Seasonal decoupling of water supply and CO2 fluxes of urban greening shrubs is driven by leaf area index.

Abstract The development and functioning of landscapes in different regions of the world, especially at polar latitudes, may be significantly affected by the increased frequency of extreme weather events associated with modern climate change. These events can influence regional biogeochemical cycles, including water, carbon, and nitrogen cycles, with serious implications for ecosystem functioning and canopy production. The main objective of this study is to assess the spatial variability in the response of daily net ecosystem CO2 exchange (NEE) of Northern Hemisphere tundra and forest-tundra landscapes to anomalous temperature and precipitation events during the growing season. These landscape types are considered to be among the most vulnerable to changes in environmental conditions under a changing climate. For our data analysis, we use meteorological and CO2 flux data from the global FLUXNET and regional AmeriFlux networks, as well as the ERA5 reanalysis dataset. Analysis of CO2 flux anomalies in tundra and forest-tundra ecosystems revealed a wide range of observed NEE responses to anomalous temperature and precipitation events during the growing season, depending on geographic location and landscape type. In contrast to most previous studies, the stressed CO2 uptake and higher CO2 emissions under anomalously high temperatures were mostly detected at the southern boundary of the polar region, where heat waves are more frequent. Prevailing CO2 uptake during anomalously high temperature days was found in deciduous broadleaf forests and open shrublands. The effect of anomalously low temperature is manifested by an increase in CO2 emissions. The response of CO2 fluxes to anomalously high and low precipitation is quite similar regardless of the time scale (short-term or long-term response). In most tundra and forest-tundra ecosystems, heavy precipitation typically results in increased CO2 emissions to the atmosphere. The prolonged precipitation deficit is accompanied by a prevailing CO2 uptake.

ABSTRACTDrought events are increasing in frequency and intensity due to climate change, causing lasting impacts on plant communities and ecosystem functioning. In the sub‐arctic, climate is changing at a rate above the global average with amplifying effects on the carbon cycle. Drought‐induced shifts in the balance between productivity and respiration might have important implications for climate change feedbacks in these regions. However, little is known about how carbon fluxes in sub‐arctic ecosystems respond to drought, hampering predictions. Here, we test how two important but contrasting sub‐arctic ecosystem types, Sphagnum peatland and tundra heath, respond to experimental drought. Mesocosms were exposed to a full precipitation exclusion for 7 weeks, decreasing gravimetric water content by 66% and 53% for Sphagnum peatland and tundra heath, respectively. Drought suppressed all CO2 flux components. Gross primary productivity was on average reduced by 47% and 64%, and ecosystem respiration by 40% and 53% in Sphagnum peatland and tundra heath, respectively. Concomitantly with the ecosystem fluxes, leaf photosynthesis of the three most abundant vascular plant species per ecosystem type was on average suppressed by 40% (peatland) and 77% (tundra heath). Drought resulted in high plant mortality, with up to 54% (peatland) and 73% (tundra heath) dead shoots, which might represent a significant legacy effect suppressing CO2 uptake in subsequent growing seasons. In summary, tundra heath was overall more responsive to drought than peatland. This differential sensitivity, previously unaccounted for, might be important in the future under intensifying drought events. Considering that tundra heath covers more than half of the sub‐arctic land area, its drought responsiveness might induce significant reductions in total arctic net CO2 uptake. This would move the arctic carbon balance further toward a net CO2 source.

