Measurement of net ecosystem exchange, productivity and respiration in three spruce forests in Sweden shows unexpectedly large soil carbon losses

摘要

Measurement of net ecosystem exchange was made using the eddy covariance method above three forests along a north-south climatic gradient in Sweden: Flakaliden in the north, KnottcEsen in central and Asa in south Sweden. Data were obtained for 2 years at Flakaliden and KnottcEsen and for one year at Asa. The net fluxes (Nep) were separated into their main components, total ecosystem respiration (Rt) and gross primary productivity (Pg). The maximum half-hourly net uptake during the heart of the growing season was highest in the southernmost site with -0.787 mg CO mpo sp# followed by KnottcEsen with -0.631 mg CO mpo sp# and Flakaliden with -0.429 mg CO mpo sp#. The maximum respiration rates during the summer were highest in KnottcEsen with 0.245 mg CO mpo sp# while it was similar at the two other sites with 0.183 mg CO mpo sp#. The annual Nep ranged between uptake of -304 g C mpo yearp# (Asa) and emission of 84 g C mpo yearp# (KnottcEsen). The annual Rt and Pg ranged between 793 to 1253 g C mpo yearp# and -875 to -1317 g C mpo yearp#, respectively. Biomass increment measurements in the footprint area of the towers in combination with the measured net ecosystem productivity were used to estimate the changes in soil carbon and it was found that the soils were losing on average 96-125 g C mpo yearp#. The most plausible explanation for these losses was that the studied years were much warmer than normal causing larger respiratory losses. The comparison of net primary productivity and Pg showed that ca 60% of Pg was utilized for autotrophic respiration.