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Calibration of a recently developed water flow and storage model based on experimental data for a young diffuse-porous beech tree (Fagus sylvatica L.) and a young ringporous oak tree (Quercus robur L.) revealed that differences in stem wood anatomy between species strongly affect the calibrated values of the hydraulic model parameters. The hydraulic capacitance (C) of the stem storage tissue was higher in oak than in beech (939.8 versus 212.3 mg MPa-1). Model simulation of the elastic modulus (ε) revealed that this difference was linked to the higher elasticity of the stem storage tissue of oak compared with beech. Furthermore, the hydraulic resistance (Rx) of beech was about twice that of oak (0.1829 versus 0.1072 MPa s mg-1). To determine the physiological meaning of the Rx parameter identified by model calibration, we analyzed the stem wood anatomy of the beech and oak trees. Calculation of stem specific hydraulic conductivity (ks) of beech and oak with the Hagen-Poiseuille equation confirmed the differences in Rx predicted by the model. The contributions of different vessel diameter classes to the total hydraulic conductivity of the xylem were calculated. As expected, the few big vessels contributed much more to total conductivity than the many small vessels. Compared with beech, the larger vessels of oak resulted in a higher ks (10.66 versus 4.90 kg m-1 s-1 MPa-1). The calculated ratio of ks of oak to beech was 2, confirming the Rx ratio obtained by model calibration. Thus, validation of the Rx parameter of the model led to identification of its physiological meaning. [ABSTRACT FROM AUTHOR]

Abstract: The use of plant water status indicators such as midday stem water potential (Ψ stem) and maximum daily trunk shrinkage (MDS) in irrigation scheduling requires the definition of a reference or threshold value, beyond which irrigation is necessary. These reference values are generally obtained by comparing the seasonal variation of plant water status with the environmental conditions under non-limiting soil water availability. In the present study an alternative approach is presented based on the plant’s response to water deficit. A drought experiment was carried out on two apple cultivars (Malus domestica Borkh. ‘Mutsu’ and ‘Cox Orange’) in which both indicators (Ψ stem and MDS) were related to several plant physiological responses. Sap flow rates, maximum net photosynthesis rates and daily radial stem growth (DRSG) (derived from continuous stem diameter variation measurements) were considered in the assessment of the approach. Depending on the chosen plant response in relationship with Ψ stem or MDS, the obtained reference values varied between −1.04 and −1.46MPa for Ψ stem and between 0.17 and 0.28mm for MDS. In both cultivars, the approach based on maximum photosynthesis rates resulted in less negative Ψ stem values and smaller MDS values, compared to the approaches with sap flow and daily radial stem growth. In the well-irrigated apple trees, day-to-day variations in midday Ψ stem and MDS were related to the evaporative demand. These variations were more substantial for MDS than for midday Ψ stem. [Copyright &y& Elsevier]

The spatial and diurnal variability of sap flow in a mature beech tree (Fagus sylvatica L.) was investigated on days with different climatic conditions (sunny, cloudy, rainy), during the summer. Sap flux density (v) was measured with six heat field deformation probes placed around the stem circumference. Each probe measured v at six sapwood depths. Daily v exhibited clear radial variation, and the shape of the radial profile of v differed substantially among circumferential positions. At some positions, daily v decreased monotonically towards the stem centre, whereas at others it showed an almost monotonic increase. Hence, the conducting sapwood area of the beech was highly asymmetrical. At all positions, conducting sapwood reached beyond the deepest sampled sapwood depth, precluding correct estimations of total sap flow. The radial profile of v also differed among measuring days. A general trend was that the inner sapwood contributed relatively more to total sap flow under better weather conditions. Besides variations among days, the shape of the radial profile of v also showed within-day variations. The contribution of the inner sapwood to total sap flow increased in the afternoon, with increasing vapour pressure deficit and photosynthetic active radiation. Because of large circumferential and temporal variability, no general function for the radial profile of v could be developed. [ABSTRACT FROM AUTHOR]

