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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]

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: 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]

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]

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]