Year-round observations of the net ecosystem exchange of carbon dioxide in a native tallgrass prairie

An Ameriflux site was established in mid 1996 to study the exchange of CO₂ in a native tallgrass prairie of north‐central Oklahoma, USA. Approximately the first 20 months of measurements (using eddy covariance) are described here. This prairie, dominated by warm season C4 grasses, is typical of the central Kansas/northern Oklahoma region. During the first three weeks of the measurement period (mid‐July–early August 1996), moisture‐stress conditions prevailed. For the remainder of the period (until March 1998), however, soil moisture was nonlimiting. Mid‐day net ecosystem CO₂ exchange (NEE), under well‐watered conditions, reached a maximum magnitude of 1.4 mg CO₂ m^?2 s^?1 (flux toward the surface is positive) during peak growth (mid‐July 1997), with green leaf area index of 2.8. In contrast, under moisture‐stress conditions in the same growth stage in 1996, mid‐day NEE was reduced to near‐zero. Average night NEE ranged from near‐zero, during winter dormancy, to ? 0.50 mg CO₂ m^?2 s^?1, during peak growth. Most of the variance in average night NEE was explained by changes in soil temperature (0.1 m depth) and green leaf area. The daytime NEE measurements were examined in terms of a rectangular hyperbolic relationship with incident photosynthetically active radiation. The analysis showed that the quantum yield during peak growth was similar to those measured in other prairies and the y‐intercept, so obtained, can be potentially used as an estimate of night‐time CO₂ emissions when eddy covariance data are unavailable. Daily integrated NEE reached its peak magnitude of 30.8 g CO₂ m^?2 d^?1 (8.4 g C m^?2 d^?1) in mid‐July when the green LAI was the largest (about 2.8). In general, the seasonal trend of daily NEE (on relatively clear days) followed that of green LAI. Annually integrated carbon exchange, between prescribed burns in 1997 and 1998, was 268 g C m^?2 y^?1. After incorporating carbon loss during the prescribed burn , the net annual carbon exchange in this prairie was near‐zero in 1998.     

Partitioning net ecosystem carbon exchange and the carbon isotopic disequilibrium in a subalpine forest

We investigate the utility of an improved isotopic method to partition the net ecosystem exchange of CO₂ (F) into net photosynthesis (F_A) and nonfoliar respiration (F_R). Measurements of F and the carbon isotopic content in air at a high‐elevation coniferous forest (the Niwot Ridge AmeriFlux site) were used to partition F into F_A and F_R. Isotopically partitioned fluxes were then compared with an independent flux partitioning method that estimated gross photosynthesis (GEE) and total ecosystem respiration (TER) based on statistical regressions of night‐time F and air temperature. We compared the estimates of F_A and F_R with expected canopy physiological relationships with light (photosynthetically active radiation) and air temperature. Estimates of F_A and GEE were dependent on light as expected, and TER, but not F_R, exhibited the expected dependence on temperature. Estimates of the isotopic disequilibrium D , or the difference between the isotopic signatures of net photosynthesis (δ_A, mean value ?24.6‰) and ecosystem respiration (δ_R, mean value ?25.1‰) were generally positive (δ_A>δ_R). The sign of D observed here is inconsistent with many other studies. The key parameters of the improved isotopic flux partitioning method presented here are ecosystem scale mesophyll conductance (g_m) and maximal vegetative stomatal conductance (g_cmax). The sensitivity analyses of F_A, F_R, and D to g_cmax indicated a critical value of g_cmax (0.15 mol m^?2 s^?1) above which estimates of F_A and F_R became larger in magnitude relative to GEE and TER. The value of D decreased with increasing values of g_m and g_cmax, but was still positive across all values of g_m and g_cmax. We conclude that the characterization of canopy‐scale mesophyll and stomatal conductances are important for further progress with the isotope partitioning method, and to confirm our anomalous isotopic disequilibrium findings.     