Abstract: Validation of ecological models is not a straightforward task and much controversy exists in the published literature on the exact definition. A lot of approaches have been suggested, without one being clearly superior. In this paper, we use the earlier published dynamic ‘RCGro’ tree water flow and storage model as an illustrative example to address some model validation issues. We show that the model fails validation when confronted with a data set not used during model building and calibration. We apply and discuss therefore several validation criteria, including visual comparison between model output and measured data using both time series and scatter plots, calculation of the slope, the intercept and the R 2 statistic of the regression line, computation of the model efficiency, and visual analysis of the residual errors. We then investigate Rykiel''s [Rykiel Jr., E.J., 1996. Testing ecological models: the meaning of validation. Ecol. Model. 90, 229–244] suggestion to re-calibrate the model in order to deal with validation failures. Using three different data sets representing spring, summer and late summer growth of beech (Fagus sylvatica L.) and oak (Quercus robur L.), we show that model re-calibration is necessary to successfully pass both the visual and the quantitative validation tests. Recalibration caused the values of some of the hydraulic parameters to change (C(stem) and C(crown)) as well as parameters related to radial stem growth (β and ϕ). From these changes, information regarding the overall seasonal behaviour could be derived (e.g., cessation of growth). Additionally, it was shown that general conclusions about the differences between beech and oak (e.g., a higher hydraulic capacitance and a lower hydraulic xylem resistance for oak compared to beech) remained valid across the seasons as well. We finally demonstrate that the recalibrated model parameter values are not transferable across the growing season and underscore the need for caution when applying a model or application developed for one temporal or spatial situation to another. [Copyright &y& Elsevier]

Because of the increasing worldwide shortage of freshwater and costs of irrigation, a new plant-based irrigation scheduling method is proposed. In this method, two real-time plant-based measurements (sap flow and stem diameter variations) are used in combination with a mathematical water flow and storage model in order to predict the stem water potential. The amount of required irrigation water is derived from a time integration of the sap flow profile, while the timing of the irrigation is controlled based on a reference value for the predicted stem water potential. This reference value is derived from the relationship between midday values of maximum photosynthesis rates and stem water potential. Since modelling is an important part of the proposed methodology, a thorough mathematical analysis (identifiability analysis) of the model was performed. This analysis showed that an initial (offline) model calibration was needed based on measurements of sap flow, stem diameter variation and stem water potential. Regarding irrigation scheduling, however, only sap flow and stem diameter variation measurements are needed for online simulation and daily model calibration. Model calibration is performed using a moving window of 4 days of past data of stem diameter variations. The research tool STACI (Software Tool for Automatic Control of Irrigation) was used to optimally combine the continuous measurements, the mathematical modelling and the real-time irrigation scheduling. The new methodology was successfully tested in a pilot-scale setup with young potted apple trees ( Malus domestica Borkh) and its performance was critically evaluated. [ABSTRACT FROM AUTHOR]

A young potted oak (Quercus robur L.) tree was subjected to drought by interrupting the water supply for 9 days. The tree was placed in a growth chamber in which daily patterns of temperature and radiation were constant. The effects of drought on the water and carbon status of the stem were examined by measuring stem sap flow rate, stem water potential, stem diameter variations, stem CO2 efflux rate (FCO2 ) and xylem CO2 concentration ([CO2* ]). Before and after the drought treatment, diurnal fluctuations in FCO2 and [CO2*] corresponded well with variations in stem temperature (Tst). Daytime depressions in FCO2 did not occur. During the drought treatment, FCO2 still responded to stepwise changes in temperature, but diurnal fluctuations in FCO2 were no longer correlated with diurnal fluctuations in Tst. From the moment daily growth rate of the stem became zero, diurnal fluctuations in FCO2 became closely correlated with diameter variations, exhibiting clear daytime depressions. The depressions in FCO2 were likely the result of a reduction in metabolic activity caused by the lowered daytime stem water status. Xylem [CO2*] showed clear daytime depressions in response to drought. When the tree was re-watered, FCO2 and [CO2*] exhibited sharp increases, coinciding with an increase in stem diameter. After resumption of the water supply, daytime depressions in FCO2 and [CO2*] disappeared and diurnal fluctuations in FCO2 and [CO2*] corresponded again with variations in Tst. [ABSTRACT FROM AUTHOR]

A greenhouse experiment was performed in order to investigate the effects of different levels of water stress on leaf water potential (Ψw), stomatal resistance (rs), protein content and chlorophyll (Chl) content of tomato plants ( Lycopersicon esculentum Mill. cv. Nikita). Water stress was induced by adding polyethylene glycol (PEG 6 000) to the nutrient solution to reduce the osmotic potential (Ψs). We investigated the behavior of anti-oxidant enzymes, such as catalase (CAT) and superoxide dismutase (SOD), during the development of water stress. Moderate and severe water stress (i.e. Ψs= -0.51 and -1.22 MPa, respectively) caused a decrease in Ψw for all treated (water-stressed) plants compared with control plants, with the reduction being more pronounced for severely stressed plants. In addition, rs was significantly affected by the induced water stress and a decrease in leaf soluble proteins and Chl content was observed. Whereas CAT activity remained constant, SOD activity was increased in water-stressed plants compared with unstressed plants. These results indicate the possible role of SOD as an anti-oxidant protector system for plants under water stress conditions. Moreover, it suggests the possibility of using this enzyme as an additional screening criterion for detecting water stress in plants. (Managing editor: Ping He) [ABSTRACT FROM AUTHOR]

