Net Ecosystem Exchange Measurements Research Articles

Wind regimes above and below a dense oil palm canopy: Detection of decoupling and its implications on CO2 flux estimates

In tall vegetation canopies, calm weather conditions may result in the formation of an isolated below-canopy air layer. Especially during night, below-canopy carbon dioxide (CO2) fluxes may be concealed from above-canopy eddy covariance (EC) measurements, masking the magnitude of CO2 net ecosystem exchange (NEE). Particular attention to this “nighttime problem” needs to be paid in oil palm (Elaeis guineensis Jacq.) plantations, which can form a dense top canopy and which are grown in tropical regions where wind speeds are often low. Here, we investigated EC-based NEE measurements, wind and micrometeorological patterns, and LiDAR-derived terrain and canopy structure in a mature commercial oil palm plantation in the tropical lowlands of Jambi province, Sumatra, Indonesia. Over 3 years, we assessed the strength of turbulent and thermal mixing and tested five different methods to determine decoupling: (i) single-level (above-canopy) and (ii) two-level (above- and below-canopy) friction velocity (u*), (iii) above- and below-canopy standard deviation of the vertical wind (σw), (iv) bulk Richardson number (Rib), and (v) vertical turbulence intensity (ITurb). Our results show that decoupling is a common phenomenon in this plantation, with the two-level filters of σw and u* being most sensitive in its determination, showing ∼93% nocturnal decoupling. Decoupling is driven by the oil palm canopy structure, which develops a marked blocking layer at ∼10 m above the ground, high frequency of calm weather conditions, inverse temperature gradients and a terrain slope of 3° which was enough to create a thermally-induced drainage flow. Filtering NEE data, especially the two-level filters of σw and u*, challenged estimations of the plantation's CO2 balance due to high uncertainties in nocturnal respiration rates which could not be reduced by adding storage components to the measured fluxes highlighting the importance of additional below-canopy CO2 flux measurements, especially in such a weak-wind and dense-canopy tropical environment.

Dynamics of short-term ecosystem carbon fluxes induced by precipitation events in a semiarid grassland

Abstract. Infrequent and small precipitation (PPT) events characterize PPT patterns in semiarid grasslands; however, plants and soil microorganisms are adapted to use the unpredictable small pulses of water. Several studies have shown short-term responses of carbon and nitrogen mineralization rates (called the “priming effect” or the Birch effect) stimulated by wet–dry cycles; however, dynamics, drivers, and the contribution of the priming effect to the annual C balance are poorly understood. Thus, we analyzed 6 years of continuous net ecosystem exchange measurements to evaluate the effect of the PPT periodicity and magnitude of individual PPT events on the daily/annual net ecosystem C exchange (NEE) in a semiarid grassland. We included the period between PPT events, previous daytime NEE rate, and previous soil moisture content as the main drivers of the priming effect. Ecosystem respiration (ER) responded within a few hours following a PPT event, whereas it took 5–9 d for gross ecosystem exchange (GEE; where −NEE = GEE + ER) to respond. Precipitation events as low as 0.25 mm increased ER, but cumulative PPT > 40 mm infiltrating deep into the soil profile stimulated GEE. Overall, ER fluxes following PPT events were related to the change in soil water content at shallow depth and previous soil conditions (e.g., previous NEE rate, previous soil water content) and the size of the stimulus (e.g., PPT event size). Carbon effluxes from the priming effect accounted for less than 5 % of ecosystem respiration but were significantly high with respect to the carbon balance. In the long term, changes in PPT regimes to more intense and less frequent PPT events, as expected due to the effects of climate change, could convert the semiarid grassland from a small C sink to a C source.

