Field Guide to the Lichens of White Rocks

University Press of Colorado

Contents

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 in elevation 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 prairies 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. This book is documentation of a sandstone outcropping 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 Field Guide are encountered commonly throughout central and western North America. There exist several additional Fox Hills outcrops in Colorado and neighboring states as well as sandstone outcrops of other geological time periods. The present guide will be especially useful in helping to identify the lichen constituents of those formations. 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 attributable in part to its geological history, climatological history, and degree of preservation but also to the fact that 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 outcrop itself consists of a large one- to two-tier sandstone shelf with horizontal and vertical exposed surfaces. It is approximately two-thirds of a mile 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 approximately one-half of a mile 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 biotic crusts — 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, Crawford, and Jennings 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 builds upon 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 (ESA). However, at present, lichens are more or less excluded from federal conservation measures (only two species are currently protected by the ESA). Rare lichens at White Rocks do, however, receive local protection through the Open Space and Mountain Parks (OSMP) conservation practices. To protect the many sensitive natural resources that occur at this site, including federally regulated bird nesting habitat, White Rocks can be accessed only through permitted research and scheduled educational tours and is otherwise closed to the public(additional information on public access is available on the OSMP website www.bouldercolorado.gov/osmp).

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 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 and Anderson (1969) presented a lichen checklist for the state, but this list represents a small fraction of the state's total lichen biodiversity, is based entirely on literature reports, and is outdated taxonomically. The manuscript from which this Field Guide draws (Tripp 2015) is based on new field collections and adds to a list of important regional inventories in western North America that, together, will help scientists stitch together a better understanding of lichenology of the Great American West. Most immediately, this Field Guide and associated publication provide the initial steps toward 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 and Lendemer (2015) for descriptions of two new species from the site.

Why Lichens?

Every component of an ecosystem functions in some way vital to that ecosystem (Braun 1950). Lichens are for the most part Ascomycete fungi (the "mycobiont") with an obligate symbiotic relationship with one or more green algae or cyanobacterium (the "photobiont"). Lichens are among the most diverse and ecologically important obligate symbioses and represent important components of terrestrial ecosystems worldwide (Hawksworth 1991; Brodo et al. 2001; Cornelissen et al. 2007). In some regions of the world, lichens (together with bryophytes) contribute more to the total biotic diversity than do vascular plants (Kantvilas 1990; Jarman and Kantvilas 1995). In the relatively arid state of Colorado, the ratio of flowering plant species to lichens probably ranges between ~3 to 1 and ~3 to 2, indicating the importance of lichens to the total biota of the state. However, unlike the ±3,000 species of flowering plants in Colorado, we have only the most rudimentary knowledge of the identity, let alone ecology, of the ±1,000 to 2,000 species of Colorado lichens. Yet the pace of new species description of North American lichens far exceeds the pace of description of new plants — despite the fact that major, large-scale plant floristic projects that would seem to correlate with new species discovery exist or have been recently completed (e.g., Flora of North America Editorial Committee 1993; Jepson Flora Project 2016; Conquista et al. 2013; Weakley 2015) whereas large-scale efforts in lichenology are few (Nash et al. 2002, 2004, and 2007 represents one of the few modern lichen floras in North America).

Just like plants and animals, lichens grow and respire; just like plants, lichens also photosynthesize because of the presence of the photobiont. In addition, lichens with a cyanobacterial photobiont and/or specialized lineages of endolichenic bacteria contribute to the "fixation" of nitrogen — that is, the process by which atmospheric nitrogen is converted into a form usable by living organisms. At White Rocks, only two genera of lichens harbor cyanobacteria: Enchylium and Lichinella. Elsewhere in Boulder County, cyanolichens include Leptogium, Lobaria, Nephroma, Peltigera, and, if you are really lucky, Sticta. Genomic investigation of bacterial communities of lichens is in its infancy, but it is likely that other nitrogen-fixing prokaryotes in addition to cyanobacteria are far more prevalent in lichens than previously recognized (Kane, Tripp, Lendemer, and McCain, in progress).

