Abstract
To investigate whether nitrogen (N) load affects the ozone (O3) stomatal flux-effect relationship for birch biomass, three-year old birch saplings were exposed to seven different O3 profiles (24 h mean of 35–66 ppb) and four different N loads (10, 30, 50 and 70 kg ha−1 yr−1) in precision-controlled hemispherical glasshouses (solardomes) in 2012 and 2013. Stomatal conductance (gs) under optimal growth conditions was stimulated by enhanced N supply but was not significantly affected by enhanced O3 exposure. Birch root, woody (stem + branches) and total biomass (root + woody) were not affected by the Phytotoxic Ozone Dose (POD1SPEC) after two seasons of O3 exposure, and enhanced N supply stimulated biomass production independent of POD1SPEC (i.e. there were no POD1SPEC × N interactions). There was a strong linear relationship between the stem cross-sectional area and tree biomass at the end of the experiment, which was not affected by O3 exposure or N load. Enhanced N supply stimulated the stem cross-sectional area at the end of season 2, but not at the end of season 1, which suggests a time lag before tree biomass responded to enhanced N supply. There was no significant effect of POD1SPEC on stem cross-sectional area after either the first or second growing season of the experiment. Contrasting results reported in the literature on the interactive impacts of O3 and N load on tree physiology and growth are likely due to species-specific responses, different duration of the experiments and/or a limitation of the number of O3 and N levels tested.
Highlights
Tropospheric ozone (O3) is the most important air pollutant in terms of adverse effects onnatural vegetation and cultivated crops and it is of primary interest due to its strict relation with climate change (Lefohn et al, 2018)
Analysis of the gs measurements made during the exposure showed that the parameterisations of light, temperature, water vapour pressure deficit, and soil water potential for the DO3SE model were not affected by either N dose (Fig. S2) or O3 treatment (Fig. S3)
A compilation of data for Fagus sylvatica L and Betula pendula for different experiments conducted in Europe indicated a high variation in the total tree biomass response to O3 up to a POD1SPEC of ca. 30 mmol m−2 yr−1, with a reduction in total biomass becoming more pronounced at higher POD1SPEC (LRTAP Convention, 2017)
Summary
Tropospheric ozone (O3) is the most important air pollutant in terms of adverse effects on (semi-)natural vegetation and cultivated crops and it is of primary interest due to its strict relation with climate change (Lefohn et al, 2018). Rising tropospheric O3 concentration is an important air pollution problem in northern mid-latitudes, with levels rising since the Industrial Revolution, when O3 concentrations were approximately 10 ppb (Cooper et al, 2014; Hartmann et al, 2013). In the mid-20th century, the emissions of anthropogenic reactive nitrogen to the atmosphere accelerated because of increased fossil fuel combustion and intensification of agricultural activities. This resulted in a large increase in nitrogen (N) deposition to ecosystems through dry and wet processes that has approximately doubled since 1900 (Li et al, 2016b; Sutton et al, 2011). In Europe, the total deposition of N remains high (typically 5–30 kg N ha−1 yr−1, depending on locality and vegetation type) and control of ammonia emissions is hard to achieve, with European emissions stable since 2000 and predicted to remain stable at current levels in the 2020s and 2030s (Winiwarter et al, 2011)
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