The photochemical oxidant ozone (O3) and pollutants such as sulfur dioxide (SO2) have been shown to damage plants (Kita et al. 2000; Potter et al. 2002; Ashmore 2005). However, the combined effects of pollutants, CO2 levels, temperature, and changes in precipitation are not mechanistically well understood (Kirschbaum 2004; Ashmore 2005; DeLucia et al. 2000, 1994). In localized studies, higher levels of O3 and other pollutants were associated with insect-related disturbances (Jones et al. 2004), suggesting an increase in pollutant-related stress increases the likelihood of occurrence of other stressors. O3 was also found to interact with frost (Oksanen et al. 2005), increasing the negative effects of frost on pigment loss and stomatal conductance. Integrating O3 with CO2, temperature, and precipitation changes within models, results in varying productivity predictions (Hanson et al. 2005). Single factor studies may help us understand mechanisms but they may not be useful in determining the magnitude of long-term ecosystem responses, since combined factors may cancel or compound each other (we discuss this concept in depth in a subsequent section). There is no doubt that the increase in atmospheric O3 will modify the response of forest to elevated CO2, temperature, precipitation, and radiation. But few multifactor experiments currently exist and our modeling efforts are bound by our understanding of the interactions.
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