Evolution of genotypes during range expansion is driven in part by colonization dynamics. I investigated genetic patterns of colonization and dispersal during initial expansion of an invasive bunchgrass, Brachypodium sylvaticum, into Oregon. Using microsatellite markers, I sampled plants at two different scales: at regular intervals along three parallel roads spanning about 30km, and in populations identified throughout Oregon. I also collected field-generated progeny from a subset of populations and used molecular identification of outcrossing events to estimate selfing rates in both central and peripheral populations. Dispersal patterns were similar at both scales, with non-contiguous dispersal responsible for colonization of new populations. High levels of differentiation were observed at all scales, though newly-colonized populations were more differentiated than older populations. Corvallis populations were responsible for colonization of a majority of populations throughout Oregon, while individuals from Eugene were only occasionally found in new populations. Admixture occurs between Corvallis and Eugene populations, decreasing differentiation, and potentially creating novel phenotypes and increasing evolutionary potential of populations. Selfing rates were high, but two populations in the areas of original introduction had lower rates of selfing, suggesting that selfing rates may decrease as population density and diversity increases with age. The influences of founder effects and bottlenecks on phenotypic evolution during range expansion require further investigation, as inbreeding, lag times, and selection may influence evolutionary trajectories of populations.

Rapid range expansion of newly invasive species provides a unique opportunity for studying patterns of dispersal and gene flow. In this thesis, I examined the effect of landscape features on gene flow in the invasive grass Brachypodium sylvaticum at the edge of its expanding range. I used genome-wide Single Nucleotide Polymorphism (SNP) surveys of individuals from 22 locations in the Clackamas Watershed in the Portland, Oregon metropolitan region to assess genetic diversity and structure, to identify putative source populations, and to conduct landscape genetic analyses. Resistance surfaces were created for each landscape feature, using ResistanceGA to optimize resistance parameters. My STRUCTURE analysis identified three distinct clusters, and diversity analyses support the existence of at least two local introductions. Multiple Regression on distance Matrices (MRM) showed no evidence that development, roads, canopy cover, or agriculture had a significant influence on genetic distance in B. sylvaticum. The effect of geographic distance was marginal and reflected geographic clustering. The model of rivers acting as a conduit explained a large portion of variation in genetic distance. Results indicate that rivers influence patterns of dispersal of B. sylvaticum by human recreational activity centering on use of rivers, and possibly due to movement of deer.

Dynamics of soil water use by two cool-season Agropyron bunchgrasses during the warm season depletion of soil water reserves were monitored for two years in experimental plots in the field. Agropyron desertorum, an introduced, competitive species from Eurasia, extracted more water from the deeper ( > 50 cm) soil layers than the native, less competitive Agropyron spicatum. Agropyron desertorum both extracts this water earlier and to lower soil water potentials than Agropyron spicatum. From the water extraction dynamics of the grasses in monocultures and in their two-way (50:50) mixtures with a shrub they commonly co-occur with, Artemisia tridentata, partitioning of the soil water resource between the grasses and the shrub was inferred. This indicated that Artemisia tridentata and Agropyron desertorum partitioned the soil water resource fairly evenly, while considerable quantities of water in the deeper soil layers under Agropyron spicatum seemed to be available to the shrub without direct competition. The implications of this difference in water resource partitioning for competition of the grasses with Artemisia tridentata are discussed. Predawn and midday xylem pressure potentials were not different between the two grasses in spite of different fluxes through the plants. Agropyron desertorum initiated new adventitious roots in fall and early spring while Agropyron spicatum did so only during spring. Observations from a root observation chamber indicated essentially parallel pattern of lateral root elongation during the depletion phase through top 200 cm of the profile. In both species the number of active tips, and the rate of elongation of active tips, decreased as the soil dried out. Root tips at all depths were inactive by the middle of September. Agropyron desertorum maintained root elongation at 50-110 cm for two weeks longer than A. spicatum.

