Annual Respiration Research Articles

Comparison of different objective functions for parameterization of simple respiration models

The eddy covariance measurements of carbon dioxide fluxes collected around the world offer a rich source for detailed data analysis. Simple, aggregated models are attractive tools for gap filling, budget calculation, and upscaling in space and time. Key in the application of these models is their parameterization and a robust estimate of the uncertainty and reliability of their predictions. In this study we compared the use of ordinary least squares (OLS) and weighted absolute deviations (WAD, which is the objective function yielding maximum likelihood parameter estimates with a double exponential error distribution) as objective functions within the annual parameterization of two respiration models: the Q10 model and the Lloyd and Taylor model. We introduce a new parameterization method based on two nonparametric tests in which model deviation (Wilcoxon test) and residual trend analyses (Spearman test) are combined. A data set of 9 years of flux measurements was used for this study. The analysis showed that the choice of the objective function is crucial, resulting in differences in the estimated annual respiration budget of up to 40%. The objective function should be tested thoroughly to determine whether it is appropriate for the application for which the model will be used. If simple models are used to estimate a respiration budget, a trend test is essential to achieve unbiased estimates over the year. The analyses also showed that the parameters of the Lloyd and Taylor model are highly correlated and difficult to determine precisely, thereby limiting the physiological interpretability of the parameters.

Modeling analysis of primary controls on net ecosystem productivity of seven boreal and temperate coniferous forests across a continental transect

AbstractProcess‐based models are effective tools to synthesize and/or extrapolate measured carbon (C) exchanges from individual sites to large scales. In this study, we used a C‐ and nitrogen (N)‐cycle coupled ecosystem model named CN‐CLASS (Carbon Nitrogen‐Canadian Land Surface Scheme) to study the role of primary climatic controls and site‐specific C stocks on the net ecosystem productivity (NEP) of seven intermediate‐aged to mature coniferous forest sites across an east–west continental transect in Canada. The model was parameterized using a common set of parameters, except for two used in empirical canopy conductance–assimilation, and leaf area–sapwood relationships, and then validated using observed eddy covariance flux data. Leaf Rubisco‐N dynamics that are associated with soil–plant N cycling, and depend on canopy temperature, enabled the model to simulate site‐specific gross ecosystem productivity (GEP) reasonably well for all seven sites. Overall GEP simulations had relatively smaller differences compared with observations vs. ecosystem respiration (RE), which was the sum of many plant and soil components with larger variability and/or uncertainty associated with them. Both observed and simulated data showed that, on an annual basis, boreal forest sites were either carbon‐neutral or a weak C sink, ranging from 30 to 180 g C m−2 yr−1; while temperate forests were either a medium or strong C sink, ranging from 150 to 500 g C m−2 yr−1, depending on forest age and climatic regime. Model sensitivity tests illustrated that air temperature, among climate variables, and aboveground biomass, among major C stocks, were dominant factors impacting annual NEP. Vegetation biomass effects on annual GEP, RE and NEP showed similar patterns of variability at four boreal and three temperate forests. Air temperature showed different impacts on GEP and RE, and the response varied considerably from site to site. Higher solar radiation enhanced GEP, while precipitation differences had a minor effect. Magnitude of forest litter content and soil organic matter (SOM) affected RE. SOM also affected GEP, but only at low levels of SOM, because of low N mineralization that limited soil nutrient (N) availability. The results of this study will help to evaluate the impact of future climatic changes and/or forest C stock variations on C uptake and loss in forest ecosystems growing in diverse environments.

