Decrease In Gross Primary Productivity Research Articles

Effects of drought on ecosystem evapotranspiration and gross primary productivity in the Haihe River Basin

Evapotranspiration is a crucial component of the hydrological cycle and Gross Primary Productivity (GPP) is the largest flux in the global carbon cycle. However, both evapotranspiration and GPP are significantly affected by climate disasters, especially extreme drought events. Therefore, in this study, MODIS product data from 2002 to 2021 was used to analyze the spatiotemporal variations of actual evapotranspiration (ET), potential evapotranspiration (PET) and GPP within the Haihe River Basin. Then extreme drought events were identified by using EDDI drought index. A statistical analysis was performed to assess the impact of extreme drought on ecosystem evapotranspiration and GPP. The results showed that: (1) the annual evapotranspiration in the Haihe River Basin fluctuated greatly, ET showed a general increasing trend which was higher in the eastern region and lower in the central and western regions, PET showed an overall decreasing trend which was higher in the central and western regions and lower in the eastern region; (2) the annual GPP in the Haihe River Basin from 2002 to 2021 was 646.88 g C/m2/a and the increase rate was 14.75 g C/m2/a, which exhibited a decreasing trend from the northern to the southern and central-western regions; (3) under extreme drought conditions, ET decreased compared with the mean value, while PET increased, the Zhangweihe and the Tuhaimajiahe sub-basins were the most affected, with ET decreasing by 16.36% and PET increasing by 7.00% compared to the reference year. Additionally, extreme drought led to a decrease in GPP, with a reduction ranging from 7% to 18%.

Impacts of Compound Hot–Dry Events on Vegetation Productivity over Northern East Asia

Climate extremes, such as heatwaves and droughts, significantly impact terrestrial ecosystems. This study investigates the influence of compound hot–dry (CHD) events on vegetation productivity in northern East Asia. Four of the most widespread CHD events occurring during the summer from 2003 to 2019 were selected as the focus of this research. We first verified the performance of the Community Land Model version 5 (CLM5) in the region and then conducted factor-controlled experiments using CLM5 to assess the effects of different climate factors on gross primary productivity (GPP) changes during CHD events. Our results show that vegetation productivity exhibits greater sensitivity to CHD events within the transitional climatic zone (TCZ) than in other affected areas. In grassland areas within the TCZ, precipitation deficit is the primary factor leading to the decrease in GPP (explaining 56%–90% of GPP anomalies), while high temperatures serve as a secondary detrimental factor (explaining 13%–32% of GPP anomalies). In high-latitude forests outside the TCZ, high temperature has a more significant impact on suppressing GPP, while the decrease in soil moisture has a synchronously negligible impact on GPP. There are differences in the effects of high solar radiation on grasslands and woodlands during CHD events. It was observed that high radiation benefits trees by increasing the maximum carboxylation rate (Vcmax) and maximum electron transport rate (Jmax), as well as enhancing photosynthesis, but has a negligible impact on grasses. Furthermore, this study highlights the potential for compound events to impact vegetation productivity more than expected from individual events due to confounding nonlinear effects between meteorological factors. More than 10% of the negative anomalies in GPP during two CHD events in 2017 and 2010 were attributed to these nonlinear effects. These research findings are significant for understanding ecosystem responses to climate extremes and their influence on carbon cycling in terrestrial ecosystems. They can also contribute to more precisely evaluating and predicting carbon dynamics in these regions.

An abrupt shift in gross primary productivity over Eastern China-Mongolia and its inter-model diversity in land surface models

The terrestrial ecosystem in East Asia mainly consists of semi-arid regions that are sensitive to climate change. Therefore, gross primary productivity (GPP) in East Asia could be highly variable and vulnerable to climate change, which can significantly affect the local carbon budget. Here, we examine the spatial and temporal characteristics of GPP variability in East Asia and its relationship with climate factors over the last three decades. We detect an abrupt decrease in GPP over Eastern China-Mongolia region around the year 2000. This is attributed to an abrupt decrease in precipitation associated with the phase shift of the Pacific decadal oscillation (PDO). We also evaluate the reproducibility of offline land surface models to simulate these abrupt changes. Of the twelve models, eight were able to simulate this abrupt response, while the others failed due to the combination of an exaggerated CO2 fertilization effect and an underrated climate impact. For accurate prediction, it is necessary to improve the sensitivity of the GPP to changes in CO2 concentrations and the climate system.

