Interannual Variability In Net Ecosystem Exchange Research Articles

Sustainability in Natural Grassland in the Brazilian Pampa Biome: Livestock Production with CO2 Absorption

The Brazilian Pampa biome has natural pastures that have been used for centuries for cattle grazing. This is considered a sustainable system because it combines the conservation of natural vegetation and high-quality meat production, protecting the biome from commercial agriculture’s advances. However, whether it is a source or a sink of carbon dioxide (CO2) has yet to be evaluated. Hence, this study aimed to quantify the net ecosystem exchange (NEE) of the CO2 of a natural pasture of the Pampa biome used for livestock production. The experimental area is located in a subtropical region of southern Brazil, where eddy covariance (EC) measurements were conducted from 2015 to 2021 in a rotational cattle grazing system. The seven months of the warm season (September to March) were characterized as CO2 absorbers, while the five months of the cold season (April to August) were CO2 emitters. Throughout the six years and with complete data, the ecosystem was an absorber of atmospheric CO2, with an average value of −207.6 g C m−2 year−1. However, the significant interannual variability in NEE was observed, with cumulative values ranging from −82.0 to −385.3 g C m−2 year−1. The results suggest the coupling of climatic conditions to pasture management can be the factor that modulated the NEE interannual variability. The cattle raising system on the natural pastures of the Pampa absorbs CO2, which is further evidence of its sustainability and need for conservation.

Atmospheric water vapor and soil moisture jointly determine the spatiotemporal variations of CO2 fluxes and evapotranspiration across the Qinghai-Tibetan Plateau grasslands

Alpine grasslands play important functions in mitigating climate change and regulating water resources. However, the spatiotemporal variability of their carbon and water budgets remains unquantified. Here, 47 site-year observations of CO2 and water vapor fluxes (ET) are analyzed at sites situated along a hydrothermal gradient across the Qinghai-Tibetan Plateau, including an alpine wetland (wettest), an alpine shrub (coldest), an alpine meadow, an alpine meadow-steppe, and an alpine steppe (driest and warmest). The results show that the benchmarks for annual net ecosystem exchange (NEE) are −79.3, −77.8, −66.7, 20.2, and 100.9 g C m−2 year−1 at the meadow, shrub, meadow-steppe, steppe, and wetland, respectively. The peak daily NEE normalized by peak leaf area index converges to 0.93 g C m−2 d−1 at the 5 sites. Except in the wetland (722.8 mm), the benchmarks of annual ET fluctuate from 511.0 mm in the steppe to 589.2 mm in the meadow. Boosted regression trees-based analysis suggests that the enhanced vegetation index (EVI) and net radiation (Rn) determine the variations of growing season monthly CO2 fluxes and ET, respectively, although the effect is to some extent site-specific. Inter-annual variability in NEE, ecosystem respiration (RES), and ET are tightly (R2 > 0.60) related to the inter-growing season NEE, RES, and ET, respectively. Both annual RES and annual NEE are significantly constrained by annual gross primary productivity (GPP), with 85% of the per-unit GPP contributing to RES (R2 = 0.84) and 15% to NEE (R2 = 0.12). Annual GPP significantly correlates with annual ET alone at the drier sites of the meadow-steppe and the steppe, suggesting the coupling of carbon and water is moisture-dependent in alpine grasslands. Over half of the inter-annual spatial variability in GPP, RES, NEE, and ET is explained by EVI, atmospheric water vapor, topsoil water content, and bulk surface resistance (rs), respectively. Because the spatial variations of EVI and rs are strongly regulated by atmospheric water vapor (R2 = 0.48) and topsoil water content (R2 = 0.54), respectively, we conclude that atmospheric water vapor and topsoil water content, rather than the expected air/soil temperatures, drive the spatiotemporal variations in CO2 fluxes and ET across temperature-limited grasslands. These findings are critical for improving predictions of the carbon sequestration and water holding capacity of alpine grasslands.

