Causes and consequences of woody encroachment into grass dominated systems have been widely studied, however functional mechanisms which promote encroachment are largely unknown. Many expansive woody species are shrubs with rhizobial or actinorhizal N-fixing symbiotic associations. Morella cerifera L. (Myricaceae) is an actinorhizal N-fixing shrub which rapidly expands into grasslands on the barrier islands off the coast of Virginia, USA. The objective of this research was to determine physiological drivers of woody encroachment resulting in increased woody cover of M. cerifera on Southeastern, US barrier islands. Variations in physiology and resource use efficiencies (RUE) of M. cerifera and co-occurring shrubs were determined, and edaphic characteristics beneath shrub thicket canopies and in open areas were quantified as indications of resource availability. Analysis of dune vegetation and soils showed severe freshwater limitation and reduced plant height of dune forbs suggesting dunes represent an upper elevational boundary for M. cerifera distribution. Soil N availability was higher beneath shrubs compared to open areas, and both physiology and isotope effects showed facilitation of the non-fixing shrub, Baccharis halimifolia, by M. cerifera which may lead to increased rates of woody encroachment as B. halimifolia colonizes expanding thicket edges. Morella cerifera and other N-fixers had higher %refixation within stems which resulted in higher carbon use efficiency (CUE) and water use efficiency of N-fixing shrubs compared to non-fixers. Results of an N-fertilization experiment suggest B. halimifolia has higher dependence on and demand for soil nutrients compared to M. cerifera. Morella cerifera showed no signs of resource deficiency or reduced physiological capacity even at 0 ppm total Nsoil. Morella cerifera transitioned from utilizing solely fixation derived N to soil N as N concentrations increased providing another mechanism leading to increased CUE and, indirectly, overall RUE. In summary greater RUE, lower resource demand, and greater resource availability for M. cerifera compared to co-occurring shrubs may result from symbiotic root associations with bacteria and fungi. While expansion of M. cerifera thickets is limited to lower elevational interdunal depressions, expansion may continue and result in increased rates of woody encroachment through facilitation of co-occurring shrubs.

It is important to understand mechanisms that facilitate expansion of two common shrubs, Morella cerifera and Baccharis halimifolia in coastal environments. The purpose of my study was to investigate the physiological and structural changes that occur as leaves age. Photosynthesis, incident light, chlorophyll, and leaf C:N ratios were quantified for young, intermediate, and old leaves (distal, central and proximal leaves, respectively). Leaf structural differences were also compared. Leaves did not change morphologically with age. Light decreased with leaf age and during winter months. Photosynthesis showed no seasonal or age related patterns. Chlorophyll increased initially and then declined with age due to self shading. Nitrogen content was highest during spring. Seasonality and leaf age had unique effects on the two study species. Understanding senescence adaptations of these two shrubs can help explain their abundance in coastal ecosystems.

Invasive plants have a significant detrimental effect on ecosystems globally, with impacts estimated at millions of dollars per invasive species each year. Biological control has long been used as a management tool for invasive plants, as it is considered a long–term cost–effective control strategy. Surprisingly, the impact of biological agents is rarely quantified. Any form of impact evaluation is generally conducted soon after agent release and establishment; with few studies examining the impact of the agents on the population dynamics of the invader, particularly once the agents have been established for a long time. The aim of the research in my thesis is to evaluate the biological control program of groundsel bush (Baccharis halimifolia L. Asteraceae) in Australia. The groundsel bush biological control agents were released up to 40 years ago and no quantitative assessment of agent impact has ever been conducted, despite the fact that the program has cost about $9.6 million. More specifically, the overall aim of this thesis is to investigate the impact of the released biological control agents on individual plants and populations of groundsel bush. In addition, my thesis aims to examine the impacts of climate as a potential confounding factor of the biological control program. My thesis provides a unique example of biological control evaluation by using a combination of observational damage studies, insect exclusion experiments, and statistical, population and climate modelling to assess, a posteriori, the effectiveness of biological control. This is the first time a long term biological control program has ever been evaluated. To assess the efficacy of the agents, I conducted a large field survey to examine whether the agents were distributed throughout the entire range of groundsel bush and if any biotic or abiotic factors influenced their effectiveness. In addition to this, I assessed the effect of the agents on the growth, survival and fecundity of individual plants under field conditions, and subsequently population growth rate. To do this, I used statistical models of observed effects of biological control agent damage and insect exclusion experiments on plant growth and fecundity to parameterise matrix population models. My results indicate that the groundsel bush biological control agents may be patchy in their effectiveness due to factors such as rainfall and plant size. At their current rate of damage, the groundsel bush biological control agents do not reduce plant growth or fecundity significantly. However, simulation models demonstrated that the agents have the potential to reduce individual plant and population growth when damage is at high levels. A reduction in an invader’s population growth rate, following the introduction of biological control agents, does not necessarily signify that the agents were responsible for the reduction. Factors such as land clearing, chemical and mechanical control, ecosystem health and climate may reduce populations of invasive plant species. With this in mind, I developed a series of climate models to examine how the favourability for growth of groundsel bush may change under different climate scenarios. The climate simulations demonstrated that the distribution and abundance of groundsel bush populations may have contracted in the past 50 years (post–biological control agent release) due to changing rainfall and temperature patterns. The results of the research in my thesis clearly show the need for thorough biological control evaluations, and for detailed data to be collected on the target plant’s demography and population sizes pre- and post-agent release. At a minimum, this should enable biological control practitioners to determine some level of agent impact and demonstrate support for further agent releases or integrative management strategies if necessary. Groundsel bush is a significant invader in Europe where biological control has not yet been carried out. Lessons from the evaluation of the Australian biological control program could be applied to new biological control programs elsewhere such as Europe. Overall, my research findings contribute to a better understanding of how to best evaluate a post-release biological control program, using groundsel bush as a case study. This is the first study to demonstrate an effective set of strategies and quantitative tools to evaluate a biological control program, which can be similarly applied to any biological control program and contributes significantly to an area of biological control which has only recently received significant attention.

Methods for rapid, safe and effective detection of unmapped buried ordnance are vital to the protection of humans and environmental quality throughout the world. This study aimed to investigate the use of phytosensing and to understand the physiological response of woody plants to 2,4,6-trinitrotoluene (TNT) contamination. Baccharis halimifolia were potted in soils containing various concentrations of TNT and physiological responses were observed over a 9-week experimental period. Measurements included the collection of remotely sensed data, such as hyperspectral reflectance and chlorophyll fluorescence, and traditional plant-level physiological data. In accordance with the hypothesis, low levels of TNT improved physiological response in plants due to the slight increase in nitrogen, while high levels of TNT induced stress. Key markers in stress responses were identified, specifically with reflectance indices and derivatives, which may separate TNT-contaminated plants from naturally stressed plants and would allow for accurate detection of buried ordnance at the landscape level.