Forest Inventory and Analysis National Program

Lichen Communities

---

Background

Using Lichens to Monitor Air Quality and Forest Health

Lichens are organisms consisting of both fungi and algae. Lichens are very responsive to environmental stressors in forests, including changes in forest structure, air quality, and climate. A large body of literature published during the last century has documented the close relationship between lichen communities and air pollution, especially SO2 and acidifying or fertilizing nitrogen and sulfur-based pollutants. The composition of an epiphytic lichen community is one of the best biological indicators of air pollution in forests, because epiphytic lichens rely totally on atmospheric sources of nutrition.

In several studies (e.g., Muir and McCune 1988), lichens have given much clearer responses to N- and S pollutants (in terms of diversity, total abundance, and community composition) than either leaf symptoms or tree growth, and have been one of the few components of terrestrial ecosystems to show a clear relationship to gradients of acidic deposition in the eastern United States.

Although trees may respond to moderate, chronic levels of air pollution deposition, all of the other influences on tree growth, such as variation in soils, make the responses of trees to pollutants difficult to measure in the field. Epiphytic lichens may be used to assess potential air quality impacts on forest ecosystem health and productivity that are difficult to measure directly. Long-term observation of lichen community change provides early indication of improving or deteriorating air quality.

Conceptual Model

The conceptual model, shows how the Lichen Community Indicator relates to the condition of the forest resource, and to some environmental stressors. The FIA Lichen Indicator Fact Sheet and most of the publications and reports provide additional details on the rationale for the indicator. 

Lichen Diversity and Forest Structure

Epiphytic lichens represent a significant portion of forest biodiversity in many regions, and typically we encounter between 5 and 50 species on an acre plot. Sixty or more species may be found on plots in particularly rich areas of the Pacific Northwest and southeastern US. In addition to air quality, the diversity of lichens on plots depends on many factors including climate, forest type, forest age, tree density, spatial arrangement of trees and proximity to lichen propagule sources. Factors tending to enhance diversity are late-successional status, open structure with high light levels, lack of dominance by bryophytes (e.g., mosses and liverworts), and high moisture levels. Factors tending to decrease diversity are early successional status, dense stands with poor light, and either very dry climates or extremely wet climates promoting dominance by bryophytes.

Lichens play many important biological roles in temperate forests including winter forage for mammals, arthropod forage and habitat, and nesting material for birds. Among the animals that eat high volumes of lichens during parts of the year are flying squirrels, several species of deer, woodland caribou, mountain goats and moose. In the boreal forest of the western US and Alaska, the northern flying squirrel forages predominantly on lichens (predominantly Bryoria, or "old man's beard") during 7 months of the year, eating the sporocarps and truffles of mycorrhizal fungi for the other five. * Lichens are also important in nutrient cycling, especially nitrogen fixation in moist areas. In the west-side conifer forests of the Pacific Northwest, lichens contribute between 5-25 kg/ha of fixed nitrogen to the ecosystem each year.

For more information on lichens and wildlife, see http://www.lichen.com/animals.html and http://www.lichen.com/fauna.html.

Several special studies have used "off-frame" (i.e., off of the systematic sampling grid) plots to address detailed questions about the relationship of lichen diversity to forest structure. The Bureau of Land Management Density Management Study used approximately 70 off-frame plots in two several hundred acre blocks to assess the factors associated with high lichen diversity in young managed forests, concluding that gaps and old-growth remnants harbor the greatest lichen diversity in these forests. Other studies have used off-frame plots in comparisons between old-growth forests and young managed stands.

FIA lichen plots have been useful in contributing to our understanding of the lichen flora and its distribution, particularly in areas that have received little previous study. The program provides detailed distribution and abundance data for hundreds of species nationwide, and has added taxa to state lichen lists and databases for species of special management concern. *Mycorrhizae are critical to the health and growth of the forests in which they are found. The spores of many mycorrhizal fungi are dispersed and facilitated in germination by passage through animals' digestive tracts. The trees, in turn, create the habitat needed for abundant lichen communities. Over millennia, the trees, lichens, mycorrhizal fungi and squirrels have co-evolved into a tight circle of interrelationship.

The Lichen Community Indicator in FIA and FHM - A Brief History

The lichen community indicator was developed in the Southeast in 1990-93 by Bruce McCune and Jonathan Dey, funded jointly by the U.S. EPA and the USDA Forest Service (EMAP Program). Forest Health Monitoring (FHM) pilot projects were run in 1994-96 in a variety of eastern states and Colorado, Oregon, Washington and California in the west. The lichen community indicator was included in regular permanent plot surveys starting in 1997 (FHM 1997-1999, FIA 2000 on). Regional gradient models have been developed for the southeast (1994), the northeast (1997), and Colorado (1998), and are being developed for the Midatlantic States (1999-2000) the Pacific Northwest (2001), and California (2003). Lichen communities are now a Phase 3 indicator in the Forest Inventory and Analysis Program (FIA).