An Empirical Model of Gross Primary Productivity (GPP) and Relations between GPP and Its Driving Factors, Biogenic Volatile Organic Compounds in a Subtropical Conifer Plantation in China

Measurements of net ecosystem exchange (NEE), solar global radiation, photosynthetically active radiation (PAR) and meteorological parameters were carried out on a subtropical conifer plantation in China from 2013 to 2016. These observations were used to develop and evaluate an empirical model of gross primary production (GPP) (EMGPP) with 3-factor and 2-factor models. Using a 3-factor model, the simulated hourly GPP values were consistent with observations with a relative bias of 9.96% and normalized mean square error values of 0.07 mg CO2 m−2 s−1 for the scattering factor S/Q (S and Q are diffuse and global solar radiation) < 0.5 and 15.52% and 0.15 mg CO2 m−2 s−1 for S/Q ≥ 0.5. Validations of the EMGPP for hourly, daily, monthly, and annual GPP values were carried out and showed that both 3-factor and 2-factor EMGPP models can accurately capture diurnal, seasonal and interannual variations in GPP, but most simulated GPP overestimated the observed value. When the scattering factor is not available, the 2-factor EMGPP can be used. The EMGPP using 3-factor and 2-factor models was applied to simulate GPP under all sky conditions from 2013–2016, and the estimated GPP were in reasonable agreement with the measured values and showed systematic overestimations of 31% and 29% for mean hourly GPP and 41% and 29% for annual amounts, respectively. The sensitivity test demonstrated that GPP values were more sensitive to changes in PAR than to changes in water vapor and scattering factor at low S/Q, but were more sensitive to changes in water vapor than to PAR and S/Q at high S/Q. The sensitivity test revealed some mechanisms of GPP and its related processes, including the relationships between GPP and scattering of PAR, GPP and water vapor, which were in good agreement with other observations and model studies. An empirical model based on PAR energy balance can better describe the multiple interactions between GPP and its driving factors (PAR, water vapor, S/Q). The ratio of the emissions of biogenic volatile organic compounds (BVOCs) to net ecosystem exchange clearly varied between forests in different climate zones.

Carbon dioxide exchanges in an alpine tundra ecosystem (Gran Paradiso National Park, Italy): A comparison of results from different measurement and modelling approaches

Mountain ecosystems are particularly sensitive to the effects of climate change and require a detailed understanding of how their components respond to the varying environmental conditions. However, the knowledge of crucial biogeochemical variables, such as carbon dioxide (CO2) fluxes in the Critical Zone of high-altitude grassland and Alpine tundra, is still patchy. In this framework, two methods are typically adopted to measure CO2 exchanges between the atmosphere and the underlying ecosystem: eddy covariance and accumulation chambers. Potential drawbacks affect both approaches, also depending on the scale at which studies are performed. On the one hand, eddy covariance method provides large spatial coverage, despite requiring specific statistical assumptions that are not always valid in remote environments. In addition, the eddy covariance method allows measuring only the CO2 net ecosystem exchange (NEE), while the separation of CO2 emission (ecosystem respiration, ER) and uptake (gross primary production, GPP), needed for understanding ecosystem functioning and its possible future changes, requires partitioning procedures based on regressive models. On the other hand, accumulation chambers allow for direct measurements of both NEE and ER but are usually limited to sampling small areas and the chambers themselves could potentially disturb plant-soil dynamics. The present study investigates such issues by combining three complementary methods (i.e., eddy covariance, portable and automated accumulation chambers) to measure summer CO2 fluxes at the soil-vegetation-atmosphere interface in a heavily instrumented high-altitude Alpine tundra site. While eddy covariance and portable chamber measurements provide consistent estimates of the NEE, a significant discrepancy between the results of the different methodologies was found for daytime ER. In particular, the eddy covariance daytime and nighttime partitioning methods failed in representing ER at the ecosystem scale. The discrepancy between modelled and measured ER suggests that different processes are involved during day and night, which are not fully represented in models. These aspects would require further explorations to achieve site-specific assessments of daytime ER.

Modeled production, oxidation, and transport processes of wetland methane emissions in temperate, boreal, and Arctic regions.

