Synchronized slowdown of climate warming and carbon sink enhancement over deciduous broadleaf forests based on FLUXNET analysis

The temperate deciduous broadleaf forests (DBFs) are widely distributed in middle and high latitudes and play vital role in providing ecosystem services and mitigating human induced climate change, however, the responses of the carbon sink of DBFs to recent warming slowdown were not well known at regional scale. Here, we investigated the temporal pattern of carbon sink in DBFs and its drivers based on the eddy covariance carbon flux observations at 15 FLUXNET sites from 1997 to 2014. The annual mean net ecosystem production (NEP) was 272 +/- 43 g C m(-2) yr(-1) during 1997-2005 and it increased to 445 +/- 13 g C m(-2) yr(-1) during 2006-2014, as a result of boosted gross primary production (GPP) and weakened ecosystem respiration, respectively. We found slowdown in trends of temperature and carbon sink enhancement during 2006-2014. The NEP decreased at the rate of -11.9 g C m(-2) yr(-1) during the warming slowdown period (2006-2014, p < 0.05) compared to a prior strong increased trend during the warming period (1997-2005, 47.6 g C m(-2) yr(-1), p < 0.05). The mean annual temperature dominated the inter-annual variation of NEP (46.9 %), and it worked together with seasonal water availability to control the NEP changes. The slowdown of peak photosynthesis growth rate and carbon uptake period (CUP) extension may contribute to the NEP reversal during the later period. Our results demonstrated that the carbon uptake potential of temperate DBFs may highly depend on warming trends.