Non-stomatal processes are responsible for the decrease in gross primary production of a potato crop during edaphic drought

Soil water stress is one of the main constraints on agrosystem functioning, causing a reduction in gross primary production (GPP). It is a key factor for the selection of drought-tolerant plant varieties and the adaptation of irrigation management strategies. Therefore, identifying the physiological factors limiting GPP is crucial. Land surface models commonly implement a beta (β) stress function to reproduce the effects of drought on GPP. It is still unclear whether GPP limitations originate from a direct stomatal response (SOL) or from other non-stomatal causes (NSOL). Moreover, the shape and thresholds of the β function are a major cause of uncertainty in LSMs. This study investigated the effects of edaphic drought on GPP limitations of a potato crop by using eddy covariance data from the Lonzée ICOS station (BE-Lon) in Belgium for four years (2006, 2010, 2014 and 2018) and by calibrating the β function on NSOL and SOL.The decrease in the relative extractable water in the soil (REW) induced a decline in the apparent maximum carboxylation rate (NSOL) from similar REW thresholds for all growing seasons of potato crops. A sensitivity analysis showed that the non-inclusion of REW decrease on NSOL in the modeling of GPP led to important overestimations of carbon sequestration from a threshold corresponding to 46 % of the maximum available soil water. The stomatal sensitivity to photosynthesis (SOL) remained constant or even increased in 2010 and 2018. The carbon and water fluxes were decoupled when REW decreased, exhibiting a strict control of NSOL on hourly dynamics of GPP. This study provides REW thresholds to identify drought stress episodes and to help for designing irrigation management strategies. Our results advocate for a better representation of the influence of drought on photosynthesis processes of potato crops to improve the accuracy of model predictions during drought.

Drought-induced resource use efficiency responses in an alpine meadow ecosystem of northern Tibet

Drought is a threat universally faced by global terrestrial ecosystems under global climate change. The habitat of alpine ecosystems is harsh, and they necessitate to use the limited resources efficiently to survive. However, drought occurred frequently during the short growing season, which greatly affected their resource use efficiency (RUE). In this study, based on long-term flux and microclimate observations in a typical alpine meadow ecosystem in northern Tibet, the effects of drought on RUE, including water use efficiency (WUE), light use efficiency (LUE), and carbon use efficiency (CUE), were studied. The results indicate that droughts could change the driving paths of abiotic and biotic factors on carbon and water fluxes and hence affect RUE. When vegetation exhibits robust physiological activity, droughts could enhance the control effects of biotic factors. The droughts that occurred at different periods of the growing season showed divergent effects on RUE. In the early-growing season, the strong photosynthetic capacity of the nascent leaves facilitated enhancements in WUE and LUE in response to drought conditions. However, the CUE did not increase significantly due to the heightened consumption of organic matter by respiration. The mid-growing season droughts led to a sharp decrease in gross primary productivity (GPP), consequently leading to a decline in WUE, LUE and CUE. During the late-growing season, droughts together with the decomposition of chlorophyll reduced the photosynthetic capacity and led to a significant decrease in LUE. The diminished physiological activity of the senescent leaves led to insensitive responses of WUE and CUE to droughts. These findings indicate that RUE might primarily be regulated by factors affecting GPP rather than others. This study would be beneficial for assessing the responses and adaptation characteristics of alpine meadow ecosystems to future climate change and could help accurately predict their possible evolution trends.

