Carbon dioxide exchange in a high-arctic fen estimated by eddy covariance measurements and modelling

The high-arctic environment is an environment where the consequences of global warming may be significant. In this paper we report on findings on carbon dioxide and water vapour fluxes above a sedge-dominated fen at Zackenberg (74°28′N, 20°34′ W) in The National Park of North and East Greenland. Eddy covariance measurements were initiated at the start of the growing season and terminated shortly before its end lasting 45 days. The net CO₂ flux during daytime reaches a high of 10 μmol m^–2s^–1, and around the summer solstice, net CO₂ assimilation occurred at midnight, resulting in net carbon gain during the night. The measured carbon dioxide fluxes compare well to estimates based on the photosynthesis model by Collatz et al. (1991 ). The total growing-season net ecosystem CO₂ exchange was estimated to be 96 g C m^–2 based on the carbon dioxide model and micrometeorological data. Finally, the combined CO₂ assimilation and soil respiration models are used for examining the dependence of the carbon dioxide budget on temperature. The ecosystem is found to function optimally given the present temperature conditions whereas either an increase or a decrease in temperature would reduce the ecosystem CO₂ accumulation. An increase in temperature by 5 °C would turn the ecosystem into a carbon dioxide source.     

Landscape and Ecosystem-Level Controls on Net Carbon Dioxide Exchange along a Natural Moisture Gradient in Canadian Low Arctic Tundra

Our understanding of the controls and magnitudes of regional CO₂ exchanges in the Arctic are limited by uncertainties due to spatial heterogeneity in vegetation across the landscape and temporal variation in environmental conditions through the seasons. We measured daytime net ecosystem CO₂ exchange and each of its component fluxes in the three major tundra ecosystem-types that typically occur along natural moisture gradients in the Canadian Low Arctic biweekly during the full snow-free season of 2004. In addition, we used a plant-removal treatment to compare the contribution of bulk soil organic matter to total respiratory CO₂ loss among these ecosystems. Net CO₂ exchange rates varied strongly, but not consistently, among ecosystems in the spring and summer phases as a result of ecosystem-specific and differing responses of gross photosynthesis and respiration to temporal variation in environmental conditions. Overall, net carbon gain was largest in the wet sedge ecosystem and smallest in the dry heath. Our measures of CO₂ flux variation within each ecosystem were frequently most closely correlated with air or soil temperatures during each seasonal phase. Nevertheless, a particularly large rainfall event in early August rapidly decreased respiration rates and stimulated gross photosynthetic rates, resulting in peak rates of net carbon gain in all ecosystems. Finally, the bulk soil carbon contribution to total respiration was relatively high in the birch hummock ecosystem. Together, these results demonstrate that the relative influences of moisture and temperature as primary controls on daytime net ecosystem CO₂ exchange and its component fluxes differ in fundamental ways between the landscape and ecosystem scales. Furthermore, they strongly suggest that carbon cycling responses to environmental change are likely to be highly ecosystem-specific, and thus to vary substantially across the low arctic landscape. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Estimates of CO2 uptake and release among European forests based on eddy covariance data

The net ecosystem exchange (NEE) of forests represents the balance of gross primary productivity (GPP) and respiration (R). Methods to estimate these two components from eddy covariance flux measurements are usually based on a functional relationship between respiration and temperature that is calibrated for night‐time (respiration) fluxes and subsequently extrapolated using daytime temperature measurements. However, respiration fluxes originate from different parts of the ecosystem, each of which experiences its own course of temperature. Moreover, if the temperature–respiration function is fitted to combined data from different stages of biological development or seasons, a spurious temperature effect may be included that will lead to overestimation of the direct effect of temperature and therefore to overestimates of daytime respiration. We used the EUROFLUX eddy covariance data set for 15 European forests and pooled data per site, month and for conditions of low and sufficient soil moisture, respectively. We found that using air temperature (measured above the canopy) rather than soil temperature (measured 5 cm below the surface) yielded the most reliable and consistent exponential (Q₁₀) temperature–respiration relationship. A fundamental difference in air temperature‐based Q₁₀ values for different sites, times of year or soil moisture conditions could not be established; all were in the range 1.6–2.5. However, base respiration (R₀, i.e. respiration rate scaled to 0°C) did vary significantly among sites and over the course of the year, with increased base respiration rates during the growing season. We used the overall mean Q₁₀ of 2.0 to estimate annual GPP and R. Testing suggested that the uncertainty in total GPP and R associated with the method of separation was generally well within 15%. For the sites investigated, we found a positive relationship between GPP and R, indicating that there is a latitudinal trend in NEE because the absolute decrease in GPP towards the pole is greater than in R.     

