The tipburn symptom, especially in white pine, usually affects all needles in a fascicle equally. Young, rapidly growing needles that are directly exposed to sunlight are the most sensitive, but older needles can exhibit mottle and premature senescence as a result of prolonged low dose exposures. As is true of broadleaf species, symptoms on plants within a single conifer species can vary considerably. Ozone-sensitive plants exhibit premature defoliation of older needles, general needle chlorosis, and needle shortening. Such trees, known as chlorotic dwarfs, have thin chlorotic crowns and the foliage has a tufted appearance.
Black cherry appears to be another good indicator of ozone.
Other forest species that may serve as bioindicators of ozone pollution are blackberry, milkweed and sorrel. Blackberry and sorrel exhibit a dark purple-red stippling of the upper leaf surface that often coalesces over most of the leaf surface. The symptom on milkweed is similar to that on blackberry, except that the colouration is purple-black. Plants often respond to different stresses in a similar fashion. Several incitants other than air pollution are known to cause symptoms similar to those induced by O 3 .