Painted Rocks: The Orange Lichens

     One of the more striking experiences with lichens anyone is likely to have in Colorado is encountering splashes of DayGlo orange or chartreuse on an otherwise brown, black, and green lichenized rock face. This post explores tricks for identifying down to the genus level what species of shockingly orange lichen you may have encountered, and some interesting facts about them as well. The genera of interest are Xanthoria/Xanthomendoza and Caloplaca. 

Xanthoria/Xanthomendoza (orange) species, possibly X. elegans, growing on Pike's Peak granite. The violent coloration can be missed due to the small size of the thalli: the larger crystals in this picture are not more than 1cm across. The brilliantly yellow-green lichen is Pleopsidium flavum, which I will look at in a future post.

Xanthoria/Xanthomendoza vs. Caloplaca 

     Orange lichens that you encounter in Colorado belong one of two (now three) genera. The genera of Xanthoria/Xanthomendoza discussed below represent eight species, while Caloplaca weighs in at a whopping 54 species. Not all species in these genera are orange, but all orange species in Colorado belong to these genera, and all of them owe their orange coloration to the same remarkable pigment: parietin(1). Parietin functions to protect the lichen from UV radiation(2) and to chemically beat down competing microorganisms on the rock surface(3). It has also demonstrated some weak anti-cancer properties in being able to induce apoptosis in certain cell lines, but not as well as some other lichen secondary metabolites(4). Parietin holds its color well after the death of the lichen, and some people use it for dying(5). From a lichen identification perspective, a spot test using a solution of potassium hydroxide will yield a brilliant purple color in the presence of parietin(1). This doesn't help to differentiate between any of Colorado's orange genera as they all have the same primary pigment, but it can be handy for other genera. A good photo of a KOH spot-test can be found here (scroll down for image). 

     Note that I have been separating Xanthoria and Xanthomendoza genera from each other. All Xanthomendozae used to be classified under Xanthoria, until it was noticed that the two bear different conidia, and that those which became Xanthomendoza have either rhizenes or other holdfasts while Xanthoria do not(6). Other than those differences, the two genera look essentially the same. 

     If color is of no use, how then to differentiate Xanthoria/Xanthomendoza ssp. from Caloplaca ssp.? All three genera belong to the same family, Teloschistaceae(7). All three genera have many species which tend to be orange, and may grow on rocks or on bark. The species with orange thalli all have rounded, cup-shaped lecanorine apothecia(6,8). The best feature to distinguish between Caloplaca and Xanthoria/Xanthomendoza ssp. appears to be the habit of the thallus: Xanthoria and Xanthomendoza ssp. tend to have foliose thalli(9), while Caloplaca ssp. tend to be crustose or occasionally fruticose(9,8). A wikipedia page which does not cite its source for the information also notes that Xanthoria ssp. may be squamulose(9). 

     In practice this means that an orange lichen which can be cleanly separated from its substrate with a knife blade probably belongs to Xanthoria or Xanthomendoza, while orange lichens which cling tenaciously to their substrates and do not have a lower cortex (which may be a different color than the thallus) are more likely to be Caloplaca ssp. This is hardly a foolproof test, however, since squamulose lichens also do not have a lower cortex, and since some Caloplaca ssp. such as Caloplaca coralloides have such tightly bound fruticose structures as to appear crustose(8). Also, based on my own observations of photos of each genus, it seems that Xanthoria/Xanthomendoza lichens tend to have a more flowing, consistent thallus while the thalli of Caloplaca ssp. tend to be more warty, fissured, or grainy. Below are some pictures of lichens that I'm pretty sure I have identified correctly from each genus. Click image to enlarge. 

Genus Caloplaca

Caloplaca trachyphylla growing on sandstone in the Utah desert. This lichen almost always grows on sandstone, and forms beautiful, prominent rosettes like this one. The crustose habit and "warty" thallus help to separate it from Xanthoria elegans. 