Winter respiration of allochthonous and autochthonous organic carbon in a subarctic clear‐water lake

We studied a small subarctic lake to assess the magnitude of winter respiration and the organic carbon (OC) source for this respiration. The concentration and stable isotopic composition (δ13C) of dissolved inorganic carbon (DIC) accumulating in the lake water under ice was analyzed over one winter (7 months). The DIC concentration increased and the δ13C of DIC decreased over time, with the greatest changes at the lake bottom. Winter respiration was 26% of annual respiration in the lake. Keeling plot analysis demonstrated that the δ13C of respired DIC varied spatially, high δ13C values occurring at shallow (2.5 m, 221.7‰%) compared with intermediate (4 m, 225.1‰) and deep (6 m, 227.8‰) locations in the lake. The variation in the δ13C of respired DIC was related to the variation in the δ13C of the sediments between locations, suggesting that sediment OC supported much of the winter respiration and that the dominant OC source for respiration was OC from benthic algae at shallow locations and settled OC, of predominately terrestrial origin, at deep locations. The respiration of OC from benthic algae constituted 55% of the winter respiration, equaling 54% of the primary production by benthic algae the previous summer. The study indicates the importance of temporal and spatial variation in respiration for the metabolism and net DIC production in unproductive high‐latitude lakes; both allochthonous and autochthonous carbon can contribute to winter DIC accumulation and, consequently, to spring CO2 emissions from lakes.

Spatial distribution of carbon balance in forest ecosystems across East Asia

The objective of this paper is to clarify what kind of environmental factors that regulate net ecosystem production (NEP), gross primary production (GPP), and ecosystem respiration (RE) in forest ecosystems across East Asia. Study sites were widely distributed and included diverse ecosystems, such as evergreen and deciduous, coniferous and broadleaf, planted and natural forests, from subarctic to tropical zones. We measured NEP using the eddy covariance technique at 13 forest sites in East Asia. Annual values of GPP and RE are simply regulated by annual mean air temperature across East Asia. There is a clear linear relationship between annual GPP and annual mean air temperature because the air temperature influences both growing period length and the seasonal variation of the maximum photosynthetic capacity, which, together, regulate the annual GPP. On the other hand, there is a strong exponential relationship between annual RE and annual mean air temperature on an East Asia scale, which is quite similar to the relation obtained on a canopy scale. The dependency of annual RE on air temperature on the East Asia scale was similar to that of monthly RE on air temperature on an individual site scale excepting for temperate larch and mixed forests in northern Japan. The reason why the relation is simple is that severe stress, which affects GPP or RE, is small in East Asia. The present study suggests that RE is sensitive to non-climate environmental factors when compared to GPP, thus the annual RE–air temperature relationship is more scattered than the annual GPP–air temperature relationship. The NEP is small at high latitude, relatively large at mid-latitude, and scattered at low latitude. As a whole, the NEP is more influenced by RE than GPP in East Asia. Compared to North America and Europe, the increase in the ratio of GPP to air temperature is slightly higher in East Asia. One of the possible reasons for this is that GPP in East Asia is not exposed to severe environmental stresses, such as summer drought.

Mechanism and bio‐environmental controls of ecosystem respiration in a cropland in the North China plains

CO2 flux was measured continuously using the eddy covariance technique in a wheat‐maize rotation system in the North China Plains from October 2002 to October 2006. The annual and seasonal variation of ecosystem respiration and the bio‐environmental controls on them were investigated. The results show that ecosystem respiration (Rec) in the cropland increased exponentially with soil temperature at 5 cm depth. The temperature sensitivity coefficient (Q10) for ecosystem respiration varied from 3.5 to 5.4 for wheat and from 2.4 to 4.5 for maize. In the wheat growing season, monthly average R0 (ecosystem respiration at 0°C) increased linearly with soil temperature and logarithmically with leaf area index (LAI). Monthly average Q10 decreased logarithmically with R 0. Residual R ec was significantly correlated with LAI. After considering LAI, the modified Q10 model could estimate R ec better than before. The simulation results show that annual ecosystem respiration in the wheat‐maize rotation system in the North China Plains was 1327, 1348, 1040 and 1171 gC m‐2 yr‐1 for the 4 years of the study. As a 4‐year average, seasonal mean ecosystem respiration in wheat (2.60 gC m‐2 day‐1) was much lower than in maize (6.09 gC m‐2 day‐1). However, integrated ecosystem respiration for the wheat growing season (566 gC m‐2) was slightly higher than that for maize (520 gC m‐2). These account for 46.4 and 42.6% of the annual values, respectively.

