The Spruce and Peatland Responses Under Changing Environments (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 northern 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. This talk focuses on the responses of the two dominant ericaceous shrubs (Rhododendron groenlandicum and Chamaedaphne calyculata), quantifying the seasonal patterns of photosynthesis and respiration to the first two years of temperature and CO2 treatments. These two species dominate the understory at this site (~80% of biomass) and are 35% more productive than the trees in this open canopy forest.
Results/Conclusions
Gas-exchange results from the first treatment year exhibited some photosynthetic acclimation to CO2 treatments and respiratory acclimation to temperature, although the degree of acclimation was species-specific in each case. Nitrogen per unit leaf mass of R. groenlandicum decreased under elevated CO2, but nitrogen per unit leaf area was maintained by a concurrent increase in leaf mass per area. Detailed gas exchange measurements across a temperature range of 10 to 45oC revealed the trade-off between photosynthetic and respiratory rates that underpin a broad thermal optimum of net assimilation rates observed in the second full treatment year. Whole ecosystem warming extended the physiologically active season in both spring and fall for these years, increasing the period of active carbon assimilation. We discuss how these results will be incorporated into modeling efforts for northern peatlands in global dynamic vegetation models, including how choices such as functional forms for photosynthetic and respiratory responses to temperature may have large effects on projections of boreal ecosystem responses to global change.