San Francisco State University
Department of Geography

Geography 316:  Biogeography

The Biogeography of  Toxicodendron diversilobum  or 
Western poison oak

by  Emily Meriam student in Geography 316, Fall 2001

Taxonomy KINGDOM: Plant DIVISION: Spermatophyta CLASS: Dicotyledoneae ORDER: Sapindales FAMILY: Anacardiaceae GENUS: formally Rhus now Toxicodendron SPECIES: Toxicodendron diversilobum (Hauser 2001)     The Latin name means: toxico, poison and dendron, plant or tree; in Greek diversi is the diversity of leaf shape, lobum or lobata is for loba  and describes the appearance of the  poison oak leaf (Hauser 2001).

Figure 1: Poison oak in the fall at Mt. Diablo State Park, California. Photo by A. McTavish (2001).

Figure 2: Poison oak stalks after leaf loss at Mt. Diablo.
Photo by A. McTavish (2001).

Introduction 

    As a small child, “Leaves of three, let them be” was a warning I always heard from my grandmother prior to going out for a walk in the woods.  My grandparents lived in a remote portion of the Sierra Nevada at about 3500 feet, and poison oak grew all over the place.  It was my grandfather’s nemesis, as he was constantly trying to remove the plant away from the small trails he cleared for people to explore the wonderful woods on his property. 
    For all of those who have experienced the pernicious wrath from either poison oak, poison sumac, or poison ivy, it is universally understood how uncomfortable and unforgettable the sensation can be.  The skin irritation that develops from the interaction with our skin, due to an oil on the plant called urushiol, is wretched and agonizing.  It can cause complete anguish for an individual, even though they may have had just had a brief encounter with the virulent plant.  Unfortunately for humans, and not all of humans are allergic, we are the most apt to suffer the affects within the natural world of this plant. 
    Although poison oak, poison sumac, and poison ivy all share the same noxious toxicity, general appearance, and family,  this paper is a comprehensive inquiry into Toxicodendron diversilobum or Western Poison oak.  There are many fascinating things to learn about poison oak.  It is a despised plant, yet visually beautiful.  It has its place within its natural environment and has a purpose in the ecosystem.
    Over two hundred years ago, Carl Linnaeus placed poison sumac, ivies and oaks in the same category along with the other less toxic members of the Rhus genus (Brooks 2001). This has caused a great deal of confusion over the years.  It was not until mid last century that William T. Gillis, from Michigan State University, proposed that the the toxic varieties containing urushiol be placed within their own genus, Toxicodendron (Hauser 2001).  Despite the fact that the previously used Rhus label is still used from time to time,now the three plants formally identified as Rhus radicans (poison ivy), Rhus toxicodendron (poison oak), and Rhus vernix (poison sumac), are now separated into five Toxicodendron species (Hauser 2001).  It was botanist David Douglas that scientifically distinguished this western variety of Toxicodendron in 1830 on Vancouver Island. (Brooks 2001)

Diversity of the Genus

(Hauser 1996)

POISON IVIES: Previously divided into one species called either Rhus radicans and Rhus toxicodendron.  Now in the Toxicodendron genus and is divided into two species.

     1. Toxicodendron radicans; shrub or climbing vine; eastern US and Mexico.
     2. Toxicodendron rydbergii; dwarf shrub; central and west central US and Canada.

POISON OAKS: Previously divided into one species called either Rhus toxicodendron, Rhus quercifolia, Rhus lobata, and Rhus diversilobum.  Currently in the Toxicodendron genus and is divided into two species.

     1. Toxicodendron pubescens; non climbing shrub; southeastern and south central US.
     2. Toxicodendron diversilobum; bush or climbing shrub; west coast US and British Columbia.

POISON SUMAC: Still considered one species formally called either Rhus vernix, Rhus venenata, and Rhus glabrum.  Currently all are in the Toxicodendron genus.

