Investigating the Effects of Nitric Acid on Ramalina menziesii Tayl.

Jennifer Riddell Arizona State University School of Life Sciences, Tempe
Pamela Padgett USDA F.S. Pacific Southwest Research Station, Riverside, CA
Thomas H. Nash III Arizona State University School of Life Sciences, Tempe


Lichens have been used as biomonitors of changes in air quality for over a century and a half. Naturalists in industrializing Europe noted the loss of lichen species and dubbed it the “city effect,” later connecting the disappearances with SO2 pollution. The differential responses that lichen species have to pollutants have allowed biologists to develop biomonitoring programs that use inventories of community composition in potentially effected areas and compare them to baseline data or predicted community makeup. Based on current knowledge, presence or absence of particular species can indicate levels of NH3 or SO2 pollution, but little is known about community responses to ozone and HNO3.

Surveys in the mountains of the LA air basin in the 1980’s showed that approximately 50% of the lichen species collected in the early 1900’s were extirpated, and many extant species had declined in abundance as well as showing signs physical damage. A 2004 resurvey in the San Bernardino Mts. showed further decline in the abundance of formerly tolerant species. The researchers who described this phenomenon found that ozone concentration gradients overlaid the patterns of species extirpation. More recent research in the air basin has shown that nitric acid deposition gradients run parallel to the ozone concentration gradient, and that nitrate and nitric acid can have significant effects on forest health. In the Los Angeles (LA) air basin, air quality has been a concern for more than 35 years, and nitrogen dry deposition rates in forests downwind of the urban areas can reach 35-40 kg ha-1 year-1, exceeding currently understood critical loads.

Our research examines the effects of nitric acid dry deposition on the lichen Ramalina menziesii Tayl. in an effort to understand the significant loss of species in southern California, and increase the usefulness of lichens as biomonitors of nitrogen pollutants.

We transplanted healthy R. menziesii thalli from the UCSB Sedgwick Reserve, north of Santa Barbara, CA, into constantly stirred fumigation chambers in a climate-controlled greenhouse at UCR, Riverside, CA, containing varying levels of gaseous nitric acid. Ramalina menziesii thalli treated with nitric acid in month-long fumigations show a marked decline in chlorophyll content and carbon exchange capacity compared to thalli in control chambers. The leachate conductivity, nitrate and potassium concentrations increased with nitric acid fumigation levels and time, while sodium content decreased with time, not exposure. We conclude that R. menziesii has an unequivocally negative response to nitric acid gas concentrations common to ambient summer conditions in the LA air basin. Our next line of investigation will examine the threshold concentrations of damage to lichen physiology using several species, as well as responses to factorial fumigations with O3 and HNO3.