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.     

Model-data synthesis of diurnal and seasonal CO2 fluxes at Niwot Ridge, Colorado

Eddy covariance records hold great promise for understanding the processes controlling the net ecosystem exchange of CO₂ (NEE). However, NEE is the small difference between two large fluxes: photosynthesis and ecosystem respiration. Consequently, separating NEE into its component fluxes, and determining the process‐level controls over these fluxes, is a difficult problem. In this study, we used a model‐data synthesis approach with the Simplified PnET (SIPNET) flux model to extract process‐level information from 5 years of eddy covariance data at an evergreen forest in the Colorado Rocky Mountains. SIPNET runs at a twice‐daily time step, and has two vegetation carbon pools, a single aggregated soil carbon pool, and a soil moisture submodel that models both evaporation and transpiration. By optimizing the model parameters before evaluating model‐data mismatches, we were able to probe the model structure independent of any arbitrary parameter set. In doing so, we were able to learn about the primary controls over NEE in this ecosystem, and in particular the respiration component of NEE. We also used this parameter optimization, coupled with a formal model selection criterion, to investigate the effects of making hypothesis‐driven changes to the model structure. These experiments lent support to the hypotheses that (1) photosynthesis, and possibly foliar respiration, are down‐regulated when the soil is frozen and (2) the metabolic processes of soil microbes vary in the summer and winter, possibly because of the existence of distinct microbial communities at these two times. Finally, we found that including water vapor fluxes, in addition to carbon fluxes, in the parameter optimization did not yield significantly more information about the partitioning of NEE into gross photosynthesis and ecosystem respiration.     

Variations in daytime net carbon and water exchange in a montane shrubland ecosystem in southeast Spain

Carbon and water fluxes in a semiarid shrubland ecosystem located in the southeast of Spain (province of Almería) were measured continuously over one year using the eddy covariance technique. We examined the influence of environmental variables on daytime (photosynthetically active photons, F _P >10 μmol m⁻² s⁻¹) ecosystem gas exchange and tested the ability of an empirical eco-physiological model based on F _P to estimate carbon fluxes over the whole year. The daytime ecosystem fluxes showed strong seasonality. During two solstitial periods, summer with warm temperatures (>15 °C) and sufficient soil moisture (>10 % vol.) and winter with mild temperatures (>5 °C) and high soil moisture contents (>15 % vol.), the photosynthetic rate was higher than the daytime respiration rate and mean daytime CO₂ fluxes were ca. −1.75 and −0.60 μmol m⁻² s⁻¹, respectively. Daytime evapotranspiration fluxes averaged ca. 2.20 and 0.24 mmol m⁻² s⁻¹, respectively. By contrast, in summer and early autumn with warm daytime temperatures (>10 °C) and dry soil (<10 % vol.), and also in mid-winter with near-freezing daytime temperatures the shrubland behaved as a net carbon source (mean daytime CO₂ release of ca. 0.60 and 0.20 μmol m⁻² s⁻¹, respectively). Furthermore, the comparison of water and carbon fluxes over a week in June 2004 and June 2005 suggests that the timing—rather than amount—of spring rainfall may be crucial in determining growing season water and carbon exchange. Due to strongly limiting environmental variables other than F _P, the model applied here failed to describe daytime carbon exchange only as a function of F _P and could not be used over most of the year to fill gaps in the data.     