A close-up of C. trachyphylla. Note how the lecanorine apothecia become more flattened and even convex towards the thallus' center. The white growth toward the top center-left may be the beginnings of a colony of Acarospora stapfiana, another lichen which is parasitic on C. tracyphylla.

Caloplaca arenaria, also growing on sandstone. The salient parietin-bearing structures in this picture are actually the apothecia. This species often has a near non-existent thallus, which if present is gray(12). The black, blurry marks at left are millimeters.  Seen on the macro scale this species appears as tiny, blood-red granules.

Genus Xanthoria 

   

Xanthoria elegans growing on Pike's Peak granite with an unidentified gray foliose lichen. X. elegans may form more rosette-like structures, but this admittedly blurry photo does a nice job showing its more flowing structure.  

If you see vertical streaks of orange like this on rocks, odds are you're dealing with X. elegans. It enjoys drainage channels and seeps that come off of nitrogen-rich cracks in rocks where rodents or birds habitually poop(1). The purple flowers barely visible in this photo are the relatively rare wildflower Boykinia jamesii, which grows in abundance in the Crags area on the West side of Pike's Peak.

 

Genus Xanthomendoza 

A magnified view of a Xanthomendoza species I found on the wooden fence in my back yard. I'm not certain on the ID, but it appears to be either Xanthomendoza ulophyllodes or Xanthomendoza fulva. The presence of apothecia seems to indicate Xanthomendoza ulophyllodes.

Xanthomendoza montana growing on an exposed rock face on Mount Evans. As best I can tell, X. montana is best differentiated from Xanthoria elegans by its more lumpy, mounded habit. 

Xanthomendoza montana growing in luxuriant lumps near the summit of Mount Evans. The thallus can hardly be seen through the prolific apothecia. 

Xanthomendoza montana growing on granite at approx. 12.000 ft. The white flowers are Minuartia macrantha, the alpine sandwort. 

     Thank you for reading about orange lichens! Think of them as you enjoy your orange Thanksgiving foods such as Ipomoea batatas, although in that case the orange pigments are carotenoids, not parietins. 

References

1. Corbridge, J. N., & Weber, W. A. (1998). A Colorado Lichen Primer. Niwot, CO: University Press of Colorado.
2. Nybakken, L., Solhaug, K. A., Bilger, W., & Gauslaa, Y. (2004). The lichens Xanthoria elegans and Cetraria islandica maintain a high protection against UV-B radiation in Arctic habitats. Oecologia, 140(2), 211-216. doi:10.1007/s00442-004-1583-6
3. Gazzano, C., Favero-Longo, S. E., Iacomussi, P., & Piervittori, R. (2013). Biocidal effect of lichen secondary metabolites against rock-dwelling microcolonial fungi, cyanobacteria and green algae. International Biodeterioration & Biodegradation, 84300-306. doi:10.1016/j.ibiod.2012.05.033
4. Bačkorová, M., Jendželovský, R., Kello, M., Bačkor, M., Mikeš, J., & Fedoročko, P. (2012). Lichen secondary metabolites are responsible for induction of apoptosis in HT-29 and A2780 human cancer cell lines. Toxicology In Vitro, 26(3), 462-468. doi:10.1016/j.tiv.2012.01.017
5. Mycopigments (2013). Lichen dyes [web log post]. Retrieved from http://mycopigments.com/lichen-dyes/
6. Sharnoff, S. (2014). Field Guide to California Lichens. New Haven, CT: Yale University Press.
7. KARNEFELT, I. (1989). MORPHOLOGY AND PHYLOGENY IN THE TELOSCHISTALES. Cryptogamic Botany, 1(2), 147-203.
8. Kelso, M. (2005). Caloplaca coralloides. Retrieved from: http://www.mbari.org/staff/conn/botany/lichens/taxonomy.htm
9. Wade, A. E. (1965). The genus Caloplaca th. fr. in the British Isles. The Lichenologist: 3(1), pp. 1-28. DOI: http://dx.doi.org/10.1017/S0024282965000038  
10. Xanthoria (n.d.) [article entry in wiki]. Retrieved from: https://en.wikipedia.org/wiki/Xanthoria
11. Consortium of North American Lichen Herbaria [CNALH](n.d.) Colorado. Retrieved from: http://lichenportal.org/portal/checklists/checklist.php?pagenumber=2&cl=1202&dynclid=0&pid=510&searchsynonyms=1
12. Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.)(2001). Lichen Flora of the Greater Sonoran Desert Region. Vol 3. Tempe, AZ. Retrieved from: http://lichenportal.org/portal/taxa/index.php?taxauthid=1&taxon=56217&cl=1202

