Other parameters

Other reviews and meta-analyses cover other parameters such as secondary metabolites and performance of insect herbivores. Valkama et al. (2007) indicated that effects of elevated O3

tends to increase tree foliage quality for herbivores, but elevated CO2 may alleviate these effects of O3. Finally, there is great interest in determining the consequences of enhanced CO2 and O3

concentrations on ecosystem effects like N cycling and carbon storage. After 11 years of

exposure of stands within the ‘aspenFACE’ study ecosystem C content was enhanced by 11% by elevated CO2 whereas it was decreased by 9% by elevated O3 (Talhelm et al., 2014). Thus, the authors concluded that the long-term effect on elevated O3 on net primary production (NPP) will be smaller than expected. There is some evidence that both CO2 and O3 have the potential to change litter production rates via affecting C assimilation and leaf senescence and this will

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significantly alter litter and chemical inputs into the soil (Kasurinen et al., 2007; Liu et al., 2005, 2007).

Kubiske et al. (2007) attended to shifts in the community composition and ascertained that elevated O3 hastened the conversion of aspen stands favouring paper birch and sugar maple, whereas the presence of elevated CO2 delayed it.

Major results: O3 x CO2

For O3-sensitive crop and tree species the predominant effect of simultaneously enhanced concentrations of CO2 is to mitigate adverse effects of O3 on growth and photosynthesis, while O3

is limiting some positive growth responses to elevated CO2.

The protective effect of elevated CO2 on O3-induced injuries becomes less pronounced the greater the O3 sensitivity of a species.

Depending on the concentrations used in the studies, the combined treatment may even result in a growth stimulation compared to the control with low O3 at ambient CO2.

Since CO2 and O3 generally affect physiology and biomass accumulation in an opposite way, the ameliorative effect of the gases in combination may exist even without a statistically significant interaction.

Decreasing the stomatal conductance plays a major role by which CO2 attained a mitigation of adverse O3-effects but enhancing the antioxidative capacity of foliage may be an additional factor.

Modifying effects of CO2 on plant’s response to O3 may also apply to edaphic processes, trophic systems, and species composition dynamics.

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