Häberle, KH; Werner, H; Fabian, P; Pretzch, H; Reiter, I; Matyssek, R
The definition of exposure indices which may reflect, in a quantitative way, tree responses to ozone (O3) impact and risks of O3 injury under stand conditions has been an issue of debate for decades. During the early nineties, the concept of 'Critical Levels for Ozone' was re-defined by UN-ECE (Fuhrer & Achermann 1994; Karenlampi & Skiirby 1996). The 'critical level' was defined as an AOT4O dose (i.e. the 'accumulated exposure to ozone above a threshold of 40 nl O3 l-1) which would indicate - when surpassed - the biomass production to decline by more than 10 % relative to pre-industrial O3 regimes. For the time being, the critical AOT40 dose was set to 10 Ál O3 l-1 h, assuming O3 exposure below 40 nl F' and at night.to be irrelevant, but regarding O3 fluctuations above 40 nl 1-1 and during the daylight hours as a constraint on tree development. It is acknowledged that the assumption of 10 Á1 O3 l-1 h as the critical AOT40 dose is preliminary (Fuhrer et al. 1997), given its derivation from experiments with young trees grown in exposure chambers. There is increasing awareness that young individuals may distinctly differ in their physiology, and thus in O3 response, from mature trees (Kelly et al. 1995), and that chamber conditions (altered micro-climate, in most cases exclusion of competitors/parasites, often high water/nutrient supply) may strongly bias the sensitivity to ozone (Chappelka & Chevone 1992). It is not surprising, therefore, that the regional distribution of (often estimated) AOT4O is rather inconsistent with forest condition, being aware also that the latter is typically concluded from the unspecific crown appearance, that AOT4O does not reflect the actual O3 uptake, that trees may be sensitive to O3 uptake by shade crowns and at night - and that proposing only one 'critical level' does probably not cover the different kinds of tree species, forest types and site conditions: A critical AOT4O dose of 10 Ál l-1 h apparently is too low for O3-tolerant, but may be too high for O3-sensitive species/genotypes. In addition, it is not clear whether the definition of AOT4O in relation to biomass production is ecologically meaningful - and in what ways AOT4O may be linked, in more mechanistic terms, to the whole-tree performance.