Key to Lichen Sensitivity Ratings

Nitrogen Requirement: O = oligotroph, M = mesotroph, E = eutroph


Oligotrophs are most frequently encountered where total nitrogen deposition is between 0.5 to 2.5 kg N ha-1 y-1. They are increasingly difficult to find as deposition increases above 2.5 kg/ha/y. Oligotrophs are generally adapted to acidic substrates in cold, high-precipitation, nutrient-poor, typically montane environments. Many of the species listed here as oligotrophic have also been categorized as 'acidophytes' by other researchers. However our ratings are based solely on nitrogen deposition and not on measurements of substrate or precipitation pH. Many of the large pendant, filamentous lichens in the genera Alectoria, Bryoria, Ramalina, and Usnea are oligotrophs. They achieve a rather large biomass in old growth forests where they play important functional roles as winter forage, nesting material, insect habitat, and help moderate humidity and nutrient deposition. Many of the largest, leafy lichens in the nitrogen-fixing genera Lobaria, Nephroma, Pseudocyphellaria, and Sticta are also oligotrophs. In old growth and riparian forests that are otherwise nitrogen-limited, they contribute fixed N and provide nutritious forage for insects and mammals.


Mesotrophs are most frequently encountered in moderate nutrient regimes, 2.5 to 4.5 kg N ha-1 y-1, and the probability of encountering them declines as N deposition exceeds 4.5 or drops below 2.5 kg ha-1 y-1. These species are most often found in moderate environments, on either hardwoods or conifers, typically on valley floors and low to mid elevation forests with moderate nutrient availability, less leaching from precipitation, and warmer minimum temperatures. Those that prefer hardwood substrates have been called 'neutrophytes'; the pH of hardwood bark is typically more neutral relative to typically acidic conifer bark. The mesotrophs include some ecologically important species in otherwise mainly oligotrophic genera, and also some of the smaller forage lichens.


Eutrophs tolerate and even thrive at N deposition loadings above 4.5 kg N ha-1 y-1. Also known as 'nitrophytes' or 'nitrophiles', in nature they thrive in environments with enhanced nutrient availability. Some of their natural habitats include mineral rich, alkaline substrates such as limestone or seashore rocks, and environments with concentrated nitrogen input such as rocks and branches under bird or small animal nests or perches. These lichens are often seen growing on barns or on trees in agricultural fields where there is an ample supply of ammonia from animal wastes or volatilized fertilizers. Most prefer hardwood substrates; but, if N deposition is high enough, and especially if the source is ammonia, many will also grow on conifer bark. Eutrophs include the orange lichens (Xanthoria, Xanthomendoza) as well as the small gray rosette lichens (Physcia, Physconia, Phaeophyscia), some of the camouflage lichens (Melanelia, Melanohalea, Melanelixia) and others. Eutrophic lichens tend to be rather small in size and adhere tightly to their substrates, producing a relatively small biomass compared to oligotrophic and mesotrophic species.

For more information about lichen response to N deposition in the US Pacific Northwest click here.

National Emissions Inventory 2002 for Oregon and Washington, US EPA - AirData Emissions by Category Report - Criteria Air Pollutants:

Sulfur Dioxide Sensitivity: S = sensitive, I = intermediate, T = tolerant.


SO2-'sensitive' lichens tolerate mean annual SO2 concentrations from 5 to 15 ppb SO2.


Similarly, lichens of 'intermediate' SO2 sensitivity tolerate 16-30 ppb SO2.


Lichens rated 'tolerant' can tolerate SO2 concentrations above 30 ppb.

Within each category, some lichens may tolerate only part of the range, e.g. some 'sensitive' species may tolerate only 5 ppb while others may tolerate up to 15 ppb. Sensitivity at a given site may be moderated or intensified by climate, degree of exposure (forest floor vs. ridgetop), and other environmental stressors and influences. For more..

Ozone, Acid Deposition, Fluorine, Metals

These ratings follow author's designations and tend to be qualitative rather than quantitative. I.e., as the respective pollutant increases, sensitive species might be expected to disappear of show signs of harm first whereas tolerant species should be the most resistant. For more..