Wetlands are the largest natural source of methane (CH4 ) to the atmosphere. The eddy covariance method provides robust measurements of net ecosystem exchange of CH4 , but interpreting its spatiotemporal variations is challenging due to the co-occurrence of CH4 production, oxidation, and transport dynamics. Here, we estimate these three processes using a data-model fusion approach across 25 wetlands in temperate, boreal, and Arctic regions. Our data-constrained model-iPEACE-reasonably reproduced CH4 emissions at 19 of the 25 sites with normalized root mean square error of 0.59, correlation coefficient of 0.82, and normalized standard deviation of 0.87. Among the three processes, CH4 production appeared to be the most important process, followed by oxidation in explaining inter-site variations in CH4 emissions. Based on a sensitivity analysis, CH4 emissions were generally more sensitive to decreased water table than to increased gross primary productivity or soil temperature. For periods with leaf area index (LAI) of ≥20% of its annual peak, plant-mediated transport appeared to be the major pathway for CH4 transport. Contributions from ebullition and diffusion were relatively high during low LAI (<20%) periods. The lag time between CH4 production and CH4 emissions tended to be short in fen sites (3 ± 2 days) and long in bog sites (13 ± 10 days). Based on a principal component analysis, we found that parameters for CH4 production, plant-mediated transport, and diffusion through water explained 77% of the variance in the parameters across the 19 sites, highlighting the importance of these parameters for predicting wetland CH4 emissions across biomes. These processes and associated parameters for CH4 emissions among and within the wetlands provide useful insights for interpreting observed net CH4 fluxes, estimating sensitivities to biophysical variables, and modeling global CH4 fluxes.

Ecosystem-atmosphere CO2 exchange from semiarid mangroves in the Gulf of California

Mangrove wetlands play an essential role as blue carbon ecosystems. However, worldwide they also face imminent threats caused by anthropogenic activities and climate change. Mangroves in the semiarid region of the Gulf of California are highly vulnerable to both threats. In this study, we present eddy covariance measurements of net ecosystem exchange of CO2 (NEE), gross primary productivity (GPP), and ecosystem respiration (Reco) at a preserved mangrove (Estero El Sargento) and a site influenced by anthropogenic activities (Bahia del Tobari) located in northwest Mexico. Both sites were net annual carbon sinks but had different dynamics. The net annual carbon gain for Estero El Sargento was −717 g C m−2 y−1 while for Bahia deTobari was −247 g C m−2 y−1. Over the studied year, Estero El Sargento had lower Reco(329 g C m−2) during the winter-spring months and an overall lower annual Reco/GPP (0.42) than Bahia de Tobari. The Reco was notably high during the winter-spring period at Bahia de Tobari when agriculture drainage from the intensive food production activities at the Yaqui Valley reaches the bay. The contrasting NEE patterns and magnitude between both mangrove sites suggest that anthropogenic activities influencing coastal ecosystems exert an important control on the CO2 sink potential.

Annual carbon sequestration and loss rates under altered hydrology and fire regimes in southeastern USA pocosin peatlands.

Peatlands drained for agriculture or forestry are susceptible to the rapid release of greenhouse gases (GHGs) through enhanced microbial decomposition and increased frequency of deep peat fires. We present evidence that rewetting drained subtropical wooded peatlands (STWPs) along the southeastern USA coast, primarily pocosin bogs, could prevent significant carbon (C) losses. To quantify GHG emissions and storage from drained and rewetted pocosin we used eddy covariance techniques, the first such estimates that have been applied to this major bog type, on a private drained (PD) site supplemented by static chamber measurements at PD and Pocosin Lakes National Wildlife Refuge. Net ecosystem exchange measurements showed that the loss was 21.2Mg CO2 ha-1 year-1 (1Mg=106 g) in the drained pocosin. Under a rewetted scenario, where the annual mean water table depth (WTD) decreased from 60 to 30 cm, the C loss was projected to fall to 2Mg CO2 ha-1 year-1 , a 94% reduction. If the WTD was 20 cm, the peatlands became a net carbon sink (-3.3Mg CO2 ha-1 year-1 ). Hence, net C reductions could reach 24.5Mg CO2 ha-1 year-1 , and when scaled up to the 4000 ha PD site nearly 100,000 Mg CO2 year-1 of creditable C could be amassed. We conservatively estimate among the 0.75million ha of southeastern STWPs, between 450 and 770 km2 could be rewet, reducing annual GHG emissions by 0.96-1.6Tg (1Tg=1012 g) of CO2 , through suppressed microbial decomposition and 1.7-2.8Tg via fire prevention, respectively. Despite covering <0.01% of US land area, rewetting drained pocosin can potentially provide 2.4% of the annual CO2 nationwide reduction target of 0.18 Pg (1Pg=1015 g). Suggesting pocosin restoration can contribute disproportionately to the US goal of achieving net-zero emission by 2050.

