at the base", "stem diameter 30 cm from the base", and "height of the tree in its natural growth form". Chlorophyll fluorescence induction (JIP test) was measured on seedlings in both trials, using the multi–parametric expression, namely performance index (PIABS), as a measure of the overall vitality of the plants of each species–treatment combination. Physical and chemical analyses were carried out on the soil inside the experimental exclosure. Basic descriptive statistics were used to analyse seedling survival and germination rates, and a two–way analysis of variance (ANOVA) was used to determine the statistical significant effects of the various enhancement treatments on diameter growth in each species (p < 0.05). Fluorescence data were processed using the Biolyzer software and significant effects in each species were determined using the Student's t–test (p < 0.05). Multivariate data ordinations using the CANOCO package were used to determine the differences in soil types inside the experimental exclosure. Moisture stress due to transplantation shock, competition with dense grass cover and herbivory, resulted in an overall 55.8% seedling survival rate and negative stem diameter growth for transplanted seedlings in the field. In comparison, seedlings cultivated in the greenhouse had much higher survival rates and showed positive stem diameter growth. Most species in the greenhouse showed higher growth rates and significantly higher vitality values when planted with enhancement treatments. The responses of transplanted seedlings to the enhancement treatments were very species–specific in the field trials. Based on these results, it was concluded that the enhancement treatments were beneficial with regard to the establishment and growth of most of the species. The beneficial effect was however cancelled out by the various abiotic and biotic factors encountered in the natural environment. Seedlings transplanted in the understory of established pioneer A. tortilis trees had much lower survival rates as the extensive root system of A. tortilis most likely out–competed the transplanted seedlings for moisture and nutrients. Many seedlings were also predated by insects or small mammals which reduced the growing potential. The germination trials recorded the highest germination rates for most species when germinated in the compost–containing treatments. These trials also indicated that all of the investigated species showed higher survival rates when pre–sowing treatments, such as soaking, mechanical scarification and removing the seed from fruit, were applied. Various recommendations emphasising long–term monitoring, proper maintenance and after–care of future restoration efforts are made. These include experimental layout of exclosure plots and pretransplantation treatments of seedlings while cultivated in the nursery. During this study, the experimental exclosure was also used as a demonstration site for training and capacity building for SANParks personnel and students from academic institutions.
Thesis (M.Sc. (Environmental Science))--North-West University, Potchefstroom Campus, 2011.

The process of environmental degradation that has been occurred in the lower region of Sao Francisco s river is reducing the permanent preservation areas. The artificial regeneration is a method of riparian restoration widely used to reconstitute the native vegetation in degraded areas. For the reasons set was a work of restoration in an area of riparian forest in Santana do São Francisco s county Sergipe, with the objective of proposing models to restore the natural balance of the environment. The studied area after tested was monitored for 60 months, using the experimental design in randomized blocks in three models restoration the riparian forest (seedlings growth of forest species in spacing 3x1,5m and 3x3m and direct seeding in spacing 3x1,5m) and twelve native forest species. There were evaluated seedling emergence, surviving and development of species tested (height, base diameter e the relative growth rate). Was also performed to evaluated phenological aspects of species tested and identified the presence of colonizing species in the studied area. The results after 60 months old of the implanted species indicated a higher percentage of surviving of species in the model with growth in spacing 3x1,5m (62,2%). It was observed that the spacement used not able to influence the development of species. To TCR (relative growth rate) was identified as the pioneer species Schinus terebinthifolius (Raddi.) had the highest rates of growth in height in three models implanted. With respect to the diameter base was observed that the species that showed the highest growth rates were Vitex polygama (Cham.), Schinus terebinthifolius (Raddi.) e Cassia grandis (L.f). Considering the phenological aspects were observed that five species (40%) showed flowering and fruiting during the 60 months of evaluation. The species studied showed up in the potential restoration of riparian forests and may be the recommended direct seeding method to reduce the costs of deployment in the lower region of Sao Francisco s river.
O processo de degradação ambiental que vem ocorrendo na região do Baixo Rio São Francisco vem reduzindo as áreas de preservação permanente. A regeneração artificial é um método de restauração ciliar muito utilizado para recompor a vegetação nativa em áreas degradadas. Pelos motivos apresentados foi realizado um trabalho de restauração em uma área de mata ciliar no município de Santana do São Francisco SE, com o objetivo de propor modelos de restabelecimento do equilíbrio natural do ambiente. A área de estudo, após implantada foi monitorada por 60 meses, sendo utilizado o delineamento experimental em blocos casualizados em três modelos de restauração da mata ciliar (plantio de mudas de espécies florestais em espaçamentos 3x1,5m e 3x3m e semeadura direta em espaçamento 3x1,5m) e doze espécies florestais nativas. Foram feitas avaliações de emergência de plântulas, sobrevivência e desenvolvimento das espécies implantadas (altura, diâmetro do colo e taxa de crescimento relativo). Também foi realizada a avaliação dos aspectos fenológicos das espécies implantadas e identificada a presença de espécies colonizadoras na área de estudo. Os resultados obtidos após 60 meses de idade das espécies implantadas indicaram um maior percentual de sobrevivência no modelo com plantio de mudas no espaçamento 3x1,5m (62,2%). Observou-se que os espaçamentos utilizados não exerceram influência no desenvolvimento das espécies. Para a TCR (taxa de crescimento relativo) foi identificado que a espécie pioneira Schinus terebinthifolius (Raddi.) apresentou as maiores taxas de crescimento em altura nos três modelos implantados. Com relação ao diâmetro do colo observou-se que as espécies que apresentaram as maiores taxas de crescimento foram Vitex polygama (Cham.), Schinus terebinthifolius (Raddi.) e Cassia grandis (L.f). Considerando-se os aspectos fenológicos foram observados que cinco espécies (40%) apresentaram floração e frutificação durante os 60 meses de avaliação. As espécies estudadas apresentaram-se potenciais no processo de restauração de matas ciliares, podendo ser recomendado o método de semeadura direta para reduzir os custos da implantação na Região do Baixo Rio São Francisco.

Airborne lidar (light detecting and ranging) is a useful tool for probing the structure of forest canopies. Such information is not readily available from other remote sensing methods and is essential for modern forest inventories. In this study, small-footprint lidar data were used to estimate biophysical properties of young, mature, and old cottonwood trees in the Upper San Pedro River Basin, Arizona, USA. The lidar data were acquired in June 2003 and 2004, using Optech's 1233 ALTM (Optech Incorporated, Toronto, Canada). Canopy height, crown diameter, stem diameter at breast height (dbh), canopy cover, and mean intensity of return laser pulses from the canopy surface are estimated for the cottonwood trees from lidar data. The lidar estimates show a good degree of correlation with ground-based measurements. This study also demonstrates that other parameters of young, mature, and old cottonwood trees such as height and canopy cover, when derived from lidar, are significantly different (p < 0.05). These lidar-derived canopy metrics provided the basis for a supervised image classification of cottonwood age categories, using a maximum likelihood algorithm. The results of classification illustrate the potential of airborne lidar data to differentiate age classes of cottonwood trees for riparian areas quickly and quantitatively.In addition, four metrics (tree height, height of median energy, ground return ratio, and canopy return ratio) were derived by synthetically constructing a large footprint lidar waveform from small-footprint lidar data (we summed up a series of Gaussian pulses that vertically stacked at the elevations produced by the small-footprint elevation data to create a modeled large-footprint return waveform and compared the synthetic waveforms with ground-based Intelligent Laser Ranging and Imaging System (ILRIS) scanner images in cottonwood trees). These four metrics were incorporated into a stepwise regression procedure to predict field-derived LAI for different age classes of cottonwoods.Additionally, this study applied the Penman-Monteith model to estimate transpiration of the cottonwood clusters using lidar-derived canopy metrics, such as height and LAI, and compared it with transpiration measured by sap flow, so that improved riparian water use estimates could be made.

