Carbon Dioxide Flux Measurements Research Articles

Carbon-scaled nitrous oxide emissions better reflect the impacts of land use changes than raw nitrous oxide emissions in the Dry Chaco region

In the Dry Chaco region, agriculture expansion has caused significant land use change hotspots. However, the post-impact of land use change on nitrous oxide (N2O) emissions and carbon (C) budgets remains unknown. This study aimed to contrast the impacts of the main land use systems on N2O emissions related to its C inputs and C budgets by comparing them with those of a native forest at two sites of the Dry Chaco region of Argentina. At Site 1, the land use system were soybean-fallow-soybean and maize-fallow-maize sequences, whereas at Site 2, it was a soybean-wheat sequence. Measurements of soil N2O and carbon dioxide (CO2) fluxes were carried out monthly using the static chamber method. The C budgets of each system were determined for the annual crop-fallow cycle by the difference between the C inputs (from annual aboveground (ABG), belowground (BG), and rhizodeposition) and C outputs (defined as cumulative CO2-C emissions). At Site 1, the native forest showed 168 and 50 % more cumulative N2O emissions than maize and soybean, respectively. However, most land use differences were based on C inputs. Thus, when the cumulative N2O emissions of each system were related to their C inputs, the N2O emissions per ton of C entered of the native forest were lower than those of soybean and similar to those of maize. The C budgets (± standard error) at Site 1 were 6.4 ± 1.3, 1.0 ± 0.3 and −0.7 ± 0.6 t C ha−1 yr−1 for native forest, maize and soybean, respectively. At Site 2, they were 3.1 ± 0.7 and −4.0 ± 0.6 t C ha−1 yr−1 for the native forest and the soybean-wheat sequence, respectively. This paper proposes a comprehensive approach that integrates C inputs and budgets when evaluating N2O emissions from different land uses as a guide to define mitigating management practices and considers a native vegetation system to unmask the real impacts of agroecosystems.

Interannual Variation and Trend of Carbon Budget Observed for More Than Two Decades at Takayama in a Cool‐Temperate Deciduous Forest in Central Japan

AbstractLong‐term carbon dioxide (CO2) flux measurements between the atmosphere and the ecosystem have been made since 1993 at a cool‐temperate deciduous forest site (Takayama) in Japan influenced by the Asian Monsoon, constituting the longest data set among all the AsiaFlux sites. Interannual variations (IAVs) and trends in the annual carbon budget components and related environmental factors were examined. Annual net ecosystem production (NEP) (mean ± 1σ) during the period of eddy covariance measurements in 1999–2021 was 265 ± 86 gC m−2 yr−1, and its IAV was dependent more on gross primary production (GPP) than on ecosystem respiration. The annual NEP and GPP were correlated with the monthly mean NEP, GPP and leaf area index from June to September, as well as with the length of the net carbon uptake period. Significantly increasing and decreasing trends in the annual NEP and GPP were detected during 2004–2013 and 2013–2021, respectively. The increasing trends were mainly caused by the vegetation recovery from typhoon disturbances. On the other hand, although the cause of the decreasing trends was not clearly identified, recent extreme weather events and/or forest succession might be related to the trends. Significant positive correlations between the start and the end of the net carbon uptake period, and between the leaf expansion and leaf fall were found. These may be attributed to phenological characteristics and negative correlation between air temperature in spring and solar radiation in early fall associated with ENSO events that can also influence IAVs in annual NEP and GPP.

Validation and field application of a low-cost device to measure CO2 and evapotranspiration (ET) fluxes

