Strong impacts of autumn phenology on grassland ecosystem water use efficiency on the Tibetan Plateau

Vegetation phenology is a commonly used indicator of climate-vegetation interactions, which also plays a crucial role in regulating gross primary productivity (GPP), evapotranspiration (ET) and ecosystem water use efficiency (WUE, defined as the ratio between GPP and ET). Actually, quantitative evaluation of the impact of phenology on WUE is critical yet lacking. In this study we detected the interannual variations of autumn WUE, GPP and ET and conducted Redundancy analysis (RDA) to quantitatively assess their responses to autumn end of growing season (EOS) in the grassland ecosystem across the Tibetan Plateau. Our study found that the delayed EOS increased the daily mean GPP but meanwhile increased the ET in autumn, rather than enhanced the WUE. Over the entire Tibetan Plateau, the daily GPP and ET in the autumn of the steppe were dominated by the delayed EOS, with the explanatory power of EOS to GPP and ET of 75.90% (P < 0.01) and 21.80% (P < 0.05). An one day delay of the EOS increased the daily GPP and ET by 0.05 ? 10-1 gC m-1 day- 1 (P < 0.01) and 0.27 ? 10-1 mm day-1 (P < 0.01), respectively. Furthermore, the EOS agreed on a positive correlation with daily GPP and ET in different ecoregions and the interannual variations of WUE for the meadow and steppe in the different ecoregions across the Tibetan Plateau did not show significant trend because of this same increasing pattern of GPP and ET form the delayed EOS. This study highlights the impacts of EOS on the interannual changes of the GPP, ET and WUE in autumn in a large cold and semiarid/arid area, which indicated that phenology should be regard as a crucial indicator in the process of carbon water coupling cycles and their biological regulatory mechanisms in terrestrial ecosystems.

Automated summary

This study found that the delayed end of growing season (EOS) increased daily mean GPP but also increased ET in autumn, without enhancing WUE. The delay in EOS had a significant impact on daily GPP and ET in the steppe ecosystem across the Tibetan Plateau.