The effect of HNO₃ gas on the lichen Ramalina menziesii

Abstract

Nitric acid (HNO₃) and ozone (O₃), secondary products of photochemical reactions of nitrogen oxides (NO_x) and volatile organic compounds, are important pollutants in arid regions with large outputs from petrol combustion. In the Los Angeles (LA) air basin, nitrogen dry deposition rates in forests downwind of the urban areas can reach 35–40 kg ha⁻¹ year⁻¹, roughly equivalent to the amount of N used to fertilize agricultural fields. The marked decline in the lichen population of the LA air basin has previously been attributed to local O₃ concentration gradients, which overlaid the patterns of species extirpation. Recent research in the air basin has shown that nitrate (NO₃⁻) deposition gradients run parallel to the O₃ concentration gradient, and that deposition of NO₃⁻ and HNO₃ can have significant effects on forest health. Our research examines the effects of HNO₃ dry deposition on the lichen Ramalina menziesii Tayl. in an effort to understand the loss of lichen species in southern California, and increase the usefulness of lichens as biomonitors of nitrogen pollutants. We transplanted healthy R. menziesii thalli from a “pristine” location into fumigation chambers and exposed them to HNO₃ under humid and dry conditions, and moderate and high HNO₃ fumigations. R. menziesii thalli treated with HNO₃ in month-long fumigations experienced a significant decline in chlorophyll content and carbon exchange capacity compared to thalli in control chambers. Leachate conductivity, NO₃⁻ and K⁺ concentrations increased with HNO₃ fumigation levels and time. We conclude that R. menziesii has an unequivocally negative response to HNO₃ gas concentrations common to ambient summer conditions in the LA air basin.

Introduction

Just as canaries provide warnings of toxic gases to coal miners, so can the investigation of lichen communities provide information on potential deterioration of ecosystems stressed by air pollutants (Nash, 2008). Lichen species are well known to be differentially sensitive to air pollutants. The most sensitive species may become locally extirpated in urban areas or near industrial facilities, while a few very tolerant species will survive and even flourish. Except for SO₂, the mechanisms underlying this differential sensitivity are poorly understood.

In the case of southern California, we know that approximately half the epiphytic lichen species known to occur in the late 1800s and early 1900s (Hasse, 1913) have subsequently disappeared (Ross, 1982; Sigal and Nash, 1983). Twenty-five years ago the apparent cause of the lichen decline in the Los Angeles (LA) region seemed clear – namely oxidant air pollutants with an emphasis on ozone (O₃). O₃ is widely recognized as the major phytotoxic air pollutant in the United States (Lefohn, 1991) in general, and in the San Bernardino Mountains in particular (Miller and McBride, 1999). Along gradients of increasing oxidants, lichen communities on both conifers (Nash and Sigal, 1998) and oaks (Sigal and Nash, 1983) exhibited marked declines in species richness and relative health.

However, over the past decade, a previously unrecognized high N-deposition pattern has been documented with a gradient overlapping the O₃ gradient (Fenn and Bytnerowicz, 1993). In fact, it is suggested that the forests near LA are N-saturated (Breiner et al., 2007; Fenn et al., 1996) and the total N-deposition, variously estimated as up to 35–50 kg ha⁻¹ year⁻¹ (Bytnerowicz and Fenn, 1996; Padgett and Bytnerowicz, 2001), is nearly equal to the highest deposition rates observed in Europe (e.g. the Netherlands and northern Germany), but with one major difference. In southern California oxidized forms of N predominate, whereas in Europe reduced forms of N predominate (Bytnerowicz and Fenn, 1996). Major components of the southern California N-deposition components include two strong gas phase acids, nitric acid (HNO₃) and HNO₂. Of these two, the HNO₃ gas phase occurs in the highest concentrations, and it exhibits diurnal patterns that parallel that of O₃ (Seinfeld and Pandis, 1998). Field measurements of gaseous HNO₃ in the mountains downwind of LA have measured levels as high as 27.3 ppb (24-h average), whereas remote locations elsewhere may see levels in the range of 0.00025 ppb 24-h averages (avg.) (Bytnerowicz and Fenn, 1996). Unlike O₃, once HNO₃ is created, it no longer participates in atmospheric chemical reactions, and it rapidly deposits to exposed surfaces. Thus, our operational question is whether gaseous HNO₃ alone is sufficiently phytotoxic to contribute to the observed lichen decline in southern California. Herein, we report results of our initial experimentation with Ramalina menziesii Taylor, one of the most sensitive lichen species in the LA urban area. It is almost unknown today in this area (Ross, 1982; Mount Palomar to the south was the closest extant location in the late 1980s) but was abundant earlier (Hasse, 1913), as 35 herbarium specimens document its relative abundance across the LA basin in the early 1900s (Ross, 1982; Sigal and Nash, 1983). As far as we know, this is the first report on the effects of HNO₃ gas phase on any lichen and one of the first dealing with any organism.