We are conducting an in situ warming by elevated CO2 manipulation located in a high-carbon, ombrotrophic peatland in northern Minnesota. Methods to warm 10 large plots (12-m diameter) combined with aboveground enclosure walls create an internally recirculating warm air envelope. Together with soil deep heating to simulate a broad range of future warming treatments of as much as +9 °C, whole-ecosystem warming was initiated in August 2015, followed by elevated CO2 atmospheres (eCO2 at + 500 ppm) in June 2016 (half the plots). Net surface C flux estimates as CO2 and CH4 were measured from 1.2 m diameter in situ collars monthly from April through December. Annual assessments of tree growth were made with circumference observations at dbh, automated dendrometer bands, and height evaluations via terrestrial LIDAR. Shrub-level vegetation growth above the Sphagnum bog surface was obtained from destructive harvest of a pair of 0.25 m2 plots in each enclosure. Sphagnum growth was measured from sampled populations. Growth data were tallied and extrapolated to the plot scale for annual estimates of plot net primary production by species (Trees: Picea and Larix; Shrubs: Rhododendron, Chamaedaphne, Vaccinium, Maianthemum, and sedges as a group; Sphagnum as a combined community).
Results/Conclusions
The net flux of CO2 and CH4 followed exponential patterns with warming temperatures, and the CO2 flux was larger than CH4. After a lag period of several months in 2016, eCO2 treatments also led to enhanced net CO2 emissions in September 2016 and enhanced CH4 emissions just before the onset of winter freeze events (December 2016). Updated flux information will be provided for 2017. Following one year of warming treatments (plus 5 months of elevated CO2 exposures), we have not yet observed clear warming or eCO2 responses for aboveground tree or shrub growth across the warming gradient. Plot-level carbon budget contributions from vegetation growth data were extrapolated to the plot scale yielding estimates of tree, shrub, and Sphagnum net primary production of 24, 104, and 208 gC m-2 y-1, respectively. Cumulative losses of C from dark-heterotrophic CO2 and CH4 efflux were obtained by interpolating flux data and yielded losses of 357 and 16 gC m-2 y-1, respectively. Although we perceive the historical bog to be a net carbon sink, varying the assumptions regarding the fraction of CO2 leaving the bog from autotrophic sources (rapidly cycling current year photosynthate), can produce alternate conclusions.