Variability In Net Ecosystem Exchange Research Articles

Testing a land model in ecosystem functional space via a comparison of observed and modeled ecosystem flux responses to precipitation regimes and associated stresses in a Central U.S. forest

AbstractTesting complex land surface models has often proceeded by asking the question: does the model prediction agree with the observation? Such an approach has yet led to high‐performance terrestrial models that meet the challenges of climate and ecological studies. Here we test the Community Land Model (CLM) by asking the question: does the model behave like an ecosystem? We pursue its answer by testing CLM in the ecosystem functional space (EFS) at the Missouri Ozark AmeriFlux (MOFLUX) forest site in the Central U.S., focusing on carbon and water flux responses to precipitation regimes and associated stresses. In the observed EFS, precipitation regimes and associated water and heat stresses controlled seasonal and interannual variations of net ecosystem exchange (NEE) of CO2 and evapotranspiration in this deciduous forest ecosystem. Such controls were exerted more strongly by precipitation variability than by the total precipitation amount per se. A few simply constructed climate variability indices captured these controls, suggesting a high degree of potential predictability. While the interannual fluctuation in NEE was large, a net carbon sink was maintained even during an extreme drought year. Although CLM predicted seasonal and interanual variations in evapotranspiration reasonably well, its predictions of net carbon uptake were too small across the observed range of climate variability. Also, the model systematically underestimated the sensitivities of NEE and evapotranspiration to climate variability and overestimated the coupling strength between carbon and water fluxes. We suspect that the modeled and observed trajectories of ecosystem fluxes did not overlap in the EFS and the model did not behave like the ecosystem it attempted to simulate. A definitive conclusion will require comprehensive parameter and structural sensitivity tests in a rigorous mathematical framework. We suggest that future model improvements should focus on better representation and parameterization of process responses to environmental stresses and on more complete and robust representations of carbon‐specific processes so that adequate responses to climate variability and a proper degree of coupling between carbon and water exchanges are captured.

Carbon and energy fluxes in cropland ecosystems: a model-data comparison

Croplands are highly productive ecosystems that contribute to land–atmosphere exchange of carbon, energy, and water during their short growing seasons. We evaluated and compared net ecosystem exchange (NEE), latent heat flux (LE), and sensible heat flux (H) simulated by a suite of ecosystem models at five agricultural eddy covariance flux tower sites in the central United States as part of the North American Carbon Program Site Synthesis project. Most of the models overestimated H and underestimated LE during the growing season, leading to overall higher Bowen ratios compared to the observations. Most models systematically under predicted NEE, especially at rain-fed sites. Certain crop-specific models that were developed considering the high productivity and associated physiological changes in specific crops better predicted the NEE and LE at both rain-fed and irrigated sites. Models with specific parameterization for different crops better simulated the inter-annual variability of NEE for maize-soybean rotation compared to those models with a single generic crop type. Stratification according to basic model formulation and phenological methodology did not explain significant variation in model performance across these sites and crops. The under prediction of NEE and LE and over prediction of H by most of the models suggests that models developed and parameterized for natural ecosystems cannot accurately predict the more robust physiology of highly bred and intensively managed crop ecosystems. When coupled in Earth System Models, it is likely that the excessive physiological stress simulated in many land surface component models leads to overestimation of temperature and atmospheric boundary layer depth, and underestimation of humidity and CO2 seasonal uptake over agricultural regions.

Differential responses of carbon and water vapor fluxes to climate among evergreen needleleaf forests in the USA