Divergent impacts of VPD and SWC on ecosystem carbon-water coupling under different dryness conditions

Ecosystem water use efficiency (WUE) is an indicator of carbon-water interactions and is defined as the ratio of gross primary productivity (GPP) to evapotranspiration (ET). However, it is currently unclear how WUE responds to atmospheric and soil drought events in terrestrial ecosystems with different dryness conditions. Additionally, the contributions of GPP and ET to the WUE response remain poorly understood. Based on measurements from 26 flux tower sites distributed worldwide, the binning method and random forest model were employed to separate the sensitivities of daily ecosystem WUE, GPP, and ET to vapor pressure deficit (VPD) and soil water content (SWC) under different dryness conditions (dryness index = potential evapotranspiration/precipitation, DI). Results showed that the sensitivity of WUE to VPD was negative at humid sites (DI < 1), while the sensitivity of WUE to SWC was positive at arid sites (DI > 2). Furthermore, the contribution of GPP to VPD-induced WUE variability was 63 % at humid sites, and the contribution of ET to SWC-induced WUE variability was 68 % when SWC was less than the 60th percentile at arid sites. Consequently, one increasing VPD-induced decrease in GPP was generally linked to a decrease in WUE at humid sites, and one drying soil moisture-caused decrease in ET was linked to a WUE increase under low SWC conditions at arid sites. Finally, VPD had a stronger effect on WUE than SWC when VPD was less than the 90th percentile or SWC was greater than the 50th percentile. Our findings underscore the importance of considering ecosystem dryness when investigating the impacts of VPD and SWC on ecosystem carbon-water coupling.

Pan-Arctic soil moisture control on tundra carbon sequestration and plant productivity.

Long-term atmospheric CO2 concentration records have suggested a reduction in the positive effect of warming on high-latitude carbon uptake since the 1990s. A variety of mechanisms have been proposed to explain the reduced net carbon sink of northern ecosystems with increased air temperature, including water stress on vegetation and increased respiration over recent decades. However, the lack of consistent long-term carbon flux and in situ soil moisture data has severely limited our ability to identify the mechanisms responsible for the recent reduced carbon sink strength. In this study, we used a record of nearly 100 site-years of eddy covariance data from 11 continuous permafrost tundra sites distributed across the circumpolar Arctic to test the temperature (expressed as growing degree days, GDD) responses of gross primary production (GPP), net ecosystem exchange (NEE), and ecosystem respiration (ER) at different periods of the summer (early, peak, and late summer) including dominant tundra vegetation classes (graminoids and mosses, and shrubs). We further tested GPP, NEE, and ER relationships with soil moisture and vapor pressure deficit to identify potential moisture limitations on plant productivity and net carbon exchange. Our results show a decrease in GPP with rising GDD during the peak summer (July) for both vegetation classes, and a significant relationship between the peak summer GPP and soil moisture after statistically controlling for GDD in a partial correlation analysis. These results suggest that tundra ecosystems might not benefit from increased temperature as much as suggested by several terrestrial biosphere models, if decreased soil moisture limits the peak summer plant productivity, reducing the ability of these ecosystems to sequester carbon during the summer.

Microwave-based soil moisture improves estimates of vegetation response to drought in China

The increased frequency and severity of drought has heightened concerns over the risk of hydraulic vegetative stress and the premature mortality of ecosystems globally. Unfortunately, most land surface models (LSMs) continue to underestimate ecosystem resilience to drought – which degrades the credibility of model-predicted ecohydrological responses to climate change. This study investigates the response of vegetation gross productivity to water-stress conditions using microwave-based vegetation optical depth (VOD) and soil moisture retrievals. Based on the estimated isohydric/anisohydric spectrum, we find that vegetation at isohydric state exhibits a larger decrease in gross primary productivity and higher water use efficiency than anisohydric vegetation due to their more rigorous stomatal control and higher tolerance of carbon starvation risk. In addition, the introduction of microwave soil moisture improves the accuracy of isohydricity/anisohydricity estimates compared to those obtained using microwave VOD alone (i.e., increases their Spearman rank correlation versus the benchmark of Global Biodiversity Information Facility dataset from 0.12 to 0.63). Results of this study provide clear justification for the use of microwave-based soil moisture retrievals to enhance stomatal conductance parameterization within LSMs.

From sink to source: high inter-annual variability in the carbon budget of a Southern African wetland.

