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

Abstract

As global observations of solar-induced chlorophyll fluorescence (SIF) have become available from multiple satellite platforms, 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. The PhotoSpec system consisted of two narrowband spectrometers, for the retrieval of SIF in the red (680-686 nm) and far-red (745-758 nm) regions of the electromagnetic spectrum, and a broadband spectrometer for the assessment of vegetation indices. We found that SIF correlated with GPP across diurnal and seasonal cycles, but that SIF irradiances were more strongly related to downwelling radiation than GPP. However, while this dependence of SIF on radiation obscured drought signals in SIF itself, we demonstrate that a SIF response to severe drought was apparent as a decrease in relative SIF. 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 drought stress, but was largely driven by seasonal and interannual changes in canopy structure, as well as by synoptic changes in downwelling radiation.

Quantifying Seasonal and Diurnal Cycles of Solar‐Induced Fluorescence With a Novel Hyperspectral Imager

“…Variations in SIF are closely linked to irradiance and f esc . To account for irradiance and canopy structure effects, we also calculate relative SIF (unitless) using total reflected radiance surrounding the absorption feature (RAD 757−762 ) (Butterfield et al., 2023; Pierrat et al., 2021, 2022): $$$relative\ SIF=\frac{SI{F}_{760}}{RA{D}_{\mathit{757\mbox{-}762}}}$$$ …”

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

“…We built and deployed a PhotoSpec spectrometer system (Grossmann et al, 2018) at the US-UMB tower during the 2018 and 2019 growing seasons (Butterfield et al, 2022). The PhotoSpec system consisted of two narrowband spectrometers (QEPro, Ocean Optics Inc.): one with a wavelength range of 670-732 nm and a resolution of 0.074 nm/pixel, 0.3 nm full width half maximum (FWHM), for measuring SIF in the red region of the spectrum, and a second QEPro (729-784 nm, 0.067 nm/pixel, 0.3 nm FWHM) optimized for measuring SIF in the far-red.…”