Yucca Yucca Yucca

Some plant names are just fun to say. The genus Yucca falls very much into this category, hence the title. Go ahead, try it. 

I decided to devote a post to our local Yucca plants as I kept coming across them in both nature and a series of alternately fascinating and bizarre writings on biology. Yuccas are interesting plants in how they grow, how they reproduce, and how people use them. 

Distribution and Description

The yuccas are a prevalent local genus which are relatively easy to identify in the Denver area despite there being considerable confusion among botanists about the subtleties of hybridized varieties(1). This is because in the front range there is but one species (discounting others which may have escaped cultivation) which is Yucca glauca. Y. glauca are the familiar sprays of sword-like leaves that populate the pastures along I-70 outside of Denver, and may be found as far east as Kansas and Nebraska(2). It is hardly endangered, and is even considered a rangeland weed by some ranchers. Two other Yucca varieties, Y. baccata and Y. harrimanieae, may be found farther to the south and west on this side of the Continental Divide but seldom, if ever, in the Denver area(1). 

This photo could be of pretty much anywhere in the front range as far as the presence of yuccas is concerned. In this case, Yucca glauca (the spiny green plants about 1/3 of the way up the photo) are growing in the Deer Creek Canyon area near Littleton. Yucca are almost as emblematic of the landscape of the high planes as Saguaro cacti are of the Sonoran Desert. 

Morphologically, Yucca glauca presents as spiky lumps which gradually develop a caudex (stem) as successive generations of tough, fibrous, ensiform (sword-shaped) evergreen leaves rise from the apical meristem. This distinctive habit is characteristic of the family Agavaceae, a primarily tropical family which helps explain Y. glauca's almost exotic appearance(1). Once the leafy rosette reaches a sufficient size of about a 3-6cm diameter at the lowest green leaves, it blooms once and then dies(2). The flowers are three-sepaled and three-petaled with six anthers and a prominent three-sectioned style*. These flowers are abundant and pendant on tall rarely branched racemes that rise high above their parent plants. The fruits are dehiscent(3), beginning as fleshy, three-chambered green capsules before becoming desiccated and splitting open. 

*If you are noticing a lot of threes here, it is because monocot plants such as Yucca will typically have flowering parts in multiples of three, while dicots are in fours or fives. This is a handy trick in the field, as it can help to narrow down families and genera fairly quickly. 

Larger plants (caudex >5cm diameter at lowest green leaves) which do not flower in a given year seem to have a greater chance of reproducing by growing ramets ("pups") around their bases(2). This accounts for Y. glauca's tendency to appear as dense groups as well as individual plants. When going about the important business of reproduction, it apparently doesn't hurt to "hedge" one's bets.   

A clonal colony of Y. glauca growing near Littleton, CO. The dry spikes are from previous flowerings 

by now deceased individuals, and the clumping habit is due to clonal reproduction.  Note the tendency of older plants to form shaggy stalks as they grow in an effort to reach flowering and death. Valar morghulis. 

The border between green, living leaves and those which die to contribute to the caudex. You can get a feel for the intense toughness of the fibrous leaves (click to enlarge) which lend themselves to a wide variety of human uses (see below). 

The flowering racemes of Y. glauca. Compared to the austere parent plant, the flowers are luxuriant and fleshy. I used to think that the color of the sepals was a useful field character, but as far as I can tell it is actually not indicative of anything: we only have one front range species, but I have run into flowers with sepals ranging from purple to white. 

Yucca Ecology: Animals Using Yuccas

The inflorescences of Y. glauca are fascinating as they represent evanescent but robust ecological communities. All of our native Yucca species are obligate symbiotes with the moth Tegeticula yuccasella(1). The moth bores into the ovary of the Yucca flower and deposits an egg inside. Then it (the moth) gathers pollen from the yucca flower and stuffs it deep inside the ovary, ensuring both food for its young and pollination for hundreds of yucca ovules. This relationship is critical as Yucca flowers cannot be pollinated by simple contact of pollen with the stigma, and must have the pollen placed inside their ovaries directly to ensure fertilization. 

Yucca flowers are also frequented by parasitic aphids, which in turn are tended and protected by diligent armies of ants(4). The ants "farm" the aphids for their sugary secretions in a fascinating process that you can read more about here. While they are on the flowers, the ants can in turn have a limiting effect on populations of parasitic moths which bore into Yucca stems or which prey upon their seeds(5). In this way, aphids can ironically improve the reproductive success of the flowers they are parasitising! 