We report on three years of continuous monitoring of carbon dioxide (CO2) and methane (CH4) emissions in two contrasting wetland areas of the Okavango Delta, Botswana: a perennial swamp and a seasonal floodplain. The hydrographic zones of the Okavango Delta possess distinct attributes (e.g. vegetation zonation, hydrology) which dictate their respective greenhouse gas (GHG) temporal emission patterns and magnitude. The perennial swamp was a net source of carbon (expressed in CO2-eq units), while the seasonal swamp was a sink in 2018. Despite differences in vegetation types and lifecycles, the net CO2 uptake was comparable at the two sites studied in 2018/2020 (−894.2 ± 127.4 g m−2 yr−1 at the perennial swamp, average of the 2018 and 2020 budgets, and −1024.5 ± 134.7 g m−2 yr−1 at the seasonal floodplain). The annual budgets of CH4 were however a factor of three larger at the permanent swamp in 2018 compared to the seasonal floodplain. Both ecosystems were sensitive to drought, which switched these sinks of atmospheric CO2 into sources in 2019. This phenomenon was particularly strong at the seasonal floodplain (net annual loss of CO2 of 1572.4 ± 158.1 g m−2), due to a sharp decrease in gross primary productivity. Similarly, drought caused CH4 emissions at the seasonal floodplain to decrease by a factor of 4 in 2019 compared to the previous year, but emissions from the perennial swamp were unaffected. Our study demonstrates that complex and divergent processes can coexist within the same landscape, and that meteorological anomalies can significantly perturb the balance of the individual terms of the GHG budget. Seasonal floodplains are particularly sensitive to drought, which exacerbate carbon losses to the atmosphere, and it is crucial to improve our understanding of the role played by such wetlands in order to better forecast how their emissions might evolve in a changing climate. Studying such hydro-ecosystems, particularly in the data-poor tropics, and how natural stressors such as drought affect them, can also inform on the potential impacts of man-made perturbations (e.g. construction of hydro-electric dams) and how these might be mitigated. Given the contrasting effects of drought on the CO2 and CH4 flux terms, it is crucial to evaluate an ecosystem's complete carbon budget instead of treating these GHGs in isolation.This article is part of a discussion meeting issue ‘Rising methane: is warming feeding warming? (part 2)’.

Impacts of heterogeneous CO2 on water and carbon fluxes across the global land surface

ABSTRACT Over the recent decades, the increase of atmospheric carbon dioxide (CO2) concentration has caused large effects on the earth system. However, few studies have paid attention to the effects of heterogeneity of CO2 on the biosphere and the hydrosphere. Using a coupled diagnostic biophysical model (PML-V2) and comparing three heterogeneous CO2 datasets (GOSAT, CMIP6 and CarbonTracker) against a baseline homogeneous data (ESRL), this study investigated the effects of heterogeneous CO2 on gross primary production (GPP), actual evapotranspiration (ET) and water use efficiency (WUE) across the global. The results show that among the three heterogeneity CO2, CarbonTracker produced the highest CO2 concentration and showed the largest difference in ET (−6% to 2%), GPP (−2% to 5%) and WUE (4% to 11%) compared to those from the baseline. The most effects of the CO2 heterogeneity occurred in summer. Russia was identified as a vulnerable region with prominent decrease in GPP and an increase in ET due to CO2 heterogeneity. An obvious increase in GPP and a decrease in ET appeared in the Amazon rainforest, the Congo rainforest, and eastern Asia. On global scale, the effects of the CO2 heterogeneity on ET/GPP/WUE were not significant.

Competing effects of nitrogen deposition and ozone exposure on northern hemispheric terrestrial carbon uptake and storage, 1850–2099

