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Ecosystem respiration budget for a Pinus sylvestris stand in central Siberia
Abstract
abstractUsing a ground-based and an above-canopy eddy covariance system in addition to stem respiration measurements, the annual respiratory fluxes attributable to soil, stems and foliage were determined for a Scots pine (Pinus sylvestris L.) forest growing in central Siberia. Night-time foliar respiration was estimated on the basis of the difference between fluxes measured below and above the canopy and the stem respiration measurements. Comparison of the effects of night-time turbulence on measured CO2 fluxes showed flux loss above the canopy at low wind speeds, but no such effect was observed for the ground-based eddy system. This suggests that problems with flow homogeneity or flux divergence (both of which would be expected to be greater above the canopy than below) were responsible for above-canopy losses under these conditions. After correcting for this, a strong seasonality in foliar respiration was observed. This was not solely attributable to temperature variations, with intrinsic foliar respiratory capacities being much greater in spring and autumn. The opposite pattern was observed for stem respiration, with the intrinsic respiratory capacity being lower from autumn through early spring. Maximum respiratory activity was observed in early summer. This was not simply associated with a response to higher temperatures but seemed closely linked with cambial activity and the development of new xylem elements. Soil respiration rates exhibited an apparent high sensitivity to temperature, with seasonal data implying a Q10 of about 7. We interpret this as reflecting covarying changes in soil microbial activity and soil temperatures throughout the snow-free season. Averaged over the two study years (1999 and 2000), the annual respiratory flux was estimated at 38.3 mol C m−2 a−1. Of this 0.61 was attributable to soil respiration, with stem respiration accounting for 0.21 and foliar respiration 0.18.
... Noticeable peak periods of soil CO2 efflux are observed in the summer or early autumn whereas soil efflux is lowest during the often-long winters (e.g. Rayment and Jarvis 2000;Högberg et al. 2001;Shibistova et al. 2002a;Pumpanen et al. 2003a;Domisch et al. 2006). In addition to seasonal changes in temperature and moisture, the seasonal pattern of soil CO2 efflux is influenced by many factors; root production of boreal plants and as well as mycelial production of ectomycorrhizal fungi have been found to vary seasonally in northern ecosystems (Wallander et al. 1997;2001;Steinaker et al. 2010), which most likely also influence the temporal variation of forest soil CO2 efflux through root respiration and root-associated heterotrophic respiration. ...
... The peak CO2 efflux occurred in July-August as observed in many previous studies in boreal coniferous forests (e.g. Morén and Lindroth 2000;Högberg et al. 2001;Shibistova et al. 2002a;Domisch et al. 2006;Kolari et al. 2009). The highest soil CO2 efflux at 10°C was found in August as well and the lowest in May, similar to the temperature response pattern observed in a Siberian Scots pine forest (Shibistova et al. 2002a). ...
... Morén and Lindroth 2000;Högberg et al. 2001;Shibistova et al. 2002a;Domisch et al. 2006;Kolari et al. 2009). The highest soil CO2 efflux at 10°C was found in August as well and the lowest in May, similar to the temperature response pattern observed in a Siberian Scots pine forest (Shibistova et al. 2002a). The observed seasonality of temperature response in monthly models corresponded also well to the pattern reported for a temperate forest (Janssens and Pilegaard 2003), with greater Q10's and lower base respiration (i.e. ...
... We studied the seasonal pattern of soil CO 2 efflux rates using both eddy covariance and chamber techniques, also examining seasonal changes in microbial population structure and sources of spatial variability in soil CO 2 efflux rates. Data from this study form the basis of an accompanying paper, in which estimates of the annual respiratory balance of the stand at the ecosystem level are given (Shibistova et al., 2002). ...
... The inter-relationships between soil temperature and soil moisture in determining overall rates of soil CO 2 efflux are discussed further in Section 3.5. This lack of a fit of the model of Lloyd and Taylor (1994) is considered in much more detail in an accompanying paper with alternative formulations for describing the temperature dependence of soil respiration for this ecosystem also being given (Shibistova et al., 2002). A second possibility for the differences between the chamber and eddy covariance measurements might be that the eddy covariance system was systematically understimating fluxes due to suppressed or intermittent turbulence characteristics close to the forest floor (Janssens et al., 2001). ...
... Even when eddy covariance measurements are made above a canopy, there are sometimes indications that some " loss of flux " may occur for under conditions of low wind speeds at night (Goulden et al., 1996; Jarvis et al., 1997). We have examined this problem closely elsewhere (Shibistova et al., 2002) and conclude that, in contrast to the eddy covariance system installed above the forest, flux " losses " at low turbulence were not a problem for the ground eddy covariance system in this experiment. It is, however, well known that different soil CO 2 efflux measurement methodologies or even instruments can give vastly different soil CO 2 efflux estimates (Healy et al., 1996; Norman et al., 1997; Arneth et al., 1998; Le Dantec et al., 1999; Janssens et al., 2001), and the basis of these differences is not always well known or even consistent. ...
Rates of CO 2 efflux from the floor of a central Siberian Scots pine (Pinus sylvestris) forest were measured using a dynamic closed chamber system and by a eddy covariance system placed 2.5 m above the forest floor. Measurements were undertaken for a full growing season: from early May to early October 1999. Spatial variability as determined by the chamber measurements showed the rate of CO 2 efflux to depend on location, with rates from relatively open areas ("glades") only being about 50% those observed below or around trees. This was despite generally higher temperatures in the glade during the day. A strong relationship between CO 2 efflux rate and root density was observed in early spring, suggesting that lower rates in open areas may have been attributable to fewer roots there. Continuous measurements with the eddy covariance system provided good temporal coverage. This method, however, provided estimates of ground CO 2 efflux rate rates that were about 50% lower than chamber measurements that were undertaken in areas considered to be representative of the forest as a whole. An examination of the seasonal pattern of soil CO 2 efflux rates suggests that much of the variability in CO 2 efflux rate could be accounted for by variations in soil temperature. Nevertheless, there were also some indications that the soil water deficits served to reduce soil CO 2 efflux rates during mid-summer. Overall the sensitivity of CO 2 efflux rate to temperature seems to be greater for this boreal ecosystem than has been the case for most other studies.
... Our interest lies with the underlying controls on canopy physiology, rather than reporting annual totals (for these c.f. above referenced publications), and we therefore divide the measured ecosystem-atmosphere exchange of CO 2 (NEE) into its component fluxes assimilation (A) and respiration (R). In case of the Siberian ecosystems, Arrhenius-type relationships of measured night-time NEE (=R) with soil temperature were established, and by using these, R were extrapolated to daylight hours (Lloyd and Taylor, 1994;Arneth et al., 2002a;Shibistova et al., 2002). In the Mopane woodland, ecosystem respiration rates were frequently confined by soil moisture rather than by temperature. ...
... This period thus exceeds the green or growing season, as it can commence before new leaf-growth and continue beyond senescence. We also explicitly acknowledge continuation of some ecosystem activity during the dormant period, particularly heterotrophic respiration, since the low rates of CO 2 efflux observed in the cold or dry season can sum up to a sizeable portion of the annual budget and must not be ignored (Hanan et al., 1998;Lafleur et al., 2001;Arneth et al., 2002a;Shibistova et al., 2002;Aurela et al., 2004;Veenendaal et al., 2004). To alleviate comparison of the seasonality in the observed carbon exchange rates in ecosystems that represent diverse biomes from both hemispheres we define the onset of the active period as the first day following the month of August when rain exceeds 10 mm (Botswana), and the first day after January with average air temperature exceeding 0 @BULLET C (Siberia), respectively. ...
... Still, separate cuvette measurements on soil, stem and leaf level would be required to investigate these processes in more detail. Throughout the year, an ecosystems' " capacity " for respiration varies since plant growth and maintenance respiration, and heterotrophic activity respond rather plastically to varying environmental conditions (e.g.,Law et al., 1999;Arneth et al., 2002a;Shibistova et al., 2002;Atkin and Tjoelker, 2003;Pendall et al., 2004). Over a period of weeks to months, the exponential respiration-temperature response which typically dominates the short-term variation in respiration rates is thus mediated by additional factors, like available soil moisture or phenology. ...
We compare assimilation and respiration rates, and water use strategies in four divergent ecosystems located in cold-continental central Siberia and in semi-arid southern Africa. These seemingly unrelated systems have in common a harsh and highly seasonal environment with a very sharp transition between the dormant and the active season, and with vegetation facing dry air and soil conditions for at least part of the year. Moreover, the northern high latitudes and the semi-arid tropics will likely experience changes in key environmental parameters (e.g., air temperature and precipitation) in the future; indeed, in some regions marked climate trends have already been observed over the last decade or so. The magnitude of instantaneous or daily assimilation and respiration rates, derived from one to two years of eddy covariance measurements in each of the four ecosystems, was not related to the growth environment. For instance, respiration rates were clearly highest in the two deciduous systems included in the analysis (a Mopane woodland in northern Botswana and a Downy birch forest in Siberia; > 300 mmol m-2Td-1), while assimilation rates in the Mopane woodland were relatively similar to a Siberian Scots pine canopy for a large part of the active season (ca. 420 mmol m -2d-1). Acknowledging the limited number of ecosystems compared here, these data nevertheless suggest that factors like vegetation type, canopy phenology or ecosystem age can override larger-scale climate differences in terms of their effects on carbon assimilation and respiration rates. By far the highest rates of assimilation were observed in Downy birch, an early successional species. These were achieved at a rather conservative water use, as indicated by relatively low levels of A, the marginal water cost of plant carbon gain. Surprisingly, the Mopane woodland growing in the semi-arid environment had significantly higher values of ¿. However, its water use strategy included a very plastic response to intermittently dry periods, and values of A were much more conservative overall during a rainy season with low precipitation and high air saturation deficits. Our comparison demonstrates that forest ecosystems can respond very dynamically in terms of water use strategy, both on interannual and much shorter time scales. It remains to be evaluated whether and in which ecosystems this plasticity is mainly due to a short-term stomatal response, or goes hand in hand with changes in canopy photosynthetic capacity
... The contribution of forest floor respiration to the total ecosystem respiration observed in our study was close to the higher end of the range of 40-90% estimated for other boreal forests (Bergeron et al., 2009;Black et al., 1996;Goulden and Crill, 1997;Ikawa et al., 2015;Launiainen et al., 2005;Morén and Lindroth, 2000;Shibistova et al., 2002). The stem and foliage respiration estimated as R eco minus R ff , accounting for 10-32% of R eco at our pine stand, compares with 39% of R eco estimated in a 200-year-old Scots pine stand in central Siberia (Shibistova et al., 2002) and 15-45% of R eco reported in a temperate 70-year-old white pine (Pinus strobues L.) forest in Canada (Peichl et al., 2010). ...
... The contribution of forest floor respiration to the total ecosystem respiration observed in our study was close to the higher end of the range of 40-90% estimated for other boreal forests (Bergeron et al., 2009;Black et al., 1996;Goulden and Crill, 1997;Ikawa et al., 2015;Launiainen et al., 2005;Morén and Lindroth, 2000;Shibistova et al., 2002). The stem and foliage respiration estimated as R eco minus R ff , accounting for 10-32% of R eco at our pine stand, compares with 39% of R eco estimated in a 200-year-old Scots pine stand in central Siberia (Shibistova et al., 2002) and 15-45% of R eco reported in a temperate 70-year-old white pine (Pinus strobues L.) forest in Canada (Peichl et al., 2010). However, the stem and foliage respiration might be slightly underestimated in this study because R ff measured using the EC method includes small fractions of the autotrophic respiration from stems, branches, foliage, and seedlings existing in the lower part of the forest below the EC mounting height (2.5 m), which is however likely small compared to the forest floor contribution (Tarvainen et al., 2017). ...
The forest floor provides an important interface of soil-atmosphere CO2 exchanges but their controls and contributions to the ecosystem-scale carbon budget are uncertain due to measurement limitations. In this study, we deployed eddy covariance systems below- and above-canopy to measure the spatially integrated net forest floor CO2 exchange (NFFE) and the entire net ecosystem CO2 exchange (NEE) at two mature contrasting stands located in close vicinity in boreal Sweden. We first developed an improved cospectra model to correct below-canopy flux data. Our empirical below-canopy cospectra models revealed a greater contribution of large- and small-scale eddies in the trunk space compared to their distribution in the above-canopy turbulence cospectra. We found that applying the above-canopy cospectra model did not affect the below-canopy annual CO2 fluxes at the sparse pine forest but significantly underestimated fluxes at the dense mixed spruce-pine stand. At the mixed spruce-pine stand, forest floor respiration (Rff) was higher and photosynthesis (GPPff) was lower, leading to a 1.4 times stronger net CO2 source compared to the pine stand. We further found that drought enhanced Rff more than GPPff, leading to increased NFFE. Averaged across the six site-years, forest floor fluxes contributed 82% to ecosystem-scale respiration (Reco) and 12% to gross primary production (GPP). Since the annual GPP was similar between both stands, the considerable difference in their annual NEE was due to contrasting Reco, the latter being primarily driven by the variations in NFFE. This implies that NFFE acted as the driver for the differences in NEE between these two contrasting stands. This study therefore highlights the important role of forest floor CO2 fluxes in regulating the boreal forest carbon balance. It further calls for extended efforts in acquiring high spatiotemporal resolution data of forest floor fluxes to improve predictions of global change impacts on the forest carbon cycle.
... Nonetheless, several studies suggest the importance of starch accumulation in forest ecosystems, and this may account for discrepancies between interannual variations in eddy-covariance-based and biometricbased estimates. For example, interannual changes in tower-based GPP estimates did not correlate with changes in tree rings in a Scots pine forest growing in central Siberia (Shibistova et al. 2002). These data suggested that interannual changes in the demand of carbohydrates for new stem production might be compensated by changes in growth rates of other parts of the tree or by increased starch accumulation. ...
Net primary production (NPP) refers to the net amount of the carbon and energy fixed by green plants through photosynthetic activity. Estimates of NPP are of fundamental human importance, because food supply is predominantly dependent on plant productivity. Moreover, measurements of spatiotemporal variations of forest NPP provide important information for understanding and projecting the global carbon cycle, because forest ecosystems are a major terrestrial carbon sink. Here we discuss methods for estimating NPP in forest ecosystems using inventory data, and describe the “summation method”, which was developed in Japan in the 1960s at the International Biological Program (IBP) to facilitate standardization in the absence of complex instruments under field conditions. Global climate change prompted to development of this “summation method” as an improved “biometric method” in the 1990s. Biometric-based estimates of NPP are conceptually defined as the total amount of new organic matter produced during an interval per unit area at the ecosystem scale, and are expressed as the sum of stand increments of living biomass (SI), newly produced aboveground litter (L
an
), and fine root production (P
fr
). The SI of above- and belowground (coarse roots) biomass can be estimated by tracking the survival and diameter of individual tree stems in a permanent plot. Aboveground litter of short-lived organs (L
an
), such as deciduous leaves, flowers, and fruits, can be determined using litter traps that are set on the forest floor. Although methods for determining P
fr
remain unstandardized, 1-year turnover of fine roots is often considered an estimate of fine root dynamics (P
fr
≈ mean fine root biomass). In a study of the Takayama Experimental Forest, we demonstrated correlations of biometric-based NPP estimates with various methods, such as simulated canopy photosynthesis by scaling up leaf photosynthesis and incorporating values obtained using eddy covariance. The resulting biometric method has sufficient sensitivity to demonstrate climate-induced year-to-year variations of tree growth and allocation of carbon inputs by NPP in forest ecosystems.
... Correspondingly, in the Russian Federation, this method is constantly, temporarily, or occasionally used only in some sites, mainly in boreal forests. These studies were organized in the Central Forest Reserve, Tver oblast (Ol'chev et al., 2008); Krasnoyarsk krai, Zotino and Khakassia (Kelliher et al., 1998; Knohl et al., 2002; Shibistova et al., 2002); Central and Northern Yakutia (Maksii mov et al., 2005); ...
