Toxicodendron diversilobum

Toxicodendron diversilobum (syn. Rhus diversiloba), commonly named Pacific poison oak<ref>Template:PLANTS</ref> or western poison oak, is a woody vine or shrub in the sumac family, Anacardiaceae.

It is widely distributed in western North America, inhabiting conifer and mixed broadleaf forests, woodlands, grasslands, and chaparral biomes.<ref name="Hogan">C. Michael Hogan (2008); "Western poison-oak: Toxicodendron diversilobum" Template:Webarchive, GlobalTwitcher, ed. Nicklas Strömberg</ref> Peak flowering occurs in May.<ref name="inaturalist">iNaturalist: Toxicodendron diversilobum</ref> Like other members of the genus Toxicodendron, T. diversilobum causes itching and allergic rashes in most people after contact by touch or smoke inhalation. Despite its name, it is not closely related to oaks, nor is it a true tree.

Description

Toxicodendron diversilobum is extremely variable in growth habit and leaf appearance. It grows as a dense Template:Convert tall shrub in open sunlight, a treelike vine Template:Convert and may be more than Template:Convert long with an Template:Convert trunk, as dense thickets in shaded areas, or any form in between.<ref name="usfs">U.S. Forest Service: Toxicodendron diversilobum</ref><ref>Integrated Taxonomic Information System (ITIS): Toxicodendron diversilobum (Western Poison-oak) - Overview</ref> It reproduces by spreading rhizomes and by seeds.<ref name=Hogan/>

T. diversilobum foliage at Samuel P. Taylor State Park, California

The plant is winter deciduous, so that after cold weather sets in, the stems are leafless and bear only the occasional cluster of mature fruit. Without leaves the stems may sometimes be identified by occasional black marks where its milky sap may have oozed and dried.

The leaves are divided into three (rarely 5, 7, or 9) leaflets, Template:Convert long, with scalloped, toothed, or lobed edges.<ref name="jepson">Jepson</ref> They generally resemble the lobed leaves of a true oak, though tend to be more glossy. Leaves are typically bronze when first unfolding in February to March, bright green in the spring, yellow-green to reddish in the summer, and bright red or pink from late July to October.<ref name="usfs"/>

White flowers form in the spring, from March to June.<ref name="usfs"/> If they are fertilized, they develop into greenish-white or tan drupes.<ref name="jepson"/>

Botanist John Howell observed that the plant's toxicity obscures its aesthetic values:

In spring, the ivory flowers bloom on the sunny hill or in sheltered glade, in summer its fine green leaves contrast refreshingly with dried and tawny grassland, in autumn its colors flame more brilliantly than in any other native, but one great fault, its poisonous juice, nullifies its every other virtue and renders this beautiful shrub the most disparaged of all within our region.<ref>Template:Cite book</ref>

Distribution and habitat

Toxicodendron diversilobum is found in California (Los Angeles was built on the site of a village named Yaangna or iyáanga’, meaning "poison oak place"),<ref>Roots of native names, by Ron Sullivan, in the San Francisco Chronicle; published December 7, 2002; retrieved June 20, 2017</ref> the Baja California Peninsula, Nevada, Oregon, Washington, and British Columbia.<ref>Template:PLANTS</ref> The related T. pubescens (eastern poison oak) is native to the Southeastern United States. T. diversilobum and T. rydbergii (western poison ivy) hybridize in the Columbia River Gorge area.<ref name="usfs" />

Toxicodendron diversilobum is common in various habitats, from mesic riparian zones to xeric chaparral.<ref name="jepson" /> It thrives in shady and dappled light through full and direct sunlight conditions, at elevations below Template:Convert.<ref name="usfs" /> The vining form can climb up large shrub and tree trunks into their canopies. Sometimes it kills the support plant by smothering or breaking it.<ref name="usfs" /> The plant often occurs in chaparral and woodlands, coastal sage scrub, grasslands, and oak woodlands; and Douglas-fir (Pseudotsuga menzesii), hemlock–Sitka spruce, Sequoia sempervirens (coast redwood), Pinus ponderosa (Ponderosa pine), and mixed evergreen forests.<ref>Calflora</ref>