Representative estimates of soil and ecosystem respiration in an old beech forest

Respiration has been proposed to be the main determinant of the carbon balance in European forests and is thus essential for our understanding of the carbon cycle. However, the choice of experimental design strongly affects estimates of annual respiration and of the contribution of soil respiration to total ecosystem respiration. In a detailed study of ecosystem and soil respiration fluxes in an old unmanaged deciduous forest in Central Germany over 3 years (2000–2002), we combined soil chamber and eddy covariance measurements to obtain a comprehensive picture of respiration in this forest. The closed portable chambers offered to investigate spatial variability of soil respiration and its controls while the eddy covariance system offered continuous measurements of ecosystem respiration. Over the year, both fluxes were mainly correlated with temperature. However, when soil moisture sank below 23 vol.% in the upper 6 cm, water limitations also became apparent. The temporal resolution of the eddy covariance system revealed that relatively high respiration rates occurred during budbreak due to increased metabolic activity and after leaf fall because of increased decomposition. Spatial variability in soil respiration rates was large and correlated with fine root biomass (r2 = 0.56) resulting in estimates of annual efflux varying across plots from 730 to 1,258 (mean 898) g C m−2 year−1. Power function calculations showed that achieving a precision in the soil respiration estimate of 20% of the full population mean at a confidence level of 95%, requires about eight sampling locations. Our results can be used as guidelines to improve the representativeness of soil respiration measurements by nested sampling designs, being applied in long-term and large-scale carbon sequestration projects such as FLUXNET and CarboEurope.

Examination of model-estimated ecosystem respiration using flux measurements from a cool-temperate deciduous broad-leaved forest in central Japan

Reducing uncertainty in the emission of carbon dioxide (CO2) from plants and microbes is critically important in determining carbon budgets. We examined properties of net ecosystem CO2 exchange (NEE) derived from a processbased model that simulates an ecosystem carbon cycle, focusing on nighttime flux determined from ecosystem respiration and soil efflux. The model simulated autotrophic and heterotrophic respiration using semi-empirical ecophysiological parameterizations. In a cool-temperate deciduous broad-leaved forest in central Japan, simulation results from 1998 to 2005 were compared with measurement of the forest made using eddy-covariance and chamber methods. The model estimated annual ecosystem respiration as 1397 g Cm-2 yr-1, of which 67% was from the soil surface, with a clear seasonal cycle. Compared to flux observations, the model appropriately captured daytime NEE, but produced substantial differences from the observed nighttime NEE. The differences were evident under stable atmospheric conditions (at low friction velocity), implying a problem with the observations. With regard to soil-surface CO2 efflux (soil respiration), the model estimation was consistent with chamber observations, except in winter periods with thick snow cover. We discuss whether the model is applicable for estimating ecosystem respiration rates, and what is required to improve the predictability of the model.

Problematiche di inventariazione del carbonio nella biomassa forestale ipogea

Signatory countries of Kyoto Protocol are engaged in carrying out national inventories to quantify greenhouse gas emission and potentiality of C sinks. Forests represent the terrestrial ecosystem with the highest C sequestration capacity taking up CO2 from the atmosphere and fixing it in vegetal biomass through photosynthesis process; C stocks can be divided in aboveground and belowground ones. In inventorial processes, root biomass is empirically extrapolated from aboveground biomass using a 0.2 factor, which underestimate the real value. Some authors suggest that total underground C allocation can be assessed from the difference between annual respiration rate and litter fall. Belowground biomass can be divided in permanent biomass (structural roots) and temporary one (fine roots). Models allow a valuation of structural roots biomass from stand dendrometrical characteristics. Literature reveals that underground biomass, as fine roots than structural ones, highly varies with local conditions. The development of models that take into account these station parameters and therefore able to reproduce this variability seems to be obligatory to deal with inventory processes with an acceptable precision.