A lot of studies on the impact of global climate changes on natural communities deal with cryogenic ecosystems, tundra in particular, since they are delimited by low air temperature and permafrost, thus being extremely sensitive to long-term climate fluctuations. Continuous warming in Northern Hemisphere is unmasking all the more details concerning complex system of direct relationships, feedbacks, and interactions of carbon balance factors as the main response function. While the set of such factors may be viewed as more or less complete, their relative contribution to C-balance, as is becoming clear with accumulating results of field observations, directly depends on temporal scale of observations and is not constant. As the results of field observations and modeling of tundra ecosystems show, any one of significant factors can become the leading one within the boundaries determined by the given scale of observations. Even the least significant factor can become the determining one for direction of carbon annual net flux in an ecosystem, if contributions of more significant factors canceled each other during the period of observations. In the most general situation, the greater is the variation of a significant factor during the period of observations, the larger is its partial contribution. The complete set of independent variables of C-balance is not limited by abiotic factors but should include such an important factor as a stock of plants living top mass, which can be treated as not only the natural product of C-balance but also as its independent parameter.
... Moreover, chambers can change the soil-plant environment (air temperature and humidity) and affect the measurement results. For high precision and the ability to measure plant carbon flux at different heights, the eddy covariance technique is the main tool used to measure carbon flux in many FLUXNET sites (Valentini et al. 2000; Baldocchi 2003; giasson et al. 2006). in eddy covariance research, daytime ecosystem respiration is obtained from the empirical function of night time Co 2 exchange versus soil temperature (the Q 10 model) (Shibistova et al. 2002; Zamolodchikov et al. 2003; Zha et al. 2004; Xu & Baldocchi 2004; Van Dijk & Dolman 2004). This method is suitable for simulating ecosystem respiration of forest, grassland and single cropping systems. ...
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 R0. Residual Rec was significantly correlated with LAI. After considering LAI, the modified Q10 model could estimate Rec 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 yr for the 4 years of the study. As a 4‐year average, seasonal mean ecosystem respiration in wheat (2.60 gC m day) was much lower than in maize (6.09 gC m day). However, integrated ecosystem respiration for the wheat growing season (566 gC m) was slightly higher than that for maize (520 gC m). These account for 46.4 and 42.6% of the annual values, respectively.
... Monthly averages and models agreed with earlier findings of greater efflux in autumn compared to spring at comparable temperatures in a temperate forest (Crill, 1991). Our monthly models resulted in a pattern of predicted soil CO 2 efflux identical to that found in a Siberian pine forest study (Shibistova et al., 2002), with the month of May having the lowest CO 2 efflux at 10 @BULLET C and August having the highest. Variation in soil moisture did not explain the seasonality of the temperature response. ...
Our objectives were to identify factors related to temporal variation of soil CO2 efflux in a boreal pine forest
and to evaluate simple predictive models of temporal variation of soil CO2 efflux. Soil CO2 efflux was measured with a portable chamber in a Finnish Scots pine forest for three years, with a fourth year for model evaluation. Plot averages for soil CO2 efflux ranged from 0.04 to 0.90 gCO2 m−2 h−1 during the snow-free period, i.e. May–October, and from 0.04 to 0.13 gCO2 m−2 h−1 in winter. Soil temperature was a good predictor of soil CO2 efflux. A quadratic model of lntransformed efflux explained 76–82% of the variation over
the snow-free period. The results revealed an effect of season: at a given temperature of the organic layer, soil CO2 efflux was higher later in the snow-free period (in August and September) than in spring and early summer (in May and June). Regression coefficients for temperature (approximations of a Q10 value) of month-specific models decreased with increasing average soil temperatures. Efflux in July, the month of peak photosynthesis, showed no clear response to temperature or moisture. Inclusion of a seasonality index, degree days, improved the accuracy of temperature response models to predict efflux for the fourth year of measurements, which was not used in building of regression models. During peak efflux from mid-July to late-August, efflux was underestimated with the models that included degree days as well as with the models
that did not. The strong influence of the flux of photosynthates belowground and the importance of root respiration could explain the relative temperature insensitivity observed in July and together with seasonality of growth of root and root-associated mycorrhizal fungi could explain partial failure of models to predict magnitude of efflux in the peak season from mid-July to August.
http://www.biogeosciences.net/8/3169/2011/bg-8-3169-2011.pdf
... data reported by Quegan et al. ( 2011 ) – between 73 and 59 % of NPP. Similarly, Wirth et al. ( 2002 ) estimated 48 % for post fi re chronosequences in pine forests of Siberian middle taiga, although reported values for NEP (or NEE) for taiga forests of Central Siberia vary greatly – from 50–60 to 250–270 gC m −2 year −1 (e.g., Röser et al . 2002 ; Shibistova et al . 2002 ; Schulze 2002 ) . Substantial differences between NPP and soil respiration can occur, particularly for crops, since a major part of the NPP is removed as yield, e.g., the sink of agro-ecosystems of the taiga zone of the region was estimated to be 111 gC m −2 year −1 , or 42 % of NPP (Vedrova 2002 ) . Such effects usually are not represen ...
This chapter considers the current state of Siberian terrestrial ecosystems, their spatial distribution, and major biometric characteristics. Ongoing climate change and the dramatic increase of accompanying anthropogenic pressure provide different but mostly negative impacts on Siberian ecosystems. Future climates of the region may lead to substantial drying on large territories, acceleration of disturbance regimes, deterioration of ecosystems, and positive feedback to global warming. The region requires urgent development and implementation of strategies of adaptation to, and mitigation of, negative consequences of climate change.
... As the flux of melt water increases, the processes of freezing and thawing maintain a constant temperature near 0 1C in the upper soil layer. This phenomenon has been observed over extended periods mostly during autumn freeze-back in permafrost areas (Outcalt et al., 1990; Boike et al., 1998; Hinkel et al., 2001; Kane et al., 2001 ), and has also been previously observed throughout entire winters (Milyukova et al., 2002) or for extended periods after the first snowmelt in nonpermafrost boreal forest areas (Shibistova et al., 2002b). In our study, a zero-curtain period of 2–4 weeks preceded soil thaw in most springs with sub-zero soil temperatures. ...
The timing of the commencement of photosynthesis (P*) in spring is an important determinant of growing-season length and thus of the productivity of boreal forests. Although controlled experiments have shed light on environmental mechanisms triggering release from photoinhibition after winter, quantitative research for trees growing naturally in the field is scarce. In this study, we investigated the environmental cues initiating the spring recovery of boreal coniferous forest ecosystems under field conditions. We used meteorological data and above-canopy eddy covariance measurements of the net ecosystem CO2 exchange (NEE) from five field stations located in northern and southern Finland, northern and southern Sweden, and central Siberia. The within- and intersite variability for P* was large, 30–60 days. Of the different climate variables examined, air temperature emerged as the best predictor for P* in spring. We also found that ‘soil thaw’, defined as the time when near-surface soil temperature rapidly increases above 0°C, is not a useful criterion for P*. In one case, photosynthesis commenced 1.5 months before soil temperatures increased significantly above 0°C. At most sites, we were able to determine a threshold for air-temperature-related variables, the exceeding of which was required for P*. A 5-day running-average temperature (T5) produced the best predictions, but a developmental-stage model (S) utilizing a modified temperature sum concept also worked well. But for both T5 and S, the threshold values varied from site to site, perhaps reflecting genetic differences among the stands or climate-induced differences in the physiological state of trees in late winter/early spring. Only at the warmest site, in southern Sweden, could we obtain no threshold values for T5 or S that could predict P* reliably. This suggests that although air temperature appears to be a good predictor for P* at high latitudes, there may be no unifying ecophysiological relationship applicable across the entire boreal zone.
... The final two scenarios have been taken in order to reflect a typical situation for daytime canopy photosynthesis and for a nighttime ecosystem respectively. Nocturnal respiration of forest ecosystems is typically dominated by the soil CO 2 efflux (Law et al., 1999; Janssens et al., 2001; Milyukova et al., 2002; Shibistova et al., 2002a). As can be seen inFigure 4B these different assumptions regarding the source distribution lead to significantly different footprint estimates. ...
A knowledge of the distribution of the contribution of upwind sources to measurements of vertical scalar flux densities is important for the correct interpretation of eddy covariance data. Several approaches have been developed to estimate this so-called footprint function. Here a new approach based on the ensemble-averaged Navier—Stokes equations is presented. Comparisons of numerical results using this approach with results from other studies under a range of environmental conditions show that the model predictions are robust. Moreover, the approach outlined here has the advantage of a potential wide applicability, due to an ability to take into account the heterogeneous nature of underlying surfaces. For example, the model showed that any variations in surface drag, such as must occur in real life heterogeneous canopies, can exert a marked influence of the shape and extent of flux footprints. Indeed, it seems likely that under such circumstances, estimates of surface fluxes will be weighted towards areas of highest foliage density (and therefore quite likely higher photosynthetic rates) close to the measurement sensor.
Three-dimensional footprints during the day and night were also determined for a mixed coniferous forest in european Russia. A marked asymmetry of the footprint in the crosswind direction was observed, this being especially pronounced for non-uniform plant distributions involving vegetation types with different morphological and physiological properties. The model also found that, other things being equal, the footprint peak for forest soil respiration is typically over twice the distance from the above canopy measurement sensor compared to that for canopy photosynthesis. This result has important consequences for the interpretation of annual ecosystem carbon balances by the eddy covariance method.
... change in CO 2 concentration in the canopy airspace between the two devices (DC store is provided by gas profile measurements). The trunk respiration R st is estimated at 0.3 and 0.4 mmol m À2 s À1 for 1999 and 2000, respectively [Shibistova et al., 2002]. We follow previous authors in designating skyward carbon flow as positive, although, when quantifying photosynthesis and respiration, we discuss them in terms of their magnitude. ...
This paper was published as Journal of Geophysical Research, 2005, 110, D23209. Copyright © 2005 American Geophysical Union. It is also available from the publisher's website at http://www.agu.org/pubs/crossref/2005/2005JD006060.shtml. Doi: 10.1029/2005JD006060 We have expanded the Monte Carlo, ray-tracing model FLIGHT in order to simulate photosynthesis within three-dimensional, heterogeneous tree canopies. In contrast to the simple radiative transfer schemes adopted in many land-surface models (e.g., the Big Leaf approximation), our simulation calculates explicitly the leaf irradiance at different heights within the canopy and thus produces an accurate scale-up in photosynthesis from leaf to canopy level. We also account for both diffuse and direct sunlight. For a Siberian stand of Scots pine Pinus sylvestris, FLIGHT predicts observed carbon assimilation, across the full range of sky radiance, with an r.m.s. error of 12%. Our main findings for this sparse canopy, using both measurements and model, are as follows: (1) Observationally, we detect a light-use efficiency (LUE) increase of only ≤10% for the canopy when the proportion of diffuse sky radiance is 75% rather than 25%. The corresponding enhancement predicted by our simulations is 10–20%. With such small increases in LUE, our site will not assimilate more carbon on overcast days compared to seasonally equivalent sunny days; (2) the scale-up in photosynthesis from top-leaf to canopy is less than unity. The Big Leaf approximation, based on Beer's law and light-acclimated leaf nitrogen, overpredicts this scale-up by ≥60% for low sky radiance (≤500 μmolPAR m−2 s−1); (3) when leaf nitrogen is distributed so as to maximize canopy photosynthesis, the increase in the canopy carbon assimilation, compared with a uniform nitrogen distribution, is small (≅4%). Maximum assimilation occurs when the vertical gradient of leaf nitrogen is slightly shallower than that of the light profile.
... [13] Several studies were previously conducted close to the ZOTTO site. Eddy covariance flux measurements of CO 2 , H 2 O, and energy exchanges were made in a nearby pine forest Shibistova et al., 2002b], dark taiga [Röser et al., 2002], and bogs [Kurbatova et al., 2002]. These measurements were accompanied by process studies on soil respiration [Shibistova et al., 2002a] and detailed forest inventories Wirth et al., 1999]. ...
We present first results from 19 months of semicontinuous concentration measurements of biogeochemical trace gases (CO2, CO, and CH4) and O2, measured at the Zotino Tall Tower Observatory (ZOTTO) in the boreal forest of central Siberia. We estimated CO2 and O2 seasonal cycle amplitudes of 26.6 ppm and 134 per meg, respectively. An observed west-east gradient of about −7 ppm (in July 2006) between Shetland Islands, Scotland, and ZOTTO reflects summertime continental uptake of CO2 and is consistent with regional modeling studies. We found the oceanic component of the O2 seasonal amplitude (Atmospheric Potential Oxygen, or APO) to be 51 per meg, significantly smaller than the 95 per meg observed at Shetlands, illustrating a strong attenuation of the oceanic O2 signal in the continental interior. Comparison with the Tracer Model 3 (TM3) atmospheric transport model showed good agreement with the observed phasing and seasonal amplitude in CO2; however, the model exhibited greater O2 (43 per meg, 32%) and smaller APO (9 per meg, 18%) amplitudes. This seeming inconsistency in model comparisons between O2 and APO appears to be the result of phasing differences in land and ocean signals observed at ZOTTO, where ocean signals have a significant lag. In the first 2 months of measurements on the fully constructed tower (November and December 2006), we observed several events with clear vertical concentration gradients in all measured species except CO. During “cold events” (below −30°C) in November 2006, we observed large vertical gradients in CO2 (up to 22 ppm), suggesting a strong local source. The same pattern was observed in CH4 concentrations for the same events. Diurnal vertical CO2 gradients in April to May 2007 gave estimates for average nighttime respiration fluxes of 0.04 ± 0.02 mol C m−2 d−1, consistent with earlier eddy covariance measurements in 1999–2000 in the vicinity of the tower.
... The observed 4-week period of constant near-zero temperatures during snow melting in the spring—the zero curtain—is a manifestation of the temperature-balancing effect of latent heat absorption by melting water and release by freezing water until all the water in the soil has melted or frozen completely. This behavior has been reported in permafrost areas mostly during autumn (Outcalt et al. 1990, Boike et al. 1998) and also throughout entire winters (Milyukova et al. 2002) or after the first snowmelt in non-permafrost boreal forest areas (Shibistova et al. 2002 ), but has not been observed at our site before , probably because the soil has never before frozen so completely during the monitoring period. Suni et al. (2003a) reported a zero-curtain effect at several boreal sites after wintertime soil temperatures several degrees below zero, but never when wintertime temperatures remained close to or above zero. ...
Warm air in combination with frozen soil is a major cause of wintertime drought damage in evergreen plants in subalpine and boreal environments. We analyzed diurnal tree stem diameter variation (SDV), which reflects soil water uptake, canopy-level water vapor flux (Fw), stand photosynthesis (Ps), photosynthetically active radiation (PAR), soil and air temperatures (Ts and T air, respectively) and soil liquid water content (theta) to determine under what conditions photosynthesis is possible in wintertime and how crucial water uptake from soil is for photosynthesis. Measurements were made under field conditions in a Scots pine forest in southern Finland during winter 2002-2003. We found four wintertime periods when there was measurable Ps and SDV, the latter always starting 2-7 days after photosynthesis and both usually ending on the same day. Stand photosynthesis began when T air reached 3-4 degrees C and ended when T air dropped below -7 degrees C. The trees appeared to rely on stored stem water first and started taking up water from the soil a few days later, when the transpirational demand became strong enough. The more difficult it was to access soil water because of low Ts or low theta, the longer the trees used water stored in their stems. Even partial stem freezing did not prevent photosynthesis or soil water uptake.
... ld then be sealed to the body of a container mounted on a tree stem by two locking clips. The bodies of the containers were sealed to tree stems with a silicon sealant, and remained in place between stem CO 2 efflux measurements. We checked the chambers for leaks by exhaling near the seals and monitoring the CO 2 concentration inside the chambers (Shibistova et al . 2002). The chamber volumes were determined following the method of Edwards & Hanson (1996); the mean chamber volume was 793 ± 20 mL (mean ± 1 SD). The mean total system volume including the infrared gas analyser (IRGA) was 958 ± 21 mL (mean ± 1 SD). Each chamber covered an area of 150 cm 2 of stem surface. Thermocouples were inserted to appr ...
