The convective boundary layer over pasture and forest in Amazonia

Summary The coupling between different types of surface (tropical forest or grass) and the Convective Boundary Layer (CBL) has been investigated using observational (rawinsoundings) data collected over Rondônia in southwest Amazonia. The data reported here support the notion that deforestation may modify the dynamics of the boundary layer, in particular during the dry season. In this period the sensible heat fluxes are very high over pasture, creating a CBL around 550m deeper compared to that over the forest. The measurements showed the height of the fully developed CBL for pasture to be 1650m, compared to around 1100m for forest. During the wet season the height of the CBL is lower than during the dry season and has the same height (around 1000m) for forest and pasture sites. The CBL over pasture is hotter and drier than over forest during the dry season, but during the wet season the air temperatures and humidities are similar. Comparing the CBL growth during the dry and wet season, there is evidence that the CBL properties over the forest are not dependent on the surface characteristics, but over the pasture they are.     

Quantification of ecosystem carbon exchange characteristics in a dominant subtropical evergreen forest ecosystem

CO₂ fluxes were measured continuously for three years (2003?C2005) using the eddy covariance technique for the canopy layer with a height of 27 m above the ground in a dominant subtropical evergreen forest in Dinghushan, South China. By applying gapfilling methods, we quantified the different components of the carbon fluxes (net ecosystem exchange (NEE)), gross primary production (GPP) and ecosystem respiration (R_eco) in order to assess the effects of meteorological variables on these fluxes and the atmospherecanopy interactions on the forest carbon cycle. Our results showed that monthly average daily maximum net CO₂ exchange of the whole ecosystem varied from ?3.79 to ?14.24 ??mol m^?2 s^?1 and was linearly related to photosynthetic active radiation. The Dinghushan forest acted as a net carbon sink of ?488 g C m^?2 y^?1, with a GPP of 1448 g Cm^?2 y^?1, and a R_eco of 961 g C m^?2 y^?1. Using a carboxylase-based model, we compared the predicted fluxes of CO₂ with measurements. GPP was modelled as 1443 g C m^?2 y^?1, and the model inversion results helped to explain ca. 90% of temporal variability of the measured ecosystem fluxes. Contribution of CO₂ fluxes in the subtropical forest in the dry season (October-March) was 62.2% of the annual total from the whole forest ecosystem. On average, 43.3% of the net annual carbon sink occurred between October and December, indicating that this time period is an important stage for uptake of CO₂ by the forest ecosystem from the atmosphere. Carbon uptake in the evergreen forest ecosystem is an indicator of the interaction of between the atmosphere and the canopy, especially in terms of driving climate factors such as temperature and rainfall events. We found that the Dinghushan evergreen forest is acting as a carbon sink almost year-round. The study can improve the evaluation of the net carbon uptake of tropical monsoon evergreen forest ecosystem in south China region under climate change conditions.     

An Alternative Approach for CO2 Flux Correction Caused by Heat and Water Vapour Transfer

Energy and CO₂ fluxes are commonly measured above plant canopies using an eddy covariance system that consists of a three-dimensional sonic anemometer and an H₂O/CO₂ infrared gas analyzer. By assuming that the dry air is conserved and inducing mean vertical velocity, Webb et al. (Quart. J. Roy. Meteorol. Soc. 106, 85-100, 1980) obtained two equations to account for density effects due to heat and water vapour transfer on H₂O/CO₂ fluxes. In this paper, directly starting with physical consideration of air-parcel expansion/compression, we derive two alternative equations to correct for these effects that do not require the assumption that dry air is conserved and the use of the mean vertical velocity. We then applied these equations to eddy flux observations from a black spruce forest in interior Alaska during the summer of 2002. In this ecosystem, the equations developed here led to increased estimates of CO₂ uptake by the vegetation during the day (up to about 20%), and decreased estimates of CO₂ respiration by the ecosystem during the night (approximately 4%) as compared with estimates obtained using the Webb et al. approach.     