Thesis (Ph. D.)--School of Geosciences, Faculty of Science, University of Sydney, 2004.
Degree awarded 2004; thesis submitted 2003. Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Geosciences, Faculty of Science. Title from title screen (viewed 13 January 2009). Includes bibliographical references. Also available in print form.

Magister Scientiae (Biodiversity and Conservation Biology) - MSc (Biodiv and Cons Biol)
Heavy metals and nutrients have long been regarded as pollutants to freshwater ecosystems. These elements have a detrimental effect on plants, animals and the water quality of rivers in South Africa. The Berg River flows from the mountains of Franschhoek to the West Coast of the Western Cape. It is an important river in Cape Town, as it is essential for water distribution to town, for agriculture and industry and also supports a rich diversity of organisms in the ecosystem. Along the river, many farms and towns are situated and many tributaries enter the river. The Berg River dam provides for a water supply during the drier periods of the year. Therefore it is crucial to maintain a good water quality. The study was driven by the need to increase the knowledge of water quality in the upper Berg River after the construction of a new major Berg River dam, constructed in 2007. This study investigated oxygen, water temperature, electrical conductivity, pH, ammonium, nitrate, nitrite in the water and cadmium, copper, lead, iron, zinc, potassium, sodium, calcium, magnesium and phosphorus found in water, sediment and three plant species at ten sites along the upper Berg River, Western Cape. The results showed that the electrical conductivity, pH and the concentrations of nitrate, calcium and magnesium increased downstream, whereas the water temperature decreased downstream. Nitrate, cadmium, copper, potassium, sodium, calcium and magnesium displayed a general increase towards the colder period in the water. Seasonally, copper and magnesium showed significant winter increase within the sediment. Nitrogen, iron and calcium levels within Salix sp., Acacia mearnsii and Brabejum stellatifolium increased downstream. Nitrogen, cadmium, copper, potassium, calcium, magnesium and phosphorus in the three species were higher in the warmer seasons and decreased in the colder. Sources of pollution stem from the Franschhoek and Dwars tributaries, urban and farm runoff.

Ephemeral rivers are a particular type of dryland river which only flow during and immediately after rain events. Despite their important social and ecological roles in drylands we know very little about the ecosystems or their sensitivity to human activities. This PhD investigates drivers of riparian tree distribution, recruitment and dieback along a dammed ephemeral river; and also investigates wider changes that have occurred to the riparian ecosystem and stakeholders’ perceptions of these changes. A variety of methods are used such as ecological and social surveys, remote sensing and archival searches. The study system is the Swakop River in Namibia; the largest catchment of the country’s westerly flowing ephemeral rivers. In the 1970s two large dams were installed in the upper catchment of the river to supply water to central Namibia, including the capital city of Windhoek. In order of abundance, the riparian woody vegetation community of the Swakop River is composed of Prosopis species (an invasive species introduced in the 1900s), Tamarix usneoides, Faidherbia albida, Salvadora persica, Vachellia erioloba, Euclea pseudebenus and Vachellia tortilis. Although no net change in woodland extent is observed, the woodlands decreased in density by 18%. This reduction in density is consistent with the considerable mortality observed in the riparian trees: 51% of all F. albida, 29% of V. tortilis, 26% of Prosopis spp., 25% of E. pseudebenus and 23% of V. erioloba. The river’s longitudinal profile from the interior of Namibia towards the coast is associated with an increasingly drier climate. This climatic gradient is the most consistently implicated driver of riparian tree occurrence, recruitment and dieback. This study also indicates that tributaries may play an important role in tree population dynamics along ephemeral rivers and may help mitigate the negative effects of damming.

The primary objectives of this study were to address how stream temperature is influenced by (1) spatial variability in energy exchanges, (2) reach-scale stream-subsurface water interactions and (3) the net radiation dynamics associated with standing dead riparian vegetation. Stream temperature, riparian microclimate, and hydrology were characterized for a 1.5 km reach of Fishtrap Creek, located north of Kamloops, British Columbia. Within-reach air temperature and humidity variability was small, while wind speed, net radiation and surface-subsurface interactions exhibited considerable spatially variability. The field data were used to drive a deterministic energy budget model to predict stream temperature. The model was evaluated against measured stream temperature and performed well. The model indicated that the spatially complex hydrology was a significant control on the observed stream temperature patterns. A modelling exercise using three canopy cover scenarios revealed that post-disturbance standing dead trees reduce daytime net radiation reaching the stream surface by one third compared to complete vegetation removal. However, standing dead trees doubled daytime net radiation reaching the stream compared to pre-wildfire conditions. The results of this study have highlighted the need to account for the spatial variability of energy exchange processes, specifically net radiation and surface-subsurface water interactions, when understanding and predicting stream thermal regimes.

Managed forest landscapes in Sweden are dominated by boreal coniferous forest and are deficient in broad-leaved trees. Both beavers (Castor fiber) and ungulate browsers prefer broad-leaved trees to conifers, and the interaction between multiple herbivores may have a considerable impact on the recruitment of deciduous trees in riparian areas. To evaluate the relative importance of beavers and browsers on tree recruitment, I conducted an inventory of broad-leaved tree species at five beaver sites in Färnebofjärden National Park, Sweden. At each site, five transects were established perpendicular to the shoreline in the riparian zone, resulting in a grid of 35 sample plots at each beaver site. Foraging activity of both beavers and browsers (e.g. ungulates) was concentrated to a narrow band in the riparian zone and there was a decrease in the proportion of beaver-cut and browsed trees further away from the shoreline. Beavers preferred birch (Betula spp.) and grey willow (Salix cinerea) while browsers preferred oak (Quercus robur). Beavers targeted larger trees while browsers targeted seedlings <1 cm in diameter. Old-growth trees of high conservation value, such as aspen (Populus tremula) and birch, are at risk to be cut by beaver, yet long-term protection of broad-leaved stands requires a shift in focus towards strategies that enhance tree recruitment by considering and mitigating pressure from multiple herbivores.
Svenska skogslandskap domineras av boreal barrskog med en brist på lövträd. Både bäver (Castor fiber) och betande klövvilt föredrar lövträd framför barrträd, och interaktionen mellan dessa herbivorer kan ha en betydande påverkan på föryngring av lövträd i strandnära skogar. För att utvärdera den relativa betydelsen av bävrar och betare på trädföryngring så genomförde jag en inventering av lövträdsarter och betestryck vid fem bäverlokaler i Färnebofjärden Nationalpark, Sverige. Vid varje bäverlokal lades fem transekter i rät vinkel mot strandkanten i den strandnära zonen, med provrutor var femte meter längs varje transekt, vilket resulterade i ett nätverk av 35 provrutor vid varje lokal. Både bävrars och betares (dvs. klövvilts) födosökande var begränsat till ett smalt band längs strandkanten, och andelen bävergnagda och betade trädstammar minskade med ökat avstånd från strandkanten. Bäver föredrog björk och gråvide medan betare föredrog ek. Bäver föredrog större träd medan betare föredrog groddplantor <1 cm i diameter. Gamla lövträd som är viktiga att bevara, såsom asp och björk, hotas av bäveraktivitet, men långsiktigt bevarande av lövträdsbestånd kräver ett fokus på strategier som främjar trädföryngring, bland annat genom att beakta och begränsa den sammantagna påverkan av olika herbivorers betning.

