Verification of a threshold concept of ecologically effective precipitation pulse: From plant individuals to ecosystem

Ecosystem water use efficiency and carbon use efficiency respond oppositely to vegetation greening in China's Loess Plateau

Ecosystem water use efficiency (WUE) and carbon use efficiency (CUE) are critical parameters to determine the trade-off between water consumption and carbon sequestration in drylands. However, the roles of vegetation cover, climate factors and soil moisture in affecting the coupling of WUE and CUE were still poorly understood. This study combined the spatial random forest model and structural equation model to detect the drivers of WUE and CUE variations in China's Loess Plateau during 2001-2020, a typical water-limited region with about 87 % of area experiencing significant vegetation greening. The WUE increased significantly (P < 0.05) in 92.7 % of the greening area (0.024 ± 0.020 gC kg-1H2O yr-1), and CUE decreased (-0.0011 ± 0.0011 yr-1) and increased (0.0010 ± 0.0012 yr-1) slightly in half and half of the greening area. The vegetation greening trend was a dominant factor positively influencing the variation of WUE (with 42.0 % explanation), whereas CUE exhibited a negative response to the greening trend (with 25.0 % explanation). However, the regions with dense vegetation cover (NDVI >0.5) were unfavourable for the increase of WUE but favourable for the increase of CUE. Vegetation greening exerted effect on WUE mainly via the path of gross primary productivity and ratio of transpiration to evapotranspiration (path coefficient: 0.83), whereas it had direct effect on CUE (path coefficient: 0.48). An increase in VPD facilitated the enhancement of both WUE and CUE. There was an optimal annual precipitation range (400-600 mm) that maximized WUE enhancement, and annual temperature (∼10 °C) optimized CUE increase. The findings indicate that despite the improvement in WUE, the vegetation greening may not necessarily enhance carbon sequestration potential in drylands. This study enhances the knowledge of the interaction between vegetation dynamics and water‑carbon coupling in drylands, contributing to ecological restoration practices and sustainable ecosystem management.

Divergent responses of CO2 and CH4 fluxes to changes in the precipitation regime on the Tibetan Plateau: Evidence from soil enzyme activities and microbial communities

Carbon fluxes (CO₂ and CH₄) are important indicators of the response of alpine meadow ecosystems to global climate change. Alpine meadows on the Qinghai-Tibet Plateau are sensitive to climate change. Although the temporal allocation of precipitation can vary, its intensity is expected to increase, and its frequency is expected to decrease in the future. In this study, a manipulative field experiment was conducted to investigate how carbon fluxes are altered in response to moderate and severe changes in the precipitation regime. Fluctuations in CH₄ flux were large under a severely altered precipitation regime (range of -0.048-0.038 mg m^-2 h^-1). Severe changes in the precipitation regime significantly reduced soil CH₄ uptake by approximately 54.3%. This was probably affected by the decrease in the dissolved organic carbon concentration and changes in the microbial community (mainly Gammaproteobacteria), which were induced by variation in soil water conditions under various precipitation regimes. Under moderate changes in the precipitation regime, the average value of CO₂ fluxes (ecosystem respiration) was 698.21 ± 35.19 mg m^-2 h^-1, which was significantly decreased by 20.7% compared with the control. This likely stems from the suppression of enzyme activity (particularly α-1,4-glucosidase and β-1,4-glucosidase) and the alteration of microbial community structure in this treatment, which led to a decrease in organic matter breakdown and a reduction in the release of CO₂ to the atmosphere. However, CO₂ fluxes were slightly (i.e., not significantly) decreased under the severely altered precipitation regime. Such different responses of CO₂ flux are probably driven by differences in microbial strategies. This study not only increases our understanding of the mechanisms underlying the adaptation of alpine meadow ecosystems to global climate change but also provides new insight into the carbon source/sink functions of alpine meadows.

