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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]

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]

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]

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]