Project "Validation of phytotoxic ozone fluxes in needles and leaves as a prerequisite for a realistic, integrated risk assessment for forest ecosystem services in Germany" (valORTree).

Manuela Baumgarten, Bálint Georg Jákli, Ludger Grünhage, Marc Goebel

PL Dr. Manuela Baumgarten

The damaging effect of elevated concentrations of ground-level ozone (O3) on plants has been demonstrated in numerous studies. Increasing exposure to ground-level O3 can therefore be expected to reduce the productive capacity and carbon storage capacity of natural ecosystems and impair their functions and services at global and national levels. Anthropogenic O3, in addition to the "classical" greenhouse gases, not only directly contributes to an increase in radiative forcing in the atmosphere and thus to climate warming, but also indirectly affects the carbon cycle between land surfaces and the atmosphere by limiting plant carbon fixation and storage.
As a robust basis for future O3 risk assessments for forest trees, validation of modeled ozone uptake and subsequent adjustment of model parameterization are prerequisites.
The main goal of our new research project, whose experimental phase started in spring 2019, is to further develop and improve ozone risk assessment and modeling for forest trees starting from future climate scenarios based on the recently established RCP scenarios (representative concentration pathways, IPCC 2013) taking into account the change in tropospheric O3 and CO2.
For the calculation of ozone doses taken up by plants, stomatal ozone fluxes and other gas exchange parameters need to be determined. To assess risks to forest trees, tree biomass and allometric data, N and C allocation, fine root development, and changes in ecto-mycorrhizal diversity, among others, will be studied.
The experiment will be conducted for two growing seasons in the new phytotrons of TUMmesa with 5-8 year old beech and spruce (natural regeneration from the forest site, cultivated in their natural soil monolith). A low mountain region (Spessart) was chosen as the "model" forest region for Germany for the simulations.

Experimental design in phytotrons:

  1. Simulation of an O3 gradient under standard climate / CO2 conditions.
    To obtain ozone dose-response functions adapted to current conditions and to derive realistic  target values for spruce and beech, four different O3 concentration regimes are used (Fig. 1: OG - pre-industrial, ambient, moderately elevated, and high, respectively, in seasonal and diurnal variation).
  2. Simulation of a future climate including the corresponding O3 -/ CO2 regime:
    To assess the impact of a changing climate and emissions scenario in the future, daily cycles are simulated for a "best case" and a "worst case" future scenarios according to RCP 2.6 and RCP8.5, respectively, for the period around 2100. Since a deficit water supply will have a determining role for productivity and species composition in the climate of the future, the future scenarios are also simulated including moderate dry periods. (Fig. 1: CCO, CCO + drought).

Realistic simulation of the corresponding O3 and CO2 regimes will provide insight into combined effects under projected climate change conditions. Thus, future dose-response functions for forest trees can be developed.
Using the drought variants, the future effect of elevated O3 and CO2 concentrations on stomatal regulation and water use efficiency, and thus drought tolerance of trees, can be predicted. The results will be implemented in future growth models.
All simulation data are regionalized with respect to the sample forest site and are applied at an hourly resolution, as it is important to generate scenarios as "realistically" as possible while not generating "non-physiological" conditions for the plants. Thus, the hourly meteorological data correspond to the corresponding O3 concentration values.

The simulation data were generated by the company MeteoSolutions GmbH as characteristic time series for representative years in the future (different RCP scenarios) based on the reference site. For experimental climate impact research, regionally related, scientifically valid climate and emission data for the future are thus available in high temporal resolution for the first time.

The main objectives of our research project are:

  • Assessment of the ozone risk for forest trees in Germany
  • Further development and re-parameterization of ozone flux modeling for forest trees considering increasing O3 as well as future changing O3 and CO2 concentrations according to climate change projections
  • Assessment of forest growth and CO2 sink strength of forests by incorporating the impacts of different future climate and emission scenarios
  • Establishing the basis for timely development of silvicultural adaptation strategies
  • Defining realistic target values for O3 risk assessment for forest trees

For this purpose, the following requirements must be met:

  • Experimental verification of the existing ozone dose-response relationship through empirical, systematically derived measurements in controlled environments
  • Determination of stomatal ozone uptake and validation ozone flux modeling considering O3 and CO2 dose-response relationships of future climate change scenarios based on recently established representative concentration pathways (RCP, IPCC 2013) including changing tropospheric O3 and CO2 concentrations
  • Adjustment of the parameterization of the FO3REST ozone risk model

The project is funded by the German Federal Environmental Agency UBA.

Experiment design:

Simulation data are simulated according to the RCPs for the period around 2100 and regionalized on the basis of long-term measurement series from a measurement station in the Spessart region.

Model forecasts for the development of CO2, O3 and temperature development

Surface Ozone (global average in ppb)

Preparatory work from April 2018:

valORTree experiment during the 2019 growing season in the TUMmesa phytotrons:

Simulation data series (April-September): Variant - future Climate Change, scenario RCP8.5 ("worst case"), 2071-2100
Seasonal variation during the experiment in TUMmesa (sample trees: beech, spruce).


  • Soil moisture (10 HS, Meter Group)
  • Stem circumference dendrometer (Ecomatik)
  • Leaf temperature and transpiration (TransP, Ecomatik)
  • Leaf gas exchange measurements (LI-6400 and LI-6800, Licor)
  • Precision weighing, continuous
  • Mini-Rhizotron (Vienna Scientific) - detection of root growth

Further analyses:

  • Micorrhizal studies
  • Soil and leaf analyses
  • Bonitures (shoots, leaf fall, damage symptoms, etc.)
  • Allometry
Recording of evapotranspiration with precision balances; transpiration sensors on leaves and needles.
Example of a daily variation of the difference between reference body temperature and leaf temperature as a function of external parameters; recorded with an Ecomatik TransP sensor. Shown are mean values of 10 individual sensors ± SE (gray).
Example of transpiration and conductivity of a beech leaf in the diurnal cycle; calculated via an Ecomatik TransP sensor.