Seasonal variation of carbon exchange of typical forest ecosystems along the eastern forest transect in China

The long-term and continuous carbon fluxes of Changbaishan temperate mixed forest (CBS), Qianyanzhou subtropical evergreen coniferous forest (QYZ), Dinghushan subtropical evergreen mixed forest (DHS) and Xishuangbana tropical rainforest (XSBN) have been measured with eddy covariance techniques. In 2003, different responses of carbon exchange to the environment appeared across the four ecosystems. At CBS, the carbon exchange was mainly determined by radiation and temperature. 0 degrees C and 10 degrees C were two important temperature thresholds; the former determined the length of the growing season and the latter affected the magnitude of carbon exchange. The maximum net ecosystem exchange (NEE) of CBS occurred in early summer because maximum ecosystem photosynthesis (G(PP)) occurred earlier than maximum ecosystem respiration (Re). During summer, QYZ experienced severe drought and NEE decreased significantly mainly as a result of the depression of GPP. At DHS and XSBN, NEE was higher in the drought season than the wet season, especially the conversion between carbon sink and source occurring during the transition season at XSBN. During the wet season, increased fog and humid weather resulted from the plentiful rainfall, the ecosystem G(PP) was dispressed. The Q(10) and annual respiration of XSBN were the highest among the four ecosystems, while the average daily respiration of CBS during the growing season was the highest. Annual NEE of CBS, QYZ, DHS and XSBN were 181.5, 360.9, 536.2 and -320.0 g(.)C(.)m(-2.)a(-1), respectively. From CBS to DHS, the temperature and precipitation increased with the decrease in latitude. The ratio of N-EE/R-e increased with latitude, while R-e/G(PP), ecosystem light use efficiency (L-UE), precipitation use efficiency and average daily GPP decreased gradually. However, XSBN usually escaped such latitude trend probably because of the influence of the south-west monsoon climate which does not affect the other ecosystems. Long-term measurement and more research were necessary to understand the adaptation of forest ecosystems to climate change and to evaluate the ecosystem carbon balance due to the complexity of structure and function of forest ecosystems.

Evaluation of satellite based primary production modelling in the semi-arid Sahel

The MODIS (Moderate Resolution Imaging Spectroradiometer) primary productivity products are evaluated against observed Above-ground Net Primary Production (AGNPP) in the semi-arid Senegal 2001. MODIS net primary productivity (NPP) modelling is a light use efficiency (LUE) based approach incorporating constraints on vegetation productivity arising from simulated radiation, water demand and temperature data from NASA's Data Assimilation Office (DAO). Annually integrated MODIS PSN (MOD17A2 net photosynthesis, Collection 4) explains more of the observed biomass variation ( r 2 = 0.77) than MODIS fAPAR (fraction Absorbed Photosynthetically Active Radiation, Collection 4) ( r 2 = 0.72), indicating the effect of including the canopy stress scalar ( ε s) based on DAO data combined with modelled maintenance respiration costs (of leaf and fine roots). Annual MODIS NPP (MOD17A3, Collection 4 (C4) and Collection 4.5 (C4.5)) including growth respiration and live wood maintenance respiration costs and modified DAO input (C4.5) however increases the residual unexplained observed AGNPP variance (C4 NPP; r 2 = 0.49) (C4.5 NPP; r 2 = 0.37). The overall quality of the annual NPP MODIS C4 and C4.5 products are moderate for the semi-arid Senegal because of the annual respiration cost modelling and a change in C4.5 biome-specific parameters stored in a Biome Properties Look-Up Table (BPLUT) is the main contributor to the observed discrepancy between C4 and C4.5 NPP. The dynamic range of the values of all MOD17 products was too low when compared to observed AGNPP. An estimate of canopy water stress (SIWSI; Shortwave Infrared Water Stress Index) derived from MODIS channels 2 and 6 and photosynthetically active radiation (PAR) irradiance derived from geostationary METEOSAT data were tested for primary production modelling using a stepwise linear regression analysis. PAR irradiance was combined with MODIS fAPAR into APAR (Absorbed Photosynthetically Active Radiation) explaining 79% of the observed AGNPP variation. Introducing SIWSI significantly increased the explained variance of observed AGNPP ( r 2 = 0.89). MODIS-derived percentage tree cover was tested as a predictor based on the hypothesis that tree cover provides information on differences in respiratory costs between trees and grasses thereby accounting for variations in the LUE conversion efficiency ε. No significant reduction in residual unexplained AGNPP variance was found. Earth observation based derivation of PAR and canopy water stress from SIWSI suggest potential improvements to primary production models in semi-arid biomes that can be implemented in general NPP modelling LUE methodology.

