## 1 Introduction  
Forest ecosystems are the largest carbon reservoirs on land and serve as the main drivers of terrestrial carbon sinks, playing a crucial role in regulating terrestrial ecosystems and the global carbon cycle [1, 2]. Photosynthesis is a key process in forest ecosystems, with the amount of carbon dioxide (CO\({}_{2}\)) assimilated per unit time and area through photosynthesis defined as gross primary productivity (GPP), representing the carbon sequestration capacity of forest ecosystems [3, 4]. Precisely estimating the GPP of forest ecosystems across varying climate conditions is essential in comprehending the carbon storage potential of terrestrial ecosystems amid global changes. Solar-induced chlorophyll fluorescence (SIF) provides an essential method for the tracking of photosynthesis and estimation of GPP [5; 6; 7]. Light energy absorbed by photosynthetic pigments is partly re-emitted as SIF, and the SIF spectrum ranges from 650 to 800 nm, with two peaks at around 687 nm (red SIF) and 760 nm (far-red SIF) [8]. The absorbed light energy is primarily used in two key processes, namely photosynthesis and non-photochemical quenching (NPQ), with a small portion released as SIF [9; 10]. Thus, SIF is biologically linked to the photosynthetic mechanism, and it is a potential indicator of photosynthetic activity in remote sensing applications [11]. Previous studies have reported that SIF from various observation platforms, including satellite, airborne, and ground-based ones, can allow the estimation of GPP at the leaf, canopy, and ecosystem scales [5; 6; 7; 12; 13; 14; 15; 16]. The intensity and frequency of drought events are projected to increase further with future climate change, exacerbating the negative impacts on photosynthesis and productivity in forests [17; 18]. Consequently, it is crucial to analyze the impact of drought on the relationship between SIF and GPP to improve the accuracy of GPP estimation in forest ecosystems. Under drought stress, vegetation exhibits both stomatal and non-stomatal responses. The reduction in stomatal conductance caused by drought limits CO\({}_{2}\) exchange, resulting in a lower carboxylation rate and a higher photorespiration rate (stomatal response); severe drought affects the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and enzymes related to the Calvin cycle, leading to a decrease in the electron transport rate (ETR) and inhibiting or downregulating photosynthesis (non-stomatal response) [19]. These physiological responses decouple light and carbon reactions in photosynthesis over different time scales [20]. Since SIF is directly associated with light rather than carbon reactions, the GPP-SIF correlation may weaken or even disappear under drought conditions, as already proven in previous studies. [PERSON] et al. [21] reported that SIF initially declined slightly and then dropped sharply during a heatwave, while GPP decreased linearly. According to [PERSON] et al. [22], drought conditions notably weakened the linear correlation between SIF and GPP, highlighting the limitations of using SIF to monitor GPP under drought stress. [PERSON] et al. [23] revealed that heatwaves disrupted the SIF-GPP relationship via NPQ interference. Similarly, [PERSON] and [PERSON] [24] highlighted that the TROPOMI SIF-GPP correlation was robust under normal or wet conditions but diminished significantly under drought conditions. However, [PERSON] et al. [25] and [PERSON] et al. [26] observed no significant change in the GPP-SIF relationship before and after drought events. [PERSON] et al. [27] found that SIF and GPP exhibited consistent seasonal trends under drought conditions due to drought stress activating photoprotective mechanisms in light reactions, such as the xanthophyll de-epoxidation cycle, leading to increased NPQ. As NPQ competes with photosynthesis and SIF for absorbed light energy, its rise leads to decreased photochemical activity and reduced SIF, often occurring in parallel with this reduction.