Methane emission from Siberian arctic polygonal tundra: eddy covariance measurements and modeling

Eddy covariance measurements of methane flux were carried out in an arctic tundra landscape in the central Lena River Delta at 72°N. The measurements covered the seasonal course of mid‐summer to early winter in 2003 and early spring to mid‐summer in 2004, including the periods of spring thaw and autumnal freeze back. The study site is characterized by very cold and deep permafrost and a continental climate with a mean annual air temperature of ?14.7 °C. The surface is characterized by wet polygonal tundra, with a micro‐relief consisting of raised moderately dry sites, depressed wet sites, polygonal ponds, and lakes. We found relatively low fluxes of typically 30 mg CH₄ m^?2 day^?1 during mid‐summer and identified soil temperature and near‐surface atmospheric turbulence as the factors controlling methane emission. The influence of atmospheric turbulence was attributed to the high coverage of open water surfaces in the tundra. The soil thaw depth and water table position were found to have no clear effect on methane fluxes. The excess emission during spring thaw was estimated to be about 3% of the total flux measured during June–October. Winter emissions were modeled based on the functional relationships found in the measured data. The annual methane emission was estimated to be 3.15 g m^?2. This is low compared with values reported for similar ecosystems. Reason for this were thought to be the very low permafrost temperature in the study region, the sandy soil texture and low bio‐availability of nutrients in the soils, and the high surface coverage of moist to dry micro‐sites. The methane emission accounted for about 14% of the annual ecosystem carbon balance. Considering the global warming potential of methane, the methane emission turned the tundra into an effective greenhouse gas source.     

Carbon dioxide and methane exchange of a north-east Siberian tussock tundra

Carbon dioxide, energy flux measurements and methane chamber measurements were carried out in an arctic wet tussock grassland located on a flood plane of the Kolyma river in NE Siberia over a summer period of 155 days in 2002 and early 2003. Respiration was also measured in April 2004. The study region is characterized by late thaw of the top soil (mid of June) and periodic spring floods. A stagnant water table below the grass canopy is fed by thawing of the active layer of permafrost and by flood water. The climate is continental with average daily temperature in the warmest months of 13°C (maximum temperature at midday: 28°C by the end of July), dry air (maximum vapour pressure deficit at midday: 28 hPa) and low rainfall of 50 mm during summer (July–September). Summer evaporation (July–September: 103 mm) exceeded rainfall by a factor of 2. The daily average Bowen ratio (H/LE) was 0.62 during the growing season. Net ecosystem CO₂ uptake reached 10 μmol m⁻² s⁻¹ and was related to photon flux density (PFD) and vapour pressure deficit (VPD). The cumulative annual net carbon flux from the atmosphere to the terrestrial surface was estimated to be about −38 g C m⁻² yr⁻¹ (negative flux depicts net carbon sink). Winter respiration was extrapolated using the Lloyd and Taylor function. The net carbon balance is composed of a high rate of assimilation in a short summer and a fairly large but uncertain respiration mainly during autumn and spring. Methane flux (about 12 g C m⁻² measured over 60 days) was 25% of C uptake during the same period of time (end of July to end of September). Assuming that CH₄ was emitted only in summer, and taking the greenhouse gas warming potential of CH₄ vs. CO₂ into account (factor 23), the study site was a greenhouse gas source (at least 200 g C_equivalent m⁻² yr⁻¹). Comparing different studies in wetlands and tundra ecosystems as related to latitude, we expect that global warming would rather increase than decrease the CO₂-C sink.     

