Phenology plays an important role in the regulation of terrestrial ecosystem water-use efficiency in the Northern Hemisphere

Ecosystem-scale water use efficiency (WUE), defined as the ratio of gross primary productivity (GPP) to evapotranspiration (ET), is an important indicator of coupled carbon-water cycles. Relationships between WUE and environmental factors have been widely investigated, but the variations in WUE in response to biotic factors remain little understood. Here, we argue that phenology cannot be ignored in the regulation of WUE by analyzing seasonal and annual WUE responses to variability of photosynthetic phenological factors in terrestrial ecosystems of the Northern Hemisphere using MODIS satellite observations during 2000-2014. Our results show that WUE during spring and autumn is widely significantly correlated to the photosynthetic onset (PO) and end (PE), respectively, after controlling for environmental factors (including temperature, precipitation, solar radiation and CO2 concentration), and the WUE sensitivities clearly enhance from warm regions to cold areas. The main patterns of WUE response to phenology suggest that an increase in spring (autumn) WUE with an earlier PO (later PE) are mainly because the increase in GPP is relatively larger in magnitude compared to that of ET, or due to an increase in GPP accompanied by a decrease in ET, resulting from an advanced PO (delayed PE). The phenology is widely identified as the dominant control of the WUE during spring and autumn, although climatic factors were previously recognized as the main drivers over seasonal WUE in the Northern Hemisphere. However, WUE during summer as well as the entire year shows a relatively weak correlation with the photosynthetic phenology. Our analyses will complement a mechanistic understanding of the carbon-water interactions over terrestrial ecosystems under global change.