Changing temperature response of respiration turns boreal forest from carbon sink into carbon source

• A boreal forest site turned from a net carbon sink into a net carbon source.
• Annual net carbon losses were driven by an increase in total ecosystem respiration.
• A change in ecosystem respiration response to temperature increased net losses.
• Changes in ecosystem functioning had a large impact on the carbon balance.

Seventeen years (1997–2013) of carbon dioxide (CO2) fluxes were measured in a boreal forest stand in northern Sweden using the eddy covariance technique. During the measurement period the forest turned from a net carbon sink into a net carbon source. The net ecosystem exchange (NEE) was separated using values from periods of darkness into the gross components of total ecosystem respiration (TER) and gross primary productivity (GPP), which was calculated as GPP = −NEE + TER. From the gross components we could determine that an increase in TER during the autumn (September to end of November) and spring (March to end of May) periods resulted in the forest becoming a net source of CO2. We observed no increase in the GPP from the eddy covariance measurements. This was further supported by measurements of tree growth rings. The increased TER was attributed to a change in the forest’s temperature response at lower temperatures (−5 to 10 °C) rather than to a temperature increase. This study shows that changes in ecosystem functioning can have a larger impact on the carbon balance than climate warming per se.