The deforestation of riparian areas is a major concern in southern Africa. These areas are characterized as fragile ecosystems which contribute largely to the regional and global biodiversity of the world. It is therefore important to restore these degraded areas along the natural rivers of South Africa to ensure the sustainability and biodiversity of riparian corridors. Riparian areas inside the National Parks of South Africa, and especially in Mapungubwe National Park, have a high esthetical value and should be preserved for future generations. The study was conducted in the Mapungubwe National Park, which is listed as a cultural world heritage site. Plans are in place to convert it into one of Africa's biggest Transfrontier Parks, called the Limpopo/Shashe Transfrontier Conservation Area (TFCA), which will be situated between neighbouring countries Zimbabwe, Botswana and South Africa. The main purpose of this project was to establish a demonstration site for the restoration of degraded, previously cultivated lands in the deforested riparian areas in the Mapungubwe National Park, Limpopo Province. Another aim of the project was to evaluate the theoretical assumption that the growth of trees on so called "activity lines" in the environment due to geological and soil characteristics is enhanced. "Activity lines" were identified by Mr. Lynn van Rooyen of South African National Parks (SANParks) and trees of which the growth was tested, were planted both on and off "activity lines". The selection of the right type of trees for the restoration of the deforested riparian areas during active restoration applications is very important and depends on a multitude of factors. These factors include aspects such as the location with its specific vegetation, soil type and climatic conditions, the historical background of the management practices such as previous land uses, as well as other environmental impacts that previously occurred in the area to be restored. The latter can be gained through interviews with previous and present managers of the area, as well as maps, reports and aerial photographs. Ten different indigenous tree species that previously occurred in the area were planted in an experimental demonstration site of approximately 70ha, which was enclosed by an electrical game fence. The ten tree species that were evaluated included: Faidherbia albida (Ana tree), Acacia nigrescens (Knob thorn), Acacia tortilis (Umbrella thorn), Schotia brachypetala (Weeping boer-bean), Acacia xanthophloea (Fever tree), Lonchocarpus capassa, recently renamed Philenoptera violacea (Apple-leaf), Salvadora australis (Narrow-leaved mustard tree), Adansonia digitata (Baobab), Combretum imberbe (Leadwood) and Xanthocercis zambesiaca (Nyala tree). With the aid of aerial photographs, phytosociological studies, interviews with previous and present land users and managers, as well as existing surrounding vegetation, four different zones within the enclosure were identified according to ecotones. The establishment, growth and survival rate of the different tree species were monitored using morphological and physiological vegetation sampling techniques, as well as leaf component analyses on individuals of selected species. Soil physical and chemical analyses were carried out in the four different blocks identified within the experimental site. Data analysis was carried out on both the soil and leaf component analyses using the CANOCO-package. The establishment of the experimental site was successful, and important information was collected on various aspects of restoration activities. Positive growth effects were also observed in certain indigenous tree species concerning the "activity line" effect, especially with regard to Acacia tortilis and Combretum imberbe. However, the preliminary results obtained through this pilot study showed no conclusive evidence to what exactly stimulated the enhanced growth phenomena observed in certain individual tree species planted on "activity lines". Additional watering was identified as the most important factor contributing to successful establishment and growth of indigenous tree species in this semi-arid area. Various results showed a multiplying effect when a combination of additional watering and "activity lines" was applied. It was concluded that, should any further restoration work be conducted in the degraded areas of the Mapungubwe National Park, the planting of trees should be done on "activity lines" and with the addition of water. This will result in higher establishment rates of transplanted trees and speed up the succession processes involved in the natural "healing process" of degraded areas. Parameters that should be used for monitoring tree growth include the trunk thickness at the base, trunk thickness at 30cm from the base, and the length of the tree in its natural growth form. Recommendations were also made as reference for future restoration practices to ensure better and more successful and sustainable outcomes in the planting of trees. These include the use of nurse plants such as Acacia tortilis and Salvadora australis to establish a more favourable microclimate for climax species, as well as the establishment of a preferred herbaceous layer. Care should be taken in the period required for the cultivation of indigenous trees before they are transplanted into the field, as a prolonged cultivation period could lead to a circular growth form of the root system, preventing sufficient penetration ability of the roots into deeper, more nutrient rich soils. Before trees can be planted into the field, a hardening period must be applied to all seedlings for at least a three week period. This entails the exposure to more direct sunlight for longer periods as well as a reduction in the water applied weekly. Special attention should be paid to the stresses caused by herbivory, especially that of termites and porcupines. The maintenance of the exclosure is a critical factor contributing to the successful outcomes of this particular restoration project. Problem animals, especially elephants, should be kept out of the exclosure at all costs. The results of this project can be used in this ongoing restoration program, as well as in other related projects in semi-arid, degraded savannah areas over the long-term.
Thesis (M.Sc. (Botany))--North-West University, Potchefstroom Campus, 2008.

