## 1 Introduction
A study on changes in GPP during drought in rubber plantations in northern and southern Thailand showed that northern plantations depend on soil moisture content, and high light levels can offset the negative effects of drought on rubber GPP. In contrast, southern plantations in humid regions are more sensitive to PAR [23]. Using flux observations from rubber plantations in Hainan Island, China, the EC-LUE and SEIB-DGVM models indicate that the GPP of rubber trees is significantly affected by extreme and prolonged droughts, showing greater sensitivity to droughts in the early rainy season [24; 25]. A study on rubber plantations in Xishuangbanna, China, found that winter cold stress causes rubber trees to concentrate leaf shedding in January and February, which is a dominant factor affecting the net ecosystem exchange (NEE) of rubber plantations [25]. In recent years, sun-induced chlorophyll fluorescence (SIF) has been widely used as a more reliable method for directly monitoring vegetation activity. Studies have shown that SIF measures the photosynthetic capacity determined by chlorophyll concentration and environmental conditions, providing significant improvements over reflectance-based vegetation indices in estimating GPP [26]. Traditional vegetation indices (such as NDVI and EVI) only reflect static information during vegetation growth (such as leaf number, pigment content, and canopy structure) [27]. Tropical vegetation, often being evergreen, shows relatively small annual changes in greenness, and the same greenness index can correspond to large variations in photosynthetic activity [28; 29], making these indices less sensitive for tropical evergreen crops. Compared to traditional vegetation indices, SIF can reveal changes in vegetation health and environmental stress impacts earlier [30; 31; 32]. SIF can detect subtle spring changes in evergreen forests that traditional greenness indices cannot differentiate [33], and it can also reflect rapid changes in canopy water stress [34]. During the early stages of drought and heatwave events, SIF levels decrease earlier and more significantly than the traditional vegetation index EVI [35]. For example, during the severe meteorological drought in the Amazon from August 2015 to July 2016, overall forest greenness slightly increased due to enhanced solar radiation, with 21.6% of the forest becoming even greener. However, SIF, as an indicator of photosynthetic capacity, significantly decreased [36]. In monitoring drought effects in rubber plantations, the GOSIF product derived from the OCO-2 satellite outperforms the normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI) in detecting GPP and drought effects. Additionally, incorporating near-infrared reflectance of vegetation (NIRv) allows for a more accurate representation of the relationship between SIF and GPP [37]. To explore the potential impact of drought on GPP in different rubber plantation zones, we utilized carbon and water flux data from four rubber plantation sites in China and Thailand, specifically situated in the traditional southern region of Thailand, the northern region of Thailand, Hainan Island in China, and Xishuangbanna in China. Initially, we investigated the relationship between GPP measured at rubber forest flux observation stations across the four regions and satellite remote sensing indices, subsequently establishing a model for the inversion of GPP using remote sensing indices. Second, we integrated the long time series of GPP inverted from satellite remote sensing with drought indices. The specific questions addressed in this study include the following: (a) assessing the feasibility and accuracy of using remote sensing indices to estimate GPP in rubber plantations; and (b) quantifying the seasonal impacts of drought on GPP across different rubber forest ecosystems.