PUB - Publikationen an der Universität Bielefeld

Non-native plant species and invaded habitats are expanding more rapidly around the globe. Certain species have successfully colonized areas beyond their native habitats, while others have experienced rapid population increases. Some of these species have emerged as the main contributors to the loss of global biodiversity, posing also serious risks to the world's economy and health security. The ability of invasive species to successfully establish across a broad spectrum of abiotic and biotic environmental conditions has been partially attributed to significant variation in several life-history traits. Some of these traits may be useful pre-adaptations, while others diverge among populations due to distinct post-introduction evolution. For example, invasive plant species are often distinguished from their native congeners by their rapid growth in the invaded area, which can result in higher vegetative biomass and reproductive output. The extent to which invading species can maintain their enhanced biomass and reproductive output across the whole environmental gradient of their invaded territory, particularly during range expansion, remains unclear. Furthermore, there has been a significant increase in the number of naturalized species that have developed into invasive species. As a result, it is expected that invasive species will expand even more in the future, with no evident signs of decline. However, our understanding of the traits that contribute to the success of non-native species throughout the stages of introduction, naturalization, and invasion is limited, especially in the presence of abiotic and biotic stressors.

The aim of this thesis was to investigate the ability of various plant populations of a successful range expander species in the Brassicaceae family, Bunias orientalis, to establish in new environments. Therefore, this study explored trait variation and the effects of both abiotic and biotic stressors on numerous aspects of growth, defense, and reproduction-related traits. In order to comprehensively understand the variation in traits, a diverse range of analytical tools was applied and experiments were performed in the field and in the laboratory. Multivariate statistical techniques were used to analyze the data.

In Chapter II, I investigated the trait variation among different plant populations of B. orientalis that were either of native, invasive, or naturalized status. This was accomplished by growing seeds collected from different countries in common environmental conditions in the field and studying their traits in relation to the environmental conditions of their local origins. Several trailts related to phenology, growth, and reproduction were compared across status and populations. The analysis of the field data revealed that the phenology did not change with regards to status. However, numerous plants from native populations that originated in places with low yearly temperatures did not start flowering. Plants from invasive populations produced more leaves than those from native populations, suggesting a potential advantage in their ability to accumulate vegetative biomass. There was no significant difference in the number and mass of silicles and other growth traits among the different status groups. However, notable variation was found in these traits among the different populations. Functional trait variation may be associated with long-term adaptations to local environments at the place of origin as well as high genetic diversity. Additionally, distinct environmental factors in the new environment may have also accounted for the significant variation in traits.

In Chapter III, I used a laboratory experiment to gain insight into how competition between plants in a non-invasive naturalized versus an invasive population influences population growth patterns. I expected that these populations may differ in their intra- and inter-population competitive effects owing to their distinct impacts on the introduced habitats. In addition, this chapter expands on the findings of Chapter II by investigating how morphological and physiological traits respond to intra- and interpopulation competition, which may contribute to the successful establishment and range expansion of B. orientalis in its introduced habitat. Multiple traits were measured twice using plants of Bunias orientalis from a naturalized and an invasive population grown alone or in groups of two or three, after an initial phase of growth and after a re-growth phase following a cut-back, with the aim of exploring the allocation of resources towards growth and regrowth. The findings indicated that competition primarily affected traits associated with growth rather than those related to physiology. Furthermore, the magnitude of competition within and between populations varied across plant traits at the initial and the second harvest. The invasive population showed a superior performance in comparison to the naturalized population by exhibiting a greater allocation of resources to the aboveground biomass and producing more and longer leaves especially when grown in competition of three plants. Furthermore, I revealed that the competitive ability of plants from the invasive population surpassed that of plants from the naturalized population, which could contribute to the successful establishment and range expansion of B. orientalis in the areas where it is invasive.

In Chapter IV, I examined the impact of low and high nitrate fertilization on the responses of an invasive population of B. orientalis to the pathogenic fungus Alternaria brassicae and the herbivorous insect species Mamestra brassicae. I measured various plant traits, including biomass, water content, carbon and nitrogen content, sugar composition as well as the concentrations of leaf glucosinolates, known for their chemical defense properties and commonly found in the Brassicaceae family.
Additionally, I analyzed the leaf peroxidase activity, which is known to play a role in plant-pathogen interactions, and compared the trichome densities on leaves in response to the two levels of fertilization. Plants grown under high fertilization had a higher aboveground biomass and leaf trichome density than plants grown under low fertilization. Fertilization and antagonist attack had no influence on leaf water, nitrogen content, or glucosinolate concentrations. However, following fungal infection, the overall soluble sugar content, particularly fructose, as well as leaf peroxidase activity increased substantially in leaves, although this was independent of fertilization. In summary, this study indicates that in conditions where fertilization levels are elevated, B. orientalis plants exhibit a greater allocation of resources towards growth and morphological defenses rather than chemical defenses. On the other hand, the induced responses to a short antagonist attack appeared to be unrelated to the availability of nitrate within this specific population.

In Chapter V, I generally discuss all results and present an outlook. In conclusion, this thesis demonstrates that the invasion success of B. orientalis is dependent on a broad spectrum of factors, and no single hypothesis can predict the species' invasion success. I revealed that having more leaves in invasive B. orientalis populations may provide advantages in terms of growth, light resource utilization and competitive ability over conspecific neighbors. The influence of abiotic factors such as increased nutrient availability revealed that resources in the B. orientalis population studied here were allocated more to growth and morphological defenses than chemical defenses. However, short-term effects of biotic stressors such as fungal pathogens and herbivorous insects on plant responses were independent of nutrition availability. Furthermore, I highlighted that B. orientalis plants exhibit multiple functional traits associated with pre-adaptation and post-introduction mechanisms that are likely advantageous for successful range expansion.