Lichens that grow on rock also influence nutrient distribution in their environments in another manner: as primary decomposers of parent rock material into what will ultimately become soil. The lichen mycobiont manufactures and secretes various chemicals that aid fungal hyphae in the penetration of rock surfaces. Once below the surface, hyphae grow among the crystalline structures and between rock cleavage lines, further fracturing the parent material. As lichens grow and decompose, they also trap fine soil particles among their surfaces, encouraging saprophytic fungi and bacterial growth, further adding to the process of soil formation. Lichens that grow on trees influence local distribution of nutrients by trapping airborne nutrients including water, thereby helping to maintain relative humidity as well as providing shelter for microorganisms. Whether their presence is detrimental to living substrates (e.g., trees) is a question unanswered. On the whole, however, it is widely appreciated that healthy ecosystems have ample lichen cover, and unhealthy ecosystems lack lichen cover. A stop along a forest bordering northern stretches of Interstate 95 will demonstrate the latter. A hike at 13,000 feet in Rocky Mountain National Park will demonstrate the former.

Beyond their roles in ecosystem nutrient cycling, lichens also function prominently in many food webs. In Colorado, lichens figure substantially in the diets of both large- and small-bodied animals ranging from elk to moose, deer, squirrels, birds, snails, mites, and insects (Sharnoff 1994; Pettersson et al. 1995). In boreal ecozones, lichens serve as primary food sources for mountain goats, and caribou cannot live without them. Lichens are also used by numerous animals in nest construction (Ladd 1998) or medicinally in some human societies (Wei et al. 1982). In fact, lichens manufacture hundreds of secondary compounds not known elsewhere in nature and whose ecological functions are at best minimally understood. This chemical diversity has (perhaps unsurprisingly) been exploited by lichenologists as a source of taxonomic information: thin layer chromatography (TLC) to determine lichen secondary chemistry is necessary to identify at least a third of all North American species with confidence.

Finally, lichen abundance and species richness have long been appreciated as indicators of the richness of other taxa, habitat quality, and air quality of a particular region (De Wit 1983; Nilsson et al. 1995; Bergamini et al. 2007; Tripp and Lendemer 2012; Lendemer et al. 2013). Nordén and colleagues (2007) found a significant correlation between temperate and deciduous forest lichen, bryophyte, and wood fungi diversity and the number of rare, Red Listed species in these groups. In the southeastern United States, McCune and colleagues (1997) found lower lichen diversity and abundance in areas of higher air pollution. Great Smoky Mountains National Park, which contains some of the most extensive tracts of old growth forest remaining in eastern North America, is lichenologically the most diverse park in the United States and contains upward of half of all species present in eastern North America (Tripp and Lendemer 2012; Lendemer et al. 2013). For the most part, relatively common species have been used as forest indicators (McCune et al. 1998). However, recent studies are also bringing new recognition to the rarer component of the lichen biota, and inventories have indicated that occurrence of rare plants and animals correlates to occurrence of rare lichens (Lendemer et al. 2013).

Lichen Biology: The Basics

The traditional concept of the lichen symbiosis has been that the mycobiont and photobiont form a mutualist relationship, with the fungus providing a protected environment in which the photobiont can thrive and the photobiont supplying nutritional products of photosynthesis to feed the fungus. More recent perspectives suggest this traditional view may not be accurate, with one or both of the partners functioning parasitically at times (reviewed in Richardson 1999). Recent research has also demonstrated that the lichen symbiosis itself is far more complex than previously understood, with the discovery of multiple photobiont genotypes in a single organism (Muggia et al. 2013), as well as additional partners including endolichenic fungi or bacteria, whose functions are for the most part still under investigation (Arnold et al. 2009; Spribille et al. 2016).

The vegetative or non-sexual portion of the lichen body is termed a thallus (plural: thalli) and consists generally of four layers. The upper and lower layers comprise the cortex and are composed of densely packed fungal hyphae. The upper cortex is where many accessory pigments that give lichens their color reside. Just underneath the upper cortex is the photobiont layer, which varies from light to dark green to orange to blue, depending on photobiont type (e.g., coccoid green alga, Trentepohlia, a cyanobacterium). The layer below the photobiont is termed the medulla; it is usually bright white in cross section (but can be brightly pigmented in some species, such as in Vulpicia pinastri, which is common in Boulder County and has a bright yellow medulla) and consists of loosely arranged fungal hyphae. Below the medulla, the lower cortex (where present, see below) is generally white, gray, brown, or black in color.

