Ponderosa pine is one of the most sensitive species to ozone and a dominant species in the Sierra Nevada Mountains. To assess the factors controlling ozone deposition to the forest and to study the forest response, we established a research site in a ponderosa pine plantation ~75 km northeast of Sacramento. Ozone concentration and ozone flux, along with relevant environmental variables, were measured from June to September in 1997 and from May to November in 1998. Summer of 1997 had very low soil moisture and an early budbreak, while summer of 1998 had very high soil moisture and later budbreak. Soil moisture and vapor pressure deficit exerted a strong control on ozone deposition in the dry year (1997), but the relationship was less clear in the wet year (1998). During the dry year ozone concentration and flux became decoupled owing to stomatal closure, but this did not occur explicitly in 1998. Phenology also proved to be important in controlling ozone deposition. Early in summer 1997 cumulative ozone flux was 50% higher than that of 1998: the difference can be attributed to the late budbreak in 1998. Further, the highest ozone deposition velocity in both years occurred 3--4 weeks after budbreak. Total cumulative ozone flux during the summer was 6% lower in 1998 due mainly to later budbreak even though drought stress reduced ozone flux late in summer 1997. Our results show that interannual climate variability impacts temporal patterns, physiological controls, and magnitude of ozone deposition to sensitive Mediterranean-type ecosystems. ¿ 2000 American Geophysical Union