Seasonal changes in water use of ash trees exposed to ozone episodes

Young ash trees (Fraxinus excelsior L.) growing in the field were exposed to episodes of 150 nl 1^-1 ozone, or to clean air, in open-top chambers at the University of Nottingham, Sucton Bonington Campus, UK, in the summer of 1992. The episodes were for 8 h daily and for 1-4 in succession, with a seasonal total of 27 d. From late-June until mid-September 1942, flow of water in the stems of individual trees was measured using a heat balance method. Short-term effects of ozone exposure on stem flow were not detected. However, in the longer-term, daily integrated stem flow values for the ozone treatment decreased throughout the measurement period relative to the clear air treatment. Further data analysis showed that integrated stem flow values for morning, evening and for the more stable central part of the day changed in a similar way, but this was statistically significant only for the evening period. No treatment differences, however, were detected in night-time water use values. This response was probably mediated by changes in stomatal resistance and, if applicable to other species, has important implications for the long-term growth of trees in regions where photochemical ozone is common.