Thesis (PhD)--Stellenbosch University, 2012.
ENGLISH ABSTRACT: Invasive alien species are widely considered to be the second most significant threat to biodiversity globally following direct habitat destruction. The invasion of riparian systems worldwide by alien plants has contributed to profound changes in biodiversity and ecosystem functioning. In South Africa, river banks and river beds are amongst the most severely invaded landscapes, with the most damaging invaders, especially in the Fynbos Biome, being trees and shrubs of the Australian genera Acacia and Eucalyptus. Although large-scale management operations are underway to clear invasive trees and restore ecosystems, little is known regarding opportunities and constraints of native species recovery after alien clearing. The core aim of this thesis is to consider whether key aspects of two widely cited restoration models (successional and alternative-state models) are useful for guiding effective management of severely-invaded riparian vegetation. As a study system, I used the Berg River in the Western Cape, South Africa which is severely impacted by invasive trees, especially Eucalyptus camaldulensis. By linking the studies of constraints for restoration and opportunities for native species recovery, the aim was to provide new possibilities for restoration in riparian zones. The thesis starts by examining constraints to restoration following alien invasion, in particular allelopathy which is one of the factors that exacerbate the impacts of Eucalyptus invasion and inhibit recovery of natural vegetation after clearing. I further assess opportunities for both passive (based on the successional model) and active restoration (based on the alternative-state model) following different strategies for removing invasive trees. The aim is to determine the effectiveness of the different models for sustainable, goal-directed management. Finally, I investigate soil-related properties namely water repellency, soil moisture and infiltration that benefit from alien clearing and subsequent recovery of native vegetation. Work on allelopathy as a restoration constrain showed that the presence of E. camaldulensis along the Berg River negatively affects the recovery of native species. Eucalyptus camaldulensis is allelopathic and induces soil water repellency. I recommend the removal of E. camaldulensis from riparian systems as this has the potential to restore soils to a non-allelopathic and non-repellent state that can pave way for native vegetation recovery. Native vegetation recovery showed mixed results. Restoration based on the successional model was generally efficient, whereas restoration based on tenets of the alternative-state model was inefficient mainly due to the several constraints active restoration faced. Native species recovery was successful on both completely cleared and thinned sites that were treated four years ago. Cover of native trees and shrubs was higher in both completely cleared and thinned sites compared to invaded sites, indicating that both methods promote indigenous vegetation recovery and set the ecosystem on a trajectory towards recovery. To improve recovery through thinning, I propose a new four-stage process to guide management in ensuring good recovery of key native species. Numerous challenges associated with active restoration following fell & stack burning and fell & removal were observed on sites that were treated one year ago. Germination of introduced native species was low in both fell & removal and fell & stack burning sites. Secondary invasion of alien herbs and graminoids, dry summer conditions and low seed germination hindered early native species establishment and recovery. Therefore, for active restoration to achieve its goals, effective recruitment and propagation strategies need to be established. Recruitment of native species was non-existent in the sites that were not seeded; this is attributed to the dominance of alien herbaceous species and graminoids and the depletion of native species in the soil seed bank. Reduction of water repellency of soils after removal of the invasive trees is important as it has the potential to affect the success of native vegetation recovery. On sites where native vegetation was recovering well, soil water repellency ranged from moderately repellent in thinned sites to non-repellent in completely cleared sites. Therefore, successful native species recovery has the potential to improve soil-related ecosystem functions, which will possibly help towards restoring indigenous vegetation. I conclude that the invasive alien tree E. camaldulensis negatively affects the native riparian ecosystem and that strategies to remove the species are needed. Recovery of native vegetation composition, structure and ecosystem function depends on the degree of ecosystem degradation and remaining ecosystem resilience. Besides having clear and effective restoration goals, restoration efforts should also develop realistic solutions to overcome numerous challenges and constraints, before any restoration plan is implemented. Successfully restored riparian ecosystems have potential to increase river flow and may lead to increased availability of water to agriculture, recreation, conservation and for domestic use, resulting in significant water security in South Africa. Both the successional model and the alternative-state model emphasize the need to identify restoration constraints. This study identified allelopathy as an important constrain for restoration and recommends measures to address it so as to facilitate restoration. Recovery based on the successional model was more effective than recovery based on the alternative-state model, which faced several constraints. Models of alternative-states incorporate system thresholds and feedbacks that might explain why the degraded system faced recovery challenges and remained resilient to restoration.
AFRIKAANSE OPSOMMING: Naas habitatverlies word indringer spesies as die grootste bedreiging vir biodiversiteit beskou. Die indringing van riviersisteme wêreldwyd deur uitheemse plante dra by tot groot veranderinge in die biodiversiteit en ekosisteem funksie. In Suid-Afrika, veral in die Fynbos Bioom, is rivieroewers en -beddings van die landskappe wat die meeste ingedring word, meestal deur skadelike indringers soos bome en struike van Australiese genera soos bv. Acacia en Eucalyptus. Alhoewel grootskaalse bestuursoperasies besig is om die indringers te verwyder en ekosisteme te herstel, is min bekend omtrent die geleenthede en beperkinge vir die herstel van inheemse spesies na die verwydering van indringers. Die hoofdoel van hierdie tesis is om die nut te bepaal van die sleutel faktore van twee wyd aangehaalde restorasie modelle (suksessie en alternatiewe-toestand modelle) om die effektiewe bestuur van hewig ingedringde oewers te lei. Die Berg Rivier in die Wes Kaap, Suid-Afrika, is gebruik as studie area. Die Berg Rivier is hewig geimpakteer deur indringers, veral deur Eucalyptus camaldulensis. Die doel was om nuwe geleenthede vir restorasie in rivier areas te voorsien, deur die studies oor beperkinge vir restorasie en geleenthede vir inheemse spesie herstel te verbind. Hierdie tesis begin deur die beperkinge van restorasie na indringing te ondersoek, veral allelopatie wat een van die faktore is wat die impakte van Eucalyptus indringing verhoog en die herstel van natuurlike plantegroei na verwydering van indringer inhibeer. Verder bepaal ek die geleenthede vir beide passiewe (gebaseer op die suksessie model) en aktiewe restorasie (gebaseer op die alternatiewe-toestand model) wat volg op verskillende strategieë van verwydering van indringer bome. Die doel is om die effektiwiteit van die verskillende modelle vir volhoubare, doel georiënteerde bestuur te bepaal. Laastens het ek die grond verwante eienskappe ondersoek naamlik, water terugdrywing, grondvog en infiltrasie wat voordeel trek uit indringer verwydering en die daaropvolgende herstel van inheemse plantegroei. Resultate van allelopatie as ʼn restorasie beperking het getoon dat die teenwoordigheid van E. camaldulensis langs die Berg Rivier die herstel van inheemse spesies negatief beïnvloed. Eucalyptus camaldulensis is allelopaties en gee aanleiding tot grondwater terugdrywing. Ek beveel aan die verwydering van E. camaldulensis vanuit rivier sisteme omdat dit die potensiaal het om grond na nie-allelopatiese en nie-terugdrywende toestand te herstel wat die weg kan baan vir die herstel van inheemse plante groei. Die herstel van inheemse plantegroei het gemengde resultate gewys. Restorasie gebaseer op die suksessie model was oor die algemeen meer doelmatig, teenoor restorasie gebaseer op die idee van ʼn alternatiewe-toestand model, hoofsaaklik as gevolg van verskeie beperkinge wat aktiewe restorasie in die gesig staar. Inheemse spesie herstel was suksesvol op beide die totaal indringer verwyderde en uitgedunde areas, wat vier jaar vantevore behandel is. Dekking van inheemse bome en struike was hoër in beide heeltemal skoongemaakte en uitgedunde areas wanneer die vergelyk word met ingedringde areas. Dit dui daarop dat beide metodes inheemse plantegroei herstel promoveer en die ekosisteem op ʼn baan na herstel plaas. Om herstel deur uitdunning te verbeter stel ek ʼn vier-stadium proses voor om bestuurders te lei vir goeie herstel van sleutel inheemse spesies. Verskeie uitdagings geassosieer met aktiewe restorasie wat volg op val-en-stapel brand en val-en-verwyder is geobserveer in areas wat ʼn jaar van te vore behandel is. Ontkieming van aangeplante inheemse spesies se sade was laag in beide die val-en-verwyder en die val-en-stapel brand areas. Sekondêre indringing van uitheemse kruie en graminoiede, droë somers toestande en lae saad ontkieming hinder die vroeë inheemse spesie vestiging en herstel. Dus, vir aktiewe restorasie om sy doel te bereik moet effektiewe werwing en verspreidings strategieë in plek wees. Daar was geen werwing van inheemse spesies in die areas wat nie gesaai was nie. Dit kan toegeskryf word in die dominansie van uitheemse kruie spesies and graminoiede en die uitputting van inheemse spesies in die grond saadbank. Vermindering van water terugdrywing van grond ná verwydering van indringer bome is belangrik aangesien dit die potensiaal het om die sukses van inheemse plantegroei herstel te affekteer. Die areas waar inheemse plantegroei goed herstel het, het grondwater terugdrywing gevarieer van gemiddeld afstootlik in die uitgedunde areas na nie-afstootlik in die heeltemal skoongemaakte areas. Dus, suksesvolle inheemse spesie herstel het die potensiaal om die grondverwante ekosisteem funksies te verbeter, wat moontlik sal bydra tot die herstel van inheemse plantegroei. Ek kom tot die gevolgtrekking dat die indringer boom E. camaldulensis die inheemse rivier ekosisteem negatief affekteer en dat strategieë om hierdie spesie te verwyder nodig is. Herstel van inheemse plantegroei samestelling, struktuur en ekosisteem funksie hang af van die graad van ekosisteem verval en die oorblywende ekosisteem weerstandigheid. Behalwe die verwyderings en effektiewe restorerings doelwitte, moet restorasie pogings ook realistiese oplossings vir die oorkombaarheid van verskeie uitdagings en beperkinge ontwikkel voor enige restorasie plan geïmplementeer kan word. Suksesvolle herstel van rivier ekosisteme het die potensiaal vir verhoogde rivier vloei en mag moontlik lei tot ʼn verhoogde beskikbaarheid van water vir landbou, ontspanning, natuurbewaring en vir huishoudelike gebruik, en kan dus ʼn beduidende bydrae kan lewer tot water sekuriteit in Suid Afrika. Beide die suksessie model en die alternatiewe-toestand model beklemtoon die noodsaaklikheid om restorasie beperkinge te identifiseer. Hierdie studie identifiseer allelopatie as ʼn belangrike beperking tot restorasie en maak aanbevelings om dit aan te spreek en om restorasie te fasiliteer. Herstel gebaseer op die suksessie model was meer effektief as herstel gebaseer op die alternatiewe-toestand model wat verskeie beperkings in die gesig staar. Die alternatiewe-toestand modelle inkorporeer sisteemdrumpels en terugvoer wat moontlik kan verduidelik waarom gedegradeerde sisteme herstel uitdagings getoon het en weerstandig teenoor restorasie gebly het.