Understanding the differences in carbon and water vapor fluxes of spatially distributed evergreen needleleaf forests (ENFs) is crucial for accurately estimating regional or global carbon and water budgets and when predicting the responses of ENFs to current and future climate. We compared the fluxes of ten AmeriFlux ENF sites to investigate cross-site variability in net ecosystem exchange of carbon (NEE), gross primary production (GPP), and evapotranspiration (ET). We used wavelet cross-correlation analysis to examine responses of NEE and ET to common climatic drivers over multiple timescales and also determined optimum values of air temperature (T a) and vapor pressure deficit (VPD) for NEE and ET. We found larger differences in the NEE spectra than in the ET spectra across sites, demonstrating that spatial (site-to-site) variability was larger for NEE than for ET. The NEE and ET were decoupled differently across ENF sites because the wavelet cospectra between ET and climate variables were similar at all sites, while the wavelet cospectra between NEE and climate variables were higher (i.e., closer coupling between NEE and climatic drivers) in semi-arid and Mediterranean sites than in other sites. Ecosystem water use efficiency (EWUE) based on annual GPP/ET ranged from 1.3 ± 0.18 to 4.08 ± 0.62 g C mm−1 ET, while EWUE based on annual net ecosystem production (NEP)/ET ranged from 0.06 ± 0.04 to 1.02 ± 0.16 g C mm−1 ET) among ENFs. Responses of NEE and ET to T a varied across climatic zones. In particular, for ENF sites in semi-arid and Mediterranean climates, the maximum NEE and ET occurred at lower ranges of T a than in sites with warm and humid summers. The optimum T a and VPD values were higher for ET than for NEE, and ET was less sensitive to high values of T a and VPD. Large spatial variability in carbon and water vapor fluxes among ENFs and large variations in responses of NEE and ET to major climate variables among climatic zones necessitate sub-plant functional type parameterization based on climatic zones to better represent climate sensitivity of ENFs and to reduce uncertainty in model predictions.

Direct and indirect effects of climatic variations on the interannual variability in net ecosystem exchange across terrestrial ecosystems

Climatic variables not only directly affect the interannual variability (IAV) in net ecosystem exchange of CO2 (NEE) but also indirectly drive it by changing the physiological parameters. Identifying these direct and indirect paths can reveal the underlying mechanisms of carbon (C) dynamics. In this study, we applied a path analysis using flux data from 65 sites to quantify the direct and indirect climatic effects on IAV in NEE and to evaluate the potential relationships among the climatic variables and physiological parameters that represent physiology and phenology of ecosystems. We found that the maximum photosynthetic rate was the most important factor for the IAV in gross primary productivity (GPP), which was mainly induced by the variation in vapour pressure deficit. For ecosystem respiration (RE), the most important drivers were GPP and the reference respiratory rate. The biome type regulated the direct and indirect paths, with distinctive differences between forests and non-forests, evergreen needleleaf forests and deciduous broadleaf forests, and between grasslands and croplands. Different paths were also found among wet, moist and dry ecosystems. However, the climatic variables can only partly explain the IAV in physiological parameters, suggesting that the latter may also result from other biotic and disturbance factors. In addition, the climatic variables related to NEE were not necessarily the same as those related to GPP and RE, indicating the emerging difficulty encountered when studying the IAV in NEE. Overall, our results highlight the contribution of certain physiological parameters to the IAV in C fluxes and the importance of biome type and multi-year water conditions, which should receive more attention in future experimental and modelling research.

Carbon Dioxide Fluxes and Their Environmental Control in a Reclaimed Coastal Wetland in the Yangtze Estuary

Large areas of natural coastal wetlands have suffered severely from human-driven damages or conversions (e.g., land reclamations), but coastal carbon flux responses in reclaimed wetlands are largely unknown. The lack of knowledge of the environmental control mechanisms of carbon fluxes also limits the carbon budget management of reclaimed wetlands. The net ecosystem exchange (NEE) in a coastal wetland at Dongtan of Chongming Island in the Yangtze estuary was monitored throughout 2012 using the eddy covariance technique more than 14 years after this wetland was reclaimed using dykes to stop tidal flooding. The driving biophysical variables of NEE were also examined. The results showed that NEE displayed marked diurnal and seasonal variations. The monthly mean NEE showed that this ecosystem functioned as a CO2 sink during 9 months of the year, with a maximum value in September (−101.2 g C m−2) and a minimum value in November (−8.2 g C m−2). The annual CO2 balance of the reclaimed coastal wetland was −558.4 g C m−2 year−1. The ratio of ecosystem respiration (ER) to gross primary production (GPP) was 0.57, which suggests that 57 % of the organic carbon assimilated by wetland plants was consumed by plant respiration and soil heterotrophic respiration. Stepwise multiple linear regressions suggested that temperature and photosynthetically active radiation (PAR) were the two dominant micrometeorological variables driving seasonal variations in NEE, while soil moisture (M s) and soil salinity (PSs) played minor roles. For the entire year, PAR and daytime NEE were significantly correlated, as well as temperature and nighttime NEE. These nonlinear relationships varied seasonally: the maximum ecosystem photosynthetic rate (A max), apparent quantum yield (∂), and Q 10 reached their peak values during summer (17.09 μmol CO2 m−2 s−1), autumn (0.13 μmol CO2 μmol−1 photon), and spring (2.16), respectively. Exceptionally high M s or PSs values indirectly restricted ecosystem CO2 fixation capacity by reducing the PAR sensitivity of the NEE. The leaf area index (LAI) and live aboveground biomass (AGBL) were significantly correlated with NEE during the growing season. Although the annual net CO2 fixation rate of the coastal reclaimed wetland was distinctly lower than the unreclaimed coastal wetland in the same region, it was quite high relative to many inland freshwater wetlands and estuarine/coastal wetlands located at latitudes higher than this site. Thus, it is concluded that although the net CO2 fixation capacity of the coastal wetland was reduced by land reclamation, it can still perform as an important CO2 sink.