A Primrose by Any Other Name

     I was recently surprised to learn that a primrose is not, in fact, necessarily a primrose. What we refer as merely primroses vs. evening primroses are two botanically distinct families, Primulaceae and Onagraceae, respectively. Neither of these families include actual roses, which belong to the family Rosaceae. Both families of primrose are well-represented in the Colorado flora, and I thought it would be nice to post a selection from each family and talk about their characteristics and quirks. I have reported such interesting research as I have been able to find on each species, but in many cases the value of these flowers seems to be for their ecological niche and gorgeous blossoms rather than their chemistry or ethnobotanical backgrounds. 

     The word "primrose" itself evolved from the Latin prima rosa, which simply means "first rose(1)." The genus name Primulus, which was the diminutive form of primus, simply means "first." Both etymologies refer to the tendency of some primroses to be an early-blooming flower in the spring, although this is not necessarily true for species here in Colorado.  

Primulaceae, the "True" Primrose Family

     The most familiar members of this family are probably the common garden primrose, Primula vulgaris, which sprouts overnight in garden centers in little plastic pots around Easter. Cyclamens also hail from this family. Members of Primulaceae tend to share certain characteristics: leaf attachment is either basal or opposite; flowers have five petals joined at the base, five sepals joined at the base, and five stamens opposite the petals; flowers frequently cluster above a bract. Some Primulaceae species that inhabit the Colorado Rocky Mountains include Parry's Primrose, Alpine Primrose, and Shooting Star. 

Primula parryi, Parry's Primrose 

P parryi growing in the forests on Loveland Pass at about 11,000 ft, just below

timberline. The fuchsia flowers strike a strong contrast with the surrounding woods. 

     Parry's primrose, named for the prolific plant collector Charles Parry who was active in Colorado during the late 1800s(3). A single stalk on each plant with matte textured flesh supports several brilliant pink flowers. The base of the plant is a rosette of fleshy, elliptic to lanceolate leaves. Parry's primrose blooms in the summer in wet habitats, although it can grow other places as well. Closer investigation of the flowers will yield a fetid odor, which the flower uses to attract pollinators. 

Primula angustifolia, Fairy Primrose, Alpine Primrose

P. angustifolia growing just above timberline on Pike's Peak. This flower is tiny, growing no 

more than a four inches tall. 

     P. angustifolia's family relationship with P. parryi is immediately apparent. Recent phylogenetic research suggests that the two species share a common, hybrid ancestor(5). I mistook this flower for a smaller version of Parry's primrose growing at high altitude. Larger, low-altitude flowers will often have dwarf incarnations in the alpine zone, but P. angustifolia is actually a distinct species. Tiny rosettes of lanceolate leaves yield one to several flowers that are colossal compared to the rosettes supporting them (see photo above). Like their lower altitude cousins, P. angustifolia blooms in summer and emits a rancorous odor which attracts pollinating flies. 

Dodecatheon pulchellum, Shooting Star 

D. pulchellum flowers growing near a stream on Pike's Peak. 

Another angle. This flower has an unusual brilliantly red band where the petals join.