Reduction of ecosystem productivity and respiration during the European summer 2003 climate anomaly: a joint flux tower, remote sensing and modelling analysis

The European CARBOEUROPE/FLUXNET monitoring sites, spatial remote sensing observations via the EOS‐MODIS sensor and ecosystem modelling provide independent and complementary views on the effect of the 2003 heatwave on the European biosphere's productivity and carbon balance. In our analysis, these data streams consistently demonstrate a strong negative anomaly of the primary productivity during the summer of 2003. FLUXNET eddy‐covariance data indicate that the drop in productivity was not primarily caused by high temperatures (‘heat stress’) but rather by limitation of water (drought stress) and that, contrary to the classical expectation about a heat wave, not only gross primary productivity but also ecosystem respiration declined by up to more than to 80 gC m⁻² month⁻¹. Anomalies of carbon and water fluxes were strongly correlated. While there are large between‐site differences in water‐use efficiency (WUE, 1–6 kg C kg⁻¹ H₂O) here defined as gross carbon uptake divided by evapotranspiration (WUE=GPP/ET), the year‐to‐year changes in WUE were small (<1 g kg⁻¹) and quite similar for most sites (i.e. WUE decreased during the year of the heatwave). Remote sensing data from MODIS and AVHRR both indicate a strong negative anomaly of the fraction of absorbed photosynthetically active radiation in summer 2003, at more than five standard deviations of the previous years. The spatial differentiation of this anomaly follows climatic and land‐use patterns: Largest anomalies occur in the centre of the meteorological anomaly (central Western Europe) and in areas dominated by crops or grassland. A preliminary model intercomparison along a gradient from data‐oriented models to process‐oriented models indicates that all approaches are similarly describing the spatial pattern of ecosystem sensitivity to the climatic 2003 event with major exceptions in the Alps and parts of Eastern Europe, but differed with respect to their interannual variability.     

The carbon balance of tropical, temperate and boreal forests

Forest biomes are major reserves for terrestrial carbon, and major components of global primary productivity. The carbon balance of forests is determined by a number of component processes of carbon acquisition and carbon loss, and a small shift in the magnitude of these processes would have a large impact on the global carbon cycle. In this paper, we discuss the climatic influences on the carbon dynamics of boreal, temperate and tropical forests by presenting a new synthesis of micrometeorological, ecophysiological and forestry data, concentrating on three case-study sites. Historical changes in the carbon balance of each biome are also reviewed, and the evidence for a carbon sink in each forest biome and its likely behaviour under future global change are discussed. We conclude that there have been significant advances in determining the carbon balance of forests, but there are still critical uncertainties remaining, particularly in the behaviour of soil carbon stocks.     

Environmental controls on CH4 emission from polygonal tundra on the microsite scale in the Lena river delta,Siberia

The carbon budgets of the atmosphere and terrestrial ecosystems are closely coupled by vertical gas exchange fluxes. Uncertainties remain with respect to high latitude ecosystems and the processes driving their temporally and spatially highly variable methane (CH₄) exchange. Problems associated with scaling plot measurements to larger areas in heterogeneous environments are addressed based on intensive field studies on two nested spatial scales in Northern Siberia. CH₄ fluxes on the microsite scale (0.1–100 m²) were measured in the Lena River Delta from July through September 2006 by closed chambers and were compared with simultaneous ecosystem scale (10⁴–10⁶ m²) flux measurements by the eddy covariance (EC) method. Closed chamber measurements were conducted almost daily on 15 plots in four differently developed polygon centers and on a polygon rim. Controls on CH₄ emission were identified by stepwise multiple regression. In contrast to relatively low ecosystem‐scale fluxes controlled mainly by near‐surface turbulence, fluxes on the microsite scale were almost an order of magnitude higher at the wet polygon centers and near zero at the drier polygon rim and high‐center polygon. Microsite scale CH₄ fluxes varied strongly even within the same microsites. The only statistically significant control on chamber‐based fluxes was surface temperature calculated using the Stefan–Boltzmann equation in the wet polygon centers, whereas no significant control was found for the low emissions from the dry sites. The comparison with the EC measurements reveals differences in controls and the seasonal dynamics between the two measurement scales, which may have consequences for scaling and process‐based models. Despite those differences, closed chamber measurements from within the EC footprint could be scaled by an area‐weighting approach of landcover classes based on high‐resolution imagery to match the total ecosystem‐scale emission. Our nested sampling design allowed for checking scaling results against measurements and to identify potentially missed sources or sinks.     

