FOLIAR INJURY RESPONSES OF PONDEROSA PINE-SEEDLINGS TO OZONE, WET AND DRY ACIDIC DEPOSITION, AND DROUGHT

A 3-year field study of the potential interactions of ozone (O3), wet and dry acidic deposition, and soil water availability on foliar injury responses of 18 ponderosa pine Pinus ponderosa Laws.) families was conducted in the Sierra Nevadas of California. Thirty-six 2-year-old seedlings in each of 30 open-top chambers (plus six ambient air plots) were exposed to combinations of three levels of O3 [charcoal-filtered (CF), non-charcoal filtered (NF), and NF plus 1.5 times ambient O3 (NF150)]; three simulated rain pH treatments (pH 3.5, 4.4, 5.3); two levels of dry deposition (60% filtration of ambient dry deposition and 90% filtration), and two irrigation regimes (approximately weekly watering vs irrigation every other week) for three growing seasons. One-third of the trees were harvested at the end of each year. O3, irrigation level, amount of dry deposition, and family (genotype) significantly affected degree of foliar injury responses to O3. The interactions of O3 with irrigation amount and O3 with dry deposition were also statistically significant. Drought-stressed seedlings had significantly less O3 injury than well-watered trees, but seedlings exposed to 60% filtration of dry deposition had significantly greater O3 injury than those in the 90% filtered treatments. Ponderosa pine families differed greatly in susceptibility to O3, ranging from two with an average of > 20% O3 injury to several with nearly no O3 injury. These results reflect the complex patterns of ponderosa pine responses to natural and pollutant stresses and emphasize the importance of long-term, multifactorial experiments to elucidate those patterns.