Despite What You May Think Based On This Photo, Lichens Really Do Have ‘Charm’

Introduction. The Southern Rocky Mountains and adjacent prairies of Colorado represent a biological mosaic of environments typical of many montane areas of North America, especially western North America. Boulder County alone ranges elevationally from ca.  5,000 ft. to 14,000 ft., traversing one of the greatest elevational gradients of any single county in North America and hosting a range of habitats including mixed grass prairie with tallgrass relicts, submontane forested foothills, and alpine environments above treeline. Among these dominant vegetation zones are patches of rarer habitat such as geological outcroppings of sandstone or shale, eastern woodland relict forests, and fens. The present contribution is documentation of the lichen biota of one such rare outcropping: a sandstone formation within the city limits of Boulder. The lichen biota of White Rocks represents an assemblage of species from the High Plains and Mesas of Colorado as well as from mid to low-elevation montane habitats throughout the Rocky Mountains. As such, many species in this Guide are encountered commonly throughout central and western North America. Importantly, this Guide treats fully the diverse crustose lichen biota in addition to the more conspicuous macrolichens.

White Rocks represents an ~100-acre ecologically important protected area within Boulder. Its biological significance is in part attributable to its geological history, climatological history, and degree of preservation, but also because it is a biodiversity reservoir within a sea of agriculture and urban development (i.e., the Boulder-Denver-Longmont urban triangle). White Rocks is a rare and fragile outcropping of sandstone that rises directly above the northern margin of Boulder Creek. The outcropping itself consists of a large, one to two-tiered sandstone shelf with horizontal and vertical exposed surfaces. It is ~1000 m in length oriented in an east-west manner. This outcropping is flanked by a more minor, adjacent sandstone exposure directly to the east, which is ~800 m in length. White Rocks belongs to the Fox Hills Laramie Formation, dating to ca. 67 million years before present. The sandstone at White Rocks is, as the name implies, very white in color and is composed primarily of quartz with small amounts of montmorillinite clay. The sandstone is extremely fragile and susceptible to weathering by foot travel or natural phenomena such as strong rains or high winds, but its erosion is slowed substantially by “case hardening” of the rock, which derives from hardened clay strengthened by a biotic crust – primarily, lichens.

Despite the relatively small geographical size of White Rocks, the preserve is known to harbor numerous common as well as rare vascular plants and animals (Byars 1936; Weber 1949; Clark 2001). This relates to the high microhabitat diversity represented at White Rocks, which is attributable to small-scale variation in relative humidity and available water, exposure to wind and sun, mineral content, aspect and steepness of slopes, and the biotic environment itself. White Rocks similarly hosts a community of common lichens (seen throughout the High Plains and Rocky Mountains) as well as rare lichens that are un- or underrepresented in Boulder County or much of Colorado. The latter reflects prior discoveries of rare or unusual lichens present at other sandstone outcrops in North America (Skorepa 1973; Showman 1987).

Several species at White Rocks warrant conservation protection. A few may even deserve protection under the Federal Endangered Species Act. However, lichens are more or less excluded from federal conservation measures (there are only two species currently protected by the ESA) although myself together with colleague James Lendemer (New York Botanical Garden) and the Center for Biological Diversity are working diligently to change this. For now, we are fortunate that White Rocks is protected by local conservation measures and monitoring by the dedicated and wonderful staff of Boulder’s Open Space and Mountain Parks Program.

Although a history exists of research and general interest in White Rocks Open Space, no inventory or assessment of lichens of this unique outcropping has been conducted. Thus, the primary objective of this project was to conduct a comprehensive inventory of the lichens of White Rocks. This inventory builds baseline information about the biodiversity of this important preserve as well as similar sandstone formations across western North America, enables long-term conservation planning and resource management in a data driven manner, facilitates future lichen taxonomic and ecological research, and improves our capacity to educate the public about the importance of lichens in our urban environments.

Finally, while the total lichen biota of Colorado is expected to be particularly rich given the mosaic of environments and sharp elevational and climatological gradients, a comprehensive account of Colorado lichens is lacking. Shushan & Anderson (1969) presented a lichen checklist for the state, but this list represents a small fraction of the state’s total lichen biodiversity, was based entirely on literature reports, and is outdated taxonomically. The manuscript upon which this Field Guide draws from (E. Tripp in press [2015], Western North American Naturalist) is based on new field collections and adds to a list of important regional inventories in western North America that, together, will help scientists stich together a better understanding of lichenology of the Great American West. Most immediately, this field guide and associated publication provide the initial steps towards a revision of the lichen biota of Colorado. Readers should refer to Tripp (2015) for more extensive information and background on White Rocks, as well as Tripp & Lendemer (2015) for descriptions of two new species from White Rocks.