Climatic warming enhances soil respiration resilience in an arid ecosystem

Precipitation plays a vital role in maintaining desert ecosystems in which rain events after drought cause soil respiration (R_s) pulses. However, this process and its underlying mechanism remain ambiguous, particularly under climatic warming conditions. This study aims to determine the magnitude and drivers of R_s resilience to rewetting. We conducted a warming experiment in situ in a desert steppe with three climatic warming scenarios-ambient temperature as the control, long-term and moderate warming treatment, and short-term and acute warming treatment. Our findings showed that the average R_s over the measurement period in the control, moderate and acute warming plots were 0.51, 0.30 and 0.30 μmol·CO₂·m^-2·s^-1, respectively, and significantly increased to 1.72, 1.41 and 1.72 μmol·CO₂·m^-2·s^-1, respectively, after rewetting. Both microbial and root respiration substantially increased by rewetting; microbial respiration contributed more than root respiration to total R_s. The R_s significantly increased with microbial biomass carbon and soil organic carbon (SOC) contents. The R_s increase by rewetting might be due to the greater microbial respiration relying heavily on microbial biomass and the larger amount of available SOC after rewetting. A trackable pattern of R_s resilience changes occurred during the daytime. The resilience of R_s in acute warming plots was significantly higher than those in both moderate warming and no warming plots, indicating that R_s resilience might be enhanced with drought severity induced by climatic warming. These results suggest that climatic warming treatment would enhance the drought resilience of soil carbon effluxes following rewatering in arid ecosystems, consequently accelerating the positive feedback of climate change. Therefore, this information should be included in carbon cycle models to accurately assess ecosystem carbon budgets with future climate change scenarios in terrestrial ecosystems, particularly in arid areas.

Characterizing Growing Season Length of Subtropical Coniferous Forests with a Phenological Model

Understanding plant phenological change is of great concern in the context of global climate change. Phenological models can aid in understanding and predicting growing season changes and can be parameterized with gross primary production (GPP) estimated using the eddy covariance (EC) technique. This study used nine years of EC-derived GPP data from three mature subtropical longleaf pine forests in the southeastern United States with differing soil water holding capacity in combination with site-specific micrometeorological data to parameterize a photosynthesis-based phenological model. We evaluated how weather conditions and prescribed fire led to variation in the ecosystem phenological processes. The results suggest that soil water availability had an effect on phenology, and greater soil water availability was associated with a longer growing season (LOS). We also observed that prescribed fire, a common forest management activity in the region, had a limited impact on phenological processes. Dormant season fire had no significant effect on phenological processes by site, but we observed differences in the start of the growing season (SOS) between fire and non-fire years. Fire delayed SOS by 10 d ± 5 d (SE), and this effect was greater with higher soil water availability, extending SOS by 18 d on average. Fire was also associated with increased sensitivity of spring phenology to radiation and air temperature. We found that interannual climate change and periodic weather anomalies (flood, short-term drought, and long-term drought), controlled annual ecosystem phenological processes more than prescribed fire. When water availability increased following short-term summer drought, the growing season was extended. With future climate change, subtropical areas of the Southeastern US are expected to experience more frequent short-term droughts, which could shorten the region’s growing season and lead to a reduction in the longleaf pine ecosystem’s carbon sequestration capacity.

Long-Term Changes and Variability of Ecologically-Based Climate Indices along an Altitudinal Gradient on the Qinghai-Tibetan Plateau

Extreme climate events are typically defined based on the statistical distributions of climatic variables; their ecological significance is often ignored. In this study, precipitation and temperature data from 78 weather stations spanning from 1960 to 2015 on the Qinghai-Tibetan Plateau were examined. Specifically, long-term and altitudinal variability in ecologically relevant climate indices and their seasonal differences was assessed. The results show that indices of daily temperatures greater than 10 °C and 25 °C show positive annual change trends during the growing season (May to September). Indices of daily rainfall greater than 2 mm, 3 mm and 5 mm positively alternate with years both in and around the growing season (May–September, April and October). In contrast, the index of daily snowfall greater than 2 mm shows opposite annual variability. Additionally, a higher altitude significantly leads to fewer days with temperature deviations above 20 °C, except for in October. The three abovementioned rainfall indices present significantly positive variability with increasing altitude during the growing season. In contrast, the snow index shows similar altitudinal changes in the months surrounding the growing season. This study allows us to better cope with the threats of climate change to vulnerable ecosystems.

