Wind as a main driver of the net ecosystem carbon balance of a semiarid Mediterranean steppe in the South East of Spain

Despite the advance in our understanding of the carbon exchange between terrestrial ecosystems and the atmosphere, semiarid ecosystems have been poorly investigated and little is known about their role in the global carbon balance. We used eddy covariance measurements to determine the exchange of CO₂ between a semiarid steppe and the atmosphere over 3 years. The vegetation is a perennial grassland of Stipa tenacissima L. located in the SE of Spain. We examined diurnal, seasonal and interannual variations in the net ecosystem carbon balance (NECB) in relation to biophysical variables. Cumulative NECB was a net source of 65.7, 143.6 and 92.1 g C m^?2 yr^?1 for the 3 years studied, respectively. We separated the year into two distinctive periods: dry period and growing season. The ecosystem was a net source of CO₂ to the atmosphere, particularly during the dry period when large CO₂ positive fluxes of up to 15 μmol m^?2 s^?1 were observed in concomitance with large wind speeds. Over the growing season, the ecosystem was a slight sink or neutral with maximum rates of ?2.3 μmol m^?2 s^?1. Rainfall events caused large fluxes of CO₂ to the atmosphere and determined the length of the growing season. In this season, photosynthetic photon flux density controlled day‐time NECB just below 1000 μmol m^?2 s^?1. The analyses of the diurnal and seasonal data and preliminary geological and gas‐geochemical evaluations, including C isotopic analyses, suggest that the CO₂ released was not only biogenic but most likely included a component of geothermal origin, presumably related to deep fluids occurring in the area. These results highlight the importance of considering geological carbon sources, as well as the need to carefully interpret the results of eddy covariance partitioning techniques when applied in geologically active areas potentially affected by CO₂‐rich geofluid circulation.     

Severe drought effects on ecosystem CO2 and H2O fluxes at three Mediterranean evergreen sites: revision of current hypotheses?

Eddy covariance and sapflow data from three Mediterranean ecosystems were analysed via top‐down approaches in conjunction with a mechanistic ecosystem gas‐exchange model to test current assumptions about drought effects on ecosystem respiration and canopy CO₂/H₂O exchange. The three sites include two nearly monospecific Quercus ilex L. forests – one on karstic limestone (Puéchabon), the other on fluvial sand with access to ground water (Castelporziano) – and a typical mixed macchia on limestone (Arca di Noè). Estimates of ecosystem respiration were derived from light response curves of net ecosystem CO₂ exchange. Subsequently, values of ecosystem gross carbon uptake were computed from eddy covariance CO₂ fluxes and estimates of ecosystem respiration as a function of soil temperature and moisture. Bulk canopy conductance was calculated by inversion of the Penman‐Monteith equation. In a top‐down analysis, it was shown that all three sites exhibit similar behaviour in terms of their overall response to drought. In contrast to common assumptions, at all sites ecosystem respiration revealed a decreasing temperature sensitivity ( Q ₁₀) in response to drought. Soil temperature and soil water content explained 70–80% of the seasonal variability of ecosystem respiration. During the drought, light‐saturated ecosystem gross carbon uptake and day‐time averaged canopy conductance declined by up to 90%. These changes were closely related to soil water content. Ecosystem water‐use efficiency of gross carbon uptake decreased during the drought, regardless whether evapotranspiration from eddy covariance or transpiration from sapflow had been used for the calculation. We evidence that this clearly contrasts current models of canopy function which predict increasing ecosystem water‐use efficiency (WUE) during the drought. Four potential explanations to those results were identified (patchy stomatal closure, changes in physiological capacities of photosynthesis, decreases in mesophyll conductance for CO₂, and photoinhibition), which will be tested in a forthcoming paper. It is suggested to incorporate the new findings into current biogeochemical models after further testing as this will improve estimates of climate change effects on (semi)arid ecosystems' carbon balances.     