     The shooting star is instantly recognizable and always a pleasing find. The distinctive, swept-back petals look like virtually no other flower in the region, making it easy to identify. As with the other primroses, D. pulchellum's flowers are supported by a single stalk rising from a basal rosette of lanceolate to elliptic fleshy leaves. The pointed purple projections on the front of the flower are united stamens, which require insects to force their probosces past the pollen-rich stamens to access the nectar(6). D. pulchellum may bloom in Spring or Summer, and seems to prefer wet habitats. Every time I have seen it has been by a stream or other body of water.   

  

Onagraceae, the Evening Primroses 

     The most familiar evening primroses are again the garden varieties, such as the Missouri evening primrose and the Mexican evening primrose. Many evening primroses are perennial, although some are annuals. Plants in Onagraceae tend to have four petals, four stamens, and a stigma with as many lobes as there are carpals in the ovary (typically four, although may be two or five)(7). Locally, three species of interest include the tufted evening primrose, fireweed, and Hooker's evening primrose. 

Oenothera caespitosa, Tufted Evening Primrose 

C. caespitosa growing on an arid slope in the foothills near Denver. The blossoms 

are enormous compared to the rest of the plant. 

Another tufted evening primrose growing in lava rock scree on North Table Mountain. 

This picture was taken around 4:00 pm as the flowers were just opening. 

     This gorgeous flower is easy to identify and provides a striking contrast to its dun-colored background. At a distance it could be mistaken for a prickly poppy, but closer inspection reveals much different foliage and a yellow center on the poppy which is absent from the tufted evening primrose. The 3-4 inch wide flowers sport four heart-shaped petals with a prominent stigma with four lobes(6). The petals are often white, although I have seen pink variations. The flower rises from basally attached, lanceolate leaves which may be dentate to deeply lobed. True to the name, tufted evening primroses bloom later in the day so that they can be pollinated by night-flying insects.  The buds of the flowers have an almost pyramidal shape which is a good field mark when blossoms are not all the way out. Tufted evening primroses grow on dry, south-facing slopes during spring and summer.  

     Interestingly, C. caespitosa has been identified as a source of gallic acid, which may be useful in the treatment of some cancers(7). The research I have found is quite limited, indicating that this primrose is probably a far cry from a cancer cure, but it is interesting nonetheless. 

Chamerion angustifolius, Fireweed

Fireweed growing on Mount Evans around 10,000 feet. Note the prominent stigma.

     Fireweed is a gorgeous, summer-blooming flower that can turn entire alpine meadows pink. It is named not for its color, but for the fact that it is often one of the first plants to colonize burn scars(6). Flowers have four pink petals with a prominent four-lobed stigma. The claw-like curved pink structures between the petals are the sepals. Leaves are oppositely attached, lanceolate, and have prominent veins which tend to be lighter in coloration than the surrounding leaf. Unlike the tufted evening primrose, fireweed's flowers persist through all times of day. There is precious little literature on fireweed, which indicates that researchers don't like pretty flowers. 

Oenothera elata, Hooker's Evening Primrose

Oenothera elata growing near a lake. Note the exaggerated hypanthium.

     Hooker's evening primrose has a confused naming history, hence Hooker appearing in the common name but not the binomial name. Apparently one proposed name for the plant involved honoring Sir Joseph Dalton Hooker, the director of the Royal Botanic Gardens, who was in turn a fan of the work of Charles Parry (he of P. parryi!), whom he dubbed "King of Colorado Botany(9)." However, the species proved to be quite variable, and the Hooker name wound up attached to a subspecies out in California. Our own local variant is O. elata subspecies hirsutissima, likely in reference to the tiny hairs which are faintly visible in the photo above on the flower buds(10). 

     O. elata has flowers mounted on a long hypanthium. The petals are heart-shaped and yellow, with stamens and a stigma roughly equal in length. Leaves are lanceolate, oppositely attached, and feature a more lightly colored vein down the center. The plant overall has a shrubby appearance, with the one I have seen growing about 2.5 feet tall. The hypanthium is a useful field mark for differentiating O. elata from a highly similar local species, O. longissima. While O. elata has petals around the same length as the hypanthium, O. longissima has a hypanthium about three times the length of its petals, giving it a decidedly lanky appearance.    

