• climate;
  • forests;
  • growth;
  • models;
  • ozone (O3);
  • water use


  • • 
    A lack of data on responses of mature tree growth and water use to ambient ozone (O3) concentrations has been a major limitation in efforts to understand and model responses of forests to current and future changes in climate.
  • • 
    Here, hourly to seasonal patterns of stem growth and sap flow velocity were examined in mature trees from a mixed deciduous forest in eastern Tennessee (USA) to evaluate the effects of variations in ambient O3 exposure and climate on patterns of stem growth and water use.
  • • 
    Ambient O3 caused a periodic slowdown in seasonal growth patterns that was attributable in part to amplification of diurnal patterns of water loss in tree stems. This response was mediated by statistically significant increases in O3-induced daily sap flow and led to seasonal losses in stem growth of 30–50% for most species in a high-O3 year.
  • • 
    Decreased growth and increased water use of mature forest trees under episodically high ambient O3 concentrations suggest that O3 will amplify the adverse effects of increasing temperatures on forest growth and forest hydrology.