Some foliar characteristics of ash trees (Fraxinus excelsior) exposed to ozone episodes.

Young ash trees (Fraxinus excelsior L.) growing in the field were exposed to episodes of 150 nl 1 ozone, or to clean air, in open-top chambers at the University of Nottingham, Sutton Bonington Campus, UK, May-October, 1992-1994. The episodes were for 8 h daily and for 1-4 d in succession, with seasonal totals of 24-27 d. A nondestructive method for estimating whole-tree leaf area was developed in 1992 and used to monitor leaf area, leaf number and leaf size in 1993 and 1994. Stomatal density and leaf abscission were also studied. No effects of exposure to ozone on leaf area and leaf number were detected. In 1994 there were consistent trends towards greater mean leaf size and higher stomatal density in trees exposed to ozone, but these effects were not significant at the 95% confidence level. Leaf abscission was slower in trees exposed to ozone in 1992 and 1994, but not in 1993 when there were fewer exposure days late in the season.