Western riparian zones have undergone significant landscape changes over the past several decades, with introduced saltcedar (Tamarix spp.) as a crucial component of this transformation. Saltcedar, now a dominating presence along many western rivers, due to its high tolerance to drought, salinity and stress, is considered to be a high-water-use plant that can desiccate disturbed river systems. Where native and saltcedar plant communities occur together, it is important to understand water use patterns and the physiological responses of each species to environmental stress factors, as a way to project an eventual course of succession processes and management options at a given site. Stress and disturbance in the form of reduced stream flows and land use changes may influence these interactions. Understanding the conditions that allow for saltcedar dominance is critical in determining riparian water budgets, and developing effective management strategies. Sap flux sensors were used to measure the physiological response of co-occurring communities of saltcedar and native trees to these environmental stress factors during the pre-monsoon period in early summer, a time of maximum stress for riparian vegetation. The results suggest that native trees are still competitive with salt cedar so that a mixed plant community is likely to continue on the San Pedro River on the condition that current groundwater levels and river flows are maintained. If base flows and depth to groundwater continue to decline, this competitive balance between saltcedar and native trees likely could change.

In this study I investigate how the biodiversity of vascular plants differs between three slow flowing parts of the stream Juktån in northern Sweden where hydropower has impacted the flow regime and thus the conditions for riparian vegetation. The hypothesis was that species richness would increase with the distance from the hydropower plant due to increasing naturalness of the flow regime. Species richness was investigated since it is a fundamental and important function for the maintenance and quality of ecosystems (Naiman and Décamps 1997). Juktån, where data for the study was collected, receives a static minimum flow of barely 12 % of the yearly natural medium flow released from the hydropower station into the channel (Wisaeus 2014). The minimum flow follows the variation set by the hydropower which deviates from natural seasonal flow variation, which is essential for the germination, growth, and reproduction of many plant species (Poff et al. 1997). The investigation includes a comparison between species richness and richness of 4 functional groups among the three study locations. In addition, analysis of the relationship between environmental variables such as position on the riparian zone (height) and substrate composition were performed. Sikselet that was the slow flowing reach closest to the hydropower station had the highest species richness while Bredselet further downstream had the lowest species richness with Långselet having intermediate species richness. One reason for the pattern in species richness could be that seeds get trapped in weirs located between Sikselet and Långselet. In that case, reintroducing a natural flow regime may not suffice to increase species richness.

Doctor of Philosophy(PhD)
It is generally accepted that tree roots can reinforce soil and improve the stability of vegetated slopes. Tree root reinforcement is also recognised in riverbanks although the contribution that the roots make to bank stability has rarely been assessed due to the reluctance of geomorphologists to examine riverbank stability by geomechanical methods that allow for the inclusion of quantified biotechnical parameters. This study investigates the interaction between alluvial soil and the roots of four southeastern Australian riparian trees. It quantifies the amount and distribution of root reinforcement present beneath typically vegetated riverbanks of the upper Nepean River, New South Wales, and examines the effect of the reinforcement on the stability of these banks. The ability of a tree to reinforce the soil is limited by the spatial distribution of its root system and the strength that the roots impart to the soil during shear. These two parameters were determined for the following four species of native riparian tree: Casuarina glauca, Eucalyptus amplifolia, Eucalyptus elata, and Acacia floribunda. The four species all exhibit a progressive reduction in the quantity of root material both with increasing depth and with increasing lateral distance from the tree stem. In the vertical direction there are two distinct zones that can be described. The first occurs from between 0 and approximately 15 % of the maximum vertical depth and consists of approximately 80 % of the total root material quantity. In this zone the root system consists of both vertical and lateral roots, the size and density of which varies between species. The second zone occurs below approximately 15 % of the maximum vertical depth and consists primarily of vertical roots. The quantity of root material in this zone decreases exponentially with depth due to the taper of individual roots. The earth reinforcement potential in terms of both geometric extent and the quantity of root material expressed as the Root Area Ratio (RAR) varies significantly from species to species. E. elata exhibited the highest values of RAR in soil zones beneath it while E. amplifolia reinforced a greater volume of soil than any of the other species examined. The increased shear resistance (Sr) of alluvial soil containing roots was measured by direct in-situ shear tests on soil blocks beneath a plantation. For three of the species (C. glauca, E. amplifolia, E. elata) Sr increased with increasing RAR measured at the shear plane, in a similar linear relationship. The shear resistance provided by A. floribunda roots also increased with increasing RAR at the shear plane but at a much greater rate than for the other three species. This is attributable to A. floribunda’s greater root tensile strength and therefore pull-out resistance, as well as its smaller root diameters at comparative RARs which resulted in a greater proportion of roots reaching full tensile strength within the confines of the test. Tree roots fail progressively in this system. Therefore determining the increased shear strength from the sum of the pull-out or tensile strengths of all individual roots and Waldron’s (1977) and Wu et al’s (1979) simple root model, would result in substantial over estimates of the overall strength of the soil-root system. The average difference between Sr calculated in this manner and that measured from direct in-situ shear tests is 10.9 kPa for C. glauca, 19.0 kPa for E. amplifolia, 19.3 kPa for E. elata, and 8.8 kPa for A. floribunda. A riverbank stability analysis incorporating the root reinforcement effect was conducted using a predictive model of the spatial distribution of root reinforcement beneath riparian trees within the study area. The model is based on measurements of juveniles and observations of the rooting habits of mature trees. It indicates that while the presence of vegetation on riverbank profiles has the potential to increase stability by up to 105 %, the relative increase depends heavily on the actual vegetation type, density, and location on the bank profile. Of the species examined in this study the greatest potential for improved riverbank stability is provided by E. amplifolia, followed by E. elata, A. floribunda, and C. glauca. The presence of trees on banks of the Nepean River has the potential to raise the critical factor of safety (FoS) from a value that is very unstable (0.85) to significantly above 1.00 even when the banks are completely saturated and subject to rapid draw-down. It is likely then that the period of intense bank instability observed within this environment between 1947 and 1992 would not have taken place had the riparian vegetation not been cleared prior to the onset of wetter climatic conditions. Typical ‘present-day’ profiles are critically to marginally stable. The introduction of vegetation could improve stability by raising the FoS up to 1.68 however the selection of revegetation species is crucial. With the placement of a large growing Eucalypt at a suitable spacing (around 3-5 m) the choice of smaller understorey trees and shrubs is less important. The effect of riparian vegetation on bank stability has important implications for channel morphological change. This study quantifies the mechanical earth reinforcing effect of some native riparian trees, thus allowing for improved deterministic assessment of historical channel change and an improved basis for future riverine management.