We measured stem CO2 efflux and leaf gas exchange in a tropical savanna ecosystem in northern Australia, and assessed the impact of fire on these processes. Gas exchange of mature leaves that flushed after a fire showed only slight differences from that of mature leaves on unburned trees. Expanding leaves typically showed net losses of CO2 to the atmosphere in both burned and unburned trees, even under saturating irradiance. Fire caused stem CO2 efflux to decline in overstory trees, when measured 8 weeks post-fire. This decline was thought to have resulted from reduced availability of C substrate for respiration, due to reduced canopy photosynthesis caused by leaf scorching, and to priority allocation of fixed C towards reconstruction of a new canopy. At the ecosystem scale, we estimated the annual above-ground woody-tissue CO2 efflux to be 275 g C m−2 ground area year−1 in a non-fire year, or approximately 13% of the annual gross primary production.
We contrasted the canopy physiology of two co-dominant overstory tree species, one of which has a smooth bark on its branches capable of photosynthetic re-fixation (Eucalyptus miniata), and the other of which has a thick, rough bark incapable of re-fixation (Eucalyptus tetrodonta). Eucalyptus miniata supported a larger branch sapwood cross-sectional area in the crown per unit subtending leaf area, and had higher leaf stomatal conductance and photosynthesis than E. tetrodonta. Re-fixation by photosynthetic bark reduces the C cost of delivering water to evaporative sites in leaves, because it reduces the net C cost of constructing and maintaining sapwood. We suggest that re-fixation allowed leaves of E. miniata to photosynthesize at higher rates than those of E. tetrodonta, while the two invested similar amounts of C in the maintenance of branch sapwood.
In the past 30 years, observational climate datasets reveal a significant a drying and warming trend over in North China. Understanding of climatic variability over North China and its driving mechanism in a long-term perspective is, however, limited to a few sites only, especially the lack of temperature reconstructions based on latewood density and blue intensity. In this study, we developed a 281-year latewood blue intensity chronology based on 45 cores of Picea meyeri in western North China. Based on the discovery that the warm season (May-August) mean maximum temperature is the main controlling factor affecting the change in blue light reflection intensity, we established a regression model that explained 37% of the variance during the calibration period (1950-2020), allowing to trace the mean maximum temperature up to 1760 CE. From the past 261 years, we identified seven persistent high temperature periods a nearby temperature reconstructions and climate gridded data indicate that our reconstruction record a wide range of temperature variations in North China. The analysis of links between large-scale climatic variation and the temperature reconstruction showed that there is a relationship between extremes in the warm season temperature and anomalous SSTs in the equatorial eastern Pacific, and implied that the extremes in the warm season temperature in North China will be intensified under future global warming.
Inter-annual and seasonal variations of energy, vapor water, and carbon fluxes and associated climate variables in a middle taiga pine (Pinus sylvestris) forest on sandy soils and in a northern taiga larch (Larix gmelinii) forest on permafrost in central Siberia were studied from eddy covariance measurements acquired during the growing seasons of 1998–2000 and 2004–2008, respectively. Both the pure Scots pine of 215-year-old and pure Gmelin larch of 105-year-old forests naturally regenerated after forest fires, differed by their tree stand characteristics, and grew in extremely contrasting environments with distinctive climatic and soil conditions. Net radiation was greater in the pine forest due to higher values in the summer months and a longer growing season. Sensible heat flux was the larger term in the radiation balance in both forests. The Bowen ratio stayed between 1 and 2 during the growing season and was as high as 8–10 in dry spring in both forests. In the dry summers, latent heat explained 70%–80% of the daily net ecosystem CO2 exchange (NEE) variation in both forests. The average NEE was significantly smaller in the larch ecosystem at −4 µmol m−2s−1 compared to −7 µmol m−2s−1 in the pine forest. NEP for the growing season was 83 in the larch forest on continuous permafrost and 228 g C m−2 in the pine forest on warm sandy soils. Water use efficiency was 5.8 mg CO2 g−1H2O in the larch forest and 11 mg CO2 g−1H2O in the pine forest and appeared to be consistent with that in boreal forests. As a result of the forest structure change from Gmelin larch to Scots pine due to the permafrost retreat in a warming climate, the boreal forest C-sink may be expected to increase. Thus, potential feedback to the climate system in these “hot spots” of forest-forming replacement species may promote C-uptake from the atmosphere. However, as many studies suggest, in the pace of transition from permafrost to non-permafrost, C-sink would turn into C-source in hot spots of permafrost retreat.
A model is a simplification of our understanding of reality, but how much simplification is too much? We assessed model sensitivity for a new spatially-explicit individual-based gap model (IBM) SIBBORK to determine how spatial resolution (i.e., plot size) and explicit representation of light and space affect the simulated vegetation characteristics (e.g., structure and composition). The 3-dimensional (3-D) representation of light is the most computationally-expensive part of the simulation, so it is worthwhile to evaluate whether the time, resource and effort of spatial explicity improve predictive capabilities. In the simulation, gap dynamics occur at the plot-level, wherein a certain level of spatial homogeneity is assumed. Therefore, for the simulation of each ecosystem it is imperative to select the appropriate scale at which gap dynamics are simulated, i.e. the plot size. We conducted these sensitivity analyses and qualitatively compared model output to the descriptions of forest structure and composition at two different sites in central and southern Siberia. We systematically varied the plot size (100-900 m(2)) and found that a 100 m2 plot is most appropriate for the simulation of the central Siberian boreal forest ecosystem. Moreover, we found that the forest structure and composition simulated under the light environment generated using a 3-dimensional light ray tracing subroutine more closely resembled local and regional forest characteristics at both sites, whereas the growth and stand processes computed based on the simplified (1-D) light conditions approximated only large-scale (continental) average forest structure and biomass for the Eurasian boreal forest.
A model is a simplification of our understanding of reality, but how much simplification is too much? We assessed model sensitivity for a new spatially-explicit individual-based gap model (IBM) SIBBORK to determine how spatial resolution (i.e., plot size) and explicit representation of light and space affect the simulated vegetation characteristics (e.g., structure and composition). The 3-dimensional (3-D) representation of light is the most computationally-expensive part of the simulation, so it is worthwhile to evaluate whether the time, resource and effort of spatial explicity improve predictive capabilities. In the simulation, gap dynamics occur at the plot-level, wherein a certain level of spatial homogeneity is assumed. Therefore, for the simulation of each ecosystem it is imperative to select the appropriate scale at which gap dynamics are simulated, i.e. the plot size. We conducted these sensitivity analyses and qualitatively compared model output to the descriptions of forest structure and composition at two different sites in central and southern Siberia. We systematically varied the plot size (100–900 m²) and found that a 100 m² plot is most appropriate for the simulation of the central Siberian boreal forest ecosystem. Moreover, we found that the forest structure and composition simulated under the light environment generated using a 3-dimensional light ray tracing subroutine more closely resembled local and regional forest characteristics at both sites, whereas the growth and stand processes computed based on the simplified (1-D) light conditions approximated only large-scale (continental) average forest structure and biomass for the Eurasian boreal forest.
The objective of this paper is to analyse the impacts of climate change on a pine forest stand in Central Siberia (Zotino) to assess benefits and risks for such forests in the future.
We use the regional statistical climate model STARS to develop a set of climate change scenarios assuming a temperature increase by mid-century of 1, 2, 3 and 4 K. The process-based forest growth model 4C is applied to a 200-year-old pine forest to analyse impacts on carbon and water balance as well as the risk of fire under these climate change scenarios. The climate scenarios indicate precipitation increases mainly during winter and decreases during summer with increasing temperature trend. They cause rising forest productivity up to about 20 % in spite of increasing respiration losses. At the same time, the water-use efficiency increases slightly from 2.0 g C l−1 H2O under current climate to 2.1 g C l−1 H2O under 4 K scenario indicating that higher water losses from increasing evapotranspiration do not appear to lead to water limitations for the productivity at this site. The simulated actual evaporation increases by up to 32 %, but the climatic water balance decreases by up to 20 % with increasing temperature trend. In contrast, the risk of fire indicated by the Nesterov index clearly increases. Our analysis confirms increasing productivity of the boreal pine stand but also highlights increasing drought stress and risks from abiotic disturbances which could cancel out productivity gains.
The future of the land carbon sink is a significant uncertainty in global change projections. Here, key controls on global terrestrial carbon storage are examined using a simple model of vegetation and soil. Equilibrium solutions are derived as a function of atmospheric CO2 and global temperature, these environmental variables are then linked in an idealized global change trajectory, and the lag between the dynamic and equilibrium solutions is derived for different linear rates of increase in atmospheric CO2. Terrestrial carbon storage is departing significantly from equilibrium because CO2 and temperature are increasing on a similar timescale to ecosystem change, and the lag is found to be proportional to the rate of forcing. Thus peak sizes of the land carbon sink, and any future land carbon source, are proportional to the rate of increase of CO2. A switch from a land carbon sink to a source occurs at a higher CO2 and temperature under more rapid forcing. The effects of parameter uncertainty in temperature sensitivities of photosynthesis, plant respiration and soil respiration, and structural uncertainty through the effect of fixing the ratio of plant respiration to photosynthesis are explored. In each case, the CO2 fertilization effect on photosynthesis is constrained to reproduce the 1990 atmospheric CO2 concentration within a closed global model. New literature compilations are presented for the temperature sensitivities of plant and soil respiration. A lower limit, Q10=1.29, for soil respiration significantly increases future land carbon storage. An upper limit, Q10=3.63, for soil respiration underpredicts the increase in carbon storage since the Last Glacial Maximum. Fixing the ratio of plant respiration to photosynthesis (R/P) at 0.5 generates the largest and most persistent land carbon sink, followed by the weakest land carbon source.
Soil, tree stems, and ecosystem carbon dioxide fluxes were measured by chambers and eddy covariance methods in a paludified shallow-peat spruce forest in the southern taiga of European Russia (Tver region, 56° N 33° E) during the growing seasons of 2002–2012. The site was established in 1998 as part of the EUROSIBERIAN CARBONFLUX project, an international field experiment examining atmosphere–biosphere interaction in Siberia and European Russia. In all years the observed annual cumulative net ecosystem flux was positive (the forest was a source of carbon to the atmosphere). Soil and tree stem respiration was a significant part of the total ecosystem respiration (ER) in this paludified shallow-peat spruce forest. On average, 49% of the ER came from soil respiration. We found that the soil fluxes exhibited high seasonal variability, ranging from 0.7 to 10 μmol m−2 s−1. Generally, the soil respiration depended on the soil temperature and ground water level. In drought conditions, the soil respiration was low and did not depend on temperature. The stem respiration of spruces grew intensively in May, had permanently high values from June to the end of September, and in October it dramatically decreased. The tree stem respiration in midsummer was about 3–5 μmol m−2 s−1 for dominant trees and about 1–2 μmol m−2 s−1 for subdominant trees. The respiration of living tree stems was about 10–20% of the ER.
This paper studies the structure and dynamics of the major pools of organic matter in clearings in the lichen pine forests of the middle taiga subzone of Yenisei Siberia, in the range of measurements of the international observatory Zotto, a high-tech research platform for long-term (>30 years) monitoring of the atmosphere and climate changes. The contribution of phytocenoses at different stages of post-clearing succession in the process of deposition of atmospheric carbon is analyzed. In conjunction with the data on atmospheric measurements of a wide range of greenhouse gases carried out at the base of the observatory, the results of this work will be used for modeling the stocks and flows of carbon in forest ecosystems of Central Siberia.
Changes in the intra-annual cycle of atmospheric CO2 at Mauna Loa, Hawaii, are investigated by comparing monthly observations to the mean value of each year's waxing and waning phase of the cycle. Results identify a previously unnoticed change between 1965 and 2004 during October in which October concentrations declined relative to the cycle's mean value. Results confirm a previously noted change in spring (April), but unlike previous analyses, this result indicates that between 1965 and 2004 April concentrations rose relative to the cycle's mean value. The timing of these changes is not caused by commonly recognized patterns of atmospheric circulation, such as ENSO events. Instead, these changes may be generated by asymmetric effects of warming. Warming in early spring may increase heterotrophic respiration relative to net primary production while warming in late summer/early fall may enhance net primary production relative to heterotrophic respiration.
The effects of air flow rate and chamber position on measuring stem respiration rate (Rstem) were examined using an open flow system on six Japanese red pine (Pinus densiflora Sieb. and Zucc) trees. Rstem was more closely correlated with stem temperature observed 2–6h earlier than with current stem temperature. Rstem was not affected by the air flow rate passing through the stem chamber. Although there were no differences in stem temperatures between azimuth angles, there were significant differences in Rstem between azimuth angles (except for only one case) of each sample stem. The distance from below the bark to the stem centre varied at each azimuth in the samples, and was significantly positively correlated with R0 (p
Variability of soil CO2 efflux strongly depends on soil temperature, soil moisture and plant phenology. Separating the effects of these factors is critical to understand the belowground carbon dynamics of forest ecosystem. In Ethiopia with its unreliable seasonal rainfall, variability of soil CO2 efflux may be particularly associated with seasonal variation. In this study, soil respiration was measured in nine plots under the canopies of three indigenous trees (Croton macrostachys, Podocarpus falcatus and Prunus africana) growing in an Afromontane forest of south-eastern Ethiopia. Our objectives were to investigate seasonal and diurnal variation in soil CO2 flux rate as a function of soil temperature and soil moisture, and to investigate the impact of tree species composition on soil respiration. Results showed that soil respiration displayed strong seasonal patterns, being lower during dry periods and higher during wet periods. The dependence of soil respiration on soil moisture under the three tree species explained about 50% of the seasonal variability. The relation followed a Gaussian function, and indicated a decrease in soil respiration at soil volumetric water contents exceeding a threshold of about 30%. Under more moist conditions soil respiration is tentatively limited by low oxygen supply. On a diurnal basis temperature dependency was observed, but not during dry periods when plant and soil microbial activities were restrained by moisture deficiency. Tree species influenced soil respiration, and there was a significant interaction effect of tree species and soil moisture on soil CO2 efflux variability. During wet (and cloudy) period, when shade tolerant late successional P. falcatus is having a physiological advantage, soil respiration under this tree species exceeded that under the other two species. In contrast, soil CO2 efflux rates under light demanding pioneer C. macrostachys appeared to be least sensitive to dry (but sunny) conditions. This is probably related to the relatively higher carbon assimilation rates and associated root respiration. We conclude that besides the anticipated changes in precipitation pattern in Ethiopia any anthropogenic disturbance fostering the pioneer species may alter the future ecosystem carbon balance by its impact on soil respiration.
Mean residence time (MRT) of topsoil organic carbon is one critical parameter for predicting future land carbon sink dynamics. Large uncertainties remain about controls on the variability in global MRT of soil organic carbon. We estimated global MRT of topsoil (0-20 cm) organic carbon in terrestrial ecosystems and found that mean annual air temperature, annual precipitation, and topsoil nitrogen storage were responsible for the variability in MRT. An empirical climate and soil nitrogen - based (Clim&SN) model could be used to explain the temporal and spatial variability in MRT across various ecosystems. Estimated MRT was lowest in the low-latitude zones, and increased toward high-latitude zones. Global MRT of topsoil organic carbon showed a significant declining tendency between 1960 and 2008, particularly in the high-latitude zone of the northern hemisphere. The largest absolute and relative changes (0.2% per yr) in MRT of topsoil organic carbon from 1960 to 2008 occurred in high-latitude regions, consistent with large carbon stocks in, and greater degree of climate change being experienced by, these areas. Overall, global MRT anomalies (differences between MRT in each year and averaged value of MRT from 1960 to 2008) of terrestrial topsoil organic carbon were decreasing from 1960 to 2008. Global MRT anomalies decreased significantly (P<0.001) with the increase of global temperature anomalies, indicating that global warming resulted in faster turnover rates of topsoil organic carbon.
