Environmental control of profiles of ozone concentration in a grassland canopy

In multi-species grassland communities, ozone exposure of individual species may differ due to their location inside the canopy. In this study, it was tested how the ozone profile inside a grassland canopy is influenced by the vertical distribution of leaf area (LAI) and by meteorological parameters, specifically by the turbulence intensity quantified by the friction velocity (u*). Multi-level ozone profiles were measured continuously during the first growth period of intensively managed grassland, and stomatal conductance (gs) was measured in the two dominant species, i.e. white clover (Trifolium repens) at 0.25 m and meadow foxtail (Alopecurus pratensis) at 0.50 m. In the two-layer structure of the canopy, with <20% of LAI located in the upper half, radiation (PAR) was only moderately reduced throughout the canopy thus favouring high gs also in the lower part. Daytime ozone concentration at 0.25 m, where white clover was dominant, was reduced on average by 36% of the value at the reference height of 0.90 m. When scaled with canopy height, this reduction was considerably larger than most comparable published data for other vegetation types. The shape of the ozone profile in the canopy was not affected by increasing LAI from 4.7 to 6.8 m2 m−2 or by the changing vertical distribution of LAI during the observation period. The main environmental influence on the shape of the profile could be attributed to u*. Generally, relative in-canopy concentrations of ozone increased with u*. This could explain the stronger and more variable ozone depletion during nighttimes. The different levels of ozone exposure of species predominantly located in the upper and lower parts of the canopy supports a multi-layer approach to modelling uptake of ozone, which is important for the assessment of ozone risks for individual plants growing in grasslands.