The 2030 Renewable Energy Targets of the EU foresee to increase the share of renewable energy in the gross final energy consumption to at least 27% by 2030. Bio-energy is the most important single source of energy from renewables. Poplar (Populus spp.) is the most commonly cultivated tree genera in experimental and commercial short-rotation coppice (SRC) plantations for bio-energy production. The success of these highly productive SRC plantations with poplar strongly depends on soil water availability and on the sensitivity of poplar to tropospheric ozone (O3) pollution. Both concerns are strongly related to the high stomatal conductance and considerable water consumption of poplar. One of the consequences of global climatic changes is the altered water availability, increasing the intensity and the frequency of extreme events and representing an important risk for ecosystems.
The sensitivity of poplar to water shortage and to high O3 concentrations limits the future development of its cultivation in SRC bio-energy plantations. This makes the study of the physiological and environmental controls of water loss and of O3 uptake (in particular their stomatal control) particularly timely.
The general objective of PHYSIO-POP was to study the physiological adaptations that climate change is imposing on different poplar genotypes – in SRC bio-energy plantations – via the transpirational water loss and O3 fluxes at all relevant scales (leaf, tree and ecosystem). The project made full use of an existing SRC plantation in Flanders, Belgium. During the entire grant period there has been only one deviation from the original objectives and tasks. This concerns the quantification of ozone (O3) fluxes in poplar. Ozone data (concentrations and fluxes) were collected and recorded during an entire growing season, but the analysis and interpretation are still in progress. In the presentation of the results, we therefore primarily focused on the quantification of the transpirational water loss of the bio-energy plantation at the leaf, tree and ecosystem levels. The general objective has been translated into three specific objectives.
Obj. 1. To quantify the transpirational water loss at leaf level for poplar genotypes using gas exchange and water relation measurements throughout the growing season.
Obj. 2. To quantify the transpirational water loss at tree level by continuously recorded sap flow measurements. These sap flow measurements allow to quantify the daily transpiration rates (Ec) throughout the entire growing season.
Obj. 3. To obtain the transpirational water loss at ecosystem level by continuously recorded eddy covariance flux measurements.
All measurements were performed in a subset of four (commercially available) poplar genotypes specifically selected to cover a wide genetic background and from within the footprint of the flux measurements. With their conservative water behavior poplar genotypes Bakan and Koster are better suited for SRC plantations in low water input systems with little or no irrigation, for example temperate Mediterranean or warm Oceanic/Continental climates. In contrast, in regions where water availability is not a concern (e.g. Flanders, Belgium) genotype Grimminge might achieve an effective drainage of the flooded lands due to its water spending behaviour (high transpiration rates). This also implies that the studied genotypes might better tolerate environmental changes linked to climate change as drought or flooding. The stand water balance analysis showed that there was no negative impact of the SRC plantation on the regional water cycle. The SRC poplar trees were not water restricted at any time during the growing season and consumed less water as compared to a reference grassland.
