## 1 Introduction  
Currently, the carbon cycles of terrestrial ecosystems have already been significantly influenced by global warming through altered water cycles [1, 2]. Ecosystem-scale water-use efficiency (WUE), defined as the rate of carbon assimilation (e.g., gross primary productivity (GPP)) per unit of water loss (e.g., evapotranspiration (ET), which is the sum of evaporation and plant transportation from the Earth's land to the atmosphere), is an important indicator of ecological function and the coupled carbon-water cycle of terrestrial ecosystems [3, 4]. A deeper understanding of how WUE has varied and the related underlying regulatory mechanisms will provide valuable insight into how carbon and water cycles will change with future climate change [5]. To achieve this aim, temporal-spatial variations in WUE and their relationships with environmental factors, such as climatic factors, CO\({}_{2}\) concentration and nitrogen deposition, have been investigated by manipulation experiments, site and satellite observations, and data-driven and process-based ecosystem models [3; 5; 6; 7; 8; 9; 10]. However, the variations in WUE in response to biotic factors remain inadequately understood, and this will eventually result in uncertainties in modeling of global carbon and water budgets. Phenology is the periodic plant life cycle, which includes not only the dates of life cycle events or phenomena (e.g., the sprouting and coloring of deciduous plants), but the state-transition times of annual cycles of ecosystem processes (e.g., the seasonality of photosynthesis in winter-dormant/summer-active evergreen ecosystems) [11]. From individual plants to the entire ecosystem, phenology directly or indirectly regulates carbon (e.g., photosynthesis and respiration) and water (e.g., transpiration and evaporation) fluxes between the land surface and the atmosphere [12] by altering physiological and structural characteristics, such as photosynthetic rate, canopy conductance, albedo and surface roughness, etc. [13]. Based on both ground observations and remotely sensed data, it has been widely reported that land surface phenology has significantly changed as a result of current climate change in terrestrial ecosystems of the Northern Hemisphere [13; 14; 15; 16; 17; 18; 19], and this modification is likely to affect processes of carbon and water cycles and consequent WUE [13; 20; 21; 22]. However, phenology has not been fully considered in the regulation of WUE at the ecosystem scale. First, although previous studies have shown variations in WUE with phenology (e.g., leaf age and canopy leaf area index) at leaf scale [23; 24; 25], it is difficult to upscale these findings from leaf to ecosystem level [9]. Second, WUE responses to yearly anomalies in phenology for a range of plant functional types (PFTs) remain little understood [26]. It is well known that responses of carbon assimilation and uptake to variability of phenology vary with climate condition and plant type [12; 27; 28; 29]. Due to the diverse sensitivities of carbon and water fluxes to interannual fluctuations of phenology, the variations in WUE with phenology can potentially be quite dynamic across PFTs [13]. Third, phenology may play different roles in regulating ecosystem processes. For example, the conceptual scenarios proposed by [PERSON] et al. [22] exhibited direct and/or indirect, positive and/or negative, and synchronized and/or lagged effects of earlier spring phenology on plant activities. Considering that a variability of WUE is determined by changes of both GPP and ET, the pattern by which WUE responds to phenology and the underlying mechanisms should be more complicated. Taking into account the important role and future changes of phenology, awareness of temporal-spatial WUE responses to critical phenological factors in terrestrial ecosystems is critical and valuable. Remote sensing data are very useful for investigating long-term, large-scale variability of ecosystem-scale WUE from a real-world perspective [8; 10].