Environmental and biotic controls on the interannual variations in CO2 fluxes of a continental monsoon temperate forest

The interannual variability (IAV) of CO2 fluxes of temperate forests in continental East Asia was rarely explored via direct measurements. To explore the IAV of net ecosystem exchange of CO2 (NEE) and its driving factors, we continuously measured the CO2 fluxes of a temperate deciduous broadleaved forest at the Maoershan site, northeast China with a continental monsoon climate, by eddy covariance over 11 years (2008 – 2018). The means of NEE, gross primary production (GPP), and ecosystem respiration (Re) were −157 ± 64 (mean ± standard deviation), 1356 ± 148, and 1200 ± 138 g C m−2 yr−1, respectively. For annual fluxes, the IAV of NEE was jointly controlled by the length of net CO2 uptake period and the summer peak of net CO2 uptake, whereas those of GPP and Re were predominated by the summer peaks (GPPmax and Remax). Meanwhile, the dominant role of GPPmax on the IAV of GPP was attributed to the leaf-level photosynthetic capacity rather than the maximum leaf area index. However, the environmental factors showed weak impacts on NEE, largely because of the offset between the positive responses of GPP and Re to the spring and autumn soil water content, respectively. For particular seasons, phenology was the dominant biotic driver of the spring and autumn NEE, whereas spring precipitation and autumn photosynthetically active radiation were the main environmental drivers of the spring and autumn NEE, respectively. Overall, this work filled the gap of the relatively long-term CO2 fluxes measured in the temperate forests of continental East Asia. Taking the influences of ecosystem- and leaf-level physiology, canopy structure, and phenology, together with their environmental interactions, on the CO2 fluxes into account will improve the understanding and prediction of the forest carbon dynanics.

On what scales can GOSAT flux inversions constrain anomalies in terrestrial ecosystems?

Abstract. Interannual variations in temperature and precipitation impact the carbon balance of terrestrial ecosystems, leaving an imprint in atmospheric CO2. Quantifying the impact of climate anomalies on the net ecosystem exchange (NEE) of terrestrial ecosystems can provide a constraint to evaluate terrestrial biosphere models against and may provide an emergent constraint on the response of terrestrial ecosystems to climate change. We investigate the spatial scales over which interannual variability in NEE can be constrained using atmospheric CO2 observations from the Greenhouse Gases Observing Satellite (GOSAT). NEE anomalies are calculated by performing a series of inversion analyses using the GEOS-Chem adjoint model to assimilate GOSAT observations. Monthly NEE anomalies are compared to “proxies”, variables that are associated with anomalies in the terrestrial carbon cycle, and to upscaled NEE estimates from FLUXCOM. Statistically significant correlations (P<0.05) are obtained between posterior NEE anomalies and anomalies in soil temperature and FLUXCOM NEE on continental and larger scales in the tropics, as well as in the northern extratropics on subcontinental scales during the summer (R2≥0.49), suggesting that GOSAT measurements provide a constraint on NEE interannual variability (IAV) on these spatial scales. Furthermore, we show that GOSAT flux inversions are generally better correlated with the environmental proxies and FLUXCOM NEE than NEE anomalies produced by a set of terrestrial biosphere models (TBMs), suggesting that GOSAT flux inversions could be used to evaluate TBM NEE fluxes.

