Graduate Thesis Or Dissertation

Yosemite region nitrogen deposition and patterns in the composition of lichen communities

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  • This dissertation describes patterns in epiphytic macrolichen community composition along a nitrogen deposition gradient in the Yosemite region of the Sierra Nevada in California. This region is part of the largest federal air quality Class 1 designated air-shed in the western United States, covering the entire crest of the Sierra Nevada, and is protected under the Clean Air Act. Aside from the westernmost foothills, most of the region is federally designated Wilderness and has been subjected to few direct human impacts. The main air pollutants are nitrogen compounds which are carried eastward by prevailing winds from emission sources in the Central Valley. For more than a decade, measurements of nitrogen deposition at the Yosemite National Atmospheric Deposition Program station have been near or above the published lichen critical load; the quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge. A California-wide deposition model predicts that critical load exceedance has become the norm throughout the western slope of the central and southern Sierra Nevada. The principal objectives of the study were to determine the threshold response level of epiphytic lichen communities to nitrogen deposition and to map where that deposition level is exceeded in the Yosemite area. I measured throughfall nitrogen deposition at 12 sites for one year and related it to the accumulation of nitrogen in the thalli of the lichens Letharia vulpina, Letharia columbiana, and Evernia prunastri. I then measured the accumulation of nitrogen by lichens at nearly 300 additional locations throughout the study area to calibrate a model for lichen nitrogen accumulation at the landscape level. I extrapolated the calibration model to estimate N inputs at a high level of resolution throughout the study area. In 38 intensive study sites along the nitrogen deposition gradient, I conducted detailed assessments of the composition of epiphytic macrolichen communities to evaluate the apparent responses to elevated nitrogen deposition. I combined the data from two independent sampling techniques: a standard, semi-quantitative, stand-level method; and a new quantitative, branch-level assessment method to determine a lichen critical load for the coniferous forests in the study area. Concurrent with the branch lichen community assessment, I measured the pH and conductivity of the branch bark to address persistent questions in the literature about the role of the ionic environment of the substrate in mediating the response of lichen communities to nitrogen deposition. Both lichen community assessment methods revealed similar trends in the composition of lichen communities along the nitrogen deposition gradient. Lichen communities exposed to low N deposition levels were dominated by species adapted to low nitrogen inputs and had a low species richness and abundance of lichen eutrophs. Exposure to elevated nitrogen deposition was associated with lichen communities that were increasingly eutroph-dominated. There was weak evidence for a decline of lichen oligotrophs on conifer branches with increasing nitrogen deposition. Lichen substrate pH and electrical conductivity were not correlated with nitrogen deposition, and the abundance of individual lichen species, including eutrophs, was not correlated with substrate pH or conductivity. Therefore, the increase in eutrophs was likely a direct consequence elevated nitrogen deposition. The lichen community work demonstrated that lichen thallus nitrogen concentrations standardized to Letharia vulpina equivalents exceeding 1.1% dry-weight were associated with major changes in the composition of lichen communities. This corresponded to 2.9 kg inorganic nitrogen ha⁻¹ yr⁻¹ (wet + dry); the empirical critical load determined in this study. Throughfall deposition measurements from this study, and deposition measurements at the Yosemite National Atmosphere Deposition Program station verify that nitrogen deposition reached or exceeded that critical load at 12 locations in the study area. The lichen thallus nitrogen accumulation model, which included hundreds of local calibration points, suggested the critical load is exceeded in at least 68% of the study area, including more than 70% of the area designated as federal air quality Class 1 air-sheds.
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