珠海凤凰山陆气相互作用与碳通量观测塔的基本观测及晴天主要观测量的日变化特征

珠海凤凰山地处北回归线以南,森林植被覆盖率达90%,植被类型为南亚热带常绿阔叶林群落,是岭南地区典型的城市或村庄周边的再生森林,我们选择在凤凰山麓森林冠层较为平缓的低矮坡地建立了陆-气相互作用和碳通量的观测铁塔塔站。本文详细介绍了观测塔的地理环境、初步的仪器布设和基本观测,并利用已获得的资料分析了旱季典型晴天主要观测量的日变化特征。太阳总辐射及其分光辐射和反射辐射的日变化都是比较常规的中午最高的对称结构;冠层接收到的长波辐射比向上长波辐射低;气温日变化的峰值比太阳辐射滞后,白天达到最高值前的气温是低层高于高层,达到最高值后到落日前气温陡然下降,夜晚的气温是低层低于高层。相对湿度凌晨最大,下午最小,夜晚是低层相对偏湿,白天正好相反;11月份,珠海地区盛行旱季的偏北季风,有明显的海陆风的作用,白天的海风较强,夜晚的陆风较弱;森林冠层向大气释放的感热和潜热的量值基本相当,潜热基本为正;感热白天为正,夜晚基本为负;森林冠层吸收的二氧化碳的最高值出现在午后,此时空气中水汽浓度达到最低,向大气释放的二氧化碳在日出后的清晨最大,此时空气中的二氧化碳浓度达到最大,同时空气密度也最大;由于森林冠层高、密度大,土壤湿度基本没有日变化;表层土壤温度日变化的振幅随土壤深度加深而变小,土壤热流的变化是下午高,清晨低。本文还发现了一些值得深入探讨的现象,需要以后根据充沛的资料分析论证。     

A Case Study of Carbon Pools Under Three Different Land-Uses in Panamá

This paper examines changes in carbon (C) pools associated with land-use, synthesizing data from two experiments dealing with different aspects of tree plantation establishment in Central Panamá. First, we analysed soil profiles in a grazed pasture and an adjacent 5-year-old teak (Tectona grandis) plantation. There were small differences in soil C mass in the top 10 cm of the pasture and the plantation, though analysis of paired profiles suggested larger differences at greater depth. Analysis of the ¹³C signatures in the pasture soils and litter showed that 90% to 95% of the organic matter in the surface 5 cm was derived from C₄ pasture plants, over the 45 years since the pasture was converted from forest. Comparison of the ¹³C signatures in the pasture and teak plantation profiles indicated substantial replacement of C₄—derived organic matter with the dominantly C₃—derived plantation tissues. Organic matter turnover times in the upper 10 cm of the soils ranged from 8 to 34 years and from 11 to 58 years in the upper 30 cm, depending on topographic location. We also present preliminary results, and technical challenges, for an eddy covariance experiment set up to provide a direct comparison between a grazed pasture and a native tree plantation. The two ecosystems studied are estimated to be small CO₂ sinks, 92 g,C,m^–2 yr^–1 for the pasture, and 57 g,C,m^–2 yr^–1 for native species plantation in the first year after establishment. The pastures response to seasonal change was more pronounced, both in term of CO₂ fluxes and in term of herbaceous productivity, than the plantations response. The storage below ground systems contained up 40% of the total sapling biomass.     

Using a GCM analogue model to investigate the potential for Amazonian forest dieback

Summary A combined GCM analogue model and GCM land surface representation is used to investigate the influences of climatology and land surface parameterisation on modelled Amazonian vegetation change. This modelling structure (called IMOGEN) captures the main features of the changes in surface climate as estimated by a GCM with enhanced atmospheric greenhouse gas concentrations. Advantage is taken of IMOGENs computational speed which allows multiple simulations to be carried out to assess the robustness of the GCM results.The timing of forest dieback is found to be sensitive to the initial pre-industrial climate, as well as uncertainties in the representation of land-atmosphere CO₂ exchange. Changing from a Q ₁₀ form for plant dark and maintanence respiration (as used in the coupled GCM runs) to a respiration proportional to maximum photosynthesis, reduces the biomass lost from Amazonia in the 21st century. Replacing the GCM control climate (which has about 25% too little rain in the annual mean over Amazonia) with an observed climatology increases the CO₂ concentration at which rainfall drops to critical levels, and thereby further delays the dieback. On the other hand, calibration of the canopy photosynthesis model against Amazonian flux data tends to lead to earlier forest dieback. Further advances are required in both GCM rainfall simulation and land-surface process representation before a clearer picture will emerge on the timing of possible Amazonian forest dieback. However, it seems likely that these advances will overall lead to projections of later forest dieback as GCM control climates become more realistic.     