Net uptake of atmospheric CO2 by coastal submerged aquatic vegetation

‘Blue Carbon’, which is carbon captured by marine living organisms, has recently been highlighted as a new option for climate change mitigation initiatives. In particular, coastal ecosystems have been recognized as significant carbon stocks because of their high burial rates and long‐term sequestration of carbon. However, the direct contribution of Blue Carbon to the uptake of atmospheric CO₂ through air‐sea gas exchange remains unclear. We performed in situ measurements of carbon flows, including air‐sea CO₂ fluxes, dissolved inorganic carbon changes, net ecosystem production, and carbon burial rates in the boreal (Furen), temperate (Kurihama), and subtropical (Fukido) seagrass meadows of Japan from 2010 to 2013. In particular, the air‐sea CO₂ flux was measured using three methods: the bulk formula method, the floating chamber method, and the eddy covariance method. Our empirical results show that submerged autotrophic vegetation in shallow coastal waters can be functionally a sink for atmospheric CO_2. This finding is contrary to the conventional perception that most near‐shore ecosystems are sources of atmospheric CO₂. The key factor determining whether or not coastal ecosystems directly decrease the concentration of atmospheric CO₂ may be net ecosystem production. This study thus identifies a new ecosystem function of coastal vegetated systems; they are direct sinks of atmospheric CO₂.     

Terrestrial carbon balance in a drier world: the effects of water availability in southwestern North America

Global modeling efforts indicate semiarid regions dominate the increasing trend and interannual variation of net CO₂ exchange with the atmosphere, mainly driven by water availability. Many semiarid regions are expected to undergo climatic drying, but the impacts on net CO₂ exchange are poorly understood due to limited semiarid flux observations. Here we evaluated 121 site‐years of annual eddy covariance measurements of net and gross CO₂ exchange (photosynthesis and respiration), precipitation, and evapotranspiration (ET) in 21 semiarid North American ecosystems with an observed range of 100 – 1000 mm in annual precipitation and records of 4–9 years each. In addition to evaluating spatial relationships among CO₂ and water fluxes across sites, we separately quantified site‐level temporal relationships, representing sensitivity to interannual variation. Across the climatic and ecological gradient, photosynthesis showed a saturating spatial relationship to precipitation, whereas the photosynthesis–ET relationship was linear, suggesting ET was a better proxy for water available to drive CO₂ exchanges after hydrologic losses. Both photosynthesis and respiration showed similar site‐level sensitivity to interannual changes in ET among the 21 ecosystems. Furthermore, these temporal relationships were not different from the spatial relationships of long‐term mean CO₂ exchanges with climatic ET. Consequently, a hypothetical 100‐mm change in ET, whether short term or long term, was predicted to alter net ecosystem production (NEP) by 64 gCm^?2 yr^?1. Most of the unexplained NEP variability was related to persistent, site‐specific function, suggesting prioritization of research on slow‐changing controls. Common temporal and spatial sensitivity to water availability increases our confidence that site‐level responses to interannual weather can be extrapolated for prediction of CO₂ exchanges over decadal and longer timescales relevant to societal response to climate change.     

Radon fluxes in tropical forest ecosystems of Brazilian Amazonia: night-time CO2 net ecosystem exchange derived from radon and eddy covariance methods

Radon‐222 (Rn‐222) is used as a transport tracer of forest canopy–atmosphere CO₂ exchange in an old‐growth, tropical rain forest site near km 67 of the Tapajós National Forest, Pará, Brazil. Initial results, from month‐long periods at the end of the wet season (June–July) and the end of the dry season (November–December) in 2001, demonstrate the potential of new Rn measurement instruments and methods to quantify mass transport processes between forest canopies and the atmosphere. Gas exchange rates yield mean canopy air residence times ranging from minutes during turbulent daytime hours to greater than 12 h during calm nights. Rn is an effective tracer for net ecosystem exchange of CO₂ (CO₂ NEE) during calm, night‐time hours when eddy covariance‐based NEE measurements are less certain because of low atmospheric turbulence. Rn‐derived night‐time CO₂ NEE (9.00±0.99 μmol m^?2 s^?1 in the wet season, 6.39±0.59 in the dry season) was significantly higher than raw uncorrected, eddy covariance‐derived CO₂ NEE (5.96±0.51 wet season, 5.57±0.53 dry season), but agrees with corrected eddy covariance results (8.65±1.07 wet season, 6.56±0.73 dry season) derived by filtering out lower NEE values obtained during calm periods using independent meteorological criteria. The Rn CO₂ results suggest that uncorrected eddy covariance values underestimate night‐time CO₂ loss at this site. If generalizable to other sites, these observations indicate that previous reports of strong net CO₂ uptake in Amazonian terra firme forest may be overestimated.     

