Doctor of Philosophy / Department of Agronomy / Johanna A. Dille / Success of plants depends largely on their environment. A survey, field and greenhouse studies were conducted from 2006 to 2010 to characterize the dynamics of giant ragweed (AMBTR) and common sunflower (HELAN) in KS through determining their distribution, plant-soil feedback response and demography. The survey revealed a distinct distribution pattern within the state with HELAN being more dominant than AMBTR. Populations of AMBTR and HELAN from KS could be grouped by their emergence characteristics. Plant-soil feedback response of AMBTR and HELAN varied across sites with HELAN having a consistent positive feedback response while AMBTR had negative feedback response in IL, KS, MI-a, MI-B, OR and SD but not in MT. Interaction coefficient analysis was neutral for IL, KS and MI-b while positive for OR and SD and negative for MI-a and MT. Plant-soil feedback response of KS-derived populations of AMBTR and HELAN indicated that both KS-AMBTR and KS-HELAN seemed to grow best in soil preconditioned by another species. Seed survivorship over winter and summer, emergence, and population growth rates of both weed species varied over three years with HELAN having higher growth rates that AMBTR. This study showed that demographic success of HELAN and AMBTR was greatly influenced by the climatic conditions more than any other factors. AMBTR from IL, if introduced to the state can adapt to KS conditions. Further studies including impacts of soil biota, nutrient dynamics and biochemical processes such as allelopathy are needed to better understand the mechanism behind the plant-soil feedback response of the two weed species and its contribution to the demographic success of AMBTR and HELAN.

The difference in the ability of plants to obtain resources has been used to predict the competition outcomes. Competitive interactions between plants can be influenced by trophic interactions. If mycorrhizae increase the growth of inferior competitors, then it can prevent competitive exclusion. I examined the effect of mycorrhizae on competitive interactions and found that inferior competitors had slow growth in the absence of mycorrhizae, but greater growth with mycorrhizae. By providing greater growth responses to inferior versus superior competitors, mycorrhizae promoted coexistence. In a separate experiment, I looked at the effects of whole soil biota on competitive ability. Coexistence can occur if dominant competitors experience negative feedback and or if inferior competitors experience positive feedback. I examined Plantago lanceolata L. specific feedback effects on competitive ability of 21 co-occurring species. I found that feedback effects did not affect hierarchies, and that positive feedback did not improve species competitive ability. / NSERC (Natural Science and Engineering Research Council), University of Guelph, Queen Elizabeth II Graduate Scholarship for Science and Technology

Plant populations near the northern limits of their geographic distribution may experience different biotic pressures than southern populations. For instance, if herbivores are scarce in northern populations, performance of their host plants may benefit. In this study, I looked at populations of burdock (Arctium minus) along an 815 km latitudinal gradient from the northern range limit to more southern populations. I found that plant height, stem diameter, and number of seeds all increased on approaching the northern limit. In addition, I also found significant decreases in herbivory by generalist and specialist leaf and seed predators, even though northern plants invested less in physical and possibly chemical defenses. In an experiment in which seeds were planted in different soils, marginal soil initially produced smaller plants, but subsequently larger plants than soil from southern populations. These results indicate northern populations experience relaxed pressure from natural enemies and may benefit as a result.

Plant populations near the northern limits of their geographic distribution may experience different biotic pressures than southern populations. For instance, if herbivores are scarce in northern populations, performance of their host plants may benefit. In this study, I looked at populations of burdock (Arctium minus) along an 815 km latitudinal gradient from the northern range limit to more southern populations. I found that plant height, stem diameter, and number of seeds all increased on approaching the northern limit. In addition, I also found significant decreases in herbivory by generalist and specialist leaf and seed predators, even though northern plants invested less in physical and possibly chemical defenses. In an experiment in which seeds were planted in different soils, marginal soil initially produced smaller plants, but subsequently larger plants than soil from southern populations. These results indicate northern populations experience relaxed pressure from natural enemies and may benefit as a result.

Plant-soil feedback has been a well-studied mechanism in recent years of the success of invasive plants, the shift of species in succession, and the structure of plant communities in general. It is a process during which the plant affects the soil with its growth and these changes are reflected in the growth of other plants. Despite the large number of previous studies, relatively little attention is paid to the interaction of plant-soil feedback with other factors, which I consider important for understanding its role in natural plant communities. The aim of the work was to clarify the influence of plant-soil feedback on model species Arrhenatherum elatius and Centaurea scabiosa and to compare its influence with other factors - interspecific competition and herbivory (simulated loss of aboveground biomass). The influence of factors was investigated using biomass and changes in plant physiology, specifically chlorophyll fluorescence and the content of elements in aboveground biomass. The plant-soil feedback mechanism of the model species was assessed using the content of elements in the soil after cultivation. In the biomass of the species Arrhenatherum elatius, the factors of plant-soil feedback and competition manifested themselves in mutual interaction, when the presence of a competitor changed...

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Plant-soil feedback is an important mechanism in plant communities affecting their species composition. Understanding plant-soil feedback is crucial for describing the interactions between plant communities and their soil communities. The effects of plant-soil feedback can be positive, negative or neutral and because of these changes plants are either coexisting or competing. Plant-soil feedback is often studied within plant succession, plant invasion and plant dominance in a plant community. In these cases, the nature and strength of influence of each species is studied. Despite the large number of previous studies on plant-soil feedback, very little is known about the temporal and spatial changes of the intensity of plant-soil feedback and the intraspecific variability in plant-soil feedback. The aim of my work was to observe the temporal changes in the plant-soil feedback of a species growing in soil which was conditioned by the same or other plant species. Another goal was to explore spatial changes in plant-soil feedback. The last objective was to test differences in the intensity of plant-soil feedback among different genotypes of one species. The results of my work demonstrated that the intensity of intraspecific plant-soil feedback of Rorippa austriaca is negative and increases with...

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The processes that determine the structure of plant communities are of considerable practical and theoretical interest. Natural enemies such as herbivores, seed predators and pathogens provide one potentially important influence on plant diversity. I investigated the effects of natural enemies on plant diversity in two contrasting, species-rich plant communities (tropical forests in Panama and temperate grasslands in the UK), focusing on pre-dispersal seed predation by insects, and the mortality of seeds and seedlings caused by soil fungi. In Panama I found that pre-dispersal insect seed predators generate significant levels of mortality in multiple tropical tree species, with high heterogeneity in predation rates among individuals and at different forest sites. Insect seed predators were highly host-specific, consistent with a role in enhancing plant diversity. At Upper Seeds, a calcareous grassland site in the UK, I used manipulative experiments to show that soil fungi increase the diversity of plants propagating from soil seed banks. A parallel experiment in Panama, mimicking germination under light gap conditions, revealed differential effects of fungi among sites, with fungicide treatment appearing to increase the diversity of propagated seedlings at some sites but reducing it at others. These results suggest that the influence of soil fungi on pre-emergence mortality can alter plant diversity, even when post-emergence mortality from fungal pathogens is limited. In Panama, I also tested whether enemy-mediated mortality increases with rainfall, potentially contributing to the positive regional correlations widely observed between precipitation and plant diversity. In contrast to predictions, neither pre-dispersal insect seed predation nor the influence of soil fungi on seedling recruitment were affected significantly by site humidity, or (for soil fungi) with experimentally manipulated soil moisture levels. Overall, my results provide evidence that pre-dispersal seed predators and soil fungi can affect plant recruitment and diversity at early life stages, with potential consequences for the community structure of adult plants.