Future Expansion Of Agriculture and Pasture Acts to Amplify Atmospheric CO2 Levels in Response to Fossil-Fuel and Land-Use Change Emissions

The expansion of crop and pastures to the detriment of forests results in an increase in atmospheric CO₂. The first obvious cause is the loss of forest biomass and soil carbon during and after conversion. The second, generally ignored cause, is the reduction of the residence time of carbon when, for example, forests or grasslands are converted to cultivated land. This decreases the sink capacity of the global terrestrial biosphere, and thereby may amplify the atmospheric CO₂ rise due to fossil and land-use carbon release. For the IPCC A2 future scenario, characterized by high fossil and high land-use emissions, we show that the land-use amplifier effect adds 61 ppm extra CO₂ in the atmosphere by 2100 as compared to former treatment of land-use processes in carbon models. Investigating the individual contribution of each of the six land-use transitions (forest ↔ crop, forest ↔ pasture, grassland crop) to the amplifier effect indicates that the clearing of forest and grasslands to arable lands explains most of the CO₂ amplification. The amplification effect is 50% higher than in a previous analysis by the same authors which considered neither the deforestation of pastures nor the ploughing of grasslands. Such an amplification effect is further examined in sensitivity tests where the net primary productivity is considered independent of the atmospheric CO₂. We also show that the land-use changes, which have already occurred in the recent past, have a strong inertia at releasing CO₂, and will contribute to about 1/3 of the amplification effect by 2100. These results suggest that there is an additional atmospheric benefit of preserving pristine ecosystems with high turnover times.     

Future Expansion Of Agriculture and Pasture Acts to Amplify Atmospheric CO<Subscript>2</Subscript> Levels in Response to Fossil-Fuel and Land-Use Change Emissions

The expansion of crop and pastures to the detriment of forests results in an increase in atmospheric CO₂. The first obvious cause is the loss of forest biomass and soil carbon during and after conversion. The second, generally ignored cause, is the reduction of the residence time of carbon when, for example, forests or grasslands are converted to cultivated land. This decreases the sink capacity of the global terrestrial biosphere, and thereby may amplify the atmospheric CO₂ rise due to fossil and land-use carbon release. For the IPCC A2 future scenario, characterized by high fossil and high land-use emissions, we show that the land-use amplifier effect adds 61 ppm extra CO₂ in the atmosphere by 2100 as compared to former treatment of land-use processes in carbon models. Investigating the individual contribution of each of the six land-use transitions (forest &#x02194; crop, forest &#x02194; pasture, grassland crop) to the amplifier effect indicates that the clearing of forest and grasslands to arable lands explains most of the CO₂ amplification. The amplification effect is 50% higher than in a previous analysis by the same authors which considered neither the deforestation of pastures nor the ploughing of grasslands. Such an amplification effect is further examined in sensitivity tests where the net primary productivity is considered independent of the atmospheric CO₂. We also show that the land-use changes, which have already occurred in the recent past, have a strong inertia at releasing CO₂, and will contribute to about 1/3 of the amplification effect by 2100. These results suggest that there is an additional atmospheric benefit of preserving pristine ecosystems with high turnover times.     

Comparing surface energy, water and carbon cycle in dry and wet regions simulated by a land-surface model

In this study, we analyze results from 47-year (1954?C2000) offline simulations using an Australian land-surface model CSIRO Atmosphere Biosphere Land Exchange. We focus on exploring its surface mean climatology, interannual and decadal variations in Australia and Amazonia basin in South America which are distinguished by dry and wet climates respectively. Its skill is assessed by using observational datasets and four model products from the Global Land-surface Data Assimilation System. Surface evaporation and runoff climatologies are satisfactorily simulated, including surface energy and water partitions in dry and wet climates. In the Australian continent dominated by dry climate, slowly varying soil moisture processes are simulated in the southeast during austral winter. The model is skilful in reproducing the nonlinear relationship between rainfall and runoff variations in the southwestern part of the Australia. It shows that the significant downward trend of river inflow in the region is associated with enhanced surface evaporation which is caused by increased surface radiation and wind speed. In its carbon-cycle modeling, the model simulates an upward trend of NPP by about 0.69%/year over the Amazonia forest region in the 47-year period. By comparing two sets of the model results with/without CO₂ variations, it shows that 35% of such increases are caused by changes in climatic conditions, while 65% is due to the increase in atmospheric CO₂ concentration. Given the close linkage between climate, water and vegetation (carbon cycle), this work promotes an integrated modeling and evaluation approach for better representation of land-surface processes in Earth system studies.     