Thesis (PhD)--Stellenbosch University, 2012.
ENGLISH ABSTRACT: Worldwide, invasive alien plants (IAPs) alter aspects of invaded ecosystems including geomorphology, above-ground vegetation, soil seed banks and soil nutrient regimes, thereby affecting the long-term stability of ecosystems. In many cases these invasions call for various management interventions, including restoration. Effects of alien plant invaders on native ecosystems are widely acknowledged, but compared to terrestrial ecosystems, riparian habitats are poorly studied. Riparian habitats are inherently rich in biodiversity, but are particularly prone to invasion by IAPs because of their dynamic nature and because these ecosystems are affected by many anthropogenic activities. This enhances the proliferation of IAPs, especially trees such as Tamarix spp. (e.g. in the southwestern United States), Salix spp. (e.g. in Australia), and Australian Acacia spp. and Eucalyptus camaldulensis (in South Africa). Initiatives such as the Working for Water (WfW) programme in South Africa that are focusing on clearing IAPs, particularly in riparian zones, have reported much success in the short term, but ecosystem recovery remains limited. There is a poor understanding of the nature of the impacts of invasive trees, and of the opportunities that exist for ecosystem restoration. The objective of my thesis was to investigate the effects of the widespread invasions of Eucalyptus camaldulensis in riparian zones. Methodological constraints have been identified as one reason for limited success of restoration projects. Many studies examining the effects of IAPs used a comparative approach of uninvaded (reference) and invaded sites. However, this approach often fails to separate cause from effect. An experimental removal approach has been suggested as an alternative, but it is often inappropriate for large invasive trees owing to time and cost constraints. I used a comparative gradient approach, involving sampling along a continuum from uninvaded sites, and varying E. camaldulensis invasion densities categorised as “lightly”, “moderately” and “heavily” invaded (hereafter the gradient of invasion), each with four replicates (n = 16). This approach enabled me to detect trends that could be reliably attributed to invasion. Furthermore, my study attempted to provide a comprehensive understanding of the effects of IAPs in invaded ecosystems by considering various components of the ecosystem and integrating their feedback relationships to establish the net effects on ecosystem processes. The riparian zone is a unique ecosystem which provides an opportunity for such a multifaceted study examining and integrating the effects of invasive trees on riparian geomorphology, above-ground vegetation, soil seed banks, and soil properties. I studied the effects of E. camaldulensis invasion in riparian zones along the Berg River in the Mediterranean-type climate zone of the Western Cape, South Africa. I asked the questions 1) Are environmental conditions (i.e. the riparian environment created by geomorphological processes) altered by Eucalyptus invasion and if so, are these conditions less suitable for recruitment of indigenous species? 2. Has Eucalyptus invasion altered the composition of indigenous species (potentially limiting post-clearing propagule supply)? 3: How does Eucalyptus invasion change the composition of the riparian soil seed bank; and its potential to re-initiate ecosystem recovery? 4: Do soil properties (physical and chemical) change with Eucalyptus invasion? Data for all four questions were collected in the same sites to enable cross comparisons. For the first question, I employed photogrammetry techniques in a GIS and remote sensing environment to analyse repeated aerial photographs (1938 -2010) to reconstruct the invasion history and riparian geomorphology evolutionary dynamics. For the second and third questions, I compared richness, diversity, evenness and composition of resident above-ground and seed bank vegetation between uninvaded sites and the gradient of invaded sites. For the fourth question, I compared a set of variables describing physico-chemical properties in uninvaded sites to the gradient of invaded sites seasonally. Results of the geomorphology study showed that the riparian zone has been very dynamic over the 70 years, with a 13.5% net decrease in area. There was evidence of channel narrowing, riverbank steepening and river bed incision in areas that were densely invaded. No significant trends were detected in hydrometeorological data. The above-ground vegetation study revealed that species richness, diversity and structural attributes (e.g. height, relative cover and mean basal diameter) of native species decreased consistently along the invasion gradient. Invasion also altered native and alien plant species composition, both of which showed a high preference for lightly invaded sites. The seed bank study revealed that E. camaldulensis invasion had no significant effect on total and native species richness, diversity and evenness, however the effects were bigger on native than alien species. Alien species density was significantly higher than native species density. Invasion also influenced the composition of the native seed bank. However, native seed banks were more diverse than above-ground plant assemblages with some native fynbos species occurring only in the seed bank. In the soil study, I found that soil pH levels were significantly lower in invaded sites than in uninvaded sites in all seasons. Litter cover and thickness increased significantly with invasion intensity. Soil moisture decreased consistently with invasion intensity while temperature increased in winter and spring. Total macro, micro and available nutrients did not vary significantly along the invasion gradient (p > 0.05), but exchangeable cation content was significantly higher in uninvaded than in invaded sites, especially in winter and spring. Narrowing of river channel and steepening of river banks reduces the suitability of the riparian zone to perform certain ecological functions such as soil seed storage and providing space for vegetation to grow. This reduces opportunities for seedling recruitment and consequently reduces species diversity. Conversely, the reduction of species diversity in above-ground vegetation reduces seed input into the soil leading to depauperate soil-stored seed banks. The abundance of seeds of Acacia mearnsii in the soil-stored seed bank poses a threat of secondary invasions post clearing of E. camaldulensis if this is adopted as a management action. The study was successful in addressing the stated objectives. The transition from a native-dominated riparian plant community to a monoculture of E. camaldulensis over c. 50 years has resulted in marked changes to riparian geomorphology and above-ground vegetation, but has yet to radically change the total soil seed bank and soil nutrient concentrations. The findings offer support to the WfW clearing initiatives and show that potential for recovery of native species after removal of the invasive eucalypts still exists. This knowledge not only enhances effectiveness of restoration and long term management of riparian ecosystems, but also advances the field of riparian ecology and restoration. Further experimental work is needed to establish the mechanisms responsible for the changes associated with Eucalyptus invasion. The contribution of agriculture in shaping channel morphology also needs to be investigated.