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

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

FLUXNET and modelling the global carbon cycle

Measurements of the net CO₂ flux between terrestrial ecosystems and the atmosphere using the eddy covariance technique have the potential to underpin our interpretation of regional CO₂ source–sink patterns, CO₂ flux responses to forcings, and predictions of the future terrestrial C balance. Information contained in FLUXNET eddy covariance data has multiple uses for the development and application of global carbon models, including evaluation/validation, calibration, process parameterization, and data assimilation. This paper reviews examples of these uses, compares global estimates of the dynamics of the global carbon cycle, and suggests ways of improving the utility of such data for global carbon modelling. Net ecosystem exchange of CO₂ (NEE) predicted by different terrestrial biosphere models compares favourably with FLUXNET observations at diurnal and seasonal timescales. However, complete model validation, particularly over the full annual cycle, requires information on the balance between assimilation and decomposition processes, information not readily available for most FLUXNET sites. Site history, when known, can greatly help constrain the model‐data comparison. Flux measurements made over four vegetation types were used to calibrate the land‐surface scheme of the Goddard Institute for Space Studies global climate model, significantly improving simulated climate and demonstrating the utility of diurnal FLUXNET data for climate modelling. Land‐surface temperatures in many regions cool due to higher canopy conductances and latent heat fluxes, and the spatial distribution of CO₂ uptake provides a significant additional constraint on the realism of simulated surface fluxes. FLUXNET data are used to calibrate a global production efficiency model (PEM). This model is forced by satellite‐measured absorbed radiation and suggests that global net primary production (NPP) increased 6.2% over 1982–1999. Good agreement is found between global trends in NPP estimated by the PEM and a dynamic global vegetation model (DGVM), and between the DGVM and estimates of global NEE derived from a global inversion of atmospheric CO₂ measurements. Combining the PEM, DGVM, and inversion results suggests that CO₂ fertilization is playing a major role in current increases in NPP, with lesser impacts from increasing N deposition and growing season length. Both the PEM and the inversion identify the Amazon basin as a key region for the current net terrestrial CO₂ uptake (i.e. 33% of global NEE), as well as its interannual variability. The inversion's global NEE estimate of −1.2 Pg [C] yr⁻¹ for 1982–1995 is compatible with the PEM‐ and DGVM‐predicted trends in NPP. There is, thus, a convergence in understanding derived from process‐based models, remote‐sensing‐based observations, and inversion of atmospheric data. Future advances in field measurement techniques, including eddy covariance (particularly concerning the problem of night‐time fluxes in dense canopies and of advection or flow distortion over complex terrain), will result in improved constraints on land‐atmosphere CO₂ fluxes and the rigorous attribution of mechanisms to the current terrestrial net CO₂ uptake and its spatial and temporal heterogeneity. Global ecosystem models play a fundamental role in linking information derived from FLUXNET measurements to atmospheric CO₂ variability. A number of recommendations concerning FLUXNET data are made, including a request for more comprehensive site data (particularly historical information), more measurements in undisturbed ecosystems, and the systematic provision of error estimates. The greatest value of current FLUXNET data for global carbon cycle modelling is in evaluating process representations, rather than in providing an unbiased estimate of net CO₂ exchange.     

