## 1 Introduction  
Anthropogenic climate change has become one of the most important issues nowadays. Biosphere-atmosphere interactions under this background have attracted increasing attention [1; 2; 3; 4; 5; 6]. On one hand, vegetation can mitigate global warming through photosynthesis, which absorbs carbon dioxide (CO\({}_{2}\)) in the atmosphere. On the other hand, the changed climate can directly or indirectly impact vegetation, which vegetation may or may not be able to adopt. Among all the interactions, forest-climate interactions are critical because forests have high photosynthesis capacities and are also sensitive to climate change [7; 8]. Two processes of particular interest are photosynthesis and transpiration. Taking the pathway of stomata, forests absorb CO\({}_{2}\) through photosynthesis and release water through transpiration [9] to regulate the carbon-water balance between ecosystems and the atmosphere. Therefore, photosynthesis and transpiration are tightly coupled [10; 11]. The relationship between these carbon and water fluxes has been the subject of multiple studies. For example, [PERSON] et al. [12] stated that atmospheric demand plays an increasingly important role in modulating the carbon and water fluxes.  
With advances in the eddy covariance (EC) measurement, the gross primary productivity (GPP) and evapotranspiration (ET) can be measured over forest sites worldwide [13]. GPP is a good measure of photosynthesis [14], while for forests with high vegetation closure, a large portion of ET comes from transpiration on no-rain days, as soil evaporation is weak. Therefore, GPP and ET are widely used in studying the carbon-water relationship [4; 15], in terms of water use efficiency (WUE, a ratio of GPP to ET), or some GPP-ET coupling indices. Previous studies suggested that GPP is linearly correlated with ET in most ecosystems at yearly or monthly time steps [3; 16; 14]. However, GPP-ET relationships at daily or hourly time steps can be different [2; 17].  
The GPP and ET of an ecosystem are collectively modulated by a diversity of climatic factors. Each ecosystem has its special responses to climate conditions. In an arid grassland, the coupling between GPP and ET is limited by water [1; 18]. For forests in China, vapor pressure deficit (VPD) has dominant influence on the carbon-water relationship [2]. Radiation and air temperature have a negative effect on the coupling between GPP and ET in tea gardens over southeastern China [19]. Drought can exacerbate the morning shift of GPP compared to ET in forests and can increase the frequency of decoupling of diurnal GPP and ET in grasslands [17].  
Subtropical forests in China are the only oasis at the same latitudes in the world and play an important role in global carbon and water cycles [20; 21]. Subtropical forests are rich in tree species, complex stand structure, and diverse environmental conditions [22; 23]. Yet, how climatic conditions affect the GPP-ET relationship in China's subtropical forests remains to be explored. Therefore, in this study, we selected two typical subtropical forests in southeast China: Dinghushan evergreen broadleaved forest and Qianyanzhou evergreen needleleaf forest. Based on the FLUXNET2015 dataset, we analyzed the short-term GPP-ET relationships using two measures: (1) daily WUE, which is the ratio of GPP to ET, and (2) \(r\), which is the correlation coefficient of hourly GPP and ET and a measure of C-W coupling. Specific objectives were (1) to characterize daily WUE and C-W coupling in the subtropical evergreen broadleaved and evergreen needleleaf forests, and (2) to identify meteorological impacts on daily WUE and C-W coupling in the two subtropical forests.