Responses of net ecosystem CO2 exchange in managed grassland to long-term CO2 enrichment, N fertilization and plant species

The effects of elevated pCO₂ on net ecosystem CO₂ exchange were investigated in managed Lolium perenne (perennial ryegrass) and Trifolium repens (white clover) monocultures that had been exposed continuously to elevated pCO₂ (60 Pa) for nine growing seasons using Free Air CO₂ Enrichment (FACE) technology. Two levels of nitrogen (N) fertilization were applied. Midday net ecosystem CO₂ exchange (mNEE) and night-time ecosystem respiration (NER) were measured in three growing seasons using an open-flow chamber system. The annual net ecosystem carbon (C) input resulting from the net CO₂ fluxes was estimated for one growing season. In both monocultures and at both levels of N supply, elevated pCO₂ stimulated mNEE by up to 32%, the exact amount depending on intercepted PAR. The response of mNEE to elevated pCO₂ was larger than that of harvestable biomass. Elevated pCO₂ increased NER by up to 39% in both species at both levels of N supply. NER, which was affected by mNEE of the preceding day, was higher in T. repens than in L. perenne. High N increased NER compared to low N supply. According to treatment, the annual net ecosystem C input ranged between 210 and 631 g C m⁻² year⁻¹ and was not significantly affected by the level of pCO₂. Low N supply led to a higher net C input than high N supply. We demonstrated that at the ecosystem level, there was a long-term stimulation in the net C assimilation during daytime by elevated pCO₂. However, because NER was also stimulated, net ecosystem C input was not significantly increased at elevated pCO₂. The annual net ecosystem C input was primarily affected by the amount of N supplied.     

Separation of net ecosystem exchange into assimilation and respiration using a light response curve approach: critical issues and global evaluation

The measured net ecosystem exchange (NEE) of CO₂ between the ecosystem and the atmosphere reflects the balance between gross CO₂ assimilation [gross primary production (GPP)] and ecosystem respiration (R_eco). For understanding the mechanistic responses of ecosystem processes to environmental change it is important to separate these two flux components. Two approaches are conventionally used: (1) respiration measurements made at night are extrapolated to the daytime or (2) light–response curves are fit to daytime NEE measurements and respiration is estimated from the intercept of the ordinate, which avoids the use of potentially problematic nighttime data. We demonstrate that this approach is subject to biases if the effect of vapor pressure deficit (VPD) modifying the light response is not included. We introduce an algorithm for NEE partitioning that uses a hyperbolic light response curve fit to daytime NEE, modified to account for the temperature sensitivity of respiration and the VPD limitation of photosynthesis. Including the VPD dependency strongly improved the model's ability to reproduce the asymmetric diurnal cycle during periods with high VPD, and enhances the reliability of R_eco estimates given that the reduction of GPP by VPD may be otherwise incorrectly attributed to higher R_eco. Results from this improved algorithm are compared against estimates based on the conventional nighttime approach. The comparison demonstrates that the uncertainty arising from systematic errors dominates the overall uncertainty of annual sums (median absolute deviation of GPP: 47 g C m^?2 yr^?1), while errors arising from the random error (median absolute deviation: ～2 g C m^?2 yr^?1) are negligible. Despite site‐specific differences between the methods, overall patterns remain robust, adding confidence to statistical studies based on the FLUXNET database. In particular, we show that the strong correlation between GPP and R_eco is not spurious but holds true when quasi‐independent, i.e. daytime and nighttime based estimates are compared.     