    1. Toxicodendron vernix; shrub or tree shape; eastern US and Canada.  

Habitat and Distribution

United States

Green areas indicate confirmed presence.
Figure 3: United States Distribution of Toxicodendron diversilobum
(Source: BONAP Distribution Data 1998)  

California

Figure 4: California distribution of Toxicodendron diversilobum
(Source: CalFlora Database 2001)

    Poison Oak seems to be widespread all over the state of California (see Figure 4 for California distribution of Toxicodendron diversilobum.)  It can be called a  dexterous plant, as it is certainly able to live in a variety of places.  Western poison oak’s distribution pattern is continuous throughout all of its adjacent regions.  It is California’s most distributed shrub and occurs in mixed green evergreen forests, woodlands, chaparral, coastal sage scrub and riparian zones (Harris & Howard 1994).  It is also found from British Columbia through to Baja California, and west of the Cascade Range in Washington and Oregon (Harris & Howard 1994).  The Bureau of Land Management places it in four physiographic regions: Northern Pacific Border, Cascade Mountains, Southern Pacific Border, and Sierra Mountains (Harris & Howard 1994).  The ecosystems it prefers to live in are Douglas-fir, Ponderosa Pine, Hemlock-Sitka spruce, Redwood, Western hardwoods, and Chaparral-mountain shrub (Harris & Howard 1994).  In the Columbia River Gorge area, western poison ivy hybridizes with poison oak (Howard & Harris 1994). 
    Poison oak is very resourceful and versatile.  Its range extends from sea level to 4000 feet.  The soils it prefers are dry, but it can also adjust to living in moist environments.  It has been shown that they grow especially well in areas where the ground has been disturbed such as forest trails, city parks, suburban backyards, and roadside embankments (Hauser 2001). Essentially, poison oak is not limited to particular soils or drainage patterns.   It does opt for sunnier locations, but the adroit plant can dwell in shady or non direct sun areas.  It should be noted that when in vine form, poison oak may kill its support plant by smothering or breaking it due to its opportunistic stem and root system (Harris & Howard 1994).     

Description of Species

Figure 5: Poison oak in a beautiful red display at Mt. Diablo. Photo by Barbara Holzman (2001).	Figure 6: Poison oak berries Photo by A. McTavish (2001).

    Poison oak loses its leaves in the winter (deciduous) and can be vine or shrub in appearance (DiTomaso & Lanini 2001).  In winter it is often difficult to recognize poison oak because it looks like dead slender and elongated gray sticks that just stick out of the ground (see Figure 2).  The leaf buds open February to March and the stems grow March to April; Flowering is March through June and the leaves drop late July to the beginning of October; the fruits disperse in summer and fall (Harris & Howard 1994).  The leaves of the poison oak plant resemble the oak tree leaf.  The leaf is usually three leaflets and the stalk and center leaf is longer than the two on either side, and each leaf is 1 to 4 inches long with toothed and lobed edges (DiTomaso & Lanini 2001).  The colors of the leaves vary seasonally.  The leaf begins green and eventually turns stunningly brilliant hues of red, orange, pink, and yellow prior to browning and falling off.  The fruits hang in small groups near the leaves and are round and white in color. 
    Poison oak is dioecious, it has to have male and female plants to reproduce.  Armstrong and Epstein (1995) explain, “Male flowers contain five stamens and a rudimentary pistil surrounded by five cream-colored petals and five sepals.  Female flowers have a fertile pistil (gynoecium) and reduced, sterile stamens.”   They also add that unisexual and bisexual flowers are possible on the same plant.  Plants are thought not to produce flowers and fruit until their third year (Hauser 1996).  On male plants, the flowers fall away, are more fragent than the females, and often attract honeybees, but on female plants, the flowers are pollinated and become fruit (Hauser 1996).
     The flowers are mostly pollinated by insects, and the single-seeded fruit is eaten by birds and the passage of the seed through the birds digestive tract  facilitates germination (DiTomaso & Lanini 2001).  In addition, bird droppings help to disperse the seeds to other places.  Poison oak is not dependent on fire for regeneration because seedlings sprout up before and after a fire has occurred.  (Harris & Howard 1994).  Underground horizontal root stalks grow slowly and this single root system has the ability to cover large areas (DiTomaso & Lanini 2001).  Poison oak will sprout from the rhizomes and root crown if a fire or animal has removed top growth (Harris & Howard 1994). 