BackgroundForest ecosystems are in the spotlight for their potential to mitigate anthropogenic carbon dioxide (CO2) emissions through net photosynthesis. However, this mitigation potential can be counteracted by respiratory losses, e.g., from soils and the forest floor. With global warming, soil respiration (SR) rates are expected to increase, unless acclimation occurs. Using manual and automated chambers as well as a below-canopy eddy-covariance system, we quantified SR and forest floor net CO2 exchange (NEEff) for 13 years throughout an 18-year study period (2006–2010, 2015–2016, 2018–2023) in a mixed deciduous forest ecosystem in Switzerland. We identified the contribution of environmental drivers for SR and NEEff using Extreme Gradient Boosting models and Shapley additive explanations (SHAP) analyses and assessed the long-term temperature sensitivity of SR and NEEff.ResultsOver the 18-year study period, soil temperature increased significantly and was the main driver of both SR and NEEff, explaining over 50% of their temporal variability. Differences in drivers and magnitudes of SR vs. NEEff were only found in early spring, when the forest floor vegetation showed net CO2 uptake. Finally, we found no evidence that SR or NEEff (at mean annual temperatures) had increased between 2006 and 2023. Similarly, no significant change in the temperature sensitivity of SR and NEEff was observed.ConclusionsCombining multiple techniques to assess long-term responses of CO2 fluxes to environmental conditions with machine learning approaches enhanced our understanding of forest responses to climate change. Moreover, our findings suggest that soil and forest floor respiration already acclimated to warmer conditions, highly relevant for predicting future mitigation potentials of forest ecosystems.

Environmental changes, such as climate warming and higher herbivory pressure, are altering the carbon balance of Arctic ecosystems; yet, how these drivers modify the carbon balance among different habitats remains uncertain. This hampers our ability to predict changes in the carbon sink strength of tundra ecosystems. We investigated how spring goose grubbing and summer warming-two key environmental-change drivers in the Arctic-alter CO2 fluxes in three tundra habitats varying in soil moisture and plant-community composition. In a full-factorial experiment in high-Arctic Svalbard, we simulated grubbing and warming over two years and determined summer net ecosystem exchange (NEE) alongside its components: gross ecosystem productivity (GEP) and ecosystem respiration (ER). After two years, we found net CO2 uptake to be suppressed by both drivers depending on habitat. CO2 uptake was reduced by warming in mesic habitats, by warming and grubbing in moist habitats, and by grubbing in wet habitats. In mesic habitats, warming stimulated ER (+75%) more than GEP (+30%), leading to a 7.5-fold increase in their CO2 source strength. In moist habitats, grubbing decreased GEP and ER by ~55%, while warming increased them by ~35%, with no changes in summer-long NEE. Nevertheless, grubbing offset peak summer CO2 uptake and warming led to a twofold increase in late summer CO2 source strength. In wet habitats, grubbing reduced GEP (-40%) more than ER (-30%), weakening their CO2 sink strength by 70%. One-year CO2-flux responses were similar to two-year responses, and the effect of simulated grubbing was consistent with that of natural grubbing. CO2-flux rates were positively related to aboveground net primary productivity and temperature. Net ecosystem CO2 uptake started occurring above ~70% soil moisture content, primarily due to a decline in ER. Herein, we reveal that key environmental-change drivers-goose grubbing by decreasing GEP more than ER and warming by enhancing ER more than GEP-consistently suppress net tundra CO2 uptake, although their relative strength differs among habitats. By identifying how and where grubbing and higher temperatures alter CO2 fluxes across the heterogeneous Arctic landscape, our results have implications for predicting the tundra carbon balance under increasing numbers of geese in a warmer Arctic.

