EFFECTS OF SIMULATED ACID-RAIN AND OZONE ON FOLIAR CHEMISTRY OF FIELD-GROWN PINUS-PONDEROSA SEEDLINGS AND MATURE TREES

We investigated the additive and interactive effects of simulated acid rain and elevated ozone on C and N contents, and the C:N ratio of one -year-old and current-year foliage offield-grown mature trees and thei r half-sib seedlings of a stress tolerant genotype of ponderosa pine. Acid rain levels (pH 5.1 and 3.0) were applied weekly to foliage only (no soil acidification or N addition), from January to April, 1992. Pl ants were exposed to two ozone levels (ambient and twice-ambient) duri ng the day from September 1991 to November 1992. The sequential applic ation of acid rain and elevated ozone mimicked the natural conditions. Twice-ambient ozone significantly decreased foliar N content (by 12-1 4%) and increased the C:N ratio of both one-year-old and cuurent-year foliage of seedlings. Although similar ozone effects were also observe d on one-year-old foliage of mature trees, the only statistically sign ificant effect was an increased C:N ratio when twice-ambient ozone com bined with pH 3.0 rain (acid rain by ozone interaction). Enhancing the effect of twice-ambient ozone in increasing the C:N ratio of one year -old foliage of mature trees in June was the only significant effect o f acid rain.