Translated abstract:
In a field experiment at "Kranzberger Forst" (near Freising/Germany) the impact of a chronically enhanced O3 regime on about 60-year-old beech (Fagus sylvatica L.) and spruce (Picea abies (L.) Karst.) trees was assessed throughout three growing seasons. Site conditions of the mixed beech/spruce forest were edaphically non-limiting in terms of the nutrient and water availability. The aim of the study was to quantify the chronic ozone impact on and the responses of the adult forest trees. To this end, the ozone sensitivity of biochemical, physiological and structural tree parameters was compared between an experimentally enhanced, twice-ambient ozone regime (2xO3; through an free-air fumigation approach) and the unchanged ambient air (1xO3 = control) at the forest site. Responses of the gas exchange, transpiration, stomatal regulation and structural differentiation of leaves/needles and twigs as well as biochemical leaf parameters (free and conjugated 1-aminocyclopropane-1-carbonic acid (ACC), free polyamines, salicylic- and gentisic acid) to the O3 regimes were examined, regarding consistency within each tree species and their respective sun and shade crowns. Given this set of investigated parameters, an ecologically relevant risk assessment was performed, regarding effects of chronic O3 regimes on the adult forest trees under site conditions. The ozone-flux model currently promoted by UNECE policies (Level II concept; Emberson et al., 2000a, b) was parameterised for the adult beech and spruce trees at "Kranzberger Forst" and validated versus the ozone uptake into the leaf organs as assessed through gas exchange measurements of beech and spruce twigs. Threshold concepts for ozone based on AOT40 (= accumulated ozone dose over a threshold of 40 nl l-1) and O3 flux were evaluated for their ecological relevance. Emphasis was on the quantification of the ozone sensitivity of sun versus shade leaves and on the comparison between findings from the adult trees at the site and the prevailing knowledge in the literature about juvenile beech and spruce as exposed to controlled chamber conditions. Suggestions are made towards improvement of the O3-flux concept and revised "Critical Levels for Ozone" for forest trees in terms of dose-response relationships. Conclusions: 1. In this case study, spruce was not substantially less ozone sensitive than beech. 2. Parameters affected by ozone differed between sun and shade leaves each of beech and spruce. A general difference in ozone sensitivity between sun and shade leaves was not evident. 3. Even though the current threshold of AOT40 = 10 µl l-1 h per growing season in trees was exceeded during the three years of investigation, no reduction in the radial stem increment of beech and spruce was found. 4. Adult beech and spruce at "Kranzberger Forst" were less ozone-sensitive in comparison with reports in the literature on juvenile trees. 5. O3 uptake (resulting from O3 flux) as a measure of the physiologically effective O3 dose better reflects potential risks of ozone injury in trees as does AOT40, as the latter is solely based on O3 exposure. Capsule: The comparison between the two O3 regimes does not appear to indicate existential risks for adult beech and spruce at the short-term scale. However, leaf-level responses do reflect adverse O3 effects under 2xO3. On these grounds, it is concluded that risks by chronic ozone impact cannot be out ruled for adult beech and spruce trees on the long-term scale.