The occurrence of compensatory photosynthesis was examined in the field during the spring for all foliage elements on two Agropyron bunchgrass species that differ in their evolutionary history of grazing pressure. Compensatory photosynthesis did occur in many individual foliage elements during at least part of their ontogeny. For both species, compensatory photosynthesis was related primarily to delayed leaf senescence and increased soluble protein concentrations, but not to an improvement in the water status of clipped plants. Photosynthetic water use efficiency and photosynthetic rates per unit soluble protein of foliage on partially defoliated plants were not increased following the clipping treatments. Light and temperature dependencies of gas exchange measurements were usually very similar between the two Agropyron species. However, gas exchange rates per unit foliage area of leaves exserted late in the spring on~ spicatum plants were significantly different from those on A. desertorum plants when these leaves were senescing. To determine the ecological significance of these differences between species for light and temperature dependencies, the average carbon gain and water loss rate per tiller were estimated. The differences between species for carbon gain and water loss rates per tiller in this environment were substantially less than the individual leaf gas exchange differences between species. Photosynthetic activity and survival of leaves were also determined during the fall, winter, and early spring for the two Agropyron species in the field. A large proportion of the leaves of both species survived the winter. Photosynthetic rates of both species declined as air temperature dropped during the fall, were slightly positive during the winter between periods of snow cover, and increased during the early spring. Even though there is potential for photosynthesis during a winter with intermittent snow cover, total plant saccharide pools were barely maintained over such a winter. Although A. desertorum and A. spicatum were exposed to different levels of grazing pressure during their evolutionary history, the phenology, water status, and gas exchange rates of foliage and tillers were very similar both for undefoliated as well as partially defoliated plants. Therefore, we conclude that compensatory photosynthesis does not appear to be an important ecological component of herbivory tolerance for these species.

It is common knowledge that invasive species cause worldwide ecological and economic damage, and are nearly impossible to eradicate. However, upon introduction to a novel environment, alien species should be the underdogs: They are present in small numbers, possess low genetic diversity, and have not adapted to the climate and competitors present in the new habitat. So, how are alien species able to invade an environment occupied by native species that have already adapted to the local environment? To discover some answers to this apparent paradox I conducted four ecological genetic studies that utilized the invasive species Brachypodium sylvaticum (Hudson) Beauv. to determine mechanisms contributing to adaptation and success in the novel habitat. The first study used simulations and experiments to test the hypothesis that genetic purging, the process where genetic load is reduced by selection against the recessive deleterious alleles expressed in the homozygous state, promotes invasive range expansion. I found that homozygous populations on B. sylvaticum's range periphery displayed lower inbreeding depression compared to heterozygous populations near introduction sites. Empirical tests with B. sylvaticum further demonstrate that purging of genetic load is a plausible scenario promoting range expansion during invasion. Next, I explored how the interaction between population genetic diversity and the environment contributed to the establishment and spread of Brachypodium sylvaticum. I found that nitrogen application increases both final size and shoot biomass for B. sylvaticum individuals from source populations with low HS levels to levels found in individuals from populations with high HS. A coefficient of relative competition intensity index (RCI) displayed reduced effects of interspecific competition on B. sylvaticum biomass in high nitrogen plots. Results show that elevated nitrogen deposition is a factor that increases establishment of introduced species with historically small effective population sizes. Thirdly, I investigated phenotypic differentiation during the establishment and range expansion of Brachypodium sylvaticum. Utilizing a novel approach, unique alleles were used to determine the genetic probability of contribution from native source regions to invasive regions. These probabilities were integrated into QST-FST comparisons to determine the influence of selection and genetic drift on twelve physiological and anatomical traits associated with drought stress. Phenotypic divergence greater than neutral expectations was found for five traits between native and invasive populations, indicating selective divergence. Results from this study show that the majority of divergence in B. sylvaticum occurred after introduction to the novel environment, but prior to invasive range expansion. The final chapter of my dissertation investigates the adaptive role of genetic differentiation and plasticity for Brachypodium sylvaticum invasion. Plasticity was measured across treatments of contrasting water availability. Linear and nonlinear selection gradients determined the effect of directional and quadratic selection on plasticity and genetic differentiation. Invasive trait divergence was a consequence of post-introduction selection leading to genetic differentiation, as there were no plastic responses to contrasting water availability for any measured traits. Genetic divergence of invasive plants was not consistently in the direction indicated by selection, suggesting limitations of selection that may be a consequence of physical constraints and/or tradeoffs between growth and abiotic tolerance. Results suggest that selection, rather than plasticity, is driving phenotypic change in the invaded environment. The combined volume of these studies contributes significantly to the field of invasion and plant biology by providing novel insights into the processes underlying range expansion, adaptation, and ultimately, evolution of introduced species.