Lichen upper cortices are modified or ornamented in myriad ways. Specialized features include cyphellae and pseudocyphellae, maculae, perforations, cilia, and pruina. Pruina is a whitish coating on the upper surfaces of some lichens, giving an appearance of powdered sugar. Pruina is very common among species at White Rocks and can often be seen on surfaces of apothecia (e.g., Diplotomma venusta) and/or on surfaces of thalli (e.g., Psora tuckermanii). Degree of pruinosity can vary to such an extent even over a single thallus as to completely obscure the true color of the upper cortex by giving the lichen the appearance of a white color (as in Acarospora strigata: dark brown when epruinose but bright white when pruinose). Lichen lower cortices are differentiated by color, texture, and presence of attachment structures such as rhizines, which anchor lichens to their substrate and are typical of many foliose lichens.

Not all lichens have differentiated upper and lower cortices, and not all lichens have both cortices. First, by definition, most fruticose lichens (see growth form information below) have only one type of cortex, that is, an upper cortex is non-differentiable from a lower cortex. We have only one fruticose lichen at White Rocks: Lichinella stipatula, which is micro-fruticose. Second, by definition, crustose lichens lack a lower cortex. The majority of the lichen biota at White Rocks is crustose (e.g., Caloplaca trachyphylla, Lepraria finkii, Acarospora spp.). Finally, most foliose lichens have well-differentiated upper and lower cortices, but a small number of foliose lichens are characterized by having "ecorticate" lower surfaces (like crustose species), such as species in the genus Peltigera and Heterodermia (not present at White Rocks but commonly encountered elsewhere in Boulder County).

Lichen Reproduction

Lichens reproduce sexually and/or asexually. Most commonly, a given species reproduces primarily through one but not both means, that is, a species is either sexual or asexual (Tripp 2016); however, asexual species are on occasion encountered with sexual reproductive structures (see below). At White Rocks, Psora tuckermanii is almost always found with sexual reproductive structures, whereas Verrucaria furfuracea is always found with asexual reproductive structures. Just like all other Ascomycete fungi, the basic unit of sexual reproduction in lichens is the ascoma. Lichen ascoma (plural: ascomata) occur primarily in two forms: disc-shaped apothecia and flask-shaped perithecia. Inside the ascomata are sacs that contain the products of meiosis: ascospores. Spores are released from either the open discs of an apothecium (as in Candelariella rosulans) or from a pore-like opening at the top of a perithecium (as in Staurothele areolata). Because sexual reproduction involves only the fungus and not the alga, dispersed fungal ascospores must encounter a non-lichens-compatible alga (or steal an alga from a different lichen, which surely happens with some frequency) to give rise to a new thallus. Lichen apothecia, the most common type of reproductive structure, are extremely diverse morphologically. They can occur with or without thalline margins, and when present, these are sometimes ornamented. Thalline margins are typically described as lecideine if they lack algae and are carbonized or black in color (as in Lecidea hoganii) or lecanorine if algae are present and margins are the same color as the thallus (as in Lecanora muralis). Apothecia may be concave at maturity (Xanthoparmelia coloradoensis), convex (Lecidella carpathica), flat (Candelariella clarkiae), lirellate or lip-like (this modification not present at White Rocks), or raised on stalks or podetia (this modification also not present at White Rocks but characteristic of the diverse and widespread genus Cladonia). Sexual reproductive structures can range from comprising nearly the entire visible lichen (Lecanora flowersiana) to being scattered across the thallus to being rare (Caloplaca decipiens) or completely unknown (Lepraria finkii) for a given species. Finally, sexual ascospores are extremely important in lichen identification. They range from hyaline to brown at maturity and simple to transversely sepatate to muriform. A given sexually reproducing species generally has a diagnostic number of ascospores per ascus. The most typical number is eight spores per ascus, reflecting meiosis followed by a single mitosis event, but severe reductions in the number of spores per ascus are possible (as in one spore/ascus in Rhizocarpon disporum), as are additional mitotic events to yield many more spores per ascus (as in Acarospora, with thirty-two to hundreds of tiny spores/ascus). Sexually reproducing crustose lichens almost always need to be sectioned by hand and studied under a compound microscope to identify species with confidence.