Spatiotemporal estimation of gross primary production for terrestrial wetlands using satellite and field data

The main goal of this study was to determine the GPP at the Biebrza Wetlands for sedges, reeds and grasses habitats, applying satellite data. The ground measurements of Net Ecosystem Exchange (NEE) and respiration (RESP) were conducted at 26 sites at the peatland using chamber method during the vegetation growth from April to October 2015–2020. Applying round-the-clock registrations of Photosynthetically Active Radiation (PAR) and one ground measurement of NEE and RESP the simulation of the daily course of GPP was performed. GPP daily mean values were the object of estimation applying the models based on the satellite data derived from Sentinel 2, Sentinel 3 and MODIS. The main factors of photosynthetic carbon uptake – greenness and moisture were described by satellite vegetation indices Normalized Difference Vegetation Index (NDVI), Normalized Difference Infrared Index (NDII), Accumulated Photosynthetically Active Radiation (APAR), evapotranspiration, latent heat and the differences of surface temperature (Ts) and air temperature (Ta). The models presented as a result of this research can be used applying solely the satellite data. The results of the analysis revealed that NDVI index is the driving factor for the assessment of GPP independently on the type of satellite data (r = 0.64) Up to 40% of GPP variance is explained by the variable connected with the vegetation greenness. The contribution of vegetation moisture indices is about two times smaller. The best fit has been achieved for the Sentinel-3 models with inclusion of the environmental parameters, as NDVI, temperature difference (Ts–Ta), latent heat. The differences of uptake the CO2 between the habitats depending on their phases of phenological development have been presented.

Theory, method and application advance of isotopic flux partitioning of ecosystem photosynthesis and respiration

Photosynthesis and respiration are two important components of net ecosystem exchange (NEE). NEE can be directly observed by eddy covariance (EC) technique, and statistically separated into ecosystem assimilation and respiration based on the statistical flux partitioning of temperature response function or light-response curves. However, these methods would result in auto-correlation between assimilation and respiration, and overestimate daytime respiration. Recently-developed isotope ratio infrared spectroscopy permits high-resolution measurement of atmospheric CO2 and its stable carbon isotope composition (δ13C) under field conditions, and achieves diurnal and seasonal partitioning of ecosystem photosynthesis and respiration by matching with NEE measurements from EC. We expounded the fundamental theories and assumptions of isotopic flux partitioning of ecosystem photosynthesis and respiration, elaborated the development and application advance of techniques in isotopic flux measurement, summarized the advance of isotopic flux partitioning to provide new insight into the assimilation and respiration processes, and prospected the uncertainty of isotopic flux partitioning theory and the necessity of comparative researches of various methods.

Comparison of eddy covariance and automatic chamber‐based methods for measuring carbon flux

AbstractAn automatic chamber (AC) is an alternative method to the eddy covariance (EC) technique for estimating of CO2 fluxes. However, few direct comparisons between two techniques have been performed on nature alpine meadows. In order to determine diurnal pattern of net ecosystem exchange (NEE) of CO2 in summer on alpine meadows, and test the NEE measurement performance of AC under site conditions, we conducted simultaneous measurements of NEE using both EC and AC methods. The NEEAC can be estimated with the information from the initial slope of the time series of CO2 mixing ratio. We found that the application of linear regression, when photosynthesis processes were considered, could lead to serious deviations, even if closure times were short. The linear regression, however, was adequate for estimating CO2 fluxes when photosynthesis processes were not involved in, at least for short chamber closing times. Using appropriate fitting method, the fluxes of AC were relatively close to that of EC, but presented a daytime underestimation and nighttime overestimation. With the subsequent friction velocity (u*) filtering (&lt;m s–1), the nighttime overestimation was eliminated, and the fluxes of AC showed a similar diurnal pattern of NEE with that of EC, but an overall underestimation all day long. Here, we may provide a side evidence of the overestimation of chamber fluxes at low atmospheric turbulence. Finally, we assume that the overall underestimation of AC is caused by the absence of detection of effective volume of chamber.

