Acidic atmospheric deposition interacts with tree type and impacts the cryptogamic epiphytes in Acadia National Park, Maine, USA

Abstract

We examined plot-level interactions among nitrogen and sulfur deposition, tree type, and epiphytic diversity, biomass, and abundance in Acadia National Park (ACAD), Maine, that receives pollutant deposition and is of particular concern for biological conservation. Spruce appeared to acidify incoming deposition resulting in more concentrated inputs of nitrogen and sulfur within the canopy as well as to the soil relative to maple. This resulted in more acidic and sulfur-rich substrates and reduced biomass and species richness for cryptogamic epiphytes on spruce relative to maple. Position vertically on the tree bole was more important for epiphytes on maples than on spruces because spruces were uniform in their bark chemistry. Apparent overlap between the bark chemistry of spruce and maple, particularly for samples from higher on maple boles, suggests a reduction in the area of chemically suitable substratum for epiphytes in ACAD. Because tree species vary markedly in habitat variables important to epiphytes, stands with a mix of deciduous and evergreen trees provide the widest gradient of habitats, which appears to be reflected in the highest species richness for epiphytes occurring in mixed stands. Air quality and forest health monitoring studies utilizing epiphytes as indicators should account for the complex effects of stand-level tree species composition to better detect biological effects of deposition at the landscape scale.