This study conducted a comprehensive assessment of the response of wetland ecosystems in temperate and polar latitudes, located on different continents, to extreme weather events. These events included temperature anomalies (unusually high/low temperatures) and precipitation anomalies (droughts/intense precipitation). The analysis of the response net ecosystem exchange (NEE) of CO2 and latent heat (LE) fluxes to extreme temperature and precipitation events used ERA5 reanalysis data [Smith, 2011] and observations of CO2 and LE fluxes from the global FLUXNET database [https://fluxnet.org/data/]. Fifteen greenhouse gas flux monitoring stations were selected for the study, representing the longest and most continuous time series of observations. These stations are located on different continents, with eight stations in temperate latitudes and seven in polar regions. It should be noted that this study focused exclusively on the warm season. The beginning and end of the warm season were defined as the sustained crossing of the daily mean air temperature above 0°C for at least seven consecutive days. For each station, daily anomalies of CO2 and LE fluxes were calculated as the deviation from the long-term mean values for the corresponding day of the year. Extremely high/low values of flux anomalies were identified as exceeding one standard deviation from the overall time series for each calendar month individually. To identify periods with extreme air temperature values, ERA5 reanalysis data on two-meter air temperature every 3 hours with a spatial resolution of 0.25°×0.25° from 1991 to 2021 were used. To estimate extreme precipitation amounts, data from half-hourly station observations were used. Daily means were calculated from these data in a first step. Thresholds for defining extremely hot/cold periods were calculated as daily mean air temperature exceeding the 95th percentile (for anomalously hot periods) or not exceeding the 5th percentile (for anomalously cold periods) of a normal distribution with mean and standard deviation. The distribution was constructed for a specific month of the year and then averaged over the entire period considered. Two approaches were used to determine the extreme precipitation threshold. In the first approach, extreme precipitation days were defined as days with daily precipitation exceeding the 95th percentile of the probability density function (the Weibull distribution was used for precipitation). The second approach was based on the assessment of the Antecedent Precipitation Index (API), which determines the cumulative effect of precipitation on CO2 fluxes. For the quantitative assessment of the relationship between temperature and precipitation extremes and flux anomalies, the percentages of days on which both the NEE/LE anomaly exceeded the standard deviation and the temperature/precipitation exceeded the 95th percentile for the upper threshold or the temperature did not reach the 5th percentile for the lower threshold were calculated. The percentage was calculated based on the total number of days when one of the characteristics (air temperature, daily sum of precipitation) exceeded the threshold. The analysis showed that temperate and polar wetland ecosystems can respond differently to temperature and precipitation anomalies. These differences can be attributed to the geographic location of the ecosystem, regional climatic conditions, plant species composition, and the intensity of temperature and precipitation extremes. During the warm half of the year, periods of extremely high temperatures in temperate latitudes were associated with a positive CO2 flux anomaly, corresponding to an increased emission of CO2 into the atmosphere. In contrast, polar latitudes showed an opposite response - an increase in CO2 uptake by wetland ecosystems under anomalously high temperatures. This opposite response of CO2 fluxes may be related to the different soil moisture regimes in polar wetland ecosystems and the different plant species composition. Extremely high temperatures were accompanied by positive LE anomalies due to the intensification of evaporation processes with rising temperatures, a trend observed in all wetland ecosystems analyzed. The immediate response of wetland ecosystems to intense precipitation (above the 95th percentile) was manifested as an increase in CO2 flux to the atmosphere at almost all stations analyzed. This observed response could be related to the "Birch effect" [Birch, 1964], which is characterized by an intensification of soil respiration due to a sudden increase in soil moisture and, consequently, an increase in the rate of decomposition and mineralization of organic matter during heavy precipitation and rising groundwater levels. LE flux decreases during intense precipitation, indicating suppression of evaporation due to high humidity and reduced incoming solar radiation. The cumulative effect (API index) of extremely high precipitation is characterized by a predominance of extremely positive CO2 flux anomalies over negative ones in wetland ecosystems at both temperate and polar latitudes. It should also be noted that the percentage of days with increased CO2 uptake during the two weeks following intense precipitation is significantly higher than for the immediate response (10-25% of days in temperate latitudes and 5-20% of days in polar latitudes). The increase in CO2 uptake after heavy precipitation may be related to enhanced photosynthetic rates of the vegetation cover under sunny weather and optimal soil moisture conditions. A prolonged absence of precipitation, represented by extremely low API values, is accompanied by negative CO2 flux anomalies (enhanced uptake) at most of the studied wetland ecosystem stations, indicating a high adaptive potential of the studied wetland ecosystems to short-term (less than 14 days) dry periods. On the other hand, enhanced CO2 uptake could be facilitated by clear weather conditions, which prevail during dry periods and are accompanied by an increase in direct solar radiation and corresponding acceleration of photosynthetic processes. It is noteworthy that flux anomalies often did not coincide with temperature or precipitation extremes, indicating that the functioning of wetland ecosystems is strongly influenced by multiple abiotic and biotic factors, which vary among different plant communities.

