Climate change impacts upon plants and soils along environmental gradients

Climate change is altering ecosystems worldwide. Despite advances in our understanding of the effects of increasing temperature, little is known about how increased temperatures will impact upon plants and soils across environmental gradients. This thesis investigated soil legacies and plant defense along a subarctic tundra elevational gradient as well as soil microbial and nematode community and litter decomposition responses to interactions between plant functional group removal and warming along a post-fire successional chronosequence in boreal forest. In subarctic tundra, soil legacies were strongly temperature-driven, but these effects varied between vegetation types, with plants grown in meadow vegetation showing a strong unidirectional decline in growth when grown in soils from increasing elevation and plants grown in heath soils showing no response to elevation. Positive legacy effects were observed in soils from the lowest (i.e., warmest) elevation and these effects were primarily the result of soil abiotic, as opposed to biotic, effects. Subarctic plant defense was reduced by nitrogen (N) fertilization, while phosphorous (P) fertilization had few effects. Nitrogen fertilization reduced plant defense most at the lowest elevation. Along a boreal forest successional gradient, warming and plant functional group removal favored bacteria in the youngest forests. In contrast, the nematode community was impaired by plant functional group removal, but was not responsive to warming or successional stage. Litter decomposition was strongly influenced by understory plant functional groups. Mosses increased litter mass loss and reduced P loss, while shrubs decreased litter mass loss and immobilized more litter P; warming and successional stage were less significant drivers. Taken together, these results demonstrate that above- and belowground responses to temperature can vary considerably in subarctic tundra and boreal forest ecosystems. Therefore, the work presented in this thesis highlights the importance of considering environmental context (i.e., changes associated with increasing elevation/succession) when making predictions about how global climate change will affect plant and soil-mediated ecosystem processes