## 1 Introduction
Whole plant-level WUE refers to the rate of carbon accumulation in dry matter relative to cumulative water loss [20; 21]. The ecosystem level WUE is typically defined as the ratio of gain in gross primary productivity to water consumption through evapotranspiration (ET), reflecting the intricate relationship between carbon and water in the ecosystem [15; 16]. Age-related dynamics of water-use efficiency are more complex at the ecosystem scale than they are at the individual scale, because of factors such as ecological succession, interspecific nutrient competition, and self-thinning processes [22; 23; 24; 25]. The intricate link between climate change and forest WUE varies across climate zones. For instance, variations in temperature and precipitation can affect species composition and water availability in temperate forests [26; 27; 28], which can affect WUE overall. However, in northern wetter regions of boreal forests, warmer temperatures boost forest growth, but in drier southern regions, the rise in temperature leads to increased water stress, which hinders forest productivity [29]. Similarly, changes in humidity and precipitation patterns in tropical forests have an impact on transpiration rates and the WUE [30]. In Amazonia soil, water stress reduces ET and enhances WUE [31]; however, increased iWUE is likely driven by adaptations to water stress rather than growth, suggesting that higher temperatures and drought conditions may impair radial growth [32]. Different forest types and structures, management regimes, as well as age groups, also exhibit varying responses to climate change [33; 34; 35]. Young, rapidly expanding forests may differ in WUE compared to mature and structurally complex forests [33]. On the other hand, recent investigations have demonstrated that elevated CO\({}_{2}\) in the atmosphere leads to a considerable increase in instantaneous WUE across different forest types and climate zones [36; 37; 38; 39]. Climate change has a significant effect on forests by changing their WUE, which is a critical factor for their health and resilience. Furthermore, increased frequency of extreme weather events can exacerbate forest vulnerability to various disturbances, ultimately disrupting the delicate balance of forest ecosystems. For instance fires have significant impacts on ecosystem functions such as regulation of the hydrological cycle and on many other ecosystem services, including soil fertility, food, timber, and biodiversity conservation [40]. Forest fires reduce forest cover due to tree mortality and lessen its ability to store carbon [41]. Fire-induced water repellency and significant loss of forest canopy are the primary factors contributing to higher runoff and faster erosion in burnt soils [42]. Moreover, post-fires also intensify peak flow, leading to increased flooding [43]. A recent dendrochronology and stable isotope analyses showed that defoliated trees exhibit altered water-use patterns after fire, indicating a significant decrease in WUE [41]. Similarly, paired catchment experiments show that changes in forest cover have a significant impact on water yield and evapotranspiration [44; 45]. Timber harvesting can also significantly alter other components of streamflow, such as accelerating snow melting, sedimentation, and flooding [11]. Challenges associated with understanding and managing forest WUE under climate change include the intricate connections between climate factors, forest types, and water dynamics [6]. This adoption approach may contain climate-resilient tree species, implementation of sustainable forest-management practices, and leveraging of advanced monitoring technologies. To address these issues, a comprehensive approach is required that combines scientific study and practical management to increase forest resilience and protect vital ecosystem functions in the face of ongoing climate change.