Data from the Forest Inventory and Analysis Program were used to determine three empirical critical loads (CL) of nitrogen (N) deposition for epiphytic macrolichens in the Sierra Nevada. A CL for a biological receptor, like lichens, is the loading of a pollutant below which no unacceptable harm is known to occur. While defining unacceptable is indeed subjective, CLs and the response curves they’re derived from help us to quantitatively link pollutant levels with ecological impact for use in air quality regulation and monitoring. Measurements of N in canopy throughfall (n = 11) were regressed against two types of lichen-based response, N accumulated in the lichen Letharia vulpina ((L.) Hue) and abundance of N sensitive and N tolerant lichen species. Letharia vulpina was collected extensively (n = 535) from forests spanning heavily polluted to clean (e.g. remote) sites in the Pacific Northwest, allowing us to determine the natural range of N accumulation in this species. We used the uppermost limit (100% quantile) of expected % N in collections from clean sites to calculate a “clean site threshold” of 1.0% N. This threshold was used to define the most conservative CL (3.1 kg N ha^-1 yr^-1; 95% confidence interval: 0.70 to 5.5 kg ha^-1 yr^-1) within the series of three.

Results/Conclusions

While enhanced N accumulation is not necessarily harmful to L. vulpina, our data suggest that management according to the clean-site CL would preempt major loss of N-sensitive lichens known as acidophytes. This was determined by fitting the CL to a simple linear regression (SLR) between throughfall N and the proportional contribution of acidophytes to lichen abundance in the community (r² = 0.64; p = 0.0019). This ecologically significant group includes the ‘beard lichens,’ which are utilized by wildlife for forage and nesting material in the Sierra Nevada. With SLR we identified 2 additional CLs corresponding with major shifts in community composition. At a fairly moderate N-loading (5.2 kg N ha^-1 yr^-1) communities will likely shift from acidophyte dominance to dominance by N-tolerant species known as neutrophytes (95% inverse prediction interval: 4.0 to 6.4 kg N ha^-1 yr^-1). At a more extreme N loading of about 10.2 kg N ha^-1 yr^-1, our data predict complete extirpation of acidophytes from the lichen community (95% inverse prediction interval: 7.7 to 12.7 kg N ha^-1 yr^-1).