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

Abstract

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, Knottåsen in central and Asa in south Sweden. Data were obtained for 2 years at Flakaliden and Knottåsen and for one year at Asa. The net fluxes (N_ep) were separated into their main components, total ecosystem respiration (R_t) and gross primary productivity (P_g). The maximum half-hourly net uptake during the heart of the growing season was highest in the southernmost site with −0.787 mg CO₂ m⁻² s⁻¹ followed by Knottåsen with −0.631 mg CO₂ m⁻² s⁻¹ and Flakaliden with −0.429 mg CO₂ m⁻² s⁻¹. The maximum respiration rates during the summer were highest in Knottåsen with 0.245 mg CO₂ m⁻² s⁻¹ while it was similar at the two other sites with 0.183 mg CO₂ m⁻² s⁻¹. The annual N_ep ranged between uptake of −304 g C m⁻² year⁻¹ (Asa) and emission of 84 g C m⁻² year⁻¹ (Knottåsen). The annual R_t and P_g ranged between 793 to 1253 g C m⁻² year⁻¹ and −875 to −1317 g C m⁻² year⁻¹, 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 m⁻² year⁻¹. 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 P_g showed that ca 60% of P_g was utilized for autotrophic respiration.