Drought limits alpine meadow productivity in northern Tibet

Under global climate change, warmer temperatures and changing precipitation patterns will increase the relative importance of soil and atmospheric droughts in limiting productivities across different ecosystems, especially in the fragile and sensitive ecosystem on the Tibetan Plateau. Therefore, the present study examined the effects of atmospheric drought, soil drought and compound drought on alpine meadow productivity based on seven years of continuous observational data. The results demonstrated that water conditions dominated the variations in alpine meadow productivity in northern Tibet. Gross prime productivity (GPP) in the growing season was significantly inhibited during soil drought (defined as soil water content (SWC) < 0.11 m³ m⁻³) or atmospheric drought (defined as vapor pressure deficit (VPD) > 0.61 kPa) conditions. Pooling of the moisture data according to these thresholds revealed that soil drought occurred more frequently and accounted for 20.4% of all observed days. Atmospheric drought occurred in 2.4% of all days, and the frequency of compound drought (when SWC < 0.11 m³ m⁻³ and VPD > 0.61 kPa) was 8.7%. Compound drought reduced GPP by the largest extent to as much as 1.47 g C m⁻² d⁻¹. Atmospheric drought reduced GPP by 1.10 g C m⁻² d⁻¹. However, the more frequent soil drought reduced GPP by the smallest extent, with a value of 0.69 g C m⁻² d⁻¹. In spite of this, soil drought played an important role in restricting the accumulated GPP of this alpine meadow via its higher occurrence frequency and longer duration. All three types of drought significantly increased sensible heat flux (H), which provided positive feedbacks to droughts and led to further decreases in GPP. The results of the present study suggest that drought greatly threatens the alpine meadow ecosystem in northern Tibet. Therefore, more attention should be paid to water conditions under the global climate change context.