Spatial variability and major controlling factors of CO2 sink strength in Asian terrestrial ecosystems: evidence from eddy covariance data

Asian terrestrial ecosystems cover an extensive area characterized by a large variety in climates and ecosystem properties. The observations of ecosystem CO₂ flux in this area are increasing both in duration and spatial density, but no synthesis has yet been conducted. We surveyed CO₂ flux observation data obtained by eddy covariance methods at 49 sites in terrestrial Asia. The measurements at most sites (44 of 49) began after 2000. The net ecosystem uptake of CO₂ (NEE) varied greatly among sites and years and averaged −132.6±73.7, −250.1±206.1, and −180.1±361.7 g C m⁻² yr⁻¹, in boreal, temperate, and tropical Asia, respectively, and the coefficient of variation among sites increased from boreal to tropical Asia. The site-averaged annual NEE was correlated linearly with the mean annual temperature (T_air) and also correlated logarithmically with the precipitation. Multiple regression analysis and stepwise analysis indicated that photosynthetic active radiation (PAR) and T_air were the most significant predictors of the annual NEE. The study results suggest that Asian terrestrial ecosystems are currently significant net CO₂ sinks and that the sink strength is largely controlled by temperature, moisture, and light conditions.     

Mature semiarid chaparral ecosystems can be a significant sink for atmospheric carbon dioxide

Carbon flux in arid and semiarid area shrublands, especially in old‐growth shrub ecosystems, has been rarely studied using eddy covariance techniques. In this study, eddy covariance measurements over a 100‐year old‐growth chamise‐dominated chaparral shrub ecosystem were conducted for 7 years from 1996 to 2003. A carbon sink, from −96 to −155 g C m⁻² yr⁻¹, was determined under normal weather conditions, while a weak sink of −18 g C m⁻² yr⁻¹ and a strong source of 207 g C m⁻² yr⁻¹ were observed as a consequence of a severe drought. The annual sink strength of carbon in the 7‐year measurement period was −52 g C m⁻² yr⁻¹. The results from our study indicate that, in contrast to previous thought, the old‐growth chaparral shrub ecosystem can be a significant sink of carbon under normal weather conditions and, therefore, be an important component of the global carbon budget.     

Assessing the eddy covariance technique for evaluating carbon dioxide exchange rates of ecosystems: past, present and future

The eddy covariance technique ascertains the exchange rate of CO₂ across the interface between the atmosphere and a plant canopy by measuring the covariance between fluctuations in vertical wind velocity and CO₂ mixing ratio. Two decades ago, the method was employed to study CO₂ exchange of agricultural crops under ideal conditions during short field campaigns. During the past decade the eddy covariance method has emerged as an important tool for evaluating fluxes of carbon dioxide between terrestrial ecosystems and the atmosphere over the course of a year, and more. At present, the method is being applied in a nearly continuous mode to study carbon dioxide and water vapor exchange at over a hundred and eighty field sites, worldwide. The objective of this review is to assess the eddy covariance method as it is being applied by the global change community on increasingly longer time scales and over less than ideal surfaces. The eddy covariance method is most accurate when the atmospheric conditions (wind, temperature, humidity, CO₂) are steady, the underlying vegetation is homogeneous and it is situated on flat terrain for an extended distance upwind. When the eddy covariance method is applied over natural and complex landscapes or during atmospheric conditions that vary with time, the quantification of CO₂ exchange between the biosphere and atmosphere must include measurements of atmospheric storage, flux divergence and advection. Averaging CO₂ flux measurements over long periods (days to year) reduces random sampling error to relatively small values. Unfortunately, data gaps are inevitable when constructing long data records. Data gaps are generally filled with values produced from statistical and empirical models to produce daily and annual sums of CO₂ exchange. Filling data gaps with empirical estimates do not introduce significant bias errors because the empirical algorithms are derived from large statistical populations. On the other hand, flux measurement errors can be biased at night when winds are light and intermittent. Nighttime bias errors tend to produce an underestimate in the measurement of ecosystem respiration. Despite the sources of errors associated with long‐term eddy flux measurements, many investigators are producing defensible estimates of annual carbon exchange. When measurements come from nearly ideal sites the error bound on the net annual exchange of CO₂ is less than ±50 g C m^?2 yr^?1. Additional confidence in long‐term measurements is growing because investigators are producing values of net ecosystem productivity that are converging with independent values produced by measuring changes in biomass and soil carbon, as long as the biomass inventory studies are conducted over multiple years.     