This image gives a good idea of Y glauca's flowering structure, as well as showing the presence of the aforementioned ants and aphids in the top center (click to enlarge). 

Last year's fruit. Note the dehiscence of two of the three lobes from each other (third lobe not visible). You can also clearly see one of the bore holes left by Tegeticula yuccasella just to the right of the image center, as well as another more obscured one on the lower lobe.

Yucca Ethnobotany: People Using Yuccas

 

Members of the genus Yucca are culturally important plants to virtually all of the indigenous peoples in their native ranges, and Y. glauca is no exception. It is used as a medicine, food, material for cordage/basketry, as soap, and as a cow fart mitigation factor. 

Medicine 

The Blackfoot and Cheyenne tribes used the roots for a number of topical applications including easing arthritis pain and as a dressing for cuts and inflammation(6). Interestingly, the Dakota and Isleta peoples used the root as an aid to "make hair grow." Presumably the latter application was more psychosomatic in effect than anything, or else the male pattern baldness drug industry has missed a serious holy grail. In modern research, Y. glauca has been investigated for the cytotoxic activities of the steroidal saponins (soap-like substances) found in its roots(7), i.e. when we throw this plant at cancer, does it kill it? The authors did find two substances which induced cell death in cancer cells, so presumably research is ongoing. 

 

Food 

Many Native Americans such as the Apache, Lakota, and Navajo tribes ate the green fruits following a number of methods of preparation including roasting, pickling, eating raw, and sun-drying(6). The flowers are reportedly edible raw as well.

Cordage/Basketry 

Virtually every tribe that lived anywhere near Y. glauca's native range used the leaf fibers to make ropes, baskets, and nets. Of particular note are the Tewa of Hano Indians, who used the leaf fibers to make whips used for beating novices in initiation ceremonies. Thank you, sir, may I have another! This blog post gives a good feel for how relatively easy it is to harvest and transform yucca leaves into ropes. 

Soap 

This is the other use for Y. glauca that virtually every tribe engaged in. Apparently all parts of the plant produce some amount of saponins, as the preparation methods for many tribes involve macerating (soaking) the whole plant and using the water for washing. The part that is richest in saponins appears to be the roots, as this was both the more popular preparation method among Native Americans and the preferred method for isolating saponins for research purposes. 

A post demonstrating how yucca roots may be used as soap to clean wool for spinning into yarn may be found here. I recommend checking this link out, as it really gives a good feel for how soapy a single yucca root can be. 

Cow fart mitigation factor 

This use falls very much under the category of things that are not known until they are sought after. While I was combing research databases for info on yucca-derived saponins, I discovered that there is a burgeoning body of literature addressing the ability of yucca-derived saponins placed in cow feed to reduce a cow's methane emissions. This is not just one odd study, but actually represents a hot area of research large enough to warrant its own meta-analysis. The meta-analysis is free here for those interested. I find it heartening that these researchers are doing their part to reduce global warming (methane is a major greenhouse gas), but I was taken off guard by this particular line of scientific inquiry. 

Thank you for reading about Yucca glauca! I didn't even know how much I didn't know about this surprisingly information-dense native plant, and it was a pleasure to study. I hope that you enjoyed this excursion into the pokey world of yuccas. 

References 

1. Weber, W. A., & Wittman, R. C. (2012). Colorado Flora: Eastern Slope Boulder, Colorado: University Press of Colorado.
2. KINGSOLVER R, W. (1986). VEGETATIVE REPRODUCTION AS A STABILIZING FEATURE OF THE POPULATION DYNAMICS OF YUCCA-GLAUCA. Oecologia (Berlin), 69(3), 380-387.
3. Colorado Plant Database (2015). Yucca glauca. Retrieved from http://jeffco.us/coopext/plantdetail.do?sna=Yucca%20glauca&image=1
4. SNELL, R. S., & ADDICOTT, J. F. (2008). Limiting the success of stem borers ( Prodoxus quinquepunctellus) in yuccas: indirect effects of ants, aphids, and fruit position. Ecological Entomology, 33(1), 119-126. doi:10.1111/j.1365-2311.2007.00946.x
5. Perry, J., Addicott, J., & Mondor, E. (2004). An indirect mutualism: ants deter seed predators from ovipositing in yucca fruit. Canadian Journal Of Zoology, 82(5), 823-827.
6. Moerman, D. E. (1998). Native American Ethnobotany. Portland, OR: Timber Press, Inc.
7. Yokosuka, A., Suzuki, T., Tatsuno, S., & Mimaki, Y. (2014). Steroidal glycosides from the underground parts of Yucca glauca and their cytotoxic activities. Phytochemistry, 101109-115. doi:10.1016/j.phytochem.2014.02.002