Nitrogen effects on net ecosystem carbon exchange in a temperate steppe

It has widely been documented that nitrogen (N) enrichment stimulates plant growth and net primary production. However, there is still dispute on how N addition affects net ecosystem CO₂ exchange (NEE), which represents the balance between ecosystem carbon (C) uptake and release. We conducted an experimental study to examine effects of N addition on NEE in a temperate steppe in northern China from 2005 to 2008. N was added at a rate of 10 g N m⁻² yr⁻¹ with NH₄NO₃ alone or in combination with phosphorous (P, 5 g P₂O₅ m⁻² yr⁻¹) in both clipped and unclipped plots. Over the 4 years, N addition significantly stimulated growing-season NEE, on average, by 27%. Neither the main effects of P addition or clipping nor their interactions with N addition were statistically significant on NEE in any of the 4 years. However, the magnitude of N stimulation on NEE declined over time. N addition significantly increased NEE by 60% in 2005 and 21% in 2006, but its effect was not significant in 2007 and 2008. N-induced shift in species composition was primarily responsible for the declined N stimulation over time. The gradually increasing coverage of the upper canopy species ( Stipa krylovii ) and standing litter accumulation induced light limitation on the lower canopy species ( Artemisia frigida ). Thus, N-induced shifts in plant species composition strongly regulated the direct effects of N addition on C sequestration in the temperate steppe.     

太阳辐射变化对千烟洲亚热带人工针叶林净CO_2交换量的影响

太阳辐射是植被进行光合作用的重要驱动因子,影响着森林生态系统的净CO2吸收。本研究基于千烟洲亚热带人工针叶林30 min的通量和常规气象数据,以晴空指数为指标,分析了2003—2010年太阳辐射变化对该森林生态系统净CO2交换(NEE)的影响。结果表明:在晴空指数为0.4~0.6的中等辐射条件下NEE达到最大;与晴朗天空的强太阳辐射相比,中等辐射条件下散射辐射增加、气温以及饱和水汽压差的下降是NEE增加的原因;晴空指数在0.4~0.5和0.5~0.6的NEE显著大于晴空指数在0.7~0.8的NEE;该地晴空指数在0.6~0.7和0~0.1出现的频率较大,表明该地经常出现的天空状况以及对应的太阳辐射强度不是提高该生态系统净CO2吸收的最适条件。     

Seasonal net carbon dioxide exchange of a beech forest with the atmosphere

The seasonal carbon dioxide exchange of a beech forest of Central Italy was studied by means of the eddy covariance technique. Additional measurements of biomass respiration with cuvettes and relationship of carbon dioxide exchanges with temperature and light were used to interpolate missing data during the dormant and part of the growing season. The net ecosystem production of the forest equals 472 g C m^?2 y^?1 while the gross ecosystem production 1016 g C m^?2 y^?1 and respiration 544 g C m^?2 y^?1. These estimates are compared with the net primary production determined by direct biomass sampling which amounts to 802 g C m^?2 y^?1.     

Characteristics of net ecosystem carbon dioxide exchange(NEE) from August to October of Alpine meadow on the Tibetan Plateau,China

The Alpine meadow is one of the vegetation types widely distributed on the Tibetan Plateau in China with an area of about 1.2 million square kilometers.The Damxung rangeland station,located in the hinterland of the Tibetan Plateau,is covered with an typical vegetation.The continuous carbon flux data (from August to middle October,2003) measured with the open-path eddy covariance system was used to analyze the diurnal variation pattern of net ecosystem carbon dioxide exchange (NEE) and its relationship with the environmental factors,such as photosynthetically active radiation (PAR),precipitation,and temperature.Results showed that NEE presented obvious diurnal variation pattern with single-peak of diurnal maximum carbon assimilation at 11:00-12:00 (local time) with an CO2.m-2-s-1.During the daytime,NEE fitted fairly well with PAR in a rectangular hyperbola function with the apparent the night-time,NEE showed a good exponential relation with the soil temperature at 5 cm depth.     

Characteristics of net ecosystem carbon dioxide exchange (NEE) from August to October of Alpine meadow on the Tibetan Plateau, China

The Alpine meadow is one of the vegetation types widely distributed on the Tibetan Plateau in China with an area of about 1.2 million square kilometers. The Damxung rangeland station, located in the hinterland of the Tibetan Plateau, is covered with an typical vegetation. The continuous carbon flux data (from August to middle October, 2003) measured with the open-path eddy covariance system was used to analyze the diurnal variation pattern of net ecosystem carbon dioxide exchange (NEE) and its relationship with the environmental factors, such as photosynthetically active radiation (PAR), precipitation, and temperature. Results showed that NEE presented obvious diurnal variation pattern with single-peak of diurnal maximum carbon assimilation at 11: 00–12: 00 (local time) with an average of −0.268 mg CO₂·m⁻²·s⁻¹, i.e., −6.08 μmol CO₂·m⁻²·s⁻¹. During the daytime, NEE fitted fairly well with PAR in a rectangular hyperbola function with the apparent quantum yield (0.020 3 μmol CO₂ μmol⁻¹ PAR) and maximum ecosystem assimilation (9.741 1 μmol CO₂·m⁻²·s⁻¹). During the night-time, NEE showed a good exponential relation with the soil temperature at 5 cm depth. __________ Translated from Acta Ecologica Sinica 2005, 25(8): 1948–1952 [译自: 生态学报, 2005, 25(8): 1948–1952]     

