The study that prompts us to think along these lines is that of Herman et al. (2001), who note that air pollution by SO2, Pb, NOx and NH3 has been significantly reduced in central Europe over the past two decades, but that ozone concentrations there have been on the rise, based on trends derived from European databases that include ozone measurements from about 100 stations in Austria and Germany.  The parameter Herman et al. use to express the significance of the ozone trends is the AOT40 Critical Level set by the UN-ECE (1994), which has a value of 10 ppm.h and is defined as accumulated ozone exposure above a threshold of 40 ppb, 24 hours per day, over a six-month growing season (e.g. April-September).  In controlled experiments, such an ozone regime has been documented to result in approximate 10% reductions in tree biomass production.

How bad has the ozone pollution become in central Europe?  In most of the grid plots of the Austrian Forest Inventory Grid, and based on 1993 ozone data, Herman et al. report "the Critical Level of 10 ppm.h had been exceeded up to sevenfold," which to us sure sounds alarming.  Herman et al. concur, stating that "where standards had been exceeded to such an alarming extent, serious damage of forest trees should be expected."

So what do the trees in these highly-ozone-polluted grid plots look like?  Are they absolutely devastated?  Even dead?  In the words of Herman et al., "the results of the Austrian monitoring surveys do not reflect such damage."  They note, for example - and in superlative understatement - that "neither the general evaluation of the foliage losses in the context of the crown condition inventories nor the development of the growing stock reflect a dramatic situation."  In fact, they report that not only are there no "dramatic" reductions in tree health and productivity, there are typically none at all; and in many areas there are actually improvements, as they state that "crown conditions in Austria have slightly improved" and "the growing stock has increased."

Continuing, Herman et al. acknowledge that although ozone-related losses of biomass "could not be confirmed on old trees up to now," under present ozone levels they do show some reduction of photosynthetic CO2 uptake.  This phenomenon is particularly evident in old trees "at high altitudes," where AOT40 values are much more extreme, and in trees that are experiencing "additional climatic stress."  But the researchers are careful to add that "the reductions of the CO2 uptake are in no proportion to the massive excess of the AOT40."

What is one to conclude from these dichotomous observations?  Ozone exposures more than sevenfold greater than the Critical Level - which Critical Level alone should decrease tree productivity by 10% - are occurring all across the Austrian Forest Inventory Grid; and such conditions could well be expected to reduce the growth rates of the exposed trees by 70% or more.  Yet there is no evidence of any widespread damage or productivity reduction.  In fact, growth conditions seem to have improved almost everywhere, except at high altitudes and under conditions of more-than-usual climatic stress.

Herman et al. suggest these observations imply that the once-adequate Critical Level of ozone exposure is no longer suitable for application.  But why?  They provide the answer in correctly stating that "the significant parameter for the assessment of the risk" is not the atmospheric concentration of ozone, but "the absorbed dose."  Hence, they advise the creation of a new Critical Level that "takes into account leaf conductance and the environmental parameters influencing it."

This latter statement is a reasonable rendering of what we believe is the proper approach to the issue, i.e., there are many concurrent and ongoing changes in earth's atmospheric environment, and the net result of all of them acting in unison must be considered in predicting the consequences of changes in any individual factor.  In the case of earth's climate, for example, the surface air temperature consequences of an increase in the air's CO2 content cannot be adequately evaluated without considering the effects of concurrent changes in atmospheric aerosol quantities and properties.  Likewise, in the case of forest health, the biological consequences of rising tropospheric ozone concentrations cannot be adequately evaluated without considering the effects of the concurrent and ongoing rise in the air's CO2 content, which is known to have a significant impact on leaf conductance and, hence, largely determines a tree's critical "absorbed dose" of ozone [see Stomates (Conductance and Density) in our Subject Index].

When this more rational approach is followed, it has been shown in numerous laboratory and field experiments that realistically-scaled concurrent increases in atmospheric CO2 and ozone concentrations typically lead to very little change in plant productivity [see Ozone (Effects on Plants) in our Subject Index].  It is thus our opinion that the lack of substantial negative ozone-induced impacts on the forests of central Europe, as described by Herman et al., may well be the result of the compensatory beneficial impacts of the historical and still-ongoing rise in the air's CO2 content.  We can only imagine what would be the wretched state of these highly-valued ecosystems in the absence of an upward-trending atmospheric CO2 concentration.

References
Herman, F., Smidt, S., Huber, S., Englisch, M. and Knoflacher, M.  2001.  Evaluation of pollution-related stress factors for forest ecosystems in central Europe.  Environmental Science & Pollution Research 8: 231-242.

UN-ECE.  1994.  Critical Levels for Ozone.  A UN-ECE Workshop Report. Fuhrer, J. and Achermann, B. (Eds.).  Swiss Federal Research Station of Agricultural Chemistry and Environmental Health, No. 16. ISSN-1013-154X.

Wahid, A., Milne, E., Shamsi, S.R.A., Ashmore, M.R. and Marshall, F.M.  2001.  Effects of oxidants on soybean growth and yield in the Pakistan Punjab.  Environmental Pollution 113: 271-280.