Evolution and Natural History

    The Archives in Dermatology in 1987 announced that a 35 million year old fossil of a poison sumac leaf had been found in volcanic ash deposits in central Oregon (Hauser 1996).  This could mean that Homo sapiens have been potentially feeling the virulent and annoying effects from the Toxicodendron species for quite some time.  Since poison ivy no longer grows on the west coast, William T. Gillis in 1975 thought that other poison ivy fossils that had been found  "resembled more closely the poison ivies of eastern Asia that they do extant poison ivies from the country [US] today” (Hauser 1996).  Gillis reckoned that the eastern Asian and North American poison ivies most likely “originated in North America about 80 million years ago and migrated across the Bering Straits when there was a land connection between North America and Asia, and when the climate was much milder.” (Hauser 1996)  With changes in temperature and separation of the two continents, distinct subspecies evolved on each continent (Hauser 1996).
    The east Asian connection is considerable.  Of the approximately thirty Toxicodendron species in the world, more than twenty are indigenous to eastern Asia, and only a small number are indigenous to the western hemisphere (Hauser 1996).  There have been literary references of these toxic plants in writings of Chinese scholars back to the seventh century; seeds were found in the medicine bag of a thirteenth century southwestern US American Indian, and some seeds found in the cliff dwellings at Mesa Verde National Park in Colorado were radiocarbon-dated as to having grown in the 13th century (Hauser 1996).   In addition, it was not until 1624 that Captain John Smith became the first European to write about the plant (Hauser 1996).  Later, Captain Frederick Beechey in the 19th century took samples of Western poison oak back to England and it was planted in English gardens, although, later gardeners were humiliated and disappointed (Armstrong & Epstein 1995).  Due to many other individuals lured by its beautiful display of colors in the fall and its seductive and curvy climbing vine, it has spread from America to Continental Europe, England, Australia, and South Africa (Brooks 2001).
    The poison oak resin urushiol may have an evolutionary significance by sealing wounds on the plant, or it may retard the growth of infectious fungal or bacterial spores (Armstrong & Epstein 1995).  Since the leaves are eaten by a wide variety of animals such as deer, goats, horses, cattle, and birds, urushiol as a chemical defense strategy is an un-favored scenario (Armstrong & Epstein 1995).

The noxious poison


Figure 7:
This is the chemical structure of urushiol.  Poison oak contains a combination of  four heptadecylcatechols with a 17-carbon side chain
and poison ivy contains a mixture of four pentadecylcetechols with a 15-carbon side chain (Armstong & Epstein 1995).
Illustration by David Smith (2001).

Figure 8:
This is the chemical structure of poison oak uruishol and its oxidized reactive quinone.  The reactive quinone bonds to the human skin white blood cell membranes (Armstong & Epstein 1995).
Illustration by David Smith (2001).

      Urushiol is the toxic poison in poison oak, and is the common term for the substance in the sap that causes contact dermatitis in humans (Armstrong & Epstein 1995).   What is amazing is specimens 100 years old have been known to cause dermatitis in humans, because urushiol is a relatively stable compound, and can remain potent for years in the absence of oxidation (Armstrong & Epstein 1995).  Urushiol got its name from from the Japanese word “urushi” which was the lacquer produced from the sap of the Japanese lacquer tree (Armstrong & Epstein 1995).  Poison oak plants that are cut or damaged anywhere on their stems, roots, and flowers ooze a terpene oleoresin that oxidizes and polymerizes into a black resinous sap (Armstrong & Epstein 1995).  On the poison oak plant, urishiol is in the resin canals only, and will only get on plant surfaces if the leaves or stems are attacked by insects, or if bruised (Armstrong & Epstein 1995).  Urushiol is not found in honey or pollen that is made from poison oak flowers, but it can be found in ash, dust particles, or minute droplets in smoke when the plant is burned (Armstrong & Epstein 1995). 
    Urushiol is a combination of phenolic compounds called catechols and potent benzene ring compounds with a long side-chain of 15 or 17 carbon atoms (Armstrong & Epstein 1995).  Urushiol is transferred from the plant to human skin by direct contact, contaminated clothing or objects, and animal fur, by penetrating to the epidermal layer of the skin where it binds to proteins of deeper skin cell membranes (Armstrong & Epstein 1995).    Dr. Lisa A. Gartner (1999) explains that, “These substances elicit a classic type 4 cell mediated immune reaction....The typical clinical lesions are erythematous papules and plaques, often with vesticles or bullae; the lesions commonly occur in a linear patters indicating lines of contact with the plant.”  The reaction of urushiol and humans is a red crusty, weeping, bumpy and annoying rash.  It usually lasts from two to five weeks, although some have had it for longer.  Dr. Gartner (1999) also added that, “Blister fluid does not contain urushiol and is therefore not contagious.”  However,  urushiol residue is difficult to wash off and may be spread by scratching (Armstong & Epstein 1995).  Urushiol causes a delayed dermatitis reaction with the human body, and the most uncomfortable of the effects may not be felt for days or weeks (Armstrong & Epstein 1995).    
    Armstrong and Epstein (1995) note, “Poison oak and ivy do not spare age, sex, race or economic status.  Each year thousands of people are afflicted with moderate to severe dermatitis from touching the foliage of these plants.”  One of the most serious ways of contacting poison oak is through fire.  In California, Oregon, and Washington, close to one third of firefighters and forestry workers will suffer from rashes and lung irritations due to burning poison oak (Stehlin 1997).  It is the smoke from burning poison oak that is toxic.    The effects from urushiol will result in more workers compensation claims for lumberjacks, park rangers, and firefighters than any other source (Brooks 2001).
    Fifty to seventy percent of the population are prone to dermatitis from exposure to urushiol (Arriola & Lee 1999).  Although many people feel the allergic reaction from poison oak, there are many people who do not.  Studies have shown that if both a child’s parents react to urushiol, there is an eighty percent chance that they will also (Garner 1999).  Urushiol is so incredibly toxic that it would take only one ounce of it to effect everyone on the earth with a rash (Brooks 2001).
    There are other species in the Sumac family that have urushiol.  The cashew nut shell, mango, Rengas tree, Burmese lacquer tree, India marking nut tree,  and Ginkgo biloba have all been reported to cause dermatitis (Armstrong & Epstein 1995).  