Controls of spatial and temporal variability in CH4 flux in a high arctic fen over three years

The aim of this study was to establish the main drivers of the spatial variability in growing season CH4 flux within an arctic wetland ecosystem. During 3 years (2011–2013) we measured CH4 flux and potential drivers, e.g., CO2 fluxes (net ecosystem exchange (NEE), gross primary productivity (GPP) and ecosystem respiration), temperature, water table depth, pore-water concentration of organic acids (e.g., acetate) and the vascular plant composition and density. The study included 16–20 main plots (Cmain) and in 2013 also experimental plots (10 excluded muskoxen grazing, 9 snow fence and 10 automated chamber plots) distributed over 0.3 km2. The results show a 1.8-times difference in CH4 flux magnitude inter-annually and 9- to 35-times spatially (depending on year and treatment). During all 3 years GPP was a strong driver of the variability in Cmain plots. Accordingly, the plant productivity related variables NEE, GPP and acetate were singled out as the strongest drivers of the variability in 2013, when all variables were measured on a majority of the plots. These variables were equally strong drivers of the spatial variability in CH4 flux regardless of whether experimental plots were included in the analysis or not. The density of Eriophorum scheuchzeri was the strongest driver of the spatial variability in NEE, GPP and acetate. In conclusion, changes in vegetation composition or productivity of wet arctic ecosystems will have large impacts on their carbon balance and CH4 flux, irrespective of whether these changes are driven directly by climate change or by biotic interactions, such as grazing.

Spatio-temporal drivers of soil and ecosystem carbon fluxes at field scale in an upland grassland in Germany

Ecosystem carbon (C) fluxes in terrestrial ecosystems are affected by varying environmental conditions (e.g., soil heterogeneity and weather) and land management. However, the interactions between soil respiration (Rs) and net ecosystem exchange (NEE) and their spatio-temporal dependence on environmental conditions and land management at field scale is not well understood. We performed repeated C flux measurement at 21 sites during the 2013 growing season in a temperate upland grassland in Germany, which was fertilized and cut three times according to the agricultural practice typical of the region. Repeated measurements included determination of NEE, Rs, leaf area index (LAI), meteorological conditions as well as physical and chemical soil properties. Temporal variability of Rs was controlled by air temperature, while LAI influenced the temporal variability of NEE. The three grass cuts reduced LAI and affected NEE markedly. More than 50% of NEE variability was explained by defoliation at field scale. Additionally, soil heterogeneity affected NEE, but to a lower extent (>30%), while Rs remained unaffected. We conclude that grassland management (i.e., repeated defoliation) and soil heterogeneity affects the spatio-temporal variability of NEE at field scale.

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.

Long term carbon storage potential and CO2 sink strength of a restored salt marsh in New Jersey

The study compares the amounts of carbon fixed via photosynthesis of a restored tidal marsh to the total organic carbon remaining in sediments of a natural tidal marsh and arrives at preliminary baselines for carbon sequestration and storage over time. The Eddy-covariance method (indirect method) was used to estimate marsh canopy net ecosystem exchange (NEE) and measured an annual gross primary production of 979gCm−2, while the loss through respiration was 766gCm−2, resulting in a net uptake of 213gCm−2yr−1. Time of the day, solar irradiation, air temperature, humidity and wind direction all together explained 66% of the variation in NEE. The high marsh community of Spartina patens showed NEE to be significantly higher than the low marsh community. The net ecosystem carbon balance (NECB) over long time scales was estimated by measuring the actual amount of total organic carbon contained in dated sediment cores from a natural marsh (direct method), which resulted in a carbon accumulation rate of 192.2gm−2yr−1. Changes in total organic carbon content over time in the core sample showed that 78% of organic carbon remained stored in the sediments after 130 years and only the most recalcitrant carbon (50%) remained under storage beyond 645 years. Overall the study showed that temperate macrotidal salt marshes are net sinks of carbon with potential for long term carbon storage. The marsh turned into a carbon sink at the beginning of May and switched back to being a source in late November. The average sedimentation rate estimated from the 137 CS dating (1950s to present) was 1.4mmyr−1 which is similar to accretion rates of comparable S. patens patches in the east coast. Accretion rates derived from our study are slightly lower than the 60+ year rate of sea level rise (2.6mmyr−1) recorded by tide gauge measurements in the Northeast.