Patterns and controls of inter-annual variability in the terrestrial carbon budget

Abstract. The terrestrial carbon fluxes show the largest variability among the components of the global carbon cycle and drive most of the temporal variations in the growth rate of atmospheric CO2. Understanding the environmental controls and trends of the terrestrial carbon budget is therefore essential to predict the future trajectories of the CO2 airborne fraction and atmospheric concentrations. In the present work, patterns and controls of the inter-annual variability (IAV) of carbon net ecosystem exchange (NEE) have been analysed using three different data streams: ecosystem-level observations from the FLUXNET database (La Thuile and 2015 releases), the MPI-MTE (model tree ensemble) bottom–up product resulting from the global upscaling of site-level fluxes, and the Jena CarboScope Inversion, a top–down estimate of surface fluxes obtained from observed CO2 concentrations and an atmospheric transport model. Consistencies and discrepancies in the temporal and spatial patterns and in the climatic and physiological controls of IAV were investigated between the three data sources. Results show that the global average of IAV at FLUXNET sites, quantified as the standard deviation of annual NEE, peaks in arid ecosystems and amounts to ∼ 120 gC m−2 y−1, almost 6 times more than the values calculated from the two global products (15 and 20 gC m−2 y−1 for MPI-MTE and the Jena Inversion, respectively). Most of the temporal variability observed in the last three decades of the MPI-MTE and Jena Inversion products is due to yearly anomalies, whereas the temporal trends explain only about 15 and 20 % of the variability, respectively. Both at the site level and on a global scale, the IAV of NEE is driven by the gross primary productivity and in particular by the cumulative carbon flux during the months when land acts as a sink. Altogether these results offer a broad view on the magnitude, spatial patterns and environmental drivers of IAV from a variety of data sources that can be instrumental to improve our understanding of the terrestrial carbon budget and to validate the predictions of land surface models.

Long term trend and interannual variability of land carbon uptake—the attribution and processes

Ecosystem carbon (C) uptake in terrestrial ecosystems has increased over the past five decades, but with large interannual variability (IAV). However, we are not clear on the attribution and the processes that control the long-term trend and IAV of land C uptake. Using atmospheric inversion net ecosystem exchange (NEE) data, we quantified the trend and IAV of NEE across the globe, the Northern Hemisphere (NH), and the Southern Hemisphere (SH), and decomposed NEE into carbon uptake amplitude and duration during each year from 1979–2013. We found the NH rather than the SH determined the IAV, while both hemispheres contributed equivalently to the global NEE trend. Different ecosystems in the NH and SH had differential relative contributions to their trend and IAV. The long-term trends of increased C uptake across the globe and the SH were attributed to both extended duration and increasing amplitude of C uptake. The shortened duration of uptake in the NH partly offsets the effects of increased NEE amplitude, making the net C uptake trend the same as that of the SH. The change in NEE IAV was also linked to changes in the amplitude and duration of uptake, but they worked in different ways in the NH, SH and globe. The fundamental attributions of amplitude and duration of C uptake revealed in this study are helpful to better understand the mechanisms underlying the trend and IAV of land C uptake. Our findings also suggest the critical roles of grassland and croplands in the NH in contributing to the trend and IAV of land C uptake.

Drivers of long-term variability in CO<sub>2</sub> net ecosystem exchange in a temperate peatland

Abstract. Land–atmosphere exchange of carbon dioxide (CO2) in peatlands exhibits marked seasonal and inter-annual variability, which subsequently affects the carbon (C) sink strength of catchments across multiple temporal scales. Long-term studies are needed to fully capture the natural variability and therefore identify the key hydrometeorological drivers in the net ecosystem exchange (NEE) of CO2. Since 2002, NEE has been measured continuously by eddy-covariance at Auchencorth Moss, a temperate lowland peatland in central Scotland. Hence this is one of the longest peatland NEE studies to date. For 11 years, the site was a consistent, yet variable, atmospheric CO2 sink ranging from −5.2 to −135.9 g CO2-C m−2 yr−1 (mean of −64.1 ± 33.6 g CO2-C m−2 yr−1). Inter-annual variability in NEE was positively correlated to the length of the growing season. Mean winter air temperature explained 87% of the inter-annual variability in the sink strength of the following summer, indicating an effect of winter climate on local phenology. Ecosystem respiration (Reco) was enhanced by drought, which also depressed gross primary productivity (GPP). The CO2 uptake rate during the growing season was comparable to three other sites with long-term NEE records; however, the emission rate during the dormant season was significantly higher. To summarise, the NEE of the peatland studied is modulated by two dominant factors: - phenology of the plant community, which is driven by winter air temperature and impacts photosynthetic potential and net CO2 uptake during the growing season (colder winters are linked to lower summer NEE), - water table level, which enhanced soil respiration and decreased GPP during dry spells. Although summer dry spells were sporadic during the study period, the positive effects of the current climatic trend towards milder winters on the site's CO2 sink strength could be offset by changes in precipitation patterns especially during the growing season.

