Diurnal variations of solar-induced fluorescence and photosynthesis in a heterogeneous peatland ecosystem under heatwave and non-heatwave conditions

Abstract

Peatlands are pivotal carbon sinks, storing approximately one-third of the terrestrial carbon. Nevertheless, peatland vegetation is vulnerable to environmental stressors, particularly heatwaves. Delving into the diurnal course of photosynthesis and its relationship with solar-induced fluorescence (SIF) unravels valuable insights into peatland vegetation's physiological responses to environmental stress. The study examines the correlation between SIF and gross primary production (GPP), examining the photosynthetic vigour of peatland vegetation across heatwave and non-heatwave periods in the Rzecin peatland, Poland, from June to October 2019. CO2 fluxes were measured by manual chambers, and GPP was calculated from consecutive net ecosystem exchange and ecosystem respiration measurements. Each campaign's GPP data was modelled with a Michaelis-Menten rectangular hyperbola model. SIF in the O2-A band was retrieved by the improved Fraunhofer Line Depth (iFLD) method from the hyperspectral data measured by the FloX system. The SIF-GPP relationship was further examined based on dates and periods (before noon, noon, and afternoon), with the heatwave and non-heatwave scenarios. Results showed the differences in correlations dependent on date, period, and heatwave scenarios. Generally, correlations increased under cloudy or partially cloudy conditions, where low light intensity and temperature alleviated plant stress, increasing photosynthetic efficiency. The SIF and GPP exhibit positive correlations in the morning and afternoon, but the relationship is broken during midday, underscoring the impact of factors such as intense light and high temperature on the peatland vegetation physiology. Diurnal dynamics reveal a robust linear relationship between GPP and SIF O2-A across non-heatwave days, losing coherence during heatwaves. A notable midday depression during heatwaves, characterized by a dip in SIF-GPP correlation at noon, points to changes in energy distribution in photosynthetic apparatus. The findings stress the significance of considering diurnal variations of SIF and GPP under heatwave conditions when assessing photosynthesis-climate interactions. The findings showed that SIF is a good indicator of changes in plant physiology during midday depression caused by the high intensity of solar radiation and high temperature, mainly during the heatwave periods.   The Research was founded by National Science Centre, Poland: 2020/39/O/ST10/00775.