Seasonal and annual respiration of a ponderosa pine ecosystem

The net ecosystem exchange of CO₂ between forests and the atmosphere, measured by eddy covariance, is the small difference between two large fluxes of photosynthesis and respiration. Chamber measurements of soil surface CO₂ efflux (F_s), wood respiration (F_w) and foliage respiration (F_f) help identify the contributions of these individual components to net ecosystem exchange. Models developed from the chamber data also provide independent estimates of respiration costs. We measured CO₂ efflux with chambers periodically in 1996–97 in a ponderosa pine forest in Oregon, scaled these measurements to the ecosystem, and computed annual totals for respiration by component. We also compared estimated half-hourly ecosystem respiration at night (F_nc) with eddy covariance measurements. Mean foliage respiration normalized to 10 °C was 0.20 μmol m^–2 (hemi-leaf surface area) s^–1, and reached a maximum of 0.24 μmol m^–2 HSA s^–1 between days 162 and 208. Mean wood respiration normalized to 10 °C was 5.9 μmol m^–3 sapwood s^–1, with slightly higher rates in mid-summer, when growth occurs. There was no significant difference (P > 0.10) between wood respiration of young (45 years) and old trees (250 years). Soil surface respiration normalized to 10 °C ranged from 0.7 to 3.0 μmol m^–2 (ground) s^–1 from days 23 to 329, with the lowest rates in winter and highest rates in late spring. Annual CO₂ flux from soil surface, foliage and wood was 683, 157, and 54 g C m^–2 y^–1, with soil fluxes responsible for 76% of ecosystem respiration. The ratio of net primary production to gross primary production was 0.45, consistent with values for conifer sites in Oregon and Australia, but higher than values reported for boreal coniferous forests. Below-ground carbon allocation (root turnover and respiration, estimated as F_s– litterfall carbon) consumed 61% of GPP; high ratios such as this are typical of sites with more water and nutrient constraints. The chamber estimates were moderately correlated with change in CO₂ storage in the canopy (F_stor) on calm nights (friction velocity u* < 0.25 m s^–1; R² = 0.60); F_stor was not significantly different from summed chamber estimates. On windy nights (u* > 0.25 m s^–1), the sum of turbulent flux measured above the canopy by eddy covariance and F_stor was only weakly correlated with summed chamber estimates (R² = 0.14); the eddy covariance estimates were lower than chamber estimates by 50%.     

Do dynamic global vegetation models capture the seasonality of carbon fluxes in the Amazon basin? A data‐model intercomparison

To predict forest response to long‐term climate change with high confidence requires that dynamic global vegetation models (DGVMs) be successfully tested against ecosystem response to short‐term variations in environmental drivers, including regular seasonal patterns. Here, we used an integrated dataset from four forests in the Brasil flux network, spanning a range of dry‐season intensities and lengths, to determine how well four state‐of‐the‐art models (IBIS, ED2, JULES, and CLM3.5) simulated the seasonality of carbon exchanges in Amazonian tropical forests. We found that most DGVMs poorly represented the annual cycle of gross primary productivity (GPP), of photosynthetic capacity (Pc), and of other fluxes and pools. Models simulated consistent dry‐season declines in GPP in the equatorial Amazon (Manaus K34, Santarem K67, and Caxiuanã CAX); a contrast to observed GPP increases. Model simulated dry‐season GPP reductions were driven by an external environmental factor, ‘soil water stress’ and consequently by a constant or decreasing photosynthetic infrastructure (Pc), while observed dry‐season GPP resulted from a combination of internal biological (leaf‐flush and abscission and increased Pc) and environmental (incoming radiation) causes. Moreover, we found models generally overestimated observed seasonal net ecosystem exchange (NEE) and respiration (R_e) at equatorial locations. In contrast, a southern Amazon forest (Jarú RJA) exhibited dry‐season declines in GPP and R_e consistent with most DGVMs simulations. While water limitation was represented in models and the primary driver of seasonal photosynthesis in southern Amazonia, changes in internal biophysical processes, light‐harvesting adaptations (e.g., variations in leaf area index (LAI) and increasing leaf‐level assimilation rate related to leaf demography), and allocation lags between leaf and wood, dominated equatorial Amazon carbon flux dynamics and were deficient or absent from current model formulations. Correctly simulating flux seasonality at tropical forests requires a greater understanding and the incorporation of internal biophysical mechanisms in future model developments.     