Responses of greenhouse gas emissions to increased precipitation events in different ecosystems: A meta-analysis

Abstract Saline lakes contributions to the carbon cycle is crucial to the Qinghai‐Tibetan Plateau (QTP) carbon budget. Here, based on the 8‐year direct measurement of CO2 flux over the Qinghai Lake (QHL) and 83 collected CO2 flux data estimated by pCO2 sampling from 45 lakes over the QTP, we identified the interannual variations of CO2 flux and its response to the extreme climate events. Results showed: (a) the QHL CO2 absorption weakened in the spring, autumn and winter and turn to CO2 emissions in the summer during 2013–2020; (b) with higher Ts and less precipitation, coupling of positive Pacific Decadal Oscillation (PDO) and El Niño enhanced the summer CO2 emissions; and (c) the PDO and ENSO had obvious superposition effect on the decrease of CO2 absorption in autumn. Our results show the potential mechanism of lake CO2 flux responses to extreme climate and further defines the significance of the QTP carbon budget and cycling.

Modern climate change, accompanied by rapidly increasing global air temperature, changing precipitation patterns, frequency and severity of extreme weather events, may have a significant impact on the functioning, growth, and development of forest ecosystems. The aim of the study was to assess the effects of extreme weather events (significant positive and negative anomalies in air temperature and precipitation) on the carbon dioxide (CO2) fluxes in boreal and temperate forest ecosystems in the Northern Hemisphere. 26 greenhouse gas (GHG) flux monitoring stations of the global FLUXNET network with the most continuous observations were selected for the analysis of CO2 fluxes. The stations are located in forest ecosystems of different biome types according to the IGBP classification. Meteorological conditions were analyzed using observations from meteorological stations and ERA5 reanalysis data. The results showed that the response of CO2 fluxes to temperature and precipitation anomalies varies depending mainly on the type of forest ecosystem, its geographical location and regional climatic conditions. Extremely high air temperature in any season leads to increased CO2 emissions to the atmosphere in all forest types studied, with the most pronounced response in coniferous forests. Negative air temperature anomalies in the warm season could have the opposite effect, either increasing or decreasing the CO2 uptake by forest ecosystems, depending on the forest ecosystem type. No significant response of CO2 fluxes to extremely low temperatures in the cold season was found. During periods of heavy precipitation, the emission of CO2 to the atmosphere dominated in all forest ecosystems under study. At the same time, after a large amount of precipitation during the warm season, a cumulative effect of available soil moisture in the plant root zone on CO2 fluxes was revealed. As a result the CO2 uptake by vegetation increases due to the growing rate of plant photosynthesis under sufficient soil moisture conditions.

The effects of anomalous weather conditions (such as extreme temperatures and precipitation) on CO2 flux variability in different tropical ecosystems were assessed using available reanalysis data, as well as information about daily net CO2 fluxes from the global FLUXNET database. A working hypothesis of the study suggests that the response of tropical vegetation can differ depending on local geographical conditions and intensity of temperature and precipitation anomalies. The results highlighted the large diversity of CO2 flux responses to the fluctuations of temperature and precipitation in tropical ecosystems that may differ significantly from some previously documented relationships (e.g., higher CO2 emission under the drier and hotter weather, higher CO2 uptake under colder and wetter weather conditions). They showed that heavy precipitation mainly leads to the strong intensification of mean daily CO2 release into the atmosphere at almost all stations and in all types of study biomes. For the majority of considered tropical ecosystems, the intensification of daily CO2 emission during cold and wet weather was found, whereas the ecosystems were predominantly served as CO2 sinks from the atmosphere under hot/dry conditions. Such disparate responses suggested that positive and negative temperature and precipitation anomalies influence Gross Primary Production (GPP) and Ecosystem Respiration (ER) rates differently that may result in various responses of Net Ecosystem Exchanges (NEE) of CO2 to external impacts. Their responses may also depend on various local biotic and abiotic factors, including plant canopy age and structure, plant biodiversity and plasticity, soil organic carbon and water availability, surface topography, solar radiation fluctuation, etc.

