The Forestry Commission open top chamber (OTC) research programme (Headley I) ran from 1986 to 1993. A total of five species were exposed to either ambient air, or air with reduced pollution inputs through the use of charcoal filters at three sites in the UK. Headley in Hampshire (beech, Norway spruce and Scots pine) represented a high ozone exposure, Chatsworth in the English Midlands (Beech, Scots pine, Sitka spruce) historically had a high NOx and SO2 input, whilst Glendevon in the Scottish uplands (Scots pine, Norway spruce, Sitka spruce) was assumed to have a low pollution load.
Fig.1. The effect of ambient air pollution on height growth of beech, Scots pine and Norway spruce at Headley in Hampshire (Durrant et al., 1992).
Initially, beech and Norway spruce grew significantly better in the filtered treatments at Headley (Fig.1) with height growth enhanced by approximately 20% in both species, and biomass production of Norway spruce by up to 49% after 3 years exposure (Durrant et al., 1992). However, by the end of the experiment (when tree numbers were greatly reduced), only the growth of beech was (statistically) significantly reduced by ambient air pollution at Headley. This enhancement of growth in filtered air was accompanied by improved needle retention in Scots pine (Fig. 2).
Fig. 2. The effect of ambient air pollution on needle retention of Scots pine at Headley in Hampshire.
There was no enhancement of growth in filtered air at the other two sites, and most species grew better in ambient air at those sites, possibly indicating a lower N input in the charcoal filtered treatments leading to the development of sub-optimal nitrogen levels. The overall conclusion from this long-running experiment is thus that ambient air pollution did not affect the growth of the production forestry species investigated, except for beech in southern England, a region exposed to a relatively high ozone dose (AOT40=2-8 ppm h; 1986-1993). This experiment was superceded by Headley II, a factorial investigation of the interactions between ozone pollution, elevated atmospheric CO2 and water supply in 1994 (see Broadmeadow and Jackson, 2000), which provides further support for the approach of modelling ozone exposure as a physiologically effective dose or flux (Fig. 3).
Fig. 3. Relationship between foliar chlorophyll content (reflecting physiological integrity) and mean stomatal conductance for ambient (open circles) and elevated ozone treatments (closed circles). The variation in stomatal conductance is the result of varying irrigation and CO 2 concentration treatments (Broadmeadow and Jackson, 2000).
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.
Durrant, D.W.H., Waddell, D.A., Benham, S.E. & Houston, T.J. (1992) Air quality and tree growth: results of the open-top chamber experiments 1991. Research Information Note 221. Forestry Commission, Edinburgh.