The Fate of Ozone at a Ponderosa Pine Plantation: Partitioning Between Stomatal and Non-stomatal Deposition Using Sap Flow and Eddy Covariance Techniques

Major advances in quantifying ozone deposition to vegetated ecosystems have been made using above-canopy techniques -- such as eddy covariance -- that allow for the direct measure of ozone flux into natural systems. However, from above-canopy flux measurements alone, it is impossible to differentiate between deposition through stomatal openings of trees versus non-stomatal surfaces or within canopy chemical loss. Therefore, there is a need to partition ozone fluxes into plant stomatal and non-stomatal components. Sap flow measurements provide a direct measurement of stomatal conductance from which we can infer ozone uptake by trees: this represents a novel way to determine pollutant loading on stomatal surfaces of trees that is inexpensive, reliable, and can be deployed in a multitude of environments. Sap flow measurements were used to determine ozone uptake by ponderosa pine trees in the Sierra Nevada Mountains year-round starting in June 2000 at Blodgett Forest, an Ameriflux site located ~75 miles downwind of Sacramento, CA. Concurrently, total ecosystem ozone flux was measured using eddy covariance. Mean total ozone flux to the ecosystem was 46.6 μ mol m^-2 h^-1 (+/-15.1) in summer 2000, 27.6 μ mol m^-2 h^-1 (+/-14.2) in fall 2000, 8.2 μ mol m^-2 h^-1 (+/-5.1) in winter 2001, and 21.1 μ mol m^-2 h^-1 (+/-11.6) in spring 2001. Mean ozone flux through the stomata was 14.6 μ mol m^-2 h^-1 (+/-4.1) during summer 2000, 12.9 μ mol m^-2 h^-1 (+/-5.8) during fall 2000, 5.6 μ mol m^-2 h^-1 (+/-2.8) during winter 2001, and 12.7 μ mol m^-2 h^-1 (+/-3.7) during spring 2001. The percentage of total ozone deposition which occurred through the stomata was 31% in summer, 47% in fall, 69% but highly variable in winter, and 60% in spring. The difference between total ozone flux to the ecosystem and stomatal ozone flux to the trees varied exponentially with air temperature, suggesting that much of the non-stomatal deposition was actually due to chemical loss either on surfaces or within the canopy. The influence of biogenic VOC and/or NOx emissions in controlling the non-stomatal ozone deposition will be further explored.