The effect of grassland degeneration on the emissions and sinks of carbon dioxide (CO2) received extensive attention because of the increase of degraded area and the degenerationlevel in alpine meadow. To quantify its effect, we investigated net ecosystem exchange (NEE), gross primary productivity (GPP), ecosystem respiration (Reco), plant respiration (Rplant) and heterotrophic respiration (Rh), as well as environmental variability (soil water content (SWC), soil total carbon (STC), soil total nitrogen content (STN) and aboveground biomass (AGB)) from different degraded grasslands and non-degraded (ND) grassland in alpine meadow, Qinghai-Tibetan Plateau in growing season. The results indicated that compared with ND grassland, land degradation significantly decreased net CO2 uptake (-NEE), GPP, Reco, Rplant and Rh by 60.61%, 63.22%, 67.53% 78.82% and 43.56%, respectively in extremely degraded (ED) grassland. These consistent responses suggested that the ecosystem CO2 fluxes were very sensitive to grassland degradation. Degradation also decreased Rplant/Reco and Rplant/GPP by 16.8% and 8.1%, respectively, while increased Rh/Reco, Rh/Rplant and Rh/GPP by 16.8%, 41.4% and 3.8%, respectively, suggested that grassland degradation could eventually shifted Reco from autotrophic dominated to heterotrophic dominated. Four structural equation models (SEM) indicated that decline in NEE, GPP and Reco directly related to AGB and STC and decline in Rplant directly related to AGB, and indirectly related to SWC and STN. Our study highlighted that the consistent responses of CO2 fluxes to grassland degradation could alter the ecosystem’s carbon balance, and further would be influence carbon-climate feedbacks under deterioration of ecological environment.

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

In this study, CO2 exchange over sugarcane and wheat growing season was quantified by continuous measurement of CO2 fluxes using eddy covariance (EC) system from January 2014 to June 2015. We also elaborated on the response of CO2 fluxes to environmental variables. The results show that the ecosystem has seasonal and diurnal dynamics of CO2 with a distinctive U-shaped curve in both growing seasons with maximal CO2 absorption reaching up to -8.94 g C m-2 day-1 and -6.08 g C m-2 day-1 over sugarcane and wheat crop, respectively. The ecosystem as a whole acted as a carbon sink during the active growing season while it exhibits a carbon source prior to sowing and post-harvesting of crops. The cumulative net ecosystem exchange (NEE), gross primary productivity (GPP), and ecosystem respiration (Reco) were -923.04, 3316.65, and 2433.18 g C m-2 over the sugarcane growing season while the values were -192.30, 621.47, and 488.34 g C m-2 over the wheat growing season. The sesbania (green manure) appeared to be a carbon source once it is incorporated into soil. The response of day-time NEE to photosynthetically active radiation (PAR) under two vapor pressure deficit (VPD) sections (0-20 h Pa and 20-40 h Pa) seems more effective over sugarcane (R2 = 0.41-0.61) as compared to the wheat crop (R2 = 0.25-0.40). A decrease in net CO2 uptake was observed under higher VPD conditions. Similarly, night-time NEE was exponentially related to temperature at different soil moisture conditions and showed higher response to optimum soil moisture conditions for sugarcane (R2 = 0.87, 0.33 ≤ SWC < 0.42 m3 m-3) and wheat (R2 = 0.75, 0.31 ≤ SWC < 0.37 m3 m-3) crop seasons. The response of daily averaged NEE to environmental variables through path analysis indicates that PAR was the dominant predictor with the direct path coefficient of -0.65 and -0.74 over sugarcane and wheat growing season, respectively. Satellite-based GPP products from Moderate Resolution Imaging Spectroradiometer (GPPMOD) and Vegetation Photosynthetic model (GPPVPM) were also compared with the GPP obtained from EC (GPPEC) technique. The seasonal dynamics of GPPEC and GPPVPM agreed well with each other. This study covers the broad aspects ranging from micro-meteorology to remote sensing over C4-C3 cropping system.