Pan-Arctic modelling of net ecosystem exchange of CO2

Net ecosystem exchange (NEE) of C varies greatly among Arctic ecosystems. Here, we show that approximately 75 per cent of this variation can be accounted for in a single regression model that predicts NEE as a function of leaf area index (LAI), air temperature and photosynthetically active radiation (PAR). The model was developed in concert with a survey of the light response of NEE in Arctic and subarctic tundras in Alaska, Greenland, Svalbard and Sweden. Model parametrizations based on data collected in one part of the Arctic can be used to predict NEE in other parts of the Arctic with accuracy similar to that of predictions based on data collected in the same site where NEE is predicted. The principal requirement for the dataset is that it should contain a sufficiently wide range of measurements of NEE at both high and low values of LAI, air temperature and PAR, to properly constrain the estimates of model parameters. Canopy N content can also be substituted for leaf area in predicting NEE, with equal or greater accuracy, but substitution of soil temperature for air temperature does not improve predictions. Overall, the results suggest a remarkable convergence in regulation of NEE in diverse ecosystem types throughout the Arctic.     

Variations in daytime net carbon and water exchange in a montane shrubland ecosystem in southeast Spain

Carbon and water fluxes in a semiarid shrubland ecosystem located in the southeast of Spain (province of Almería) were measured continuously over one year using the eddy covariance technique. We examined the influence of environmental variables on daytime (photosynthetically active photons, F _P >10 μmol m⁻² s⁻¹) ecosystem gas exchange and tested the ability of an empirical eco-physiological model based on F _P to estimate carbon fluxes over the whole year. The daytime ecosystem fluxes showed strong seasonality. During two solstitial periods, summer with warm temperatures (>15 °C) and sufficient soil moisture (>10 % vol.) and winter with mild temperatures (>5 °C) and high soil moisture contents (>15 % vol.), the photosynthetic rate was higher than the daytime respiration rate and mean daytime CO₂ fluxes were ca. −1.75 and −0.60 μmol m⁻² s⁻¹, respectively. Daytime evapotranspiration fluxes averaged ca. 2.20 and 0.24 mmol m⁻² s⁻¹, respectively. By contrast, in summer and early autumn with warm daytime temperatures (>10 °C) and dry soil (<10 % vol.), and also in mid-winter with near-freezing daytime temperatures the shrubland behaved as a net carbon source (mean daytime CO₂ release of ca. 0.60 and 0.20 μmol m⁻² s⁻¹, respectively). Furthermore, the comparison of water and carbon fluxes over a week in June 2004 and June 2005 suggests that the timing—rather than amount—of spring rainfall may be crucial in determining growing season water and carbon exchange. Due to strongly limiting environmental variables other than F _P, the model applied here failed to describe daytime carbon exchange only as a function of F _P and could not be used over most of the year to fill gaps in the data.     

Estimating diurnal to annual ecosystem parameters by synthesis of a carbon flux model with eddy covariance net ecosystem exchange observations

We performed a synthetic analysis of Harvard Forest net ecosystem exchange of CO₂ (NEE) time series and a simple ecosystem carbon flux model, the simplified Photosynthesis and Evapo‐Transpiration model (SIPNET). SIPNET runs at a half‐daily time step, and has two vegetation carbon pools, a single aggregated soil carbon pool, and a simple soil moisture sub‐model. We used a stochastic Bayesian parameter estimation technique that provided posterior distributions of the model parameters, conditioned on the observed fluxes and the model equations. In this analysis, we estimated the values of all quantities that govern model behavior, including both rate constants and initial conditions for carbon pools. The purpose of this analysis was not to calibrate the model to make predictions about future fluxes but rather to understand how much information about process controls can be derived directly from the NEE observations. A wavelet decomposition enabled us to assess model performance at multiple time scales from diurnal to decadal. The model parameters are most highly constrained by eddy flux data at daily to seasonal time scales, suggesting that this approach is not useful for calculating annual integrals. However, the ability of the model to fit both the diurnal and seasonal variability patterns in the data simultaneously, using the same parameter set, indicates the effectiveness of this parameter estimation method. Our results quantify the extent to which the eddy covariance data contain information about the ecosystem process parameters represented in the model, and suggest several next steps in model development and observations for improved synthesis of models with flux observations.     

