SOME FOLIAR CHARACTERISTICS OF ASH TREES (FRAXINUS-EXCELSIOR) EXPOSEDTO OZONE EPISODES

Young ash trees (Fraxinus excelsior L.) growing in the field were expo sed to episodes of 150 nl l(-1) ozone, or to clean air, in open-top ch ambers at the University of Nottingham, Sutton Bonington Campus, UK, M ay-October, 1992-1994. The episodes were for 8 h daily and for 1-4 d i n 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. Stomat al density and leaf abscission were also studied. No effects of exposu re to ozone on leaf area and leaf number mere detected. In 1994 there were consistent trends towards greater mean leaf size and higher stoma tal density in trees exposed to ozone, but these effects were not sign ificant at the 95 % confidence level. Leaf abscission was slower in tr ees exposed to ozone in 1992 and 1994, but not in 1993 when there were fewer exposure days late in the season.