Eddy correlation measurements of CO2, latent heat,and sensible heat fluxes over a crop surface

Eddy correlation equipment was used to measure mass and energy fluxes over a soybean crop. A rapid response CO₂ sensor, a drag anemometer, a Lyman-alpha hygrometer and a fine wire thermocouple were used to sense the fluctuating quantities.Diurnal fluxes of sensible heat, latent heat and CO₂ were calculated from these data. Energy budget closure was obtained by summing the sensible and latent heat fluxes determined by eddy correlation which balanced the sum of net radiation and soil heat flux. Peak daytime CO₂ fluxes were near 1.0 mg m^–2 (ground area) s^–1.The eddy correlation technique was also employed in this study to measure nocturnal CO₂ fluxes caused by respiration from plants, soil, and roots. These CO₂ fluxes ranged from - 0.1 to - 0.25 mg m^–2s^–1.From the data collected over mature soybeans, a relationship between CO₂ flux and photosynthetically active radiation (PAR) was developed. The crop did not appear to be light-saturated at PAR flux densities < 1800 Ei m^–2 s^–1. The light compensation point was found to be about 160 Ei m^–2 s^–1.Published as Paper No. 7402, Journal Series, Nebraska Agricultural Experiment Station. The work reported here was conducted under Nebraska Agricultural Experiment Station Project 27-003 and Regional Research Project 11–33.Post-doctoral Research Associate, Professor and Professor, respectively. Center for Agricultural Meteorology and Climatology, Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, NE 68583-0728.     

土壤水分条件对内蒙古典型草原水汽和二氧化碳通量的影响研究

本文基于2007年和2008年生长季内蒙古羊草和大针茅草原湍流观测资料,分析了两种典型草原下垫面生长季的不同土壤水分条件下水汽和二氧化碳通量交换特征及其控制因子。主要结果如下:（1）在植被生长峰值期,日尺度上,干旱条件下土壤湿度是潜热通量的主要控制因子,而土壤水分条件较好时潜热通量主要受净辐射控制。（2）与大针茅草原相比,羊草草原叶面积指数较大,水分条件较好时,其潜热通量平均值更大,CO₂吸收能力更强,吸收CO₂更多;但在土壤水分胁迫出现时,羊草草原叶面的气孔闭合度急剧增加,大针茅草原的潜热通量、和CO₂吸收反而更大,表现出更为耐旱的植被特性。（3）地表导度可以用来解释土壤水分条件对羊草和大针茅草原碳水通量的影响。     

Leaf-level gas exchange and scaling-up of forest understory carbon fixation rates with a “patch-scale” canopy model

Summary During the Hartheim experiment (HartX) 1992, conducted in the Upper Rhine Valley, Germany, we estimated water vapor flux from the understory by several methods as reported in Wedler et al. (this issue). We also examined the photosynthetic gas exchange of the dominant understory speciesBrachypodium pinnatum, Carex alba, andCarex flacca at the leaf level with an CO₂/H₂O porometer. A mechanisticallybased leaf gas exchange model was parameterized for these understory species and validated via the measured diurnal courses of carbon dioxide exchange. Leaf CO₂ gas exchange was scaled-up to patch- and then to stand-level utilizing the leaf gas exchange model as a component of the canopy light interception/energy balance model GAS-FLUX, and by further considering variation in vegetation patch-type distribution, patch-specific spatial structure, patch-type leaf area index, and microclimate beneath the tree canopy.At patch-level,C. alba exhibited the lowest net CO₂ uptake of ca. 75 mmol m^–2 d^–1 due to a low leaf-level photosynthetic capacity, whereas net CO₂ fixation ofB. pinnatum- andC. flacca-patches was approx. 178 and 184 mmol m^–2 d^–1, respectively. Highest CO₂ uptake was estimated for mixed patches whereB. pinnatum grew together with the sedge speciesC. alba orC. flacca. Scaling-up of leaf gas exchange to stand level resulted in an estimated average rate of total CO₂ fixation by the graminoid understory patches of approximately 93 mmol m^–2 d^–1 during the HartX period. The conservative gas exchange behavior ofC. alba at Hartheim and its apparent success in space capture seems to affect overall functioning of this pine forest ecosystem by limiting understory CO₂ uptake. The CO₂ uptake by the understory is approximately 20% of stand total CO₂ uptake. CO₂ uptake fluxes mirror the relative differences in water loss from the understory and crown layer during the HartX period. Comparative measurements indicate that understory vegetation in spruce and pine forests is not greatly different from that of other low-statured natural ecosystems such as tundra or marshes under high light conditions, although CO₂ capture by the understory at Hartheim is at the low extreme of the estimates, apparently due to the success ofC. alba. With 6 Figures     