For three forest canopies (a sparse, boreal needleleaf; a temperate broadleaf; and a dense, tropical, broadleaf stand) light-use efficiency (LUE) is found to be 6–33% higher when sky radiance is dominated by diffuse rather than direct sunlight. This enhancement is much less than that reported previously for both crops (110%; Choudbury, 2001) and moderately dense temperate woodland (50–180%). We use the land-surface scheme JULES to interpret the observed canopy response. Once sunflecks and leaf orientation are incorporated explicitly into the scheme, our simulations reproduce convincingly the overall level of canopy gross photosynthetic product (GPP), its enhancement with respect to diffuse sunlight and the mean 15% reduction in productivity observed during the afternoon due to stomatal closure. The LUE enhancement under diffuse sunlight can be explained by sharing of the canopy radiation-load, which is reduced under direct sky radiance. Once sunflecks are accounted for the advantage of implementing more sophisticated calculations of stomatal conductance (e.g. Ball–Berry and SPA submodels) is less obvious even for afternoon assimilation. Empirical relations are developed between observed carbon flux and the environmental variables total downwelling shortwave radiation (SW), canopy temperature (T) and the fraction of diffuse sky radiance (fDIF). These relations allow us to gauge the impact of increased/reduced insolation on GPP and net ecosystem exchange (NEE). Overall the three stands appear to be fairly stable within global trends and typical interannual variability (SW changing by <15%). Greatest sensitivity is exhibited by the boreal site, Zotino, where NEE falls by 9±4% for a 15% reduction in SW.
Tower‐based eddy covariance measurements were used to quantify the effect of fire on subsequent carbon dioxide fluxes and water and surface energy balance characteristics for campo sujo savanna located near Brasília in Central Brazil (15°56′ S, 47°51′ W). Campo sujo is a xeromorphic, open shrub savanna with very scattered but definitely visible shrubs and tree‐like shrub elements. We studied two areas, one exposed to a prescribed fire late in the dry season, and a second that had not been burned for the previous 4 years.
The fire on 22 September 1998 consumed an estimated 26 mol C m ⁻² . Immediately after the fire, evapotranspiration rates decreased and the savanna became a stronger net source of CO 2 to the atmosphere. This was attributed to the removal of the still slightly physiologically active grass layer and higher soil CO 2 efflux rates as a consequence of elevated surface soil temperatures post‐burning.
On the commencement of the first rains in early October 1998, this situation was reversed, with the burned area rapidly becoming a stronger sink for CO 2 and with higher evapotranspiration rates than a nearby unburned (control) area. This difference persisted throughout the wet season (until at least June 1999) and was attributable to greater physiological activity of the regrowing vegetation in the burned area. Early in the growing season, higher soil evaporation rates may also have contributed to faster water use by the previously burned area.
Overall, we estimate an annual gross primary productivity for the burned area of 135 mol C m ⁻² year ⁻¹ , with that for the unburned area being 106 mol C m ⁻² year ⁻¹ . Estimated ecosystem respiration rates were more similar on an annual basis (96 and 82 mol C m ⁻² year ⁻¹ for the burned and unburned areas, respectively), giving rise to a substantially higher net ecosystem productivity for the previously burned area (38 vs 24 mol C m ⁻² year ⁻¹ ).
Stimulation of photosynthetic activity in the rapid post‐fire growth phase means that the negative effects of fire on the ecosystem carbon balance were more or less neutralized after only 12 months.
Our objectives were to identify factors related to temporal variation of soil CO2 efflux in a boreal pine forest and to evaluate simple predictive models of temporal variation of soil CO2 efflux. Soil CO2 efflux was measured with a portable chamber in a Finnish Scots pine forest for three years, with a fourth year for model evaluation. Plot averages for soil CO2 efflux ranged from 0.04 to 0.90 g CO2 m-2 h-1 during the snow-free period, i.e. May-October, and from 0.04 to 0.13 g CO2 m-2 h-1 in winter. Soil temperature was a good predictor of soil CO2 efflux. A quadratic model of ln-transformed efflux explained 76-82% of the variation over the snow-free period. The results revealed strong seasonality: at a given soil temperature, soil CO2 efflux was higher later in the snow-free period than in spring and early summer. Regression coefficients for temperature (approximations of a Q10 value) of month-specific models decreased with increasing average soil temperatures. Efflux in July, the month of peak photosynthesis, showed no clear response to temperature or moisture. Inclusion of a seasonality index, degree days, improved the accuracy of temperature response models to predict efflux for the fourth year of measurements, which was not used in building of regression models. Underestimation during peak efflux (mid-July to late-August) remained uncorrected. The strong influence of the flux of photosynthates belowground and the importance of root respiration could explain the relative temperature insensitivity observed in July and together with seasonality of growth of root and root-associated mycorrhizal fungi could explain partial failure of models to predict magnitude of efflux in the peak season from mid-July to August. The effect of moisture early in the season was confounded by simultaneous advancement of the growing season and increase in temperature. In a dry year, however, the effect of drought was evident as soil CO2 efflux was some 30% smaller in September than in the previous wet year. Although soil temperature was a good overall predictor of soil CO2 efflux, possibly partly due to its proxy-like quality for covarying processes, strong seasonality of the temperature response observed in this boreal forest corroborates recent findings concerning the importance of seasonal changes in carbon inputs to processes producing CO2 in soil.
Annual measurements of the diameter growth and litter fall of trees began in 1998 using a 1.0 ha permanent plot beneath a flux tower at the Takayama flux site, central Japan. This opened up an opportunity for studies that compare the interannual variability in tree growth with eddy covariance-based net ecosystem production (NEP). A possible link between multiyear biometric-based net primary production (NPP) and eddy covariance-based NEP was investigated to determine the contribution of autotrophic production and heterotrophic respiration (HR) to the interannual variability of NEP in deciduous forest ecosystems. We also defined the NEP* as the measurable organic matter stored in an ecosystem during the interval in which soil respiration (SR) measurements were taken. The difference of biometric-based NEP* from eddy covariance-based NEP within a given year varied between 55% and 105%. Woody tissue NPP (stems and coarse roots) varied markedly from 0.88 to 1.96 Mg C ha−1 yr−1 during the 8-year study period (1999–2006). Annual woody tissue NPP was positively correlated with eddy covariance-based NEP (r2=0.52, P<0.05). However, neither foliage NPP (r2=0.03) nor HR (r2=0.06) were correlated with eddy covariance-based NEP. Therefore, it was hypothesized that interannual variability in the ecosystem carbon exchange was directly responsible for much of the interannual variation in autotrophic production, especially carbon accumulation in the woody components of the ecosystem. Moreover, similar interannual variations of biometric-based NEP* and eddy covariance-based NEP with small variations in SR and foliage NPP suggest a constant net accumulation of carbon in nonliving pools at the Takayama site.
Northern Eurasia is the largest terrestrial reservoir of carbon, and its dynamics and interactions with climate are globally significant. We present five independent estimates of the contemporary carbon balance of central Siberia using three different methodologies: a landscape-ecosystem approach (LEA) that amalgamates comprehensive vegetation, soil, hydrological and morphological information into a Geographical Information System, linked to regression-based estimates of carbon flux; two Dynamic Global Vegetation Models (DGVMs); and two atmospheric inversions. Apart from one of the DGVMs, all methods produce estimates of the net biome productivity (NBP) that are consistent both amongst themselves and with a range of other estimates. They indicate the region to be a carbon sink with a NBP of 27.5 ± 7.2 g C m−2 yr−1, which is equivalent to 352 ± 92 Mt C yr−1 if considered representative for boreal Asia. This is comparable with fossil fuel emissions for the Russian Federation, currently estimated as 427 MtC yr−1, and implies that boreal Asia does not play the major role in the northern hemisphere land sink, typically estimated to be of magnitude 1.5–2.9 Gt C yr−1. The LEA and DGVM approaches produce very different partitioning of NBP into its component fluxes. The DGVMs find net primary production (NPP) to be nearly balanced by heterotrophic respiration, disturbance being a relatively small term pushing the system closer to equilibrium. In the LEA, heterotrophic respiration is significantly less than NPP, and disturbance plays a much larger role in the overall carbon balance. The use in the LEA of observationally based estimates of heterotrophic respiration and fire disturbance, along with a more complete description of disturbance fluxes, suggests that the partitioning derived by the LEA is more likely, and that improved process descriptions and constraints by data are needed in the DGVMs.
abstractUsing light aircraft and at intervals of approximately 14 days, vertical profiles of temperature, humidity, CO2 concentration and 13C/12C and 18O/16O ratio, as well as concentrations of CH4, CO, H2 and N2O, from about 80 to 3000 m above ground level have been determined for the atmosphere above a flux measurement tower located near the village of Zotino in central Siberia (60°45′N, 89°23′E). As well as being determined from flask measurements (typically at heights of 100, 500, 1000, 1500, 2000, 2500 and 3000 m) continuous CO2 concentration profiles at 1 Hz have also been obtained using an infrared gas analyser. This measurement program is ongoing and has been in existence since July 1998. Data to November 2000 are presented and show a seasonal cycle for CO2 concentration of about 25 μmol mol−1 within the atmospheric boundary layer (ABL) and about 15 μmol mol−1 in the free troposphere. Marked seasonal cycles in the isotopic compositions of CO2 are also observed, with that of oxygen-18 in CO2 being unusual: always being depleted in the ABL with respect to the free troposphere above. This is irrespective of whether the CO2 concentration is higher or lower in the free troposphere. We interpret this as indicating a net negative discrimination being associated with the net terrestrial carbon exchange, irrespective of whether photosynthesis or respiration dominates the net carbon flux in this region. During winter flights, large fluctuations in CO2 concentration with height are often observed both within and above the stable ABL. Usually (but not always) these variations in CO2 concentrations are associated with more or less stoichiometrically constant variations in CO and CH4 concentrations. We interpret this as reflecting the frequent transport of polluted air from Europe with very little vertical mixing having occurred, despite the large horizontal distances traversed. This notion is supported by back-trajectory analyses. Vertical profiles of CO2 concentration with supplementary flask measurements allow more information on the structure and composition of an air mass to be obtained than is the case for flask measurements or for ground-based measurements only. In particular, our data question the notion that there is usually anything like “well mixed background air” in the mid-to-high northern latitudes during the winter months.
abstractWe present a first analysis of data (June 1998 to December 2000) from the long-term eddy covariance site established in a Pinus sylvestris stand near Zotino in central Siberia as part of the EUROSIBERIAN CARBONFLUX project. As well as examining seasonal patterns in net ecosystem exchange (NE), daily, seasonal and annual estimates of the canopy photosynthesis (or gross primary productivity, GP) were obtained using NE and ecosystem respiration measurements.Although the forest was a small (but significant) source of CO2 throughout the snow season (typically mid-October to early May) there was a rapid commencement of photosynthetic capacity shortly following the commencement of above-zero air temperatures in spring: in 1999 the forest went from a quiescent state to significant photosynthetic activity in only a few days. Nevertheless, canopy photosynthetic capacity was observed to continue to increase slowly throughout the summer months for both 1999 and 2000, reaching a maximum capacity in early August. During September there was a marked decline in canopy photosynthesis which was only partially attributable to less favourable environmental conditions. This suggests a reduction in canopy photosynthetic capacity in autumn, perhaps associated with the cold hardening process. For individual time periods the canopy photosynthetic rate was mostly dependent upon incoming photon irradiance. However, reductions in both canopy conductance and overall photosynthetic rate in response to high canopy-to-air vapour differences were clearly evident on hot dry days. The relationship between canopy conductance and photosynthesis was examined using Cowan's notion of optimality in which stomata serve to maximise the marginal evaporative cost of plant carbon gain. The associated Lagrangian multiplier (λ) was surprisingly constant throughout the growing season. Somewhat remarkably, however, its value was markedly different between years, being 416 mol mol−1 in 1999 but 815 mol mol−1 in 2000. Overall the forest was a substantial sink for CO2 in both 1999 and 2000: around 13 mol C m−2 a−1. Data from this experiment, when combined with estimates of net primary productivity from biomass sampling suggest that about 20% of this sink was associated with increasing plant biomass and about 80% with an increase in the litter and soil organic carbon pools. This high implied rate of carbon accumulation in the litter soil organic matter pool seems unsustainable in the long term and is hard to explain on the basis of current knowledge.
Results of measurements and calculations of carbon budget parameters of forests and swamps of Siberia are reported. The zonal
variability of reserves (and an increment in reserves) of carbon in forest and swamp ecosystems is characterized, carbon dioxide
fluxes are measured directly by means of microeddy pulsations, and an uncertainty brought into the calculation of carbon budget
parameters by forest fires is estimated.
Active as well as passive spaceborne sensors can be used to monitor spring snowmelt on regional to continental scale. Change
detection methods are used to determine dates related to the thaw period. They comprise initial thaw, primary thaw, start
of diurnal thaw/refreeze period, mean date of thaw, end of thaw and start of greening-up of vegetation. Only the latter is
determined by use of passive optical sensors and combines measurements of visible and infrared radiation. All other approaches
use microwave data. Some instruments such as the scatterometer Seawinds on QuikScat and the radiometer AMSR-E on Aqua make
several measurements per day allowing the detection of diurnal thaw and refreeze, which is characteristic of the spring snowmelt
period in northern latitudes. A specific, diurnal difference approach developed for QuikScat allows the determination of the
length of the final period of diurnal thaw/refreeze. This duration and the spatial dynamics are closely linked to surface
hydrology and ecosystem processes.
KeywordsSnowmelt-Active microwaves-Satellite data
Carbon dioxide exchange between the ecosystem and the atmosphere would be a major component for carbon budget at boreal forests.
In this chapter, net ecosystem exchange (NEE) of CO2 at a permafrost larch ecosystem will be discussed, based on a micrometeorological (tower flux) measurement. Movement of CO2 from ecosystem to atmosphere is customarily labeled as positive. The micrometeorological measurement can obtain NEE with
a half-hourly time-resolution at an ecosystem scale. These temporal and spatial scales are advantages in carbon, water, and
energy budget studies over ecological measurements.
Zonal patterns of above-ground phytomass dynamics and carbon storage in above-ground vegetation, phytodetritus and humus were
revealed based on the study of the carbon balance and its components in forest ecosystems of the Yenisei meridian transect.
Results indicate that the carbon storage ratio in different plant layers is determined by climatic regimes. For example pine
stands were used to calculate the full carbon budget using data on its fluxes and storage in different biogeocenosis components.
Biological productivity indices and the carbon pool of hydromorphic complexes are highly dependant on the mineral nutrition
regime and morphological characteristics of the stands. Experimental study results show the importance of forest and bog ecosystems
as carbon cycle regulators is determined by the complex interaction of zonal-climatic and forest conditions as well as by
forest vegetation characteristics (which depend on varying carbon balance structure and energy- mass exchange processes).
We have built the first comprehensive global three-dimensional model of δ18O in atmospheric CO2. The constructed model goes beyond all other approaches made until now, by simulating the diurnal variations and transport of CO2, δ18O of water, and δ18O of CO2. The CO18O fluxes are thereby dependent on the atmospheric CO18O composition. We have validated the model surface processes, showing that it compares well to other estimates and measurements of NPP, NEE, and stomata-internal CO2 mixing ratio (ci), except for high northern latitudes. Here, the model is considerably lower in NPP and higher in ci than other model estimates. However, estimates derived indirectly from observations tend to support our model findings. The water isotopes of rain are reproduced very well at all latitudes. The soil bucket model used in the model integrates incoming rain in one single value. The bucket approach overattenuates the isotopic variations of rain, and hence our isotopic source signature of respiration shows almost no seasonal cycle and is thus isotopically too depleted during summer.