Polyethylene glycol (PEG 6000)-induced water deficit causes physiological as well as biochemical changes in plants. The present study reports on the results of such changes in hydroponically grown tomato plants ( Ly coper sicon esculentum Mill. cv. Nikita). Plants were subjected to moderate and severe levels of water stress (i.e. water potentials in the nutrient solution of-0.51 and-1.22 MPa, respectively). Water stress markedly affected the parameters of gas exchange. Net photosynthetic rate (Pn) decreased with the induction of water stress. Accordingly, a decrease in the transpiration rate (E) was observed. The ratio of both (Pn/E) resulted in a decrease in water use efficiency. One of the possible reasons for the reduction in Pn is structural damage to the thylakoids, which affects the photosynthetic transport of electrons. This was indicated by an increase in non-photochemical quenching and a reduction in the quantum yield of photosystem II. Furthermore, a decrease in both leaf water potential and leaf osmotic potential was observed, which resulted in a significant osmotic adjustment during stress conditions. Analysis of the physiological responses was complemented with a study on changes in proline content. In stressed plants, a 10-fold increase in proline content was detected compared with control plants. It is clear that water stress tolerance is the result of a cumulative action of various physiological and biochemical processes, all of which were affected by PEG 6000-induced water stress. (Managing editor: Ping HE) [ABSTRACT FROM AUTHOR]

Abstract: Stem diameter variations are not solely driven by the water status of a tree, but also by the carbon status in the phloem. We mechanically changed the carbon status in young oak trees (Quercus robur L.) by girdling the stem in order to investigate its effect on stem diameter variations. The stem was girdled at two heights by removing the bark which contains the carbon conducting phloem. The upper stem zone (U) still received new assimilates from the leaves, while the lowest stem zone (L) received only stored carbon from the roots. The middle stem zone (M) was completely isolated from crown and roots. As downward carbon transport was interrupted after girdling, the stem expansion and the roots. We argue that expedited growth in U was driven by both irreversible radial growth and reversible swelling. In contrast to U, stem expansion and carbohydrate content decreased in the two lower stem zones (M and L). Time lags were observed between the moments of morning shrinkage when comparing the three stem zones (U, M and L). Since these time lags correlated with the soluble carbohydrate content, it seems that daily dynamics in stem diameter variations were influenced by changes in carbon status. Furthermore, a decrease in xylem sap flow rate was observed, which could be attributed to an indirect effect of girdling. This decrease seems to be provoked by a feedback inhibition of net photosynthesis rate via stomatal closure. Such feedback inhibitions are often described in literature as a consequence of the reduced sink strength after girdling. This study, hence, demonstrates that a disturbance in tree carbon status not only changes carbon-related processes (i.e. radial stem growth, photosynthesis, carbon storage), but also water-related processes (i.e. stem swelling and shrinkage, transpiration and sap flow). [Copyright &y& Elsevier]

Mesic–dry tundras are widespread in the Arctic but detailed assessments of net primary production (NPP) and ecosystem carbon (C) stocks are lacking. We addressed this lack of knowledge by determining the seasonal dynamics of aboveground vascular NPP, annual NPP, and whole-ecosystem C stocks in five mesic–dry tundras in Northern Sweden with contrasting microtopography, altitude, and dominant species. Those measurements were paralleled by the stock assessments of nitrogen (N), the limiting nutrient. The vascular production was determined by harvest or in situ growing units, whereas the nonvascular production was obtained from average species growth rates, previously assessed at the sites. Results showed that aboveground vascular NPP (15–270 g m−2), annual NPP (214–282 g m−2 or 102–137 g C m−2) and vegetation biomass (330–2450 g m−2) varied greatly among communities. Vegetation dominated by Empetrum hermaphroditum is more productive than Cassiope tetragona vegetation. Although the large majority of the apical NPP occurred in early-mid season (85%), production of stems and evergreen leaves proceeded until about 2 weeks before senescence. Most of the vascular vegetation was belowground (80%), whereas most of the vegetation production occurred aboveground (85%). Ecosystem C and N stocks were 2100–8200 g C m−2 and 80–330 g N m−2, respectively, stored mainly in the soil turf and in the fine organic soil. Such stocks are comparable to the C and N stocks of moister tundra types, such as tussock tundra. [ABSTRACT FROM AUTHOR]

