Disentangling Plant Interactions with Tropospheric Ozone

G-02: Ozone and Forest Health

 

Photo by Marco Simola for Center for International Forestry Research (CIFOR).
Photo by Marco Simola for Center for International Forestry Research (CIFOR).

Tropospheric ozone (O3) is a criteria pollutant that is formed by the reaction of sunlight with ozone precursor molecules such as NOx and VOCs. While the formation of ozone is a natural process, anthropogenic production of ozone precursors through combustion and industrial processes can increase the concentration of O3 in the troposphere.

Andrzej Bytnerowicz summarized the trends in O3 concentrations over the past few decades in North America.   He showed that, there, concentrations of O3 have drastically decreased over the past 10-20 years due to the implementation of strict control measures, but intercontinental transport of O3 from developing countries was contributing to higher concentrations on the west coast. Bytnerowicz presented some case studies illustrating that concentrations of O3 can be extremely variable in space and time and it is still common for O3 to exceed air quality standards. Because of this variability, predicting future trends will be difficult.

Increased ozone concentrations are not only bad for humans, but also for plants. When plants open their stomata to uptake CO2, they can also uptake O3 which can lead to chlorotic mottling, decreased levels of photosynthesis, and a general decline in tree health. In the context of climate change, there is increasing interest in discerning not only how trees respond to ozone, but also what the combined effects of increased ozone with other climate change scenarios will be.

Rainer Matyssek investigated how different tree species respond to ozone in combination with pathogens, drought, nutrient deficiency, altitude, and increased NO2 or CO2 concentrations, in a series of controlled experiments. He found that genotype was an important driver of plant responses and that those responses were species specific. He found a high degree of plasticity within plant species, making it very difficult to determine the mechanisms responsible at the plant-level.

Similarly, Jeffrey Chieppa examined the combined effects of O3 and a fungus on southern pine in the southeastern United States. He studied two groups of southern pine – one that was resistant to the fungus and one that was susceptible – and subjected these groups to different concentrations of O3. Like Matyssek, he found that genotype was important. Trees that were susceptible to the fungus were also more susceptible to O3 damage.

Due to the complexities of trying to understand the effect of O3 on forest health at the plant-level, Matyssek stressed the need to step back and approach the issue from a larger-scale. Once there is an understanding of the effects of ozone at the ecosystem scale, these results can inform plant-level research. He warned that using the current bottom-up approach, it is easy to get lost in the details, and difficult to form a processed-based understanding of ozone’s effects on plants.

It is quite evident from the collective research in this session that trees are being negatively affected by ozone, but perhaps the approach that is being taken to evaluate these effects needs to be reconsidered.

 

Written by: Allison Chan

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