Other members of the Anacardiaceae Family

Table 1.  Species List for Family: Anacardiaceae
(Source: University of Idaho)


Other interesting issues

    Although the poison oak plant causes many ill feelings among those who have experienced its wrath, I felt it was important to include some of the other more positive ways that it is utilized.  In DNA sequence studies, urishiol has been found to mediate DNA strand scission (Harris & Howard 2001).  The poison oak leaves are rich in tannin and can be used for candles, dyes, ink, and varnish for shoes (Calflora 2001).  Leaves have been used to treat paralysis, palsy, ringworm, and herpes (Calflora 2001).  As Armstrong and Epstein (1995) said at the end of their article, “research in poison oak may lead to a better understanding of the human immune system and the treatment of renegade viruses and tumor cells.  In final analysis, poison oak might be a blessing in disguise.”
    Native Americans found a bounty of resources with poison oak.  Pomo Indians used the fresh resin from the plant to cure warts, ringworm,  rattlesnake bites, and dye their baskets (Armstrong and Epstein 1995).  Other tribes wove baskets with the stems and Native Americans in northern California covered the bulbs of soap lilies or acorn meal with poison oak leaves, and baked them in earth ovens to eat (Armstrong & Epstein 1995).  Native Americans also made poultices to alleviate the dermatitis effects from boiled leaves of yerba santa, manzanita, mule ears, balsam-scented sunflowers, basal leaves, and North American jewel weed (Armstrong & Epstein 1995).  One method used here in California by the Indians and early settlers to relieve the rash   was a poultice made from the leaves of the gum plant (Armstrong and Epstein 1995).
    Many animals utilize the plant.  Black-tailed deer and livestock eat poison oak and its palatability is rated good to fair for horses and deer; fair to poor for cattle, sheep, and goats (Harris & Howard 1994).  Chickadees, warblers, flickers, some rodents and honey bees utilize the plant as well in different ways (Hauser 2001).  These animals eat it because it has nutrition.  Percent crude protein in poison oak foliage from plants sampled all over California was 24.2/March, 20.6/May, 10.1/July, and 6.5/September (Harris & Howard 1994).  The mineral content of the leaves is in a percentage: Ca 1.00, P 0.23, K 1.13, Mg 0.59, and S 0.19  (Harris and Howard 1994). 

                                                    
                                                                          Figure 9: Poison Oak growing wild at Mt. Diablo.
                                                                               Photo by Barbara Holzman (2001).