Pseudoroegneria spicata and Elymus wawawaiensis are two native perennial bunchgrasses of North America's Intermountain West. Frequent drought, past overgrazing practices, subsequent weed invasions, and increased wildfire frequency have combined to severely degrade natural landscapes in the region, leading to a decline in the abundance of native vegetation. Being formerly widespread throughout the region, P. spicata is a favorite for restoration purposes in the Intermountain West. Elymus wawawaiensis, which occupies a more restricted distribution in the Intermountain West, is often used as a restoration surrogate for P. spicata. However, since most restoration sites are outside the native range of E. wawawaiensis and as the use of native plant material may be more desirable than a surrogate, the use of E. wawawaiensis as a restoration plant material has been somewhat controversial. The main goal of my research was to identify plant materials of these species with superior seedling growth, drought tolerance, and defoliation tolerance, traits that may contribute to enhanced ecological function in restored rangeland plant communities. I conducted a growth-chamber study to evaluate morphological and growth-related traits of germinating seedlings of these two species. My study suggested that, while the two bunchgrasses are similar in many ways, they display fundamentally different strategies at the very-young seedling stage. While P. spicata exhibited greater shoot and root biomass to enhance establishment, E. wawawaiensis displayed high specific leaf area (SLA) and specific root length (SRL), two traits commonly associated with faster growth. According to the eco-physiology literature, plants with greater stress tolerance display lesser growth potential. However, my greenhouse study showed that E. wawawaiensis was relatively more drought tolerant than P. spicata, despite higher expression of growth-related traits, e.g., SLA and SRL. While the two species displayed similar water use efficiency when water was abundant, E. wawawaiensis was also more efficient in its water use when drought stress was imposed. In a field study, I found E. wawawaiensis to be twice as defoliation tolerant as P. spicata. This study showed that P. spicata is typically more productive in the absence of defoliation, but E. wawawaiensis was more productive after defoliation due to its superior ability to recover and hence is a better candidate for rangelands that will be grazed. Hence, my study showed that E. wawawaiensis, despite being regarded as a surrogate for P. spicata, exhibits superior seedling establishment, drought tolerance, and defoliation tolerance. Therefore, E. wawawaiensis has advantages as a restoration species for the Intermountain West.

The effects that mutualistic soil biota have on invasive species success is a growing topic of inquiry. Studies of the interactions between invasive plants and arbuscular mycorrhizal fungi (AMF) have shown changes in AMF community composition, reductions in AMF associations in invasive plants, and changes in native species fitness and competitive outcomes in invasive-shifted AMF communities. These findings support the degraded mutualist hypothesis, where invasive species alter the mutualist community composition, resulting in detrimental associations with the new mutualist community for native species. Here I present two studies that examine various aspects of the arbuscular mycorrhizal fungal (AMF) mutualism in the success of a newly invasive bunchgrass, Brachypodium sylvaticum. The first chapter is a field survey of AMF associations between a native bunchgrass, Elymus glaucus and B. sylvaticum in the invaded range. The second chapter presents a test of reduced mycorrhizal dependence between invasive and native-range populations of B. sylvaticum. For the field survey, AMF colonization and spore density of root and soil rhizosphere samples from B. sylvaticum and E. glaucus from the two regions of introduction of the B. sylvaticum invasion were measured. In this survey I found lower AMF colonization and spore density in B. sylvaticum compared to the native species in the invaded ranges. The reduction in AMF associations in B. sylvaticum was predicted to be due to the evolution of reduced mycorrhizal dependence in invasive populations compared to native populations of B. sylvaticum. I tested the prediction for reduced mycorrhizal dependence by measuring the fitness gains or losses with AMF inoculation compared to sterile conditions in both fertilized and unfertilized treatments for individuals of B. sylvaticum from each of the introduction sites in Oregon, USA and source populations from the native range in Europe. There were no differences in plant or AMF fitness between the invasive and native populations of B. sylvaticum. Under high nutrients the interaction between all B. sylvaticum plants and AMF was mutualistic. Under low nutrient treatments both B. sylvaticum and AMF had reduced fitness measures, suggesting a competitive interaction. Nutrient levels of inoculated unfertilized soils are similar to field conditions. It is likely that the reduction in AMF associations in B. sylvaticum observed in the field is due antagonistic interactions between AMF and B. sylvaticum.