Maximum carbon uptake rate dominates the interannual variability of global net ecosystem exchange

Terrestrial ecosystems contribute most of the interannual variability (IAV) in atmospheric carbon dioxide (CO₂) concentrations, but processes driving the IAV of net ecosystem CO₂ exchange (NEE) remain elusive. For a predictive understanding of the global C cycle, it is imperative to identify indicators associated with ecological processes that determine the IAV of NEE. Here, we decompose the annual NEE of global terrestrial ecosystems into their phenological and physiological components, namely maximum carbon uptake (MCU) and release (MCR), the carbon uptake period (CUP), and two parameters, α and β, that describe the ratio between actual versus hypothetical maximum C sink and source, respectively. Using long‐term observed NEE from 66 eddy covariance sites and global products derived from FLUXNET observations, we found that the IAV of NEE is determined predominately by MCU at the global scale, which explains 48% of the IAV of NEE on average while α, CUP, β, and MCR explain 14%, 25%, 2%, and 8%, respectively. These patterns differ in water‐limited ecosystems versus temperature‐ and radiation‐limited ecosystems; 31% of the IAV of NEE is determined by the IAV of CUP in water‐limited ecosystems, and 60% of the IAV of NEE is determined by the IAV of MCU in temperature‐ and radiation‐limited ecosystems. The Lund‐Potsdam‐Jena (LPJ) model and the Multi‐scale Synthesis and Terrestrial Model Inter‐comparison Project (MsTMIP) models underestimate the contribution of MCU to the IAV of NEE by about 18% on average, and overestimate the contribution of CUP by about 25%. This study provides a new perspective on the proximate causes of the IAV of NEE, which suggest that capturing the variability of MCU is critical for modeling the IAV of NEE across most of the global land surface.     

Highlights for Agave Productivity

Agaves can grow in marginal arid and semiarid lands where their special ecological and physiological adaptations to environmental conditions give them the potential to produce substantial biomass. Agave americana was the first agave species shown to be a Crassulacean Acid Metabolism plant, with CO₂ uptake occurring primarily at night and with high water‐use efficiency (photosynthesis/transpiration). A. salmiana and A. mapisaga can have high nocturnal net CO₂ uptake rates and high productivities averaging 40 tonnes dry weight ha^?1 yr^?1. Agaves can benefit from the increases in temperature and atmospheric CO₂ levels accompanying global climate change. An Environmental Productivity Index can predict the effects of soil and environmental factors on CO₂ uptake and hence on the regions appropriate for cultivating agaves. In turn, their increased cultivation can support the production of innovative earth‐friendly commodities that can be used as new bioenergy feedstocks.     

Growing season net ecosystem CO2 exchange of two desert ecosystems with alkaline soils in Kazakhstan

Central Asia is covered by vast desert ecosystems, and the majority of these ecosystems have alkaline soils. Their contribution to global net ecosystem CO₂ exchange (NEE) is of significance simply because of their immense spatial extent. Some of the latest research reported considerable abiotic CO₂ absorption by alkaline soil, but the rate of CO₂ absorption has been questioned by peer communities. To investigate the issue of carbon cycle in Central Asian desert ecosystems with alkaline soils, we have measured the NEE using eddy covariance (EC) method at two alkaline sites during growing season in Kazakhstan. The diurnal course of mean monthly NEE followed a clear sinusoidal pattern during growing season at both sites. Both sites showed significant net carbon uptake during daytime on sunny days with high photosynthetically active radiation (PAR) but net carbon loss at nighttime and on cloudy and rainy days. NEE has strong dependency on PAR and the response of NEE to precipitation resulted in an initial and significant carbon release to the atmosphere, similar to other ecosystems. These findings indicate that biotic processes dominated the carbon processes, and the contribution of abiotic carbon process to net ecosystem CO₂ exchange may be trivial in alkaline soil desert ecosystems over Central Asia.     

