Stefanski, Artur2024-01-052024-01-052021-08https://hdl.handle.net/11299/259661University of Minnesota Ph.D. dissertation. August 2021. Major: Natural Resources Science and Management. Advisors: Peter Reich, Rebecca Montgomery. 1 computer file (PDF); xiv, 160 pages.Photosynthesis is a physiological process, without which, life as we know it would not be possible—period. Photosynthesis allows plants to harvest and store energy in carbon bonds; however, like any biological process, it is constrained by several abiotic and biotic conditions. This dissertation was inspired by my fascination with the process of photosynthesis, as I sought insights into: i) its complexity and whether and how the fast pace of changing climate might affect it, and ii) understanding whether signals of plant responses to an ecologically realistic in situ warming and reduced rainfall experiment are consistent across multiple years with varying climatic conditions (e.g., wet vs. dry and hot vs. cold years) warranting confidence and generalization of findings.Leveraging the B4WarmED (Boreal Forest [4] Warming at an Ecotone in Danger) experiment, my collaborators and I investigated whether and how the photosynthetic process of tree species from the boreal-temperate ecotone—as a whole and its subcomponents—will be affected by future climate. B4WarmED is an experimental platform that implements ecologically realistic open-air warming and summer rainfall reduction in juvenile tree species native to the boreal-temperate ecotone in North America. My dissertation is in three parts, i.e., three chapters. First, I show that juvenile tree species reveal a surprising lack of sensitivity of the biochemical limitation of the photosynthetic process in response to warming and water limitation. Second, I demonstrate that stomatal behavior changes under warming such that plants become more prolific water users but only when water is ample. The stomatal behavior becomes more conservative when water becomes more limiting and marginal water cost of carbon gain increases due to either rainfall reduction or an increase of evaporative demand caused by warming. Moreover, the weak effect of elevated temperatures becomes detectable, but only when thermal effects are separated from indirect effects of warming on soil water content. Third, I examine a complex set of interactions that act together to shape carbon assimilation, including its component processes and climatic variables on a long-time scale. As a result, I demonstrate that carbon assimilation of tree species from the boreal-temperate ecotone will be affected by future climate change where temperate species will have a primarily positive response while the boreal will have a negative response. This change in carbon assimilation will be dictated by the alleviating effect that warming will have on photosynthetic enzymes especially, for temperate species, and reduction of stomatal conductance that reduces water loss resulting in increased water use efficiency in all species. Further, I also show that the effects are directionally consistent within species across years and vary in magnitude on an intra and interannual scale due to realized climatic conditions. Together, this dissertation research adds to our understanding of when and how climatic change—in terms of elevated temperatures and altered precipitation—will affect the photosynthetic process and its components. This provides additional insight to basic plant physiology, carbon cycle modeling in response to climate change, and demonstrates how tree species from the boreal-temperate forests will fare in the future climate.enThesis or Dissertation