Abstract. Mitigating the global climate crisis and its consequences, such as more frequent and severe droughts, is one of the major challenges for future agriculture. Therefore, identifying land use systems and management practices that reduce greenhouse gas (GHG) emissions and promote water use efficiency (WUE) is crucial. This, however, requires accurate and precise measurements of carbon dioxide (CO2) fluxes and evapotranspiration (ET). Despite that, commercial systems to measure CO2 and ET fluxes are expensive and thus often exclude research in ecosystems within the Global South. This is especially true for research and data of agroecosystems in these areas, which are to date still widely underrepresented. Here, we present a newly developed low-cost, non-dispersive infrared (NDIR)-based CO2 and ET flux measurement device (∼ EUR 200) that provides reliable, accurate and precise CO2 and ET flux measurements in conjunction with manually operated closed chambers. To validate the system, laboratory and field validation experiments were performed, testing multiple different low-cost sensors. We demonstrate that the system delivers accurate and precise CO2 and ET flux measurements using the K30 FR NDIR (CO2) and SHT31 (RH, relative humidity) sensor. An additional field trial application demonstrated its longer-term stability (> 3 months) and ability to obtain valid net ecosystem C balances (NECBs) and WUE. This was the case, even though environmental conditions at the field trial application site in sub-Saharan Africa were rather challenging (e.g., extremely high temperatures, humidity and rainfall). Consequently, the developed low-cost CO2 and ET flux measurement device not only provides reasonable results but also might help with democratizing science and closing current data gaps.

The influence of the hydrometeorological factors on the CO2 fluxes from the oligotrophic bog surface.

The influence of the hydrometeorological factors on the CO2 fluxes from the oligotrophic bog surface.

Measurements of Carbon Assimilation in Aquatic Systems.

The recent development of an infrared gas analyzer capable of making carbon dioxide flux measurements from aquatic samples has enabled a new sphere of photosynthesis research. This study details key photosynthesis measurements on four aquatic and hydrophytic species, diverse in their morphology, physiology, and habitat. This guide specifies the methods and procedures needed to make reliable and accurate gas exchange measurements, with examples of data correction and presentation.

Results of daily measurements of carbon dioxide fluxes in an oligotrophic bog in southern Karelia

Results of daily measurements of carbon dioxide fluxes in an oligotrophic bog in southern Karelia

Rewetting increases vegetation cover and net growing season carbon uptake under fen conditions after peat-extraction in Manitoba, Canada

The moss layer transfer technique has been developed to restore the carbon sequestration function and typical vegetation of Sphagnum-dominated peatlands after peat extraction in North America. However, the technique does not lead to successful bryophyte establishment when applied to peatlands with a richer residual fen peat. Therefore, we evaluated an alternative method of active rewetting and passive vegetation establishment using vegetation surveys and carbon dioxide and methane (CH4) flux measurements at a post-extracted fen in southern Manitoba, Canada. After one growing season post-rewetting, wetland vegetation established and the site was a net carbon sink over the growing season. However, high abundance of Carex lasiocarpa 10 years post-treatment led to higher CH4 emissions than the reference ecosystem. Successful establishment of wetland vegetation is attributed to the area being surrounded by undisturbed fens that can provide a local source of plant propagules. Bryophyte expansion was less successful than vascular plants, likely due to episodic flooding and shading from the sedge communities. Therefore, careful management of water levels to just below the peat surface is needed if reference vegetation community recovery is the goal of restoration. Water level management will also play a key role in controlling CH4 emissions to maximize carbon sequestration potential.

Impacts of leaks and gas accumulation on closed chamber methods for measuring methane and carbon dioxide fluxes from tree stems

Accurate measurements of methane (CH4) and carbon dioxide (CO2) fluxes from tree stems are important for understanding greenhouse gas emissions. Closed chamber methods are commonly employed for this purpose; however, leaks between the chamber and the atmosphere as well as gas accumulation, known as the concentration buildup effect, can impact flux measurements significantly. In this study, we investigated the impacts of concentration buildup and leaks on semi-rigid closed chamber methods. Field measurements were conducted on six tree species, including three species from a Mexican mangrove ecosystem and three species from a Magellanic sub-Antarctic forest. Systematic observations revealed significant leak flow rates, ranging from 0.00 to 465 L h−1, with a median value of 1.25 ± 75.67 L h−1. We tested the efficacy of using cement to reduce leaks, achieving a leak flow rate reduction of 46–98 % without complete elimination. Our study also demonstrates a clear and substantial impact of concentration buildup on CH4 flux measurements, while CO2 flux measurements were relatively less affected across all tree species studied. Our results show that the combined effects of leaks and concentration buildup can lead to an underestimation of CH4 emissions by an average of 40 ± 20 % and CO2 emissions by 22 ± 22 %, depending on the bark roughness. Based on these findings, we recall a straightforward yet effective method to minimize experimental errors associated with these phenomena, previously established, and reiterated in the current context, for calculating emissions that considers effects of leaks and concentration buildup, while eliminating the need for separate determinations of these phenomena. Overall, the results, combined with a literature review, suggest that our current estimates of GHG flux from tree stems are currently underestimated.