Evidence of increased net ecosystem productivity associated with a longer vegetated season in a deciduous forest in south‐central Indiana,USA

Observations of net ecosystem exchange (NEE) of carbon and its biophysical drivers have been collected at the AmeriFlux site in the Morgan‐Monroe State Forest (MMSF) in Indiana, USA since 1998. Thus, this is one of the few deciduous forest sites in the world, where a decadal analysis on net ecosystem productivity (NEP) trends is possible. Despite the large interannual variability in NEP, the observations show a significant increase in forest productivity over the past 10 years (by an annual increment of about 10 g C m^?2 yr^?1). There is evidence that this trend can be explained by longer vegetative seasons, caused by extension of the vegetative activity in the fall. Both phenological and flux observations indicate that the vegetative season extended later in the fall with an increase in length of about 3 days yr^?1 for the past 10 years. However, these changes are responsible for only 50% of the total annual gain in forest productivity in the past decade. A negative trend in air and soil temperature during the winter months may explain an equivalent increase in NEP through a decrease in ecosystem respiration.     

Diurnal and seasonal variation of carbon dioxide exchange from a former true raised bog

Carbon dioxide exchange was measured, using the eddy covariance technique, during a one and a half year period in 1994 and 1995. The measurements took place over a former true raised bog, characterized by a shallow peat layer and a vegetation dominated by Molinia caerulea. The growing season extended from May until late October, with a maximum LAI in August of 1.7. The carbon balance shows a net release of 97 g C m^–2 y^–1 (265 kg C ha^–1 y^–1) from the peat bog ecosystem to the atmosphere. During June, July and August there is net consumption of CO₂, while during the rest of the year there is net production of CO₂. The average daytime assimilation rates ranged between – 0.2 and – 0.5 mg CO₂ m^–2 s^–1 (– 45 and –11.3 μmol CO₂ m^–2 s^–1), in a period where the LAI ranged between 1 and 1.7. A high vapour pressure deficit (> 15 hPa) corresponding with high temperatures was found to reduce the assimilation rate by on average 50%. Apart from these factors, LAI and the soil temperature codetermine the net exchange of CO₂. The total nocturnal respiration during the growing season lies within the same order as the average daytime net assimilation rate. Temperature was found to be the main factor controlling soil respiration, with a Q₁₀ of 4.8.     

Spatial variation in regional CO2 exchange for the Kuparuk River Basin, Alaska over the summer growing season

The spatial and temporal patterns in CO₂ flux for the Kuparuk River Basin, a 9200‐km² watershed located in NE Alaska were estimated using the Regional Arctic CO₂ Exchange Simulator (RACES) for the 1994–1995 growing seasons. RACES uses non‐linear models and a Geographical Information System database (GIS) consisting of the normalized difference vegetation index (NDVI) and dynamic temperature and radiation maps. The spatial and temporal patterns in the NDVI during both growing seasons suggest that ecosystem development occurred 2–4 weeks earlier and was relatively more rapid in the southern portion of the Kuparuk River Basin. Rates of gross primary production (GPP) and whole‐ecosystem respiration (R) were 2–4 fold higher in the southern basin than along the arctic coastal plain depending on time of year. The higher rate of GPP estimated for the southern basin was primarily due to higher NDVI values, while the higher R estimated for the southern basin was due in part to higher temperature and the NDVI. While GPP and R showed strong latitudinal trends, spatial and temporal trends in net ecosystem CO₂ exchange (NEE) were much more variable. Thus, while spatial trends in carbon gain (GPP) and loss (R) were highly correlated, small spatial and temporal differences in these large fluxes (GPP and/or R) lead to corresponding large spatial variations in the NEE.     

