GROWTH-RESPONSE OF YOUNG SLASH PINE TREES TO SIMULATED ACID-RAIN AND OZONE STRESS

To assess the impact of acid rain and ozone stress on the growth and physiology of slash pine (Pinus elliottii Engelm. var. elliottii), four half-sib families of slash pine were planted in large open-top chambers and exposed 28 months to factorial combinations of three rain acidities (pH 3.3, 4.3, and 5.3) and four concentrations of ozone (subambient produced with carbon filtration, ambient, two times ambient, and three times ambient). Growth was assessed by seasonal stem diameter and height increments and by stem volume increment at steady-state conditions. Information on tree physiology was obtained indirectly from estimates of mean unit leaf rate, a measure of the net assimilation capacity of the tree. Among the growth variables, only volume increment showed a consistent year to year response to acid rain and ozone. The acid rain x ozone interaction was significant for volume increment each year and was the result of trees in the two times ambient ozone level having greater sensitivity to rain acidity. Both acid rain and ozone significantly affected mean unit leaf rate; however, this effect weakened with time, suggesting some degree of physiological acclimation. Increasing rain acidity significantly increased mean leaf area only during the first growth period. Ozone significantly decreased mean leaf area all three growth periods, with the effect intensifying each year. Complex family interactions with acid rain and ozone masked any clear family sensitivities to these pollutants.