3D-гидродинамическое моделирование потоков СО2 и CH4 в приземном слое атмосферы (На примере лесного участка «Рошни-Чу»)

Introduction: The aim of this study is to evaluate the spatial variability of carbon dioxide (CO2) and methane (CH4) fluxes at the reference site "Roshni-Chu" of the carbon measurement megasites of the Chechen Republic using modeling experiments with in-situ measurements and remote sensing data. Methods and materials: Measurements of carbon dioxide (CO2) and methane (CH4) fluxes at the soil surface at the "Roshni Chu" forest site in the mountainous forest region of the Chechen Republic were conducted using a dynamic closed chamber connected to a portable gas analyzer G4301 (Picarro, USA). Leaf photosynthesis and respiration parameters of the main edificators and sub-edificators of premontane broadleaf forests were measured using the portable photosynthetic system LI-6800 (LI-COR, USA). Landsat 8 data were used to produce digital maps of surface topography and Nor-malized Difference Vegetation Index (NDVI). A 3D process-based atmospheric transfer model was chosen to describe the spatial variability of carbon dioxide and methane fluxes within the atmospheric boundary layer. The model is based on a one-and-a-half closure scheme for the Navier-Stokes and continuity equations, solved using Reynolds averaging and the Bussiness conjecture. Results: It was shown that a three-dimensional (3D) mathematical transfer models based on the solution of the equations of thermo-hydrodynamics are among the most effective methods for estimating vertical and horizontal fluxes of greenhouse gases in the atmosphere, taking into account the heterogeneous vegetation structure and surface topography. Based on the modeling results, maps of spatial distribution of turbulent exchange coefficient and horizontal wind speed at 5 m height, and maps of spatial distribution of CO2 and CH4 methane fluxes at 5, 25 and 50 m height were created. It was revealed that the "Roshni-Chu" forest area serves as a CO2 sink from the atmosphere under warm sunny weather in summer. The greatest uptake is detected near the local elevations. The fluxes of CH4 are almost negative, the lowest values of CH4 uptake are connected with uneven topography and are observed in small depressions between the hills. Conclusions: Based on the study of the wind field and the greenhouse gas fluxes at the carbon experimental site "Roshni-Chu" in the Chechen Republic, a significant spatial heterogeneity of the vertical CO2 and CH4 fluxes was revealed. The model approach, together with field measurements and remote sensing data, can be very effective for assessing the spatial heterogeneity of greenhouse gas fluxes at sites with non-uniform topography and vegetation. The method of modeling the spatial distribution of CO2 and CH4 fluxes within the atmospheric boundary layer can be used in different forest regions of the North Caucasus to describe the regional greenhouse gas balance.

Salinization, inundation and tree mortality interact to affect greenhouse gas emissions from stressed coastal forests

Sea level rise and intensifying storms cause salinization and freshwater inundation of coastal forest soils which can result in tree mortality and altered ecosystem carbon (C) cycling. However, it is not yet clear if increased salinity and inundation will affect greenhouse gas (GHG) emissions to feed back with climate change. To assess the impacts of in situ chronic and pulsed salinity on GHG fluxes from coastal forests, we made continuous measurements of carbon dioxide and methane fluxes from intact soil cores collected in 1) an upland forest dominated by loblolly pine (Pinus taeda) and a freshwater swamp dominated by baldcypress (Taxodium distichum) 2) adjacent forest stands within forest types experiencing high versus low salinization and associated tree mortality and 3) before and after pulsed salinity from a hurricane related storm surge. In lab mesocosms, all soil cores were exposed to three levels of rainwater addition to assess potential interactive effects between salinization and inundation. We found that chronic salinization and associated tree mortality decreased soil CO2 fluxes in loblolly, but not baldcypress forest with in situ soil inundation patterns potentially driving the site effect. Additionally, in an upland loblolly forest, pulsed salinity from a storm surge exhibited the potential to increase CH4 fluxes. Finally, the effect of rainwater inundation on CH4 fluxes was greater in low compared to high salinity stands suggesting that salinization may have suppressed the effects of rainwater inundation on CH4 fluxes. Overall, we show that complex interactions between biotic and abiotic conditions in stressed coastal forests can alter GHG emissions, highlighting a need for future research focused on understanding the mechanisms driving GHG fluxes from coastal forests under changing environmental conditions.

