Flora - Morphology, Distribution, Functional Ecology of Plants
The effect of HNO3 gas on the lichen Ramalina menziesii
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.
Section snippets
Collection and transplanting
R. menziesii thalli (vegetative bodies) were collected at the University of California Sedgwick Reserve, near Santa Ynez, California, from abundant populations on Quercus douglasii Hook. & Arn branches; all collections were made within a 150-m radius of one another. Lichen thalli were collected after more than a week of dry weather, and transported in paper bags to the lab on the same day. Five randomly selected thalli were placed on Q. douglasii branches in each treatment chamber, with no more
Results
Overall in both experiments R. menziesii thalli treated with HNO3 showed visual signs of bleaching and by the end of day 28 were clearly damaged, if not dead. Thalli became brittle and yellowish brown, in contrast to control treatments where thalli remained pale yellow-green and were supple when wet.
Discussion
Our results show unequivocally that HNO3 is toxic to R. menziesii, albeit at somewhat higher levels than typically encountered in the LA air basin. While HNO3 concentration levels varied among chambers and treatments from levels commonly encountered in summer months to levels only found in extreme cases, ultimately all thalli treated with gaseous HNO3 declined physiologically. Decreasing respiration and photosynthesis indicate that both algal and fungal cells sustained damage. This damage may
Acknowledgments
The authors are grateful for the financial support of the US E.P.A.'s STAR fellowship to the senior author and the US National Forest Service Forest Health and Monitoring Program. We would also like to acknowledge Dr. Robert Heath for his continuous patience and generosity, David Jones and Phil Dawson for their technical assistance, and the UCSB Sedgwick Reserve for their cooperation and support.
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