Compared with today, the landscape in Rhode Island must have been much different prior to 1850 when the last of the primitive forests were cut down and James L. Bennett (1888) first published a list of Rhode Island's lichens. The last one-half of the 1880s saw the removal of these forests to make room for farmland and population centers. Only isolated stands of original forests, or remnant trees saved for shade around farmhouses, remained. It was these trees that gave rise to secondary-growth forests in the coming years, but nearly all of the virgin timber was then gone. Gone with it also was microhabitat of the tree and understory vegetation of mixed age, as well as standing dead trees, logs in various stages of decomposition, and scattered openings caused by fallen trees.

Remnants of several lichen species undoubtedly persisted for several years in these isolated, old-growth forest habitats that gave rise to species of Lobaria, Nephroma, Sticta, Ramalina, and Usnea that appear on Bennett's list of lichens in 1888; but these species did not survive the changed environment of the secondary, and subsequent, forest growth.

Rhode Island covers 1,214 square miles, 156 of which are water. During the past 100 years its population has increased nearly 145%, from 428,556 in 1900 to 1,048,319 in 2000 (US Census figures). Human population centers, with cities, industrial and commercial complexes, and roads, affect the lichen microclimate, especially with the production of waste gases from burning fuels. In addition to this ambient air pollution, there also exists "heat pollution". This increase in long-term, average temperature has its greatest influence upon the available atmospheric moisture (humidity) available to the lichen microclimate.

Not all the adverse conditions that affected lichen populations were from within Rhode Island. External air pollution has probably had a much greater effect, especially in recent years. From as far away as Ohio, Indiana, West Virginia, and Pennsylvania came air pollution, beginning in the early 1970s. The Clean Air Act of 1970 forced coal-burning, electricity-generating power plants to acknowledge their contribution to dirty air. These plants produced 67% of the country's sulfur dioxide, 33% of the carbon dioxide, 33% of the mercury pollution, and 20% of the nitrogen oxide (The Beacon Journal 2001). To meet the Clean Air Standards on the local level, these power-generating plants constructed tall smokestacks that discharged their pollutants into the higher levels of the atmosphere where they were caught by the prevailing westerly winds and carried eastward into New York and New England.

Sulfur dioxide and nitrogen oxide are the key chemicals that result in acid rain. Many soils, rich in lime, are capable of neutralizing much of this acid rain, but not so the soils derived from granite-rich bedrock in New England. Ozone is another pollutant created from nitrogen oxide and vehicular exhausts, facilitated by sunlight. Ozone has been shown to be detrimental to some lichen species, especially those containing the cyanobacteria (e.g. Collema, Leptogium, and Peltigera) and Phaeophyscia. Some lichens seem to thrive in nutrient-rich environments and are said to be nitrophilous. Nitrogen compounds from automobile exhausts and other fuels contribute to this type of atmosphere and probably account for the relative abundance of several nitrophilous lichen species in city parks and along streets. Among the most common of these are Parmelia sulcata, Physcia adscendens, Physcia millegrana, Punctelia rudecta, and Xanthoria parietina.

Few studies have been made of Rhode Island's lichens since Bennett's list in 1888. Except for a brief study of the lichen flora in the state by former University of Rhode Island student Anne Hebert in the early 1960s and by Don Flenniken (see Flenniken 1968a, 1968b) over 30 years ago, only incidental records were found in scattered publications (Evans 1944, Llano 1950, Culberson and Culberson 1956, Ahti 1961, Culberson 1961, Thomson 1963, Hale 1964, 1965, Lindblom 1997, Hines 1998) indicating lichen collections from Rhode Island.

Mason E. Hale, Jr. (Hale 1950) reported several lichen species known to be susceptible to air pollution in his study in western Connecticut. Among these are species of Collema, Leptogium, Nephroma, Peltigera, Menegazzia, Cetrelia, Platismatia, Tuckermannopsis, and Usnea. Erwin Brodo (1968) reported Lobaria pulmonaria, Lobaria quercizans, Leptogium cyanescens, Pseudevernia consocians, Stereocaulon saxatile, Anzia colpodes, Vulpicida viridis, Ramalina willeyi, Usnea trichodea, and Heterodermia obscurata being present on Long Island, New York. These species seem to indicate that air pollution was not a major controlling factor in lichen growth at the times of those studies.

Approximately 10 years after new regulations of the Clean Air Act were initiated, Kenneth J. Metzler (1980) began to notice the affects of air contamination on the corticolous (tree) lichen population in Connecticut, reporting many collection sites with species showing damage attributed to poor air quality, originating both within and outside of Connecticut.

This present study of the foliose and fruticose lichens in Rhode Island is intended to form a baseline of the species present in 2001. Since it is difficult to evaluate the effects of air pollution on particular lichen distribution patterns from a single survey, it is recommended that re-surveys of selected lichen communities be done at 5- to 10-year intervals to document changes in the lichen flora. The Rhode Island Nature Conservancy Preserves are protected and monitored to prevent significant deterioration of the environment and thus would be ideal study sites for future comparison with the data presented here.