San Francisco State University
Department of Geography

Geography 316:  Biogeography

The Biogeography of
Canyon Liveforever (Dudleya cymosa)

Source:  Weeden 1996
 
 

by: Tiffany Meyer

San Francisco State University
Geography 316
November 30, 1999
 
 

Canyon Liveforever:  The Inside Story

Kingdom: Plantae
Division: Magnoliophyta  (Angiosperms)
Class: Magnoliopsida
Subclass: Rosidae
Order:  Rosales
Family: Crassulaceae  (stonecrop)
Genus:  Dudleya
Species:  Dudleya cymosa

Common name:  Canyon liveforever or Hens and chickens

Natural History of Dudleya cymosa:

   Dudleya cymosa is a small, herbaceous perennial plant with furry, thick, blue-gray, teardrop shaped leaves that grow in rosettes close to the ground.  The flower, which is seen from April to June, rises above the rosette about 4 to 6 inches on a thick reddish tinted stalk (Weeden 1996).  It is brightly colored in orange and red and even at full bloom has the appearance of a flower that has not quite bloomed yet.  These odd looking blossoms that almost have beak like appearances and sit on straight, thick leggy stems have won the plant the unattractive name of Hens-and-chickens.  It has also been given the common name Canyon Liveforever.

Source: CalFlora Occurrence Database
 

    Dudleya cymosa is in the class of angiosperms; it is a flowering plant that produces a seed-filled fruit.  The genus Dudleya was described in 1903 by Britton and Rose (Jacobsen 1960), who honored W. R. Dudley of Stanford University, by naming it after the professor (Martin & Chapman 1977).  There are forty species within the genus, and many subspecies within those forty.  Some of the subspecies of D. cymosa, such as crebiflora, are uncommon, and some are even considered rare, such as the subspecies ovatiflora, which is found only on harsh granitic slopes (Jepson 1993).
    Dudleya cymosa belongs to the Crassulaceae family, of which many members fall into a well-known category of plants called succulents.  The term succulent refers to a group of plants that share similar morphologies but are not necessarily related to each other.  In fact there are many succulents that belong to different plant families and genera.  The most familiar succulents are cacti, and like these rugged desert plants, Dudleya cymosa has been very successful in using a variety of methods to survive its harsh environments of low precipitation and high light intensity.  The thick, sclerophyllous leaves of the small plant grow in little rosettes close to the ground.  This creates less surface area and allows it to conserve water.  Dudleya cymosa also has a fine layer of hairs on its thick, fleshy leaves that reduce evaporation by reflecting sunlight away and slowing down air movement across the leaf (Whitney 1979).  These hairs are what give the plant its soft, blue gray appearance.
    Dudleya cymosa also combats its dry habitat by flowering in the early spring, around April, to take advantage of the moist time of the year.  The insects that pollinate it must be quick because it is an ephemeral plant, meaning that its growing season does not last very long.  The plant becomes withered by the summer time, for like most desert shrubs, it has a short life cycle (Hickman 1993).
    Dudleya cymosa shares many physical (and physiological) similarities to cacti which makes it a superb and frequently used plant for ornamental cultivation. Dudleya cymosa requires excellent drainage and is intolerant of frequent summer water, so it should not be grown near lawns or other water loving plants.  It can be successfully grown just about anywhere with in California, except the extremely cold, high elevations of the Klamath, Cascade or Sierra Nevada mountain ranges or the high desert (Hickman 1993).  This makes it ideal for gardeners who frequently forget to water their gardens or enjoy drought tolerant plants.  I have some lovely Dudleya in my own yard that add a wild, natural look to the landscape instead of the harsh, dangerous appearance a thorny, prickly cactus can project.  The stems and leaves even become edible after cooking (Weeden 1996).

Distribution

    Dudleya cymosa is a special plant because not only is it native to California, it is also endemic to California, meaning that it is only found naturally in this State.  It is distributed throughout the more mountainous parts of the State such as the North Coast Ranges, the volcanic Cascade Range, the Sierra Nevada, San Francisco Bay Area including Mount Tamalpais, the Diablo Range, and the Santa Cruz Mountains.  There are also subspecies found in Southern California and Baja California.