     O. elata has been heavily researched, with many articles looking at genetics, viability for bioproduction of oils, and its relation to other Oenothera ssp. I can only speculate that this is because O. elata has been domesticated as a garden perennial, and that more work is done on plants which are more readily available.  

     

     Thank you for reading about primroses. I do not find them as intriguing for their novelty as lichens or poisonous plants, but sometimes it's nice to simply look at pretty flowers and learn their quirks.

References 

1. Harper, D. (n.d.) Primrose [entry in an online dictionary]. Retrieved from: http://www.etymonline.com/index.php?term=primrose
2. Elpel, J. (n.d.) Patterns of the primrose family. Retrieved from: http://www.wildflowers-and-weeds.com/Plant_Families/Primulaceae.htm  
3. Schneider, A. (n.d.) Parry Primrose. Retrieved from: http://www.swcoloradowildflowers.com/Pink%20Enlarged%20Photo%20Pages/primula%20parryi.htm 
4. Primula angustifolia (n.d.). Retrieved from: http://www.easterncoloradowildflowers.com/Primula_angustifolia.htm 
5. Cianchi, R., Arduino, P., Mosco, M. C., & Bullini, L. (2015). Evidence of hybrid speciation in the North American primroses Primula suffrutescens, P. parryi, P. rusbyi and P. angustifolia (Primulaceae). Plant Biosystems, 149(2), 229-234.
6. Mammoser, D., & Tekiela, S. (2007). Wildflowers of Colorado. Cambridge, Minnesota: Adventure Publications, Inc.
7. Onagraceae (n.d.) Retrieved from: http://www.wildflowers-and-weeds.com/Plant_Families/Onagraceae.htm
8. PETTIT G, R., SALDANA E, I., & LEHTO, E. (1974). ANTI NEOPLASTIC AGENTS PART 35 RHUS-TRILOBATA. Lloydia (Cincinnati),37(3), 539-540.
9. Schnieder, A. (n.d.) Colorado flora of Charles Parry. Retrieved from: http://www.swcoloradowildflowers.com/Slide%20Shows/Charles%20Parry's%20Colorado%20Flora/index.htm
10. Schnieder, A. (n.d.) Oenothera elata. Retrieved from:http://www.swcoloradowildflowers.com/Yellow%20Enlarged%20Photo%20Pages/oenothera%20elata.htm

Ancient of Days

     The post title is a reference to the book of Daniel, which was likely written around the 100s B.C.E., or about 2100 years ago (1). This is about the time that currently living organisms I would like to investigate in this post might have gotten their start in the high, hostile crags of the Rocky Mountains. Some, although likely not many, of the lichens and trees that grow in the alpine and subalpine zones of the mountains are so ancient of days that they have endured since the times of the later Old Testament, demonstrating extreme longevity. The two long-lived organisms that I want to highlight are the map lichen,  Rhizocarpon geographicum, and the Rocky Mountain bristlecone pine, Pinus aristata. 

Rhizocarpon geographicum, Map Lichen  

R. geographicum (yellow-green) growing on an exposed rock face on Mount Evans at around 11,000 feet. 

The gray lichen at right is Rhizoplaca  melanophthalma, a common foliose lichen, and the brown 

and black lichen surrounding both the others is Lecidea atrobrunnea. 

    An unassuming, yellow-green lichen that seems to prefer high altitude exposed rock faces, R. geographicum is distinguished by both its extreme slow growth and extreme longevity. It has a yellow-green thallus, black apothecia, and oftentimes a black margin. R. geographicum derives its common and scientific names both from its similarity to a map or a landscape viewed from high above (2). For a review on lichen terminology, feel free to scan my previous post on lichens. I have not been able to find a source that specifies a preferred altitude range for R. geographicum in Colorado, but I have mostly noticed it starting at 8,000 feet and growing all the way up to 14,000 feet high. 

