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

Abstract

SUMMARYYoung ash trees {Fraxi?ius excelsior L.) growinii in the field were exposed to episodes of 150 nl 1"' ozone, orlo t:lean air, in open-top chambers at the University of Nottingham, Sutton Bonington Campus, UK, in the summer of 1992. The episodes were fnr 8 h daily and for 1^ d in succession, with a st:asonal total of 27 d. From late-June until mid-September 1992, flow-of water in the stems of individual trees was measured using a heat balance method. Short-term effects of ozone exposure on stem How 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 cbanges in stnmatal resistance and, if applicable to other species, has important implications for the long-term growth of trees in regiuns where photochemical ozone is common-

Effect of Long-Term vs. Short-Term Ambient Ozone Exposure on Radial Stem Growth, Sap Flux and Xylem Morphology of O3-Sensitive Poplar Trees

“…Similar results were reported for O 3 -tolerant clones of Betula pendula [74] and European beech [75]. On the contrary, four-year-old ash trees exposed to episodic elevated O 3 concentration decreased the sap flow later in the season as result of an increased stomatal resistance as well as a change in leaf area due to early leaf shedding during late summer [76]. Previous results showed that whole-tree use efficiency was lower in WAT trees than in EDU trees, because carbon assimilation decreased more than water losses, while it was not affected by O 3 -induced stomatal sluggishness [21].…”

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

“…This work clearly demonstrates that ozone exposure affects at least some foliar characteristics of ash trees, and therefore this work adds to the existing evidence (Wiltshire, Wright & Unsworth, 1994) which suggests that ozone is a contributory factor in the ash dieback svndrome.…”

Impacts of ozone on forests: a European perspective

“…This was reversed, however, at the end of the season (Wiltshire, Unsworth & Wright, 1994). This suggests that longterm O $ -exposure causes stomata to respond less efficiently, because their functioning has become sluggish, and that, as a result, water use is less efficient, and trees might be more susceptible to drought.…”

“…Experiments using heat-balance techniques to measure sap flow and water use in ash (Fraxinus excelsior) seedlings during episodes of O $ also showed that O $ -exposed trees used more water than controls early in the growing season. This was reversed, however, at the end of the season (Wiltshire, Unsworth & Wright, 1994). This suggests that longterm O $ -exposure causes stomata to respond less efficiently, because their functioning has become sluggish, and that, as a result, water use is less efficient, and trees might be more susceptible to drought.…”