Continuous chlorophyll fluorescence measurements trace sudden cold spell effects on the photochemical efficiency of photosystem II in a temperate mixed forest.

The increasing frequency of air temperature extremes and short-term fluctuations poses a significant threat to the physiological functioning of temperate forests across Europe. However, the mechanisms by which these fluctuations induce stress and impair the photochemical efficiency of photosystem II (PSII) in tall tree canopies remain poorly understood. Moreover, the impact of air temperature extremes and fluctuations on ecosystem carbon fluxes remains poorly quantified. Here, we integrated real-time chlorophyll fluorescence (ChlF) and eddy covariance carbon flux measurements in a temperate mixed forest over an entire growing season. Throughout the predominantly warm and wet early-summer period, no significant stress responses were observed. In contrast, late-summer cold spells combined with high irradiance triggered significant photoinhibition, with the deciduous Fagus sylvatica exhibiting greater sensitivity than the evergreen Pseudotsuga menziesii. F. sylvatica shifted energy partitioning toward reversible non-photochemical quenching (NPQr) accompanied by a decline in photochemical efficiency of PSII indicative of chronic photoinhibition. In contrast, P. menziesii relied primarily on sustained non-photochemical quenching (NPQs​), demonstrating more efficient photoprotection and enhanced cold tolerance. Despite the pronounced photoinhibition in the F. sylvatica sun canopy and its dominance within the forest, the effect did not translate to a decline in total ecosystem carbon exchange. Our findings demonstrate that even short-term fluctuations can induce acute stress responses in temperate forest trees, which may compromise the physiological functioning of sensitive species like F. sylvatica as climate extremes intensify. Consequently, these results underscore the critical role of continuous, multi-scale in situ monitoring in resolving the species-specific responses necessary to understand and predict forest resilience.