Description

Measurements of ozone fluxes are carried out at selected plots by the micrometeorological Eddy covariance approach at forest canopy level.

The main objective is the refinement of criteria and thresholds for the forests protection, against ozone and climate change, in order to propose new standards, more appropriated, based on the quantity of ozone, absorbed by plants, as compared to external exposure implicit in the AOT40 index.

Methodology

This study will be made by micrometeorological measurements (Cieslik, 2009). The micrometeorological methods are based on the analysis of turbulent motion of lower air layers in contact with vegetation. Air transports chemical substances upward and downward by means of turbulent eddies. The molecules of the transported substances are like passengers on a train (the air motions).

The vertical flux Fi of the species i in a turbulent medium are governed by a Fick-like diffusion law (Fick, 1855), which can be written as:

where K(z) is the turbulent diffusion coefficient, Ci the local concentration of the species i and z is the height above ground level (a.g.l.). Contrary to the molecular diffusion coefficient, which is independent from space and time but depends on the nature of the transported gas, the turbulent diffusion coefficient depends on space and time but not on the nature of the gas.

The Eddy Covariance technique is a key atmospheric flux measurement technique to measure and calculate vertical turbulent fluxes within atmospheric boundary layers. It is a statistical method used in meteorology and other applications that analyses high-frequency wind and scalar atmospheric data series, and yields values of fluxes of these properties. The technique is mathematically complex and requires significant care in setting up and processing data.

The micrometeorological method is based on the calculation of the covariance of the vertical component w of the wind vector and of the concentration of the substance under consideration, both measured at very high sampling frequency (10Hz). The covariance is expressed by:

where the primed quantities represent the fluctuations around the time-averaged value of the corresponding variable and the overbar stands for the time-averaging process.

The eddy covariance technique requires measurements of very rapid turbulent fluctuations and therefore the used sensors and analysers should have a short response time combined with a high selectivity. The eddy covariance method is considered as a reliable measurement method for turbulent exchanges of momentum and heat in the atmosphere.

Ozone flux partitioning: Stomatal and non-stomatal fluxes

The data will be analysed to quantify diurnal fluxes and non-stomatal and stomatal removal will be estimated. The stomatal ozone flux is the quantity of ozone molecules transferred per unit time and area from air to plant tissues through the stomata. However, taking into account turbulent and diffusive transport, the stomatal uptake is not sufficient to predict the magnitude of the canopy sink. The so-called non-stomatal sinks have been invoked to explain the disagreement.

Abscisic Acid dosage

In order to estimate the stress state of trees in line with photochemical pollution, leaf samples will be taken and anti-stress markers determination will be made in laboratory. Indeed, ozone generates a modification of physiological effects on vegetation. On tree species strongly impacted by ozone, biochemical analyses will be led: dosages of ABA (Abscisic Acid) and chlorophylls. ABA plays an important role in plant responses to environmental stress and pathogens. Abscisic acid is also produced in the roots in response to decreased soil water potential and other situations in which the plant may be under stress. The ABA-induced stomatal closure reduces transpiration thus preventing further water loss from the leaves in times of low water availability.

Dosages of Abscisic Acid will be calculated on 5 trees per plot, on current year needles, one-year-old needles and two-years-old needles, i.e. 15 samples per plot. Comparisons will be established between asymptomatic and symptomatic foliage, i.e. finally 30 samples per plot.

Identification of forest plots, duration of the campaigns

The canopy-level ozone fluxes measurements, one measurement campaign, per year will be implemented, over two years in Italy and France.

Castelporziano (IT): “The Estate of Castelporziano covers 6000 ha close to Rome and has macchia and forest ecosystems (holm oak, mixed oaks, stone pine) distributed along the typical sea-inland belt with access to ground water table. Work performed at Castelporziano focuses on biodiversity conservation and carbon, water, biogenic trace gases, ozone and methane fluxes. A tower equipped for flux measurements run all year round managed by Dr Silvano Fares, with stations for meteorological and plant physiological parameters. The experimental site is closely connected with local research infrastructures at the Agricultural Research Council (CRA) with well equipped laboratories (mass spectrometry, chemistry, gas-exchange).”

San Rossore (IT): San Rossore is the heart of the National Park of Migliarino San Rossore Massaciuccoli that covers 4.800 ha in central-northern Tuscany. The San Rossore station, set in a typical Mediterranean pine forest, is dominated by the evergreen conifer tree Pinus pinea. The tower is managed by Carsten Gruening (JRC, Ispra).

Fontblanche (FR): The Fontblanche site, a typical Mediterranean forest, is managed by the National Institute of Agronomic Research of Avignon (Institut National de la Recherche Agronomique). The work was made possible by this technical support and contribution of the INRA-Avignon team for the ozone fluxes campaign in South-eastern France in July 2013.

Puéchabon (FR): The Puéchabon forest is representative of the most important Mediterranean ecosystems, dominated by Quercus ilex (90 %) and Quercus pubescens (10 %). The tower is managed by CEFE-Montpellier (Centre d'Écologie Fonctionnelle et Évolutive). The CEFE is currently the largest French research center in Ecology.

Validation of the DO3SE model – Refinement of criteria