Is there an impact of tropospheric ozone on the climate change mitigation potential of forests in Trentino (Northern Italy)?

Tropospheric ozone has a dual role in climate change. On the one hand, its positive radiative forcing of 0.25-0.65 W m-2 makes ozone the third most important greenhouse gas, after CO2 and CH4 (IPCC, 1997). On the other hand, ozone has been estimated to affect forest health and growth and therefore to reduce their potential C sink (e.g. Wittig et al., 2009). Failure in accounting for ozone effects on C sequestration has been estimated to increase the cost of climate change mitigation (Felzer et al., 2005). It is therefore important to have reliable information on distribution and effects of ozone, especially for mountain forests, which are considered particularly sensitive to climate change and exposed to high ozone levels. In 2007, a five-year meso-scale (6,200 km2) study was undertaken in Trentino, North Italy, where forests are estimated to stock 31x106 t C in their above-ground biomass (Tonolli and Salvagni, 2007). We (i) measured ozone concentration at 15-20 forest sites according to a systematic grid, (ii) modeled ozone exposure and associated risk for vegetation and (iii) investigated the effects on vegetation by means of field studies and statistical modeling (Gottardini et al., 2012). Results showed that ca. 76-95% of the forest area experienced an ozone concentration that exceeds the EU and UN/ECE risk thresholds, with a potential reduction of growth and therefore C sequestration. However, although specific symptoms related to ozone exposure have been identified on sensitive vegetation, measured effects on growth and health of forests were much less obvious. Despite the high exposure levels recorded, ozone was never a significant predictor of basal area increment and defoliation. Rather, frequency of tree damage and N-related variables were the most important predictors. An ad-hoc investigation based on long-term monitoring data confirmed the scarce relationship existing between ozone exposure and flux and tree defoliation and growth. A complex picture emerged, with potential high risk and early indicators of effect, but apparent limited impact of ozone on growth and therefore on C sequestration and climate mitigation potential in Trentino. There is a clear need to reconcile this picture into a consistent, meaningful frame. Felzer, B., Reilly, J., Melillo, J. et al., 2005. Future effects of ozone on carbon sequestration and climate change policy using a global biogeochemical model. Climatic Change 73, 345-373. Gottardini, E., Cristofolini, F., Cristofori et al., 2012. Ozono e foreste in Trentino: progetto Ozone EFfects on FORests in Trentino (Ozone EFFORT). Fondazione Edmund Mach, San Michele all’Adige (TN), p. 144. IPCC, 1997. The Regional Impacts of Climate Change: An Assessment of Vulnerability. R.T. Watson, M.C. Zinyowera, R.H. Moss (Eds). Cambridge University Press, UK. pp. 517. Tonolli, S., Salvagni, F., 2007. InFoCarb: Inventario Forestale del Carbonio della Provincia di Trento. Centro di Ecologia Alpina, Trento, p. 176. Wittig, V.E., Ainsworth, E.A., Naidu, S.L. et al. 2009. Quantifying the impact of current and future tropospheric ozone on tree biomass, growth, physiology and biochemistry: a quantitative meta-analysis. Global Change Biology 15, 396-424.