Soil respiration in six temperate forests in China

AbstractScaling soil respiration (RS), the major CO2 source to the atmosphere from terrestrial ecosystems, from chamber‐based measurements to ecosystems requires studies on variations and correlations of RS from various biomes and across geographic regions. However, few studies on RS are available for Chinese temperate forest despite the importance of this forest in the national and global carbon budgets. In this study, we conducted 18‐month RS measurements during 2004–2005 in six temperate forest types, representing the typical secondary forest ecosystems across various site conditions in northeastern China: Mongolian oak (Quercus mongolica Fisch.), aspen‐birch (Populous davidiana Dode and Betula platyphylla Suk.), mixed deciduous (no dominant tree species), hardwood (dominated by Fraxinus mandshurica Rupr., Juglans mandshurica Maxim., and Phellodendron amurense Rupr.) forests, Korean pine (Pinus koraiensis Sieb. et Zucc.) and Dahurian larch (Larix gmelinii Rupr.) plantations. Our specific objectives were to: (1) explore relationships of RS against soil temperature and water content for the six forest ecosystems, (2) quantify annual soil surface CO2 flux and its relations to belowground carbon storage, (3) examine seasonal variations in RS and related environmental factors, and (4) quantify among‐ and within‐ecosystem variations in RS. The RS was positively correlated to soil temperature in all forest types, and was significantly influenced by the interactions of soil temperature and water content in the pine, larch, and mixed deciduous forests. The sensitivity of RS to soil temperature at 10 cm depth (Q10) ranged from 2.61 in the oak forest to 3.75 in the aspen‐birch forests. The Q10 tended to increase with soil water content until reaching a threshold, and then decline. The annual RS for the larch, pine, hardwood, oak, mixed deciduous, and aspen‐birch forests averaged 403, 514, 781, 785, 786, and 813 g C m−2 yr−1, respectively. The annual RS of the broadleaved forests was 72% greater than that of the coniferous forests. The annual RS was positively correlated to soil organic carbon (SOC) concentration at O horizon (R2=0.868) and total biomass of roots <0.5 cm in diameter (R2=0.748). The coefficient of variation (CV) of RS among forest types averaged 25% across the 18‐month measurements. The CV of RS within plots varied from 20% to 27%, significantly (P<0.001) greater than those among plots (9–15%), indicating the importance of the fine‐scaled heterogeneity in RS. This study emphasized that variations in soil respiration and potential sampling bias should be appropriately tackled for accurate soil CO2 flux estimates.

On linking interannual tree ring variability with observations of whole‐forest CO2 flux

AbstractWe used a 10‐year record of the CO2 flux by an old growth boreal forest in central Manitoba (the Northern Old Black Spruce Site (NOBS)), a ∼150‐year‐old Picea mariana [Mill.] stand) to determine whether and how whole‐forest CO2 flux is related to tree ring width. We compared a 37‐year ring width chronology collected at NOBS to a second chronology that was collected at a nearby Black Spruce stand with a different disturbance history, and also to three measures of annual whole‐forest photosynthesis [gross ecosystem production (GEP)], two measures of annual respiration (R), and one measure of annual carbon balance [net ecosystem production (NEP)]. The year‐to‐year ring width fluctuations were well correlated between the two sites; increasing our confidence in the NOBS chronology and implying that ring width variation is driven and synchronized by the physical environment. Both chronologies exhibited serial correlation, with a fluctuation in ring width that had an apparent periodicity of ∼7 years. Neither chronology was correlated with variation in annual precipitation or temperature. Ring width and NEP increased, while R decreased from 1995 to 2004. GEP either remained constant or decreased from 1995 to 2004, depending on which measure was considered. The lack of relationship between ring width and GEP may indicate that ring growth is controlled almost entirely by something other than carbon uptake. Alternative explanations for the ring width chronologies include the possibility that wood production varies as a result of shifts in respiration, or that an unidentified aspect of the environment, rather than the balance between GEP and respiration, controls wood production. The serial correlation in ring width may be related to increases and decreases in carbohydrate pools, or to gradual changes in nutrient availability, pathogens, herbivores, soil frost or soil water table. The cause or causes of serial correlation, and the controls on the allocation of photosynthate to wood production, emerge as critical uncertainties for efforts in predicting the carbon balance of boreal ecosystems and inferring past climate from tree rings.

