Response of a white clover indicator system to tropospheric ozone at eight locations in the United States

A white clover (Trifolium repens L.) system using measured biomass to indicate effective concentrations of tropospheric ozone (O-3) has been developed. The system utilizes the relative response of an O-3-sensitive clone (NC-S) and an O-3-resistant clone (NC-R) grown in 15-liter pots. Forage (leaves, stems and flowers) is cut, dried, and weighed at 28-day intervals. Forage dry weight ratios (NC-S/NC-R) for individual or multiple harvests indicate O-3 concentrations during growth. In, 3 years of testing in open-top field chambers at Raleigh. North Carolina, O-3 always decreased growth of NC-S more than that of NC-R and the NC-S/NC-R ratio routinely decreased as the O-3 concentration increased. A national field test was performed in 1993 and 1994 to determine if the clover system can account for effects of climatic variables on clover growth per-se, and if climatic variables affect the relative response of the two clones to O-3. Eight locations (Corvallis, Oregon; Kennedy Space Center, Florida; Delaware, Ohio; Amherst, Massachusetts; Blacksburg, Virginia; Raleigh, North Carolina; Riverside, California; San Bernardino mountains. California) provided large differences in O-3 concentrations and climate. The NC-SINC-R forage ratios for three consecutive 28-day manuscript. Ratios were generally highest where mean O-3 concentrations were lowest (Oregon and Florida), lowest where mean O-3 concentrations were highest (both California locations), and intermediate at other locations.