Continuous half-hourly measurements of soil (Rs) and bole respiration (Rb), as well as whole-ecosystem CO2 exchange, were made with a non steady-state automated chamber system and with the eddy covariance (EC) technique, respectively, in a mature trembling aspen stand between January 2001 and December 2003. Our main objective was to investigate the influence of long-term variations of environmental and biological variables on component-specific and whole-ecosystem respiration (Re) processes. During the study period, the stand was exposed to severe drought conditions that affected much of the western plains of North America. Over the 3 years, daily mean Rs varied from a minimum of 0.1 μmol m−2 s−1 during winter to a maximum of 9.2 μmol m−2 s−1 in mid-summer. Seasonal variations of Rs were highly correlated with variations of soil temperature (Ts) and water content (θ) in the surface soil layers. Both variables explained 96, 95 and 90% of the variance in daily mean Rs from 2001 to 2003. Aspen daily mean Rb varied from negligible during winter to a maximum of 2.5 μmol m−2 bark s−1 (2.2 μmol m−2 ground s−1) during the growing season. Maximum Rb occurred at the end of the aspen radial growth increment and leaf emergence period during each year. This was 2 months before the peak in bole temperature (Tb) in 2001 and 2003. Nonetheless, Rb was highly correlated with Tb and this variable explained 77, 87 and 62% of the variance in Rb in the respective years. Partitioning of Rb between its maintenance (Rbm) and growth (Rbg) components using the mature tissue method showed that daily mean Rbg occurred at the same time as aspen radial growth increment during each growing season. This method led, however, to systematic over- and underestimations of Rbm and Rbg, respectively, during each year. Annual totals of Rs, Rb and estimated foliage respiration (Rf) from hazelnut and aspen trees were, on average, 829, 159 and 202 g C m−2 year−1, respectively, over the 3 years. These totals corresponded to 70, 14 and 16%, respectively, of scaled-up respiration estimates of Re from chamber measurements. Scaled Re estimates were 25% higher (1190 g C m−2 year−1) than the annual totals of Re obtained from EC (949 g C m−2 year−1). The independent effects of temperature and drought on annual totals of Re and its components were difficult to separate because the two variables co-varied during the 3 years. However, recalculation of annual totals of Rs to remove the limitations imposed by low θ, suggests that drought played a more important role than temperature in explaining interannual variations of Rs and Re.
The variability of snowmelt dates is important for the terrestrial carbon balance in boreal and subarctic environments. Scatterometers such as the Ku-Band QuikScat have been proven applicable for the detection of surface thaw. We present an improved method for the capture of thaw events based on significance of diurnal differences with respect to long term noise. For each 10 km × 10 km grid point two products are derived for the major thaw period: 1) the onset of thaw and 2) the end of daily freeze/thaw cycles at the surface. Both dates may be related to biogeochemical processes, especially carbon fluxes in boreal forests. The onset of the spring thaw period coincides with the first days of increased CO2 fluxes above the late winter baseline. The end of daily freeze/thaw cycles corresponds to the switch from source to sink in evergreen boreal forest environments as illustrated by comparison with eddy-flux tower data and xylem sap flow records from other investigators.The approach is suitable for detecting freeze/thaw cycle periods in boreal forest and tundra biomes. The mean absolute difference in end of freeze/thaw cycling date within the central Siberian study area (3 Mio km2 comprising tundra, boreal forest and steppe grassland) was 9 days for 2000 to 2004. Largest mean differences occurred in the southern taiga and all tundra regions, which were highest for spring 2000.The improved extraction method delivers more precise products from the viewpoint of carbon accounting in evergreen boreal forest environments. This widens the application potential of scatterometer data beyond the current status.
The spatial upscaling of soil respiration from field measurements to ecosystem levels will be biased without studying its spatial variation. We took advantage of the unique spatial gradients of an oak–grass savanna ecosystem in California, with widely spaced oak trees overlying a grass layer, to study the spatial variation in soil respiration and to use these natural gradients to partition soil respiration according to its autotrophic and heterotrophic components. We measured soil respiration along a 42.5 m transect between two oak trees in 2001 and 2002, and found that soil respiration under tree canopies decreased with distance from its base. In the open area, tree roots have no influence on soil respiration. Seasonally, soil respiration increased in spring until late April, and decreased in summer following the decrease in soil moisture content, despite the further increase in soil temperature. Soil respiration significantly increased following the rain events in autumn. During the grass growing season between November and mid-May, the average of CO2 efflux under trees was 2.29 µmol m-2 s-1, while CO2 efflux from the open area was 1.40 µmol m-2 s-1. We deduced that oak root respiration averaged as 0.89 µmol m-2 s-1, accounting for 39% of total soil respiration (oak root + grass root + microbes). During the dry season between mid-May and October, the average of CO2 efflux under trees was 0.87 µmol m-2 s-1, while CO2 efflux from the open areas was 0.51 µmol m-2 s-1. Oak root respiration was 0.36 µmol m-2 s-1, accounting for 41% of total soil respiration (oak root + microbes). The seasonal pattern of soil CO2 efflux under trees and in open areas was simulated by a bi-variable model driven by soil temperature and moisture. The diurnal pattern was influenced by tree physiology as well. Based on the spatial gradient of soil respiration, spatial analysis of crown closure and the simulation model, we spatially and temporally upscaled chamber measurements to the ecosystem scale. We estimated that the cumulative soil respiration in 2002 was 394 gC m-2 year-1 in the open area and 616 gC m-2 year-1 under trees with a site-average of 488 gC m-2 year-1.
Man-induced climate change has raised the need to predict the future climate and its feedback to vegetation. These are studied with global climate models; to ensure the reliability of these predictions, it is important to have a biosphere description that is based upon the latest scientific knowledge. This work concentrates on the modelling of the CO2 exchange of the boreal coniferous forest, studying also the factors controlling its growing season and how these can be used in modelling. In addition, the modelling of CO2 gas exchange at several scales was studied. A canopy-level CO2 gas exchange model was developed based on the biochemical photosynthesis model. This model was first parameterized using CO2 exchange data obtained by eddy covariance (EC) measurements from a Scots pine forest at Sodankylä. The results were compared with a semi-empirical model that was also parameterized using EC measurements. Both of the models gave satisfactory results. The biochemical canopy-level model was further parameterized at three other coniferous forest sites located in Finland and Sweden. At all the sites, the two most important biochemical model parameters showed seasonal behaviour, i.e., their temperature responses changed according to the season. Modelling results were improved when these changeover dates were related to temperature indices. During summer-time the values of the biochemical model parameters were similar at all the four sites. Different control factors for CO2 gas exchange were studied at the four coniferous forests, including how well these factors can be used to predict the initiation and cessation of the CO2 uptake. Temperature indices, atmospheric CO2 concentration, surface albedo and chlorophyll fluorescence (CF) were all found to be useful and have predictive power. In addition, a detailed simulation study of leaf stomata in order to separate physical and biochemical processes was performed. The simulation study brought to light the relative contribution and importance of the physical transport processes. The results of this work can be used in improving CO2 gas exchange models in boreal coniferous forests. The meteorological and biological variables that represent the seasonal cycle were studied, and a method for incorporating this cycle into a biochemical canopy-level model was introduced. Ilmastonmuutos aiheuttaa muutoksia ilman lämpötilaan sekä sademääriin ja näihin muutoksiin sopeutumisessa auttavat hyvät ennusteet tulevaisuuden ilmastosta. Näitä ennusteita lasketaan globaaleilla ilmastomalleilla. Kasvillisuuden ja ilmakehän kytkeytyminen voi aiheuttaa voimakkaita ilmiöitä tulevaisuudessa ja näiden arvioimiseksi kasvillisuuden ja ilmakehän väliset vuorovaikutukset on hyvä tuntea. Näitä vuorovaikutuksia tutkitaan esimerkiksi mallintamalla. Mallien kehittelyllä pyritään saamaan ymmärrystä erilaisista prosesseista, jotka tapahtuvat kasvillisuudessa ja ilmakehässä. Kun eri prosessit vaikuttavine tekijöineen saadaan hyvin mallitettua, malliennusteisiin saadaan parempi luotettavuus. Tässä työssä mallinnettiin hiilidioksidin (CO2)-kaasunvaihtoa eri skaaloilla ja CO2-kaasunvaihdon vuodenaikaisuusvaihtelun hallitsevia tekijöitä sekä kuinka näitä muuttujia voidaan käyttää mallituksessa. CO2-kaasunvaihtoa tutkittiin pienellä skaalalla käyttäen ilmarakomallia, mikä paljasti fysikaalisten siirtoilmiöiden tärkeyden biokemiallisten prosessien rinnalla. Usein tällaisessa mallinnuksessa huomioidaan vain biokemialliset prosessit. Metsikkömalleja kehitettiin ja niiden sekä mikrometeorologisten CO2-vuomittausten avulla tutkittiin boreaalisten havumetsien yhteytysmallien parametreja. Työssä havaittiin yleisesti käytetyn biokemiallisen mallin parametreissa lämpötilavasteiden vuodenaikaista vaihtelua. Löydös viittaa kasvien yhteytyskyvyn vuodenaikaisvaihteluun, jota ei yleensä oteta huomioon biokemiallisen mallin käytössä. Lisäksi havaittiin kesäaikaisten malliparametrien olevan samankaltaisia neljässä eri havumetsässä, jotka sijaitsivat Suomessa ja Ruotsissa. Tämä oikeuttaa samojen parametrien käytön kyseiselle kasvillisuusluokalle isomman skaalan malleissa. Lämpötilavasteiden muuttumisen kytkemiseksi vuodenaikaisuusvaihteluun tutkittiin erilaisia ympäristö- ja biologisia muuttujia ja niiden ennustuskykyä CO2-kaasunvaihdon alkuhetkeen keväällä ja hiipumiseen syksyllä. Näitä muuttujia olivat erilaiset lämpötilaindeksit, ilmakehän CO2-pitoisuus, maanpinnan albedo ja klorofyllifluoresenssi. Näitä kaikkia voidaan käyttää kasvillisuuden aktiivisen ajan arvioimisessa, ja lämpötilaindeksit soveltuvat hyvin mallitukseen. Saatuja tuloksia voidaan käyttää boreaalisen alueen metsien CO2-kaasunvaihtomallien kehittämiseen.
INTRODUCTION
Soil respiration is a major component in the carbon balance of terrestrial ecosystems and has been measured in the field for more than eight decades. In this chapter, we will describe the measurement of soil CO2 efflux at the soil surface that can be considered as equivalent to soil CO2 production when integrated over long time periods (week, month or season). At shorter time scales the transport of CO2 may uncouple the soil CO2 efflux from its production inside the soil. Different methods have been developed to measure this efflux. These methods can affect the object being measured by disturbing the biochemical processes involved in CO2 production, the physical properties influencing CO2 movement towards the soil surface, or by changing the environmental conditions in the soil. Therefore, soil respiration measurements in the field are one of the most difficult among the ecosystem flux measurements. So far, no single method has been established as the standard but comparisons, which give important indications on their accuracy, have been performed. The choice of the measurement methodology is not limited to that of a measurement system. The experimenter has to elaborate a protocol depending on the temporal and spatial scales studied. In this chapter, we will describe the most commonly used methodologies for measuring soil CO2 efflux and present their history, principles and constraints (Section 2.2).
A range of observations points towards earlier onset of spring in northern high latitudes. However, despite the profound effects this may have on vegetation-atmosphere exchange of carbon (NEE), vegetation-atmosphere physical coupling, or the location of the tundrataiga interface, the number of studies that investigate winter-spring transition fluxes in contrasting northern vegetation types is limited. Here, we examine spring ecosystem-atmosphere energy and carbon exchange in a Siberian pine forest and mire. Divergent surface albedo before and during snow-melt resulted in daytime net radiation (R n) above the forest exceeding Rn above the mire by up to 10 MJ m-2. Until stomata could open, absorbed radiation by the green pine canopy caused substantial daytime sensible heat fluxes (H > 10MJ m -2). H above the mire was very low, even negative (< -2 MJ m -2), during that same period. Physiological activity in both ecosystems responded rapidly to warming temperatures and snow-melt, which is essential for survival in Siberia with its very short summers. On days with above-zero temperatures, before melt was complete, low rates of forest photosynthesis (1-2 μmol m-2 s-1) were discernible. Forest and mire NEE became negative the same day, or shortly after, photosynthesis commenced. The mire lagged by about two weeks behind the forest and regained its full carbon uptake capacity at a slower rate. Our data provide empirical evidence for the importance the timing of spring and the relative proportion of forest vs. mire has for late winter/spring boundary-layer growth, and production and surface-atmosphere mixing of trace gases. Models that seek to investigate effects of increasingly earlier spring in high latitudes must correctly account for contrasting physical and biogeochemical ecosystem-atmosphere exchange in heterogeneous landscapes.
We compare assimilation and respiration rates, and water use strategies in four divergent ecosystems located in cold-continental central Siberia and in semi-arid southern Africa. These seemingly unrelated systems have in common a harsh and highly seasonal environment with a very sharp transition between the dormant and the active season, with vegetation facing dry air and soil conditions for at least part of the year. Moreover, the northern high latitudes and the semi-arid tropics will likely experience changes in key environmental parameters (e.g., air temperature and precipitation) in the future; indeed, in some regions marked climate trends have already been observed over the last decade or so.
Stem maintenance respiration was linearly related to live-cell volume for lodgepole pine from 4-36 cm dbh and for Engelmann spruce from 0-20 cm dbh. Sapwood contained >80% of the total live-cell volume in stems. Bole surface area, commonly used to estimate tree respiration costs, poorly estimated stem maintenance respiration. At 15°C, maintenance costs for lodgepole pine were 6.6 × 10-5 kg C.(Kg C sapwood)-1.d-1. Stem respiration during the growing season correlated well with annual stemwood growth. Annual stem maintenance respiration for trees and stand can be estimated using sapwood volume, sapwood temperature, and knowledge of respiratory behaviour. Total respiration (construction plus maintenance) estimated using stem growth and a model of maintenance respiration was compared with actual respiration measurements integrated over a 100-d growing season. Estimated respiration agreed with the integrated measurements for Engelmann spruce, but overestimated the integrated measurements by 73% in lodgepole pine. -from Authors
Past studies of plant-microbe interactions in the alpine nitrogen cycle have revealed a seasonal separation of N use, with plants absorbing N primarily during the summer months and microbes immobilizing N primarily during the autumn months. On the basis of these studies, it has been concluded that competition for N between plants and microbes is minimized along this seasonal gradient. In this study, we examined more deeply the links between microbial population dynamics and plant N availability in an alpine dry meadow. We conducted a year-round field study and performed experiments on isolated soil microorganisms. Based on previous work in this ecosystem, we hypothesized that microbial biomass would decline before the plant growing season and would release N that would become available to plants. Microbial biomass was highest when soils were cold, in autumn, winter, and early spring. During this time, N was immobilized in microbial biomass. After snow melt in spring, microbial biomass decreased. A peak in the soil protein concentration was seen at this time, followed by peaks in soil amino acid and ammonium concentrations in late June. Soil protease rates were initially high after snow melt, decreased to below detection limits by midsummer, and partially recovered by late summer. Proteolytic activity in soil was saturated early in the growing season and became protein limited later in the summer. We concluded that the key event controlling N availability to alpine plants occurs after snow melt, when protein is released from the winter microbial biomass. This protein pulse provides substrate for soil proteases, which supply plants with amino acids during the growing season. On average, microbial biomass was lower in the summer than at other times, although the biomass fluctuated widely during the summer. Within the summer months, maximum numbers of amino-acid-degrading microorganisms and the maximum amount of microbial biomass coincided with the peak in soil amino acids, when plants are most active. All bacterial strains isolated from this summer community had the ability to grow rapidly on low concentrations of amino acids and to degrade protein. This explains the previously observed result that the soil microbial biomass can compete strongly with plants for organic N, despite the seasonal offset of maximum plant and microbial N uptake.
Autotrophic respiration (Ra) in forest ecosystems can be >50% of the carbon fixed in photosynthesis and may regulate productivity and carbon storage in forest ecosystems, because Ra increases with temperature. We estimated annual Ra from chamber measurements in aspen, black spruce, and jack pine forests in Canada for 1994. Mean foliage respiration at 10°C for expanded leaves was 0.21–0.95 μmol m−2 (leaf surface) s−1 for all species and differed little from May to September. Wood respiration at 15°C (0.2–1 μmol m−2 (stem surface) s−1 for all species) was strongly seasonal, with high rates in midsummer that coincided with wood growth. Fine root respiration at 10°C was 2.5–7.7 μmol kg−1 s−1 for all species and declined throughout the growing season for the conifers. Annual costs of Ra for foliage, wood, and roots (overstory and understory) were 490, 610, and 450 g C m−2 (ground) yr−1 for aspen, black spruce, and jack pine (old) in northern Manitoba and 600, 480, and 310 g C m−2 yr−1 for aspen, black spruce, and jack pine (old) in central Saskatchewan. Carbon use efficiency (CUE), the ratio of net production to production plus Ra, averaged 0.44, 0.34, and 0.39 for aspen, black spruce, and jack pine (old) for all tissues and 0.61, 0.36, and 0.44 for aboveground tissues. Differences in CUE between the northern and the southern sites were small for all species, and CUE did not vary with stand biomass. Species differences in CUE suggest that models assuming a constant CUE across species may poorly estimate production and carbon balance for any given site.