Abstract. Tropospheric ozone (O3) and nitrogen deposition affect vegetation growth and, thereby, the ability of the land biosphere to take up and store carbon. However, the magnitude of these effects on the contemporary and future terrestrial carbon balance is insufficiently understood. Here, we apply an extended version of the O–CN terrestrial biosphere model that simulates the atmosphere to canopy transport of O3, its surface and stomatal uptake, the O3-induced leaf injury, and the coupled terrestrial carbon and nitrogen cycles. We use this model to simulate past and future impacts of air pollution against a background of concurrent changes in climate and carbon dioxide concentrations (CO2) for two contrasting representative concentration pathway (RCP) scenarios (RCP2.6 and RCP8.5). The simulations show that O3-related damage considerably reduced northern hemispheric gross primary production (GPP) and long-term carbon storage between 1850 and the 2010s. The simulated O3 effect on GPP in the Northern Hemisphere peaked towards the end of the 20th century, with reductions of 4 %, causing a reduction in the northern hemispheric carbon sink of 0.4 Pg C yr−1. During the 21st century, O3-induced reductions in GPP and carbon storage are projected to decline, through a combination of direct air pollution control methods that reduce near-surface O3 and the indirect effects of rising atmospheric CO2, which reduces stomatal uptake of O3 concurrent with increases of leaf-level water use efficiency. However, in hot spot regions such as East Asia, the model simulations suggest a sustained decrease in GPP by more than 8 % throughout the 21st century. O3 exposure reduces projected carbon storage at the end of the 21st century by up to 15 % in parts of Europe, the US, and East Asia. Our simulations suggest that the stimulating effect of nitrogen deposition on regional GPP and carbon storage is lower in magnitude compared to the detrimental effect of O3 during most of the simulation period for both RCPs. In the second half of the 21st century, the detrimental effect of O3 on GPP is outweighed by nitrogen deposition, but the effect of nitrogen deposition on land carbon storage remains lower than the effect of O3. Accounting for the stimulating effects of nitrogen deposition but omitting the detrimental effect of O3 may lead to an overestimation of projected carbon uptake and storage.

Attributing the changes of grass growth, water consumed and water use efficiency over the Tibetan Plateau

Degradation of grassland over the Tibetan Plateau (TP) has become increasingly intense, producing a wide impact on the local and downstream environment and society. There is a pressing demand to explore contributions of climate change or human activity. Based on the simulations at 1-km spatial resolution during 2001–2017 from the VIP distributed eco-hydrological model, the trends of grass growth (expressed as gross primary productivity (GPP)), water consumption (expressed as evapotranspiration (ET)), and water use efficiency (WUE) were studied to identify the drivers by using the ridge regressions. The results show that 18.75% and 12.43% of grassland showed upward and downward ET trends, respectively. Elevated atmospheric CO2 concentration (eCO2), increasing precipitation (PRE), leaf area index (LAI), and wind speed (U) collectively dominated the uptrend of ET, and their contributions were 62%, 17%, 9%, and 9%, respectively. eCO2 and the diminished PRE and LAI were the principal causes of the decrease of ET. Their contributions were −42%, −9% and −7%, respectively. 81.13% of the grassland showed an uptrend in GPP, while only 0.04% showed downward tendencies. Rising air temperature (Ta) and eCO2 have jointly promoted GPP, with their contributions being 36% and 58%, respectively. The water vapor pressure (EA), U, and PRE reduction and warmer Ta caused a decrease in GPP, and their contributions were −25%, −23%, −9%, and −25%, respectively. 36.60% of the grassland showed a positive WUE trend, while only 3.98% showed a downtrend. The contribution of human activities is relatively small compared to that of climate change. Clarifying the contributions to ET, GPP, and WUE in grassland will support the policy-making for ecological management and environmental protection over the plateau.

Divergent impacts of droughts on vegetation phenology and productivity in the Yungui Plateau, southwest China

Climate change has led to an increase in the severity and frequency of extreme weather events, especially droughts. Quantifying the impacts of droughts on vegetation activities is essential to study the effects of climate change on the ecosystem. Using Gross Primary Productivity (GPP), phenological metrics, and Standardized precipitation evaporation index (SPEI) with multi-timescales, we evaluated the divergent responses of vegetation activities to droughts in the Yungui Plateau from 2001 to 2017. Besides, the sensitivity of vegetation to drought was analyzed by the maximum correlation coefficients between vegetation activities and SPEI over the plateau. Results revealed that the overall changes of the start (SOS) and end (EOS) of growing season in the plateau exhibited nonsignificant changes during the study period. Meanwhile, two extreme droughts of the study period caused a tremendous decrease in GPP and severe delays in SOS and EOS. Vegetation activities increased significantly in the eastern plateau because of the strong drought resistance of vegetation. Conversely, the dynamics of vegetation activities, especially phenological metrics, were nonsignificant in the west due to the weak drought resistance of vegetation. Preseason droughts of spring phenology played a key role in determining SOS dynamics. Besides, drought before summer contributed to the delay of EOS, while summer drought leads to early EOS. Hence, preseason droughts of autumn phenology nonlinearly impacted EOS due to strong interaction between SOS and EOS. More importantly, the stronger influences of SOS on annual GPP compared to EOS suggest that preseason droughts of SOS are essential to the vegetation productivity in the plateau. Given that the continuous warming of the climate will lead to more frequent droughts, it is essential to investigate the resilience of vegetation growth to drought in future work.

