Abstract

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 NOx 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. NOx concentrations were significantly dependent on the local meteorology. NOx 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 AFst1.6 (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 AFst1.6 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.

Highlights

  • The toxicity of ozone for plants has been widely documented over the past twenty years (Benton et al, 2000; Skarby et al, 1998)

  • Even when ozone does not bring to visible damage on the leaf lamina (Bermejo et al, 2003; Novak et al, 2008; Marzuoli et al, 2008), it remains a cause for physiological alterations and a general loss in Net Primary Productivity (NPP) (Felzer et al, 2004; King et al, 2005)

  • The samples referred to completely dry canopy conditions were 1894 (45.3%), because most the samples between 8 p.m. and 8:30 a.m. were excluded due to the presence of dew on the leaves

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Summary

Introduction

The toxicity of ozone for plants has been widely documented over the past twenty years (Benton et al, 2000; Skarby et al, 1998). The EMEP model estimated for the Mediterranean areas an exposure between 40000 and 60 000 ppb h (on a six months basis, April–September) for the year 2000 (Simpson et al, 2007; Emberson et al, 2007), a value exceeding from 8 to 12 times the critical level of 5000 ppb h set by UN-ECE for the protection of forests and seminatural vegetation. Field observations never reported a strong plant injury (Bussotti and Gerosa, 2002; Bussotti et al, 2006, Paoletti et al, 2006), questioning the soundness of the exposure concept applied to ozone risk assessment. The lack of visible injuries in the Mediterranean vegetation could be due to extremely efficient physiological and biochemical defense mechanisms of these plants to oxidative stress as some experiments in controlled environments revealed (Nali et al, 2004; Elvira et al, 2004)

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