A new model of net ecosystem carbon exchange for the deciduous-dominated forest by integrating MODIS and flux data

The eddy covariance technique provides continuous measurements of plot-level net ecosystem carbon exchange (NEE) across a wide range of vegetation types. However, these NEE estimates only represent fluxes at the tower footprint scale. To quantify the NEE over regions or continents, flux tower measurements need to be up-scaled to large areas. In the present study, we propose a new NEE model solely based on Moderate Resolution Imaging Spectroradiometer data, including enhanced vegetation index (EVI), land surface water index (LWSI), land surface temperature (LST), and Terra nighttime LST′. Site-specific data from the deciduous-dominated Harvard Forest flux site were used. Analysis covered six years (2001-2006) of CO₂ flux data. The data of the first four years were used for model building and the rest as validation set. Compared with the model based solely on EVI, we also introduced LST and LSWI into the new model. The results showed that this method could further improve the precision (R² and RMSE reached 0.857 and 1.273, respectively) and generally capture the expected seasonal patterns of NEE.     

Leaf age affects the seasonal pattern of photosynthetic capacityand net ecosystem exchange of carbon in a deciduous forest

Temporal trends in photosynthetic capacity are a critical factorin determining the seasonality and magnitude of ecosystem carbonfluxes. At a mixed deciduous forest in the south‐eastern United States (Walker Branch Watershed, Oak Ridge, TN, USA), we independently measured seasonal trends in photosynthetic capacity (using single‐leaf gas exchange techniques) and the whole‐canopycarbon flux (using the eddy covariance method). Soil respiration was also measured using chambers and an eddy covariance system beneath the canopy. These independent chamber and eddy covariance measurements, along with a biophysical model (CANOAK), areused to examine how leaf age affects the seasonal pattern of carbon uptake during the growing season. When the measured seasonality in photosynthetic capacity is representedin the CANOAK simulations, there is good agreement with the eddy covariance data on the seasonal trends in carbon uptake. Removing the temporal trends in the simulations by using the early season maximum value of photosynthetic capacity over the entire growing season over estimates the annual carbon uptake by about 300 g C m^?2 year^?1– halfthe total estimated annual net ecosystem exchange. Alternatively, use of the mean value of photosynthetic capacity incorrectly simulates the seasonality in carbon uptake by the forest. In addition to changes related to leaf development and senescence, photosynthetic capacitydecreased in the middle and late summer, even when leaf nitrogenwas essentially constant. When only these middle and late summer reductions were neglected in the model simulations, CANOAK still overestimated the carbon uptake by an amount comparable to 25% ofthe total annual net ecosystem exchange.     

On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm

This paper discusses the advantages and disadvantages of the different methods that separate net ecosystem exchange (NEE) into its major components, gross ecosystem carbon uptake (GEP) and ecosystem respiration (R_eco). In particular, we analyse the effect of the extrapolation of night‐time values of ecosystem respiration into the daytime; this is usually done with a temperature response function that is derived from long‐term data sets. For this analysis, we used 16 one‐year‐long data sets of carbon dioxide exchange measurements from European and US‐American eddy covariance networks. These sites span from the boreal to Mediterranean climates, and include deciduous and evergreen forest, scrubland and crop ecosystems. We show that the temperature sensitivity of R_eco, derived from long‐term (annual) data sets, does not reflect the short‐term temperature sensitivity that is effective when extrapolating from night‐ to daytime. Specifically, in summer active ecosystems the long‐term temperature sensitivity exceeds the short‐term sensitivity. Thus, in those ecosystems, the application of a long‐term temperature sensitivity to the extrapolation of respiration from night to day leads to a systematic overestimation of ecosystem respiration from half‐hourly to annual time‐scales, which can reach >25% for an annual budget and which consequently affects estimates of GEP. Conversely, in summer passive (Mediterranean) ecosystems, the long‐term temperature sensitivity is lower than the short‐term temperature sensitivity resulting in underestimation of annual sums of respiration. We introduce a new generic algorithm that derives a short‐term temperature sensitivity of R_eco from eddy covariance data that applies this to the extrapolation from night‐ to daytime, and that further performs a filling of data gaps that exploits both, the covariance between fluxes and meteorological drivers and the temporal structure of the fluxes. While this algorithm should give less biased estimates of GEP and R_eco, we discuss the remaining biases and recommend that eddy covariance measurements are still backed by ancillary flux measurements that can reduce the uncertainties inherent in the eddy covariance data.     

