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.