Reduced magnitude and shifted seasonality of CO2 sink by experimental warming in a coastal wetland.

Coastal wetlands have the highest carbon sequestration rate per unit area among all unmanaged natural ecosystems. However, how the magnitude and seasonality of the CO2 sink in coastal wetlands will respond to future climate warming remains unclear. Here, based on measurements of ecosystem CO2 fluxes in a field experiment in the Yellow River Delta, we found that experimental warming (i.e., a 2.4°C increase in soil temperature) reduced net ecosystem productivity (NEP) by 23.7% across two growing seasons of 2017-2018. Such a reduction in NEP resulted from the greater decrease in gross primary productivity (GPP) than ecosystem respiration (ER) under warming. The negative warming effect on NEP mainly occurred in summer (-43.9%) but not in autumn (+61.3%), leading to a shifted NEP seasonality under warming. Further analyses showed that the warming effects on ecosystem CO2 exchange were mainly controlled by soil salinity and its corresponding impacts on species composition. For example, warming increased soil salinity (+35.0%), reduced total aboveground biomass (-9.9%), and benefited the growth of plant species with high salt tolerance and late peak growth. To the best of our knowledge, this study provides the first experimental evidence on the reduced magnitude and shifted seasonality of CO2 exchange under climate warming in coastal wetlands. These findings underscore the high vulnerability of wetland CO2 sink in coastal regions under future climate change.

A Model‐Based Investigation of Terrestrial Plant Carbon Uptake Response to Four Radiation Modification Approaches

AbstractA number of radiation modification approaches have been proposed to counteract anthropogenic warming by intentionally altering Earth's shortwave or longwave fluxes. While several previous studies have examined the climate effect of different radiation modification approaches, only a few have investigated the carbon cycle response. Our study examines the response of plant carbon uptake to four radiation modification approaches that are used to offset the global mean warming caused by a doubling of atmospheric CO2. Using the National Center for Atmospheric Research Community Earth System Model, we performed simulations that represent four idealized radiation modification options: solar constant reduction, sulfate aerosol increase (SAI), marine cloud brightening, and cirrus cloud thinning (CCT). Relative to the high CO2 state, all these approaches reduce gross primary production (GPP) and net primary production (NPP). In high latitudes, decrease in GPP is mainly due to the reduced plant growing season length, and in low latitudes, decrease in GPP is mainly caused by the enhanced nitrogen limitation due to surface cooling. The simulated GPP for sunlit leaves decreases for all approaches. Decrease in sunlit GPP is the largest for SAI which substantially decreases direct sunlight, and the smallest for CCT, which increases direct sunlight that reaches the land surface. GPP for shaded leaves increases in SAI associated with a substantial increase in surface diffuse sunlight, and decreases in all other cases. The combined effects of CO2 increase and radiation modification result in increases in primary production, indicating the dominant role of the CO2 fertilization effect on plant carbon uptake.

Carbon dynamics and environmental controls of a hilly tea plantation in Southeast China.

Tea plantations are widely distributed and continuously expanding across subtropical China in recent years. However, carbon flux exchanges from tea plantation ecosystems are poorly understood at the ecosystem level. In this study, we use the eddy covariance technique to quantify the magnitude and temporal variations of the net ecosystem exchange (NEE) in tea plantation in Southeast China over four years (2014–2017). The result showed that the tea plantation was a net carbon sink, with an annual NEE that ranged from −182.40 to −301.51 g C/m2, which was a much lower carbon sequestration potential than other ecosystems in subtropical China. Photosynthetic photon flux density (PPFD) explained the highest proportion of the variation in NEE and gross primary productivity (GPP) (for NEE: F = 389.89, p < .01; for GPP: F = 1,018.04, p < .01), and air temperature (Ta) explained the highest proportion of the variation in ecosystem respiration (RE) (F = 13,141.81, p < .01). The strong pruning activity in April not only reduced the carbon absorption capacity but also provided many plant residues for respiration, which switched the tea plantation to a carbon source from April to June. Suppression of NEE at higher air temperatures was due to the decrease in GPP more than the decrease in RE, which indicated that future global warming may transform this subtropical tea plantation from a carbon sink to carbon source.