Partitioning Climatic and Biotic Effects on Interannual Variability of Ecosystem Carbon Exchange in Three Ecosystems

Understanding the climatic and biotic controls of interannual variability (IAV) in net ecosystem exchange (NEE) is important for projecting future uptake of CO2 in terrestrial ecosystems. In this study, a statistical modeling approach was used to partition climatic and biotic effects on the IAV in NEE, gross primary productivity (GPP) and ecosystem respiration (RE) at a subtropical evergreen plantation in China (QYZ), a deciduous forest (MOZ), and a grassland (DK1) in the USA. The climatic effects in the study are defined as the interannual anomalies in carbon (C) fluxes directly caused by climatic variations, whereas the biotic effects are those caused by the IAV in photosynthetic and respiratory traits. The results showed that the contribution of biotic effects to the IAV in NEE increased significantly as the temporal scale got longer from daily to annual scales. At the annual scale, the contribution of biotic effects to the IAV in NEE was 47, 69, and 77% at QYZ, MOZ, and DK1, respectively. However, the IAV in NEE was mainly controlled by GPP at QYZ, and by RE at DK1, whereas the contributions of GPP and RE to the IAV in NEE were similar at MOZ, indicating different mechanisms regulating the IAV in NEE among ecosystems. Interestingly, there was a strong negative correlation between the climatic and biotic effects at the annual scale from 2003 to 2009 at QYZ (r 2 = 0.80, P < 0.01), suggesting these two effects counteracted each other and resulted in a relatively stable C sink, whereas no correlations were found at the other two sites. Overall, our study revealed the relative importance of climatic and biotic effects on the IAV in NEE and contributed to our understanding of their underlying mechanisms.

Net ecosystem exchange of CO2 and carbon balance for eight temperate organic soils under agricultural management

This study presents the first annual estimates of net ecosystem exchange (NEE) of CO2 and net ecosystem carbon balances (NECB) of contrasting Danish agricultural peatlands. Studies were done at eight sites representing permanent grasslands (PG) and rotational (RT) arable soils cropped to barley, potato or forage grasses in three geo-regional settings. Using an advanced flux-chamber technique, NEE was derived from modelling of ecosystem respiration (ER) and gross primary production (GPP) with temperature and photosynthetically active radiation as driving variables. At PG (n=3) and RT (n=5) sites, NEE (mean±standard error, SE) was 5.1±0.9 and 8.6±2.0MgCha−1yr−1, respectively, but with the overall lowest value observed for potato cropping (3.5MgCha−1yr−1). This was partly attributed to a short-duration vegetation period and drying of the soil especially in potato ridges. NECB, derived from NEE and C-removal in harvested biomass, was equivalent to 8.4±1.0 and 11.5±2.0MgCha−1 for the PG and RT land-use types, respectively. Means were not significantly different, P=0.214, and were comparable to a wider range of high-end emission factors for managed organic soils in boreal and temperate climate zones. It was stressed that evaluation of emission factors should explicitly differentiate between data representing net C balance from a soil perspective and CO2-C balance from an atmospheric perspective. Modelling of inter-annual variability in NEE for three selected sites during a 21-year meteorological period indicated a range of 18–67% (coefficients of variation). Yet, the robustness of these estimates and their importance for the derived emission factors needs to be substantiated experimentally.