Carbon dioxide fluxes over a temperate meadow in eastern Inner Mongolia, China

Understanding the carbon dynamics in grassland is essential to precisely estimate global atmospheric carbon budget in response to climatic change. Eddy flux measurements were carried out during 2011 and 2012 to characterize seasonal and annual variability of carbon exchanges above a temperate meadow in eastern Inner Mongolia, China. The CO2 flux showed obvious diurnal variations and the monthly mean amplitudes of diurnal course followed June/July > August > May > September. The daily maximum NEE reached up to −8.0 and −7.7 g C m−2 for 2011 and 2012, respectively. CO2 uptake was mainly from May to August, with seasonal peaks of −16.0 g C m−2 day−1 in both two years. Gross primary production (GPP) and ecosystem respiration (Re) were −1,084.5, 987.1 g C m−2 year−1 in 2011, and −1,123.3, 1,040.2 g C m−2 year−1 in 2012, respectively. The meadow acted as a stable carbon sink, with integrated net ecosystem exchange (NEE) of −97.4 and −83.1 g C m−2 year−1 for 2011 and 2012, respectively. Compared with 2011, the ecosystem assimilated more carbon and meanwhile respired even more, leading to a less carbon sequestration in 2012. PAR and leaf area index (LAI) dominated the seasonal variations in NEE, with PAR explaining 61–69 % of the variance in NEE as LAI maintaining the plateau during June to July. Harvest significantly decreased ecosystem carbon uptake. The interannual variability in GPP and Re resulted primarily from the variations in temperature and its effect on biomass growth.

A 12-year record reveals pre-growing season temperature and water table level threshold effects on the net carbon dioxide exchange in a boreal fen

This study uses a 12-year time series (2001–2012) of eddy covariance measurements to investigate the long-term net ecosystem exchange (NEE) of carbon dioxide (CO2) and inter-annual variations in relation to abiotic drivers in a boreal fen in northern Sweden. The peatland was a sink for atmospheric CO2 in each of the twelve study years with a 12-year average (± standard deviation) NEE of −58 ± 21 g C m−2 yr−1. For ten out of twelve years, the cumulative annual NEE was within a range of −42 to −79 g C m−2 yr−1 suggesting a general state of resilience of NEE to moderate inter-annual climate variations. However, the annual NEE of −18 and −106 g C m−2 yr−1 in 2006 and 2008, respectively, diverged considerably from this common range. The lower annual CO2 uptake in 2006 was mainly due to late summer emissions related to an exceptional drop in water table level (WTL). A positive relationship (R2 = 0.65) between pre-growing season (January to April) air temperature (Ta) and summer (June to July) gross ecosystem production (GEP) was observed. We suggest that enhanced GEP due to mild pre-growing season air temperature in combination with air temperature constraints on ecosystem respiration (ER) during the following cooler summer explained most of the greater net CO2 uptake in 2008. Differences in the annual and growing season means of other abiotic variables (e.g. radiation, vapor pressure deficit, precipitation) and growing season properties (i.e. start date, end date, length) were unable to explain the inter-annual variations of NEE. Overall, our findings suggest that this boreal fen acts as a persistent contemporary sink for atmospheric CO2 that is, however, susceptible to severe anomalies in WTL and pre-growing season air temperature associated with predicted changes in climate patterns for the boreal region.