Spatial analysis of growing season length control over net ecosystem exchange

Using data from 28 flux measurement sites, we performed an analysis of the relationship between annual net ecosystem exchange (NEE) and the length of the carbon uptake period (CUP) (the number of days when the ecosystem is a net carbon sink). The observations suggest a linear correlation between the two quantities. The change in annual carbon exchange per day of the CUP differs significantly between deciduous and evergreen vegetation types. The sites containing vegetation with short‐lived foliage (less than 1 year) have higher carbon uptake and respiration rates than evergreen vegetation. The ratio between mean daily carbon exchange rates during carbon uptake and release periods is relatively invariant (2.73±1.08) across different vegetation types. This implies that a balance between carbon release and uptake periods exists despite different photosynthetic pathways, life forms, and leaf habits. The mean daily carbon sequestration rate for these ecosystems never exceeds the carbon emission rate by more than a factor of 3. Growing season lengths for the study sites were derived from the normalized difference vegetation index (NDVI) of advanced very‐high‐resolution radiometer and from the enhanced vegetation index (EVI) of VEGETATION SPOT‐4. NDVI and EVI were found to be closely related to the CUP, and consequently they also can be used to approximate annual carbon exchange of the ecosystems. This approach has potential for allowing extrapolation of NEE over large areas from remotely sensed data, given a certain amount of ancillary information. This method could complement the currently existing techniques for extrapolation, which rely upon modeling of the individual gross fluxes.     

Linking flux network measurements to continental scale simulations: ecosystem carbon dioxide exchange capacity under non-water-stressed conditions

This paper examines long‐term eddy covariance data from 18 European and 17 North American and Asian forest, wetland, tundra, grassland, and cropland sites under non‐water‐stressed conditions with an empirical rectangular hyperbolic light response model and a single layer two light‐class carboxylase‐based model. Relationships according to ecosystem functional type are demonstrated between empirical and physiological parameters, suggesting linkages between easily estimated parameters and those with greater potential for process interpretation. Relatively sparse documentation of leaf area index dynamics at flux tower sites is found to be a major difficulty in model inversion and flux interpretation. Therefore, a simplification of the physiological model is carried out for a subset of European network sites with extensive ancillary data. The results from these selected sites are used to derive a new parameter and means for comparing empirical and physiologically based methods across all sites, regardless of ancillary data. The results from the European analysis are then compared with results from the other Northern Hemisphere sites and similar relationships for the simplified process‐based parameter were found to hold for European, North American, and Asian temperate and boreal climate zones. This parameter is useful for bridging between flux network observations and continental scale spatial simulations of vegetation/atmosphere carbon dioxide exchange.     

Measurement of net ecosystem exchange,productivity and respiration in three spruce forests in Sweden shows unexpectedly large soil carbon losses

Measurement of net ecosystem exchange was made using the eddy covariance method above three forests along a north-south climatic gradient in Sweden: Flakaliden in the north, Knottåsen in central and Asa in south Sweden. Data were obtained for 2 years at Flakaliden and Knottåsen and for one year at Asa. The net fluxes (N_ep) were separated into their main components, total ecosystem respiration (R_t) and gross primary productivity (P_g). The maximum half-hourly net uptake during the heart of the growing season was highest in the southernmost site with ?0.787 mg CO₂ m^?2 s^?1 followed by Knottåsen with ?0.631 mg CO₂ m^?2 s^?1 and Flakaliden with ?0.429 mg CO₂ m^?2 s^?1. The maximum respiration rates during the summer were highest in Knottåsen with 0.245 mg CO₂ m^?2 s^?1 while it was similar at the two other sites with 0.183 mg CO₂ m^?2 s^?1. The annual N_ep ranged between uptake of ?304 g C m^?2 year^?1 (Asa) and emission of 84 g C m^?2 year^?1 (Knottåsen). The annual R_t and P_g ranged between 793 to 1253 g C m^?2 year^?1 and ?875 to ?1317 g C m^?2 year^?1, respectively. Biomass increment measurements in the footprint area of the towers in combination with the measured net ecosystem productivity were used to estimate the changes in soil carbon and it was found that the soils were losing on average 96–125 g C m^?2 year^?1. The most plausible explanation for these losses was that the studied years were much warmer than normal causing larger respiratory losses. The comparison of net primary productivity and P_g showed that ca 60% of P_g was utilized for autotrophic respiration.