Annual ozone deposition to a Sierra Nevada ponderosa pine plantation

Ozone concentration and ecosystem scale fluxes were measured continuously from June 1999 to June 2000 above a ponderosa pine plantation at Blodgett Forest, an Ameriflux site located B75 km northeast of Sacramento, CA (1300m). The ponderosa pine trees were most active during the summer but maintained a low level of activity during the fall, winter, and spring. Cumulative ozone flux for the year was 127mmolm 2 with the contribution for each season being 37% for summer, 18% for fall, 15% for winter, and 30% for spring. The high levels of cumulative ozone deposition over non-summer seasons indicate that significant ozone damage may occur during times when ozone concentrations are not at their maximum. Ozone flux is dependent upon both ozone deposition velocity (O3 Vd; how effective the ecosystem is at taking up ozone) and ambient ozone concentration but was found to be more closely related to O3 Vd than to ozone concentration. The relationships between O3 Vd (and therefore ozone flux) and the controlling climatic variables were dynamic over the year, changing mainly with water status and phenology. Understanding how the relationship between ozone deposition and its driving variables interact and change over the year is therefore critical to understanding potential ozone damage to vegetated ecosystems. Additionally, we found that commonly used ozone exposure metrics such as SUM0 (sum of all ozone exposure during the day) were poor predictors of ozone uptake (flux) unless periods of ecosystem stress, such as drought, were excluded. r 2002 Elsevier Science Ltd. All rights reserved.