Greenhouse gas emissions from natural ecosystems of the Norilsk Industrial District

This paper describes the results of field measurements of methane and carbon dioxide fluxes from natural and anthropogenic-modified ecosystems located on the Norilsk industrial district. Previously, such studies have not been conducted in the area. The study points are located in different landscape regions determined by various conditions of permafrost formation. Most of the study area is located within the zone of predominantly continuous permafrost distribution. Six landscape regions were identified. Methane and carbon dioxide fluxes were measured at selected key sites typical for each region. When selecting the study points, the variety of conditions affecting methane and carbon dioxide emissions was considered. First, these include soil type (mineral or peat) and local moisture conditions. Methane and carbon dioxide fluxes were measured by dart static chamber method. Measurements of greenhouse gas concentrations in the chamber were carried out by a portable gas analyzer Li-7810 (Li-COR, USA). The results obtained show significant variability in greenhouse gas emissions for different ecosystem types. Methane uptake by soils is recorded on mineral soils and dry parts of bog ecosystems. Positive methane emissions are typical for watered areas of bog ecosystems and lakes with maximum values in the hollows. Methane fluxes range from slightly negative in dry bog areas with a median of -0.026 mgCH4/m2/h to emissions of 0.802 mgCH4/m2/h as the median for watered areas of bog ecosystems. Carbon dioxide fluxes are inversely correlated with the ecosystem moisture content and have a range from 51.6 mgCO2/m2/h (median for lakes) to 576 mgCO2/m2/h (median for mineral soils). A medium strength correlation was found between surface air temperature and intensity of methane uptake by mineral soils was found. Probability density distributions of methane and carbon dioxide fluxes have different forms.

Fluxes of energy, water and CO2 above a tropical monsoon forest in Dong Nai: results of 10 years of measurements

This paper explores more than 10-year continuous measurements of energy, water and carbon dioxide fluxes on a first-established eddy covariance site in Vietnam above a mature semi-evergreen tropical seasonal forest. Daily, seasonal and interannual variations of turbulent fluxes and meteorological parameters were studied. Evapotranspiration decreased in the second half of a dry season and net ecosystem exchange of CO2 increased to positive values (the tropical forest becomes a source of CO2 instead of a sink) in a hot-dry period of a year (March-May). Net radiation was the parameter with minor interannual and seasonal variation. Precipitation varied significantly from year to year and the variation was great between dry seasons. Variations of precipitation affected the annual rate of evapotranspiration but had no proven effect on the annual net ecosystem exchange of CO2. The ecosystem of tropical forest was the considerable net sink of CO2 from the atmosphere.

Environmental controls on the light use efficiency of terrestrial gross primary production.

Gross primary production (GPP) by terrestrial ecosystems is a key quantity in the global carbon cycle. The instantaneous controls of leaf-level photosynthesis are well established, but there is still no consensus on the mechanisms by which canopy-level GPP depends on spatial and temporal variation in the environment. The standard model of photosynthesis provides a robust mechanistic representation for C3 species; however, additional assumptions are required to "scale up" from leaf to canopy. As a consequence, competing models make inconsistent predictions about how GPP will respond to continuing environmental change. This problem is addressed here by means of an empirical analysis of the light use efficiency (LUE) of GPP inferred from eddy covariance carbon dioxide flux measurements, in situ measurements of photosynthetically active radiation (PAR), and remotely sensed estimates of the fraction of PAR (fAPAR) absorbed by the vegetation canopy. Focusing on LUE allows potential drivers of GPP to be separated from its overriding dependence on light. GPP data from over 100 sites, collated over 20 years and located in a range of biomes and climate zones, were extracted from the FLUXNET2015 database and combined with remotely sensed fAPAR data to estimate daily LUE. Daytime air temperature, vapor pressure deficit, diffuse fraction of solar radiation, and soil moisture were shown to be salient predictors of LUE in a generalized linear mixed-effects model. The same model design was fitted to site-based LUE estimates generated by 16 terrestrial ecosystem models. The published models showed wide variation in the shape, the strength, and even the sign of the environmental effects on modeled LUE. These findings highlight important model deficiencies and suggest a need to progress beyond simple "goodness of fit" comparisons of inferred and predicted carbon fluxes toward an approach focused on the functional responses of the underlying dependencies.

