Modelling stomatal ozone flux across Europe
Introduction
Current European levels of tropospheric ozone have been shown to cause damage to forest trees, agricultural crops and semi-natural vegetation (Kärenlampi and Skärby, 1996). In order to address this, the United Nations Economic Commission for Europe (UN-ECE) has adopted an effects-based approach, using the critical loads/levels concept. Such critical levels for ozone will play an important role in the multi-pollutant/multi-effect protocols which are being developed to aid policy formulation to control ozone precursor emissions and reduce European ozone concentrations to acceptable levels (Bull, 1991). The existing effects-based research for ozone has resulted in the establishment of critical levels to a so-called Level I standard (Kärenlampi and Skärby, 1996). These Level I values relate ozone injury to ozone exposure using a cumulative exposure over a threshold concentration of 40 ppb (nl l−1) for daylight hours (AOT40), with the necessary exposure–response relationships being derived from open-top chamber experiments. These experiments demonstrated linear relationships between cumulative seasonal ozone exposure, expressed as AOT40, and a variety of plant responses (Fuhrer et al., 1997). From these relationships it has been possible to derive a critical level based on a given level of plant response (e.g. a 5% yield reduction). However, extrapolating from these chamber experiments to ambient conditions is difficult for two reasons. Firstly, the differences in microclimate between the chamber-grown plants and those growing outside may lead to differences in plant response to the same ozone exposure (e.g. Sanders et al., 1991, Pleijel et al., 1994), because of differences in aerodynamic and boundary layer conductances, and differences in plant sensitivity. Secondly, the maintenance of chamber plants, e.g. through watering or the application of systemic fungicides, may alter plant sensitivity to air pollutants. These factors create uncertainty in the value of the Level I critical levels that have been proposed to protect vegetation.
It is also important to note that the exposure–response relationships from which the Level I critical level values have been derived are based on applying a range of different ozone exposures to replicate chambers in the same location and under the same climatic conditions. It has been emphasised by a number of authors (e.g. Fugrer et al., 1997, Grünhage and Haenel, 1997) that these chamber-based relationships cannot be used to assess the relative size of ozone impacts at different locations across Europe. In particular, it has long been recognised that plant response is more closely related to the internal ozone dose, or the instantaneous flux of ozone through the stomata, than the ambient ozone exposure (e.g. Amiro et al., 1984, Fuhrer et al., 1992). It is important to note that high concentrations of ozone are often associated with factors leading to reduced ozone flux, such as high vapour pressure deficits (VPDs) (Grünhage and Jäger, 1994, Grünhage et al., 1997). Furthermore, the highest ozone concentrations in areas such as southern Europe occur during seasons when non-irrigated vegetation experiences high soil moisture deficits (SMDs), resulting in reduced stomatal conductance and hence low ozone flux.
As such, the Level I critical level values, which are being used in developing European ozone control strategies both by UN-ECE and the European Union (EU), are intended to protect the most sensitive vegetation types under the most sensitive conditions. Exceedance of these critical levels only provides an indication that some risk exists of damage to vegetation from ozone; the degree of exceedance cannot be used to provide a measure of the relative risk of damage to vegetation in different areas of Europe.
It is thus clear that an assessment based on ozone flux to receptor sites within the leaf, rather than ozone exposure, could provide an improved estimate of the relative degree of risk of ozone damage to vegetation across Europe, and hence allow more cost-effective control strategies to be identified. Existing models of ozone flux (e.g. Baldocchi et al., 1987, Körner et al., 1995, Grünhage and Haenel, 1997) require detailed micrometeorological information, or are applicable to only a limited number of species, and cannot be readily applied at a European scale. Alternatively, models designed to model ozone deposition on a regional scale (e.g. Wesely, 1989, Erisman et al., 1994) have a limited description of the stomatal responses, which are not species-specific and which do not consider the effects of VPD or SMD.
Our objectives in this study were to develop a modelling approach, which could be applied to estimate and map stomatal ozone flux to major vegetation types across Europe. The methodology we report in this paper enables the calculation of ozone flux to physiologically active leaves of the upper canopy as a function of ozone exposure, meteorology, stomatal function and soil data. This allows the importance of different model parameters in determining ozone uptake to be estimated, and enables comparisons of the temporal and spatial variation in modelled ozone fluxes with those of the AOT40 index. Application of the model over such a large spatial scale gives an indication of the temporal and spatial variability of absorbed ozone dose, taking into account the systemic spatial variation in climate across Europe. It should be noted that the development of this model has largely concentrated on the stomatal component of ozone uptake since this is recognised as the predominant factor limiting ozone flux into the leaves. However, the atmospheric and boundary layer components are included in a simplified manner to give an indication of the relative effect of these resistances on ozone uptake. Thus, although the present calculations can be expected to improve in future, the current approach allows us for the first time to present regional-scale comparisons of modelled stomatal flux for two major species with the ozone indices currently used to determine ozone risk. Such comparisons give a clear indication of the possible implications of adopting a flux-based approach for future policy evaluation.
Section snippets
Methodology
The modelling approach for stomatal resistance (rs) is based on the multiplicative stomatal conductance model (where conductance is the reciprocal of resistance) described by Jarvis (1976) and modified by Körner et al. (1995). This model has been further developed to enable the evaluation of species-specific stomatal conductance (gs) on a European scale. The model has been parameterised for a number of different European species, including 10 tree species, 7 agricultural crops and 1 type of
Model application
The methodology described above has been applied on a pan-European scale. Model runs have been performed for the duration of the species-specific growing season during 1994. The results are described here to show the potential applications of the model and also to give an indication of the spatial and temporal variation between this flux modelling approach and the existing Level I exposure approach. These runs are performed for only two species, beech (Fagus sylvatica) and spring wheat (
Results and discussion
The modelling procedure was used to generate maps of monthly mean ozone fluxes for both beech and wheat across Europe. These flux maps were compared with equivalent AOT40 maps in order to compare the spatial variability between the flux and exposure-based risk assessment approaches. Fig. 1, Fig. 2, Fig. 3 show the AOT40 and flux maps for both wheat and beech, for June during 1994.
One of the main findings from the comparisons of the flux and exposure maps were that the areas experiencing the
Conclusions
A new deposition model has been presented, which is intended to allow the calculation of stomatal ozone uptake for major vegetation types across Europe. The model was designed to link to European-scale photo-oxidant models, enabling estimation of stomatal fluxes over the whole of Europe and for multi-annual time-periods, in order to improve future damage estimates for vegetation, and hence improve the data available for policy analysis. The flux model takes into account the major factors
Acknowledgements
This project was supported by the UK Department of the Environment, Transport and the Regions (DETR), the Nordic Council of Ministers (NMR), and the Co-operative Programme for Monitoring and Evaluation of the Long-Range Transmission of Air Pollutants in Europe (EMEP).
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