Ghosts on the Mountain

I have recently had the privilege of being able to hike more days out of the week than not due to our unseasonably warm weather and the fact that I am waiting for some paperwork to go through before I can apply for more therapist positions. This freedom has allowed me to get on intimate terms with some of the more geologically interesting formations on the West side of Denver, including the sedimentary Mount Glennon, volcanic South Table Mountain, and metamorphic/granitic Mount Falcon Park. What most struck me across these environments was how the geological and ecological past, present, and future of each place seemed to be written in its flora. I would like to visit each of these places in time Christmas Carol-style by looking at evidence for what was, what is, and what yet may be. I've enjoyed the grounded sense of the passage of time that I've found in observing what the flora around the Denver area can tell me. It is my hope to convey some of that sense in this post.

Ghosts of Life Past 

Honestly I considered not adding anything about this, as it is a thematic departure from past posts. However, I found some plant-related things that are simply too cool not to share, and having something from the distant past seemed to round out the analogy with A Christmas Carol, so I went for it. Please hang with me for a moment to delve into the likely non-existent world of amateur paleobotany. 

West Denver affords some interesting opportunities to a person who is interested in rocks and also in plants. The uplift which created the Rocky Mountains also revealed in a step-wise fashion ancient layers of sediment. The oldest sedimentary layer, the Fountain Formation, traces its venerable heritage to between 290 and 296 million years of age. Anyone who has visited Red Rocks Park or Garden of the Gods in Colorado Springs has been looking at parts of the Fountain formation(1). A few layers above the Fountain Formation lies a belt of red to white sand and clay which constitutes the famous 146.3-146.8 million-year-old (Jurassic period) Morrison Formation(2). Even if you haven't heard of the Morrison Formation, you have almost certainly heard of the dinosaur fossils discovered within it both at Dinosaur National Monument in Utah and at Dinosaur Ridge in Denver. Sitting above the Morrison Formation is the layer that I spent some time exploring on a sunny February's day, the younger but still ancient 100.5 to 66 million year-old Dakota Formation(3). All three major layers are present in the photo below. 

The Fountain Formation is visible as Red Rocks Park to the top center and center-left. The Morrison formation is difficult to see, but is present in the lowest rock strata of the hogback ridge at right (Dinosaur Ridge). The Dakota Formation is visible at far right in the crest of white sandstone along the top of the hogback, as well as in the heavily lichenized sandstone in the foreground of the photo. The town is Morrison, Colorado. 

The cool thing about having these layers readily exposed and nearby is that evidence of ancient life and landscapes can be found with only a little determined rock scrambling. The photos below illustrate some of the more interesting things I found poking around the Dakota Formation near the summit of Mount Glennon, a hogback designated as park land just south of Morrison. 

Ancient ripples in sand are preserved on multiple rock faces along the top of a sandstone ridge, here covered in a layer of the lichens including Dimelaena oreina. The Dakota Formation sat partially submerged in or along the edge of a vast inland sea (which is one of my favorite tropes in museum educational films)(3). This also meant that much of what is now the Dakota formation was honeycombed with river drainages leading to said inland sea. Based on some other fossils I found (see below), I'm guessing that these are the ripples of freshwater body and not an ocean. 

Charcoal on the West of side of South Table Mountain, across a small valley from Mount Glennon at about the same altitude. Gigantic lava flows formed the caps of each table mountain, sealing the Late Cretaceous Dakota Sandstone under a blanket of igneous rock(4). This charcoal was embedded in the  very bottom of the lava cap and was integrated into the rock strata, suggesting that it is the remains of an ancient tree which underwent pyrolysis as it was submerged in lava. I have no idea how to go about identifying the species, or whether it is even possible at this point. I'm not sure that charcoal can fossilize, but it certainly appears to be quite stable as the location of this charred branch would have the tree growing 66-ish million years ago. Presumably the shallow depth and low pressures found below the lava flow disallowed conversion into coal or oil.

Two fossils from the Dakota formation exposed near the summit of Mount Glennon. Most of the fossils I saw in this area were plant fossils preserved as red-brown iron oxide in white sandstone. The fossil at left contains two lanceolate leaves, while the fossil at right has the texture of rough linear tree bark, such as might be found on a tree belonging to the genus Populus. Both features are suggestive of a member of the family Salicaceae (the family containing willows, aspens, and cottonwoods), which was already established here in the Late Cretaceous period(5). 

