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a b s t r a c tWe used the eddy-covariance technique to measure the temporal dynamics and the relationships between leaf area index (LAI) and exchanges of carbon dioxide (CO 2 ), latent heat (LE) and sensible heat (H) in a multi-genotype short-rotation poplar coppice (SRC) located in East-Flanders (Belgium). The study was carried out over four years (2010)(2011)(2012)(2013) corresponding to the first two rotations of the plantation. The net carbon (C) balance during the first two-year rotation was 75.2 (±4.4) g C m −2 in the establishment year 2010 and −95.6 (±5.9) g C m −2 in 2011. After the harvest (second two-year rotation) the coppice was a net source of carbon, 151.0 (±10.5) g C m −2 in 2012, but a sink of −274.6 (±18.8) g C m −2 in 2013. Overall, at the end of the second rotation this SRC, was a net CO 2 sink with a cumulative uptake of −144.0 (±22.8) g C m −2 . The temporal dynamics and the magnitude of the ratio between gross primary production (GPP) and ecosystem respiration (R eco ) were similar to a deciduous forest. The evolution of LAI showed values ranging from 0.96 (±0.4) to 2.0 (±1.2) and from 5.1 (±1.5) to 4.5 (±0.84) during the first and the second rotation, respectively. The GPP (measured close to light saturation) was significantly related to LAI (r 2 of 0.76, p < 0.001). The cumulative evapotranspiration (ET) measured during the first rotation was 241.7 mm and 349.9 mm for 2010 and 2011, respectively, and 464.6 mm and 372.1 mm for 2012 and 2013. The average value of surface conductance (G s ) was 0.35 mol m −2 s −1 and 0.24 mol m −2 s −1 for the foliated and unfoliated periods, respectively. The mean decoupling factors ( ) were 0.35 and 0.23 for the foliated and unfoliated periods, respectively, indicating that ET was primarily controlled by vapor pressure deficit (VPD) and G s (a well-coupled system). The mean Priestley-Taylor coefficient (˛) was 0.77 and 0.53 for the foliated and the unfoliated periods, respectively. Such low values indicate that ET was significantly lower than the equilibrium evaporation and thus also lower than the ET of a hypothetical reference crop. During the whole experiment only two short episodes of drought were identified when in May-June 2011 and June 2013 the evaporative fraction dropped below 0.4. The analysis of G s showed a rather low stomatal control (anisohydric stomatal response) that put the poplar SRC at greater risk during severe drought conditions. All the three mentioned parameters related to ET and GPP (G s , and ) were significantly and positively correlated to LAI (r 2 from 0.14 to 0.2, p < 0.0001), suggesting that LAI was the main biophysical driver controlling the carbon and water balances in this bioenergy production system.

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© iForest -Biogeosciences and Forestry IntroductionEnergy security and greenhouse gas emission reductions are major challenges to meet the global energy demand and to mitigate climate change (IPCC 2014). In the light of these global concerns bioenergy could play a crucial role to achieve the EU's energy and climate targets (European Council 2009). The cultivation of lignocellulosic biomass, mostly poplar and willow in short rotation coppice (SRC) systems, has a high potential for the production of renewable electricity and the generation of "green" heat (Fischer et al. 2010). Despite the long-term experience with the commercial production in northern Europe (Rosenqvist et al. 2000, Langeveld et al. 2012, the implementation of SRC as a renewable energy crop is limited in European agriculture (AEBIOM 2010, Dimitriou et al. 2011, Don et al. 2011.Productivity is determined by light interception and by the efficiency of converting the intercepted radiation into biomass, i.e., the radiation use efficiency (RUE), as reported for both herbaceous (Sinclair & Horie 1989) and woody (tree) species (Cannell et al. 1988, Medlyn 1998. The crop's capacity to intercept radiation is determined by its photosynthetic area, generally assessed via the leaf area index (LAI), which is linearly related to the biomass production in poplar and willow plantations (Larson & Isebrands 1972, Cannell et al. 1988, Taylor et al. 2001a. In SRC systems, faster canopy closure and increased growth rates of sprouts from an established root system result in enhanced productivity of coppiced versus noncoppiced tree stands . Substantial genotypic variation exists in aboveground woody biomass productivity (AGWB) of SRC cultures in poplar (Ceulemans & Deraedt 1999, Dillen et al. 2011, Paris et al. 2011, Benetka et al. 2014). Variation in AGWB has been explained by variation in light interception, in biomass allocation, in leaf physiological factors related to RUE or in a combination of the aforementioned (Cannell et al. 1988, Green et al. 2001, Tharakan et al. 2008, suggesting inconclusive results on the relative importance of productivity determining factors in SRC. The quantification of genetic diversity and of genetic control contributes to future tree improvement and to yield maximization efforts towards sustainable bioenergy cultivation.In the present study, we analyzed the leaf area development, light interception and RUE of 12 poplar genotypes in a high-density SRC culture, before and after the first coppice of an experimental plantation. Assessment of LAI and woody biomass productivity was performed in a single versus a multi-stem culture, to analyze the effect of coppicing. We hypothesized significant genotypic, parentage and provenance variation in LAI, in leaf area duration (LAD), in RUE, in intercepted radiation (Iint) and AGWB. The objectives of this study were: (i) to determine the main differences between the first (R1) and the second rotation (R2) in the above mentioned parameters and in their relationships, i.e., before and after coppice,...

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The productivity of short‐rotation coppice (SRC) plantations with poplar (Populus spp.) strongly depends on soil water availability, which limits the future development of its cultivation, and makes the study of the transpirational water loss particularly timely under the ongoing climate change (more frequent drought and floods). This study assesses the transpiration at different scales (leaf, tree and stand) of four poplar genotypes belonging to different species and from a different genetic background grown under an SRC regime. Measurements were performed for an entire growing season during the third year of the third rotation in a commercial scale multigenotype SRC plantation in Flanders (Belgium). Measurements at leaf level were performed on specific days with a contrasted evaporative demand, temperature and incoming shortwave radiation and included stomatal conductance, stem and leaf water potential. Leaf transpiration and leaf hydraulic conductance were obtained from these measurements. To determine the transpiration at the tree level, single‐stem sap flow using the stem heat balance (SHB) method and daily stem diameter variations were measured during the entire growing season. Sap flow‐based canopy transpiration (E c), seasonal dry biomass yield, and water use efficiency (WUE; g aboveground dry matter/kg water transpired) of the four poplar genotypes were also calculated. The genotypes had contrasting physiological responses to environmental drivers and to soil conditions. Sap flow was tightly linked to the phenological stage of the trees and to the environmental variables (photosynthetically active radiation and vapor pressure deficit). The total E c for the 2016 growing season was of 334, 350, 483 and 618 mm for the four poplar genotypes, Bakan, Koster, Oudenberg and Grimminge, respectively. The differences in physiological traits and in transpiration of the four genotypes resulted in different responses of WUE.