Splotches of R. geographicum sharing crumbling Pike's Peak granite with an unidentified moss. 

R. geographicum is frequently subtle until you're right on top of it: the thalli in this picture are about dime-sized.

     R. geographicum may owe its longevity to its decidedly unhurried growth rate. The growth rates of crustose lichens tend to be measured in yearly gains of the thallus' diameter, and R geographicum gains size very slowly. Smaller thalli (<10mm) cruise along at a relatively quick 0.8 mm/year, before slowly diminishing to around 0.64mm/year (± .24mm/yr) in larger thalli (>50mm)(3). At any rate this is what researchers discovered in Iceland; different studies have found similar results with small variations depending on the growth conditions with respect to substrate, climate, etc. Using understandings of growth rates such as these, researchers have dated R. geographicum thalli in the arctic to up to 8,600 years old(4). This kind of research is what allows for the performance of lichenometry, or using lichen growth (especially of R. geographicum) to make inferences about the age rock faces.

     I have been unable to locate any literature on R. geographicum growth rates in Colorado in particular, nor have I been able to locate any information that the particular lichen thalli present in the alpine tundra of Colorado are as old as their arctic counterparts. However, the alpine environment of Colorado does mimic the arctic effectively enough that other arctic species can be found here(5). It is reasonable to surmise that some of the larger splatters of R. geographicum seen at high altitudes in Colorado are at least several hundred if not thousand years old. Try taking a metric ruler with you next time you head to the tundra to see for yourself! 

Pinus aristata, Rocky Mountain Bristlecone Pine 

P. aristata growing on the flanks of Mount Evans. Note the straight and twisted habits in the wood, as well as the stripped, exposed wood on one side of the tree. Bristlecones can take on an assymetrical "flagpole" appearance due to consistent, powerful prevailing winds from the summits of the mountains they grow on.

Bristlecone wood can take on many hues as it weathers over centuries. Very old trees frequently have large tracts of exposed deadwood, despite the living parts being in good health.

     The Rocky Mountain Bristlecone tends to prefer the windswept altitudes near timberline at 8,000-11,000 feet(6). The harsh environment with the short growing season may actually be part of the reason for the bristlecone's extreme longevity, with maturity reached at approximately 200-250 years, and most trees living between 800 and 1,700 years. Some live beyond 2,000 years, however. 

A stand of  Rocky Mountain Bristlecone Pine, again on Mount Evans. The gnarled, tortured appearance of the trees is due to the harsh environment. The much younger saplings (bottom left and far right) have a much more normal, "Christmas-tree" appearance than their ancient counterparts. 

     P. aristata is easily identified as it often grows in pure stands near timberline(7). The habit of the trunk may be relatively straight or twisted, and whether the trunk twists or not appears to be determined by genetics rather than environmental factors(8). The environment is responsible for the partially stripped appearance of many of the trees, with exposed deadwood being the rule rather than exception on older trees. Only 10 inches of healthy bark is needed to sustain a large crown of needles(7). Deadwood in bristlecone forests decays extraordinarily slowly, and dead trees can remain in place for hundreds or thousands of years(10). The crown of the bristlecone is often spreading and unruly-looking, which can give the trees a top-heavy appearance. 

    The needles themselves are one of the bristlecone's most telling features. Needles grow in fascicles of five and are .75-1.5 inches long. The needles exude speckles of white resin (see photos below), the most important field mark to differentiate bristlecones from the limber pine, which is highly similar in having fascicles of five needles, having a similar habit, and growing in the same habitat(6). 

Needles and cones of P. aristata. Note the white granules of sap on the needles. The purple cone at top is an immature female cone with the eponymous bristles. At bottom are the pollen-bearing male cones.  

