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Responses of Epiphytic Lichens to Atmospheric Reactive Nitrogen – Development of the Critical Level Approach for the Western United States

Andrzej Bytnerowicz¹, Sarah Jovan², Haiganoush Preisler³ and Ricardo Cisneros⁴

Atmospheric reactive nitrogen (N_r), including nitrogen oxides (NO_x), nitric acid vapor (HNO₃), and ammonia (NH₃), can have direct phytotoxic effects on vegetation at highly elevated ambient concentrations. Elevated levels of Nr species also lead to increased nitrogen (N) deposition that influences growth and health of forest and other ecosystems. Direct phytotoxic effects of N_r on vascular plants are quite rare and occur only at very high ambient concentrations. In contrast, long-term exposures to elevated N_r and resulting N deposition are common and may result in serious ecological changes, such as promotion of growth of the nitrophyllous vascular plants at the expense of the oligotrophic ones and shifts in ecosystem species composition. Because of their direct uptake of nutrient from the atmosphere, epiphytic lichens proved to be highly sensitive to elevated N deposition which has been demonstrated by shifts from N-sensitive lichens to the dominance by N-tolerant and nitorphyllous species. Such responses have been described in several terrestrial ecosystems in the western US and have been used as a tool for determining critical loads of N deposition for protection of these ecosystems. In our approach we link changes in the epiphytic lichen communities directly to ambient concentrations of the main drivers of elevated N deposition, NH₃ and HNO₃. Statistical models that relate changes in the occurrence of the acidophyllous vs. nitrophyllous species to ambient concentrations of NH₃ and HNO₃ have been developed. These models have been used for evaluation of critical levels of these pollutants for the mixed conifer forests of the south-western Sierra Nevada in California. Similar efforts are also taking place in the mixed conifer forests of the San Bernardino Mountains of southern California and boreal forests in the Athabasca Oil Sands Region in Alberta, Canada.

¹ US Forest Service, Pacific Southwest Research Station, Riverside, CA;
² US Forest Service, Region 6, Portland, OR;
³ US Forest Service, Pacific Southwest Research Station, Albany, CA;
⁴ US Forest Service, Region 5, Clovis, CA