When Physiological Models Fail: Fixing The Ozone Oxidation Problem
The plant physiological processes of photosynthesis and transpiration control the transfer of carbon dioxide and water, two powerful greenhouse gases between the biosphere and atmosphere and are therefore important in regulating climate. The rates of these processes can significantly change when plants are chronically exposed to surface ozone (O3), a phytotoxic greenhouse gas that has globally increased in concentration over the past century. Since O3 is integral in altering plant interactions with the atmosphere, there is a strong motivation to incorporate it into large-scale models. However, current methods incorporating physiological responses to O3 assume that stomatal conductance changes linearly with photosynthesis despite empirical evidence suggesting otherwise. In chapter one, I developed a physiological modeling framework to modify Ball-Berry stomatal conductance predictions independently of photosynthesis and used experimental data to evaluate the results. The new model framework significantly improved the modeled ability to predict both photosynthesis and stomatal conductance responses to O3. The second chapter tests the physiological model framework on a global scale using the Community Land Model. I found that the new framework, which directly modifies conductance, reduces the effect of O3 on both transpiration and GPP on a global scale compared to the standard modeling method of indirectly modifying conductance by changing photosynthesis. To create a comprehensive dataset available for large-scale modeling, the third chapter compiles photosynthetic and stomatal responses to chronic O3 exposure through time using data from peer-reviewed literature. The results demonstrate that photosynthesis decreases more than stomatal conductance in many plant functional types, and O3 affects plant types similarly through time. The fourth chapter combines the new modeling framework from the first two chapters with data from the third chapter to predict global changes in GPP and transpiration due to O3 exposure. Through changing transpiration, O3 increases runoff in many temperate and boreal regions and decreases latent heat flux. This body of work ultimately allows for more accurate predictions of plant and climate interactions.
ozone; photosynthesis; transpiration; conductance; stomata; Ball-Berry; Community Land Model
Sparks, Jed P.
Hess, Peter George Mueller; Goodale, Christine L; Mahowald, Natalie M
Ph. D., Ecology
Doctor of Philosophy
dissertation or thesis