Investigation of ozone stomatal flux in a northern mixed hardwood forest during the 1999 to 2004 growing seasons

Abstract

Hourly ozone stomatal fluxes (Fs_O3) were calculated for the University of Michigan’s Biological Station (UMBS) forest in northern Michigan from June 15 to September 15 for each year from 1999 to 2004 based on a set of equations derived from a resistance analog model. Measurements key to the calculations include latent and sensible heat flux, wind speed, ambient ozone (O3), and leaf air index. These measurements were taken primarily at the 46 m height of the UMBS Flux Tower on site with the exception of ambient O3, taken at 35 m on the PROPHET Tower. The calculated Fs_O3 were used to investigate the effect O3 exposure might have on Net Ecosystem CO2 Exchange (NEE). Analysis of NEE and Fs_O3 required consideration of several variables known to significantly affect NEE, including photosynthetic photon flux density (PPFD), vapor pressure deficit (VPD), soil moisture, and air temperature. Statistical analyses based on these variables consistently produced results indicating that a statistically significant positive relationship exists between NEE and Fs_O3. These results are unexpected as Fs_O3 is expected to be negatively correlated with NEE as a result of pant damage from O3 exposure, but may simply reflect the strong positive relationship known to exist between NEE and stomatal conductance.