Increased Global Land Carbon Sink Due to Aerosol‐Induced Cooling

AbstractAnthropogenic aerosols have contributed to historical climate change through their interactions with radiation and clouds. In turn, climate change due to aerosols has impacted the C cycle. Here we use a set of offline simulations made with the Organising Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE) land surface model driven by bias‐corrected climate fields from simulations of three Coupled Model Intercomparison Project Phase 5 (CMIP5) Earth system models (ESMs; IPSL‐CM5A‐LR, CSIRO‐Mk3.6.0, and GISS‐E2‐R) to quantify the climate‐related impacts of aerosols on land carbon fluxes during 1860–2005. We found that climate change from anthropogenic aerosols (CCAA) globally cooled the climate, and increased land carbon storage, or cumulative net biome production (NBP), by 11.6–41.8 PgC between 1860 and 2005. The increase in NBP from CCAA mainly occurs in the tropics and northern midlatitudes, primarily due to aerosol‐induced cooling. At high latitudes, cooling caused stronger decrease in gross primary production (GPP) than in total ecosystem respiration (TER), leading to lower NBP. At midlatitudes, cooling‐induced decrease in TER is stronger than that of GPP, resulting in NBP increase. At low latitudes, NBP was also enhanced due to the cooling‐induced GPP increase, but precipitation decline from CCAA may negate the effect of temperature. The three ESMs show large divergence in low‐latitude CCAA precipitation response to aerosols, which results in considerable uncertainties in regional estimations of CCAA effects on carbon fluxes. Our results suggest that better understanding and simulation of how anthropogenic aerosols affect precipitation in ESMs is required for a more accurate attribution of aerosol effects on the terrestrial carbon cycle.

The impact of the 2009/2010 drought on vegetation growth and terrestrial carbon balance in Southwest China

The 2009/2010 drought in Southwest China was a “once in a century drought” event. This drought event had strong adverse impacts, such as water scarcity, crop failure, and economic loss, on ecosystems and the human society. Explicit representations of this drought event and associated changes in vegetation dynamics and carbon cycle, however, are still largely missed in literature. Here we used the standardized anomaly of 3-month Standard Precipitation-Evapotranspiration Index (SPEI) to characterize this 2009/2010 drought event, including its onset and end months, duration and severity. We examined the drought impacts on vegetation greenness using Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI); and assessed the terrestrial carbon flux responses to drought using data from satellite-based datasets, atmospheric inversion, one data-based model, and three ecosystem models. Our analysis showed that this “autumn-winter-spring” drought mostly suppressed vegetation growth in Yunnan, North Guangxi, Guizhou and East Sichuan. In comparisons to the same months of the reference period (2000–2015), the drought caused a large reduction in carbon uptake (4.4 ± 5 gCm−2month-1) from the decrease in gross primary production (GPP, 5.7 ± 9.5 gCm−2month-1), although ecosystem respiration (TER) also decreased by a smaller extent (1.3 ± 5.1 gCm−2month-1). Nonetheless, more than 65% of the drought-impacted area recovered in both vegetation greenness and productivity within 3 months, while about 10% of the drought-affected area failed to recover even after 6 months post the drought event. Across different vegetation types, forests and shrubs had stronger capability of drought resistance and shorter post-drought recovery time than crops and grasses. Model comparison reveals that while data from different sources generally agreed on the sign of vegetation greenness and carbon flux responses to drought, they differ significantly in the extent of such responses.