Personal experience with the lovely looking three leaved demon

    I have been a victim of the urushiol wrath.  I have suffered from poison oak many times throughout my life because I am an avid natural world explorer.  I had it as a child on a much smaller scales, however, twice I have suffered terribly from it.  The first time, I was in Santa Cruz, California at the beach hiking with a friend and we got away from the trail.  We had to make our way back to our car through a dense thicket of poison oak which we both were oblivious to for some strange reason. Six days later, the urushiol allergy would completely debilitate both of us for over 10-12 days.  He was actually from New Mexico and when he went to the hospital there in Albuquerque, the doctors were curious to know where he had contacted it,  because it was not local there.  That experience was absolutely miserable and it was difficult and painful to move.  That experience required several hospital visits and prescription drugs since my case was severe.  Lucky for me I had a trip planned to Hawaii two weeks after the rash finally went away.  That certainly helped to forget about it.     
    The second time, I was at an large outdoor Freedom Gathering near Angels Camp, California out in the middle of the woods and I carried a sleeping bag for my friends, which apparently had been laying in poison oak because I eventually got it on both my forearms.  I knew I had been meticulously careful not to walk through any, so I knew that the only way I had came in contact with the oils was from the sleeping bag.  The rash, it seemed to last forever, and as the summer went by I still had it.  One month turned into two, and I was starting to think that it was never going to go away.  It was truly awful and frustrating.  I was using Caladryl, the over the counter pink thick liquid and fresh aloe plant.  Nothing was working.  I tried a homeopathic remedy, different muds and herbs, various natural healing creams and ointments,  still nothing.    I finally went to a dermatologist who had been recommended, and when I showed him my arms, he asked me what I had been using on it.  I told him proudly, “Caladryl of course.”  He explained to me that I was experiencing an allergic reaction to calamine, which is a plant, and is an ingredient in Caladryl.  The poison oak I had been suffering from had gone away about three weeks after initial contact, but, it was my allergic reaction to Caladryl that had prolonged my rash.  I felt totally ridiculous and the rash cleared up quickly with the discontinued use of the Caladryl
    The “ leaves of three, let them be” mantra is a wise one to follow if you are susceptible to the noxious oils of poison oak.  Because the ill effects of the plant produce many negative emotions in humans, it must be recognized that the plant does serve an important function within its environment and it is visually a very attractive and alluring plant.  One of the books I used to do research for this biogeography paper was titled “Nature’s Revenge”.  I thought that was a poignant way to classify the suffering we humans experience when we interact with the natural world carelessly and unconsciously.  You cannot go into nature without looking around and being aware.  That is especially important to remember when you are hiking in California because the lovely and virulent poison oak grows all over the place.                   

Bibliography 

Armstrong, W.P. and Epstein, W.L. 1995. “More Than Just Scratching The Surface.” Herbalgram (American Botanical Council). v34: 36-42.

Arriola, Edgar R. and Plee, Nancy. 1999. “Poison ivy, oak, and sumac dermatitis.” Western Journal of Medicine v171 no5/6; 354-355.

BONAP Distribution Data. 2001. US distribution of Toxicodendron diversilobum. [On-line map]. Available: http://www.csdl.tamu.edu/FLORA/cgi/b98_map?genus=Toxicodendron &species=diversilobum [2001, 5 November].  

Brooks, Bill. The Toxicodendrons: Poison Ivy, Poison Oak, and Poison Sumac. [Online]. Available: http://nac.tamu.edu/x075bb/caddo/frameidx.html. [2001, October 5].

Cal-flora Taxon Report. 2001. Toxicodendron diversilobum. [On-line map]. Available: http://www.calflora.org/  [2001, November 5].

Garner, Lisa A.. 1999. “Poison Ivy, Oak, and Sumac Dermatitis--Identification, Treatment, and Prevention.” The Physician and Sports Medicine Journal. 33-34 41-43.

Harris, Holly T. 1994; Howard, Janet L. 1990. Toxicodendron Diversilobum.  [Online map]. Available: http://www.fs.fed.us/database/feis/plants/vine/toxdiv/ [2001, October 6].

Hauser, Susan Carol. 1996. Nature’s Revenge. New York, New York. Lyons & Burford Publishing Co.

Hauser, Susan Carol. 2001. Outwitting Poison Ivy: How to Treat the Effects of Poison Ivy, Poison Oak, and Poison Sumac--For Hikers, Campers, Gardeners, and Anyone Else Who Spends Time in the Outdoors. [Book Online]. Available: http://www.intraart.com /hauser/nrexcerpt.htm. [2001, October 6].

Holzman, Dr. Barbara. 2001. Professor in Geography and Environmental Studies-San Francisco State University. San Francisco, California. 

Lanini, Thomas and DiTomaso, Joseph M. 2001. Pest Notes: Poison Oak. [Online]. Available: http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7431.html. [6 October, 2001]

McTavish, Anne. 2001. Student in Biogeography-San Francisco State University. San Francisco, California.

Smith, David. 2001. Research Chemist-Roche Bioscience. Palo Alto, California.

Species List. 2001. Species List for Family:Anacardiaceae.  [On-line map]. Available: http://sdg.ag.uidaho.edu/rapid/ANACARDI.html  [2001, November 5].

Stehlin, Isadora B. 1997. “Protect yourself from poison ivy.” Consumers Research Magazine. v80 n8; 29-30.

     
Other source                                                                     

Light, Jan LPN, Marks, James G. Jr. MD, and Williams, Judith V. MD. “Individual Variations in ALlergic Contact Dermatitis From Urushiol”. Archives of Dermatology. v135(8):17033-0850.

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