Monitoring Nutrient Nitrogen Effects
The National Forest Exceedance Table displays exceedances of empirical CLs of nutrient N for several receptors and responses (fungi, lichens, herbaceous plants and shrubs, forests, and nitrate leaching) but the lichen receptor is generally considered the most sensitive to nitrogen pollution. Excess nitrogen causes acidification and/or eutrophication of the environment, shifts species composition within plant taxa, decreases biodiversity, increases fire frequency (e.g., favoring grasses over shrubs), and decreases growth and reproduction of sensitive species. Lichen species composition begins to shift at or below deposition ranges that affect the most sensitive species in other floral taxa (Geiser et al. 2010). Thus exceedance of lichen-based N critical loads is a reasonably conservative indication that adverse effects may already have occurred, or may eventually occur, to other plant and fungal taxa. For this reason it makes sense to consider improving the empirical CLs of nutrient N for lichens before other receptors, assuming that exceedances occur in communities known to support lichens; grasslands, rangelands, wetlands, and alpine areas are not known to support abundant fruticose and foliose lichen communities and can therefore be removed from the lichens CL exceedance assessment. In addition, protocols are available for measuring community structure and nitrogen content of lichen tissue (Forest Inventory and Analysis Protocol for Lichen Community Analysis, Region 6 protocol for lichen elemental analysis), and databases exist to store the collected information. Any forest considering lichen monitoring should coordinate with Linda Geiser, lichenologist with the Air Program in Region 6, to develop a plan that fits into national efforts underway to improve these empirical CLs of nutrient N for lichens.
Another national effort is underway to refine the other empirical CLs of nutrient nitrogen using available land cover GIS layers. Linda Pardo, research scientist with the FS Northern Research Station, is coordinating this effort. A more immediate way to refine the empirical CLs of nutrient nitrogen for your forest is to work with a research scientist to conduct an N-addition or N-gradient study for a receptor of interest. This would refine the empirical N CL range, currently set for an entire ecoregion, to be specific to your area and therefore highly reliable when calculating CL exceedance and predicting negative ecosystem effects.