Interactions among vegetation and ozone, water and nitrogen fluxes in a coastal Mediterranean maquis ecosystem

Abstract. Ozone, water and energy fluxes were measured over a Mediterranean maquis ecosystem from 5 May until 31 July 2007 by means of the eddy covariance technique. Additional measurements of NO_x fluxes were performed by the aerodynamic gradient technique. Stomatal ozone fluxes were obtained from water fluxes by a Dry Deposition Inferential Method based on a big leaf concept.

The maquis ecosystem acted as a net sink for ozone. The different water availability between late spring and summer was the major cause of the changes observed in stomatal fluxes, which decreased, together with evapotranspiration, when the season became drier.

NO_x concentrations were significantly dependent on the local meteorology. NO_x fluxes resulted less intense than the ozone fluxes. However an average upward flux of both NO and NO₂ was measured.

The non-stomatal pathways of ozone deposition were investigated. A correlation of non-stomatal deposition with air humidity and, in a minor way, with NO₂ fluxes was found.

Ozone risk assessment was performed by comparing the exposure and the dose metrics: AOT40 (Accumulated dose over a threshold of 40 ppb) and AF_st1.6 (Accumulated stomatal flux of ozone over a threshold of 1.6 nmol m⁻² s⁻¹). AOT40, both at the measurement height and at canopy height was greater than the Critical Level for the protection of forests and semi-natural vegetation (5000 ppb h) adopted by UN-ECE. Also the AF_st1.6 value (12.6 mmol m⁻² PLA, Projected Leaf Area) was higher than the provisional critical dose of 4 mmol m⁻² PLA for forests. The cumulated dose showed two different growth rates in the spring and in the summer periods, while the exposure showed a more irregular behavior in both periods.