    Although the Crassulaceae family that the Dudleya cymosa belongs to is distributed in a cosmopolitan fashion and is found throughout the world on various continents, the species Dudleya cymosa has a very limited ecological tolerance.  Since it prefers xeric sites and is found only in California, it tends to be dispersed in small aggregations giving it a stenotopic distribution.
    Being rather small, and practically the same color as its surroundings, a plant like the Dudleya cymosa would not normally be considered a very conspicuous plant, but it has chosen an environment that allows it to stand out and catch the eye immediately.  Since it is generally found on steep, rocky limestone outcrops and talus slopes, there are not many other plants around to divert ones attention from it.  It can be found at elevations ranging from 300 feet up to 8000 feet (Jepson 1993).  These types of dry, rocky sites, that Dudleya cymosa established themselves in are called xeric.  In the Sierra, xeric sites are alpine gravel patches, rock outcrops, grasslands with moist soils, chaparral, foothill woodland and sagebrush (Weeden 1996).  This type of environment can experience extreme temperatures, from freezes, snow and low, unpredictable rainfall in the winter, to hot, dry direct sun light in the summer months.
    Dudleya cymosa is considered a xerophyte because it grows in these arid climates.  Xerophytes have made unique adaptations to be able to survive areas of low precipitation, high temperatures and frequent drought (Stern 1988).  In fact the Dudleya cymosa requires and is limited to areas of high sun exposure and well draining soils.  Because it lives in these harsh conditions, it does not’t have to compete very readily with other plants.  Other xerophytes that endure these conditions are rock-cress, mountain mahogany, fremontia, bedstraw (Galium), various types of penstemons, blazing star, poison oak, and bead fern (Cheilanthes) which is a type of fern found on dry, rocky habitat.  These plants are commonly found in association with Dudleya cymosa (Bartel & Shecock 1990)

Source: CalFlora Occurrence Database

     In addition to conserving limited resources, Dudleya cymosa uses Crassulacean Acid Metabolism (CAM) photosynthesis to create its food and energy. The CAM process was first discovered in the Crassulaceae plant family so that is what it is named after (Hart, Eggli 1995).  Most plants do all of their work during the day, taking up carbon dioxide through their plant pores, called stomata, and using the energy from sun light to convert the gas into food and oxygen.  But Dudleya cymosa and its fellow CAM relatives only open their stomata at night to take up carbon dioxide.  They keep their stomata closed during the hot days, allowing the plant to conserve tremendous amounts of water (Stern 1988).  The plant assimilates the carbon dioxide at night and converts it to a variety of organic acids.  In the morning when the stomata close, the organic acids break down and carbon dioxide is released.  With the energy of the new days sun light, the plant uses the Calvin cycle to convert the carbon dioxide it has been accumulating all night into sugars (Campbell 1990).  Dudleya cymosa’s stomatal circadian rhythm is out of phase with non CAM plants by about 12 hours (Campbell 1990).  By adapting the method of CAM photosynthesis, Dudleya cymosa is able to live in areas where the air is so dry that the light intensity is extremely high because there is less atmospheric water vapor to filter out some of the sun’s rays (Stern 1988).
 