Abstract Water and CO2 flux responses (e.g., evapotranspiration [ET] and net ecosystem exchange [NEE]) to environmental conditions can provide insights into how climate change will affect the terrestrial water and carbon budgets, especially in sensitive semiarid ecosystems. Here, we evaluated sensitivity of daily ET and NEE to current and antecedent (past) environment conditions, including atmospheric (vapor pressure deficit [VPD] and air temperature [Tair]) and moisture (precipitation and soil water) drivers. We focused on two common southwestern U.S. (“Southwest”) biomes: pinyon‐juniper woodland (Pinus edulis, Juniperus monosperma) and ponderosa pine forest (Pinus ponderosa). Due to differences in aridity, rooting patterns, and plant physiological strategies (stomatal and hydraulic traits), we expected ET and NEE in these ecosystems to respond differently to atmospheric and moisture drivers, with longer response timescales in the drier pinyon‐juniper woodland. Net sensitivity to drivers varied temporally in both ecosystems, reflecting the integrated influence of interacting drivers and antecedent precipitation patterns. NEE sensitivity to VPD and soil moisture (and ET sensitivity to deep soil moisture [Sdeep]) was higher in the ponderosa forest. ET and NEE in both ecosystems responded almost instantaneously to Tair, VPD, and shallow soil moisture (Sshall), and increases in any of these drivers weakened the carbon sink and enhanced water loss. Conversely, Sdeep and precipitation influenced ET and NEE over longer timescales (days to months, respectively), and higher Sdeep enhanced the carbon sink. As climate changes, these results suggest hotter and drier conditions will weaken the carbon sink and exacerbate water loss from Southwest pinyon‐juniper and ponderosa ecosystems.

CO2 fluxes from wood decomposition represent an important source of carbon from forest ecosystems to the atmosphere, which are determined by both wood traits and climate influencing the metabolic rates of decomposers. Previous studies have quantified the effects of moisture and temperature on wood decomposition, but these effects were not separated from the potential influence of wood traits. Indeed, it is not well understood how traits and climate interact to influence wood CO2 fluxes. Here, we examined the responses of CO2 fluxes from dead wood with different traits (angiosperm and gymnosperm) to 0%, 35%, and 70% rainfall reduction across seasonal temperature gradients. Our results showed that drought significantly decreased wood CO2 fluxes, but its effects varied with both taxonomical group and drought intensity. Drought-induced reduction in wood CO2 fluxes was larger in angiosperms than gymnosperms for the 35% rainfall reduction treatment, but there was no significant difference between these groups for the 70% reduction treatment. This is because wood nitrogen density and carbon quality were significantly higher in angiosperms than gymnosperms, yielding a higher moisture sensitivity of wood decomposition. These findings were demonstrated by a significant positive interaction effect between wood nitrogen and moisture on CO2 fluxes in a structural equation model. Additionally, we ascertained that a constant temperature sensitivity of CO2 fluxes was independent of wood traits and consistent with previous estimates for extracellular enzyme kinetics. Our results highlight the key role of wood traits in regulating drought responses of wood carbon fluxes. Given that both climate and forest management might extensively modify taxonomic compositions in the future, it is critical for carbon cycle models to account for such interactions between wood traits and climate in driving dynamics of wood decomposition.

