SENSITIVITY OF SEEDLINGS OF BLACK-CHERRY (PRUNUS-SEROTINA EHRH) TO OZONE IN GREAT-SMOKY-MOUNTAINS-NATIONAL-PARK .1. EXPOSURE-RESPONSE CURVES FOR BIOMASS

The response of seedlings of black cherry (Prunus serotina Ehrh.) to o zone was evaluated in Great Smoky Mountains National Park using open-t op chambers during the growing seasons of 1989 and 1992. Two separate sets of seedlings were each exposed to various concentrations of ozone (charcoal-filtered; 0.5x (not used in 1989), 1.0x, 1.5x, and 2.0x mod ified ambient) in two different seasons. Seasonal indices of exposure (SUM00, SUM06 and AOT40) for the 1.0x treatments were 39.2, 1.9, and 1 .62 ppm h respectively, in 1989, and 63.1, 0.9, and 0.78 ppm h, respec tively, in 1992. No significant chamber effects were noted, except for reduced height growth in open plots compared with 1.0x chambers in 19 92. In both years, the 2.0x treatment reduced total, leaf, root, and s hoot + root biomass, although some of these changes were only marginal ly significant in 1992. Stem biomass was significantly reduced in 1989 , but not 1992. Leaf area, count and weight were all highly correlated , and showed significant reductions in both years. The leaf area ratio (leaf area/total weight) was reduced in 1989, but not in 1992. Height was not affected by ozone, but diameter was reduced only in 1989. Cha mber-to-chamber variation for biomass and leaf variates was greater in 1992, and as a result, significance levels were lower. Weibull functi ons were fitted to chamber means, and showed significant near-linear d eclines for most components when log-transformed data were plotted aga inst the SUM06 and AOT40 indices. Individual Weibull models for the 19 89 and 1992 data sets, and combined models over both years, were devel oped. Combined models were adequate for describing ozone responses for all biomass components, as determined by the likelihood ratio test. T he data show that the two years of exposure produced similar, but not identical effects, despite large differences in initial size of the se edlings and in seasonal ozone dynamics. Leaf and root biomass were mos t sensitive to ozone (as determined by the slope of decrease with incr easing SUM06), whereas stem biomass was least sensitive. Black cherry seedlings are shown to be among the most sensitive to elevated ozone o f the 21 tree species examined to date in Great Smoky Mountains Nation al Park.