Thinning effects on the net ecosystem carbon exchange of a Sitka spruce forest are temperature-dependent

Commercial forest plantations need to be actively managed, through tree removal, in order to improve wood quality, maintain productivity and provide an economic return, although this could compromise an important role for forests in carbon sequestration and greenhouse gas mitigation. The impact of forest thinning on net primary productivity (NPP) and net ecosystem exchange (NEE) was assessed using a combination of biometric and eddy covariance (EC) techniques. Two thinning operations were performed in close succession, which reduced the basal area of the stand by 17% and 11% and removed a timber volume of 48m3ha−1 and 50m3ha−1, respectively. Annual rates of NPP ranged from 13.24 (±3.96) to 18.94 (±4.88)tCha−1 and 13.22 (±3.72) to 17.77 (±5.30)tCha−1 for the pre- and post-thinning periods, respectively. Estimates of NEE varied between 8.44 (±1.34) to 8.87 (±1.48)tCha−1 and 6.75 (±1.19) to 10.33 (±1.41)tCha−1 in the pre- and post-thinning periods. Forest thinning did not have a significant impact on carbon stocks or fluxes when pre-thinning (2002–2006) and post-thinning (2007–2009) estimates of NPP and NEE were compared, however the range of inter-annual variability in NEE increased after thinning. The partitioning of annual NEE carbon budgets into gross primary productivity (GPP) and ecosystem respiration (Reco) together with an analysis of key physiological parameters suggested that the impacts of forest thinning are largely dependent on temperature. An expected decrease in GPP after the initial thinning in 2007 was not observed due, in part, to the higher mean annual air temperatures and incident photosynthetic active radiation (PAR) and a compensatory increase in photosynthesis by the remaining trees. A continual decline in Reco, was observed in the years subsequent to the first thinning and was attributed to both biomass removal and climatic factors.Inter-annual variations in climate had a significant impact on NEE, GPP and Reco. Annual mean air temperature, total precipitation and total incident PAR were all shown to influence the processes driving CO2 exchange. Overall, these results suggest that the impacts of the thinning practices, as implemented in this study, are dependent on climate and under similar conditions are unlikely, in the short-term, to compromise a role for forest ecosystems in carbon sequestration and greenhouse gas mitigation.

Global patterns of land-atmosphere fluxes of carbon dioxide, latent heat, and sensible heat derived from eddy covariance, satellite, and meteorological observations

We upscaled FLUXNET observations of carbon dioxide, water, and energy fluxes to the global scale using the machine learning technique, model tree ensembles (MTE). We trained MTE to predict site-level gross primary productivity (GPP), terrestrial ecosystem respiration (TER), net ecosystem exchange (NEE), latent energy (LE), and sensible heat (H) based on remote sensing indices, climate and meteorological data, and information on land use. We applied the trained MTEs to generate global flux fields at a 0.5 degrees x 0.5 degrees spatial resolution and a monthly temporal resolution from 1982 to 2008. Cross-validation analyses revealed good performance of MTE in predicting among-site flux variability with modeling efficiencies (MEf) between 0.64 and 0.84, except for NEE (MEf = 0.32). Performance was also good for predicting seasonal patterns (MEf between 0.84 and 0.89, except for NEE (0.64)). By comparison, predictions of monthly anomalies were not as strong (MEf between 0.29 and 0.52). Improved accounting of disturbance and lagged environmental effects, along with improved characterization of errors in the training data set, would contribute most to further reducing uncertainties. Our global estimates of LE (158 +/- 7 J x 10(18) yr(-1)), H (164 +/- 15 J x 10(18) yr(-1)), and GPP (119 +/- 6 Pg C yr(-1)) were similar to independent estimates. Our global TER estimate (96 +/- 6 Pg C yr(-1)) was likely underestimated by 5-10%. Hot spot regions of interannual variability in carbon fluxes occurred in semiarid to semihumid regions and were controlled by moisture supply. Overall, GPP was more important to interannual variability in NEE than TER. Our empirically derived fluxes may be used for calibration and evaluation of land surface process models and for exploratory and diagnostic assessments of the biosphere.