Another bark fossil, this one still embedded in the cliff and covered with a generous coating of lichen. The parallel grooves of the "bark" again seem to suggest Salicaceae. Or I could be way off.

A closer look at the fossil with the leaves (click to enlarge). Compare these lanceolate leaves with a prominent central vein and pinnate venation to that of a modern willow (Salix amygdaloides). It would appear that at least part of the Cretaceous landscape bore the familiar, willow-like plants that still grow along streams here today. 

Ghosts of Life Present 

Fast-forward about 66 million years the late 1980s, when a fire leveled some of the forest on an expanse of the foothills which is now Mount Falcon Park(6). I thought it would be an interesting place to take stock of "life present" since I feel like the dynamism of the ecosystem is more apparent while recovering from a natural trauma like fire. Winter is actually an interesting season to investigate new growth in a burn scar, as the more year-round organisms such as lichens and bryophytes are not obscured by larger vascular plants.

The burn scar as it appears now. The new trees taking over appear to mostly be Pseudotsuga menziesii (Douglas Fir), Pinus nigra (Austrian Pine), and Pinus ponderosa (Ponderosa Pine). The extreme slow return of forest growth underscores the harsh, dry environment of the foothills near Denver. It also demonstrates the need for other organisms to assist in maintaining soil coverage and moisture retention until the trees can provide a consistent canopy. 

Several species of mosses covered the soil of the sunny, exposed burn scar. Mosses and lichens tend to be lumped into the category of cryptogams, meaning "secret marriage," and cryptogamic crusts on the surface of the soil are ecologically important in harsh environments for maintaining soil integrity. One study demonstrated an orderly succession of cryptogamic flora devlopment following a fire in a subarctic forest, and it is reasonable to surmise that something similar happens in pine forests of the foothills(7). Mosses also help to modulate important aspects of soil chemistry such as nitrogen levels in arid climates such as ours(8). The same research indicates that such mosses might also have a buffering effect on the destructive effects of climate change on soils. The plant in this photo is actually not a true moss but a lesser club moss, which presumably performs many of the same functions. 

A Diploschistes lichen parasitizes the club moss from the previous picture. As members of cryptogamic crusts lichens help to modulate soil temperature and moisture by providing a protective coating on the soil(9). Some lichens fix nitrogen and contribute to soil fertility, but I couldn't find information on whether Diploschistes performs this function or not.

More evidence of the forest regenerating. This tree, felled by fire, demonstrates both cuboidal brown rot (deeper into the log and left) and white rot (superficially on the log across the top and center right). Cuboidal brown rot has a distinct geometric appearance (click to enlarge) and is an important factor in forest soil regeneration. It is carried out by a variety of fungi which only digest cellulose, leaving the lignin behind(11). White rot is performed by fungi which can digest both cellulose and lignin, leaving behind a brittle white veil. These processes are helping to enrich the soil in the burn scar, and an increase in plant growth and diversity could be seen in the outflow plumes of material downhill from rotting logs. 

Lichen thalli beginning to re-establish themselves on the wavy granite-based metamorphic boulders in the burn area. Boulders outside the burn area were rife with lichens, but many boulders within the burn scar were either completely sterile or only just now beginning to show signs of life.

Rhizoplaca chrysoleuca colonizing a scorched boulder face. One study noted that new lichen colonies can take up to 10 years to become established, and that R. chrysoleuca may have a growth rate of between .32-.89 mm/yr, for dry/harsh and more hospitable habitats respectively(12). The largest thallus in this image is about 7 mm across, which minus a decade of establishment time gives us a ballpark growth rate of .35 mm/yr or so. This is consistent with what we know about the rough date of the fire and the lichen's growth rate in a dry environment, which means that we just got to perform a little amateur lichenometry! Some boulders in the burn area have yet to recover any lichen flora at all.

Ghosts of Life Future

Anthropogenic climate change, a.k.a. global warming, is upon us (13). A report for the Colorado Water Conservation Board indicates that average annual temperatures in state are expected to rise by 2.5ºF by 2025, and 4ºF by 2050(14). A report submitted to the Colorado Office of Energy breaks down threats of climate change into distinct domains, which identifies ecology as one major area of concern. These changes can already be seen, as in studies which demonstrate that the ongoing proliferation of pine beetles is exacerbated by warming temperatures(15). Other studies demonstrate what to look for, such as one that found there are observable temperature-induced changes in the phenology (physical gene expression) of alpine flowering plants(16). 