Multiple gap-filling for eddy covariance datasets

With novel developments in technology, eddy covariance flux measurements have become feasible for a variety of trace gases. While the statistical properties and gap-filling strategies have been well examined for carbon dioxide, these are much less understood for other gases.Here, we propose a universal methodology deploying multiple gap-filling techniques and artificial gap scenarios to evaluate the techniques’ performances, infer the statistical flux properties, and fill the real gaps in eddy covariance datasets of any trace gas. The methodology was implemented in a gap-filling framework with techniques spanning from simple and diurnal interpolations, look-up tables, artificial neural networks, to an inferential model. For the new scheme of half-hourly and daily artificial gaps, each additional gap was superimposed one at a time (thus keeping the disturbance to a minimum) for the whole dataset and the scenarios were resampled by bootstrapping. The gap-filled sums were then estimated from the ensemble of well-performing gap-filling techniques. The gap-filling framework was applied to campaign data of three different trace gases (51 days of ammonia, 79 days of total reactive nitrogen, and 89 days of methane flux measurements). The aggregated fluxes are stated as ensemble ranges of multiple techniques plus the techniques’ uncertainties. Additionally, the framework was used to gap-fill a full year of carbon dioxide flux measurements yielding similar performances as previously reported.Based on a review of gap-filling comparison studies and on our findings, we suggest reconsidering the standard procedure of using one gap-filling technique for multi-site studies. Deploying multiple gap-filling techniques and providing ensemble results of gap-filled sums will help to minimize the influence of a single technique and thus lead to a more robust flux aggregation. Furthermore, the estimated overall uncertainty will be more realistic by accounting for the ensemble range of multiple techniques.

Intercomparison of methods to estimate gross primary production based on CO2 and COS flux measurements.

Separating the components of ecosystem-scale carbon exchange is crucial in order to develop better models and future predictions of the terrestrial carbon cycle. However, there are several uncertainties and unknowns related to current photosynthesis estimates. In this study, we evaluate four different methods for estimating photosynthesis at a boreal forest at the ecosystem scale, of which two are based on carbon dioxide (CO2) flux measurements and two on carbonyl sulfide (COS) flux measurements. The CO2-based methods use traditional flux partitioning and artificial neural networks to separate the net CO2 flux into respiration and photosynthesis. The COS-based methods make use of a unique 5-year COS flux data set and involve two different approaches to determine the leaf-scale relative uptake ratio of COS and CO2 (LRU), of which one (LRUCAP) was developed in this study. LRUCAP was based on a previously tested stomatal optimization theory (CAP), while LRUPAR was based on an empirical relation to measured radiation.For the measurement period 2013–2017, the artificial neural network method gave a GPP estimate very close to that of traditional flux partitioning at all timescales. On average, the COS-based methods gave higher GPP estimates than the CO2-based estimates on daily (23% and 7% higher, using LRUPAR and LRUCAP, respectively) and monthly scales (20% and 3% higher), as well as a higher cumulative sum over 3 months in all years (on average 25% and 3% higher). LRUCAP was higher than LRU estimated from chamber measurements at high radiation, leading to underestimation of midday GPP relative to other GPP methods. In general, however, use of LRUCAP gave closer agreement with CO2-based estimates of GPP than use of LRUPAR. When extended to other sites, LRUCAP may be more robust than LRUPAR because it is based on a physiological model whose parameters can be estimated from simple measurements or obtained from the literature. In contrast, the empirical radiation relation in LRUPAR may be more site-specific. However, this requires further testing at other measurement sites.