We investigated the effect of temperature and irradiance on leaf respiration (R, non-photorespiratory mitochondrial CO(2) release) of snow gum (Eucalyptus pauciflora Sieb. ex Spreng). Seedlings were hydroponically grown under constant 20 degrees C, controlled-environment conditions. Measurements of R (using the Laisk method) and photosynthesis (at 37 Pa CO(2)) were made at several irradiances (0-2,000 micromol photons m(-2) s(-1)) and temperatures (6 degrees C-30 degrees C). At 15 degrees C to 30 degrees C, substantial inhibition of R occurred at 12 micromol photons m(-2) s(-1), with maximum inhibition occurring at 100 to 200 micromol photons m(-2) s(-1). Higher irradiance had little additional effect on R at these moderate temperatures. The irradiance necessary to maximally inhibit R at 6 degrees C to 10 degrees C was lower than that at 15 degrees C to 30 degrees C. Moreover, although R was inhibited by low irradiance at 6 degrees C to 10 degrees C, it recovered with progressive increases in irradiance. The temperature sensitivity of R was greater in darkness than under bright light. At 30 degrees C and high irradiance, light-inhibited rates of R represented 2% of gross CO(2) uptake (v(c)), whereas photorespiratory CO(2) release was approximately 20% of v(c). If light had not inhibited leaf respiration at 30 degrees C and high irradiance, R would have represented 11% of v(c). Variations in light inhibition of R can therefore have a substantial impact on the proportion of photosynthesis that is respired. We conclude that the rate of R in the light is highly variable, being dependent on irradiance and temperature.
The turbulent exchanges of CO2 and water vapour between an aggrading deciduous forest in the north-eastern United States (Harvard Forest) and the atmosphere were measured from 1990 to 1994 using the eddy covariance technique. We present a detailed description of the methods used and a rigorous evaluation of the precision and accuracy of these measurements. We partition the sources of error into three categories: (1) uniform systematic errors are constant and independent of measurement conditions (2) selective systematic errors result when the accuracy of the exchange measurement varies as a function of the physical environment, and (3) sampling uncertainty results when summing an incomplete data set to calculate long-term exchange.
Analysis of the surface energy budget indicates a uniform systematic error in the turbulent exchange measurements of -20 to 0%. A comparison of nocturnal eddy flux with chamber measurements indicates a selective systematic underestimation during calm (friction velocity < 0.17 m s−1) nocturnal periods. We describe an approach to correct for this error. The integrated carbon sequestration in 1994 was 2.1 t C ha−1 y−1 with a 90% confidence interval due to sampling uncertainty of ±0.3 t C ha−1 y−1 determined by Monte Carlo simulation. Sampling uncertainty may be reduced by estimating the flux as a function of the physical environment during periods when direct observations are unavailable, and by minimizing the length of intervals without flux data. These analyses lead us to place an overall uncertainty on the annual carbon sequestration in 1994 of -0.3 to +0.8 t C ha−1 y−1.
Soil respiration includes soil microbial respiration, soil fauna respiration, and plant root respiration, therefore it reflects the biological activity of the soil ecosystems. The Siberian Taiga often experiences serious damage from forest fire, due to the very low precipitation in spring. We measured the soil respiration in five forest soil ecosystems with different histories of forest fire in Yakutsk in August 1997. The dominant tree species was Larix cajanderi, and the soils were Spodosols with a sandy and loamy texture. We also measured the soil respiration in a grassland. At severely burned sites, almost all the trees had fallen, litter and vegetation on the forest floor had burned, other forms of vegetation, including bryophytes or herbs, had invaded. At less severely burned forest sites, the trees were still standing but litter and vegetation on the forest floor had disappeared. Soil temperature, moisture, pH, and EC all increased after severe forest fires, the A-horizon showed a higher organic carbon content and a lower CN ratio. Soil respiration rate ranged from 18 to 397 (10 g CO2 m ) in the same order reported so far. Soil respiration in severely burned forests was significantly lower than in intact forests, and was similar to that of grassland. Furthermore, mildly burned forests showed soil respiration values intermediate between those of severely burned and intact forests. These findings suggest that tree root respiration is considerably higher than root respiration of other plants or microbial and fauna respiration in soil. Soil microbial respiration was determined by the incubation method under the same temperature and soil moisture conditions as those in situ. Multiple regression analysis for mineral soils showed that the soil microbial respiration increased with the increase of the soil temperature and organic carbon content, that the soil microbial respiration decreased with the increase of pH. Whole soil microbial respiration within 1 m depth was higher in severely burned forests than in intact forests. These findings show that forest fire increased the soil microbial respiration and confirm that the loss of tree root respiration was the main reason for the decrease in soil respiration after severe forest fire. The contribution of tree root respiration to soil respiration was estimated to exceed 50%. Severe forest fire kills trees, and consequently results in a decrease of soil respiration.
The causes of a reduced sensitivity of high-latitude tree growth to variations in summer temperature for recent decades,, compared to earlier this century, are unknown. This sensitivity change is problematic, in that relationships between tree-ring properties and temperature are widely used for reconstructing past climate. Here we report an analysis of tree-ring and climate data from the forest-tundra zone, in combination with a mechanistic model of tree-ring growth, to argue that an increasing trend of winter precipitation over the past century in many subarctic regions led to delayed snow melt in these permafrost environments. As a result, the initiation of cambial activity (necessary for the formation of wood cells) has been delayed relative to the pre-1960 period in the Siberian subarctic. Since the early 1960s, less of the growth season has been during what had previously been the period of maximal growth sensitivity to temperature. This shift results not only in slower growth, but also in a reduced correlation between growth and temperature. Our results suggest that changes in winter precipitation should be considered in seeking explanations for observed changes in the timing of the `spring greening' of high-latitude forests, and should be taken into account in the study of the role of the Siberian subarctic forest in the global carbon cycle.
The objectives of this study are to (1) characterize the carbon (C) content, leaf area index, and aboveground net primary production (ANPP) for mature aspen, black spruce, and young and mature jack pine stands at the southern and northern Boreal Ecosystem-Atmosphere Study (BOREAS) areas and (2) compare net primary production and carbon allocation coefficients for the major boreal forest types of the world. Direct estimates of leaf area index, defined as one half of the total leaf surface area, range from a minimum of 1.8 for jack pine forests to a maximum of 5.6 for black spruce forests; stems comprise 5 to 15% of the total overstory plant area. In the BOREAS study, total ecosystem (vegetation plus detritus plus soil) carbon content is greatest in the black spruce forests (445,760-479,380kgCha-1), with 87 to 88% of the C in the soil, and is lowest in the jack pine stands (68,370-68,980kgCha-1) with a similar distribution of carbon in the vegetation and soil. Forest floor carbon content and mean residence time (MRT) also vary more among forest types in a study area than between study areas for a forest type; forest floor MRT range from 16 to 19 years for aspen stands to 28 to 39 years for jack pine stands. ANPP differs significantly among the mature forests at each of the BOREAS study areas, ranging from a maximum of 3490 to 3520kgCha-1yr-1 for aspen stands to 1170 to 1220kgCha-1yr-1 for jack pine stands. Both net primary production (NPP) and carbon allocation differ between boreal evergreen and deciduous forests in the world, suggesting global primary production models should distinguish between these two forest types. On average, 56% of NPP for boreal forests occurs as detritus and illustrates the need to better understand factors controlling aboveground and below-ground detritus production in boreal forests.
We use semi-mechanistic, empirically based statistical models to predict the spatial and temporal patterns of global carbon dioxide emissions from terrestrial soils. Emissions include the respiration of both soil organisms and plant roots. At the global scale, rates of soil CO2 efflux correlate significantly with temperature and precipitation; they do not correlate well with soil carbon pools, soil nitrogen pools, or soil C:N. Wetlands cover about 3% of the land area but diminish predicted CO2 emissions by only about 1%. The estimated annual flux of CO2 from soils to the atmosphere is estimated to be 76.5 Pg C yr−1, 1–9 Pg greater than previous global estimates, and 30–60% greater than terrestrial net primary productivity. Historic land cover changes are estimated to have reduced current annual soil CO2 emissions by 0.2–2.0 Pg C yr−1 in comparison with an undisturbed vegetation cover. Soil CO2 fluxes have a pronounced seasonal pattern in most locations, with maximum emissions coinciding with periods of active plant growth. Our models suggest that soils produce CO2 throughout the year and thereby contribute to the observed wintertime increases in atmospheric CO2 concentrations. Our derivation of statistically based estimates of soil CO2 emissions at a 0.5° latitude by longitude spatial and monthly temporal resolution represents the best-resolved estimate to date of global CO2 fluxes from soils and should facilitate investigations of net carbon exchanges between the atmosphere and terrestrial biosphere.
Predicted daily fluxes from an ecosystem model for water, carbon dioxide, and methane were compared with 1994 and 1996 Boreal Ecosystem-Atmosphere Study (BOREAS) field measurements at sites dominated by old black spruce (Picea mariana (Mill.) BSP) (OBS) and boreal fen vegetation near Thompson, Man. Model settings for simulat- ing daily changes in water table depth (WTD) for both sites were designed to match observed water levels, including predictions for two microtopographic positions (hollow and hummock) within the fen study area. Water run-on to the soil profile from neighboring microtopographic units was calibrated on the basis of daily snowmelt and rainfall inputs to reproduce BOREAS site measurements for timing and magnitude of maximum daily WTD for the growing season. Model predictions for daily evapotranspiration rates closely track measured fluxes for stand water loss in patterns con- sistent with strong controls over latent heat fluxes by soil temperature during nongrowing season months and by vari- ability in relative humidity and air temperature during the growing season. Predicted annual net primary production (NPP) for the OBS site was 158 g C·m-2 during 1994 and 135 g C·m-2 during 1996, with contributions of 75% from overstory canopy production and 25% from ground cover production. Annual NPP for the wetter fen site was 250 g C·m-2 during 1994 and 270 g C·m-2 during 1996. Predicted seasonal patterns for soil CO 2 fluxes and net ecosystem production of carbon both match daily average estimates at the two sites. Model results for methane flux, which also closely match average measured flux levels of -0.5 mg CH4·m-2·day-1 for OBS and 2.8 mg CH4·m-2·day-1 for fen sites, suggest that spruce areas are net annual sinks of about -0.12 g CH4·m-2, whereas fen areas generate net annual emis- sions on the order of 0.3-0.85 g CH4·m-2, depending mainly on seasonal WTD and microtopographic position. Fen hollow areas are predicted to emit almost three times more methane during a given year than fen hummock areas. The validated model is structured for extrapolation to regional simulations of interannual trace gas fluxes over the entire North America boreal forest, with integration of satellite data to characterize properties of the land surface.
TURC, a diagnostic model for the estimation of continental gross primary productivity (GPP) and net primary productivity (NPP), is presented. This model uses a remotely sensed vegetation index to estimate the fraction of solar radiation absorbed by canopies, and an original parameterization of the relationship between absorbed solar radiation and GPP, based on measurements of CO2 fluxes above plant canopies. An independent, uncalibrated model of autotrophic maintenance and growth respiration is parameterized from literature data, and uses databases on temperature, biomass, and remotely sensed vegetation index. This model results in global estimates of GPP and NPP of 133.1 and 62.3 Gt(C) per year, respectively, which is consistent with commonly admitted values. The ratio of autotrophic respiration to GPP is about 70% for equatorial rain forests and 50% for temperate forests, as a result the highest predicted NPP are in tropical savannas of Africa and South America, and in temperate, highly cultivated zones of North America, not in equatorial rain forest zones. Conversion efficiencies defined as the ratio of yearly integrated NPP to absorbed photosynthetically active radiation (PAR) compare relatively well with a previous compilation of literature values, except for ecosystems with probable reduction of conversion efficiency due to water stress. Several sensitivity studies are performed on some input data sets, model assumptions, and model parameters.
Seasonal variations of the Scots pine (Pinussylvestris L.), dwarf shrub and grass fine-root biomass and necromass were studied in a pole stage Scots pine stand in eastern Finland during three successive growing seasons. The biomass of Scots pine fine roots varied annually and seasonally in the humus layer between 19±5 g m−2 to 139±22 g m−2, in the upper mineral soil layer between 90±14 g m−2 to 279±0 g m−2, and in the lower mineral soil layer between 68±17 g m−2 to 217±73 g m−2. The seasonal minimum and maximum of understory vegetation fine-root biomass were in the humus layer 35±6 g m−2 and 235±42 g m−2, in the upper mineral soil layer 26±0 g m−2 and 165±0 g m−2, and in the lower mineral soil layer 14±0 g m−2 and 36±5 g m−2. The seasonal fine-root necromass varied in the humus layer from 2±0 g m−2 to 1398±236 g m−2, in the upper mineral soil layer from 86±0 g m−2 to 1267±366 g m−2, and in the lower mineral soil layer from 8±0 g m−2 to 753±306 g m−2. The major part of the living Scots pine fine roots (62%) was in the mineral soil immediately below the humus layer, but almost all dwarf shrub roots and grass roots were in the humus and in the upper mineral soil layers. Most dead fine roots (82%) were in the humus layer and in the uppermost mineral soil layer. The variations of Scots pine fine-root biomass, dwarf shrub and grass fine-root biomass and necromass did not show a distinct and clear pattern within any growing season although there were significant differences between the same month during different growing seasons. Some of the observed variations could be explained by climatic factors related to drought.
4 Sukachev's Laboratory of the Institute of Evolution and Ecology Problems, Leninsky Prospect 33, 117071 Moscow, Russia; 5 V.N. Sukachev Institute of Forests, Akademgorodok, 660036 Krasnoyarsk, Russia; 6 present address: HSSE-AG National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, 637616 Singapore ABSTRACT We present soil organic carbon (SOC) inventories and carbon isotope compositions from over 900 samples collected in areas of minimally disturbed mature vegetation on freely drained soils (ex-cluding peatlands) on a 1000 km transect along the Yennisey River, central Siberia. Carbon inven-tories over 0–30 cm depth range widely from 1.71 to 7.05 kg m −2 . While an effect of changing climate or vegetation along the transect cannot be ruled out, the observed differences in SOC in-ventories are largely the result of variations in mineral soil texture, with inventories in fine-textured soils being approximately double those in coarse-textured soils. The δ 13 C values of SOC in the 0– 5 cm interval ranged from −26.3 to −28.0‰, with δ 13 C values for the 5–30 cm interval being 0.9 ± 0.8‰ (1σ) enriched in 13 C relative to the 0–5 cm samples. The average δ 13 C value for the 0–5 cm interval for all samples was −27.1 ± 0.6‰ (1σ) and for the full 0–30 cm interval the average was −26.5 ± 0.5‰ (1σ). In general, δ 13 C values were higher in coarse-textured soils and lower in fine-textured soils. The results of detailed sampling of soils in Pinus sylvestris forest growing on sand near the Zotino flux tower suggest an SOC inventory in these soils of 2.22 ± 0.35 kg m −2 over 30 cm and an average δ 13 C value of −26.3 ± 0.2‰ over the 0–5 cm depth interval and −25.9 ± 0.3‰ over 0–30 cm. Recent burning had no effect on SOC inventories, but clearing has led to an average 25% decrease on SOC inventories from 0–30 cm over 12 yr. Neither burning nor clearing had a discernible effect on the δ 13 C value of SOC.
