Physiology and genetics of nitrogen uptake and metabolism in trees

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Plants face the unique challenge of being rooted in place. They must acquire nutrients from a limited volume of soil over an entire lifetime - in the case of trees, over centuries. In Canada, forest soils are typically cold, acidic and low in available nutrients, particularly nitrogen (N). Understanding the adaptations of trees to these soils, and how these may shape tree response to changing climates is an important practical and scientific goal. In natural systems, tree roots associate with fungal partners, mycorrhizae, that are vital to nutrient mineralization, uptake and transfer to the host. Mycorrhizal species differ in their ability to break down organic matter and take up nutrients, and in their response to abiotic factors. This has important implications for tree nutrition in changing climates. We know little about variation among mycorrhizae or about the effects of temperature or CO2 on these symbioses. This proposal describes a program of research on tree roots and their associated mycorrhizal fungi, and the effects of these symbioses on N uptake and N-form preference under contrasting temperatures, N availability and CO2. Previous research in this program has focused on N uptake by tree roots, alone. We have documented genetic variation among and within species in nutrient uptake and use efficiency, and found that soil temperature affects the form in which N is taken up. As global temperatures rise, these effects will have significant implications for forest growth and community structure. Future research will investigate contrasting tree and mycorrhizal species to understand how root temperature affects the rate and form of N uptake at the cellular and whole-root level, the expression of genes integral to N uptake, and how CO2 affects the N-carbon balance between the tree and fungus. This work will deepen our understanding of the functional diversity in mycorrhizal symbioses, and will reveal potential complexities in forest response to global change. The research is designed for the training of five graduate students and a number of undergraduates who will be part of the NSERC CREATE Program in Forests and Climate Change.