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Monitoring drought stress is crucial for estimating productivity and assessing the health status of forest ecosystems under global climate change. Solar-induced chlorophyll fluorescence (SIF) is mechanistically coupled to photosynthesis and has advantages over greenness-based vegetation indices in detecting drought. In recent years, SIF has commonly been used in monitoring drought stress in crop ecosystems. However, the response of tower-based SIF to drought stress in forest ecosystems remains unclear. In this study, we investigated the potential of tower-based SIF to monitor drought, which was quantified using the plant water stress index (PWSI) in a Chinese cork oak plantation. The results show the negative effect of drought on SIF, and afternoon depression of SIF emission under drought stress was observed. Canopy SIF (F) exhibited a nonlinear relationship with PWSI, while the quantum yield of SIF (ΦF) exhibited a significant linear relationship with PWSI at 687 nm and 760 nm (ΦF687: R2 = 0.90; ΦF760: R2 = 0.85). Incident radiation (PAR) and canopy structure affected the response of SIF to drought stress, with PAR as the main factor causing the nonlinear relationship between F and PWSI. Afternoon depression was described as the afternoon/morning ratio (AMR). AMRF and AMRΦF exhibited a negative linear response to PWSI. AMRF was less affected than F by PAR and canopy structures, and AMRΦF was more physiologically representative than ΦF. Moreover, AMRΦF was sensitive to VPD and REW, and it might be a good indicator of drought. Red SIF was more sensitive to drought than far-red SIF, as the R2 of PWSI with AMRΦF687 (R2 = 0.89) was higher than that with AMRΦF687 (R2 = 0.84). These results highlight the potential of tower-based SIF, especially red SIF, for drought monitoring in a plantation, and consideration of the physiological diurnal variation in SIF under drought stress is crucial for improving the accuracy of drought stress monitoring. [ABSTRACT FROM AUTHOR]

With the development of spectrum observation technology, solar-induced chlorophyll fluorescence (SIF)—an effective substitute for photosynthesis—has been widely used to monitor crop stress, vegetation phenology and ecosystem productivity. The relationship between fluorescence and photosynthesis is complicated because they are sensitive to environmental changes. Understanding the response of SIF to environmental factors is of great significance for clarifying the variation dynamic and relationship between SIF and photosynthesis under different conditions. In this study, the canopy SIF and the environmental factors of a Quercus variabilis BI. plantation were observed simultaneously, and the response of SIF to environmental factors at a daily scale and at a half-hour scale was analyzed. The results showed that SIF had obvious seasonal and diurnal dynamics and was mainly driven by photosynthetically active radiation (PAR). The influence of PAR, air temperature (Ta), vapor pressure deficit (VPD), soil moisture (SM) and wind speed (Ws) on SIF varied with the lapse of the growing season. After eliminating the covariant effect of PAR on the Ta and VPD during the whole growing season, the relationship between VPD and SIF was found to be negative, and the effect of Ta on SIF disappeared. This study enriched the ground observation dataset and provided support for understanding the variations in the relationship between SIF and photosynthesis under different conditions. [ABSTRACT FROM AUTHOR]

• Nonlinear correlation transformed into linear correlation with aggregation. • GPP of cork oak has intense radiation "depression" while SIF has "promotion". • The GPP and GPP/SIF of poplar is higher than cork oak and which is opposite in SIF. • GPP/SIF decreases with increasing PAR. • GPP is more sensitive to instantaneous stress, and cork oak is more vulnerable. The observation of solar-induced chlorophyll fluorescence (SIF) signals on a spaceborne platform provides a new idea for estimating and monitoring the gross primary production (GPP) of terrestrial ecosystems at the regional scale. To improve the accuracy of remote sensing SIF signals to estimate GPP, the combination of ground tower-based continuous SIF signals with GPP to explore the mechanical connection in different ecosystems is needed. Based on continuous ground observations, the discrepancy of the links between SIF and GPP and the response to micrometeorology factors of two temperate deciduous broadleaf plantations was explored. The nonlinear relationship of SIF and GPP on the 30 min timescale transformed into a linear correlation with aggregation. With the increase in SIF, GPP gradually saturates and even declines in cork oak. We found SIF "promotion" by using intense incident radiation in cork oak and GPP "depression" or saturation in both species. GPP is more sensitive to instantaneous stresses such as intense radiation imposed high air temperature (Ta) in the midday/afternoon, and the stress threshold of cork oak is lower. We used GPP/SIF as a metric for the relationship, which is significantly higher in poplar, and found GPP/SIF of both species decrease with increasing photosynthetically active radiation (PAR). Since the difference in the amplitude of SIF between tree species was not prominent, the diversity of GPP/SIF performance was mainly caused by the divergence of GPP. The GPP/SIF of cork oak continues to decrease with increasing vapor pressure deficit (VPD), indicating that the relationship between SIF and GPP has a strong daily pattern in the constraint of micrometeorology factors. Our results show that characterizing the impact of micrometeorological conditions on the SIF-GPP relationship in a specific ecosystem is of great significance for the reliable estimation of GPP from remote sensing. [ABSTRACT FROM AUTHOR]

Solar-induced chlorophyll fluorescence (SIF) is a hopeful indicator, which along with remote sensing, is used to measure the photosynthetic efficiency and gross primary production (GPP) of vegetation in regional terrestrial ecosystems. Studies have found a significant linear correlation between SIF and GPP in a variety of ecosystems. However, this relationship has mainly been established using SIF and GPP data derived from satellite remote sensing and continuous ground-based observations, respectively, which are difficult to accurately match. To overcome this, some studies have begun to use tower-based automatic observation instruments to study the changes of near-surface SIF and GPP. This study conducts continuous simultaneous observation of SIF, carbon flux, and meteorological factors on the forest canopy of a cork oak plantation during the growing season to explore how meteorological factors impact on canopy SIF and its relationship with GPP. This research found that the canopy SIF has obvious diurnal and day-to-day variations during the growing season but overall is relatively stable. Furthermore, SIF is greatly affected by incident radiation in different weather conditions and can change daily. Meteorological factors have a major role in the relationship between SIF and GPP; overall, the relationship shows a significant linear regression on the 30 min scale, but weakens when aggregating to the diurnal scale. Photosynthetically active radiation (PAR) drives SIF on a daily basis and changes the relationship between SIF and GPP on a seasonal timescale. As PAR increases, the daily slopes of the linear regressions between SIF and GPP decrease. On the 30 min timescale, both SIF and GPP increase with PAR until it reaches 1250 μmol·m−2·s−1; subsequently, SIF continues to increase while GPP decreases and they show opposite trends. Soil moisture and vapor pressure deficit influence SIF and GPP, respectively. Our findings demonstrate that meteorological factors affect the relationship between SIF and GPP, thereby enhancing the understanding of the mechanistic link between chlorophyll fluorescence and photosynthesis. [ABSTRACT FROM AUTHOR]