Effects of drought on optimum temperature of carbon fluxes in temperate grasslands

Abstract

Ecosystem carbon flux components, including ecosystem gross primary productivity (GPP), ecosystem respiration (ER), and net ecosystem productivity (NEP), are the most direct indicators of ecosystem production capacity, carbon sequestration, and climate regulation strength. Nevertheless, little is known about how water availability affects the temperature responses of carbon flux components in water-limited ecosystems. Based on a long-term eddy observation dataset of temperate grasslands in northern China, we analyzed the effects of drought on the optimum temperature of carbon flux components ( $T_{{\text{opt}}}^{{\text{flux}}}$ ) in temperate grassland ecosystems and clarified the relative contributions of vegetation and climate factors to $T_{{\text{opt}}}^{{\text{flux}}}$ . We found that the optimum temperatures of carbon flux components were widespread across different grassland types, with a significant lower optimum temperature of NEP ( $T_{{\text{opt}}}^{{\text{NEP}}}$ , 17.32 ± 3.21 °C) than that of GPP ( $T_{{\text{opt}}}^{{\text{GPP}}}$ , 18.67 ± 2.53 °C) and ER ( $T_{{\text{opt}}}^{{\text{ER}}}$ , 19.30 ± 1.94 °C). Drought significantly reduced $T_{{\text{opt}}}^{{\text{NEP}}}$ , but had no significant effects on ${\text{T}}_{{\text{opt}}}^{{\text{GPP}}}\,$ and $T_{{\text{opt}}}^{{\text{ER}}}$ . Obvious shifts of occurrence date of $T_{{\text{opt}}}^{{\text{flux}}}$ were also observed in the years with different precipitation regimes. That is, $T_{{\text{opt}}}^{{\text{GPP}}}$ and $T_{{\text{opt}}}^{{\text{NEP}}}$ occurred significantly earlier than seasonal maximum temperature ( ${T_{{\text{max}}}}$ ) in the dry years, while $T_{{\text{opt}}}^{{\text{flux}}}$ significantly lagged behind the occurrence of ${T_{{\text{max}}}}$ in the wet years. Water availability and vegetation factors co-regulated the spatiotemporal variations of $T_{{\text{opt}}}^{{\text{flux}}}$ . In the dry years, precipitation and soil water content predominated the changes in $T_{{\text{opt}}}^{{\text{GPP}}}$ and $T_{{\text{opt}}}^{{\text{ER}}}$ , whereas vegetation structure (leaf area index) and physiological characteristics played a more important role in the wet years. Our study not only provided the first evidence for the widespread existence of $T_{{\text{opt}}}^{{\text{flux}}}$ of different carbon fluxes, but also addressed the remarkable impacts of drought on $T_{{\text{opt}}}^{{\text{flux}}}$ and their occurrence date in the water-limited grasslands. Therefore, incorporating the unimodality of these observed temperature responses of ecosystem carbon fluxes into land carbon models is necessary for improving the accuracy of carbon sequestration predictions.

Keywords

• carbon flux components
• optimum temperature
• drought
• occurrence date
• thermal adaptation