Ozone risk assessment for agricultural crops in Europe: Further development of stomatal flux and flux–response relationships for European wheat and potato

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Abstract

Applications of a parameterised Jarvis-type multiplicative stomatal conductance model with data collated from open-top chamber experiments on field grown wheat and potato were used to derive relationships between relative yield and stomatal ozone uptake. The relationships were based on thirteen experiments from four European countries for wheat and seven experiments from four European countries for potato. The parameterisation of the conductance model was based both on an extensive literature review and primary data. Application of the stomatal conductance models to the open-top chamber experiments resulted in improved linear regressions between relative yield and ozone uptake compared to earlier stomatal conductance models, both for wheat (r2=0.83) and potato (r2=0.76). The improvement was largest for potato. The relationships with the highest correlation were obtained using a stomatal ozone flux threshold. For both wheat and potato the best performing exposure index was AFst6 (accumulated stomatal flux of ozone above a flux rate threshold of 6 nmol ozone m−2 projected sunlit leaf area, based on hourly values of ozone flux). The results demonstrate that flux-based models are now sufficiently well calibrated to be used with confidence to predict the effects of ozone on yield loss of major arable crops across Europe. Further studies, using innovations in stomatal conductance modelling and plant exposure experimentation, are needed if these models are to be further improved.

Introduction

During the last 10 years there has been an intensive development of methods for the estimation of ozone uptake by plants in order to more accurately assess both total ozone deposition rates and certain effects of ozone such as yield loss. Cumulative ozone uptake is likely to have stronger relationships with effects than ozone concentrations, and the ozone flux through the stomata is strongly dependent on climatic conditions (Pleijel et al., 2000). Multiplicative models of stomatal conductance as a basis for calculating ozone flux (Emberson et al., 2000a, Emberson et al., 2000b; Grünhage et al., 1999, Grünhage et al., 2001) have been suggested for risk assessment in Europe, and have been used to derive relationships between yield loss and ozone uptake for wheat and potato (Danielsson et al., 2003; Pleijel et al., 2002, Pleijel et al., 2004). Similar approaches are under development in North America (Massman et al., 2000; Musselman and Massman, 1999).

The aims of the present investigation were: (1) to calibrate the multiplicative algorithms for stomatal conductance (gsto) for wheat and potato, and (2) to derive relationships between yield loss and stomatal ozone uptake for wheat and potato based on open-top chamber (OTC) experiments with field-grown crops using these calibrations. The work was based on a combination of experimental results and modelling approaches, with the latter based on a dry deposition model referred to here as the DO3SE model (for Deposition of Ozone and Stomatal Exchange). The DO3SE model is currently used within the European Monitoring and Evaluation Programme (EMEP) photo-oxidant chemical transport model, which is used by the United Nations Economic Commission for Europe (UNECE) for assessment of European air pollution abatement strategy (Simpson et al., 2003). DO3SE allows the calculation of both stomatal and non-stomatal ozone deposition for a variety of land cover types found across Europe (Emberson et al., 2000b, Emberson et al., 2001; Simpson et al., 2003). A key component of the DO3SE model is the multiplicative algorithm used to estimate stomatal conductance; this model has formed the basis for a number of studies, which have related cumulative absorbed ozone uptake to effects as previously described by Danielsson et al. (2003) and Pleijel et al., 2002, Pleijel et al., 2004.

Using the DO3SE model to estimate stomatal flux allows the application of a modelling approach which is consistent with methods used in European scale dry deposition modelling. This is important because it is then possible to apply the flux–response relationships derived from the experimental studies to estimate the risk of ozone impacts to vegetation across Europe under different policy scenarios. In this paper we present the derivation of updated flux–response relationships for two major European crop species—wheat and potato. The key improvements of the flux–response relationships presented in this paper compared to the earlier published flux–response relationships (Danielsson et al., 2003; Pleijel et al., 2002, Pleijel et al., 2004) are as follows: (1) a detailed literature review has been conducted covering data from a wider geographical extent for use in revising the species specific, stomatal conductance (gmax) and the functions representing the influence of different environmental variables on stomatal conductance; (2) complete agreement has been ensured between the parameterisation of the stomatal conductance model used to derive the flux–response functions and the DO3SE model; (3) an improved model representation has been developed of the limitation to stomatal conductance in the afternoon, when ozone concentrations frequently peak, based on increasing vapour pressure deficit (VPD) during the day; (4) the representation of phenology in terms of the period of maximum ozone sensitivity has been revised; and (5) one experimental data set for wheat has been added for the derivation of the flux–response relationship (Pleijel et al., 2006).

Section snippets

The multiplicative conductance model

Details of the revised multiplicative model used in DO3SE are available in the Mapping Manual of the LRTAP Convention (LRTAP Convention, 2004). It is based on the Jarvis-type multiplicative algorithm:gsto=gmax[min(fphen,fO3)]flightmax{fmin,(ftempfVPD)},where gsto is the actual stomatal conductance (mmol O3 m−2 PLA s−1 sunlit projected leaf area (PLA) s−1) and gmax is the species-specific maximum stomatal conductance (mmol O3 m−2 PLA s−1). The parameters fphen, fO3, flight, ftemp and fVPD are all

Parameterisation of stomatal ozone flux model

Table 2 summarises the parameterisation of the models for wheat and potato, used to derive flux–response relationships. The derivation of this parameterisation is discussed in more detail below.

Discussion

The updated stomatal conductance model described in this paper led to improved relationships between relative yield and modelled cumulative ozone uptake. The improvement in terms of correlation (Pleijel et al., 2004) was from r2=0.77 to 0.83 for wheat and from r2=0.64 to 0.76 for potato. These improvements were not based on any single change of the conductance model but of the fine tuning of the stomatal conductance model based on the literature data, including adjustments of the maximum

Acknowledgements

Thanks are due to L. De Temmerman, P. Högy, K. Ojanperä and M. Badiani for contributing experimental data to the development of the flux–response relationships presented in this study and colleagues within the LRTAP Convention who contributed to the development and review of Section 3 of the Mapping Manual. The Mistra-Research ASTA program funded Håkan Pleijel's work with this paper. The work of Lisa Emberson and Mike Ashmore on this paper was supported under contract SPU24 from the UK

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