Cross-calibration functions for soil CO2efflux measurement systems

Different soil CO2 efflux measurement systems and methodologies were used to estimate the annual soil respiration of different forest sites. To allow comparison between these annual values, this study aimed to cross-calibrate five soil CO2 efflux (RS) closed dynamic chamber systems, and compare the in situ measurement methodologies. We first assessed the impact of the measurement methodology on RS by studying the effects of three parameters: record duration, time lag before starting to record and the mode of chamber-soil contact (use of collars or insertion of the chambers into the soil). Secondly, we directly compared systems with identical methodology during field measurements on three forest sites. We observed a significant influence of the chamber-soil contact mode (no impact of the record duration and duration before starting to record). Measurements obtained by insertion led to significantly higher estimates of RS than those obtained using collars (up to 28%). Our inter-comparison showed that deviations existing between in situ measurements performed with the different systems were partly systematic and could be corrected using simple linear equations. Measurements of pressure difference between the inside and the outside of soil chambers allowed explaining a part of the observed deviations between systems. Finally, we assessed the influence of the cross-calibration equations on annual respiration of two beech forest soils. cross calibration / forest ecosystem / measurement system / pressure effect / soil CO2 efflux

Components and seasonal variation of night-time total ecosystem respiration in a Japanese broad-leaved secondary forest

The Yamashiro Experimental Forest is a broad-leaved secondary forest in Kyoto, Japan. On its complex terrain, low wind speed, high air stability, and local advection are common at night. To reduce the uncertainty in measuring woodytissue respiration at night, we used automated stem chambers to measure stem respiration continuously (for 5 min at 30 min intervals) on stems of Quercus serrata Murr. (deciduous) and Ilex pedunculosa Miq. (evergreen) throughout 2003. Using these data, we estimated night-time respiration for the total ecosystem and its various components, and we report foliar and soil respiration rates for 2003. Annual average night-time respiration of soil, evergreen leaf, deciduous leaf, evergreen woody tissue and deciduous woody tissue were estimated as 0.0794 (63.2%), 0.0101 (8.0%), 0.0160 (12.7%), 0.0064 (5.1%) and 0.0137 (10.9%) mg CO2 m-2 s-1, respectively. The contribution of soil respiration to the total ecosystem respiration rate reached its minimum (49.1%) on 12 June (DOY 163) and its maximum (82.4%) on 29 November (DOY 333). Seasonal change of growth respiration was marked, indicating that the seasonal variation of growth respiration must be evaluated carefully to estimate total ecosystem respiration. Therefore, long-term continuous measurement using automated chambers and averaging provides an effective means of evaluating the annual night-time ecosystem respiration.

Trace gas emissions through a winter snowpack in the subalpine ecosystem at Niwot Ridge, Colorado

A preliminary study of whole air measurements within a snowpack in the subalpine forest of Colorado found that lower snow layers and underlying soil were a strong source of chemically active trace gases (VOC). The emissions of trace gases were correlated with emissions of radiatively active greenhouse gases (CO2 and N2O) through the snowpack. Carbon dioxide was enhanced in the lower snowpack to over 1000 ppmv, similar to previous studies, which have shown that a thermal insulation of snow covering the soil leads to enhanced emissions of CO2 equivalent to a large fraction of annual respiration. Comparable enhancements of several VOCs reached 1275, 1670, and 1000 pptv for methyl chloride, propane, and α‐pinene, respectively. The measurements are limited to a one‐day pilot study, however, considering that previous studies have reported significant CO2 emissions through snowpacks any emissions of VOC may also be important on a local or potentially global scale.

