Atmospheric dryness dominates diurnal carbon-water coupling in mid-latitude forests

Mid-latitude forest ecosystems in the Northern Hemisphere serve as substantial carbon sinks and also hold a strong transpiration capacity, making them critical components of the global carbon and water cycles. However, the diurnal carbon-water coupling strength over mid-latitude forests and its dependence on environmental factors are unclear. Based on half-hourly flux data from 34 eddy covariance (EC) towers over forest ecosystems located between 30° N and 60° N, here we investigate the characteristics of diurnal carbon-water coupling strength and its response to environmental factors, that is, solar radiation (SR), air temperature (T_a), vapor pressure deficit (VPD), and soil water content (SWC) through correlation analysis, ridge regression, and data binning. We find that there is a distinct difference in diurnal variation between gross primary productivity (GPP) and evapotranspiration (ET), with GPP generally decreasing earlier and faster than ET after its peak. An increase in SR, T_a, or VPD can lead to a reduction in the diurnal coupling strength, while higher SWC makes the coupling stronger. The magnitudes of the negative effects of VPD and T_a vary across the day, with VPD in the afternoon being the most influential factor and T_a playing a dominant role in the early morning. Among the 34 sites, VPD is the dominant factor influencing coupling strength at 30 sites, while T_a is secondarily important over one-third of the studied sites. The weakening of coupling strength is attributed to the asynchronous responses of GPP and ET to environmental factors, particularly under conditions of high VPD and temperature, when GPP tends to decrease while ET does not follow. This study highlights the dynamics of diurnal carbon-water coupling and the complex interactions with environmental factors, emphasizing the need to consider the short-term responses of GPP and ET coupling to environmental factors across diverse ecosystems in future research.