SEASONAL-CHANGES IN WATER-USE OF ASH TREES EXPOSED TO OZONE EPISODES

Young ash trees (Fraxinus excelsior L.) growing in the field were expo sed to episodes of 150 nl 1(-1) ozone, or to clean air, in open-top ch ambers at the University of Nottingham, Sutton Bonington Campus, UK, i n the summer of 1992. The episodes were for 8 h daily and for 1-4 d in succession, with a seasonal total of 27 d. From late-June until mid-S eptember 1992, flow of water in the stems of individual trees was meas ured using a heat balance method. Short-term effects of ozone exposure on stem flow were not detected. However, in the longer-term, daily in tegrated stem flow values for the ozone treatment decreased throughout the measurement period relative to the clear air treatment. Further d ata analysis showed that integrated stem flow values for morning, even ing and for the more stable central part of the day changed in a simil ar way, but this was statistically significant only for the evening pe riod. No treatment differences, however, were detected in night-time w ater use values. This response was probably mediated by changes in sto matal resistance and, if applicable to other species, has important im plications for the long-term growth of trees in regions where photoche mical ozone is common.