AFRIKAANSE OPSOMMING: Die langtermyn stabiliteit van ekosisteme wêreldwyd word bedreig deur uitheemse indringer plante (UIPs) wat, onder andere, die geomorfologie, bo-grondse plantegroei, onder-grondse saadbanke en voedingstofstatus van ekosisteme kan verander. In baie gevalle word bestuursoperasies vir indringer plante vereis, en sluit onder andere herstel (restorasie) in. Die invloed van UIPs op inheemse ekosisteme word wyd erken, maar kennis oor oewerbank-habitatte, in vergeleke met ander terrestriële ekosisteme, skiet steeds tekort. Oewerbank-habitatte is inherent ryk aan biodiversiteit maar uiters sensitief vir indringing deur UIPs. Dit is weens die dinamiese aard van hierdie ekosisteme asook vele menslike aktiwiteite in hierdie tipe habitat. Dit dra by tot die vermeerdering van UIPs, veral bome wat byvoorbeeld insluit Tamarix (tamariske) (bv. in die suid-wes van die Verenigde State van Amerika), Salix (wilger) (in bv. Australië) en die Australiese Acacia (wattel) en Eucalyptus spesies (bloekom) in Suid Afrika. Inisiatiewe soos die Werk vir Water (WvW) program in Suid Afrika fokus juis daarop om UIPs vanuit oewerbank areas te verwyder, en alhoewel groot sukses in die kort termyn behaal is, is die herstel van ekosisteme steeds beperk. Daar is dus min inligting beskikbaar oor die impak van indringer bome en gevolglik ook oor geleenthede vir die herstel van hierdie ekosisteme. My tesis ondersoek die impak van die wydverspreide voorkoms van die indringer, Eucalyptus camaldulensis, in oewerbank-omgewings. Beperkte metodes is tans beskikbaar om die impak van indringer plante te ondersoek, en dit is geïdentifiseer as een van die oorsake vir die beperkte sukses van herstelprojekte. Baie studies ondersoek die effekte van UIPs deur natuurlike areas sonder enige indringer plante te vergelyk met areas waar UIPs voorkom. Hierdie tipe benadering kan ongelukkig, in meeste gevalle, nie onderskei tussen die oorsaak en die impak nie. ʼn Alternatiewe voorstel is om UIPs eksperimenteel te verwyder, maar is ongelukkig nie van toepassing op groot bome nie weens die tyd en kostes verbonde hieraan. Ek het gebruik gemaak van ʼn vergelykende-gradiënt benadering wat steekproefnemings langs ʼn geleidelike kontinuum van natuurlike areas sonder enige indringer plante na areas wat effens gematig, en erg ingedring is (hiernaas bekend as die indringer-gradiënt) deur die indringer, E. camaldulensis. Die steekproefnemings van elke kategorie van indringing (geen, effens, gematig en erg) is vier keer herhaal (n = 16). Hierdie benadering het my toegelaat om neigings te identifiseer wat met sekerheid toegeskryf kan word aan die teenwoordigheid van indringers. My studie het ook verder gemik om ʼn omvattende beskrywing van die impak van UIPs in ekosisteme met indringers te verskaf deur verskeie komponente van die ekosisteem, sowel as hul terugvoer-verhoudinge, in ag te neem, en sodoende die totale effek op ekosisteem prosesse te bepaal. ʼn Oewerbank is ʼn unieke ekosisteem wat ʼn geleentheid bied vir ʼn veelsydige studie wat die impakte van indringer bome op oewerbank-geomorfologie, bo-grond plantegroei, grond saadbanke en grondeienskappe, kan bestudeer en integreer. Ek het die impak van die teenwoordigheid van die indringer, E. camaldulensis in oewerbank areas langs die Bergrivier, wat ʼn Meditereense-tipe klimaat het, in die Wes Kaap, Suid Afrika, bestudeer. My studie het die volgende vrae gevra: 1) Is omgewings-omstandighede (bv. die oewerbank-omgewing geskep deur geomorfologiese prosesse) verander deur die teenwoordigheid van die Eucalyptus indringing, en indien wel, is hierdie omstandighede minder geskik vir inheemse spesies?; 2) Het die indringing van Eucalyptus gelei tot ʼn verandering in die samestelling van inheemse spesie gemeenskappe (moontlik beperkte plant voorplantings materiaal nadat die indringer verwyder is)?; 3) Hoe verander die indringing van Eucalyptus die samestelling van saad teenwoordig in die grond (saad bank) van die oewerbank en het dit die potensiaal om ekosisteem-herstel weer aan die gang te sit?; 4) Verander indringer Eucalyptus grond eienskappe (fisies en chemies)? Data vir al vier vrae is by dieselfde plek versamel om vergelykings te kan tref. Vir die eerste vraag het ek fotogrammetrie-tegnieke in ʼn Geografiese Inligting Stelsel (GIS) en afstandwaarnemings-omgewing toegepas om herhaalde lugfotos (1938-2010) te analiseer om die indringings geskiedenis sowel as die oewerbank se geomorfologiese evolusionêre dinamika saam te stel. Vir die tweede en derde vraag, het ek die aantal, diversiteit, gelykheid en samestelling van die bo-grond en saad bank plantegroei tussen natuurlike (geen indringers) en die gradiënt met indringers vergelyk. Vir die vierde en laaste vraag, het ek ʼn stel veranderlikes wat die fisies-chemiese eienskappe van die natuurlike omgewing sonder indringers beskryf, vergelyk met die eienskappe van die indringers gradiënt op ʼn seisoenale basis. Die resultate van die geomorfologiese studie wys dat die oewerbank omgewing baie dinamies was oor die afgelope 70 jaar met ʼn totale afname van 13.5% in rivieroewer-area in die gebied wat bestudeer is. Daar is ook bewyse vir ʼn vernouing van die kanaal, toename in die steilte van die rivierbank sowel as insnyding in die rivierbedding in areas wat ernstige indringing ervaar. Daar was geen beduidende neigings in die hidro-meteorologiese data nie. Die studie van die bo-grondse plantegroei het gewys dat die aantal plantspesies, diversiteit en strukturele kenmerke (bv. hoogte, dekking en gemiddelde basale diameter) van inheemse spesies afneem soos wat die indringings gradiënt toeneem. Indringing verander ook die inheemse sowel as uitheemse plant spesies samestelling. Beide inheemse en uitheemse spesies het areas wat slegs effense indringing wys, verkies. ʼn Studie van die saad bank (aantal sade in die grond) het gewys dat die indringer, E. camaldulensis geen merkwaardige invloed op die aantal, diversiteit en gelykheid van die totale en inheemse spesies teenwoordig gehad het nie. Die aantal uitheemse spesies was merkwaardig meer as die aantal inheemse spesies. Alhoewel indringing ook die samestelling van die inheemse saadbank beïnvloed het, was die inheemse saad bank, waarvan sommige spesies slegs daar voorgekom het, meer divers as die bo-grond plant samestellings. ʼn Studie van die grond het gewys dat die pH vlakke, afgesien van die seisoen, aansienlik laer is in die areas met indringers in vergeleke met areas sonder die indringers. Die hoeveelheid en dikte van plantmateriaal op die grond neem ook toe met ʼn toename in indringers. Die waterinhoud van die grond het afgeneem met ʼn toename in indringers en temperatuur het verhoog in beide winter en lente. Die totale makro- en mikrovoedingstowwe het nie merkwaardig gewissel saam met die indringingsgradiënt nie (p > 0.05), maar die uitruilbare katioon inhoud was aansienlik hoër, veral in die winter en lente-maande, in die areas sonder indringers in vergelyking met die areas met indringers. Vernouing van rivierkanaal en versteiling van rivieroewers verminder die geskiktheid van die oewersone om sekere ekologiese funksies te verrig soos grond-saadstoring en die verskaffing van ruimte vir plante om te groei. Dit verminder geleenthede vir werwing van saailinge en verminder gevolglik spesiediversiteit. Aan die ander kant, die vermindering van die spesiediversiteit in die bogrondse plantegroei verminder saadinsette in die grond en dit lei tot verarming van grondgestoorde saadbanke. Die oorvloed van sade van Acacia mearnsii in die grondgestoorde saadbank hou die bedreiging van sekondêre indringing in na die skoonmaak van E. camaldulensis indien dit wel as 'n bestuursaksie aangeneem word. Hierdie studie het die uiteengesette doelwitte suksesvol aangespreek. Die oorgang van ʼn oewerbank-gemeenskap wat hoofsaaklik uit inheemse plante bestaan na ʼn gemeenskap wat slegs uit E. camaldulensis bestaan, oor ʼn periode van ongeveer 50 jaar, het duidelike veranderinge tot gevolg gehad. Hierdie veranderinge is veral opmerklik in die oewerbank-geomorfologie en bo-grondse plantegroei, maar tot op hede is dit nog nie so opmerklik in die saadbank en grond-voedingstowwe konsentrasies nie. Hierdie bevindinge bied ondersteuning aan die WvW verwyderings-ondernemings en wys dat daar wel moontlike herstel van inheemse spesies kan wees na die verwydering van die indringer Eucalyptus. Hierdie kennis dra nie net by tot verhoogde effektiwiteit van herstelwerk en langtermyn-bestuursplanne nie, maar ook tot die navorsingsveld van oewerbankekologie en-herstel. Verdere eksperimentele navorsing word benodig om die moontlike meganismes wat verantwoordelik is vir die waargeneemde veranderinge, geassosieerd met Eucalyptus indringing, te verduidelik. Die bydrae van landbou tot die vorming van kanaal morfologie moet ook verder bestudeer word.

