Abstract
Precipitation patterns are expected to change in the semi-arid region within the next decades, with projected increasing in extreme drought events. Meanwhile, the timing of extreme drought also shows great uncertainty, suggesting that the timing of drought, especially during growing season, may subsequently impose stronger stress on ecosystem functions than drought itself. However, how the timing of extreme drought will impact on community productivity and carbon cycle is still not clear. In this study, three timing of extreme drought (a consecutive 30-day period without precipitation event) experiments were set up separately in early-, mid- and late-growing season in a temperate steppe in Inner Mongolia since 2013. The data, including soil water content (SWC), soil temperature (ST) chlorophyll fluorescence parameter (Fv/Fm), ecosystem respiration (Re), gross primary productivity (GPP), net ecosystem carbon absorption (NEE) and aboveground net primary productivity (ANPP) were collected in growing season (from May to September) of 2016. In this study, extreme drought significantly decreased SWC during the drought treatment but not for the whole growing season. Extreme drought decreased maximum quantum efficiency of plant photosystem II (Fv/Fm) under “optimum” value (0.75~0.85) of two dominant species (Leymus chinensis and Stipa grandis). While ANPP kept stable under extreme drought treatments due to the different responses of two dominant species, which brought a compensating effect in relative abundance and biomass. In addition, only early-growing season drought significantly decreased the average Re (P < 0.01) and GPP (P < 0.01) and depressed net CO2 uptake (P < 0.01) than mid- and late-growing season drought. ST and SWC influenced the changes of GPP directly and indirectly through photosynthetic ability of the dominant species by path analysis. Our results indicated that the timing of drought should be considered in carbon cycle models to accurately estimate carbon exchange and productivity of semi-arid grasslands in the context of changing climate.
Highlights
General atmospheric circulation models predict changes in precipitation patterns, for anthropogenic activity increase atmospheric CO2 concentrations and temperature [1,2,3]
The precipitation amounts of early- and late-growing season extreme drought were close to the mean value of the corresponding period of the ~60 years’ record, except the treatment of mid-growing season extreme drought, whose precipitation was less than half of the mean value in the ~60 years’ record
All treatments decreased soil water content (SWC) during the treatment significantly (Fig 1B, 1C and 1D), which was 43~46% lower than SWC in the ambient plot
Summary
General atmospheric circulation models predict changes in precipitation patterns, for anthropogenic activity increase atmospheric CO2 concentrations and temperature [1,2,3]. In semi-arid area, the stressful drought events would play a determinant role in controlling the development of ecosystem productivity [7, 12] and carbon (C) exchange [14,15,16,17]. Many ecosystem functions, such as aboveground net primary productivity (ANPP) and net ecosystem CO2 exchange [NEE: The difference between C sequestration through gross primary productivity (GPP) and release through ecosystem respiration (Re)], are the most frequently considered variables in extreme climate studies [18,19,20]. It is necessary to consider more diverse drought events, like timing of extreme drought, in manipulative experiments which could shed light on the direction and magnitude of carbon exchange in responses to extreme drought events [32]
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