Reconciling Carbon-cycle Concepts, Terminology, and Methods

Recent projections of climatic change have focused a great deal of scientific and public attention on patterns of carbon (C) cycling as well as its controls, particularly the factors that determine whether an ecosystem is a net source or sink of atmospheric carbon dioxide (CO₂). Net ecosystem production (NEP), a central concept in C-cycling research, has been used by scientists to represent two different concepts. We propose that NEP be restricted to just one of its two original definitions—the imbalance between gross primary production (GPP) and ecosystem respiration (ER). We further propose that a new term—net ecosystem carbon balance (NECB)—be applied to the net rate of C accumulation in (or loss from [negative sign]) ecosystems. Net ecosystem carbon balance differs from NEP when C fluxes other than C fixation and respiration occur, or when inorganic C enters or leaves in dissolved form. These fluxes include the leaching loss or lateral transfer of C from the ecosystem; the emission of volatile organic C, methane, and carbon monoxide; and the release of soot and CO₂ from fire. Carbon fluxes in addition to NEP are particularly important determinants of NECB over long time scales. However, even over short time scales, they are important in ecosystems such as streams, estuaries, wetlands, and cities. Recent technological advances have led to a diversity of approaches to the measurement of C fluxes at different temporal and spatial scales. These approaches frequently capture different components of NEP or NECB and can therefore be compared across scales only by carefully specifying the fluxes included in the measurements. By explicitly identifying the fluxes that comprise NECB and other components of the C cycle, such as net ecosystem exchange (NEE) and net biome production (NBP), we can provide a less ambiguous framework for understanding and communicating recent changes in the global C cycle.     

A gradient of nutrient enrichment reveals nonlinear impacts of fertilization on Arctic plant diversity and ecosystem function

Rapid environmental change at high latitudes is predicted to greatly alter the diversity, structure, and function of plant communities, resulting in changes in the pools and fluxes of nutrients. In Arctic tundra, increased nitrogen (N) and phosphorus (P) availability accompanying warming is known to impact plant diversity and ecosystem function; however, to date, most studies examining Arctic nutrient enrichment focus on the impact of relatively large (>25x estimated naturally occurring N enrichment) doses of nutrients on plant community composition and net primary productivity. To understand the impacts of Arctic nutrient enrichment, we examined plant community composition and the capacity for ecosystem function (net ecosystem exchange, ecosystem respiration, and gross primary production) across a gradient of experimental N and P addition expected to more closely approximate warming‐induced fertilization. In addition, we compared our measured ecosystem CO₂ flux data to a widely used Arctic ecosystem exchange model to investigate the ability to predict the capacity for CO₂ exchange with nutrient addition. We observed declines in abundance‐weighted plant diversity at low levels of nutrient enrichment, but species richness and the capacity for ecosystem carbon uptake did not change until the highest level of fertilization. When we compared our measured data to the model, we found that the model explained roughly 30%–50% of the variance in the observed data, depending on the flux variable, and the relationship weakened at high levels of enrichment. Our results suggest that while a relatively small amount of nutrient enrichment impacts plant diversity, only relatively large levels of fertilization—over an order of magnitude or more than warming‐induced rates—significantly alter the capacity for tundra CO₂ exchange. Overall, our findings highlight the value of measuring and modeling the impacts of a nutrient enrichment gradient, as warming‐related nutrient availability may impact ecosystems differently than single‐level fertilization experiments.     

Effects of Experimental Water Table and Temperature Manipulations on Ecosystem CO2 Fluxes in an Alaskan Rich Fen

Peatlands store 30% of the world’s terrestrial soil carbon (C) and those located at northern latitudes are expected to experience rapid climate warming. We monitored growing season carbon dioxide (CO₂) fluxes across a factorial design of in situ water table (control, drought, and flooded plots) and soil warming (control vs. warming via open top chambers) treatments for 2 years in a rich fen located just outside the Bonanza Creek Experimental Forest in interior Alaska. The drought (lowered water table position) treatment was a weak sink or small source of atmospheric CO₂ compared to the moderate atmospheric CO₂ sink at our control. This change in net ecosystem exchange was due to lower gross primary production and light-saturated photosynthesis rather than increased ecosystem respiration. The flooded (raised water table position) treatment was a greater CO₂ sink in 2006 due largely to increased early season gross primary production and higher light-saturated photosynthesis. Although flooding did not have substantial effects on rates of ecosystem respiration, this water table treatment had lower maximum respiration rates and a higher temperature sensitivity of ecosystem respiration than the control plot. Surface soil warming increased both ecosystem respiration and gross primary production by approximately 16% compared to control (ambient temperature) plots, with no net effect on net ecosystem exchange. Results from this rich fen manipulation suggest that fast responses to drought will include reduced ecosystem C storage driven by plant stress, whereas inundation will increase ecosystem C storage by stimulating plant growth.     

