CO ₂ flux characteristics of the open savanna and its response to environmental factors in the dry–hot valley of Jinsha River, China

Abstract. The dry–hot valley ecosystem of Jinsha River (JS) is a non-zonal special heat island habitat within the global temperate region. Revealing the CO2 flux (Fc) changes and the response mechanisms of this ecosystem to environmental factors is crucial for accurately predicting the carbon (C) sequestration capacity of global terrestrial ecosystems, especially temperate ecosystems, under future extreme-drought climate conditions. We focused on the open savanna, which is a core component of the JS dry–hot valley plant community, as our research subject. Using the static chamber method, we conducted long-term fixed-point observations of Fc in the dominant grassy layer, explored the influence of different environmental factors on Fc, and analyzed the trends of Fc changes in the open savanna under future extreme-drought and low-rainfall climate scenarios. Fc of the open savanna exhibits distinct seasonal characteristics. During the dry season, it is in a C emission state, with a cumulative CO2 emission of 1.3215 t ha−1. In contrast, during the rainy season, it shows significant C absorption characteristics, with a cumulative CO2 absorption of 0.6137 t ha−1. The occurrence of extreme-drought events in the study area has weakened the C absorption capacity of the open savanna, making it a weak C source with an annual cumulative CO2 emission of 0.7078 tha-1a-1, indicating a C-neutral feature. The main environmental factors affecting the net ecosystem exchange (NEE) variations in the open savanna across different seasons were different, but overall, soil water content was the key environmental factor controlling NEE. The response mechanisms of NEE to changes in different environmental factors were generally similar, with NEE being at its minimum when located at the threshold of environmental factors. When environmental conditions exceed or fall below this threshold, the C emissions of the open savanna will increase. As the frequency and severity of future extreme droughts continue to rise, the C emissions from the open savanna in the study area will also continue to increase.