Peebles, Kala Lynn2014-08-222014-08-222014-05https://hdl.handle.net/11299/165058University of Minnesota Ph.D. dissertation. May 2014. Major: Natural Resources Science and Management. Advisor: Rebecca A. Montgomery. 1 computer file (PDF); xviii, 134 pages, appendix A.As global climate warms, plant distributions are predicted to shift due to altered disturbance, precipitation and seasonal temperature regimes. Areas currently occupied by the northern temperate and boreal forests are projected to undergo significant changes in response to climate warming. Northward migration of temperate and southern boreal species could alter community composition, diversity and productivity in forested ecosystems. Critical to understanding potential forest shifts is gaining a better understanding of warming effects on young tree seedlings, the survival and recruitment of which will influence future forest composition. In this dissertation, I examine the effects of climate warming on tree seedling physiology, growth and survival in species common to the temperate-boreal forest ecotone using both glasshouse and field experiments. </DISS_para> <DISS_para>To examine tree seedling physiological response to the dual-stress of high temperature and low light, I grew six species in two temperature environments (25°C day/19°C night or 30°C day/24°C night) in glasshouses. Later, I imposed a low light treatment (<1% light) designed to move seedlings below their whole plant light compensation points. Tree seedlings germinated and grown in elevated temperature had lower plant biomass, lower non-structural carbohydrate reserve pools and tissue concentrations relative to seedlings grown in cooler temperatures. After subjected to low-light stress, seedlings in elevated temperature showed higher dark respiration rates and mortality relative to seedlings grown in a cooler temperature environment. Non-structural carbohydrates are an important means by which plants respond to environmental stress and promote survival. To examine how allocation to and use of NSC reserves varies across resource gradients, I planted tree seedlings in one of four light environments (5%, 10%, 30% and 60% canopy openness) in a field experiment. I found that NSC reserve use and accumulation varies among species, within species across a light gradient and seasonally in conjunction with growth phenology. Within species, seedlings accumulated higher reserves when growth was limited by light availability (low light) or when light was sufficient to allocate to both growth and reserve accumulation (high light). Finally, I planted six species into three light environments (10%, 30% and 60% canopy openness), and in one of two temperature treatments within light environment (ambient or passively warmed) to examine the interactive effects of warming and light environment on seedling growth and survival. I provide evidence that warming, generally, has positive effects on growth and productivity in six species common to the temperate-boreal ecotone in low and intermediate light levels, but generally negative effects in high light. Overall, this dissertation suggests that elevated temperature may inhibit tree seedling productivity and resilience to environmental stress when coupled with additional environmental stress (i.e. very low or very high light), but increase productivity in moderate resource environments.en-USThesis or Dissertation