Boreal Peatland 24-hour Solar-induced Chlorophyll Fluorescence Measurements under Continuous Daylight Conditions during the Summer Period

Continuous ground-based observations of solar-induced chlorophyll fluorescence (SIF) from northern peatlands remain uncommon, despite their potential to complement ecosystem carbon flux measurements and support satellite-based monitoring of vegetation activity. Here, we present continuous SIF estimates using data acquired with the Fluorescence Box (FloX) at two boreal peatland sites in northern Finland during the 2023 and 2025 growing seasons. SIF was retrieved using the Bayesian SIFFI algorithm, and the analysis focuses on the diurnal and seasonal variability of spectrally resolved and spectrally integrated fluorescence signals. The measurements capture complete 24-hour cycles under Arctic summer daylight conditions, providing a unique dataset for investigating peatland photosynthetic dynamics. The results reveal clear diurnal and seasonal variability in SIF and show consistent temporal patterns with ecosystem CO2 exchange measured with the eddy covariance technique. Although retrieved SIF accuracy decreases under very low illumination conditions, the observations demonstrate the potential of continuous SIF measurements to support studies of peatland carbon cycling and highlight the importance of continued ground-based SIF monitoring for the interpretation and validation of satellite SIF observations in view of the upcoming ESA FLEX mission.