Assimilation and exchange of carbon for arctic ecosystems depend strongly on leaf area index (LAI) and total foliar nitrogen (TFN). For dry–mesic tundras, the seasonality of these characteristics is unexplored. We addressed this knowledge gap by measuring variations of LAI and TFN at five contrasting subarctic heaths during the growing season 2007, from about 2 weeks after bud burst until about 2 weeks before senescence. The communities generally showed an early season LAI and TFN increase, owing to leaf development of deciduous shrubs, and limited variations later on, owing to concurrent leaf growth and mortality of evergreen shrubs. For most of the communities, the TFN:LAI ratio was constant along the entire sampling period, indicating that leaf growth and mortality, triggers for both LAI and TFN, are more important than leaf N retranslocation, which only affects TFN. A constant TFN:LAI ratio facilitates the determination of TFN from LAI, which is easier to measure or simulate. However, a sheltered community showed an increment in the LAI of the deciduous canopy in mid–late season, paralleled by a decreased TFN:LAI ratio. [ABSTRACT FROM AUTHOR]

Abstract: Reducing energy use in greenhouses contributes to the profitability of horticulture. Important energy savings can be realized through the use of temperature integration. However, such a greenhouse heating strategy is only acceptable for commercial purposes if there are no adverse effects on plant growth and quality. During this 3-month study, Hedera helix ‘Green Ripple’ and H. helix ‘Shamrock’ were subjected to a day/night temperature regime of 20/20°C (control) and two treatments with temperature integration over 24h and 4d, respectively, based on a DIF of 13.5°C, maintaining the average temperature at the same level of the control. Temperature treatments resulted in a promotion of stem elongation. After 3 months, shoot length rose up to maximum 37.3% when temperature integration was applied. However, temperature integration reduced total dry weight and particularly root dry weight was negatively affected. In addition, assessments of relative growth rate, shoot extension rate, specific leaf area, total leaf area and pigment concentrations were performed and it was concluded that dynamic temperature regimes with longer integration periods support commercial production of English ivy. [Copyright &y& Elsevier]

Abstract: In this paper gross primary production (GPP) predicted by FORUG is compared with GPP calculated from eddy covariance measurements for a beech forest in France (Hesse). Two photosynthesis formulations at leaf level are compared: the biochemically supported approach described by Farquhar et al. [Farquhar, G.D., Von Caemmerer, S., Berry, J.A., 1980. A biochemical model of photosynthetic CO2 assimilation in leaves of C3 species. Planta 12B, 549–587] (BCF) and an empirical light response curve (LRC). Five consecutive years (1996–2000) of measured GPP are compared with FORUG model predictions. Results did not discriminate between both model formulations, but good agreement between modelled and measured GPP support the reliability of FORUG for both photosynthesis approaches. Although some discrepancies appeared, the parameterization combining literature and fitted parameters can be considered as a useful strategy. Residuals were analysed to find explanations for discrepancies between model predictions and data. The increase in residuals over the years, indicate that interannual variability of GPP is not only determined by direct climatic effects. Due to interfering long-term effects, a combination of several climatic factors (drought, temperature), acclimation, environmental and management impacts account for the interannual variation in GPP. However, the long-term effect of drought appeared to be the most important driver of the interannual variability in GPP. Taking into account these long-term climate effects will be an essential step in development of better performing ecosystem models. [Copyright &y& Elsevier]

