# Precipitation-Use Efficiency and Its Conversion with Climate Types in Mainland China
## 1 Introduction  
Global warming and the increase of extreme weather events are having a serious impact on the structure, function, and processes of global ecosystems [1], and have become a focal issue of common concern to governments, the public, and the scientific community. In IPCC AR6 [2], it is reported that the frequency and intensity of some extreme weather and climate events will continue to increase under medium and high emission scenarios, and the increased extreme events (e.g., droughts, heat waves, and heavy rainfall) will affect 25\(-\)40% of global ecosystem structure and function. The terrestrial ecosystem carbon cycle is a key process driving ecosystem change, and changes in the ecosystem carbon cycle are sensitive to climate change. China is one of the most sensitive and vulnerable regions to climate change. Climate changedecreased the capacity of carbon storage [3], and extreme climate events such as drought, extreme heat, and extreme precipitation all have serious impacts on the carbon cycle of terrestrial ecosystems. Droughts have weakened vegetation growth [4], and prolonged and persistent droughts have reduced carbon accumulation, causing grassland ecosystems in Inner Mongolia to change from a carbon sink to a carbon source in a typical year [5]. Heat waves and droughts significantly reduced regional GPPs and crop yields in summer 2013 [6]. The ice storm in early 2008 also resulted in a decrease in annual evapotranspiration and GPP in southeastern China [7]. Precipitation-use efficiency (PUE) describes the response of net primary productivity (NPP) to the temporal and spatial distribution of precipitation. PUE is a comprehensive physiological and ecological index for evaluating the appropriate degree of vegetation growth while reflecting the carbon and water cycles and the carbon-water relationship in the ecosystem [8]. Research on the characteristics of PUE and its control mechanism can help evaluate and predict the impact of global changes on the carbon-water cycle of ecosystems and provide a theoretical basis for regional plant protection and restoration. PUE distribution and changes are affected by several factors, such as topography, soil conditions, climate change, and human activities. Climate change is the most important and active factor [9; 10]. Earlier studies addressed the response of the spatiotemporal PUE pattern to climate change but did not reach a consistent conclusion due to different temporal and spatial scales. Some studies concluded that the PUE spatially decreases with increasing drought and potential evapotranspiration and increases with increasing precipitation [11]. However, other studies concluded that the PUE spatially decreases with increasing precipitation [12; 13]. It has also been demonstrated that there is no obvious relationship between the spatial PUE distribution and precipitation [14]. In addition, some studies have reported that the PUE exhibits a unimodal distribution that first increases and then decreases with increasing precipitation and reaches its peak at a specific precipitation value [13; 15; 16]. The feedback mechanisms between the water-carbon cycle of ecosystems and climate are relatively complex, and the response of NPP and PUE to climate change has a large spatial and temporal heterogeneity. China's vast land area, complex topography, diverse climate and vegetation types, and high spatial and temporal variability in precipitation and ecosystem carbon fluxes hinder the accurate assessment of carbon fluxes [17; 18; 19; 20]. Earlier studies focused mostly on small local areas with precipitation or temperature limits or specific vegetation types [11; 16; 21; 22], and little attention has been paid to variations in PUE along a climatic gradient.