Continuous chlorophyll fluorescence measurements trace sudden cold spell effects on photosynthetic efficiency in a temperate mixed forest

Air temperature extremes and fluctuations are expected to become more frequent and have been shown to affect the physiological functioning of temperate forests across Europe. The exposure to sudden cold spells during summer combined with high light intensities can lead to photoinhibition of photosystem II (PSII) and thereby reduce photosynthetic efficiency. In this study, we aimed to analyse the dynamics of photoinhibition as well as related protection mechanisms of tall tree canopies during such cold spells. Therefore, we continuously assessed leaf level chlorophyll fluorescence (ChlF) and ecosystem carbon fluxes by eddy covariance in a temperate mixed forest in southern Germany during growing season 2024. While the deciduous, broadleaved F. sylvatica indicated chronic photoinhibition as a response to sudden cold spells, the evergreen, coniferous P. menziesii showed higher tolerance to low air temperatures. Both species increased thermal energy dissipation at PSII during cold spells indicating the activation of protective mechanisms. Likewise, both species exhibited mainly the sustained form of non-photochemical quenching (NPQ _s ) as a reaction to chilling temperatures and high light intensities which maintained elevated during the recovery phases. However, the dynamics of upregulation of photoprotection and recovery processes differed between the two tree species. Furthermore, the integration of ecosystem carbon exchange and continuous leaf level ChlF measurements gave valuable insights into the photosynthetic dynamics of the mixed forest canopy. This study emphasises the importance of the assessment of dynamic responses to future climate impacts on forest ecosystem on different scales.