Forest-Air Fluxes Of Carbon, Water And Energy Over Non-Flat Terrain

A field study of surface-air exchange of carbon, water, and energy was conducted at a mid-latitude, mixed forest on non-flat terrain to investigate how to best interpret biological signals from the eddy flux data that may be subject to advective influences. It is shown that during periods of Southwest winds (sector with mild topography), the eddy fluxes are well-behaved in terms of energy balance closure, the existence of a constant flux layer, consistency with chamber observations and the expected abiotic controls on the fluxes. Advective influences are evident during periods with wind from a steep (15%) slope to the Northeast of the tower. These influences appear more severe on CO₂ flux, particularly in stable air, than on the energy fluxes. Large positive flux of CO₂ (> 23 mol m^-2 s^-1) occurs frequently at night. The annual sum of the carbon flux is positive, but the issue about whether the forest is a source of atmospheric carbon remains inconclusive.Attempts are made to assess vertical advectionusing the data collected on a single tower. Over the Southwestsector, vertical advection makes a statistically significant but small contribution to the 30-min energy imbalance and CO₂ flux variations. Contributions by horizontal advection may be larger but cannot be verified directly by the current experimental method.     

GREENHOUSE GASES FROM DEFORESTATION IN BRAZILIAN AMAZONIA: NET COMMITTED EMISSIONS

Deforestation in Brazilian Amazonia is a significant source of greenhouse gases today and, with almost 90% of the originally forested area still uncleared, is a very large potential source of future emissions. The 1990 rate of loss of forest (13.8 × 10³ km²/year) and cerrado savanna (approximately 5 × 10³ km²/year) was responsible for releasing approximately 261 × 10⁶ metric tons of carbon (10⁶ t C) in the form of CO2, or 274–285 × 10⁶ t of CO2-equivalent C considering IPCC 1994 global warming potentials for trace gases over a 100-year horizon. These calculations consider conversion to a landscape of agriculture, productive pasture, degraded pasture, secondary forest, and regenerated forest in the proportions corresponding to the equilibrium condition implied by current land-use patterns. Emissions are expressed as net committed emissions, or the gases released over a period of years as the carbon stock in each hectare deforested approaches a new equilibrium in the landscape that replaces the original forest. For low and high trace gas scenarios, respectively, 1990 clearing produced net committed emissions (in 10⁶ t of gas) of 957–958 for CO2, 1.10–1.42 for CH₄, 28–35 for CO, 0.06–0.16 for N₂O, 0.74–0.74 for NO_x and 0.58–1.16 for non-methane hydrocarbons.     

Limitations of an eddy-correlation technique for the determination of the carbon dioxide and sensible heat fluxes

A modified infrared CO₂ gas analyzer, a small thermocouple assembly, a heated-thermocouple anemometer for horizontal wind, and a propeller-type vertical wind sensor were used to measure the eddy fluxes of heat and CO₂ above a corn crop. Experimental results of these fluxes are discussed. The main sources of errors of the eddy fluxes using these instruments were estimated:

1. Sensors with a time constant of 0.5 s appear to be fast enough to detect most of the vertical CO₂ transfer as long as the sensors are located at least one meter above the crop surface.
2. The deviation from steady-state conditions for 10-min periods was found to have a significant effect on the eddy flux estimates.
3. Temperature fluctuations of the air sample passing through the CO₂ infrared gas analyzer were found to be non-negligible but could be easily corrected.
4. A 1° misalignment of the vertical anemometer affected these eddy fluxes by less than 10% under all circumstances studied.

