Trees acclimate to environmental variation by altering their growth patterns, internal chemistry, and mycorrhizal associations. Growth, chemical synthesis, and mycorrhizae simultaneously compete for limiting resources, such as nitrogen and water, and their interactions can significantly influence tree defenses against biotic and abiotic stress. The Optimal Allocation, Growth-Differentiation Balance, and Treseder and Allen's models have been proposed, among others, to explain and predict patterns of tree and mycorrhizal responses to soil nutrient availability. To date, these models have been tested in isolation. This dissertation describes how these three models can be integrated into a single model that can explain patterns of tree acclimation over a nutrient availability gradient, the Mycorrhiza-Defense Allocation Synthesis (MyDAS). The MyDAS model was derived from greenhouse-based experiments, but was then tested and partly validated under conditions that more closely mimic field conditions. Testing showed that the initial patterns of acclimation to nutrient availability can have strong impacts on the mechanisms used by trees to respond to drought. An important corollary of this research is that fertilization and mycorrhizal inoculation, which are routinely used as a means to improve tree establishment and health in managed settings, can be irrelevant or even counterproductive to enhancing tree tolerance to biotic and abiotic stress.
Aside from MyDAS, key outcomes of this research were: (1) lignin deposition likely plays a role in regulating mycorrhizal colonization; (2) acclimation to nutrient availability can significantly affect the nature of tree responses to drought; (3) initial soil type can influence tree growth more than fertilization and mycorrhizal inoculation on plant growth and health; and (4) commercial mycorrhizal products may not benefit trees in managed environments, even when these products are shown to be viable.
I believe that these outcomes will open up new areas of investigation on the ecology of woody plants, from natural environments to urban landscape settings. Ultimately, a better understanding of acclimation processes will allow the development of more rational practices to alleviate biotic and abiotic stress in managed tree systems