Annual cycles of water vapour and carbon dioxide fluxes in and above a boreal aspen forest

Water vapour and CO₂ fluxes were measured using the eddy correlation method above and below the overstorey of a 21-m tall aspen stand in the boreal forest of central Saskatchewan as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Measurements were made at the 39.5-m and 4-m heights using 3-dimensional sonic anemometers (Kaijo-Denki and Solent, respectively) and closed-path gas analysers (LI-COR 6262) with 6-m and 4.7-m long heated sampling tubing, respectively. Continuous measurements were made from early October to mid-November 1993 and from early February to late-September 1994. Soil CO₂ flux (respiration) was measured using a LI-COR 6000-09 soil chamber and soil evaporation was measured using Iysimetry. The leaf area index of the aspen and hazelnut understorey reached 1.8 and 3.3, respectively. The maximum daily evapotranspiration (E) rate was 5–6 mm d^?1. Following leaf-out the hazelnut and soil accounted for 22% of the forest E. The estimated total E was 403 mm for 1994. About 88% of the precipitation in 1994 was lost as evapotranspiration. During the growing season, the magnitude of half-hourly eddy fluxes of CO₂ from the atmosphere into the forest reached 1.2 mg CO₂ m^?2 s^?1 (33 μmol C m^?2 s^?1) during the daytime. Downward eddy fluxes at the 4-m height were observed when the hazelnut was growing rapidly in June and July. Under well-ventilated night-time conditions, the eddy fluxes of CO₂ above the aspen and hazelnut, corrected for canopy storage, increased exponentially with soil temperature at the 2-cm depth. Estimates of daytime respiration rates using these relationships agreed well with soil chamber measurements. During the 1994 growing season, the cumulative net ecosystem exchange (NEE) was -3.5 t C ha^?1 y^?1 (a net gain by the system). For 1994, cumulative NEE, ecosystem respiration (R) and gross ecosystem photosynthesis (GEP = R - NEE) were estimated to be -1.3, 8.9 and 10.2 t C ha^?1 y^?1 respectively. Gross photosynthesis of the hazelnut was 32% of GEP.     

Towards physiologically meaningful water‐use efficiency estimates from eddy covariance data

Intrinsic water‐use efficiency (iWUE) characterizes the physiological control on the simultaneous exchange of water and carbon dioxide in terrestrial ecosystems. Knowledge of iWUE is commonly gained from leaf‐level gas exchange measurements, which are inevitably restricted in their spatial and temporal coverage. Flux measurements based on the eddy covariance (EC) technique can overcome these limitations, as they provide continuous and long‐term records of carbon and water fluxes at the ecosystem scale. However, vegetation gas exchange parameters derived from EC data are subject to scale‐dependent and method‐specific uncertainties that compromise their ecophysiological interpretation as well as their comparability among ecosystems and across spatial scales. Here, we use estimates of canopy conductance and gross primary productivity (GPP) derived from EC data to calculate a measure of iWUE (G₁, “stomatal slope”) at the ecosystem level at six sites comprising tropical, Mediterranean, temperate, and boreal forests. We assess the following six mechanisms potentially causing discrepancies between leaf and ecosystem‐level estimates of G₁: (i) non‐transpirational water fluxes; (ii) aerodynamic conductance; (iii) meteorological deviations between measurement height and canopy surface; (iv) energy balance non‐closure; (v) uncertainties in net ecosystem exchange partitioning; and (vi) physiological within‐canopy gradients. Our results demonstrate that an unclosed energy balance caused the largest uncertainties, in particular if it was associated with erroneous latent heat flux estimates. The effect of aerodynamic conductance on G₁ was sufficiently captured with a simple representation. G₁ was found to be less sensitive to meteorological deviations between canopy surface and measurement height and, given that data are appropriately filtered, to non‐transpirational water fluxes. Uncertainties in the derived GPP and physiological within‐canopy gradients and their implications for parameter estimates at leaf and ecosystem level are discussed. Our results highlight the importance of adequately considering the sources of uncertainty outlined here when EC‐derived water‐use efficiency is interpreted in an ecophysiological context.     

