## 1 Introduction  
Understanding how vegetation absorbs carbon dioxide is crucial for comprehending the Earth's carbon cycle. Gross primary production (GPP) measures carbon sequestration through photosynthesis and is a key terrestrial carbon exchange, driving growth and respiration [1; 2; 3]. Forests are crucial to Earth's ecosystems and are the largest carbon reservoirs, sequestering about two-thirds of the carbon fixed by land systems annually, helping mitigate rising atmospheric CO\({}_{2}\) levels [4]. Therefore, studying the carbon sequestration capacity of forests is central to achieving carbon neutrality and addressing global climate change [5]. A decline in photosynthetic productivity due to drought weakens their role as a carbon sink. Recent research has shown that droughts, especially in mainland Southeast Asia, have significantly impacted agricultural and natural ecosystems, with prolonged and frequent drought conditions observed in countries like Cambodia and Thailand. Such events have led to a decrease in vegetation health and productivity, further exacerbating the challenge of maintaining carbon sequestration capabilities in these regions [6; 7]. Rubber (_Heven brasiliensis Muell._) plantations provide essential industrial materials, have significant economic value, and possess carbon sequestration potential. Rising latex consumption has led to increased and expanding global cultivation of rubber plantations, especially in tropical rainforest regions [8; 9; 10; 11]. Southeast Asia, with its favorable climate and conditions, is the world's primary natural rubber cultivation area, accounting for over 80% of global plantations. Over the past two decades, rubber cultivation has expanded from traditional areas to nearly all tropical regions, including Malaysia, Laos, Myanmar, Thailand, Vietnam, and Cambodia, exceeding one million hectares. However, this expansion often replaces pristine or secondary forests, causing biodiversity loss, reduced soil and water conservation, regional environmental degradation, and weakened vegetation resilience to weather-related disasters [12; 13; 14; 15]. Recent studies have highlighted the ecological consequences of this expansion, including increased vulnerability to drought and extreme weather events, which threaten the sustainability of these plantations. Studies using advanced remote sensing techniques have highlighted that the expansion of rubber plantations into ecologically sensitive regions, particularly in Cambodia, Laos, and Vietnam, has led to a notable decrease in vegetation resilience. These areas are increasingly susceptible to both short-term meteorological droughts and long-term agricultural drought conditions, which further threaten the sustainability of rubber plantation ecosystems [16; 17]. Despite the importance of rubber plantations in global carbon cycling, there is a critical gap in understanding how drought specifically affects their GPP and overall carbon balance, particularly in the context of ongoing climate change. Moreover, the complex interactions between drought indices and vegetation responses in tropical regions show that different vegetation types and altitudes respond variably to drought, with rainfed crops and mixed forests being particularly vulnerable. Understanding the response of rubber plantations' GPP to drought is crucial for their rational development, forest ecosystem protection, carbon sink balance, and climate change stability [7; 16]. Drought is a critical factor contributing to the decline in forest photosynthetic productivity, thereby weakening their role as carbon sinks. The 2015/2016 El Nino event caused prolonged severe drought (drought affected area up to 75.6% [18]) and high temperatures (2.34 \({}^{\circ}\)C above average in April 2016) in Southeast Asia [19; 20]. Remote sensing and biosphere models observed abnormal carbon flux and a significant decline in photosynthesis across Southeast Asia [21; 22].