Open top chamber climate impact studies - results from the Headley II & III experiments

Results relevant to elevated CO2 research from the Headley II and III experiments were summarised as part of the Forest Research contribution to the EU Fourth Framework ECOCRAFT project (ECOCRAFT, 1999), co-ordinated by Paul Jarvis at the University of Edinburgh. This summary (PDF-113K) is available here, with further analysis given in Chapter 9 of Bulletin 125. In addition, meta-analyses of the project-wide data-set have also been published, identifying the effects of elevated CO2 on photosynthesis and stomatal conductance (Medlyn et al., 1999; 2000). In addition, the ozone element of the research has contributed to the setting of new critical loads for ozone for forest trees, with a brief summary of the approach given below:

Updating critical loads for ozone

Data from this experiment together with previous experiments conducted in the Headley OTC facility have been used to develop dose-response relationships for ozone using biomass accumulation of young trees as the measure of effect (Karlsson et al., 2004). These relationships have been used to set new critical loads for ozone which have recently been published in the UNECE Modelling and Mapping Manual. Both accumulated external exposure or concentration (AOTx) and flux-based approaches have been developed. Although each experiment involved only one elevated ozone treatment, dose response relationships were developed by extrapolating the data from the ambient and elevated ozone treatments to zero ozone exposure, providing estimated growth reduction data for the ambient ozone treatment. Leaf or needle ozone uptake (in the case of the flux-based approach) was estimated using the multiplicative stomatal conductance simulation model (Emberson et al., 2000) with the default EMEP parameterisations (UNECE, 2004) except for length of growing season and maximum stomatal conductance, where parameterisations specific to Headley were available.

These data are part of a wider data-set used in the setting of the new concentration-based Critical Load for forest trees of 5 ppm h (accumulated over one growing season). A provisional flux-based Critical Load has also been developed on the basis of this analysis. As a result of uncertainty in both the parameterisation of the stomatal conductance model and the dose response relationships, it is recommended that the approach is not used for integrated of Europe wide assessments, but its use is restricted to national evaluations, and as a starting point for further development and improvement of the approach. The benefit of using the flux-based approach when these uncertainties are reduced is that it takes into account the limitation of ozone exposure resulting from soil moisture deficit and other environmental factors. It is also physiologically more realistic, and accounts for the effects of ozone pollution that are evident at lower values of exposure typical of northern Europe.


Broadmeadow, M.S.J, Jackson, S.B. (2000). Growth responses of Quercus petraea, Fraxinus excelsior and Pinus sylvestris to elevated carbon dioxide, ozone and water supply. New Phytologist 146, 437-451.

Broadmeadow, M., Randle, T. (2002). The impacts of increased CO2 concentrations on tree growth and function. In: Climate Change and UK Forests (ed. M Broadmeadow). Forestry Commission Bulletin 125. Forestry Commission, Edinburgh.

ECOCRAFT (1999). Predicted impacts of rising carbon dioxide and temperature on forests in Europe at stand scale. Final project report (ENV4-CT95-0077).

Emberson, L.D., Ashmore, M.R., Cambridge, H.M., Simpson, D., Tuovinen, J-P. (2000). Modeling stomatal ozone flux across Europe. Environmental Pollution 109, 403–414.

Karlsson, P.E., Udding, J., Braun ,S., Broadmeadow, M., Elviro, S., Gimeno, B.S., Le thiec, D., Oksanen, E., Vandermeiren, K., Wilkinson, M., Emberson, L. (2004). New critical levels for ozone effects on young trees based on AOT40 and simulated cumulative leaf uptake of ozone. Atmospheric Environment (in press).

UNECE (2004). Mapping critical levels in Europe. In: Modelling and Mapping Manual. UNECE, Geneva.