中国东北地区阔叶红松林与兴安落叶松林的碳通量特征及其影响因子比较

北半球中高纬度的森林生态系统在全球碳循环过程中扮演着非常重要的角色。基于中国东北地区阔叶红松林与兴安落叶松林2007年和2008年2a生长季的涡度相关通量资料及气象观测资料,比较分析了两类生态系统的碳通量特征及其环境控制因子。结果表明:研究期间,阔叶红松林与兴安落叶松林都表现为碳吸收,强度分别为199gCm-2(阔叶红松林2a生长季平均值)与49gCm-2(兴安落叶松林2008年生长季);阔叶红松林碳吸收强度在生长季的大部分时段都大于兴安落叶松林。半小时尺度上,两类生态系统的呼吸作用均与10cm土壤温度呈显著的指数相关,兴安落叶松林生态系统呼吸的温度敏感性(Q10=3.44)显著大于阔叶红松林(Q10=1.90);日尺度上,阔叶红松林与兴安落叶松林碳释放/吸收的转变临界温度为10℃左右。研究期间,兴安落叶松林生态系统的水分利用效率高于阔叶红松林生态系统。     

Artificial drainage and associated carbon fluxes (CO2/CH4) in a tundra ecosystem

Ecosystem flux measurements using the eddy covariance (EC) technique were undertaken in 4 subsequent years during summer for a total of 562 days in an arctic wet tundra ecosystem, located near Cherskii, Far-Eastern Federal District, Russia. Methane (CH₄) emissions were measured using permanent chambers. The experimental field is characterized by late thawing of permafrost soils in June and periodic spring floods. A stagnant water table below the grass canopy is fed by melting of the active layer of permafrost and by flood water. Following 3 years of EC measurements, the site was drained by building a 3 m wide drainage channel surrounding the EC tower to examine possible future effects of global change on the tundra tussock ecosystem. Cumulative summertime net carbon fluxes before experimental alteration were estimated to be about +15 g C m⁻² (i.e. an ecosystem C loss) and +8 g C m⁻² after draining the study site. When taking CH₄ as another important greenhouse gas into account and considering the global warming potential (GWP) of CH₄ vs. CO₂, the ecosystem had a positive GWP during all summers. However CH₄ emissions after drainage decreased significantly and therefore the carbon related greenhouse gas flux was much smaller than beforehand (475 ± 253 g C-CO₂-e m⁻² before drainage in 2003 vs. 23 ± 26 g C-CO₂-e m⁻² after drainage in 2005).     

Carbon exchange of grazed pasture on a drained peat soil

Land‐use changes have contributed to increased atmospheric CO₂ concentrations. Conversion from natural peatlands to agricultural land has led to widespread subsidence of the peat surface caused by soil compaction and mineralization. To study the net ecosystem exchange of carbon (C) and the contribution of respiration to peat subsidence, eddy covariance measurements were made over pasture on a well‐developed, drained peat soil from 22 May 2002 to 21 May 2003. The depth to the water table fluctuated between 0.02 m in winter 2002 to 0.75 m during late summer and early autumn 2003. Peat soil moisture content varied between 0.6 and 0.7 m³ m^?3 until the water table dropped below 0.5 m, when moisture content reached 0.38 m³ m^?3. Neither depth to water table nor soil moisture was found to have an effect on the rate of night‐time respiration (ranging from 0.4–8.0 μmol CO₂ m^?2 s^?1 in winter and summer, respectively). Most of the variance in night‐time respiration was explained by changes in the 0.1 m soil temperature (r²=0.93). The highest values for daytime net ecosystem exchange were measured in September 2002, with a maximum of ?17.2 μmol CO₂ m^?2 s^?1. Grazing events and soil moisture deficiencies during a short period in summer reduced net CO₂ exchange. To establish an annual C balance for this ecosystem, non‐linear regression was used to model missing data. Annually integrated (CO₂) C exchange for this peat–pasture ecosystem was 45±500 kg C ha^?1 yr^?1. After including other C exchanges (methane emissions from cows and production of milk), the net annual C loss was 1061±500 kg C ha^?1 yr^?1.     