Magister Scientiae - MSc (Earth Science)
The increasing expansion of Acacia longifolia trees along the riparian zones in South Africa demands an urgent intervention as the species is listed in the National Environmental Management: Biodiversity Act (2004). This list includes species that are prohibited from growing, or being imported into South Africa. The detrimental effects of alien vegetation have been observed on the hydrology of the ecosystems invaded. However, the actual water use by Acacia longifolia has never been quantified. Therefore, there is inadequate knowledge of the actual rates and the differences in water use rates by A. longifolia occurring in the riparian zones and hillslopes. This study addresses this gap in knowledge by quantifying the diurnal and seasonal transpiration dynamics of hillslope and riparian A. longifolia. The variations of climate and soil water content on the hillslope and riparian zones were also examined in this study. The study was conducted on the Spanjaardskloof hills and along the Nuwejaars River (Moddervlei) in the Heuningnes Catchment, Cape Agulhas.

Dissolved organic matter (DOM) leached from five coastal forest litterfall types. Red alder, vine maple, western red cedar, western hemlock and Douglas-fir, were studied to assess their DOM chemistry and relative ability to support growth of heterotrophic, stream bacteria. Bacterial growth was measured using [ H] leucine incorporated into protein over 24 hours of exposure to nutrient-amended leachates. Bacterial growth was greatest in deciduous and western red cedar leachates when controlling for dissolved organic carbon (DOC) concentration. The bacterial growth rates on most leachate types were greatest after one hour, then declined in a negative exponential pattern. The DOM less than 10 000 nmw supported lower bacterial growth rates than DOM from whole leachates on a per mg DOC basis. The DOM carbon to nitrogen atomic ratio was the best predictor of bacterial growth (r² = 0.84). A seven day leaching experiment revealed that DOC release from western hemlock needles increased linearly while the majority of red alder and western red cedar DOC was released after one or two days, respectively. The patterns recorded in stream DOM quantity and quality indicated that riparian vegetation type may directly influence stream DOM chemistry. Through successional changes in tree species composition, riparian forests can influence the stream microbial productivity based on the changes in dissolved organic matter.

This thesis explores the hydrological factors that may contribute to the observed distribution patterns of invasive willows (Salix) and native trees (Eucalyptus camaldulensis, E. largiflorens and Acacia stenophylla) along the Lower River Murray (LRM) in southern Australia. An initial survey, establishing the diversity and flowering biology of Salix taxa was carried out to ascertain the extent of invasion, and the likelihood of hybridisation, which may accelerate invasion. S. babylonica, S. fragilis, S. × chrysochoma and S. × rubens occur in the study region, each represented by a single gender. None were present on floodplains, but the most dominant taxon, S. babylonica, occurred along the entire length of the main channel. No seed or seedlings were observed; hence reproduction is likely to be asexual. More detailed survey work was then carried out to characterise the distribution patterns of the dominant S. babylonica and co-occurring natives (Eucalyptus camaldulensis, E. largiflorens and Acacia stenophylla) along a hydrologic gradient produced by the extensive weir system in the LRM. In weir pools, variation in daily water levels of weir pools is low (± 0.1 m) immediately upstream of the weir, but higher immediately downstream (0.2-1.0 m daily). The distribution of natives was uniform across weir pools, while S. babylonica was more abundant above weir structures, suggesting low tolerance to variable water regimes. Hypotheses relating to the observed distribution patterns were then tested experimentally on juveniles of the S. babylonica, E. camaldulensis and A. stenophylla. The experiment was carried out in outdoor ponds using an orthogonal design, with four elevations in relation to water level (-25 cm, 0 cm +25 cm, + 50 cm) under each of three water regimes. Experimental water regimes manipulated the magnitude of daily water level changes (static, 0 m dayˉ¹ ; moderate, ± 0.05 - 0.15 m dayˉ¹; high, ± 0.2 -0.5 m dayˉ¹) to mimic typical hydrological conditions across weir pools in the LRM. Final biomass and mean relative growth rates (S. babylonica, 0.0403 ± 0.002 g mˉ²dayˉ¹ A. stenophylla, 0.0249 ± 0.0017 g m ˉ² dayˉ¹; E. camaldulensis, 0.0204 ± 0.0016g mˉ² dayˉ¹) of all 3 species were unaffected by water regimes (i.e. water fluctuations), but were affected by elevation. Survival of both S. babylonica and A. stenophylla was lowest at low elevations where inundation was high. At higher elevations (+25 cm, +50 cm) the RGR of S. babylonica juveniles was much higher than the native juveniles. To test if the persistence of adults of each species along hydrologic gradients were associated with differing tolerances to water deficits and water use characteristics, S. babylonica and native species were examined under typical hydrological conditions in the field and during an unusual drawdown. S. babylonica occurring at the lowest elevations on riverbanks, had the least negative predawn shoot water potential (ψpredawn), followed by the natives, which were at higher elevations. A. stenophylla had the lowest stable carbon isotope ratio (δ¹³C) values (by 1.7 ‰) on the riverbank; suggesting more profligate water use than S. babylonica and E. camaldulensis. However, all riverbank trees had significantly less negative ψpredawn and lower δ¹³C than native trees on floodplains, consistent with higher water availability on riverbanks. The position and stable oxygen isotope ratio (δ¹⁸O) values were consistent with riverbank S. babylonica sourcing their water directly from the river or from shallow soil-water sources (<0.25 m). In floodplain habitats, depth to water was > 2.5 m, and groundwater was 5 times more saline (4.97 ± 0.88 dS mˉ¹) than river-water. Native trees with deep roots, the ability to lower water potentials and alter water use efficiencies may be at an advantage in this habitat relative to S. babylonica. Extreme low flows in the LRM, over a 6-month period, provided an opportunity to assess how S. babylonica and E. camaldulensis responded to a river-water drawdown. During the drawdown, river-water levels fell at a rate of ~2 – 2.5 mm dayˉ¹ and dropped to a minimum of 0.42 m below the designated pool level. S. babylonica and E. camaldulensis maintained high ψpredawn across the drawdown period, most likely because riverbank soil water availability was not limited; as depth to water table only decreased marginally (≤0.15 m) and soil water content and soil water potential were high (<1.1 MPa). However, an above average rainfall in February 2003 significantly increased soil water potential in the upper 0.25 m of the riverbank, which correlated with a significant increase in ψpredawn in E. camaldulensis, suggesting they were able to use shallow, precipitation derived soil-water sources whereas S. babylonica were not. Also under hot, dry conditions, S. babylonica had higher transpiration rates and lower instantaneous water use efficiencies than co-occurring E. camaldulensis. This suggests that S. babylonica may consume larger volumes of water per unit leaf area than natives, if access to water is maintained.
Thesis (Ph.D.) - University of Adelaide, School of Earth and Environmental Sciences, 2010