Initial effects of post-harvest ditch cleaning on greenhouse gas fluxes in a hemiboreal peatland forest

Ditch cleaning (DC) is a well-established forestry practice across Fennoscandia to lower water table levels (WTL) and thereby facilitate the establishment of tree seedlings following clear-cutting. However, the implications from these activities for ecosystem-atmosphere greenhouse gas (GHG) exchanges are poorly understood at present. We assessed the initial DC effects on the GHG fluxes in a forest clear-cut on a drained fertile peatland in hemiboreal Sweden, by comparing chamber measurements of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) fluxes from soil and ditches in DC and uncleaned (UC) areas over the first two post-harvest years. We also evaluated spatial effects by comparing fluxes at 4 m and 40 m from ditches. We found that 2 years after DC, mean (±standard error) WTL of −65 ± 2 cm was significantly lower in the DC area compared to −56 ± 2 cm in the UC area. We further observed lower gross primary production and ecosystem respiration in the first year after DC which coincided with delayed development of herbaceous ground vegetation. We also found higher CH4 uptake but no difference in N2O fluxes after DC. Greater CH4 uptake occurred at 4 m compared to 40 m away from both cleaned and uncleaned ditches. Model extrapolation suggests that total annual GHG emissions in the second year were reduced from 49.4 ± 17.0 t-CO2-eq-ha−1-year−1 in the UC area to 27.8 ± 10.3 t-CO2-eq-ha−1-year−1 in the DC area. A flux partitioning approach suggested that this was likely caused by decreased heterotrophic respiration, possibly because of enhanced soil dryness following DC during the dry meteorological conditions. CH4 and N2O fluxes from clear-cut areas contributed <2 % to the total (soil, ditches) GHG budget. Similarly the area-weighted contributions by CO2 and CH4 emissions from both cleaned and uncleaned ditches were <2 %. Thus, our study highlights that DC may considerably alter the post-harvest GHG fluxes of drained peatland forests. However, long-term observations under various site conditions and forest rotation stages are warranted to better understand DC effects on the forest GHG balance.

New Evidence for the Importance of Non‐Stomatal Pathways in Ozone Deposition During Extreme Heat and Dry Anomalies

AbstractDry deposition could partially explain the observed response in ambient ozone to extreme hot and dry episodes. We examine the response of ozone deposition to heat and dry anomalies using three long‐term co‐located ecosystem‐scale carbon dioxide, water vapor and ozone flux measurement records. We find that, as expected, canopy stomatal conductance generally decreases during days with dry air or soil. However, during hot days, concurrent increases in non‐stomatal conductance are inferred at all three sites, which may be related to several temperature‐sensitive processes not represented in the current generation of big‐leaf models. This may offset the reduction in stomatal conductance, leading to smaller net reduction, or even net increase, in total deposition velocity. We find the response of deposition velocity to soil dryness may be related to its impact on photosynthetic activity, though considerable variability exists. Our findings emphasize the need for better understanding and representation of non‐stomatal ozone deposition.

Carbon dioxide fluxes of a mountain grassland: Drivers, anomalies and annual budgets

The potential of grasslands to mitigate climate change by carbon (C) sequestration in soils depends on agricultural management and the response of the grassland C cycle to a warming and more variable climate. A 15-year time series of eddy covariance carbon dioxide (CO2) flux measurements on a medium-intensively managed grassland at 1000 m above sea level in Switzerland was analysed to assess how CO2 fluxes respond to environmental drivers as well as grazing and cutting events. Further, the effects of management, recent warming trends and anomalous weather conditions on the annual C sequestration potential were assessed. To this end, C budgets including atmospheric and management-related C fluxes (fertilization, cutting/grazing) were estimated. Generally, the main environmental drivers of CO2 fluxes were light and temperature. The removal of the aboveground canopy by management significantly reduced photosynthesis and induced a saturation in the photosynthetic response to light at lower intensities compared to periods without management. Ecosystem respiration remained largely unaffected by management. With a net ecosystem exchange of −357 (± 76) g C m−2 year−1 and a net biome production of −154 (± 80) g C m−2 year−1 (including management C fluxes), the grassland on average acted as a net C sink. Recent climate warming favors spring CO2 assimilation due to earlier starting vegetation periods. However, the warming also resulted in an increased ecosystem respiration. Further, anomalous and especially unfavorable weather conditions for photosynthesis and regrowth following harvest events induced several periods of an anomalously increased net CO2 release. In the year 2015, such release anomalies contributed to an annual net C loss. Under future climatic conditions, the C sequestration potential at the mountain site will depend on the extent to which spring photosynthesis is offset by such climate-management interactions and a warming-induced increase in ecosystem respiration.

