Growth responses of field-grown loblolly pine to chronic doses of ozone during multiple growing seasons

Dose–response relationships were developed for ozone and four full-sib families of Pinustaeda L. Seedlings were planted in field plots in open-top chambers near Raleigh, North Carolina, and exposed daily during three growing seasons (1985, 1986, and 1987) to ozone at concentrations from 0.022 to 0.092 μL/L of air (seasonal mean concentrations for the daily exposure period 08:00 to 20:00 eastern standard time). Plants were harvested on five dates during the study to quantify effects of ozone on growth. Ozone suppressed stem height, root collar diameter, total branch length, and (or) dry weights of above-ground parts of plants in three families, but response to ozone depended upon dose, family, and the plant part measured. One family exposed during 1985 and 1986 did not exhibit significant growth responses to ozone. The family that exhibited the greatest growth suppression in 1985 and 1986 was exposed during the 3rd year, and ozone continued to reduce growth. Suppression of root dry weight was observed after three seasons of exposure. Root collar diameter and dry weight of stem + branches (without needles) may be the most useful measurements of growth response in multiple-year experiments. Dose–response models predicted that ambient levels of ozone could reduce growth relative to the growth predicted for chronic exposure to half-ambient levels (charcoal-filtered air). For aboveground woody tissue, this suppression ranged from 0 to 19% among the four families after two seasons of exposure and was 13% for the most sensitive family after three seasons.