Multi-year Measurements of Stomatal and Non-stomatal Fluxes of Ozone to a Northern Mixed Hardwood Forest

Measurements of ozone, sensible heat, and latent heat fluxes, as well as relative humidity, temperature, pressure, wind speed, leaf area index, ambient ozone, and plant physiological parameters were made at a northern mixed hardwood forest located at the University of Michigan Biological Station in northern Michigan from June 27 to September 28, 2002; August 7 to October 10, 2003; May 22 to October 16, 2004; and June 12 to September 1, 2005. An analog resistance model was used to calculate aerodynamic, boundary layer, and canopy surface conductances for water vapor and ozone, and ozone canopy conductance was partitioned into stomatal and non-stomatal components. Analysis was limited to the period when leaves were fully grown, but not yet senescing: June 15 to September 15, according to LAI measurements. For each year, data available within this time frame was binned by hour-of-day and averaged. Mean daytime (0800-0200h) ozone canopy conductance showed little year-to-year variability: 0.39 mol m-2 s-1 (2002), 0.41 mol m-2 s-1 (2003), 0.52 mol m-2 s-1 (2004), and 0.43 mol m-2 s-1 (2005). Stomatal conductance showed expected patterns of behavior with respect to photosynthetic photon flux density (PPFD) and vapor pressure deficit (VPD), and mean daytime stomatal ozone conductances were: 0.17 mol m-2 s-1 (2002), 0.33 mol m-2 s-1 (2003), 0.40 mol m-2 s-1 (2004), and 0.20 mol m-2 s-1 (2005). Ozone non-stomatal conductance increased monotonically with increasing PPFD, and increased with temperature before falling off again at high temperature. Mean daytime non-stomatal ozone conductances accounted for as much as 61% (2002), 31% (2003), 36% (2004), and 57% (2005) of canopy conductance. Total ozone flux varied diurnally, with downward flux reaching -100 μ mol m-2 h-1 at midday, and at or near zero at night. Estimated daily average stomatal ozone burden (flux) was 2.9 x105 nmol m-2 (2002), 5.6 x105 nmol m-2 (2003), 6.6 x105 nmol m-2 (2004), and 4.1 x105 nmol m-2 (2005), with the non-stomatal partition representing 59% (2002), 26% (2003), 35% (2004), and 46% (2005) of the total flux. Non-stomatal ozone conductance was neither constant nor low. While this study leaves unanswered questions as to the mechanisms of non-stomatal conductance (non-stomatal ozone conductance may be occurring on surfaces or in gas-phase), it is important in the formulation of mechanistic models of ozone fluxes to quantify their magnitude and identify patterns with respect to major environmental factors in order to determine impacts of tropospheric ozone in forest ecosystems.