     P. aristata is monoecious, bearing both male and female cones on the same plant(9). The male cones are stubby, yellow nobs as seen above which distribute pollen to the wind. The mature female cones are 3-3.5 inches long, and bear characteristic hair-like bristles at the tips of each scale which are 0.5-0.75 inches long(6). This differentiates the cones of P. aristata from the cones of other species such as Pinus ponderosa, or Pinus contorta which have sharper, shorter, more thorn-like projections from the scales of their cones(6). 

     It is worth noting that the bristlecone most people associate with extreme longevity is actually a closely related but different species, Pinus longaeva. P. longaeva lives in the White Mountains in California and can boast individual trees which have been alive for nearly 5,000 years (7). The longest-lived P. longaeva specimens are those living in the harshest conditions, around 10-11,000 ft. This is presumably true for Colorado bristlecones as well, although I have not been able to find a source to verify it. 

Honerable Mention 

     At least two other species warrant mention for being long-lived, although not so much so as R. geographicum or P. aristata. One is the limber pine, P. flexilis, which can live up to 750 years(6). The other is the lichen Lecidea atrobrunnea, which can live to a venerable 1,500 years of age(11). 

Limber pine needles and male cones. Highly similar to a bristlecone in many regards, 

but without the resin spots. 

L. attrobrunnea growing on granite. It is easily recognizable with its brown thallus, 

gray to black margin, and chocolate chip-like black apothecia. 

     Thank you for reading about old species. I would like to end this post with a plea to pay heed to climate change warnings for the Rocky Mountains. This report  (not peer-reviewed, but does cite peer-reviewed literature) details many of the problems that are currently or soon shall beset the ecosystems of the Rocky Mountains due to anthropogenic climate change. Despite their extraordinary toughness, species such as map lichens and bristlecones are picky about their climatic conditions, and rapid change could wipe out huge populations. This means that P. aristata saplings growing today may die well before they reach their potential to witness the beginning date for the show Futurama in the year 3,000. Make sure to advocate for policies that slow the progression of climate change, and to get out and enjoy these ancient organisms while they are still in their full glory. Thanks for reading! 

References

1. Antiocus IV Epiphanes (n.d.). Retrieved from https://en.wikipedia.org/wiki/Antiochus_IV_Epiphanes
2. Corbridge, J. N., & Weber, W. A. (1998). A Colorado Lichen Primer. Niwot, CO: University Press of Colorado.  
3. Bradwell, T., & Armstrong, R. A. (2007). Growth rates of Rhizocarpon geographicum lichens; a review with new data from Iceland. JQS. Journal Of Quaternary Science, 22(4), 311-320. doi:10.1002/jqs.1058
4. Lichens (n.d.). Retrieved from http://www.nps.gov/glac/learn/nature/lichens.htm
5. Mammoser, D., & Tekiela, S. (2007). Wildflowers of Colorado. Cambridge, Minnesota: Adventure Publications, Inc.
6. Tekiela, S. (2007). Trees of Colorado. Cambridge, MN: Adventure Publications Inc.
7. Blue Planet Biomes (n.d.). Bristlecone Pine Retrieved from: http://www.blueplanetbiomes.org/bristlecone_pine.htm
8. Wing, M. R., Jones, A. K., Melbostad, S. R., & Knowles, A. J. (2014). Spiral grain in bristlecone pines (Pinus longaeva) exhibits no correlation with environmental factors [electronic resource]. Trees - Structure And Function, 28(2), 487-491.
9. Fryer, J. L. (2004). Pinus aristata. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Retrieved from http://www.fs.fed.us/database/feis/plants/tree/pinari/all.html  
10. Carrara, P. E., & McGeehin, J. P. (2015). Evidence of a higher late-Holocene treeline along the Continental Divide in central Colorado. Holocene, 25(11), 1829-1837. doi:10.1177/0959683615591353
11. Miller, C. D. (1973). Chronology of Neoglacial deposits in the northern Sawatch Range, Colorado. Arctic And Alpine Research, 5(4), 385-400.