The exchange of carbon dioxide (CO 2 ) between the atmosphere and terrestrial ecosystems due to photosynthesis and respiration has been simulated using a new version of the simple biosphere model (SiB2) and the Colorado State University (CSU) general circulation model (GCM). Parameters associated with the extent and seasonality of vegetation were derived from satellite observations. The fluxes were calculated at the GCM time step of 6 min, so that the diurnal cycle of photosynthesis is well resolved. Annual net primary productivity simulated by the coupled model agrees well with previous estimates in most regions of the world. In some regions (central North America, southeastern South America, southeast Asia), the precipitation simulated by the CSU GCM is less than observed, and in those regions the simulated NPP is less than previous estimates. The amplitude of the seasonal cycle of the simulated net flux is quite similar to previous estimates, but the phase is significantly earlier in the northern temperate and boreal zones, both as simulated by the GCM and when SiB2 is driven off-line using observed meteorological forcing. At the few locations for which observational data are available, the phase of the simulated seasonal cycle of net carbon fluxes agrees well with the data, but at one temperate forest grid cell the amplitude is too low. The phase of the simulated diurnal cycle reproduces observations from a temperate forest, temperate grassland, and tropical forest. The amplitude of the simulated diurnal cycle is close to the observed amplitude early in the season at the temperate grassland site, but deteriorates by late summer because of drought stress due to the less-than-observed simulated precipitation. DOI: 10.1034/j.1600-0889.1996.t01-2-00009.x
The quantitative composition of various trophic groups of microorganisms and the enzyme activity of forest cryogenic soils in Central Evenkia was studied. The preferential development of complexes of psychrotolerant bacteria and micromycetes was found. During a short growing period, the biological activity of upper organic horizons in northern soils is comparable with that in southern taiga soils of Siberia. The microflora and biological activity of southern taiga soils in Central Siberia is understood rather completely. At the same time, information on the biogenic properties of forest soils in northern Siberian regions is scarce, though there are representative data on sylvicultural, soil, physiological, and biochemical studies of forest biogeocenoses.
Data are drawn from the first field season of a major Anglo-Brazilian collaborative study of micrometeorology and plant physiology of Amazonian rain forest. In dry daylight conditions the temperature, humidity and humidity deficit of the top two-thirds of the canopy are similar to those of the atmosphere above, but air at the base of the canopy is strongly decoupled. At night the behaviour is reversed. The response to convective storms is rapid but fairly short-lived.-D.G.Tout
One of the key questions in climate change research relates to the future dynamics of the large amount of C that is currently stored in soil organic matter. Will the amount of C in this pool increase or decrease with global warming? The future trend in amounts of soil organic C will depend on the relative temperature sensitivities of net primary productivity and soil organic matter decomposition rate. Equations for the temperature dependence of net primary productivity have been widely used, but the temperature dependence of decomposition rate is less clear. The literature was surveyed to obtain the temperature dependencies of soil respiration and N dynamics reported in different studies. Only laboratory-based measurements were used to avoid confounding effects with differences in litter input rates, litter quality, soil moisture or other environmental factors. A considerable range of values has been reported, with the greatest relative sensitivity of decomposition processes to temperature having been observed at low temperatures. A relationship fitted to the literature data indicated that the rate of decomposition increases with temperature at 0°C with a Q10 of almost 8. The temperature sensitivity of organic matter decomposition decreases with increasing temperature, indicated by the Q10 decreasing with temperature to be about 4.5 at 10°C and 2.5 at 20°C. At low temperatures, the temperature sensitivity of decomposition was consequently much greater than the temperature sensitivity of net primary productivity, whereas the temperature sensitivities became more similar at higher temperatures. The much higher temperature sensitivity of decomposition than for net primary productivity has important implications for the store of soil organic C in the soil. The data suggest that a 1°C increase in temperature could ultimately lead to a loss of over 10% of soil organic C in regions of the world with an annual mean temperature of 5°C, whereas the same temperature increase would lead to a loss of only 3% of soil organic C for a soil at 30°C. These differences are even greater in absolute amounts as cooler soils contain greater amounts of soil organic C. This analysis supports the conclusion of previous studies which indicated that soil organic C contents may decrease greatly with global warming and thereby provide a positive feed-back in the global C cycle.
On July 15 and 16, 1996, profiles of temperature, water vapor, carbon dioxide concentration, and its carbon isotopic composition were made within and above the convective boundary layer (CBL), near the village of Zotino in central Siberia (60°N, 89°E). On both days the CBL grew to a height of around 1000 m at midday after which little further growth was observed. This was despite high rates of sensible heat flux into the CBL from the predominantly coniferous vegetation below and was attributable to a high subsidence velocity. For all flights, marked discontinuities across the top of the CBL were observed for water vapor and CO2 concentrations with differences between the CBL and the free troposphere above being as high as 10 mmol mol-1 and 13 mumol mol-1, respectively. Associated with the lower CO2 concentrations within the CBL was an enrichment of the delta 13C in CO2 of up to 0.70/00. Although for any one flight, fluctuations in CO2 and delta 13C within the CBL were small (less than 3 mumol mol-1 and 0.10/00) they were well correlated and suggested a photosynthetic discrimination, Delta, by the vegetation below of ~170/00. Estimates of regional Delta based on CBL budgeting techniques suggested values ranging from 14.8 to 20.40/00. CBL budgeting techniques were also used to estimate regional ecosystem carbon fluxes (-3 to -9 mumol m-2 s-1) and evaporation rates (1-3 mmol m-2 s-1). Agreement with ground-based tower measurements was reasonable, but a bootstrap error analysis suggested that errors associated with the integral CBL technique were sometimes unacceptably large, especially for estimates of regional photosynthetic 13C discrimination and regional evaporation rates. Conditions under which CBL techniques should result in reasonably accurate estimations of regional fluxes and isotopic fractionations are evaluated.
Measurements of the fluxes of latent heat lambdaE, sensible heat H, and CO2 were made by eddy covariance in a boreal black spruce forest as part of the Boreal Ecosystem-Atmosphere Study (BOREAS) for 120 days through the growing season in 1994. BOREAS is a multiscale study in which satellite, airborne, stand-scale, and leaf-scale observations were made in relation to the major vegetation types [Sellers et al., 1995]. The eddy covariance system comprised a sonic anemometer mounted 27 m above the forest, a system for transferring air rapidly and coherently to a closed path, infrared gas analyzer and a computer with the Edinburgh EdiSol software. Over the measurement period, closure of the energy balance on a 24 hour basis was good: (H+lambdaE)/(Rn-G-B-S)=0.97. The midday Bowen ratio was typically in the range 1.0-2.5, with an average value of ~1.9 in the first Intensive Field Campaign (IFC1) and 1.3-1.4 in IFC2 and IFC3. Daily ecosystem evapotranspiration from moss, understory, and trees followed daily net radiation. Mean half-hourly net ecosystem flux followed photosynthetic photon flux density (PPFD) closely, reaching -9mumolm-2s-1 in June and August. The mean respiratory efflux on nights during which the atmosphere was well mixed (u*>0.4ms-1) reached 6mumolm-2s-1. The PPFD-saturated biotic CO2 assimilation reached 20mumolm-2s-1 and showed little response to air temperature or vapor pressure deficit (VPD). Storage of CO2 in the air column at night did not account adequately for respiration on stable nights, so nighttime efflux was modeled for periods when u*
The regulation of soil surface CO2 efflux (Fs) from a dryland Pinus radiata clear-cut in New Zealand (658-mm annual rainfall, 50-mm soil water storage over 0.5-m depth of soil) was quantified using a multiplicative, soil moisture and temperature constraint model. Model parameters were determined from measurements of Fs by eddy covariance and by two portable CO2 analyzers equipped with soil chambers during 8 consecutive days in November 1995. Fs from the two chamber systems agreed consistently, but agreement between chamber and eddy covariance measurements was confined to a very small flux-range (1.5-2.5mumolm-2s-1). When the soil surface was wet and wind speed (u, a surrogate for static pressure fluctuations) was high, eddy covariance fluxes were >3mumolm-2s-1 (maximum 5.0mumolm-2s-1), significantly higher than the chamber values. Including u as a third constraint in the model facilitated good predictions of Fs over the entire range of environmental conditions. Daily Fs during the study period ranged from 0.8 to 2.9gCm-2. Driven by weather data from the forest fire station and subroutines for soil water balance and temperature, modeled Fs for the year ending in June 1996 was 0.37kgCm-2. This year had below average (547 mm) rainfall. For a year with 640-mm rainfall, a significant increase of annual Fs (0.44kgCm-2) demonstrated the severe restriction of soil microbial activity by moisture deficit at dryland sites. Moreover, when the soil was adequately watered, Fs was frequently restricted by low wind speed.
The term zero curtain refers to the effect of latent heat in maintaining temperatures near 0°C over extended periods in freezing or thawing soils. Analysis of thermal data from an arctic site and a mid-latitude location suggests that the phenomenon may be a universal characteristic of wet, medium-textured mineral and organic soils during freezing. Nonconductive processes play an important role in formation and maintenance of the zero-curtain effect. Water advection, condensation, and evaporation events are recognizable in mappings of thermal and ion concentration records on time-depth meshes. The zero curtain appears to be produced and maintained by vapor transport and internal distillation mechanisms, driven by osmotic potential variations induced by freeze-thaw events at the soil surface. The term zero curtain is applicable to locations outside permafrost regions, but units of measurements should not be assigned to it.
ABSTRACT We investigated the relationship between daily and seasonal temperature variation and dark respiratory CO2 release by leaves of snow gum (Eucalyptus pauciflora Sieb. ex Spreng) that were grown in their natural habitat or under controlled-environment conditions. The open grassland field site in SE Australia was characterized by large seasonal and diurnal changes in air temperature. On each measurement day, leaf respiration rates in darkness were measured in situ at 2–3 h intervals over a 24 h period, with measurements being conducted at the ambient leaf temperature. The rate of respiration at a set measuring temperature (i.e. apparent ‘respiratory capacity’) was greater in seedlings grown under low average daily temperatures (i.e. acclimation occurred), both in the field and under controlled-environment conditions. The sensitivity of leaf respiration to diurnal changes in temperature (i.e. the Q10 of leaf respiration) exhibited little seasonal variation over much of the year. However, Q10 values were significantly greater on cold winter days (i.e. when daily average and minimum air temperatures were below 6° and –1 °C, respectively). These differences in Q10 values were not due to bias arizing from the contrasting daily temperature amplitudes in winter and summer, as the Q10 of leaf respiration was constant over a wide temperature range in short-term experiments. Due to the higher Q10 values in winter, there was less difference between winter and summer leaf respiration rates measured at 5 °C than at 25 °C. The net result of these changes was that there was relatively little difference in total daily leaf respiratory CO2 release per unit leaf dry mass in winter and summer. Under controlled-environment conditions, acclimation of respiration to growth temperature occurred in as little as 1–3 d. Acclimation was associated with a change in the concentration of soluble sugars under controlled conditions, but not in the field. Our data suggest that acclimation in the field may be associated with the onset of cold-induced photo-inhibition. We conclude that cold-acclimation of dark respiration in snow gum leaves is characterized by changes in both the temperature sensitivity and apparent ‘capacity’ of the respiratory apparatus, and that such changes will have an important impact on the carbon economy of snow gum plants.
Abstract We present results from two years’ net ecosystem flux measurements above a boreal forest in central Sweden. Fluxes were measured with an eddy correlation system based on a sonic anemometer and a closed path CO2 and H2O gas analyser. The measurements show that the forest acted as a source during this period, and that the annual balance is highly sensitive to changes in temperature. The accumulated flux of carbon dioxide during the full two-year period was in the range 480–1600 g CO2 m–2. The broad range is caused by uncertainty regarding assessment of the night-time fluxes. Although annual mean temperature remained close to normal, the results are partly explained by higher than normal respiration, due to abnormal temperature distribution and reduced soil moisture during one growing season. The finding that a closed forest can be a source of carbon over such a long period as two years contrasts sharply with the common belief that forests are always carbon sinks.
Modeling the terrestrial biosphere's carbon exchanges constitutes a key tool for investigation of the global carbon cycle, which has lead to the recent development of numerous terrestrial biosphere models. However, as demonstrated by recent intercomparison studies, results of plant carbon uptake, expressed as net primary productivity (NPP), still diverge to a large degree. Here, we address the question of uncertainty by conducting a series of sensitivity tests with a single, process-based model, the Biosphere Energy-Transfer Hydrology (BETHY) scheme. We calculate NPP globally for a standard model setup and various alternative model setups representing either changes in modeling strategy or approximate uncertainties of the most important model parameters. The results show that estimated uncertainties of many process parameters are still too large for reliable predictions of global NPP. The largest uncertainties come from plant respiration, photosynthesis and soil water storage. The surface radiation balance and day-to-day variations in weather, often not included into terrestrial vegetation models, are also found to contribute significantly to overall uncertainties, while stomatal behavior, the aerodynamic coupling of vegetation and atmosphere, and the choice of the vegetation map turn out to be relatively unimportant. A further comparison with field measurements of NPP suggests that such data are too unreliable for validating biosphere modél predictions. We conclude that the inherent uncertainties in process-oriented biosphere modeling are able to explain the discrepancies that have occurred when comparing the results of different models.
Initial observations of the diurnal variation of the vertical thermal structure of a loblolly pine planation are presented. Results obtained in other forests are qualitatively confirmed. On a clear day a very unstable temperature gradient occurs above the trees, while a strong inversion (∼8°C) develops below the crowns. At night the sub-crown region becomes weakly unstable, but the atmospheric layer above the trees is then stable. On a rainy day, the strength of the temperature inversion beneath the tree crowns is less than 1°C. The position of the daytime temperature maximum in the tree-tops responds to the solar elevation, eventually descending about 2 m to the region of maximum foliage density as the sun's rays penetrate deeper into the tree crowns.
We measured soil respiration during two winters in three different ecotypes of the BOREAS northern study area. The production of CO2 was continuous throughout the winter and, when totaled for the winter of 1994-1995, was equivalent to the release of --40-55 g C/m 2 from the soil surface. As soils cooled in the early winter, the CO2 production rate decreased in a manner that appeared to be exponentially related to shallow soil temperatures. This exponential relationship was not observed when soils began to warm, possibly indicating that there may be additional or different processes responsible for increased CO2 production during winter warming events. We also measured CO2 concentrations in soil gas and the A4C of the soil CO2. These measurements show that the CO2 produced in winter is not simply the return to the atmosphere of the carbon fixed during the previous growing season. We suggest that the wintertime production of CO2 originates, at least in part, from the decomposition of old organic carbon stored at depth in the soil.
Estimates of the extent of the discrimination against13CO2 during photosynthesis (?A) on a global basis were made using gridded data sets of temperature, precipitation, elevation, humidity and vegetation type. Stomatal responses to leaf-to-air vapour mole fraction difference (D, leaf-to-air vapour pressure difference divided by atmospheric pressure) were first determined by a literature review and by assuming that stomatal behaviour results in the optimisation of plant water use in relation to carbon gain. Using monthly time steps, modelled stomatal responses toD were used to calculate the ratio of stomatal cavity to ambient CO2 mole fractions and then, in association with leaf internal conductances, to calculate ?A. Weighted according to gross primary productivity (GPP, annual net CO2 asimilation per unit ground area), estimated ?A for C3 biomes ranged from 12.9‰ for xerophytic woods and shrub to 19.6‰ for cool/cold deciduous forest, with an average value from C3 plants of 17.8‰. This is slightly less than the commonly used values of 18–20‰. For C4 plants the average modelled discrimination was 3.6‰, again slightly less than would be calculated from C4 plant dry matter carbon isotopic composition (yielding around 5‰). From our model we estimate that, on a global basis, 21% of GPP is by C4 plants and for the terrestrial biosphere as a whole we calculate an average isotope discrimination during photosynthesis of 14.8‰. There are large variations in ?A across the globe, the largest of which are associated with the precence or absence of C4 plants. Due to longitudinal variations in ?A, there are problems in using latitudinally averaged terrestrial carbon isotope discriminations to calculate the ratio of net oceanic to net terrestrial carbon fluxes.
