ACID-RAIN AND OZONE EFFECTS ON GAS-EXCHANGE OF PINUS-PONDEROSA - A COMPARISON BETWEEN TREES AND SEEDLINGS

We studied the effects of increased rain acidity and ozone on net phot osynthesis (A), internal CO2 concentration (C-1), stomatal conductance (g(1)), and instantaneous water use efficiency (A/E) of tree clones a nd their half-sib seedlings of a ponderosa pine (Finns ponderosa Dougl . ex Laws.) genotype in the field. Branches of trees and whole seedlin gs were exposed to rain (pH 5.1 and 3.0) and ozone (charcoal filtered, ambient, and twice ambient) treatments using open-bottom, 1.5-m-long x 0.7-m-diameter chambers. The rain treatment was applied weekly from January 1992 to April 1992, to foliage only. The ozone treatment was a pplied from September 1991 to November 1992. Midday charcoal filtered, ambient, and twice-ambient ozone concentrations were 0.025, 0.038, an d 0.075 mu mol mol(-1) in February, peaked at 0.028, 0.058, and 0.115 mu mol mol(-1) in July, and decreased to the lowest values at 0.01, 0. 02 and 0.044 mu mol molin November, respectively. Monthly midday gas-e xchange was measured in 1992, from February to October on 1-yr old fol iage, and from June to November on current-year foliage. No significan t interactive effect of rain and ozone was detected. Twice-ambient ozo ne, when compared with the filtered ozone regime, decreased A of 1-yr- old foliage only in trees. This decrease in A was not related to g(1), which was unaffected by increased ozone. Rain with pH 3.0 decreased A /E of both current-year and 1-yr-old foliage only in seedlings. These differential effects indicate that (1) increased ozone can decrease tr ee productivity, while acid rain can adversely affect seedlings' natur al regeneration, and (2) extrapolation of results from seedling studie s may underestimate or overestimate the pollutant effects on trees dep ending on the plant response studied and pollutant considered.