Plants directly influence climate by controlling the movement of two powerful greenhouse gases, carbon dioxide and water, between the Earth?s surface and the atmosphere through the processes of photosynthesis and transpiration. Ozone (O3), a powerful air pollutant, damages plants such that the regulation of transpiration and photosynthesis is altered ultimately leading to changes in atmospheric carbon dioxide and water concentrations. This project will incorporate experimentally determined effects of O3 on plant photosynthesis and transpiration into a commonly used global climate simulation model developed by the National Center for Atmospheric Research (NCAR). This project will be the first to use a climate model to differentiate between responses of transpiration and photosynthesis to O3 exposure. Incorporating changes in both transpiration and photosynthesis will allow for more accurate representations of future changes to regional and global hydrology and climate. This will improve the predictive power of climate models and the understanding of global scale plant responses to O3. These improved predictions can be used to inform future air pollution and climate policy decisions.
Plants play a key role in regulating Earth’s climate by taking carbon dioxide (CO2) out of the atmosphere (photosynthesis) and releasing water into the atmosphere (transpiration). Carbon dioxide and water enter and exit the leaf through tiny pores (stomata) that are the common link controlling photosynthesis and transpiration. Chronic exposure to ground-level ozone, a toxic air pollutant, impairs the ability of most plants to exchange carbon and water with the atmosphere by changing the size of the stomatal opening (stomatal conductance) in addition to damaging biochemical aspects of photosynthesis. Since photosynthesis and stomatal conductance respond differently to chronic ozone exposure, we wanted to know how much each decreased over a range of ozone exposure levels and how the decreases changed carbon and water cycling through Earth’s ecosystems. To find out how both photosynthesis and stomatal conductance changed after ozone exposure, we collated responses for all types of plants from available published literature. We found that photosynthesis was reduced by 20% after exposure to ozone for more than seven days, regardless of ozone concentrations or length of exposure. Stomatal conductance was reduced on average by 8%, though reductions were smaller with higher concentrations and/or longer exposure times. We wanted to know how the changes in photosynthesis and stomatal conductance affected whole ecosystems, so we scaled the responses collated from published literature to the world using the Community Land Model (CLM), a global land model. On a global scale, the decrease in photosynthesis caused by ozone exposure reduced gross primary productivity (GPP), a measure of carbon stored in the biomass of ecosystem producers. The largest average decreases were in central North America and central Asia, where GPP decreased by 15% annually. Ozone also caused transpiration to decrease, though decreases were much smaller than in GPP. The largest decreases in transpiration were in central North America and central Asia, where average decreases were 9% annually. Though GPP decreased in most tropical locations by approximately 8%, there was no change in transpiration in these regions. Plants use water from the soil during the process of transpiration, so the decreases in transpiration also caused an increase in surface water runoff in many locations globally. Runoff increased up to 10% in the Mississippi River basin in North America and in a few locations in central Africa and Asia. Overall, the damage ozone causes to photosynthesis and transpiration lead to decreases in terrestrial ecosystem carbon storage and increases in surface water runoff, which can potentially result in increased flooding. The results of our work suggest that policies that aim to reduce ozone concentrations might help mitigate flooding potential in certain regions and can potentially increase carbon storage in ecosystems.
