Plant survey and ecosystem C flow measurements
Plants were identified at the species level and classified as shrubs, graminoids, bryophytes, and lichens. Plant biomass and net primary productivity (NPP) of each plant group were measured by a point-frame method (Walker, 1996), based on previously developed allometric equations (Schuur et al., 2007).
Soil temperature at 15-25 cm depth in winter and soil moisture in the growing season were measured in situ every 30 minutes using constantan-copper thermocouples with CR1000 data loggers (Campbell Scientific, Logan, UT, USA). Thaw depth, which is the thickness of unfrozen soil in the growing season, was measured weekly using a metal depth probe. In the growing season, net ecosystem exchange (NEE) and ecosystem respiration (R eco) were measured using an automated CO2 flux system. Gross primary productivity (GPP) was calculated as the sum of NEE and R ecoof the growing season. WinterR eco was modeled from the measured CO2 flux data of March-April 2009 using an infrared gas analyzer, following detailed description of the modeling methodology shown in the Appendix of a previous paper (Natali, Schuur, Webb, Pries, & Crummer, 2014). In brief, an exponential equation between winterR eco and soil temperature was developed, i.e., winter R eco =R ×exp(β×Ts), where R is basal respiration, β is the rate of respiration change, and Ts is soil temperature at 20 cm depth. For CiPEHR site, parameter values estimated for this model were R = 0.47 (0.03) and β = 0.18 (0.06). This model was validated by comparing the simulation results with all observed R eco values available from flux sites (R 2 = 0.70, P < 0.001). Since no photosynthesis occurs in winter, winter R ecomainly represents heterotrophic respiration. R ecoin this study is annual R eco that is a sum of winter R eco and growing seasonR eco. The period ranges of GPP and NEE data were May 2010 - September 2010. The period ranges of other environmental data, includingR eco, soil temperature, soil moisture, and thaw depth, were October 2009-September 2010.