Seven years of recent European net terrestrial carbon dioxide exchange constrained by atmospheric observations

AbstractWe present an estimate of net ecosystem exchange (NEE) of CO2 in Europe for the years 2001–2007. It is derived with a data assimilation that uses a large set of atmospheric CO2 mole fraction observations (∼70 000) to guide relatively simple descriptions of terrestrial and oceanic net exchange, while fossil fuel and fire emissions are prescribed. Weekly terrestrial sources and sinks are optimized (i.e., a flux inversion) for a set of 18 large ecosystems across Europe in which prescribed climate, weather, and surface characteristics introduce finer scale gradients. We find that the terrestrial biosphere in Europe absorbed a net average of −165 Tg C yr−1 over the period considered. This uptake is predominantly in non‐EU countries, and is found in the northern coniferous (−94 Tg C yr−1) and mixed forests (−30 Tg C yr−1) as well as the forest/field complexes of eastern Europe (−85 Tg C yr−1). An optimistic uncertainty estimate derived using three biosphere models suggests the uptake to be in a range of −122 to −258 Tg C yr−1, while a more conservative estimate derived from the a‐posteriori covariance estimates is −165±437 Tg C yr−1. Note, however, that uncertainties are hard to estimate given the nature of the system and are likely to be significantly larger than this. Interannual variability in NEE includes a reduction in uptake due to the 2003 drought followed by 3 years of more than average uptake. The largest anomaly of NEE occurred in 2005 concurrent with increased seasonal cycles of observed CO2. We speculate these changes to result from the strong negative phase of the North Atlantic Oscillation in 2005 that lead to favorable summer growth conditions, and altered horizontal and vertical mixing in the atmosphere. All our results are available through http://www.carbontracker.eu

Environmental variation is directly responsible for short‐ but not long‐term variation in forest‐atmosphere carbon exchange

AbstractTower‐based eddy covariance measurements of forest‐atmosphere carbon dioxide (CO2) exchange from many sites around the world indicate that there is considerable year‐to‐year variation in net ecosystem exchange (NEE). Here, we use a statistical modeling approach to partition the interannual variability in NEE (and its component fluxes, ecosystem respiration, Reco, and gross photosynthesis, Pgross) into two main effects: variation in environmental drivers (air and soil temperature, solar radiation, vapor pressure deficit, and soil water content) and variation in the biotic response to this environmental forcing (as characterized by the model parameters). The model is applied to a 9‐year data set from the Howland AmeriFlux site, a spruce‐dominated forest in Maine, USA. Gap‐filled flux measurements at this site indicate that the forest has been sequestering, on average, 190 g C m−2 yr−1, with a range from 130 to 270 g C m−2 yr−1. Our fitted model predicts somewhat more uptake (mean 270 g C m−2 yr−1), but interannual variation is similar, and wavelet variance analyses indicate good agreement between tower measurements and model predictions across a wide range of timescales (hours to years). Associated with the interannual variation in NEE are clear differences among years in model parameters for both Reco and Pgross. Analysis of model predictions suggests that, at the annual time step, about 40% of the variance in modeled NEE can be attributed to variation in environmental drivers, and 55% to variation in the biotic response to this forcing. As model predictions are aggregated at longer timescales (from individual days to months to calendar year), variation in environmental drivers becomes progressively less important, and variation in the biotic response becomes progressively more important, in determining the modeled flux. There is a strong negative correlation between modeled annual Pgross and Reco (r=−0.93, P≤0.001); two possible explanations for this correlation are discussed. The correlation promotes homeostasis of NEE: the interannual variation in modeled NEE is substantially less than that for either Pgross or Reco