Multi-year eddy covariance measurements of net ecosystem exchange in tropical dry deciduous forest of India

Tropical deciduous forests are unique in terms of their geographical distribution, strong seasonality and their contribution to global carbon dynamics, but yet their carbon sequestration potential is poorly sampled. In the current study, we report the carbon balance of 65-year-old tropical dry deciduous forest in central India using the multi-year eddy covariance measurements from November 2011 to May 2019. Over the study period, the forest site was observed to be a net sink of atmospheric CO2 with a mean annual net ecosystem productivity (NEP) of 524 (± 40; ±1 SD across different years) g C m−2 yr−1 with a 233 (±15) day growing season. The NEP was partitioned into gross primary productivity (GPP) of 3358 (± 167) g C m−2 yr−1 and annual carbon loss due to respiration and decomposition (Reco) of 2834 (± 157) g C m−2 yr−1. The ecosystem showed a strong seasonality as a source of carbon during the leaf-off season (March to June), and as a carbon sink during the rest of the year with significant sequestration during the winter season (October to December). The intra-annual analysis suggested that CO2 flux is primarily controlled by canopy greenness with mean Reco/GPP ratio varying from 1.90 (± 0.12) to 0.79 (± 0.04) during leaf-off to leaf-on seasons. The monthly C fluxes in the growing season are found to be strongly correlated to the environmental variables rather than in the leaf-off season, while variability in monthly ecosystem respiration was better explained by air temperature during the leaf-off season than the growing season. Further, the inter-annual variability of NEP was mainly dependent on growing season length and mean annual temperature. The variations in annual GPP and Reco were directly dependent on the increase in mean annual temperature. The eddy covariance-based NEP was complemented by close independent biometric estimates, imparting confidence in our measurements. In conclusion, this tropical dry deciduous forest site is a substantial carbon sink and would add crucial information regarding carbon budgeting of tropical forests in global carbon balance models.

Decadal variation in CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fluxes and its budget in a wheat and maize rotation cropland over the North China Plain

Abstract. Carbon sequestration in agroecosystems has great potential to mitigate global greenhouse gas emissions. To assess the decadal trend of CO2 fluxes of an irrigated wheat–maize rotation cropland over the North China Plain, the net ecosystem exchange (NEE) with the atmosphere was measured by using an eddy covariance system from 2005 to 2016. To evaluate the detailed CO2 budget components of this representative cropland, a comprehensive experiment was conducted in the full 2010–2011 wheat–maize rotation cycle by combining the eddy covariance NEE measurements, plant carbon storage samples, and a soil respiration experiment that differentiated between heterotrophic and below-ground autotrophic respirations. Over the past decade (from 2005 to 2016), the cropland exhibited a statistically nonsignificant decreasing carbon sequestration capacity; the average of total NEE, gross primary productivity (GPP), and ecosystem respiration (ER), respectively, were −364, 1174, and 810 gC m−2 for wheat and −136, 1008, and 872 gC m−2 for maize. The multiple regression revealed that air temperature and groundwater depth showed pronounced correlations with the CO2 fluxes for wheat. However, in the maize season, incoming shortwave radiation and groundwater depth showed pronounced correlations with CO2 fluxes. For the full 2010–2011 agricultural cycle, the CO2 fluxes for wheat and maize were as follows: for NEE they were −438 and −239 gC m−2, for GPP 1078 and 780 gC m−2, for ER 640 and 541 gC m−2, for soil heterotrophic respiration 377 and 292 gC m−2, for below-ground autotrophic respiration 136 and 115 gC m−2, and for above-ground autotrophic respiration 128 and 133 gC m−2. The net biome productivity was 59 gC m−2 for wheat and 5 gC m−2 for maize, indicating that wheat was a weak CO2 sink and maize was close to CO2 neutral to the atmosphere for this agricultural cycle. However, when considering the total CO2 loss in the fallow period, the net biome productivity was −40 gC m−2 yr−1 for the full 2010–2011 cycle, implying that the cropland was a weak CO2 source. The investigations of this study showed that taking cropland as a climate change mitigation tool is challenging and that further studies are required for the CO2 sequestration potential of croplands.

