It is now, widely accepted that stomatal ozone uptake provides a more reliable indicator for the assessment of risk of ozone damage to forest trees than ozone exposure. Thus, environmental factors influencing stomatal opening strongly affect the sensitivity of forest trees towards high ozone concentrations. In this study the hydrological model WAWAHAMO is used to simulate stomatal opening dependent oil air temperature, solar radiation, vapor pressure deficit and soil beater potential. Then, a statistical analysis was carried out to investigate relations between crown defoliation and ozone concentrations dependent oil stomatal opening. The results support the hypothesis that tropospheric ozone contributes to the observed crown defoliation. However, environmental factors strongly affect tree response to high ozone concentrations.