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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]

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]

To date, models for simulating sap flow dynamics in individual trees with a direct link to stem diameter variation include only the diameter fluctuation driven by a change in stem water storage. This paper reports results obtained with a comprehensive flow and storage model using whole-tree leaf transpiration as the only input variable. The model includes radial stem growth based on Lockhart’s equation for irreversible cell expansion. It was demonstrated that including growth is essential to obtaining good simulation results. To model sap flow dynamics, capacitance of storage tissues was assumed either constant (i.e., electrical analogue approach) or variable and dependent on the water content of the respective storage tissue (i.e., hydraulic system approach). These approaches resulted in different shapes for the desorption curve used to calculate the capacitance of storage tissues. Comparison of these methods allowed detection of specific differences in model simulation of sap flow at the stem base (F(stem)) and stem diameter variation (D). Sensitivity analysis was performed to select a limited subset of identifiable parameters driving most of the variability in model predictions of F(stem) and D Both the electrical analogue and the hydraulic system approach for the flow and storage model were successfully calibrated and validated for the case of a young beech tree (Fagus sylvatica L.). Use of an objective model selection criterion revealed that the flow and storage model based on the electrical analogue approach yielded better predictions. [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]

Rates of CO 2 efflux of stems and branches are highly variable among and within trees and across stands. Scaling factors have only partially succeeded in accounting for the observed variations. In this study, the resistance to radial CO 2 diffusion was quantified for tree stems of an eastern cottonwood (Populus deltoides Bartr. ex Marsh.) clone by direct manipulation of the CO 2 concentration ([CO 2 ]) of xylem sap under controlled conditions. Tree-specific linear relationships between rates of stem CO 2 efflux (J O ) and xylem [CO 2 ] were found. The resistance to radial CO 2 diffusion differed 6-fold among the trees and influenced the balance between the amount of CO 2 retained in the xylem v. that which diffused to the atmosphere. Therefore, we hypothesised that variability in the resistance to radial CO 2 diffusion might be an overlooked cause for the inconsistencies and large variations in woody tissue CO 2 efflux. It was found that transition from light to dark conditions caused a rapid increase in J O and xylem [CO 2 ], both in manipulated trees and in an intact tree with no sap manipulation. This resulted in an increased resistance to radial CO 2 diffusion during the dark, at least for trees with smaller daytime resistances. Stem diameter changes measured in the intact tree supported the idea that higher actual respiration rates occurred at night owing to higher metabolism in relation to an improved water status and higher turgor pressure.

The Monte Carlo technique can be used to propagate input variable uncertainty and parameter uncertainty through a model to determine output uncertainty. However, to carry out Monte Carlo simulations, the uncertainty distributions or the probability density functions (PDFs) of the model parameters and input variables must be known. This remains one of the bottlenecks in current uncertainty research in forest carbon flux modeling. Because forest carbon flux models involve many parameters, we questioned whether it is necessary to take into account all parameters in the uncertainty analysis. A sensitivity analysis can determine the parameters contributing most to the overall model output uncertainty. This paper illustrates the usefulness of the Monte Carlo simulation technique for ranking parameters for sensitivity and uncertainty in process-based forest flux models. The uncertainty of the output (net ecosystem exchange, NEE) of the FORUG model was estimated for the Hesse beech forest (1997). Based on the arbitrary uncertainty of ten key parameters, a standard deviation of 0.88 Mg C ha(-1) year(-1) NEE was found which is equal to 24% of the mean value of NEE. Sensitivity analysis showed that the overall output uncertainty of the FORUG model can largely be determined by accounting for the uncertainty of only a few key parameters. The results led to the identification of the key FORUG parameters and to the recommendation for a process-based description of the soil respiration process in the FORUG model.