The exchange of carbon dioxide (CO2) between the atmosphere and terrestrial ecosystems due to photosynthesis and respiration has been simulated using a new version of the simple biosphere model (SiB2) and the Colorado State University (CSU) general circulation model (GCM). Parameters associated with the extent and seasonality of vegetation were derived from satellite observations. The fluxes were calculated at the GCM time step of 6 min, so that the diurnal cycle of photosynthesis is well resolved. Annual net primary productivity simulated by the coupled model agrees well with previous estimates in most regions of the world. In some regions (central North America, southeastern South America, southeast Asia), the precipitation simulated by the CSU GCM is less than observed, and in those regions the simulated NPP is less than previous estimates. The amplitude of the seasonal cycle of the simulated net flux is quite similar to previous estimates, but the phase is significantly earlier in the northern temperate and boreal zones, both as simulated by the GCM and when SiB2 is driven off-line using observed meteorological forcing. At the few locations for which observational data are available, the phase of the simulated seasonal cycle of net carbon fluxes agrees well with the data, but at one temperate forest grid cell the amplitude is too low. The phase of the simulated diurnal cycle reproduces observations from a temperate forest, temperate grassland, and tropical forest. The amplitude of the simulated diurnal cycle is close to the observed amplitude early in the season at the temperate grassland site, but deteriorates by late summer because of drought stress due to the less-than-observed simulated precipitation.
Abstract Quantum yields of photosynthetic CO2 uptake by Pinus sylvestris (L.) shoots were measured at temperatures between −; 2°C and 35°C from September 1984 to September 1985. The ratio of variable to peak fluorescence of photosystem II (Fv/Fp) was also measured. Quantum yield measured at 25°C varied with time from a low winter value of 0.017 to a high summer value of 0.057. This variation was strongly correlated to variation in FvFp(r2= 0.91).
The response of quantum yield to temperature changed with season. During winter, quantum yield was essentially constant between 0°C and 35°C. The constancy above 5°C was associated with a strong increase in intercellular space CO2 (Ci) with temperature. In June, the quantum yield peaked at 5°C, decreased sharply below 5°C, and was rather constant between 25°C and 35°C. This insensitivity to increased temperature above 25°C was attributed to a large increase in Ci In contrast, by September, the quantum yield was less sensitive to temperature below 5°C and more sensitive above 25°C, despite an unchanged Ci response with increasing temperatures as compared with June. In August, quantum yields were lowered at 0, 5 and 15°C, apparently as a result of high carbohydrate levels in the leaves. Overall, the results suggest that there are sites other than in photosystem II or at ribulose bisphosphate carboxylase/oxygenase at which the quantum yield of photosynthetic CO2 uptake is affected. Possible causes for the changes in efficiency are discussed.
The effects of fertilization, irrigation or both on the seasonal changes of starch and soluble carbohydrates (glucose, fructose, myo-inositol, pinitol and sucrose) in needles of 20-year-old Scots pine trees (Pinus silvestris L.) were studied during three consecutive years. The starch content of the mature needles increased during spring and early summer to about 25% of dry weight. Neither fertilization nor irrigation affected the general pattern of starch accumulation during the spring. The starch reserves were mobilized when the shoot started to grow. Starch content decreased more rapidly in needles from fertilized than in those from unfertilized trees. The current needles from the control trees accumulated starch while they were still growing. The current needles of the fertilized trees did so to a lesser extent. The amount of starch was closely correlated to the air temperature and to the growth rate. Large amounts were found at low temperatures and low growth rates.
The concentrations of soluble carbohydrates showed the well-known seasonal variation, with the highest value during the winter. The levels of sugars were nearly similar, irrespective of fertilization. An exception was sucrose, which was found in small quantities in needles from fertilized plots. Small amounts of sucrose were also found in growing current needles.
The results are discussed in relation to growth limitation by assimilate availability and indicate that the ‘sink demand’ is the limiting factor.
The possible responses of ecosystem processes to rising atmospheric CO2 concentration and climate change are illustrated using six dynamic global vegetation models that explicitly represent the interactions of ecosystem carbon and water exchanges with vegetation dynamics. The models are driven by the IPCC IS92a scenario of rising CO2 (Wigley et al. 1991), and by climate changes resulting from effective CO2 concentrations corresponding to IS92a, simulated by the coupled ocean atmosphere model HadCM2-SUL. Simulations with changing CO2 alone show a widely distributed terrestrial carbon sink of 1.4–3.8 Pg C y−1 during the 1990s, rising to 3.7–8.6 Pg C y−1 a century later. Simulations including climate change show a reduced sink both today (0.6–3.0 Pg C y−1) and a century later (0.3–6.6 Pg C y−1) as a result of the impacts of climate change on NEP of tropical and southern hemisphere ecosystems. In all models, the rate of increase of NEP begins to level off around 2030 as a consequence of the ‘diminishing return’ of physiological CO2 effects at high CO2 concentrations. Four out of the six models show a further, climate-induced decline in NEP resulting from increased heterotrophic respiration and declining tropical NPP after 2050. Changes in vegetation structure influence the magnitude and spatial pattern of the carbon sink and, in combination with changing climate, also freshwater availability (runoff). It is shown that these changes, once set in motion, would continue to evolve for at least a century even if atmospheric CO2 concentration and climate could be instantaneously stabilized. The results should be considered illustrative in the sense that the choice of CO2 concentration scenario was arbitrary and only one climate model scenario was used. However, the results serve to indicate a range of possible biospheric responses to CO2 and climate change. They reveal major uncertainties about the response of NEP to climate change resulting, primarily, from differences in the way that modelled global NPP responds to a changing climate. The simulations illustrate, however, that the magnitude of possible biospheric influences on the carbon balance requires that this factor is taken into account for future scenarios of atmospheric CO2 and climate change.
Based on review and original data, this synthesis investigates carbon pools and fluxes of Siberian and European forests (600 and 300 million ha, respectively). We examine the productivity of ecosystems, expressed as positive rate when the amount of carbon in the ecosystem increases, while (following micrometeorological convention) downward fluxes from the atmosphere to the vegetation (NEE = Net Ecosystem Exchange) are expressed as negative numbers. Productivity parameters are Net Primary Productivity (NPP=whole plant growth), Net Ecosystem Productivity (NEP = CO 2 assimilation minus ecosystem respiration), and Net Biome Productivity (NBP = NEP minus carbon losses through disturbances bypassing respiration, e.g. by fire and logging). Based on chronosequence studies and national forestry statistics we estimate a low average NPP for boreal forests in Siberia: 123 gC m –2 y –1 . This contrasts with a similar calculation for Europe which suggests a much higher average NPP of 460 gC m –2 y –1 for the forests there. Despite a smaller area, European forests have a higher total NPP than Siberia (1.2–1.6 vs. 0.6–0.9 × 10 ¹⁵ gC region –1 y –1 ). This arises as a consequence of differences in growing season length, climate and nutrition.
For a chronosequence of Pinus sylvestris stands studied in central Siberia during summer, NEE was most negative in a 67‐y old stand regenerating after fire (– 192 mmol m –2 d –1 ) which is close to NEE in a cultivated forest of Germany (– 210 mmol m –2 d –1 ). Considerable net ecosystem CO 2 ‐uptake was also measured in Siberia in 200‐ and 215‐y old stands (NEE:174 and – 63 mmol m –2 d –1 ) while NEP of 7‐ and 13‐y old logging areas were close to the ecosystem compensation point. Two Siberian bogs and a bog in European Russia were also significant carbon sinks (– 102 to – 104 mmol m –2 d –1 ). Integrated over a growing season (June to September) we measured a total growing season NEE of – 14 mol m –2 summer –1 (– 168 gC m –2 summer –1 ) in a 200‐y Siberian pine stand and – 5 mol m –2 summer –1 (– 60 gC m –2 summer –1 ) in Siberian and European Russian bogs. By contrast, over the same period, a spruce forest in European Russia was a carbon source to the atmosphere of (NEE: + 7 mol m –2 summer –1 = + 84 gC m –2 summer –1 ). Two years after a windthrow in European Russia, with all trees being uplifted and few successional species, lost 16 mol C m –2 to the atmosphere over a 3‐month in summer, compared to the cumulative NEE over a growing season in a German forest of – 15.5 mol m –2 summer –1 (– 186 gC m –2 summer –1 ; European flux network annual averaged – 205 gC m –2 y –1 ).
Differences in CO 2 ‐exchange rates coincided with differences in the Bowen ratio, with logging areas partitioning most incoming radiation into sensible heat whereas bogs partitioned most into evaporation (latent heat). Effects of these different surface energy exchanges on local climate (convective storms and fires) and comparisons with the Canadian BOREAS experiment are discussed.
Following a classification of disturbances and their effects on ecosystem carbon balances, fire and logging are discussed as the main processes causing carbon losses that bypass heterotrophic respiration in Siberia. Following two approaches, NBP was estimated to be only about 13–16 mmol m –2 y –1 for Siberia. It may reach 67 mmol m –2 y –1 in North America, and about 140–400 mmol m –2 y –1 in Scandinavia.
We conclude that fire speeds up the carbon cycle, but that it results also in long‐term carbon sequestration by charcoal formation. For at least 14 years after logging, regrowth forests remain net sources of CO 2 to the atmosphere. This has important implications regarding the effects of Siberian forest management on atmospheric concentrations. For many years after logging has taken place, regrowth forests remain weaker sinks for atmospheric CO 2 than are nearby old‐growth forests.
This paper presents CO2 flux data from 18 forest ecosystems, studied in the European Union funded EUROFLUX project. Overall, mean annual gross primary productivity (GPP, the total amount of carbon (C) fixed during photosynthesis) of these forests was 1380 ± 330 gC m−2 y−1 (mean ±SD). On average, 80% of GPP was respired by autotrophs and heterotrophs and released back into the atmosphere (total ecosystem respiration, TER = 1100 ± 260 gC m−2 y−1). Mean annual soil respiration (SR) was 760 ± 340 gC m−2 y−1 (55% of GPP and 69% of TER).
Among the investigated forests, large differences were observed in annual SR and TER that were not correlated with mean annual temperature. However, a significant correlation was observed between annual SR and TER and GPP among the relatively undisturbed forests. On the assumption that (i) root respiration is constrained by the allocation of photosynthates to the roots, which is coupled to productivity, and that (ii) the largest fraction of heterotrophic soil respiration originates from decomposition of young organic matter (leaves, fine roots), whose availability also depends on primary productivity, it is hypothesized that differences in SR among forests are likely to depend more on productivity than on temperature.
At sites where soil disturbance has occurred (e.g. ploughing, drainage), soil espiration was a larger component of the ecosystem C budget and deviated from
the relationship between annual SR (and TER) and GPP observed among the less-disturbed forests. At one particular forest, carbon losses from the soil were so large, that in some years the site became a net source of carbon to the atmosphere. Excluding the disturbed sites from the present analysis reduced mean SR to 660 ± 290 gC m−2 y−1, representing 49% of GPP and 63% of TER in the relatively undisturbed forest ecosystems.
The range and source of variation in foliage respiration rate in the dormant season were investigated for plants of Lycopodium annotinum L., Pinus contorta Dougl. var. latifolia Engelm., Picea abies (L.) Karst., Andromeda polifolia L., Calluna vulgaris (L.) Hull, Vaccinium myrtillus L., Vaccinium vitis-ideae L. and Empetrum hermaphroditum Hagerup. Field-grown plants were transferred to a cold room kept at 5°C in late autumn and then analysed for the foliage respiration rate in relation to nitrogen and sugar concentration over a period of many weeks. Respiration rate varied 1.6-fold among species at a given time, and decreased with time as long as plants remained dormant. Most of both sources of variation were accounted for by the same linear and positive correlation with total soluble sugar concentration, whereas no relationship with nitrogen concentration was found. The hypothesis presented is that respiration rate correlates with sugar concentration in the dormant season because cellular sugar concentrations are much increased and, thereby, the costs of maintaining concentration gradients. Pinus contorta had a significantly higher respiration rate for a given sugar concentration than any other species, and therefore suffered larger relative losses of sugars when kept at 5°C; possible reasons and consequences of this are discussed in relation to field performance.
Eddy covariance measurements of CO 2 flux, based on four and six week campaigns in Rondôdnia, Brazil, have been used in conjunction with a model to scale up data to a whole year, and thus estimate the carbon balance of the tropical forest ecosystem, and the changes in carbon balance expected from small interannual variations in climatological conditions.
One possible source of error in this estimation arises from the difficulty in measuring fluxes under stably stratified meteorological conditions, such as occur frequently at night. Flux may be ‘lost’ because of low velocity advection, caused by nocturnal radiative cooling at sites on raised ground. Such effects may be detected by plotting the net ecosystem flux of CO 2 , F eco is a function of wind speed. If flux is ‘lost’ then F eco is expected to decline with wind speed. In the present data set, this did not occur, and F eco was similar to the nocturnal flux estimated independently from chamber measurements.
The model suggests that in 1992/3, the Gross Primary Productivity (GPP) was 203.3 mol C m ⁻² y ⁻¹ and ecosystem respiration was 194.8 mol C m ⁻² y ⁻¹ , giving an ecosystem carbon balance of 8.5 mol C m ⁻² y ⁻¹ , equivalent to a sink of 1.0 ton C ha ⁻¹ y ⁻¹ . However, the sign and magnitude of this figure is very sensitive to temperature, because of the strong influence of temperature on respiration.
The model also suggests that the effect of temperature on the net carbon balance is strongly dependent on the partial pressure of CO 2 .
Rates of CO 2 efflux from the floor of a central Siberian Scots pine (Pinus sylvestris) forest were measured using a dynamic closed chamber system and by a eddy covariance system placed 2.5 m above the forest floor. Measurements were undertaken for a full growing season: from early May to early October 1999. Spatial variability as determined by the chamber measurements showed the rate of CO 2 efflux to depend on location, with rates from relatively open areas ("glades") only being about 50% those observed below or around trees. This was despite generally higher temperatures in the glade during the day. A strong relationship between CO 2 efflux rate and root density was observed in early spring, suggesting that lower rates in open areas may have been attributable to fewer roots there. Continuous measurements with the eddy covariance system provided good temporal coverage. This method, however, provided estimates of ground CO 2 efflux rate rates that were about 50% lower than chamber measurements that were undertaken in areas considered to be representative of the forest as a whole. An examination of the seasonal pattern of soil CO 2 efflux rates suggests that much of the variability in CO 2 efflux rate could be accounted for by variations in soil temperature. Nevertheless, there were also some indications that the soil water deficits served to reduce soil CO 2 efflux rates during mid-summer. Overall the sensitivity of CO 2 efflux rate to temperature seems to be greater for this boreal ecosystem than has been the case for most other studies.
The exchange of carbon dioxide (CO 2 ) between the atmosphere and a forest after disturbance by wind throw in the western Russian taiga was investigated between July and October 1998 using the eddy covariance technique. The research area was a regenerating forest (400 m × 1000 m), in which all trees of the preceding generation were uplifted during a storm in 1996. All deadwood had remained on site after the storm and had not been extracted for commercial purposes. Because of the heterogeneity of the terrain, several micrometeorological quality tests were applied. In addition to the eddy covariance measurements, carbon pools of decaying wood in a chronosequence of three different wind throw areas were analysed and the decay rate of coarse woody debris was derived.
During daytime, the average CO 2 uptake flux was −3 µmol m ⁻² s ⁻¹ , whereas during night‐time characterised by a well‐mixed atmosphere the rates of release were typically about 6 µmol m ⁻² s ⁻¹ . Suppression of turbulent fluxes was only observed under conditions with very low friction velocity ( u * ≤ 0.08 ms ⁻¹ ). On average, 164 mmol CO 2 m ⁻² d ⁻¹ was released from the wind throw to the atmosphere, giving a total of 14.9 mol CO 2 m ⁻² (180 g CO 2 m ⁻² ) released during the 3‐month study period.
The chronosequence of dead woody debris on three different wind throw areas suggested exponential decay with a decay coefficient of −0.04 yr ⁻¹ . From the magnitude of the carbon pools and the decay rate, it is estimated that the decomposition of coarse woody debris accounted for about a third of the total ecosystem respiration at the measurement site. Hence, coarse woody debris had a long‐term influence on the net ecosystem exchange of this wind throw area.
From the analysis performed in this work, a conclusion is drawn that it is necessary to include into flux networks the ecosystems that are subject to natural disturbances and that have been widely omitted into considerations of the global carbon budget. The half‐life time of about 17 years for deadwood in the wind throw suggests a fairly long storage of carbon in the ecosystem, and indicates a very different long‐term carbon budget for naturally disturbed vs. commercially managed forests.