     

Influences on surface energy fluxes in simulated present and doubled CO2 climates

The surface energy fluxes simulated by the CSIRO9 Mark 1 GCM for present and doubled CO₂ conditions are analyzed. On the global scale the climatological flux fields are similar to those from four GCMs studied previously. A diagnostic calculation is used to provide estimates of the radiative forcing by the GCM atmosphere. For 1 × CO₂, in the global and annual mean, cloud produces a net cooling at the surface of 31 W m^–2. The clear-sky longwave surface greenhouse effect is 311 W m^–2, while the corresponding shortwave term is –79 W m^–2. As for the other GCM results, the CSIRO9 CO₂ surface warming (global mean 4.8°C) is closely related to the increased downward longwave radiation (LW ). Global mean net cloud forcing changes little. The contrast in warming between land and ocean, largely due to the increase in evaporative cooling (E) over ocean, is highlighted. In order to further the understanding of influences on the fluxes, simple physically based linear models are developed using multiple regression. Applied to both 1 × CO₂ and CO₂ December–February mean tropical fields from CSIRO9, the linear models quite accurately (3–5 W m^–2 for 1 × CO₂ and 2–3 W m^–2 for CO₂) relate LW and net shortwave radiation to temperature, surface albedo, the water vapor column, and cloud. The linear models provide alternative estimates of radiative forcing terms to those from the diagnostic calculation. Tropical mean cloud forcings are compared. Over land, E is well correlated with soil moisture, and sensible heat with air-surface temperature difference. However an attempt to relate the spatial variation of LWt within the tropics to that of the nonflux fields had little success. Regional changes in surface temperature are not linearly related to, for instance, changes in cloud or soil moisture.     

Modelling Vegetation-Atmosphere Co2 Exchange By A Coupled Eulerian-Langrangian Approach

A Eulerian-Lagrangian canopy microclimate model wasdeveloped with the aim of discerning physical frombiophysical controls of CO₂ and H₂O fluxes. The model couples radiation attenuation with mass,energy, and momentum exchange at different canopylevels. A unique feature of the model is its abilityto combine higher order Eulerian closure approachesthat compute velocity statistics with Lagrangianscalar dispersion approaches within the canopy volume. Explicit accounting for within-canopy CO₂,H₂O, and heat storage is resolved by consideringnon-steadiness in mean scalar concentration andtemperature. A seven-day experiment was conducted inAugust 1998 to investigate whether the proposedmodel can reproduce temporal evolution of scalar(CO₂, H₂O and heat) fluxes, sources andsinks, and concentration profiles within and above auniform 15-year old pine forest. The modelreproduced well the measured depth-averaged canopy surfacetemperature, CO₂ and H₂O concentrationprofiles within the canopy volume, CO₂ storageflux, net radiation above the canopy, and heat andmass fluxes above the canopy, as well as the velocitystatistics near the canopy-atmosphere interface. Implications for scaling measured leaf-levelbiophysical functions to ecosystem scale are alsodiscussed.     

An Analytical Model for the Distribution of CO2 Sources and Sinks, Fluxes, and Mean Concentration Within the Roughness Sub-Layer

A one-dimensional analytical model that predicts foliage CO₂ uptake rates, turbulent fluxes, and mean concentration throughout the roughness sub-layer (RSL), a layer that extends from the ground surface up to 5h, where h is canopy height, is proposed. The model combines the mean continuity equation for CO₂ with first-order closure principles for turbulent fluxes and simplified physiological and radiative transfer schemes for foliage uptake. This combination results in a second-order ordinary differential equation in which soil respiration (R) and CO₂ concentration well above the RSL are imposed as lower and upper boundary conditions, respectively. An inverse version of the model was tested against datasets from two contrasting ecosystems: a tropical forest (h = 40m) and a managed irrigated rice canopy (h = 0.7m), with good agreement noted between modelled and measured mean CO₂ concentration profiles within the entire RSL. Sensitivity analysis on the model parameters revealed a plausible scaling regime between them and a dimensionless parameter defined by the ratio between external (R) and internal (stomatal conductance) characteristics controlling the CO₂ exchange process. The model can be used to infer the thickness of the RSL for CO₂ exchange, the inequality in zero-plane displacement between CO₂ and momentum, and its consequences on modelled CO₂ fluxes. A simplified version of the solution is well suited for being incorporated into large-scale climate models. Furthermore, the model framework here can be used to a priori estimate relative contributions from the soil surface and the atmosphere to canopy-air CO₂ concentration, thereby making it synergetic to stable isotopes studies.