AIR QUALITY: LICHENS

Air Quality: Lichens

Air Quality: Lichens

Introduction

Lichens are composite organisms formed by a fungus and a green alga and/or a blue-green bacterium. Lichens have been used worldwide as air pollution monitors because relatively low levels of sulphur, nitrogen, and fluorine-containing pollutants (especially SO2 and F gas, and acidic or fertilizing compounds); adversely affect many species, altering lichen community composition, growth rates, reproduction, physiology, and morphological appearance. Lichens are also used as pollution monitors because they concentrate a variety of pollutants in their tissues. Lichens are long-lived and can be monitored, field conditions permitting, in any season.

Many lichens have extensive geographical ranges, allowing study of pollution gradients over large areas. These properties make them useful for spatial and temporal evaluation of pollutant accumulation in the environment. Epiphytic lichens (those that grow on trees or plants) are often best suited to the study of air pollution effects on lichen communities, lichen growth or physiology, and to the study of pollutant loading and distribution. Because they lack roots and are located above the ground, epiphytic lichens usually receive greater exposure to air pollutants and do not have access to soil nutrient pools (Birkeland, 2009, 69).

Because they depend on deposition, water seeping over substrate surfaces, atmospheric gases, and other comparatively dilute sources for their nutrition, tissue content of epiphytic lichens largely reflects atmospheric sources of nutrients and contaminants. Lichens on soils and rock substrates are more likely to be influenced by elements and chemicals from these substrates, but otherwise share morphological and physiological characteristics of epiphytes.

Sensitivity of Lichens to Air Pollutants

Lichens have species-specific response patterns to increasing levels of atmospheric pollutants, ranging from relative resistance to high sensitivity. The majority of early lichen/air pollution studies involved sulphur dioxide because lichens are especially sensitive to this pollutant. Field studies where ambient pollutant concentrations were measured, show that sensitive species are damaged or killed by annual average levels of sulphur dioxide as low as 8-30 µg/m3 (0.003-0.012 ppm) and very few lichens can tolerate levels exceeding 125 µg/m3.

For comparison, note that ambient sulphur dioxide levels monitored in urban areas of western Oregon and Washington range from 10.4-93.6 µg/ m3 (0.004-0.036 ppm) and that EPA's national annual standard for sulphur dioxide is 0.03ppm. In recent times, sensitivity to other pollutants has been explored. Lichens are adversely affected by short-term exposure to nitrogen oxides as low as 564 µg/m3 and by peak ozone concentrations as low as 20-60 µg/m3.

With regard to ozone, most reports of adverse effects on lichens have been in areas where peak ozone concentrations were at least 180-240 µg/m3. Although ozone can, in some cases, damage dry lichens, lichens are generally considered to be less susceptible to ozone damage when dry. Ruoss et al. (1995), for example, found no adverse effects on lichens in areas of Switzerland with daily summer peaks of 180-200 µg/m3 (0.09-0.10 ppm) O3. They attributed this lack of response to the fact that ozone concentrations never rose above 120 µg/m3 ...