R. Ceulemans’s research while affiliated with University of Antwerp and other places

Poplar (Populus spp.) and willow (Salix spp.) short rotation coppice (SRC) are attractive feedstock for conversion to renewable electricity. Site managers typically optimize biomass production at their sites. However, maximum biomass production does not necessarily equate an optimal CO2 balance, water use and energy production. This is because many operational actions consume water and energy and emit CO2, either on-site or off-site. Coupling a land surface model (ORCHIDEE-SRC) with life cycle assessment enabled us to determine the optimal management for SRC in Belgium. We simulated 120 different management scenarios for each of two well-studied Belgian SRC sites (i.e. Boom and Lochristi). Simulated soil carbon changes suggested substantial carbon losses of 20–30 Mg ha−1 over a time period of 20 years, which were within observation-based uncertainty bounds. Results showed that in Belgium, which has a temperate maritime climate, optimal management of SRC has a rotation cycle of two years without irrigation. Energy inputs for this optimal management were 5.2 GJ ha−1 yr−1 for the Boom site and 5.3 GJ ha−1 yr−1 for the Lochristi site, while the biomass yields at Boom and Lochristi were 9.0 Mg ha−1 yr−1 and 9.4 Mg ha−1 yr−1, respectively. The energy ratio (i.e., ratio of bioelectricity output to cumulative energy input) for this optimal management was 12, on average. Planting density turned out to be unimportant, while rotation length turned out to be most important to obtain the highest energy ratio and still maintain high biomass yield. Scenarios with high energy-input generated more bioenergy outputs, but the energy gains did not compensate for the increased energy inputs. Reductions in energy consumption per unit of bioenergy output should target the agricultural stage since it accounted for the largest energy share in the production chain.

Beside the production of biomass, short rotation coppice (SRC) poplar plantations can also contribute to carbon sequestration in the soil through their below-ground biomass. The present study evaluated the allocation of above and below-ground biomass at the end of the first rotation of four SRC plantations under Mediterranean conditions. The genotypes evaluated are commonly used for biomass plantations, i.e. genotypes ‘AF2’ and ‘I-214’ (Populus × canadensis Mönch), and ‘Monviso’ (P. × generosa Henry × P. nigra L.). No significant differences among genotypes were found with regard to below-ground biomass yield. The root:shoot ratio decreased in line with the growth in shoot basal diameter, with values ranging from 0.15 to 0.26. The accumulation of carbon in the below-ground fraction of the biomass ranged from 0.86 to 0.91 Mg C ha⁻¹ yr⁻¹, whereas the above-ground carbon accumulation ranged from 3.89 to 6.48 Mg C ha⁻¹ yr⁻¹. A general as well as a genotype-specific allometric model allowed to accurately predict the below-ground biomass yield using shoot basal diameter as the predictor variable. Both models provide an important tool to quantify the carbon accumulated in the below-ground fraction of the biomass.

Poplar (Populus spp.) is one of the most commonly cultivated genera in experimental and commercial short-rotation coppice (SRC) plantations. The genus is among the fastest growing in temperate latitudes, but the success of highly productive poplar SRC plantations strongly depends on soil water availability. We examined the transpiration at the leaf and the individual tree levels of four different poplar genotypes under an SRC regime. Measurements were performed for the entire growing season of 2016, during the seventh growth year of the plantation and before the third coppice on four poplar genotypes (three individual multiple-stem trees per genotype) belonging to different species and from a different genetic background. The experimental site was a commercial scale multi-genotype SRC plantation, established in Flanders (Belgium). Measurements at the leaf level were performed on specific days of the growing season that differed in evaporative demand, temperature and incoming radiation. To determine the transpiration at the stem level, we measured single-stem sap flow using the stem heat balance (SHB) method and daily stem diameter variation measurements. The whole-tree transpiration was estimated by summing the sap flow rates from all stems. Measurements at the stem level were continuously monitored during the entire growing season. Sap flow was tightly connected to the phenological stage of the trees, thus onset of spring (leaf area development) and late autumn (leaf fall) were easily identifiable from sap flow measurements, showing differences among the four genotypes. The dynamics of transpiration at the leaf and tree level were driven by photosynthetic photon flux density (PPFD), but the sap flow intensity was controlled by vapour pressure deficit (VPD). The four poplar genotypes showed different water use strategies, based on determination of transpiration and other plant water status indicators.

