The Spruce and Peatland Responses Under Climatic and Environmental change (SPRUCE) project is a large-scale, long-term experiment investigating the effects of warming and elevated CO2 on an ombrotrophic bog in Minnesota. Globally, such boreal peatlands store an estimated 500 ± 100 Pg C, a disproportionately large amount relative to the land area they cover. SPRUCE is utilizing 10 large (12-m diameter) enclosures to increase air and soil temperatures to a range of targets (+0 °C, +2.25 °C, +4.5 °C, +6.75 °C, +9 °C) under both ambient and elevated (+500 ppm) CO2 concentrations for 10 years. Whole ecosystem warming treatments began in August 2015 and elevated CO2 treatments began in June 2016. This talk will address the photosynthetic and respiratory responses of vascular plants to the treatments as measured with a variety of in-situ and ex-situ porometric measurements conducted throughout the 2016 growing season. We will focus on the responses of two dominant trees (Picea mariana and Larix laricina) and two dominant ericaceous shrubs (Rhododendron groenlandicum and Chamaedaphne calyculata), their implications for carbon cycling of the bog and measurements made at larger scales.
Pretreatment data collected at this site indicate that the physiologically active season typically begins in late May and extends into the fall until freezing nighttime temperatures are consistently reached, typically in October. Early season results (pre-CO2 treatment activation) show some degree of thermal acclimation of photosynthesis in L. laricina, of respiration in P. mariana and C. calyculata, and of both processes in R. groenlandicum. However, late season measurements exhibit less pronounced thermal acclimation, with the exception of respiration in P. mariana, indicating the complex interactions between phenological changes and treatment effects on physiological processes. Late season net CO2 assimilation measured at ambient CO2 show a down-regulation of photosynthesis in both ericaceous shrub species grown under elevated CO2, but that assimilation at growth CO2 was still greater in this treatment. Additional measurements made during seasonal transitions, indicate a longer active physiological season in warmer treatments. While such measurements provide early indications of ecosystem level changes that may later be observed in plot scale measurements, several complications in scaling from leaf to stand will be discussed in the context of models both at the site scale and more generally for boreal peatlands in global dynamic vegetation models.