Environmental control of land-atmosphere CO2 fluxes from temperate ecosystems: a statistical approach based on homogenized time series from five land-use types

We assembled homogenized long-term time series, up to 19 years, of measurements of net ecosystem exchange of CO2 (NEE) and its partitioning between gross primary production (GPP) and respiration (Reco) for five different ecosystems representing the main plant functional types (PFTs) in France. Part of these data was analyzed to determine the influence of the main environmental variables on carbon fluxes between temperate ecosystems and the atmosphere, and to investigate the temporal patterns of their variations. A multi-temporal statistical analysis of the time series was conducted using random forest (RF) and wavelet coherence approaches. The RF analysis showed that, in all ecosystems, the incident solar radiation was highly correlated with GPP and that GPP was better correlated with the temporal variations of NEE than Reco. The air temperature was the second most important driver in ecosystems with seasonal foliage, i.e., deciduous forest, cropland and grassland; whereas variables related to air or soil drought were prominent in evergreen forest sites. The environmental control on CO2 fluxes was tighter at high frequency suggesting an increased resilience to environmental variations at longer time spans. The spectral analysis performed on three of the five sites selected revealed contrasting temporal patterns of the cross-coherence between CO2 fluxes and climate variables among ecosystems; these were related to the respective PFT, management and soil conditions. In all PFTs, the power spectrum of GPP was well correlated with NEE and clearly different from Reco. The spectral correlation analysis showed that the canopy phenology and disturbance regime condition the spectral correlation patterns of GPP and Reco with the soil moisture and atmospheric vapour deficit.

Effects of disturbance on the carbon dioxide balance of an anthropogenic peatland in northern Patagonia

Peatlands are characterized by their large carbon (C) storage capacity and represent important C sinks globally. In southern Chile, young peatlands (few centuries old) have originated due to clearcutting or fire at forest sites with high precipitation on poorly drained soils. These novel ecosystems are called anthropogenic peatlands here. Their role in the regional C cycle remains largely unknown. Here, we present 18 months of eddy covariance measurements of net ecosystem exchange (NEE) of carbon dioxide (CO2) in an anthropogenic peatland in northern Chiloe Island, part of which is kept undisturbed for 30–40 years, by excluding human uses, and another section of the same peatland that has been disturbed by cattle grazing and Sphagnum moss extraction. Gross primary productivity (GPP) and ecosystem respiration (Reco) were modeled from NEE, based on measured photosynthetically active radiation and air temperature, separately for each section of the peatland. Uncertainties of the annual flux estimates were assessed from the variability of modelled fluxes induced by applying different time-windows for model development between 10 and 20 days. The undisturbed area of the peatland was on average (± SD) a larger net CO2 sink (NEE = − 135 ± 267 g CO2 m−2 year−1) than the disturbed area (NEE = − 33 ± 111 g CO2 m−2 year−1). These NEE CO2 balances are small even though GPP and Reco were larger compared with other peatlands. Reco had a direct relationship with water table depth (from soil surface) and a negative relationship with soil water fraction. Our results show that the disturbance by moss extraction and cattle grazing is likely to reduce the CO2 sink function of many anthropogenic and natural peatlands on Chiloe Island, which are subjected to the same impacts.

Net Ecosystem Exchange, Gross Primary Production And Ecosystem Respiration In Ridge-Hollow Complex At Mukhrino Bog

The continuous field measurements of net ecosystem exchange (NEE) of CO 2 were provided at ridge-hollow oligotrophic bog in the Middle Taiga zone of West Siberia, Russia in 2017-2018. The model of net ecosystem exchange of CO 2 was suggested to describe the influence of different environmental factors on NEE and to estimate the total carbon budget of the bog over the growing season. The model uses air and soil temperature, incoming photosynthetically active radiation (PAR) and water table depth, as the key factors influencing gross primary production (GPP) and ecosystem respiration (ER). The model coefficients were calibrated using the data collected by automated soil CO 2 flux system with two transparent long-term chambers placed at large hollow and small ridge sites. Experimental and modeling results showed that the Mukhrino bog acted over the study period as a carbon sink, with an average NEE of –87.7 gC m -2 at the hollow site and –50.2 gC m -2 at the ridge site. GPP was – 344.8 and –228.5 gC m -2 whereas ER was 287.6 and 140.9 gC m -2 at ridge and hollow sites, respectively. Despite of a large difference in NEE estimates between 2017 and 2018 the growing season variability of NEE were quite similar.