The final "ghost" of what may be is what local species can tell us about our climate and possibly what direction the climate is heading in. Which species are growing where, and when, are useful indicators for how a habitat is doing. Things are growing which should not yet be, owing presumably to this past January being the warmest on record. This year is also unusual because El Niño boosted temperatures above and beyond those from climate change alone(17), so I doubt that it can be considered representative of the new normal for Colorado winters. All the same there are some interesting things afoot in the flora around Denver for those who care to look. I can't even pretend to have the experience or the knowledge to draw conclusions about what I am seeing in the plants around Denver. However, sometimes science can been done simply by people recording what they observe, which is what I have done. The photos below illustrate some of my observations. 

Verbascum thapsus (Mullein) and Linaria dalmatica (Butter and Eggs) both showing 

robust growth in the dead of Winter. This picture was taken on the west-facing slope of South Table Mountain at around 5,500 ft or so. Both species are classified as noxious weeds in CO. I have noted these two species already sprouting on hikes in several locations, while many native species appear to still be dormant. The willingness to grow early despite the possible dangers of another freeze may increasingly give invasives such as these an adaptive advantage as temperatures warm. 

Another exotic species, Alyssum simplex. A. simplex is a widely distributed annual weed across the American west, which is originally from the Mediterranean/North Africa(18). Note the coin-shaped seed replums (replae??) of last year's seeds, which are characteristic of Brasicaceae(19). When mature it has tiny clumps of yellow flowers. It is not considered noxious in Colorado, but it is certainly not native, and even in February it is covering the foothills in a fuzzy green blanket. As temperatures warm, it is possible that plants better adapted to a warmer climate such as A. simplex will supplant the old natives that have survived here for millennia. Annuals also tend to be better-suited to toughing out variations in climate than perennials(19), which will give alien weeds like A. simplex an adaptive advantage.  

A native Physaria of some sort also showing extensive growth on South Tabletop Mountain despite the season. It is possible that species such as this are simply well-adapted to the rigors that climate change will bring, but I worry what will happen to those big leaves if we have another hard freeze, or if the perennial root is subjected to excessive heat or desiccation. 

Another native, possibly Heterotheca villosa, putting out new growth. Again, the concern is that Winter weather may reassert itself and cut this plant back to the ground, or that changing patterns of precipitation may make its perennial habit unsustainable. 

The lichen Lecidia atrobrunnea growing on sandstone around 5,500 feet or so. Note the whitish webbing around the edges of the squamulose areoles and the prothallus (review lichen terminology here). I noted that the thalli with noticeable white webbing seemed to be qualitatively mangier and more inconsistent than those without. 

50X magnification of L. atrobrunnea prothallus. The white webbing is either a 

mold or the L. atrobrunnea fungus growing. Either case could be bad for the lichen, as molds will simply kill the lichen, while lichens placed in overly-ideal conditions will essentially "eat themselves" as the fungus outperforms the algal photosymbiont or vise versa(20)*. This may become more likely as fluctuations in temperature ranges caused by climate change create novel conditions for our front range lichens. L. attrobrunea tends to grow at high altitudes and in the arctic(21), so these lower-growing colonies probably represent the more extreme climatic outliers of this species. These colonies' continued success or failure could tell us about changes in our climate: L. atrobrunnea takes quite long to grow (living up to 1500 years!)(22), and the sudden death of a colony indicates something very new happening for the first time in a long time climatically.

*Apparently this is a real problem in the laboratory cultivation of lichens, as the conditions lichens need to thrive demand moisture and temperature variability. The authors of the cited paper created a culturing device called the "Thallinator" to address this problem. These are the gems buried deep in the cryptogam literature.  

Another crustose lichen, this one in the genus Lecanora, being devoured by a simple white mold. The growth rate of the lichen shown is unknown, but these thalli will have taken a long time to reach these proportions simply by virtue of being a crustose variety. They endured for many, many years until they were abruptly destroyed by this opportunistic white mold when the climate conditions soured. 

Thank you for reading! I understand that the material here was heterogeneous, but I wanted to share the sense I experienced of seeing time through a botanical lens.

