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Potential of fast-growing poplar for bioenergy

Bioenergy is set to become the most important single source of renewable energy for the EU. Poplar trees grown in commercial short-rotation coppice (SRC) plantations will play a major role in bioenergy production.

Climate Change and Environment
Energy

The success of highly productive SRC plantations strongly depends on soil water availability and the sensitivity of the trees to tropospheric ozone (O3) production. The EU-funded PHYSIO-POP project therefore investigated the physiological and environmental controls of water and O3 fluxes in different SRC poplar genotypes at the leaf, tree and ecosystem level over a daily and seasonal time scale. Researchers sought to gain a better understanding of the physiological adaptations imposed by climate change on different poplar genotypes in SRC bioenergy plantations. ‘The sensitivity of poplar to water shortage and high O3 concentrations limits the future development of its cultivation in SRC bio-energy plantations,’ says project coordinator Professor Reinhart Ceulemans. ‘This makes the study of the physiological and environmental controls of water loss and O3 uptake, and in particular their stomatal control, particularly timely.’ Researchers collected data on water fluxes and their fluctuations for an entire growing season for an existing SRC plantation in Flanders, Belgium. Gas exchange and water relation measurements were used to quantify the transpirational water loss at leaf level for the poplar genotypes. Transpirational water loss at the tree level was quantified using continuously recorded sap flow measurements. The transpirational water loss at the ecosystem level was obtained by continuously recording eddy covariance fluxes. Greater water efficiency All measurements were conducted on four poplar genotypes specifically selected to cover a wide genetic background and were from within the footprint of the flux measurements. The poplar genotypes Bakan and Koster proved best suited for SRC plantations in low water input systems with little or no irrigation, like temperate Mediterranean or warm Oceanic/Continental climates. In regions like Flanders, where water availability is not a concern, the genotype Grimminge can help drain flooded lands due to its high transpiration rates. This also suggests that the studied genotypes might better tolerate environmental changes, such as drought or flooding, linked to climate change. One of the future selection criteria for SRC poplar genotypes should be high water-use efficiency, according to Dr Alejandra Navarro, the researcher on the PHYSIO-POP project. ‘This means high biomass production per unit of water lost; or under conditions of water stress, a limited loss of produced biomass combined with a minimum water loss via transpiration.’ The physiological control of transpiration could mitigate the increasing effects of climate change in these SRC cultivations. ‘The project showed that a poplar SRC plantation study consumed less water than reference grassland and other open-field crops. Consequently this type of cultivation is a feasible option to produce biomass for energy without an excessive water consumption,’ says Dr Navarro. Genotype matched to environmental conditions Identification of poplar hybrids with different physiological traits, water use efficiencies and transpiration responses will benefit future research. It will also contribute to the cultivation of poplar SRC on lands where soil water availability is subjected to seasonal changes with shortage and excess of water supply. Bakan and Koster showed conservative water behaviour and low transpiration rates; they might be best suited for SRC plantations in low water input systems with little or no irrigation. Instead, in regions such as frequently flooded marginal lands where available water is not a concern, Grimminge might achieve an effective drainage of the flooded lands due to its water spending behaviour (high transpiration rates). Stakeholders benefitting from PHYSIO-POP include farmers and bio-energy industries. ‘They will get an indication of the water consumption of the different poplar genotypes as well as an entire plantation,’ states Dr Navarro. To disseminate project results and technologies and transfer this knowledge to non-specialists, PHYSIO-POP has organised guided field visits to the plantations. The project also produced a short video about the sensors used in the field experiment.

Keywords

PHYSIO-POP, poplar, bioenergy, O3, SRC plantations, climate change, transpirational water loss

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