An evaluation of ozone exposure metrics for a seasonally drought-stressed ponderosa pine ecosystemJeanne A. Panek*, Meredith R. Kurpius, Allen H. GoldsteinEnvironmental Science, Policy and Management, University of California, 151 Hilgard Hall, Berkeley, CA 94720, USAReceived 1 June 2000; accepted 12 April 2001‘‘Capsule’’: Use of the SUM0 metric (sum of all daytime ozone concentrations) was only appropriate early in the growingseason for ponderosa pine but overestimated ozone uptake under drought conditions later in the season.AbstractOzone stress has become an increasingly significant factor in cases of forest decline reported throughout the world. Currentmetrics to estimate ozone exposure for forest trees are derived from atmospheric concentrations and assume that the forest isphysiologically active at all times of the growing season. This may be inaccurate in regions with a Mediterranean climate, such asCalifornia and the Pacific Northwest, where peak physiological activity occurs early in the season to take advantage of high soilmoisture and does not correspond to peak ozone concentrations. It may also misrepresent ecosystems experiencing non-averageclimate conditions such as drought years. We compared direct measurements of ozone flux into a ponderosa pine canopy with a suiteof the most common ozone exposure metrics to determine which best correlated with actual ozone uptake by the forest. Of the metricswe assessed, SUM0 (the sum of all daytime ozone concentrations>0) best corresponded to ozone uptake by ponderosa pine, howeverthe correlation was only strong at times when the stomata were unconstrained by site moisture conditions. In the early growing season(May and June), SUM0 was an adequate metric for forest ozone exposure. Later in the season, when stomatal conductance waslimited by drought, SUM0 overestimated ozone uptake. A better metric for seasonally drought-stressed forests would be one thatincorporates forest physiological activity, either through mechanistic modeling, by weighting ozone concentrations by stomatal con-ductance, or by weighting concentrations by site moisture conditions. # 2002 Elsevier Science Ltd. All rights reserved.Keywords: Forest ozone injury; Ozone monitoring and assessment; Ozone exposure metrics: SUM0, SUM06, SUM08, W126, AOT40; Forest-atmosphere interactions1. IntroductionOzone is a common, phytotoxic air pollutant that hasbecome progressively more prevalent in industrializedparts of the world and an increasingly significant factorin the decline of forest health. Ozone damage to forestshas been reported worldwide, including forests in east-ern and western Europe (Rennenberg et al., 1997;Matyssek et al., 1997), and throughout the UnitedStates (e.g. in California, Miller et al., 1998, in NewEngland, Treshow, 1984, in the southeastern UnitedStates, Skelly et al., 1997). Cause and effect relation-ships are being sought between ozone exposure andforest response. This requires a measure of biologicallymeaningful ozone exposure.Ozone concentration is routinely monitored through-out the United States. Air pollution specialists haveexplored mathematical approaches for summarizingambient air quality information into forms that canserve as a surrogate for dose (Lefohn, 1992). Theseozone metrics are being used to describe forest exposureto ozone with the end of establishing cause and effectrelationships in standing forests. Dose has been histori-cally defined as concentrationtime (O’Gara, 1922).Effective dose was further defined as the concentrationadsorbed by vegetation (Runeckles, 1974) in contrast toambient air concentration. Fowler and Cape (1982)recognized the role of stomatal conductance as a meansfor ozone to reach leaf internal surfaces and introducedpollutant adsorbed dose, defined in units of g m2ground or leaf area. Taylor et al. (1982) further added0269-7491/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.PII: S0269-7491(01)00155-5Environmental Pollution 117 (2002) 93–100www.elsevier.com/locate/envpol* Corresponding author. Tel.: +1-510-642-9732; fax: +1-510-643-5098.E-mail address: [email protected] (J.A. Panek).the term internal flux (mg m2h1) as a measure ofphysiologically relevant ozone exposure.Several indices have been developed to describe veg-etation exposure to ozone. Some have evolved from theobserved relationship that short-term high concentra-tions of ozone cause significant damage. Others char-acterize the deleterious effects of long-term moderateconcentrations. The 7-h mean (09:00–15:59) was estab-lished by the United States EPA National Crop LossAssessment Network (NCLAN) as the statistic that bestrepresented the period of greatest plant vulnerability toozone pollution and highest ozone concentrations(Heck et al., 1982; Lefohn, 1992). Later it was recog-nized that not all sites experience the maximum ozoneconcentration between those hours and the window wasexpanded to 12 h. In the late 1980s the EPA abandonedthe use of a mean concentration statistic when it wasrecognized that means were inadequate in describing theozone environment (Lee et al., 1988) and instead cumu-lative indices were introduced. Cumulative indices withvarious threshold levels (>60 ppb, >80 ppb) and indi-ces designed to give greater weight to higher concentra-tions (e.g. W126, Lefohn and Benedict, 1982) weresuccessful at capturing the variability in crop responseto ozone (Lee et al., 1988) and have been adopted by theforest research community in an attempt to relate ozoneexposure to observed injury. Throughout the UnitedStates, the indices that are routinely employed includeSUM0 (the sum of all hourly ozone concentrations in a14-h daytime period, expressed in ppb-h), SUM06 (thesum of daytime ozone concentration hours >60 ppb,ppb-h), SUM08 (the sum of daytime ozone concentra-tion hours >80 ppb, ppb-h) and W126 (an index derivedfrom sigmoidally weighting ozone concentrations, ppb-h). In Europe, the index AOT40 (the sum of all daytimeozone concentrations above 40 ppb) has become thestandard monitoring metric. Although the role of sto-matal conductance in influencing the effective dose ofozone is generally recognized (Hogsett et al., 1989;Runeckles, 1992), the physiological activity of the planthas not been explicitly incorporated into ozone exposureindices in either the United States or Europe. Stomataare the entry point for ozone into the leaf. Most damageoccurs once ozone gets