Oxidative respiration is strongly temperature driven. However, in woody stems, efflux of CO2 to the atmosphere ( E A), commonly used to estimate the rate of respiration ( R S), and stem temperature ( T st) have often been poorly correlated, which we hypothesized was due to transport of respired CO2 in xylem sap, especially under high rates of sap flow ( f s). To test this, we measured E A, T st, f s and xylem sap CO2 concentrations ([CO2*]) in 3-year-old Populus deltoides trees under different weather conditions (sunny and rainy days) in autumn. We also calculated R S by mass balance as the sum of both outward and internal CO2 fluxes and hypothesized that R S would correlate better with T st than E A. We found that E A sometimes correlated well with T st, but not on sunny mornings and afternoons or on rainy days. When the temperature effect on E A was accounted for, a clear positive relationship between E A and xylem [CO2*] was found. [CO2*] varied diurnally and increased substantially at night and during periods of rain. Changes in [CO2*] were related to changes in f s but not T st. We conclude that changes in both respiration and internal CO2 transport altered E A. The dominant component flux of R S was E A. However, on a 24-h basis, the internal transport flux represented 9–18% and 3–7% of R S on sunny and rainy days, respectively, indicating that the contribution of stem respiration to forest C balance may be larger than previously estimated based on E A measurements. Unexpectedly, the relationship between R S and T st was sometimes weak in two of the three trees. We conclude that in addition to temperature, other factors such as water deficits or substrate availability exert control on the rate of stem respiration so that simple temperature functions are not sufficient to predict stem respiration. [ABSTRACT FROM AUTHOR]

We estimated daily use of stored water by Scots pine (Pinus sylvestris L.) trees growing in a temperate climate with the ANAFORE model (ANAlysis of FORest Ecosystems) and compared the simulation results with sap flow measurements. The original model was expanded with a dynamic water flow and storage model that simulates sap flow dynamics in an individual tree. ANAFORE was able to accurately simulate diurnal patterns of measured sap flow under microclimatic conditions that differ from those of the calibration period. Strong relationships were found between stored water use and several tree characteristics (diameter at breast height, sapwood area, leaf area), but not with tree height. Relative to transpiration, stored water use varied over time (between less than 1% and 44% of daily transpiration). On days when transpiration was high, trees were more dependent on stored water, indicating that the contribution of internal water to transpiration is not a constant in the water budget of trees. [ABSTRACT FROM AUTHOR]

Soil CO2 efflux (SR) is the second largest carbon flux on earth. We investigated the driving factors of the seasonal change and short-distance spatial variation in SR in a short-rotation plantation of willow (Salix viminalis Orm). Total annual SR ranged from 723 to 1149 g Cm-2 year-1. Both an exponential and a logistic model were fitted to the data, with soil temperature at a depth of 5 cm as the independent variable. The R² values for individual sampling points ranged from 0.83 to 0.95 and from 0.85 to 0.93 for the exponential and logistic models, respectively, indicating that soil temperature largely determined the seasonal variation in SR. Modeled soil SR at 10°C ranged from 1.22 to 1.95 µmol m-2 s-1, whereas modeled annual Q10 values were between 3.31 and 6.13. These high Q10 values were attributed to the absence of drought during the study in 2005. When the coefficients of the general SR models were replaced by linear dependencies on soil and vegetation- related characteristics, the resulting spatially explicit exponential and logistic SR models explained 85 and 86%, respectively, of the variability within the dataset. The analysis indicated that soil carbon concentration, leaf area index, soil pH and root biomass caused differences in SR at the short distances considered in this study. However, incorporating information on variables considered to account for spatial variability in the model did not result in a higher R² compared with a simple temperature function. When the general SR models were applied to independent datasets from the same plantation, the logistic model provided a better fit than the exponential model when drought occurred. Drought greatly reduced the annual Q10 values of SR. [ABSTRACT FROM AUTHOR]

Abstract: Belgium, being an EU country, has committed itself to a 7.5% reduction of greenhouse gas emissions during the first commitment period of the Kyoto Protocol. Within this framework, the Flemish government aims at reaching a share of 6% of renewable electricity in the total electricity production by 2010. In this work, the biomass production of birch, maple, poplar and willow in a short-rotation forestry (SRF) plantation after a 4-year growth period served as the base to calculate the amount of (electrical) energy that could be produced by this type of bioenergy crop in Flanders. The maximum amount of electricity that could be provided by SRF biomass was estimated at 72.9GWhe year−1, which only accounts for 0.16% of the total electricity production in this region. Although the energy output was rather low, the bioenergy production process under consideration appeared to be more energy efficient than energy production processes based on fossil fuels. The high efficiency of birch compared to the other species was mainly due to the high calorific value of the birch wood. The maximum CO2 emission reduction potential of SRF plantations in Flanders was estimated at only 0.09% of the total annual CO2 emission. The most interesting application of SRF in Flanders seemed to be the establishment of small-scale plantations, linked to a local combined heat and power plant. These plantations could be established on marginal arable soils or on polluted sites, and they could be of importance in the densely populated area of Flanders because of other environmental benefits, among which their function as (temporary) habitat for many species. [Copyright &y& Elsevier]