Photodegradation leads to increased carbon dioxide losses from terrestrial organic matter

CO₂ production in terrestrial ecosystems is generally assumed to be solely biologically driven while the role of abiotic processes has been largely overlooked. In addition to microbial decomposition, photodegradation – the direct breakdown of organic matter (OM) by solar irradiance – has been found to contribute to litter mass loss in dry ecosystems. Previous small‐scale studies have shown that litter degradation by irradiance is accompanied by emissions of CO₂. However, the contribution of photodegradation to total CO₂ losses at ecosystems scales is unknown. This study determined the proportion of the total CO₂ losses caused by photodegradation in two ecosystems: a bare peatland in New Zealand and a seasonally dry grassland in California. The direct effect of solar irradiance on CO₂ production was examined by comparing daytime CO₂ fluxes measured using eddy covariance (EC) systems with simultaneous measurements made using an opaque chamber and the soil CO₂ gradient technique, and with night‐time EC measurements under the same soil temperature and moisture conditions. In addition, a transparent chamber was used to directly measure CO₂ fluxes from OM caused by solar irradiance. Photodegradation contributed 19% of the annual CO₂ flux from the peatland and almost 60% of the dry season CO₂ flux from the grassland, and up to 62% and 92% of the summer mid‐day CO₂ fluxes, respectively. Our results suggest that photodegradation may be important in a wide range of ecosystems with exposed OM. Furthermore, the practice of partitioning daytime ecosystem CO₂ exchange into its gross components by assuming that total daytime CO₂ losses can be approximated using estimates of biological respiration alone may be in error. To obtain robust estimates of global ecosystem–atmosphere carbon transfers, the contribution of photodegradation to OM decomposition must be quantified for other ecosystems and the results incorporated into coupled carbon–climate models.     

Diurnal, seasonal and annual variation in the net ecosystem CO2 exchange of a desert shrub community (Sarcocaulescent) in Baja California, Mexico

Estimates of net ecosystem exchange (NEE) of CO₂ have been measured on a variety of ecosystems world wide including grasslands, savannahs, boreal, pine, deciduous, Mediterranean and tropical rain forests as well as arctic tundra. While there have been numerous comparisons between net primary productivity of arid and semiarid grasslands and shrublands, notably lacking are estimates of NEE with a few exceptions. The objective of this study was to characterize the seasonal and annual carbon flux of a desert shrub ecosystem using the eddy covariance technique to determine the sensitivity of the system to the timing and varying amounts of precipitation. Measurements began in July of 2001, a year with 339 mm of rainfall, considerably above the long‐term average of 174 mm and preceded by 2 years of below average rainfall (50–62 mm). Over the 2 complete years of measurements, precipitation was 147 and 197 mm in 2002 and 2003, respectively. In all years, the majority of the precipitation fell between August and September. The site was a sink of ?39 g C m^?2 yr^?1 in 2002 with a relatively strong uptake in the early part of the year and reduced uptake after the suboptimal rainfall in September. This contrasts with 2003 when the ecosystem took up ?52 g C m^?2 yr^?1 concentrated in the fall after significant rain in August and September. Likely, extremely low rainfall years would result in a carbon loss while a strengthening of the typical winter secondary peak in precipitation (notably absent in the 2 years of measurements) may extend uptake into the spring resulting in more carbon accumulation. The system appears to be buffered against variations in annual rainfall attributed to water storage in the stems and roots.     