Most studies on soil CO2 fluxes focus on the upper soil layers (i.e., 0–200 mm); however, there is a lack of investigation into soil layers below 200 mm, even though about half of soil organic carbon (SOC) is stored at these depths. In order to investigate the responses of CO2 fluxes in subsurface soils to crop residue incorporation in the topsoil, a field experiment comprising two treatments (i.e., conventional tillage with and without crop residue incorporation) was carried out under a winter wheat-summer maize cropping system from 2014 to 2016 in the Yucheng Agricultural Station, Shandong Province, China. The results showed that soil CO2 fluxes had large spatiotemporal variabilities and were significantly affected by crop residue applications (P < 0.05). Soil CO2 fluxes increased with soil depth. The CO2 fluxes from 100 to 400 mm soil depths were 1.1–5.1 times higher than those from 0 to 100 mm soil depths. Incorporation of crop residues into the soil significantly increased soil CO2 fluxes in all sampled layers (P < 0.05), and the magnitude increased with increasing soil depth. Soil moisture and the ratio of soil dissolved organic carbon to nitrogen (DOC/DON) were the dominant factors regulating soil CO2 fluxes in the wheat growing season, whereas soil DON concentrations dominated in the maize growing season. Our study indicated that deep soil C was more vulnerable to the incorporation of crop residues than the C present in surface soil. Incorporation of crop residues into surface soil might not increase SOC sequestration in the subsurface soil.

In this research effect of different soil types (normal and saline), farmyard manure norms (2 ton/ha - 4 ton/ha), manure application techniques (surface and subsurface) and soil temperature levels (20-25°C, 25-30°C, 30-35°C, 35-40°C, 40-45°C and 45-50°C) were examined of the soil CO2 flux on the pots at the laboratory conditions. According to obtained results, soil type (ST), manure norm (MN), manure application technique (MAT) and soil temperature (T) values changed CO2 flux. CO2 flux value of saline soil condition smaller than the normal soil condition. As an expected result, increased the manure amount increased the CO2 flux from soil to atmosphere. However, CO2 flux on the condition that subsurface manure application was less than surface manure application. CO2 flux values at the high soil temperatures were more than low soil temperature conditions. According to the interaction (T*ST, T*MN and T*MAT) results were not statistically significant. Soil CO2 flux were affected by gradually increasing of temperature.

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 4yr 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 2yr of the 4yr 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.

The impact of rain events on CO2 emissions from contrasting land use systems in semi-arid West African savannas

Abstract Terrestrial ecosystem responses to temperature and precipitation have major implications for the global carbon cycle. Case studies demonstrate that complex terrain, which accounts for more than 50% of Earth's land surface, can affect ecological processes associated with land‐atmosphere carbon fluxes. However, no studies have addressed the role of complex terrain in mediating ecophysiological responses of land‐atmosphere carbon fluxes to climate variables. We synthesized data from AmeriFlux towers and found that for sites in complex terrain, responses of ecosystem CO2 fluxes to temperature and precipitation are organized according to terrain slope and drainage area, variables associated with water and energy availability. Specifically, we found that for tower sites in complex terrain, mean topographic slope and drainage area surrounding the tower explained between 51% and 78% of site‐to‐site variation in the response of CO2 fluxes to temperature and precipitation depending on the time scale. We found no such organization among sites in flat terrain, even though their flux responses exhibited similar ranges. These results challenge prevailing conceptual framework in terrestrial ecosystem modeling that assumes that CO2 fluxes derive from vertical soil‐plant‐climate interactions. We conclude that the terrain in which ecosystems are situated can also have important influences on CO2 responses to temperature and precipitation. This work has implications for about 14% of the total land area of the conterminous U.S. This area is considered topographically complex and contributes to approximately 15% of gross ecosystem carbon production in the conterminous U.S.