Trends and inter-relationships of ground-level ozone metrics and forest health in Lithuania.

Lithuania is representative of maritime to continental climate, no water limitation, and moderate ground-level ozone (O) pollution. We investigated the trends of meteorological variables and O and how these environmental conditions associate with tree health from 2001 onward. Ozone metrics for forest protection, based on Accumulated O exposure Over a Threshold of X ppb (AOTX) or on Phytotoxic O Dose over a Y threshold (PODY), were modeled at nine ICP-Forests plots over the time period 2001-2014. Tree-response indicators, i.e. crown defoliation and visible foliar O injury, were assessed during annual field surveys carried out at each ICP-Forests plot over the time period 2007-2017. Mann-Kendall and Sen statistical tests were applied to estimate changes over time of meteorological variables, response indicators and O metrics. Finally, the O metrics were correlated (Spearman test) to the response indicators over the common period 2007-2014. Over this time period, trend analyses revealed an increasingly hotter (+0.27 °C decade, on average) and drier climate (rainfall, -48 mm decade). A reduction was found for O annual mean (-0.28 ppb decade, on average) and AOT40 (-2540 ppb·h decade, on average) whereas an increase was found for POD0 (+0.40 mmol m decade, on average). Visible foliar O injury increased (+0.17% decade), while an improvement of the crown conditions (-5.0% decade) was observed. AOT40 was significantly associated with crown defoliation while PODY and soil water content were correlated with visible foliar O injury. As visible foliar O injury was negligible in all the studied species, the results suggest that moderate O pollution (approximately 30 ppb as annual average) does not induce biologically significant effects on this forest vegetation under the current conditions, however the overall O risk (POD0) is expected to increase in the future under a hotter and drier climate.