A scaling approach for quantifying the net CO2 flux of the Kuparuk River Basin,Alaska

Net CO₂ flux measurements conducted during the summer and winter of 1994–96 were scaled in space and time to provide estimates of net CO₂ exchange during the 1995–96 (9 May 1995–8 May 1996) annual cycle for the Kuparuk River Basin, a 9200 km² watershed located in NE Alaska. Net CO₂ flux was measured using dynamic chambers and eddy covariance in moist‐acidic, nonacidic, wet‐sedge, and shrub tundra, which comprise 95% of the terrestrial landscape of the Kuparuk Basin. CO₂ flux data were used as input to multivariate models that calculated instantaneous and daily rates of gross primary production (GPP) and whole‐ecosystem respiration (R) as a function of meteorology and ecosystem development. Net CO₂ flux was scaled up to the Kuparuk Basin using a geographical information system (GIS) consisting of a vegetation map, digital terrain map, dynamic temperature and radiation fields, and the models of GPP and R. Basin‐wide estimates of net CO₂ exchange for the summer growing season (9 May?5 September 1995) indicate that nonacidic tundra was a net sink of ?31.7 ± 21.3 GgC (1 Gg = 10⁹ g), while shrub tundra lost 32.5 ± 6.3 GgC to the atmosphere (negative values denote net ecosystem CO₂ uptake). Acidic and wet sedge tundra were in balance, and when integrated for the entire Kuparuk River Basin (including aquatic surfaces), whole basin summer net CO₂ exchange was estimated to be in balance (?0.9 ± 50.3 GgC). Autumn to winter (6 September 1995–8 May 1996) estimates of net CO₂ flux indicate that acidic, nonacidic, and shrub tundra landforms were all large sources of CO₂ to the atmosphere (75.5 ± 8.3, 96.4 ± 11.4, and 43.3 ± 4.7 GgC for acidic, nonacidic, and shrub tundra, respectively). CO₂ loss from wet sedge surfaces was not substantially different from zero, but the large losses from the other terrestrial landforms resulted in a whole basin net CO₂ loss of 217.2 ± 24.1 GgC during the 1995–96 cold season. When integrated for the 1995–96 annual cycle, acidic (66.4 + 25.25 GgC), nonacidic (64.7 ± 29.2 GgC), and shrub tundra (75.8 ± 8.4 GgC) were substantial net sources of CO₂ to the atmosphere, while wet sedge tundra was in balance (0.4 + 0.8 GgC). The Kuparuk River Basin as a whole was estimated to be a net CO₂ source of 218.1 ± 60.6 GgC over the 1995–96 annual cycle. Compared to direct measurements of regional net CO₂ flux obtained from aircraft‐based eddy covariance, the scaling procedure provided realistic estimates of CO₂ exchange during the summer growing season. Although winter estimates could not be assessed directly using aircraft measurements of net CO₂ exchange, the estimates reported here are comparable to measured values reported in the literature. Thus, we have high confidence in the summer estimates of net CO₂ exchange and reasonable confidence in the winter net CO₂ flux estimates for terrestrial landforms of the Kuparuk river basin. Although there is larger uncertainty in the aquatic estimates, the small surface area of aquatic surfaces in the Kuparuk river basin (≈ 5%) presumably reduces the potential for this uncertainty to result in large errors in basin‐wide CO₂ flux estimates.