Responses of gross primary production of grasslands and croplands under drought, pluvial, and irrigation conditions during 2010–2016, Oklahoma, USA

To accurately estimate the terrestrial carbon cycle and food production, it is essential to understand how gross primary production (GPP) of irrigated and non-irrigated grasslands and croplands respond to drought and pluvial events. This study analyzed annual GPP of irrigation-permitted and non-permitted grasslands, winter wheat (Triticum aestivum L.), other C3 croplands, and C4 croplands in Caddo County of western Oklahoma from 2010 through 2016, a period which consisted of extreme drought (2011) and pluvial events (2015). First, we compared GPP from the Vegetation Photosynthesis Model (GPPVPM) and GPP data from the Moderate Resolution Imaging Spectroradiometer (GPPMOD17) with GPP estimates from three eddy covariance towers (GPPEC) in Oklahoma. GPPVPM more accurately estimated mean daily GPPEC at each of the three sites than GPPMOD17. Second, we analyzed the seasonal and interannual dynamics of GPPVPM for eight pixels, one each for the four irrigation-permitted and non-permitted land types. The interannual variation of GPPVPM was due to the complexity of decision making and practice for irrigation, cropping intensity, and crop types. Finally, at the county scale, annual GPPVPM from the 2011 drought and pluvial 2015 were compared with mean annual GPPVPM from the other 5 years of the study period. The results show that for the 2011 drought: 1) non-permitted C4 croplands had the largest percentage decrease in GPP, but permitted C4 croplands had the smallest decrease; 2) regardless of water rights, GPP was significantly lower than the 5-year reference mean for grasslands, winter wheat, and other C3 crops; and 3) non-permitted lands were more affected by drought than irrigation-permitted lands, except for grasslands, which had similar percentage reductions in GPP. Results for the pluvial year 2015 show that: 1) GPP was significantly higher for grasslands, winter wheat, and non-permitted C3 croplands than the 5-year reference mean, but there was no significant difference in GPP for irrigation-permitted C3 croplands or non-permitted C4 croplands; and 2) GPP for C4 irrigation-permitted croplands was lower than the 5-year reference mean. Crop-specific responses to drought and pluvial events largely depend on a landowner’s ability to irrigate, and caution should be used when assessing or generalizing how crops respond to climate variability, drought, and pluvial conditions in the absence of irrigation-related data.

Impacts of droughts and extreme-temperature events on gross primary production and ecosystem respiration: a systematic assessment across ecosystems and climate zones

Abstract. Extreme climatic events, such as droughts and heat stress, induce anomalies in ecosystem–atmosphere CO2 fluxes, such as gross primary production (GPP) and ecosystem respiration (Reco), and, hence, can change the net ecosystem carbon balance. However, despite our increasing understanding of the underlying mechanisms, the magnitudes of the impacts of different types of extremes on GPP and Reco within and between ecosystems remain poorly predicted. Here we aim to identify the major factors controlling the amplitude of extreme-event impacts on GPP, Reco, and the resulting net ecosystem production (NEP). We focus on the impacts of heat and drought and their combination. We identified hydrometeorological extreme events in consistently downscaled water availability and temperature measurements over a 30-year time period. We then used FLUXNET eddy covariance flux measurements to estimate the CO2 flux anomalies during these extreme events across dominant vegetation types and climate zones. Overall, our results indicate that short-term heat extremes increased respiration more strongly than they downregulated GPP, resulting in a moderate reduction in the ecosystem's carbon sink potential. In the absence of heat stress, droughts tended to have smaller and similarly dampening effects on both GPP and Reco and, hence, often resulted in neutral NEP responses. The combination of drought and heat typically led to a strong decrease in GPP, whereas heat and drought impacts on respiration partially offset each other. Taken together, compound heat and drought events led to the strongest C sink reduction compared to any single-factor extreme. A key insight of this paper, however, is that duration matters most: for heat stress during droughts, the magnitude of impacts systematically increased with duration, whereas under heat stress without drought, the response of Reco over time turned from an initial increase to a downregulation after about 2 weeks. This confirms earlier theories that not only the magnitude but also the duration of an extreme event determines its impact. Our study corroborates the results of several local site-level case studies but as a novelty generalizes these findings on the global scale. Specifically, we find that the different response functions of the two antipodal land–atmosphere fluxes GPP and Reco can also result in increasing NEP during certain extreme conditions. Apparently counterintuitive findings of this kind bear great potential for scrutinizing the mechanisms implemented in state-of-the-art terrestrial biosphere models and provide a benchmark for future model development and testing.