Net ecosystem exchange modifies the relationship between the autotrophic and heterotrophic components of soil respiration with abiotic factors in prairie grasslands

We investigated the relationships of net ecosystem carbon exchange (NEE), soil temperature, and moisture with soil respiration rate and its components at a grassland ecosystem. Stable carbon isotopes were used to separate soil respiration into autotrophic and heterotrophic components within an eddy covariance footprint during the 2008 and 2009 growing seasons. After correction for self‐correlation, rates of soil respiration and its autotrophic and heterotrophic components for both years were found to be strongly influenced by variations in daytime NEE – the amount of C retained in the ecosystem during the daytime, as derived from NEE measurements when photosynthetically active radiation was above 0 μmol m^?2 s^?1. The time scale for correlation of variations in daytime NEE with fluctuations in respiration was longer for heterotrophic respiration (36–42 days) than for autotrophic respiration (4–6 days). In addition to daytime NEE, autotrophic respiration was also sensitive to soil moisture but not soil temperature. In contrast, heterotrophic respiration from soils was sensitive to changes in soil temperature, soil moisture, and daytime NEE. Our results show that – as for forests – plant activity is an important driver of both components of soil respiration in this tallgrass prairie grassland ecosystem. Heterotrophic respiration had a slower coupling with plant activity than did autotrophic respiration. Our findings suggest that the frequently observed variations in the sensitivity of soil respiration to temperature or moisture may stem from variations in the proportions of autotrophic and heterotrophic components of soil respiration. Rates of photosynthesis at seasonal time scales should also be considered as a driver of both autotrophic and heterotrophic soil respiration for ecosystem flux modeling.     

Patterns in Vegetation and CO<Subscript>2</Subscript> Dynamics along a Water Level Gradient in a Lowland Blanket Bog

The surface of bogs is commonly patterned and composed of different vegetation communities, defined by water level. Carbon dioxide (CO₂) dynamics vary spatially between the vegetation communities. An understanding of the controls on the spatial variation of CO₂ dynamics is required to assess the role of bogs in the global carbon cycle. The water level gradient in a blanket bog was described and the CO₂ exchange along the gradient investigated using chamber based measurements in combination with regression modelling. The aim was to investigate the controls on gross photosynthesis (P_G), ecosystem respiration (R_E) and net ecosystem CO₂ exchange (NEE) as well as the spatial and temporal variation in these fluxes. Vegetation structure was strongly controlled by water level. The species with distinctive water level optima were separated into the opposite ends of the gradient in canonical correspondence analysis. The number of species and leaf area were highest in the intermediate water level range and these communities had the highest P_G. Photosynthesis was highest when the water level was 11 cm below the surface. Ecosystem respiration, which includes decomposition, was less dependent on vegetation structure and followed the water level gradient more directly. The annual NEE varied from −115 to 768 g CO₂ m⁻², being lowest in wet and highest in dry vegetation communities. The temporal variation was most pronounced in P_G, which decreased substantially during winter, when photosynthetic photon flux density and leaf area were lowest. Ecosystem respiration, which is dependent on temperature, was less variable and wintertime R_E fluxes constituted approximately 24% of the annual flux.     

