This study aimed to evaluate ozone (O3) phytotoxic potential using AOT40F (accumulated O3 concentration over a threshold of 40 ppb for forest protection), document visible foliar O3 injury across eight forest monitoring plots, analyse MDA (malondialdehyde) content in leaves and needles, and assess the relationship between visible injury and plot conditions. Initial findings are based on data from the 2021 and 2022 vegetation seasons. AOT40F values exceeded the critical level of 5 ppmh-1 at all plots, with higher values in 2022. The correlation between AOT40F and visible injury was inconsistent; in 2021, minimal visible O3 injuries were observed, while these were more frequent in 2022, notably on Fagus sylvatica leaves. The altitude effect on O3 concentration indicates greater vegetation damage at higher altitudes. In contrast, the AOT40F-altitude relation was not significant. The 2021 vegetation season was characterised by lower temperatures and higher relative air humidity and soil moisture in comparison to 2022. Stomatal conductance conditions were similar in both years, except for lower soil moisture in 2022. Soil moisture, air humidity, and temperature together accounted for about 50% of the variance in visible injury in 2022. The findings suggest that the AOT40F capability for predicting damage to vegetation is limited and highlight the importance of future research focusing on stomatal O3 flux-based approaches.

Key message Juveniles and canopy trees may not exhibit similar nitrogen acquisition responses to soil temperature change caused by variation in snow cover over winter. The use of(15)N tracer is a powerful tool for tracking the effects of variation in soil frost on plant nitrogen acquisition. While the responses of juvenile trees to environmental change are often used to infer the responses of canopy trees, the(15)N enrichment responses of juveniles and mature canopy trees may not be comparable. We conducted a winter soil temperature manipulation study (snow exclusion, ambient snow or soil insulation) in a lowlandFagus sylvaticaforest.N-15 tracer was applied the following spring and the(15)N enrichments of soil, juvenile and mature canopy trees were examined in late fall. Within canopy trees and juveniles, the relative treatment effects on(15)N enrichment were consistent among all sampled tissues (roots, stem cores, leaves, buds and the current year's shoot growth). For juveniles,N-15 enrichment was highest under snow exclusion (coldest soil) and lowest under soil insulation (warmest soil), and lower(15)N enrichment occurred under ambient conditions than under snow exclusion. For canopy trees,N-15 enrichment also was highest under snow exclusion and lowest under soil insulation, but there was no difference in(15)N enrichment between ambient conditions and the snow exclusion treatment. Therefore, our results indicate that sampling of juveniles may overestimate the nitrogen acquisition responses of mature trees to winter temperature variation.

Aerated forest soils are a significant sink for atmospheric methane (CH4). Soil properties, local climate and tree species can affect the soil CH4 sink. A two-year field study was conducted in a deciduous mixed forest in the Hainich National Park in Germany to quantify the sink strength of this forest for atmospheric CH4 and to determine the key factors that control the seasonal, annual and spatial variability of CH4 uptake by soils in this forest. Net exchange of CH4 was measured using closed chambers on 18 plots in three stands exhibiting different beech (Fagus sylvatica L) abundance and which differed in soil acidity, soil texture, and organic layer thickness. The annual CH4 uptake ranged from 2.0 to 3.4 kg CH4-C ha(-1). The variation of CH4 uptake over time could be explained to a large extent (R-2 = 0.71, P < 0.001) by changes in soil moisture in the upper 5 cm of the mineral soil. Differences of the annual CH4 uptake between sites were primarily caused by the spatial variability of the soil clay content at a depth of 0-5 cm (R-2 = 0.5, P < 0.01). The CH4 uptake during the main growing period (May-September) increased considerably with decreasing precipitation rate. Low CH4 uptake activity during winter was further reduced by periods with soil frost and snow cover. There was no evidence of a significant effect of soil acidity, soil nutrient availability, thickness of the humus layer or abundance of beech on net-CH4 uptake in soils in this deciduous forest. The results show that detailed information on the spatial distribution of the clay content in the upper mineral soil is necessary for a reliable larger scale estimate of the CH4 sink strength in this mixed deciduous forest. The results suggest that climate change will result in increasing CH4 uptake rates in this region because of the trend to drier summers and warmer winters. (C) 2009 Elsevier Ltd. All rights reserved.