We compared four approaches to assess phenology in a short-rotation coppice culture with 12 poplar (Populus) genotypes. The four approaches quantified phenology at different spatial scales and with different temporal resolutions: (i) visual observations of bud phenology; (ii) measurements of leaf area index; (iii) webcam images; and (iv) satellite images. For validation purposes we applied the four approaches during two years: the year preceding a coppice event and the year following the coppice event. The delayed spring greenup and the faster canopy development in the year after coppicing (as compared to the year before coppicing) were similarly quantified by the four approaches. The four approaches detected very similar seasonal changes in phe-nology, although they had different spatial scales and a different temporal resolution. The onset of autumn senescence after coppicing remained the same as in the year before coppicing according to the bud set observations, but it started earlier according to the webcam images, and later according to the MODIS images. In comparison to the year before coppicing, the growing season – in terms of leaf area duration – was shorter in the year after coppicing, while the leaf area index was higher.

The contribution of selected sources of uncertainty to the total variance of model simulation results of stem biomass increment – calculated from annual stem biomass predictions – of European beech (Fagus sylvatica L.) was quantified. Sources of uncertainty were defined as the selected variables that influence the total variance of the model results. Simulations were made: (i) for ten regional climate models (RCMs) based on the IPCC scenario A1B and providing an ensemble of climate projections up to 2100; (ii) with two forest model types (FMTYPEs); (iii) for four forest management intensities (MANFORs); and (iv) for three time windows (TIMEWINDs), each spanning 15 years, starting in 2019, in 2049 and in 2079. Both models, the empirical SIBYLA model and the process-based ANAFORE model, were calibrated using experimental tree growth data from four plots in central Slovakia between 1989 and 2003. Three of these plots, representing the four MANFORs, were subject to different prior intensities of thinning while one was left untouched as a control. The FMTYPE explained most of the total variance in the simulation results (39.9%), followed by MANFOR (i.e. thinning intensity; 22.2%) and TIMEWIND (12.0%), while the effect of RCMs on model uncertainty was limited (<1%). Stem biomass increment results obtained from the two FMTYPES were different in absolute terms, but the models agreed well in their relative response to RCM, to MANFOR and to TIMEWIND. The total variance of the predictions was 10 times higher for the process-based model (ANAFORE) than for the empirical model (SIBYLA). These observations are the reason for the large contribution of FMTYPE to the total variance of the simulated stem biomass increment results.

Renewable energy is often generated from biomass, produced in short-rotation coppice (SRC) cultures. These cultures are frequently established on former agricultural land with ample availability of plant nutrients as nitrogen, phosphorous, potassium, calcium and magnesium. Nevertheless, little is known about the annual recycling of these nutrients through the leaves, as well as about the amounts that are removed at harvest. We therefore quantified soil nutrient concentrations, as well as nutrient concentrations and the gross calorific value of the proleptic branches and of the leaves of 12 poplar (Populus) genotypes in the second rotation of an operational SRC (with two-year rotations). For the produced leaf biomass, we also quantified the standing energy stock and the nutrient stock of each genotype. After four years the P, K, Ca and Mg soil concentrations had not significantly changed, while the N concentration at 30-60 cm of soil depth had significantly increased. On average, the standing aboveground woody biomass of the 12 genotypes in 2013 was 13.75 Mg ha-1 and the total leaf biomass was 3.54 Mg ha-1. This resulted in an average standing energy stock in the leaves of 64.8 GJ ha-1. Nutrient concentrations were lower in the proleptic branches as compared to the leaves, but the proleptic branches and leaf nutrient concentrations significantly varied among the genotypes.

The Flemish renewable electricity support system has struggled to address a number of problematic issues in the past. These included excessive profit margins and general malfunctioning of the green certificate market, as well as a lack of qualification of various existing renewable energy technologies. The Flemish government responded to these issues by introducing major reforms in 2013, including “banding” to differentiate the support for various technologies. However, reliable methods for differentiating renewable electricity technologies and calculating support levels have not been sufficiently developed. The main objective of the 2013 reforms was to reduce support costs, but application of German feed-in tariffs on 18 reference technologies has shown that most projects in Flanders continue to receive high levels of support. The 2013 reforms did not succeed in addressing malfunctioning of the green certificate market. On the contrary, the confidence of investors in renewable electricity plants has decreased as the terms of support can be altered retroactively by adjusting remuneration levels and through political interventions. Future adaptations are likely to be made which will further decrease the overall stability and effectiveness of the system.