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-3) pollution. We investigated the trends of meteorological variables and 03 and how these environmental conditions associate with tree health from 2001 onward. Ozone metrics for forest protection, based on Accumulated 03 exposure Over a Threshold of X ppb (AOTX) or on Phytotoxic O-3 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-3 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-3 metrics. Finally, the O-3 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 degrees C decade(-1), on average) and drier climate (rainfall, -48 mm decade I). A reduction was found for O-3 annual mean (-0.28 ppb decade(-1), on average) and AOT40 (-2540 ppb.h decade(-1), on average) whereas an increase was found for PODO (+0.40 mmol m(-2) decade(-1), on average). Visible foliar O-3 injury increased (+0.17% decade(-1)), while an improvement of the crown conditions (-5.0% decade(-1)) was observed. AOT40 was significantly associated with crown defoliation while PODY and soil water content were correlated with visible foliar O-3 injucy. As visible foliar O-3 injury was negligible in all the studied species, the results suggest that moderate O-3 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-3 risk (PODO) is expected to increase in the future under a hotter and drier climate. (C) 2018 Elsevier B.V. All rights reserved.