Response of epiphytic macrolichen communities to N deposition across major forested eco-regions of the U.S.
To assess ecological effects of nutrient nitrogen (N) deposition under a changing climate, carefully selected indicators are needed. Long regarded as sentinels of air pollution, epiphytic macrolichens co-evolved with other flora and fauna in US forests under specific climate and nutrient regimes. They are important to nutrient cycling (cyanolichens), provide forage and nesting materials for birds and mammals (forage lichens), and provide habitat for invertebrates (matrix lichens) in simple and interconnected food webs. Many are sensitive to small increments of nitrogen deposition and small changes in climate. Thus early shifts in community composition can adversely affect ecosystem health and services. From 1992-2012, the US Forest Service conducted baseline lichen surveys at ~9,000 sites across all major forested ecoregions of the US following the Forest Inventory & Analysis lichen indicator protocol. Here we, 1) calculate nitrogen deposition response curves for western and eastern lichens as probability of detection vs. CMAQ-modeled total N deposition, 2) interpret response curves in relation to ecosystem services, 3) model community response to deposition by averaging the peak detection deposition values of the lichens at each site, accounting for climate, forest type and acidic deposition, and 4) propose lichen critical loads of nutrient N by ecoregion.
Of 520 species detected, 275 western and 180 eastern species yielded unambiguous response curves. 30% -42% were oligotrophic, meaning detections decreased with every increment of N from 1.2- 4.2 kg N/ha/yr in the west and east, respectively. Mesotroph detections peaked from 4.3-12.9 kg and eutrophs increased to maximum west and east deposition loads of 13 - 18 kg N/ha/yr. 100% of abundant forage species, 83-100% of common forage species, 47-100% of common cyanolichens, and 31- 43% of matrix lichens were oligotrophic in the west and east, respectively, highlighting the air pollution vulnerability of the ecosystem services provided by these lichens. Because eutrophs are generally more stress tolerant than oligotrophs, oligotroph:eutroph abundances decreased with increasing temperature or climatic moisture deficit, even with no increase in N deposition, highlighting the vulnerability of the same ecosystem services to climate change. Lichen-community N critical loads were 1-4 kg/ha/yr depending on ecoregion. We recommend holding the CL constant and allowing the response threshold to vary with climate, acidic deposition, and forest type. Stabilization of climate and minimal air pollution are prerequisites for optimizing forest resiliency and sustainability and for avoiding losses in biodiversity and functionality of ecosystem components.