Carbon Exchange and Accumulation in an Orinoco High Plains Native Savanna Ecosystem as Measured by Eddy Covariance

Savanna ecosystems cover ∼20% of the total land surface and account for ∼30% of the terrestrial global net primary production. They are also highly sensitive to climate change, since their carbon (C) sink capacity may decline under rising temperatures and irregular rainfall. These responses, which will define the future climate role of the savanna ecosystems, are currently not well understood. The Colombian Orinoco River basin (“Llanos”) natural savannas are being rapidly converted to agriculture. The impact of this transformation on C fluxes and accumulation is not clear. It is thus urgent to understand the Llanos natural savanna ecosystem services, including their C cycle and underlying mechanisms. Here we report and analyze 2 years of measurements of carbon dioxide fluxes from a naturally-restored (secondary) Llanos High Plains savanna ecosystem, using eddy covariance. Meteorological conditions, particularly rainfall, were quite variable during the measurement period. During the first year of measurements, the savanna was a weak carbon source (35 gC m−2), while during the second year, the system was a comparatively strong carbon sink (−273 gC m−2), despite receiving less rainfall than during the first year. As expected, the savanna net ecosystem exchange (NEE) was highly dependent on global solar radiation, soil water content, and ecosystem respiration. We found that after ∼10 days of nominal drought, i.e., with less than ∼5 mm/day of precipitation, the NEE became highly dependent on drought duration. The ecosystem reached a critical condition of low photosynthetic activity after ∼60 days of nominal drought. Based on these findings, we developed and applied a simple standard meteorology-based model that properly reproduced the observations. Our results indicate that a shift to a climate with similar total precipitation but split into extreme dry and wet seasons might eventually suppress the savanna C uptake capacity.

Simulating shrubs and their energy and carbon dioxide fluxes in Canada's Low Arctic with the Canadian Land Surface Scheme Including Biogeochemical Cycles (CLASSIC)

Abstract. Climate change in the Arctic is leading to shifts in vegetation communities, permafrost degradation and alteration of tundra surface–atmosphere energy and carbon (C) fluxes, among other changes. However, year-round C and energy flux measurements at high-latitude sites remain rare. This poses a challenge for evaluating the impacts of climate change on Arctic tundra ecosystems and for developing and evaluating process-based models, which may be used to predict regional and global energy and C feedbacks to the climate system. Our study used 14 years of seasonal eddy covariance (EC) measurements of carbon dioxide (CO2), water and energy fluxes, and winter soil chamber CO2 flux measurements at a dwarf-shrub tundra site underlain by continuous permafrost in Canada’s Southern Arctic ecozone to evaluate the incorporation of shrub plant functional types (PFTs) in the Canadian Land Surface Scheme Including Biogeochemical Cycles (CLASSIC), the land surface component of the Canadian Earth System Model. In addition to new PFTs, a modification of the efficiency with which water evaporates from the ground surface was applied. This modification addressed a high ground evaporation bias that reduced model performance when soils became very dry, limited heat flow into the ground, and reduced plant productivity through water stress effects. Compared to the grass and tree PFTs previously used by CLASSIC to represent the vegetation in Arctic permafrost-affected regions, simulations with the new shrub PFTs better capture the physical and biogeochemical impact of shrubs on the magnitude and seasonality of energy and CO2 fluxes at the dwarf-shrub tundra evaluation site. The revised model, however, tends to overestimate gross primary productivity, particularly in spring, and overestimated late-winter CO2 emissions. On average, annual net ecosystem CO2 exchange was positive for all simulations, suggesting this site was a net CO2 source of 18 ± 4 g C m−2 yr−1 using shrub PFTs, 15 ± 6 g C m−2 yr−1 using grass PFTs, and 25 ± 5 g C m−2 yr−1 using tree PFTs. These results highlight the importance of using appropriate PFTs in process-based models to simulate current and future Arctic surface–atmosphere interactions.