Accounting for Changes in Radiation Improves the Ability of SIF to Track Water Stress‐Induced Losses in Summer GPP in a Temperate Deciduous Forest.

Global observations of solar‐induced chlorophyll fluorescence (SIF) are available from multiple satellite platforms, and SIF is increasingly used as a proxy for photosynthetic activity and ecosystem productivity. Because the relationship between SIF and gross primary productivity (GPP) depends on a variety of factors including ecosystem type and environmental conditions, it is necessary to study SIF observations across various spatiotemporal scales and ecosystems. To explore how SIF signals relate to productivity over a temperate deciduous forest, we deployed a PhotoSpec spectrometer system at the University of Michigan Biological Station AmeriFlux site (US‐UMB) in the northern Lower Peninsula of Michigan during the 2018 and 2019 growing seasons. We found that SIF correlated with GPP across diurnal and seasonal cycles (R2 = 0.61 and 0.64 for 90‐min‐ and daily‐averaged data), but that SIF signals were more strongly related to downwelling radiation than GPP (R2 = 0.91 for daily‐averaged data). The dependence of SIF on radiation obscured the impact of intraseasonal drought in the SIF timeseries, but drought stress was apparent as a decrease in relative SIF, which exhibited a stronger correlation with GPP (R2 = 0.56) than other remotely sensed data over the drought period. These results highlight the potential of SIF for detecting stress‐induced losses in forest productivity. Additionally, we found that the red:far‐red SIF ratio did not exhibit a response to water stress‐induced losses in productivity, but was largely driven by seasonal and interannual changes in canopy structure, as well as by synoptic changes in downwelling radiation. Plain Language Summary: Satellite measurements of solar‐induced chlorophyll fluorescence (SIF), a faint light signal emitted from vegetation during photosynthesis, are increasingly being used to estimate ecosystem productivity and carbon uptake. To accurately do so requires a robust understanding of how the relationship between SIF and plant productivity changes over time, in response to environmental stressors, and across different ecosystems. To better understand SIF signals and how they relate to carbon uptake over a temperate deciduous forest, we used a high‐precision spectrometer system to observe SIF signals at an AmeriFlux site (US‐UMB) in the northern Lower Peninsula of Michigan. While the shared dependence of SIF and ecosystem productivity on sunlight lead to strong daily and seasonal correlations, we found that SIF signals were more closely tied to the amount of incoming sunlight than to ecosystem productivity. Despite the stronger dependence of SIF on sunlight, we show that drought conditions lead to a lower SIF relative to the total light signal. Lastly, we show that the observation of SIF at multiple wavelengths may provide additional information on seasonal and interannual changes in canopy structure. Our results demonstrate the value and limitations in using SIF to assess carbon dynamics over temperate deciduous forest ecosystems. Key Points: Solar‐induced chlorophyll fluorescence above a temperate deciduous forest is more strongly tied to radiation than to productivityRelative solar‐induced fluorescence signals track water stress‐induced summer losses in productivity better than absolute fluorescenceThe ratio of red to far‐red solar‐induced fluorescence is sensitive to phenological changes in canopy structure and downwelling radiation [ABSTRACT FROM AUTHOR]