Responses of ecosystem water use efficiency to meteorological drought under different biomes and drought magnitudes in northern China

Future climate change scenarios predict that warming may increase drought stress over northern China, causing ecological deterioration and food insecurity. Water use efficiency (WUE) is an important indicator to understand the response of ecosystem productivity to water availability. At present we lack a clear picture of how ecosystem WUE responds to drought magnitude across a broad range of vegetation types and environments. Here, we applied Moderate Resolution Imaging Spectrometer (MODIS) satellite images and meteorological data to obtain a regional estimation of gross primary production (GPP) and evapotranspiration (ET) during the period of 2000-2014. We then investigated the relationships between WUE (GPP/ET) and drought magnitude (based on the Standardized Precipitation Evapotranspiration Index, SPEI) for different climate zones and biomes. The mean annual WUE decreased with increasing Aridity Index (AI) in arid regions. However, the relationships between WUE and AI followed a logarithmic function across semi-arid and humid regions. In hype-arid, sub-humid and humid regions, a drying trend resulted in increased WUE. Contrasting responses of WUE to drought were found in arid and semi-arid regions. The negative effect of drought that caused decreased WUE was most significant in the transition zone between arid and semi-arid lands. Besides, the relationships between relative changes in WUE and drought magnitude followed a logarithmic function of SPEI. Severe and extreme drought resulted in WUE reductions regardless of different hydroclimatic conditions and biomes. The ecosystem resistant to drought can be evaluated by the magnitude of WUE reductions during drought, together with the WUE-SPEI correlations. Forests were most resistant to drought, followed by croplands, grasslands, and deserts. Differences in the ecosystem resistance to water stress were attributed to diversities in drought survival traits and strategies. Given that hydroclimatic conditions break down ecosystem resistance under severe and extreme drought, we highlight an urgent need to formulate adaptation strategies aimed at reducing drought risk in the eastern and central portions of northern China.