Evolution

    Like all flowering, seed bearing angiosperms, Dudleya cymosa’s evolutionary record is difficult to trace because it abruptly appears in the fossil record 120 million years ago, during the Cretaceous period, with no sign of previous ancestral links (Campbell 1996).  Charles Darwin calls the evolutionary history of angiosperms an “abominable mystery,” one that is yet to be solved.  Once established, angiosperms diverged into the dominant plants on earth, and now make up about 99% of the plant species known today.  D. cymosa showed up somewhere during this time of rapid and expansive diversification during the Cretaceous period.  However, it is extremely hard to say exactly when, because the Dudleya fossil record is even more elusive then its angiosperm relatives.  The hot, dry climate that Dudleya cymosa prefer does not seem to be very conducive to fossil formation (Winter& Smith 1996).
    Since the Dudleya’s fossil record is not able to tell us much, scientists have turned to other methods of trying to determine the history of these sun loving plants.  Because Dudleya cymosa is a xerophytic plant that prefers hot, dry climates, it may have evolved during the early to mid Cretaceous period when the climate became drier and seasonally arid after the era of lots of rain and moisture ended. Dudleya cymosa has an incredible ability to conserve carbon dioxide because it uses CAM photosynthesis, which could be another hint as to when it evolved.  Most plants lose excessive carbon dioxide on hot days through photorespiration, but D. cymosa conserves carbon dioxide by closing its stomata at night so it requires a much smaller amount of carbon dioxide then the c3 photosynthesizers and is considered a “CO2 concentrating mechanisms.”  It probably evolved during a time when the carbon dioxide to oxygen ratio was low (Smith & Winter 1996).  There is evidence that the Silurian period had low levels of carbon dioxide, but flowering plants were not in existence 450 million years ago (MYA).  During the glacial epoch of the Pliocene Epoch, five MYA, the CO2:O2 ratios were significantly lower, which was the next advantageous time for succulents (Smith & Winter 1996).  The hypothesis that succulents evolved during periods of low carbon dioxide levels contradict the idea that they evolved during the hot, arid times of the early to mid Cretaceous period, because there were high levels of carbon dioxide during those times.  So contradicting time lines and lack of fossil records makes it very difficult to give a time for succulents’ arrival.
    Because succulents are found in a variety of families, they are thought to be polyphyletic which means they evolved out of different plant families.  Our plant of interest probably evolved from a Central American primitive Sedum taxa, a Crassulaceae with a much broader distribution then Dudleya (Winter & Smith 1996).  Because of their large scattering of relatives and very specific specialization, we can deduce that CAM plants most likely evolved from C3 photosynthesis plants, which are the most common, wide spread type of plants (Winter & Smith 1996).  Before being permanent CAM photosynthesizers, CAMs were probably originally C3 plants that started intermittently switching to CAM methods of photosynthesis during times of drought and low rainfall.  Eventually those species evolved into species that solely depended on CAM photosynthesis (Smith & Winter 1996).  Dudleya belongs to the Crassulaceae family’s western U.S. branch of the Leucosedum-clade, which includes a variety of species of its Sedum parent.  This clade share the characteristics of being glandular pubescent and having equal length, basally connate sepals (t’Hart & Eggli 1995).
    Despite its esthetic beauty and elegant method of photosynthesis, there has been little information published on Dudleya cymosa.  Its specialized habitat requirements of arid, xeric sites tend to be found in localized patches that are often steep, rocky and difficult to access.  This isolation not only initiates new, interesting speciation, like the D. cymosa sub-species minor, which is a dwarf with orange flowers (Bartel & Shevock 1983), but it also gives D. cymosa the advantage of having little competition, particularly with humans.  Hopefully it will be left to flourish timelessly as an endemic California native.
 
 

References

Barbour, M.G., Major, J. 1995 Terrestrial Vegetation of California.  UC Davis, CA.  California Native Plant Society.

Bartel, J.A., Shevock, J.R. 1983. “Dudleya calcicola (Crassulaceae), a new species from the Southern Sierra.”  Madrono 30:210-216

Bartel, J.A. Shevock, J.R. 1990 “Dudleya cymosa subsp. costafolia (Crassulaceae), a new subspecies from the southern Sierra Nevada, Tulare County, California” Aliso 12(4): 701 –704.

Britt, Rose, Biological names, October 27, 1999.[Online].  Integrated Taxanomic Information Systems (ITIS). http://www.itis.usda.gov/plantproj/itis/cgi_bin/CBI.cgi?kingdom=Plantae&input%5ftype=T&input=502170

Cal Flora Occurrence Database August 18, 1999  http://www.calflora.org/calflora/occ/

Campbell, N. A. 1990 Biology second edition.  Riverside, CA.  The Benjamin/Cummings Publishing Company, inc.

Chapman, P.R., Martin, M.I., 1977 Succulents and their Cultivation.  Charles Scribner’s Sons.

Hickman, J. C. (editor) 1993. The Jepson manual: higher plants of California. Berkeley, CA.  University of California Press.

Jacobsen, Hermann. 1954. A Handbook of Succulent Plants.  London, England, Blandford Press.

Smith, J.A.C., Winter, K. 1996. Crassulacean Acid Metabolism, Biochemistry, Ecophysiology and Evolution.  Berlin Heidelberg, Germany, Springer-Verlag.

Stern, K.R. 1988.  Introductory Plant Biology.  Dubuque, IA.  Wm. C. Brown Publishers.

‘t Hart, H., Eggli, U., 1995 Evolution and Systematics of the Crassulaceae.  Leiden, The Netherlands,  Backhuys publishers.

Weeden, Norman F. 1996. A Sierra Nevada Flora.  Berkeley, CA.  Wilderness Press.

Whitney, S. 1979.  A Sierra Club naturalist’s guide to the Sierra Nevada.  San Francisco, CA.  Sierra Club Books.
   
 
  

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