References

1. Wikipedia.org (n.d.). Fountain Formation [entry in wiki]. Retrieved from https://en.wikipedia.org/wiki/Fountain_Formation on 20 February, 2016.
2. Wikipedia.org (n.d.). Morrison Formation [entry in wiki]. Retrieved fromhttps://en.wikipedia.org/wiki/Morrison_Formation on 20 February, 2016.
3. Wikipedia.org (n.d.). Dakota Formation [entry in wiki]. Retrieved from https://en.wikipedia.org/wiki/Dakota_Formation on 20 February 2016.
4. Drewes, H. (2008). Table Mountain Shoshonite Porphyry Lava Flows and Their Vents, Golden, Colorado. Geological Survey Scientific Investigations Report 2006–5242, p. 28.
5. Enchantedlearning.com (n.d.) Cretaceous Plants. Retrieved from  http://www.enchantedlearning.com/subjects/dinosaurs/plants/Cretaceous.shtml on 20 February, 2016.
6. Summitpost.org (n.d.) Mount Falcon. Retrieved from http://www.summitpost.org/mount-falcon/184021
7. Black, A. R., & Bliss, C. L. (1978). Recovery sequence of Picea mariana –Vaccinium uliginosum forests after burning near Inuvik, Northwest Territories, Canada, Canadian Journal of Botany, 56(17): 2020-2030, 10.1139/b78-243
8. Delgado-Baquerizo, M., Maestre, F. T., Eldridge, D. J., Bowker, M. A., Ochoa, V., Gozalo, B., & ... Singh, B. K. (2016). Biocrust-forming mosses mitigate the negative impacts of increasing aridity on ecosystem multifunctionality in drylands. New Phytologist, 209(4), 1540-1552. doi:10.1111/nph.13688
9. Walewski, J. (2007). Lichens of the North Woods. Duluth, MN.: Kollath+Stensaas Publishing
10. Stucky Evenson, V. (2015). Mushrooms of the Rocky Mountain Region. Portland, OR: Timber Press.
11. Clarke.edu (n.d.). Lesson: A comparison of Brown rot and White Rot fungi (Lab curriculum)[downloaded document]. Retrieved from http://www.clarku.edu/faculty/dhibbett/tftol/lesson%20plans/white%20brown%20rot%20lesson.doc
12. Timoney, K. P., & Marsh, J. (n.d). Lichen trimlines in northern Alberta: Establishment, growth rates, and historic water levels. Bryologist, 107(4), 429-440.
13. I'm not even doing a citation for this. Statements from organizations such as NASA which do extensive review of peer-reviewed research and data may be found here: http://climate.nasa.gov/scientific-consensus/
14. Colorado Water Conservation Board. (n.d.). Climate Change in Colorado [Report compiled for a government organization]. Retrieved from http://cwcb.state.co.us/environment/climate-change/Documents/COClimateReportOnePager.pdf on 21 February, 2016
15. Hansen, E. M., Bentz, B. J., Régnière, J., Fettig, C. J., Seybold, S. J., Hayes, J. L., & ... Negrón, J. F. (2010). Climate Change and Bark Beetles of the Western United States and Canada: Direct and Indirect Effects. Bioscience, 60(8), 602-613. doi:10.1525/bio.2010.60.8.6
16. Kimball, K. D., Davis, M. L., Weihrauch, D. M., Murray, G. D., & Rancourt, K. (2014). Limited alpine climatic warming and modeled phenology advancement for three alpine species in the Northeast United States. American Journal Of Botany, 101(9), 1437-1446. doi:10.3732/ajb.1400214
17. National Aeronautics and Space Administration [NASA] (2016). NASA, NOAA Analyses Reveal Record-Shattering Global Warm Temperatures in 2015. Retrieved from http://www.nasa.gov/press-release/nasa-noaa-analyses-reveal-record-shattering-global-warm-temperatures-in-2015
18. Centre for Agriculture and Biosciences International [CABI].(2016). Alyssum simplex [entry in Invasive Species Compendium]. Retrieved from http://www.cabi.org/isc/datasheet/112179 on February 22, 2016.  
19. Weber, W. A., & Wittman, R. C. (2012). Colorado Flora: Eastern Slope Boulder, Colorado: University Press of Colorado.
20. Pearson, L., C. (1970). Varying environmental factors in order to grow intact lichens under laboratory conditions. American Journal Of Botany, 57(6), 659-664.
21. Brodo, I. M., Sharnoff, S. D., & Sharnoff, S. (2001). Lichens of North America. New Haven, Connecticut: Yale University Press.
22. Miller, C. (1973). Chronology of neoglacial deposits in the northern Sawatch Range, Colorado. Arctic And Alpine Research, (4), 385-400.