| Summary: | Nitrogen levels are predicted to rise dramatically in the coming decades as a result of increased deposition from the burning of fossil fuels and the large-scale conversion of nitrogen into a useable form for agriculture. Plant communities react strongly to increases in soil nitrogen, particularly in northern ecosystems where nitrogen levels are naturally very low. An experiment in northern Canada that began in 1990 has been investigating the effects of long-term nutrient enrichment (fertilizer addition) and release from herbivory (fencing). After 22 years of treatment, plant community composition has been substantially altered in fertilized plots, whereas exclusion of herbivores has had almost no detectable effects. I used this experiment to address two questions about the causes and consequences of long-term nitrogen enrichment in the boreal forest.
In the first data chapter I focused on four understory species that had different levels of success in fertilized plots to investigate why some species are more successful than others under nutrient enrichment. I hypothesized that successful species would be taller, have higher specific leaf area, spurt earlier in the growing season and be more morphologically plastic than their less successful counterparts. I demonstrated that each of the two species that came to dominate fertilized plots has a different combination of traits and responses that likely gave them a competitive advantage; Mertensia paniculata has the highest specific leaf area of the four species, and E. angustifolium is tallest and exhibits morphological plasticity when fertilized by increasing biomass allocation to leaf tissue.
In the second data chapter I assessed how increasing nitrogen levels has affected investment in sexual reproduction in four herbaceous understory species. Whereas plants in northern ecosystems reproduce mainly through clonal growth, rapidly changing environmental conditions and warmer temperatures will likely result in increased benefits of sexual reproduction. Fertilization increased the probability of flowering for Achillea millefolium and resulted in a higher allocation of biomass to flower parts for E. angustifolium but did not affect investment in reproduction for Festuca altaica or M. paniculata. Increased investment in sexual reproduction could give a competitive advantage to the former two species in the future. === Science, Faculty of === Botany, Department of === Graduate
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