Carbon cycle responses of semi‐arid ecosystems to positive asymmetry in rainfall

Recent evidence shows that warm semi‐arid ecosystems are playing a disproportionate role in the interannual variability and greening trend of the global carbon cycle given their mean lower productivity when compared with other biomes (Ahlström et al. 2015 Science, 348, 895). Using multiple observations (land‐atmosphere fluxes, biomass, streamflow and remotely sensed vegetation cover) and two state‐of‐the‐art biospheric models, we show that climate variability and extremes lead to positive or negative responses in the biosphere, depending on vegetation type. We find Australia to be a global hot spot for variability, with semi‐arid ecosystems in that country exhibiting increased carbon uptake due to both asymmetry in the interannual distribution of rainfall (extrinsic forcing), and asymmetry in the response of gross primary production (GPP) to rainfall change (intrinsic response). The latter is attributable to the pulse‐response behaviour of the drought‐adapted biota of these systems, a response that is estimated to be as much as half of that from the CO₂ fertilization effect during 1990–2013. Mesic ecosystems, lacking drought‐adapted species, did not show an intrinsic asymmetric response. Our findings suggest that a future more variable climate will induce large but contrasting ecosystem responses, differing among biomes globally, independent of changes in mean precipitation alone. The most significant changes are occurring in the extensive arid and semi‐arid regions, and we suggest that the reported increased carbon uptake in response to asymmetric responses might be contributing to the observed greening trends there.     

Carbon dioxide exchange of a Russian boreal forest after disturbance by wind throw

The exchange of carbon dioxide (CO₂) between the atmosphere and a forest after disturbance by wind throw in the western Russian taiga was investigated between July and October 1998 using the eddy covariance technique. The research area was a regenerating forest (400 m × 1000 m), in which all trees of the preceding generation were uplifted during a storm in 1996. All deadwood had remained on site after the storm and had not been extracted for commercial purposes. Because of the heterogeneity of the terrain, several micrometeorological quality tests were applied. In addition to the eddy covariance measurements, carbon pools of decaying wood in a chronosequence of three different wind throw areas were analysed and the decay rate of coarse woody debris was derived. During daytime, the average CO₂ uptake flux was ?3 µmol m^?2s^?1, whereas during night‐time characterised by a well‐mixed atmosphere the rates of release were typically about 6 µmol m^?2s^?1. Suppression of turbulent fluxes was only observed under conditions with very low friction velocity (u* ≤ 0.08 ms^?1). On average, 164 mmol CO₂ m^?2d^?1 was released from the wind throw to the atmosphere, giving a total of 14.9 mol CO₂ m^?2 (180 g CO₂ m^?2) released during the 3‐month study period. The chronosequence of dead woody debris on three different wind throw areas suggested exponential decay with a decay coefficient of ?0.04 yr^?1. From the magnitude of the carbon pools and the decay rate, it is estimated that the decomposition of coarse woody debris accounted for about a third of the total ecosystem respiration at the measurement site. Hence, coarse woody debris had a long‐term influence on the net ecosystem exchange of this wind throw area. From the analysis performed in this work, a conclusion is drawn that it is necessary to include into flux networks the ecosystems that are subject to natural disturbances and that have been widely omitted into considerations of the global carbon budget. The half‐life time of about 17 years for deadwood in the wind throw suggests a fairly long storage of carbon in the ecosystem, and indicates a very different long‐term carbon budget for naturally disturbed vs. commercially managed forests.     