Abstract: During the last three decades, oil crises, agricultural surpluses and global climate change enhanced the interest in short-rotation forestry (SRF). In this study, the biomass production of birch (Betula pendula Roth), maple (Acer pseudoplatanus L.—Tintigny), poplar (Populus trichocarpa ×deltoides—Hoogvorst) and willow (Salix viminalis—Orm) growing under a short-rotation (SR) management system were compared after a 4 years period. The plantation was established on former agricultural land. The sandy soil had a mean pH of 4.5 and a mean carbon content of 1.0%. Survival rates after 4 years were 75.8%, 96.8%, 86.3% and 97.6% for birch, maple, poplar and willow, respectively. The mean actual annual biomass production for these four species amounted to 2.6, 1.2, 3.5 and 3.4tDMha−1 yr−1, respectively. The large variation in biomass production at the different plots of the plantation could not be explained by the measured soil parameters. Biomass production results found here were in the lower range of values reported in literature. However, in contrast to most other studies, no weed control, fertilisation or irrigation was applied in this experiment. As marginal agricultural soils are suboptimal for the growth of poplar and willow, birch can be considered as a very interesting alternative for the establishment of SR plantations in Flanders. [Copyright &y& Elsevier]

Quantitative analysis of wood anatomical characteristics is usually performed using classical microtomy yielding optical micrographs of stained thin sections. It is time-consuming to obtain high quality cross-sections from microtomy, and sections can be damaged. This approach, therefore, is often impractical for those who need quick acquisition of quantitative data on vessel characteristics in wood. This paper reports results of a novel approach using X-ray computed microtomography (microCT) for non-invasive determination of wood anatomy. As a case study, stem wood samples of a 2-year-old beech (Fagus sylvatica L.) and a 3-year-old oak (Quercus robur L.) tree were investigated with this technique, beech being a diffuse-porous and oak a ring-porous tree species. MicroCT allowed non-invasive mapping of 2-D transverse cross-sections of both wood samples with micrometer resolution. Self-developed software `μCTanalysis'' was used for image processing of the 2-D cross-sections in order to automatically determine the inner vessel diameters, the transverse cross-sectional surface area of the vessels, the vessel density and the porosity with computer assistance. Performance of this new software was compared with manual analysis of the same micrographs. The automatically obtained results showed no significant statistical differences compared to the manual measurements. Visual inspection of the microCT slices revealed very good correspondence with the optical micrographs. Statistical analysis confirmed this observation in a more quantitative way, and it was, therefore, argued that anatomical analysis of optical micrographs can be readily substituted by automated use of microCT, and this without loss of accuracy. Furthermore, as an additional application of microCT, the 3-D renderings of the internal microstructure of the xylem vessels for both the beech and the oak sample could be reconstructed, clearly showing the complex nature of vessel networks. It can be concluded that the use of microCT in wood science offers an interesting potential for all those who need quantitative data of wood anatomical characteristics in either the 2-D or the 3-D space. [Copyright &y& Elsevier]

Abstract Evaluations of soil organic carbon (SOC) stocks are often based on assigning a carbon density to each one of a number of ecosystems or soil classes considered, using data from soil profiles within these categories. A better approach, in which the use of classification methods by which extrapolation of SOC data to larger areas is avoided, can only be used if enough data are available at a sufficiently small scale. Over 190 000 SOC measurements (0–24 cm) have been made in the Flemish cropland (the Northern part of Belgium) in the 1989–2000 period. These SOC data were grouped into 3-year periods and as means plus standard deviation per (part of) community (polygons). This large dataset was used to calculate SOC stocks and their evolution with time, without data extrapolation. Using a detailed soil map, larger spatial groups of polygons were created based on soil texture and spatial location. Linear regression analysis showed that in the entire study area, SOC stocks had decreased or at best had remained stable. In total, a yearly decrease of 354 kton OC yr-1 was calculated, which corresponds with a net CO2 emission of 1238 kton CO2 yr-1 . Specific regions with a high carbon sequestration potential were identified, based on SOC losses during the 1989–2000 period and the mean 1999 SOC content, compared to the average SOC content of soils in Flanders with a similar soil texture. When restoring the SOC stocks to their 1990 level, we estimated the carbon sequestration potential of the Flemish cropland soils to be some 300 kton CO2 yr-1 at best, which corresponds to a 40-year restoration period. In conclusion, we can say that in regions where agricultural production is very intense, carbon sequestration in the cropland may make only a very modest contribution to a country's effort to reduce greenhouse gas emissions. [ABSTRACT FROM AUTHOR]