Detection of Heat and Drought Effects on the Photosynthetic Activity Using Solar‐Induced Chlorophyll Fluorescence and a Photochemical Reflectance Index in a Cool‐Temperate Larch Forest in Japan

ABSTRACT Recent frequent heatwaves have adversely affected the carbon cycles of terrestrial ecosystems. Solar‐induced chlorophyll fluorescence (SIF) and the photochemical reflectance index (PRI) can detect heat and drought stress in vegetation. However, the applicability of these indicators in extreme cases, such as heatwaves, needs consideration. We examined diurnal changes observed in tower‐based SIF, PRI, canopy conductance, and gross primary productivity (GPP) over 3 weeks during mid‐summer in 2022 in a cool‐temperate deciduous forest covered by Larix kaempferi (Lamb.) Carrière, in Japan. We found that half‐hourly GPP and SIF declined significantly from noon to afternoon during the latter part of clear, extremely hot days (up to 31°C) when the soil water content (SWC) was low (< 21%), and the vapor pressure deficit (VPD) was high (> 15 hPa). Values of GPP and SIF peaked at 21.7°C and 26.8°C, respectively, and they declined at higher temperatures. Canopy conductance declined with increasing air temperature and VPD, suggesting that stomatal closure caused the observed reduction in GPP. To investigate the stress response without light‐dependent variations, canopy fluorescence emission efficiency ( εF ) and light use efficiency (LUE) were calculated from SIF and GPP, respectively. VPD had the most significant negative correlation with PRI, LUE, and εF . εF was also associated with SWC, particularly under lower SWC conditions. Our results demonstrate that SIF and PRI responded to heat and drought with different sensitivities and thresholds and can serve as diagnostic tools to assess the influence of moderate to extreme environments on terrestrial ecosystems.