Understanding the biotic factors influencing invasive plant performance is essential for managing invaded land and preventing further exotic establishment and spread. I studied how competition between both conspecifics and native co-habitants and arbuscular mycorrhizal fungal (AMF) impacted the success of the invasive bunchgrass Brachypodium sylvaticumin early growth stages. I examined whether invasive plants performed and competed differently when grown in soil containing AMF from adjacent invaded and noninvaded ranges in order to determine the contribution of AMF to both monoculture stability and spread of the invasive to noninvaded territory. I also directly manipulated common mycelial network (CMN) formation by AMF to determine hyphal network contribution to competitive interactions. I found that invasive plants performed most poorly (as indicated by decreased chlorophyll content, size and shoot dry mass) in invaded range soil against conspecifics. This could be two-pronged evidence for existing biotic pressure on the invasives to expand into adjacent noninvaded ranges. I also found a negative effect of AMF colonization and invasive plant performance, potentially indicating deleterious plant-soil feedbacks which could help maintain plant biodiversity at a community level. CMN effects were found to be interactive with root competition and directly affected the performance and nutrient status of B. sylvaticum. Although no direct correlations between AMF colonization levels and competition were found, CMN presence contributed significantly to plant growth and nutrient status. Therefore AMF, through infection and CMN formation, may be able to influence invasive plant growth and spread in the field.

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This work includes an introduction and literature review of mycorrhizal enhancement of the mineral nutrition of host plants. Particular attention was paid to the role of mycorrhizae in acquiring P from inorganic phosphates. Two experiments were designed to study a mechanism by which the ectomycorrhizal fungus Hebeloma westraliensis can enhance the availability and absorption of P from Ca phosphate by <u>Eucalyptus diversicolor</u> in calcareous soil. Chapter 2 reports on a study on the growth and oxalic acid production of <u>H. westraliensis</u> in calcareous and noncalcareous liquid media. Oxalic acid is thought to chelate cations from phosphate precipitates allowing the release of phosphate into solution. L-threonine was added as an additional treatment to asses the ability of this amino acid to inhibit oxalic acid production by the fungus. Growth and oxalic acid production of <u>H. westraliensis</u> were increased in the calcareous systems. L-threonine had little effect on dry weight of mycelium but substantially reduced oxalic acid production in the calcareous media. Chapter 3 reports on a study of E. diversicolor X <u>H. westraliensis</u> mycorrhizal synthesis in model calcareous and noncalcareous systems. Oxalic acid production was measured in these systems as well as various measurements of solution and host plant nutrition. Solution P concentration decreased and solution Ca increased in the calcareous systems. Mycorrhization decreased the solution concentrations of both of these nutrient elements, due partly to sequestration of these elements in fungal and plant tissues. Plant tissue P concentration was decreased and tissue Ca concentration increased in the calcareous systems. Mycorrhization increased the concentrations of both of these elements in plant tissues. Mycorrhization also increased the height and dry weight of seedlings at harvest and ameliorated symptoms of nutrient deficiencies seen in nonmycorrhizal plants in the calcareous system. Oxalic acid production by the fungus and by the plant were increased in the calcareous system. Attempts to correlate oxalic acid production with solution and plant nutrition were unsuccessful. The relationships of these results to the mycorrhizal enhancement of plant nutrition in calcareous soil is discussed. / Master of Science