Characteristics of photosynthesis and stomatal conductance in the shrubland species manuka (Leptospermum scoparium) and kanuka (Kunzea ericoides) for the estimation of annual canopy carbon uptake.

Responses of photosynthesis to carbon dioxide (CO2) partial pressure and irradiance were measured on leaves of 39-year-old trees of manuka (Leptospermum scoparium J. R. Forst. & G. Forst.) and kanuka (Kunzea ericoides var. ericoides (A. Rich.) J. Thompson) at a field site, and on leaves of young trees grown at three nitrogen supply rates in a nursery, to determine values for parameters in a model to estimate annual net carbon uptake. These secondary successional species belong to the same family and commonly co-occur. Mean (+/- standard error) values of the maximum rate of carboxylation (hemi-surface area basis) (Vcmax) and the maximum rate of electron transport (Jmax) at the field site were 47.3 +/- 1.9 micromol m(-2) s(-1) and 94.2 +/- 3.7 micromol m(-2) s(-1), respectively, with no significant differences between species. Both Vcmax and Jmax were positively related to leaf nitrogen concentration on a unit leaf area basis, and the slopes of these relationships did not differ significantly between species or between the trees in the field and young trees grown in the nursery. Mean values of Jmax/Vcmax measured at 20 degrees C were significantly lower (P < 0.01) for trees in the field (2.00 +/- 0.05) than for young trees in the nursery with similar leaf nitrogen concentrations (2.32 +/- 0.08). Stomatal conductance decreased sharply with increasing air saturation deficit, but the sensitivity of the response did not differ between species. These data were used to derive parameters for a coupled photosynthesis-stomatal conductance model to scale estimates of photosynthesis from leaves to the canopy, incorporating leaf respiration at night, site energy and water balances, to estimate net canopy carbon uptake. Over the course of a year, 76% of incident irradiance (400-700 nm) was absorbed by the canopy, annual net photosynthesis per unit ground area was 164.5 mol m(-2) (equivalent to 1.97 kg C m(-2)) and respiration loss from leaves at night was 37.5 mol m(-2) (equivalent to 0.45 kg m(-2)), or 23% of net carbon uptake. When modeled annual net carbon uptake for the trees was combined with annual respiration from the soil surface, estimated net primary productivity for the ecosystem (0.30 kg C m(-2)) was reasonably close to the annual estimate obtained from independent mensurational and biomass measurements made at the site (0.22 +/- 0.03 kg C m(-2)). The mean annual value for light-use efficiency calculated from the ratio of net carbon uptake (net photosynthesis minus respiration of leaves at night) and absorbed irradiance was 13.0 mmol C mol(-1) (equivalent to 0.72 kg C GJ(-1)). This is low compared with values reported for other temperate forests, but is consistent with limitations to photosynthesis in the canopy attributable mainly to low nitrogen availability and associated low leaf area index.

Component carbon fluxes and their contribution to ecosystem carbon exchange in a pine forest: an assessment based on eddy covariance measurements and an integrated model.

We used a combination of eddy flux, canopy, soil and environmental measurements with an integrated biophysical model to analyze the seasonality of component carbon (C) fluxes and their contribution to ecosystem C exchange in a 50-year-old Scots pine forest (Pinus sylvestris L.) in eastern Finland (62 degrees 47' N, 30 degrees 58' E) over three climatically contrasting years (2000-2002). Eddy flux measurements showed that the growing Scots pine forest was a sink for CO2, with annual net C uptakes of 131, 210 and 258 g C m-2> year-1 in 2000, 2001 and 2002, respectively. The integrated process model reproduced the annual course of daily C flux above the forest canopy as measured by the eddy covariance method once the site-specific component parameters were estimated. The model explained 72, 66 and 68% of the variation in daily net C flux in 2000, 2001 and 2002, respectively. Modeled annual C loss by respiration was 565, 629 and 640 g C m-2 year-1, accounting for 77, 77 and 65% of annual gross C uptake, respectively. Carbon fluxes from the forest floor were the dominant contributors to forest ecosystem respiration, with the fractions of annual respiration from the forest floor, foliage and wood being 46-62, 27-44 and 9-10%, respectively. The wide range in daily net C uptake during the growing season was largely attributable to day-to-day fluctuations in incident quantum irradiance. During just a few days in early spring and late autumn, ecosystem net C exchange varied between source and sink as a result of large daily changes in temperature. The forest showed a greater reduction in gross C uptake by photosynthesis than in C loss by respiration during the dry summer of 2000, indicating that interannual variability in ecosystem net C uptake at this site was modified mostly by summer rainfall and vapor pressure deficit.