Modelling night‐time ecosystem respiration by a constrained source optimization method

One of the main challenges to quantifying ecosystem carbon budgets is properly quantifying the magnitude of night‐time ecosystem respiration. Inverse Lagrangian dispersion analysis provides a promising approach to addressing such a problem when measured mean CO₂ concentration profiles and nocturnal velocity statistics are available. An inverse method, termed ‘Constrained Source Optimization’ or CSO, which couples a localized near‐field theory (LNF) of turbulent dispersion to respiratory sources, is developed to estimate seasonal and annual components of ecosystem respiration. A key advantage to the proposed method is that the effects of variable leaf area density on flow statistics are explicitly resolved via higher‐order closure principles. In CSO, the source distribution was computed after optimizing key physiological parameters to recover the measured mean concentration profile in a least‐square fashion. The proposed method was field‐tested using 1 year of 30‐min mean CO₂ concentration and CO₂ flux measurements collected within a 17‐year‐old (in 1999) even‐aged loblolly pine (Pinus taeda L.) stand in central North Carolina. Eddy‐covariance flux measurements conditioned on large friction velocity, leaf‐level porometry and forest‐floor respiration chamber measurements were used to assess the performance of the CSO model. The CSO approach produced reasonable estimates of ecosystem respiration, which permits estimation of ecosystem gross primary production when combined with daytime net ecosystem exchange (NEE) measurements. We employed the CSO approach in modelling annual respiration of above‐ground plant components (c. 214 g C m^?2 year^?1) and forest floor (c. 989 g C m^?2 year^?1) for estimating gross primary production (c. 1800 g C m^?2 year^?1) with a NEE of c. 605 g C m^?2 year^?1 for this pine forest ecosystem. We conclude that the CSO approach can utilise routine CO₂ concentration profile measurements to corroborate forest carbon balance estimates from eddy‐covariance NEE and chamber‐based component flux measurements.     

Evaluating the carbon balance estimate from an automated ground‐level flux chamber system in artificial grass mesocosms

Measuring and modeling carbon (C) stock changes in terrestrial ecosystems are pivotal in addressing global C‐cycling model uncertainties. Difficulties in detecting small short‐term changes in relatively large C stocks require the development of robust sensitive flux measurement techniques. Net ecosystem exchange (NEE) ground‐level chambers are increasingly used to assess C dynamics in low vegetation ecosystems but, to date, have lacked formal rigorous field validation against measured C stock changes. We developed and deployed an automated and multiplexed C‐flux chamber system in grassland mesocosms in order rigorously to compare ecosystem total C budget obtained using hourly C‐flux measurements versus destructive net C balance. The system combines transparent NEE and opaque respiration chambers enabling partitioning of photosynthetic and respiratory fluxes. The C‐balance comparison showed good agreement between the two methods, but only after NEE fluxes were corrected for light reductions due to chamber presence. The dark chamber fluxes allowed assessing temperature sensitivity of ecosystem respiration (R_eco) components (i.e., heterotrophic vs. autotrophic) at different growth stages. We propose that such automated flux chamber systems can provide an accurate C balance, also enabling pivotal partitioning of the different C‐flux components (e.g., photosynthesis and respiration) suitable for model evaluation and developments.     

The dependence of respiration on photosynthetic substrate supply and temperature: integrating leaf, soil and ecosystem measurements

