## 1 Introduction  
Climate change, which involves long-term changes in temperature, precipitation, and other weather patterns, presents a major global challenge. Climate change is primarily driven by human activities, including industrialization, the burning of fossil fuels, and deforestation. These actions are causing significant, long-term changes in global climate patterns and leading to a rise in Earth's average temperature [1]. Despite scientific evidence, warming trends show no signs of subsidence, causing a considerable decrease in the geographic ranges and climatic refugia for biodiversity in various regions of the world [2]. The rising temperatures and greenhouse gas levels impact plant growth and the carbon-water cycles of ecosystems by altering rates of litter decomposition and rise in CO2 emissions and affecting WUE in ecosystems [3]. Forests cover about 30% of the surface of the Earth [4]. Forests play a crucial role in regulating the carbon and water cycles, and their WUE is directly influenced by climate change. The close relationship between forest carbon sequestration and water consumption significantly affects the Earth climate and is controlled by both human activities and climate change [5; 6]. Investigating this relationship and analyzing prior research results are essential for understanding the impacts of climate change on forest ecosystems and their role in carbon and water cycles [6]. Global efforts are underway to limit the rate of deforestation, improve protection, restoration, conservation, and management of the existing forests to mitigate climate change, and enhance their resilience and sustainability [6; 7]. The role of forests in the carbon cycle is crucial because forests help to mitigate the greenhouse effect by sequestering carbon through photosynthesis process [8]. They also regulate the hydrological cycle by influencing local and regional water balance through transpiration and interception [9; 10]. Forests regulate water quantity and quality by controlling the infiltration, retention, and release of water [11]. Their ability to efficiently use water directly impacts the health of the ecosystem and other forest ecosystem services, including water regulation, soil conservation, and habitat for biodiversity [12]. Estimating forest WUE involves various approaches, each with its own advantages and limitations. These methods include large-scale assessments using stable carbon isotopic discrimination, remote sensing, and direct measurements of gas exchange through sap flux sensors and eddy covariance towers [13; 14; 15]. Each method offers insights into various aspects of WUE. They can be used to study how forests across various regions and environmental conditions adapt to climate variability. Forest WUE is a key metric in understanding how effectively a particular forest utilizes water in relation to its carbon-sequestration capacity [13]. WUE is mainly classified into leaf, plant, and ecosystem scales; these classifications help in assessing how forests respond to varying environmental conditions and management practices [6]. At the leaf scale, intrinsic water-use efficiency (iWUE) refers to the ratio between the amount of net carbon dioxide (CO2) assimilation and the transpiration from individual leaves [16]. This reflects the plant's health and is commonly used in studies to examine how water stress affected leaf WUE of diverse functional plant groups. For example, a study found that WUE improved under moderate drought and was closely linked to changes in mesophyll conductance, highlighting its role in resource optimization [17]. The drought-tolerant tree species, which are typically characterized by anisohydric stomatal control, are likely to use more water than coexisting isohydric species [18]. When these leaf-level differences in water use are considered across entire forest canopies, they can have major effects on the forest's overall water dynamics and hydrological fluxes [19].