Asymmetric responses of gross primary productivity (GPP) and ecosystem respiration (Reco) in response to long-term drought

• GPP had a higher resilience and lower sensitivity to drought than R eco . • Stepwise modeling including treatment effects on model parameters improved the model fit. • Seasonal rewetting mitigated severe summer drought effects on the annual scale. Gross primary productivity (GPP) and ecosystem respiration (R eco ) account for the largest fluxes of CO 2 between the land surface and the atmosphere but exhibit different sensitivities in response to climate change causing potentially large effects on land-climate feedback. In this study, we investigated the long-term drought effects on GPP, R eco , and soil respiration (R S ) to uncover their potential asymmetric responses to climate change. We quantified the three flux components in a temperate grassland/heathland ecosystem exposed to long-term drought and modelled annual flux rates through stepwise parameter addition and treatment-specific parameter fitting. GPP, R eco and R S rates were all suppressed during summer droughts, but seasonal rewetting partially mitigated the summer drought effects on an annual scale. Upscaled best-fit models showed asymmetry in the annual drought impacts on GPP and R eco with a stronger decrease of R eco than of GPP resulting in annual net ecosystem exchange (NEE) ranging from a carbon source of 41.2 ± 122.4 g C m −2 y −1 in ambient plots to a carbon sink of 31.8 ± 115.7 g C m −2 y −1 in drought plots. Overall, our annual results showed greater drought resilience and lower sensitivity of GPP compared to R eco . Furthermore, our year-round observations also indicated that the annual trend covers different seasonal sensitivity patterns of GPP and R eco to drought. The stepwise modelling approach revealed that drought altered model parameters, highlighting the need to account for such parameter changes in ecosystem model projections of net ecosystem carbon flux under future climate change.