Interactions between photosynthetic substrate supply and temperature in determining the rate of three respiration components (leaf, belowground and ecosystem respiration) were investigated within three environmentally controlled, Populus deltoides forest bays at Biosphere 2, Arizona. Over 2 months, the atmospheric CO₂ concentration and air temperature were manipulated to test the following hypotheses: (1) the responses of the three respiration components to changes in the rate of photosynthesis would differ both in speed and magnitude; (2) the temperature sensitivity of leaf and belowground respiration would increase in response to a rise in substrate availability; and, (3) at the ecosystem level, the ratio of respiration to photosynthesis would be conserved despite week‐to‐week changes in temperature. All three respiration rates responded to the CO₂ concentration‐induced changes in photosynthesis. However, the proportional change in the rate of leaf respiration was more than twice that of belowground respiration and, when photosynthesis was reduced, was also more rapid. The results suggest that aboveground respiration plays a key role in the overall response of ecosystem respiration to short‐term changes in canopy photosynthesis. The short‐term temperature sensitivity of leaf respiration, measured within a single night, was found to be affected more by developmental conditions than photosynthetic substrate availability, as the Q₁₀ was lower in leaves that developed at high CO₂, irrespective of substrate availability. However, the temperature sensitivity of belowground respiration, calculated between periods of differing air temperature, appeared to be positively correlated with photosynthetic substrate availability. At the ecosystem level, respiration and photosynthesis were positively correlated but the relationship was affected by temperature; for a given rate of daytime photosynthesis, the rate of respiration the following night was greater at 25 than 20°C. This result suggests that net ecosystem exchange did not acclimate to temperature changes lasting up to 3 weeks. Overall, the results of this study demonstrate that the three respiration terms differ in their dependence on photosynthesis and that, short‐ and medium‐term changes in temperature may affect net carbon storage in terrestrial ecosystems.     

Significance of cold-season respiration and photosynthesis in a subarctic heath ecosystem in Northern Sweden

While substantial cold-season respiration has been documented in most arctic and alpine ecosystems in recent years, the significance of cold-season photosynthesis in these biomes is still believed to be small. In a mesic, subartic heath during both the cold and warm season, we measured in situ ecosystem respiration and photosynthesis with a chamber technique at ambient conditions and at artificially increased frequency of freeze–thaw (FT) cycles during fall and spring. We fitted the measured ecosystem exchange rates to respiration and photosynthesis models with R²-values ranging from 0.81 to 0.85. As expected, estimated cold-season (October, November, April and May) respiration was significant and accounted for at least 22% of the annual respiratory CO₂ flux. More surprisingly, estimated photosynthesis during this period accounted for up to 19% of the annual gross CO₂ uptake, suggesting that cold-season photosynthesis partly balanced the cold-season respiratory carbon losses and can be significant for the annual cycle of carbon. Still, during the full year the ecosystem was a significant net source of 120 ± 12 g C m⁻² to the atmosphere. Neither respiration nor photosynthetic rates were much affected by the extra FT cycles, although the mean rate of net ecosystem loss decreased slightly, but significantly, in May. The results suggest only a small response of net carbon fluxes to increased frequency of FT cycles in this ecosystem.     

Deeper Snow Enhances Winter Respiration from Both Plant-associated and Bulk Soil Carbon Pools in Birch Hummock Tundra

It has only recently become apparent that biological activity during winter in seasonally snow-covered ecosystems may exert a significant influence on biogeochemical cycling and ecosystem function. One-seventh of the global soil carbon pool is stored in the bulk soil component of arctic ecosystems. Consistent climate change predictions of substantial increases in winter air temperatures and snow depths for the Arctic indicate that this region may become a significant net annual source of CO₂ to the atmosphere if its bulk soil carbon is decomposed. We used snow fences to investigate the influence of a moderate increase in snow depth from approximately 0.3 m (ambient) to approximately 1 m on winter carbon dioxide fluxes from mesic birch hummock tundra in northern Canada. We differentiated fluxes derived from the bulk soil and plant-associated carbon pools using an experimental ‘weeding’ manipulation. Increased snow depth enhanced the wintertime carbon flux from both pools, strongly suggesting that respiration from each was sensitive to warmer soil temperatures. Furthermore, deepened snow resulted in cooler and relatively stable soil temperatures during the spring-thaw period, as well as delayed and fewer freeze–thaw cycles. The snow fence treatment increased mean total winter efflux from 27 to 43 g CO₂-C m⁻². Because total 2004 growing season net ecosystem exchange for this site is estimated at 29–37 g CO₂-C m⁻², our results strongly suggest that a moderate increase in snow depth can enhance winter respiration sufficiently to switch the ecosystem annual net carbon exchange from a sink to source, resulting in net CO₂ release to the atmosphere.