Modelling Critical Levels of Ozone for the Forested Area of Austria

The ozone risk impact for trees has been calculated by modelling the AOT40 for forest areas based on the Austrian Forest Inventory, ozone measuring data 1993 (120 monitoring sites), meteorological data and an elevation model. However, the basic AOT40 approach does not reflect the health status of Norway spruce in Austria. Therefore, additional thoughts are outlined in the following approaches taking into consideration the adaption of trees to the longtime ozone level and the ozone uptake into the sub-stomata cavity.

The provisional Critical Level (AOT40) was established to allow a uniform assessment of the ozone risk for forested areas in Europe. In Austria, where ozone risk is assessed with utmost accuracy due to the dense grid of monitoring plots of the Forest Inventory and because the contunuously collected data from more than 100 air quality measuring stations, an exceedance up to the fivefold of the Critical Level was found. However, the data of the Austrian Forest Inventory and the Austrian Forest Damage Monitoring System do not reflect such an ozone impact. Therefore, various approaches were outlined including the tolerance and avoidance mechanisms of Norway spruce against ozone impact.

The 1st approach modifies the AOT40 based on the assumption that forests have adapted to the pre-industrial levels of ozone, which increase with altitude. Thus the AOT level has to be defined as altitude-dependent gradient instead of a fixed AOT40 threshold.

The 2nd approach modifies the AOT40 according to the ozone concentration in the sub-stomata cavity as response to light intensity and water vapour saturation deficit, which affect stomatal uptake.

The 3rd approach combines the other two approaches and includes forest hemeroby: The 1st approach (considering pre-industrial ozone levels) has been applied to autochthonous ("natural") forests, the 2nd (ozone uptake) approach to non-autochthonous ("altered") forests which have not been adapted to higher ozone levels at higher altitudes.

Conclusions:

The approach revealed that a plausible result concerning the ozone impact on spruce forests in Austria could be reached by combining pre-industrial ozone levels, ozone flux into the spruce needles and the hemeroby of forests.

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