Eddy Covariance vs. Biometric Based Estimates of Net Primary Productivity of Pedunculate Oak (Quercus robur L.) Forest in Croatia during Ten Years

We analysed 10 years (2008–2017) of continuous eddy covariance (EC) CO2 flux measurements of net ecosystem exchange (NEE) in a young pedunculate oak forest in Croatia. Measured NEE was gap-filled and partitioned into gross primary productivity (GPP) and ecosystem reparation (RECO) using the online tool by Max Planck Institute for Biogeochemistry in Jena, Germany. Annual NEE, GPP, and RECO were correlated with main environmental drivers. Net primary productivity was estimated from EC (NPPEC), as a sum of −NEE and Rh obtained using a constant Rh:RECO ratio, and from independent periodic biometric measurements (NPPBM). For comparing the NPP at the seasonal level, we propose a simple model that aimed at accounting for late-summer and autumn carbon storage in the non-structural carbohydrate pool. Over the study period, Jastrebarsko forest acted as a carbon sink, with an average (±std. dev.) annual NEE of −319 (±94) gC m−2 year−1, GPP of 1594 (±109) gC m−2 year−1, and RECO of 1275 (±94) gC m−2 year−1. Annual NEE showed high inter-annual variability and poor correlation with annual average global radiation, air temperature, and total precipitation, but significant (R2 = 0.501, p = 0.02) correlation with the change in soil water content between May and September. Comparison of annual NPPEC and NPPBM showed a good overall agreement (R2 = 0.463, p = 0.03), although in all years NPPBM was lower than NPPEC, with averages of 680 (±88) gC m−2 year−1 and 819 (±89) gC m−2 year−1, respectively. Lower values of NPPBM indicate that fine roots and grasses contributions to NPP, which were not measured in the study period, could have an important contribution to the overall ecosystem NPP. At a seasonal level, two NPP estimates showed differences in their dynamic, but the application of the proposed model greatly improved the agreement in the second part of the growing season. Further research is needed on the respiration partitioning and mechanisms of carbon allocation.

Effects of low thinning on carbon dioxide fluxes in a mixed hemiboreal forest

We used eddy-covariance (EC) measurements of net ecosystem exchange (NEE) above canopy to assess the effects of thinning on CO2 fluxes at the ICOS Sweden site Norunda in central Sweden. This forest site consists of mixed pine and spruce stands approx. 100 years old. The thinning during late autumn 2008, performed in a semi-circle from the mast extending 200 m outwards harvested about 25% of the volume. Measurements were conducted from 2007 to 2016 and thus, above canopy fluxes were recorded two years before and eight years after the thinning. We also measured the net flux from the forest floor with automatic chambers in three locations and with below-canopy EC during shorter periods before and after thinning. The chamber measurements during the first part of the growing season after thinning showed strongly enhanced effluxes in the order of 150–250% of the pre-thinning values. These chamber measurements were made on drier places within the thinned area because waterlogging made it impossible to use chambers at all available locations. The below-canopy EC measurements, which had a larger footprint as compared to the chambers, showed less enhanced fluxes (in the order of 35%). This footprint included also wetter areas.The above canopy EC measurements showed a reduction of daytime net flux by approx. 30% during the first summer after thinning. The median growing season fluxes then slowly increased but were not restored to the pre-thinning levels eight years after thinning. There was also a small decrease in growing season ecosystem respiration during the first summer after thinning and with a continued decreasing trend over time. It was concluded that this decrease in respiration was caused by successively decreasing decomposition of coarse organic substrates resulting from the thinning. This respiration decrease over time persisted even under gradual biomass increase, which otherwise would indicate increasing autotrophic respiration. The light-response and respiration models fitted to all data did not show any trends in daytime or nighttime fluxes so the conclusion was that the trends were caused by the thinning and not because of trends in meteorological drivers. The annual values contrasted with the summertime results since only a minor effect was observed on the annual NEE. Both ecosystem respiration and gross primary productivity were reduced as an effect of thinning. We explained the different summertime versus annual effects to be caused by the decrease in ecosystem respiration since respiration is dominating the NEE during non-growing season periods when photosynthesis is very low or even zero. Our results are a strong indication that the NEE of a forest could be maintained over time with harvesting practices that avoids clear-cutting and thereby enhance the total carbon uptake of forests.