Estimating Carbon Flux Phenology with Satellite-Derived Land Surface Phenology and Climate Drivers for Different Biomes: A Synthesis of AmeriFlux Observations

Carbon Flux Phenology (CFP) can affect the interannual variation in Net Ecosystem Exchange (NEE) of carbon between terrestrial ecosystems and the atmosphere. In this study, we proposed a methodology to estimate CFP metrics with satellite-derived Land Surface Phenology (LSP) metrics and climate drivers for 4 biomes (i.e., deciduous broadleaf forest, evergreen needleleaf forest, grasslands and croplands), using 159 site-years of NEE and climate data from 32 AmeriFlux sites and MODIS vegetation index time-series data. LSP metrics combined with optimal climate drivers can explain the variability in Start of Carbon Uptake (SCU) by more than 70% and End of Carbon Uptake (ECU) by more than 60%. The Root Mean Square Error (RMSE) of the estimations was within 8.5 days for both SCU and ECU. The estimation performance for this methodology was primarily dependent on the optimal combination of the LSP retrieval methods, the explanatory climate drivers, the biome types, and the specific CFP metric. This methodology has a potential for allowing extrapolation of CFP metrics for biomes with a distinct and detectable seasonal cycle over large areas, based on synoptic multi-temporal optical satellite data and climate data.

Evaluating the agreement between measurements and models of net ecosystem exchange at different times and timescales using wavelet coherence: an example using data from the North American Carbon Program Site-Level Interim Synthesis

Abstract. Earth system processes exhibit complex patterns across time, as do the models that seek to replicate these processes. Model output may or may not be significantly related to observations at different times and on different frequencies. Conventional model diagnostics provide an aggregate view of model–data agreement, but usually do not identify the time and frequency patterns of model–data disagreement, leaving unclear the steps required to improve model response to environmental drivers that vary on characteristic frequencies. Wavelet coherence can quantify the times and timescales at which two time series, for example time series of models and measurements, are significantly different. We applied wavelet coherence to interpret the predictions of 20 ecosystem models from the North American Carbon Program (NACP) Site-Level Interim Synthesis when confronted with eddy-covariance-measured net ecosystem exchange (NEE) from 10 ecosystems with multiple years of available data. Models were grouped into classes with similar approaches for incorporating phenology, the calculation of NEE, the inclusion of foliar nitrogen (N), and the use of model–data fusion. Models with prescribed, rather than prognostic, phenology often fit NEE observations better on annual to interannual timescales in grassland, wetland and agricultural ecosystems. Models that calculated NEE as net primary productivity (NPP) minus heterotrophic respiration (HR) rather than gross ecosystem productivity (GPP) minus ecosystem respiration (ER) fit better on annual timescales in grassland and wetland ecosystems, but models that calculated NEE as GPP minus ER were superior on monthly to seasonal timescales in two coniferous forests. Models that incorporated foliar nitrogen (N) data were successful at capturing NEE variability on interannual (multiple year) timescales at Howland Forest, Maine. The model that employed a model–data fusion approach often, but not always, resulted in improved fit to data, suggesting that improving model parameterization is important but not the only step for improving model performance. Combined with previous findings, our results suggest that the mechanisms driving daily and annual NEE variability tend to be correctly simulated, but the magnitude of these fluxes is often erroneous, suggesting that model parameterization must be improved. Few NACP models correctly predicted fluxes on seasonal and interannual timescales where spectral energy in NEE observations tends to be low, but where phenological events, multi-year oscillations in climatological drivers, and ecosystem succession are known to be important for determining ecosystem function. Mechanistic improvements to models must be made to replicate observed NEE variability on seasonal and interannual timescales.

Growing season variability of net ecosystem CO2 exchange and evapotranspiration of a sphagnum mire in the broad-leaved forest zone of European Russia