Drought-induced reduction in methane fluxes and its hydrothermal sensitivity in alpine peatland

Accurate estimation of CH₄ fluxes in alpine peatland of the Qinghai-Tibetan Plateau under extreme drought is vital for understanding the global carbon cycle and predicting future climate change. However, studies on the impacts of extreme drought on peatland CH₄ fluxes are limited. To study the effects of extreme drought on CH₄ fluxes of the Zoige alpine peatland ecosystem, the CH₄ fluxes during both extreme drought treatment (D) and control treatment (CK) were monitored using a static enclosed chamber in a control platform of extreme drought. The results showed that extreme drought significantly decreased CH₄ fluxes in the Zoige alpine peatland by 31.54% (P < 0.05). Extreme drought significantly reduced the soil water content (SWC) (P < 0.05), but had no significant effect on soil temperature (Ts). Under extreme drought and control treatments, there was a significant negative correlation between CH₄ fluxes and environmental factors (Ts and SWC), except Ts, at a depth of 5cm (P < 0.05). Extreme drought reduced the correlation between CH₄ fluxes and environmental factors and significantly weakened the sensitivity of CH₄ fluxes to SWC (P < 0.01). Moreover, it was found that the correlation between subsoil (20 cm) environmental factors and CH₄ fluxes was higher than with the topsoil (5, 10 cm) environmental factors under the control and extreme drought treatments. These results provide a better understanding of the extreme drought effects on CH₄ fluxes of alpine peatland, and their hydrothermal impact factors, which provides a reliable reference for peatland protection and management.

Ecological responses to heavy rainfall depend on seasonal timing and multi-year recurrence

Heavy rainfall events are expected to increase in frequency and severity in the future. However, their effects on natural ecosystems are largely unknown, in particular with different seasonal timing of the events and recurrence over multiple years. We conducted a 4 yr manipulative experiment to explore grassland response to heavy rainfall imposed in either the middle of, or late in, the growing season in Inner Mongolia, China. We measured hierarchical responses at individual, community and ecosystem levels. Surprisingly, above-ground biomass remained stable in the face of heavy rainfall, regardless of seasonal timing, whereas heavy rainfall late in the growing season had consistent negative impacts on below-ground and total biomass. However, such negative biomass effects were not significant for heavy rainfall in the middle of the growing season. By contrast, heavy rainfall in the middle of the growing season had greater positive effects on ecosystem CO₂ exchanges, mainly reflected in the latter 2 yr of the 4 yr experiment. This two-stage response of CO₂ fluxes was regulated by increased community-level leaf area and leaf-level photosynthesis and interannual variability of natural precipitation. Overall, our study demonstrates that ecosystem impacts of heavy rainfall events crucially depend on the seasonal timing and multiannual recurrence. Plant physiological and morphological adjustment appeared to improve the capacity of the ecosystem to respond positively to heavy rainfall.

Aboveground net primary productivity not CO2 exchange remain stable under three timing of extreme drought in a semi-arid steppe

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.

Small rainfall pulses affected leaf photosynthesis rather than biomass production of dominant species in semiarid grassland community on Loess Plateau of China

In the semiarid region Loess Plateau of China, rainfall events, typically characterised as pulses, affect photosynthesis and plant community characteristics. The response of dominant species and grassland community to rainfall pulses was evaluated through a simulation experiment with five pulse sizes (0, 5, 10, 20 and 30mm) in the semiarid Loess Plateau of China in June and August of 2013. The study was conducted in a natural grassland community dominated by Bothrichloa ischaemum (L.)Keng and Lespedeza davurica (Lax.) Schindl. In June, the leaf photosynthetic rate (Pn), transpiration rate, stomatal conductance, intercellular CO2 concentration of both species and soil water content increased rapidly after rainfall pulses. B. ischaemum was more sensitive to the pulses and responded significantly to 5mm rainfall, whereas L. davurica responded significantly only to rainfall events greater than 5mm. The magnitude and duration of the photosynthetic responses of the two species to rainfall pulse gradually increased with rainfall sizes. The maximum Pn of B. ischaemum appeared on the third day under 30mm rainfall, whereas for L. davurica it appeared on the second day under 20mm rainfall. Soil water storage (0-50cm) was significantly affected under 10, 20 and 30mm rainfall. Only large pulses (20, 30mm) increased community biomass production by 21.3 and 27.6% respectively. In August, the effect of rainfall on the maximum Pn and community characteristics was generally not significant. Rainfall pulses affected leaf photosynthesis because of a complex interplay between rainfall size, species and season, but might not induce a positive community-level feedback under changing rainfall patterns.