Atmospheric dryness effects on canopy chlorophyll fluorescence and GPP in a deciduous forest during heat waves

Sun-Induced chlorophyll Fluorescence (SIF) is the most promising optical indicator of Gross Primary Production (GPP) in terrestrial ecosystems. However, the interpretation of SIF as a proxy of GPP is challenged when plants experience abiotic stress, particularly during extreme climatic events whose frequency is projected to increase in the future. Recently, the feasibility of canopy-scale active chlorophyll fluorescence measurements (LED-induced chlorophyll fluorescence), which directly measure the apparent fluorescence yield (F_yieldLIF), has provided new perspectives on detecting the physiological responses of plants to abiotic stress. This study was conducted during summer 2022 European heat waves in a mixed temperate deciduous broadleaf forest, located in the Fontainebleau-Barbeau station (Integrated Carbon Observation System FR-Fon site), about 50 km South-East of Paris, France. Continuous measurements of carbon dioxide (CO₂) and energy exchanges, SIF, F_yieldLIF, and ancillary environmental variables were acquired. We investigated how atmospheric dryness, measured as Vapor Pressure Deficit (VPD), affected canopy chlorophyll fluorescence (both SIF and F_yieldLIF) and GPP, as well as their relationships. Our results indicated that high VPD has a negative impact on GPP and F_yieldLIF at both half-hourly and daily scales. In contrast, SIF exhibits a positive response to high VPD at the half-hourly scale, but this relationship reverses, showing a negative response at the daily scale. At the half-hourly scale, our results revealed a decrease of the correlation between SIF and GPP (R² decreased from 0.49 to 0.17) as atmospheric dryness increased. In contrast, the correlation between F_yieldLIF and GPP strengthened significantly under the same conditions (R² increased from 0.07 to 0.43). However, at the daily scale, the correlations between SIF and GPP and between F_yieldLIF and GPP showed an overall increase, suggesting a time-scale-dependent response of these relationships to atmospheric dryness. This study also highlighted the advantages of F_yieldLIF over SIF in detecting plant responses to high atmospheric dryness. This underlines the potential of canopy-level active chlorophyll fluorescence measurements for understanding and quantifying the nature of the relationship between canopy chlorophyll fluorescence and photosynthesis in ecosystems under extreme climatic conditions.