FOLIAR METABOLIC HEAT RATE OF SEEDLINGS AND MATURE TREES OF PINUS-PONDEROSA EXPOSED TO ACID-RAIN AND OZONE

We investigated the effects of acid rain and ozone on respiration rate s of 1-year-old and current-year foliage of half-sib seedlings and mat ure clones of a ponderosa pine genotype by measurement of foliar metab olic heat rates, Two rain regimes (pH 5 . 1 and 3 . 0) were applied we ekly to foliage only, from January to April 1992, Two ozone regimes (a mbient and twice-ambient) were applied from September 1991 to November 1992, Metabolic heat rate was measured in April on 1-year-old foliage , in June on both 1-year-old and current-year foliage, and in November on current-year foliage in 1992, Except for current-year foliage in J une, the metabolic heat rate was calculated per unit of both foliar dr y mass and N mass, In seedlings, both measures of metabolic heat rate increased in late June for 1-year-old foliage exposed to twice-ambient ozone, and in November for current-year foliage exposed to the combin ation of twice-ambient ozone and pH 3 . 0 rain, In mature trees, metab olic heat rate was not affected significantly by ozone, rain acidity, or their interaction, In June, when both 1-year-old and current-year t issues were examined, the metabolic heat rate of expanding, current-ye ar foliage was higher than that of fully expanded, 1-year-old foliage regardless of plant age or treatment combination.