The spatial and temporal variability of net ecosystem exchange (NEE) of CO2 and evapotranspiration (ET) of a karst-hole sphagnum peat mire situated at the boundary between broad-leaved and forest–steppe zones in the central part of European Russia in the Tula region was described using results from field measurements. NEE and ET were measured using a portable measuring system consisting of a transparent ventilated chamber combined with an infrared CO2/H2O analyzer, LI-840A (Li-Cor, USA) along a transect from the southern peripheral part of the mire to its center under sunny clear-sky weather conditions in the period from May to September of 2012 and in May 2013. The results of the field measurements showed significant spatial and temporal variability of NEE and ET that was mainly influenced by incoming solar radiation and ground water level. The seasonal patterns of NEE and ET within the mire were quite different. During the entire growing season the central part of the mire was a sink of CO2 for the atmosphere. NEE reached maximal values in June–July (−6.8 ± 4.2 μmol m−2 s−1). The southern peripheral part of the mire, due to strong shading by the surrounding forest, was a sink of CO2 for the atmosphere in June–July only. ET reached maximal values in the well-lighted central parts of the mire in May (0.34 ± 0.20 mm h−1) mainly because of high air and surface temperatures and the very wet upper peat horizon and sphagnum moss. Herbaceous species made the maximum contribution to the total gross primary production (GPP) in both the central and the peripheral parts of the mire. The contribution of sphagnum to the total GPP of these plant communities was relatively small and ranged on sunny days of July–August from −1.1 ± 1.1 mgC g−1 of dry weight (DW) per hour in the peripheral zone of the mire to −0.6 ± 0.2 mgC g−1 DW h−1 at the mire center. The sphagnum layer made the maximum contribution to total ET at the mire center (0.25 ± 0.10 mm h−1) and the herbaceous species on the peripheral part of the mire (0.03 ± 0.03 mm h−1).

Effect of environmental variables and stand structure on ecosystem respiration components in a Mediterranean beech forest

The temporal variability of ecosystem respiration (RECO) has been reported to have important effects on the temporal variability of net ecosystem exchange, the net amount of carbon exchanged between an ecosystem and the atmosphere. However, our understanding of ecosystem respiration is rather limited compared with photosynthesis or gross primary productivity, particularly in Mediterranean montane ecosystems. In order to investigate how environmental variables and forest structure (tree classes) affect different respiration components and RECO in a Mediterranean beech forest, we measured soil, stem and leaf CO2 efflux rates with dynamic chambers and RECO by the eddy-covariance technique over 1 year (2007-2008). Ecosystem respiration showed marked seasonal variation, with the highest rates in spring and autumn and the lowest in summer. We found that the soil respiration (SR) was mainly controlled by soil water content below a threshold value of 0.2 m(3) m(-3), above which the soil temperature explained temporal variation in SR. Stem CO2 effluxes were influenced by air temperature and difference between tree classes with higher rates measured in dominant trees than in co-dominant ones. Leaf respiration (LR) varied significantly between the two canopy layers considered. Non-structural carbohydrates were a very good predictor of LR variability. We used these measurements to scale up respiration components to ecosystem respiration for the whole canopy and obtained cumulative amounts of carbon losses over the year. Based on the up-scaled chamber measurements, the relative contributions of soil, stem and leaves to the total annual CO2 efflux were: 56, 8 and 36%, respectively. These results confirm that SR is the main contributor of ecosystem respiration and provided an insight on the driving factors of respiration in Mediterranean montane beech forests.

Seven years of carbon dioxide exchange over a degraded grassland and a cropland with maize ecosystems in a semiarid area of China

Based on eddy covariance measurement over a degraded grassland and a cropland with maize (Zea mays) ecosystems from 2003 to 2009, carbon exchange processes and their responses to environmental factors in different temporal scales were analyzed in semiarid of China. Accounting for carbon export and import, NBP (net biome production) of cropland with maize ranged from 54.3 to 100.6g Cm−2yr−1. NBP remained positive indicating a carbon net loss from this ecosystem although NEE (net ecosystem exchange) was negative in most of years. Due to negligible carbon import and export, NBP of degraded grassland ecosystem was equal to NEE, with an average value of 138.4g Cm−2yr−1. The grassland ecosystem behaved as carbon source during the whole period. PPFD (incident photosynthetic active radiation) was the main driver for diurnal variation of NEE during growing season in most years. NDVI (normal difference vegetation index) was in accordance with seasonal patterns of NEE especially for cropland with maize ecosystem. Soil temperature at a depth of 5cm was also a main driver for seasonal variation of NEE at the degraded grassland ecosystem in normal precipitation years (2003 and 2005). Annual peak NDVI (NDVImax) was significantly correlated with annual NEE and GPP (gross primary productivity). The amount of growing season precipitation was more responsible for annual variation of NEE. The increasing number of precipitation event (>1mmday−1) was associated with increasing annual carbon uptake. Drought in the early growing period is more critical to carbon dynamics of degraded grassland ecosystem while drought in the middle of growing season was more critical for cropland with maize ecosystem.

