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Physiological and environmental controls of water and ozone fluxes in a short rotation poplar plantation: from leaf to tree to ecosystem scale.

Periodic Reporting for period 1 - PHYSIO-POP (Physiological and environmental controls of water and ozone fluxes in a short rotation poplar plantation: from leaf to tree to ecosystem scale.)

Reporting period: 2015-09-01 to 2017-08-31

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.
Objective 1.
Determination of stomatal conductance, stem and leaf water potential, relative water content, leaf transpiration and hydraulic conductance for quantifying the transpiration uptake at leaf level, and the physiological behaviour of the four selected genotypes.
Processing and statistical analysis of data directly obtained during the field campaign (leaf level measurements) and the stem diameter inventory. A water behavior classification was made of the four poplar genotypes based on their transpiration at leaf level and their physiology.

Objective 2.
Field work campaign in 2016 and 2017 for obtaining the stem diameter inventory for all the poplar SRC genotypes studied. These data were used in the allometric relationships between diameter and dry mass.
Acquisition, processing and statistical analysis of data obtained during the continuous acquisition of tree level measurements (sap flow rates and trunk diameter fluctuations) from both sensor sets for quantifying transpiration and ozone uptake at tree level.
We identified significant differences in transpiration among genotypes Bakan and Koster (avg. = 1.6 and 1.8 mm/day and max. = 5.2 and 5.6 mm/day) versus genotype Grimminge (avg. = 3.1 mm/day and max. = 8.2 mm/day).

Objective 3.
Continuous data acquisition and continuous monitoring of water fluxes as well as environmental variables for quantifying transpiration and ozone fluxes at ecosystem level. The total cumulative transpiration per unit of ground area – estimated using the sapwood area-based approach – was 441 mm during the 2016 growing season. The total canopy evapotranspiration was 328 mm. These values indicate that the poplar SRC plantation of our study was not water limited in 2016 and consumed less water than a reference grassland (624 mm).

Our results are highly relevant and provide high-quality information, since: (i) this is one of the most detailed studies on transpiration in an SRC plantation; (ii) data were collected at all relevant scales; (iii) a wide array of transpiration related parameters were measured and different sensors were used; and (iv) we performed measurements during one entire growing season.

The dissemination activities of the project focused on presentations at scientific conferences and symposia, as well on publications in international peer-reviewed scientific journals are included in the separate section Publications.
The activities of PHYSIO-POP followed the planning proposed in the original proposal. Thanks to the support of the technicians and the scientific personnel of the host institution all instruments were installed on time without major problems. There were no changes in the legal status of any of the beneficiaries, and no major deviations from the planned milestones and deliverables. Ozone data were collected and recorded as foreseen, but the analysis and interpretation are still in progress.

No gender or ethical issues arose and no subcontracting was necessary to reach the project goals.

Socio-economic impact and the wider societal implications of the project: The social media were used to connect and share information about PHYSIO-POP, especially of the sensors used in the field experiment, via the publication of the video “Installation of SHB sap-flow sensors in a multi-genotype poplar SRC” on YouTube ( and on the website of the University of Antwerp (

Results and technologies of the project have been also transferred to non-specialists in the topic during guided field visits to the plantation.
Sensors to monitor sap flow and to monitor stem diameter installed on poplar trees
Meteorological tower to monitor fluxes of gases between the plantation and the atmosphere
Measurements of the water potential of leaves of poplar
Measurements of stomatal conductance of individual poplar leaves in the field
Measurements of the water potential of leaves of poplar
Measurements of stomatal conductance of individual poplar leaves in the field
Sensors to monitor sap flow and to monitor stem diameter installed on poplar trees
Dendrometer sensors to monitor stem diameter on individual poplar trees