LONG-TERM EFFECTS OF OZONE AND SIMULATED ACID-RAIN ON THE FOLIAGE DYNAMICS OF SLASH PINE (PINUS-ELLIOTTI VAR ELLIOTTI ENGELM)

Seedlings from four half-sib families of Pinus elliottii var. elliottii Engelm. were planted in open-top chambers, and exposed to ozone and simulated acidic precipitation. Treatments were applied for 11 months per year for 28 months. The ozone concentrations were: charcoal filtered air (CF), nonfiltered ambient air (1 x), twice the nonfiltered level (2 x) and three times the nonfiltered level (3 x). Ozone exposures were computer controlled to follow natural diurnal fluctuations in ambient ozone concentrations. Simulated acid rain at pH 3.3, 4.3 or 5.3 was delivered weekly at the same volume as average weekly rainfall to all chambers. Leaf area, bud number and fascicle survival were all highest in the CF chambers. Genetic differences between half-sib families were apparent in both leaf area and the number of live buds. Leaf and fascicle survival were reduced with every increase in ozone exposure from CF to 3 x. Leaf litterfall increased significantly with increasing ozone exposure. Leaf growth, measured by mean leaf area and mean dry weight per needle, was not affected by ozone exposure or by acid rain. It was concluded that ozone reduced leaf area mainly by causing premature leaf senescence. The only significant effect of acidic precipitation was enhanced leaf retention in trees irrigated with pH 3.3 solution. The results indicate that the increasing levels of trophospheric ozone may significantly reduce the growth of slash pine through foliage loss. The increased leaf retention caused by acid rain is unlikely to compensate for the detrimental effect of ozone.