Upscaling Tundra CO2 Exchange from Chamber to Eddy Covariance Tower

Extrapolating biosphere-atmosphere CO2 flux observations to larger scales in space, part of the so-called “upscaling” problem, is a central challenge for surface-atmosphere exchange research. Upscaling CO2 flux in tundra is complicated by the pronounced spatial variability of vegetation cover. We demonstrate that a simple model based on chamber observations with a pan-Arctic parameterization accurately describes up to 75% of the observed temporal variability of eddy covariance—measured net ecosystem exchange (NEE) during the growing season in an Abisko, Sweden, subarctic tundra ecosystem, and differed from NEE observations by less than 4% for the month of June. These results contrast with previous studies that found a 60% discrepancy between upscaled chamber and eddy covariance NEE sums. Sampling an aircraft-measured normalized difference vegetation index (NDVI) map for leaf area index (L) estimates using a dynamic flux footprint model explained less of the variability of NEE across the late June to mid-September period, but resulted in a lower root mean squared error and better replicated large flux events. Findings suggest that ecosystem structure via L is a critical input for modeling CO2 flux in tundra during the growing season. Future research should focus on quantifying microclimate, namely photosynthetically active radiation and air temperature, as well as ecosystem structure via L, to accurately model growing season tundra CO2 flux at chamber and plot scales.

Seasonal and inter-annual variations in net ecosystem exchange of two old-growth forests in southern China

Old-growth forests can accumulate carbon. However, what controls the rate of net carbon accumulation in those old-growth forests is still poorly understood. Using eddy flux measurements from two old-growth evergreen broadleaf forests (subtropical forest and tropical forest) in southern China, we compared the seasonal and inter-annual variations in the carbon fluxes of those two forests and quantified the major drivers for these temporal variations. The measured flux data showed that the annual net carbon uptake of the subtropical forest was generally much larger than that for the tropical forest. The mean net ecosystem exchange (NEE) over 6 years was −397±94gCm−2yr−1 for the subtropical forest and −166±49gCm−2yr−1 for the tropical forest with different seasonal variations. The subtropical forest was a carbon sink for most months in a year, while the tropical forest was a carbon source in wet seasons (positive NEE) and a carbon sink in dry seasons (negative NEE). Both forests were stronger carbon sink in dry years, because of much larger reduction in ER than in wet years. At the seasonal scale, GPP in wet seasons was 37.1% higher than that for dry seasons in the subtropical forest, and was only 12.4% higher in the tropical forest. The amplitude of seasonal GPP variation in the tropical forest was much weaker than in the subtropical forest, but the amplitude of the seasonal variation in ER was much larger than in the subtropical forest. The seasonal variation in NEE was largely driven by the variation in monthly ER of the tropical forest, and by both seasonal variations in monthly GPP and ER of the subtropical forest. At inter-annual scale, annual NEE varied tightly with annual rainfall from year to year. Therefore annual rainfall was suggested a fundamental driver of annual carbon sequestration in the subtropical and tropical forests in southern China.

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.

Carbon Balance of No‐Till Soybean with Winter Wheat Cover Crop in the Southeastern United States

The southeastern United States is an economically important agricultural region, yet its role in the regional C budget is not fully understood. There is concern that climate change, particularly altered precipitation patterns, may induce a shift in how crops exchange CO2with the atmosphere. This study examined the seasonal and interannual variation in net ecosystem exchange (NEE) of a winter wheat cover crop (Triticum aestivumL.) and soybean [Glycine max(L.) Merr.] using the eddy covariance (EC) method. This was conducted at Winfred Thomas Agricultural Research Station, Hazel Green, AL (2007–2009). Annual C balance ranged from a source in 2007 (NEE = 100 g C m−2yr−1) to a sink (–20 g C m−2yr−1) in 2009. Annual ecosystem respiration (Re) ranged between 750 and 1013 g C m−2yr−1, while gross ecosystem productivity was between 650 and 1034 g C m−2yr−1. Seasonal NEE for soybean ranged between 42 and –66 g C m−2. The uptake rates from the cover crop (NEE = –80.0, –80.4, and –40.0 g C m−2for 2007, 2008, and 2009, respectively) suggested the importance of winter C uptake offsetting C losses caused by summer droughts. The Revaried between 286 and 542 g C m−2for soybean and between 160 and 313 g C m−2for the cover crop. Annual variations in NEE and Rewere primarily due to precipitation and air temperature, respectively, indicating a tight coupling between biophysical factors and C uptake. Our results were compared with those from other reported NEE crop estimates using EC.