Respiration and photosynthesis characteristics of current‐year stems of Fagus sylvatica: from the seasonal pattern to an annual balance

• Temperature and light responses of CO2 efflux of Fagus sylvatica (beech) current-year stems were measured for 1 yr to estimate their annual carbon balance. • Gas exchanges were determined using infrared gas analysis. Seasonal patterns of a fluorescence parameter ((Fv /Fm )max ), nitrogen and chlorophyll contents were also assessed in stems and leaves, using standard techniques. • Basal respiration rates at 20°C (R20 ) were very high during the growing season, reaching a maximum of 17 170 µmol m-3 s-1 . Light-saturated assimilation followed the same seasonal pattern as R20 . During the winter, chlorophyll content was undiminished compared with the summer, N content was slightly increased, and despite low (Fv /Fm )max values, instantaneous maximum assimilation could account for 80-110% of the respiration. • For an average-size stem (4 mm diameter), the estimated annual respiration was 0.5 g carbon with 55% of this amount attributed to maintenance respiration. The potential assimilation contributed 0.2 g carbon and approximately compensated for the growth respiration. Information on older branches and trunks is now needed for estimations at the tree and stand levels.

Abiotic controls on nitrogen fixation and respiration in selected woody debris from the Pacific Northwest, U.S.A.

We estimated the effects of temperature, moisture, and oxygen concentration on nitrogen fixation and respiration in woody debris and used this information to model seasonal variation in these processes. We measured acetylene reduction and CO2 evolution of wood samples to determine the relative effect of these abiotic factors on nitrogen fixation and respiration. The interactions of these abiotic factors were examined in a model to test whether temperature alone can be used as a predictor of seasonal changes in nitrogen fixation and respiration rates in woody debris. Nitrogen fixation rates were optimum near 30ºC, whereas respiration rates were optimum over a broader range, from 30°C to 50°C. Nitrogen fixation and respiration rates were greatest above 175% and 100% wood moisture content, respectively, with little activity below 50%. Nitrogen fixation was optimum at 2% O2, with activity much reduced above and below this concentration. Respiration was optimal when O2 exceeded 1%. In our simulations, annual nitrogen fixation and respiration rates were 7.8 and 1.7 times greater, respectively, when only temperature limitation was included than when moisture and oxygen limitations were also included. Therefore, seasonal interactions of abiotic factors need to be considered when estimating annual nitrogen fixation and respiration rates.

Carbon cycling of alpine tundra ecosystems on Changbai Mountain and its comparison with arctic tundra

The alpine tundra on Changbai Mountain was formed as a left-over ‘island’ in higher elevations after the glacier retrieved from the mid-latitude of Northern Hemisphere to the Arctic during the fourth ice age. The alpine tundra on Changbai Mountain also represents the best-reserved tundra ecosystems and the highest biodiversity in northeast Eurasia. This paper examines the quantity of carbon assimilation, litters, respiration rate of soil, and storage of organic carbon within the alpine tundra ecosystems on Changbai Mountain. The annual net storage of organic carbon was 2092 t/a, the total storage of organic carbon was 33457 t, the annual net storage of organic carbon in soil was 1054 t/a, the total organic carbon storage was 316203 t, and the annual respiration rate of soil was 92.9% and was 0.52 times more than that of the Arctic. The tundra-soil ecosystems in alpine Changbai Mountain had 456081 t of carbon storage, of which, organic carbon accounted for 76.7% whereas the mineral carbon accounted for 23.3%.