In Situ Observations Reveal How Spectral Reflectance Responds to Growing Season Phenology of an Open Evergreen Forest in Alaska

Plant phenology timings, such as spring green-up and autumn senescence, are essential state information characterizing biological responses and terrestrial carbon cycles. Current efforts for the in situ reflectance measurements are not enough to obtain the exact interpretation of how seasonal spectral signature responds to phenological stages in boreal evergreen needleleaf forests. This study shows the first in situ continuous measurements of canopy scale (overstory + understory) and understory spectral reflectance and vegetation index in an open boreal forest in interior Alaska. Two visible and near infrared spectroradiometer systems were installed at the top of the observation tower and the forest understory, and spectral reflectance measurements were performed in 10 min intervals from early spring to late autumn. We found that canopy scale normalized difference vegetation index (NDVI) varied with the solar zenith angle. On the other hand, NDVI of understory plants was less sensitive to the solar zenith angle. Due to the influence of the solar geometry, the annual maximum canopy NDVI observed in the morning satellite overpass time (10–11 am) shifted to the spring direction compared with the standardized NDVI by the fixed solar zenith angle range (60−70°). We also found that the in situ NDVI time-series had a month-long high NDVI plateau in autumn, which was completely out of photosynthetically active periods when compared with eddy covariance net ecosystem exchange measurements. The result suggests that the onset of an autumn high NDVI plateau is likely to be the end of the growing season. In this way, our spectral measurements can serve as baseline information for the development and validation of satellite-based phenology algorithms in the northern high latitudes.

Carbon Flux Phenology from the Sky: Evaluation for Maize and Soybean

AbstractCarbon flux phenology is widely used to understand carbon flux dynamics and surface exchange processes. Vegetation phenology has been widely evaluated by remote sensors; however, very few studies have evaluated the use of vegetation phenology for identifying carbon flux phenology. Currently available techniques to derive net ecosystem exchange (NEE) from a satellite image use a single generic modeling subgroup for agricultural crops. But, carbon flux phenological processes vary highly with crop types and land management practices; this paper reexamines this assumption. Presented here are an evaluation of ground-truth remotely sensed vegetation indices with in situ NEE measurements and an identification of vegetation indices for estimating carbon flux phenology metrics by crop type. Results show that the performance of different vegetation indices as an indicator of phenology varies with crop type, particularly when identifying the start of a season and the peak of a season. Maize fields require vegetation indices that make use of the near-infrared and red reflectance bands, while soybean fields require those making use of the shortwave infrared (IR) and near-IR bands. In summary, the study identifies how to best utilize remote sensing technology as a crop-specific measurement tool.

Comparing ecosystem and soil respiration: Review and key challenges of tower-based and soil measurements

The net ecosystem exchange (NEE) is the difference between ecosystem CO2 assimilation and CO2 losses to the atmosphere. Ecosystem respiration (Reco), the efflux of CO2 from the ecosystem to the atmosphere, includes the soil-to-atmosphere carbon flux (i.e., soil respiration; Rsoil) and aboveground plant respiration. Therefore, Rsoil is a fraction of Reco and theoretically has to be smaller than Reco at daily, seasonal, and annual scales. However, several studies estimating Reco with the eddy covariance technique and measuring Rsoil within the footprint of the tower have reported higher Rsoil than Reco at different time scales. Here, we compare four different and contrasting ecosystems (from forest to grasslands, and from boreal to semiarid) to test if measurements of Reco are consistently higher than Rsoil. In general, both fluxes showed similar temporal patterns, but Reco was not consistently higher than Rsoil from daily to annual scales across sites. We identified several issues that apply for measuring NEE and measuring/upscaling Rsoil that could result in an underestimation of Reco and/or an overestimation of Rsoil. These issues are discussed based on (a) nighttime measurements of NEE, (b) Rsoil measurements, and (c) the interpretation of the functional relationships of these fluxes with temperature (i.e., Q10). We highlight that there is still a need for better integration of Rsoil with eddy covariance measurements to address challenges related to the spatial and temporal variability of Reco and Rsoil.