Effects of tropospheric ozone and elevated nitrogen input on the temperate grassland forbs Leontodon hispidus and Succisa pratensis

Atmospheric ozone (O3) and nitrogen (N) pollution have increased since pre-industrial times and pose a threat to natural vegetation. The implications of these pollutants for the perennial temperate grassland species Leontodon hispidus (Rough Hawkbit) and Succisa pratensis (Devil’s-bit Scabious) are largely unknown. Both species are important for pollinators and Succisa pratensis is the host plant for the threatened marsh fritillary butterfly (Euphydryas aurinia).

We examine growth and physiological responses (leaf cover, leaf litter, flowering, chlorophyll index [Leontodon hispidus and Succisa pratensis]; photosynthesis and stomatal conductance [Succisa pratensis]) using an outdoor Free Air Ozone Enrichment system. Plants were exposed to Low, Medium and High ozone treatments over three growing seasons (treatment means: 24, 40 and 57 ppb, respectively), with and without the addition of nitrogen (40 kg ha−1 yr−1) during the first year.

Decreases in leaf cover (p < 0.001) and chlorophyll index (p < 0.01) were observed with increased O3 for Leontodon hispidus. The addition of N resulted in a higher chlorophyll index only at the uppermost O3 level and also led to an overall increase in litter production of 6%. However, a stronger effect of both O3 and N treatments was observed with Succisa pratensis. Litter production increased with increasing O3 (p < 0.001) and an overall rise of 31% was recorded with added N (p < 0.05). However, O3 had the biggest impact on Succisa pratensis foliage leading to more damaged leaves (p < 0.05). During summer resources were prioritised to new leaves, maintaining stomatal conductance and photosynthesis rates. However, this was not sustained during autumn and accelerated senescence occurred with higher ozone, and rates declined faster with added nitrogen (p < 0.05). Elevated O3 also reduced Succisa pratensis flowering (p < 0.01).

These effects have implications for inter- and intra-specific competition, seed establishment, nutrient cycling, as well as the provision of general pollinator resources with specific issues for butterfly larvae. Results highlight the need for concerted action to reduce pre-cursor ozone emissions to go alongside habitat management efforts to protect biodiversity.