腾格里荒漠红砂-珍珠群落CO2收支变化及其不同观测方法间的比较

由于荒漠生态系统植被覆盖度低、生产力低下,其在全球碳循环中的作用被长期忽视。为探讨荒漠生态系统碳收支各组分的变化规律,以腾格里荒漠红砂(Reaumuria soongorica Maxim.)-珍珠(Salsola passerina Beg.)群落为研究对象,采用静态箱式法研究了该群落的净生态系统CO2交换量(NEE)、生态系统呼吸、土壤呼吸的日变化规律,同时将该方法所获得的NEE结果与涡动相关法观测的结果进行了比较。结果表明:(1)红砂-珍珠群落NEE的日变化表现为,在6:00—9:00左右出现一个CO2吸收的高峰值,随后在12:00—15:00左右出现一个CO2释放高峰值。红砂种群、珍珠种群和整个群落NEE的平均值分别为0.018、0.020和0.028 mg CO2m-2s-1;(2)红砂种群、珍珠种群、土壤及整个群落生态系统呼吸速率的日变化规律一致,均表现为明显的单峰变化趋势,在12:00—15:00左右出现一个CO2释放的高峰值。红砂种群、珍珠种群、土壤和整个群落的生态系统呼吸的平均值分别为:0.121、0.062、0.029和0.040 mg CO2m-2s-1。以盖度为加权因子计算得到红砂种群、珍珠种群和土壤呼吸占生态系统呼吸的比例分别为:9%、21%和70%,由此可见,生态系统呼吸主要来源于土壤呼吸。(3)将箱式法和涡动相关法观测的NEE进行比较,结果表明两种方法观测的NEE变化规律基本一致,相关系数达到0.7。采用箱式法观测的NEE高于涡动相关法观测的结果,平均值分别0.028 mg CO2m-2s-1(箱式法)和0.015 mg CO2m-2s-1(涡动相关法),涡动相关法的观测结果与箱式法观测结果的比值为0.54。综上可得,荒漠生态系统土壤呼吸的变化速率决定了生态系统呼吸的变化规律,采用箱式法可能高估了荒漠生态系统CO2的释放量。     

Elevated CO2 does not stimulate carbon sink in a semi‐arid grassland

Elevated CO₂ is widely accepted to enhance terrestrial carbon sink, especially in arid and semi‐arid regions. However, great uncertainties exist for the CO₂ fertilisation effects, particularly when its interactions with other global change factors are considered. A four‐factor (CO₂, temperature, precipitation and nitrogen) experiment revealed that elevated CO₂ did not affect either gross ecosystem productivity or ecosystem respiration, and consequently resulted in no changes of net ecosystem productivity in a semi‐arid grassland despite whether temperature, precipitation and nitrogen were elevated or not. The observations could be primarily attributable to the offset of ecosystem carbon uptake by enhanced soil carbon release under CO₂ enrichment. Our findings indicate that arid and semi‐arid ecosystems may not be sensitive to CO₂ enrichment as previously expected and highlight the urgent need to incorporate this mechanism into most IPCC carbon‐cycle models for convincing projection of terrestrial carbon sink and its feedback to climate change.     

Gross primary production controls the subsequent winter CO2 exchange in a boreal peatland

In high‐latitude regions, carbon dioxide (CO₂) emissions during the winter represent an important component of the annual ecosystem carbon budget; however, the mechanisms that control the winter CO₂ emissions are currently not well understood. It has been suggested that substrate availability from soil labile carbon pools is a main driver of winter CO₂ emissions. In ecosystems that are dominated by annual herbaceous plants, much of the biomass produced during the summer is likely to contribute to the soil labile carbon pool through litter fall and root senescence in the autumn. Thus, the summer carbon uptake in the ecosystem may have a significant influence on the subsequent winter CO₂ emissions. To test this hypothesis, we conducted a plot‐scale shading experiment in a boreal peatland to reduce the gross primary production (GPP) during the growing season. At the growing season peak, vascular plant biomass in the shaded plots was half that in the control plots. During the subsequent winter, the mean CO₂ emission rates were 21% lower in the shaded plots than in the control plots. In addition, long‐term (2001–2012) eddy covariance data from the same site showed a strong correlation between the GPP (particularly the late summer and autumn GPP) and the subsequent winter net ecosystem CO₂ exchange (NEE). In contrast, abiotic factors during the winter could not explain the interannual variation in the cumulative winter NEE. Our study demonstrates the presence of a cross‐seasonal link between the growing season biotic processes and winter CO₂ emissions, which has important implications for predicting winter CO₂ emission dynamics in response to future climate change.