Ecology

Black-tailed deer, mule deer, California ground squirrels, western gray squirrels, and other indigenous fauna feed on the leaves of the plant.<ref name="usfs" /> It is rich in phosphorus, calcium, and sulfur.<ref name="usfs" /> Bird species use the berries for food, and utilize the plant structure for shelter.<ref name="usfs" /> Neither native animals nor horses, livestock, or dogs demonstrate reactions to urushiol.<ref name="Hogan" />

Due to human allergic reactions, T. diversilobum is usually eradicated from gardens and public landscaped areas. It can be a weed in agricultural fields, orchards, and vineyards.<ref>UC Integrated Pest Management Weed Photo Gallery and information (profile of this plant as an agricultural weed).</ref> It is usually removed by pruning, herbicides, digging out, or a combination.<ref>Sunset Western Garden Book [5th edition], (Menlo Park: Sunset Publishing, 1988), p. 506</ref> Poison oak is susceptible to infection by Phytophthora ramorum.<ref name="Senchina-2008">Template:Cite journal</ref>

Toxicity

Urushiol-induced contact dermatitis from poison oak

Toxicodendron diversilobum leaves and twigs have a surface oil, urushiol, which causes an allergic reaction.<ref name=Hogan /> It causes contact dermatitis – an immune-mediated skin inflammation – in four-fifths of humans.<ref>Template:Cite journal</ref><ref>Template:Cite web</ref> Most, if not all, will become sensitized over time with repeated or more concentrated exposure to urushiol.

The active components of urushiol have been determined to be unsaturated congeners of 3-heptadecylcatechol with up to three double bonds in an unbranched C17 side chain.<ref>Template:Cite journal</ref> In poison ivy, these components are unique in that they contain a -CH2CH2- group in an unbranched alkyl side chain.<ref name=Billets2>Template:Cite journal</ref>

Exposure

Toxicodendron diversilobum skin contact first causes itching; then evolves into dermatitis with inflammation, colorless bumps, severe itching, and blistering.<ref>Poison Oak/Poison Ivy Information Center</ref> In the dormant deciduous seasons the plant can be difficult to recognize, however contact with leafless branches and twigs also causes allergic reactions.

Urushiol volatilizes when burned, and human exposure to T. diversilobum smoke is extremely hazardous, from wildfires, controlled burns, or disposal fires.<ref name="usfs"/> The smoke can poison people who thought they were immune.<ref name="usfs"/> Branches used to toast food over campfires can cause reactions internally and externally.

Urushiol is also found in the skin of mangos, posing a danger to people sensitized to T. diversilobum when eating the fruit while it is still in the rind.<ref>Template:Cite journal</ref><ref>Template:Cite web</ref>

Uses

Medicinal

Californian Native Americans used the plant's stems and shoots to make baskets, the sap to cure ringworm, and as a poultice of fresh leaves applied to rattlesnake bites.<ref name="mich">Univ. of Michigan, Dearborn – Native American Ethnobotany Database: Toxicodendron diversilobum</ref> The juice or soot was used as a black dye for sedge basket elements, tattoos, and skin darkening.<ref name="mich"/><ref>Template:Cite report</ref>

An infusion of dried roots, or buds eaten in the spring, were taken by some native peoplesTemplate:Which for an immunity from the plant poisons.<ref name="mich"/>

Chumash peoples used T. diversilobum sap to remove warts, corns, and calluses; to cauterize sores; and to stop bleeding.<ref name="mich"/> They drank a decoction made from the roots to treat dysentery.<ref>Template:Cite journal</ref>

Cultivation

Toxicodendron diversilobum can be a carefully situated component in wildlife gardens, habitat gardens, and natural landscaping.

The plant is used in habitat restoration projects.<ref name="usfs"/> It can be early stage succession where woodlands have been burned or removed, serving as a nurse plant for other species.

See also

• Toxicodendron vernix – poison sumac
• Toxicodendron radicans – eastern poison ivy

Gallery

• 
  Red phase in spring
• 
  Green phase, and flowers
• 
  Berries
• 
  Summer–autumn turning color phase
• 
  Shrub form
• 
  Vine form

External links

• U.S. Forest Service information: Toxicodendron diversilobum (Pacific poison oak)
• Calflora Database: Toxicodendron diversilobum (Pacific poison oak)
• Jepson Flora Project: Toxicodendron diversilobum
• American Academy of Dermatology – Poison Oak information
• Toxicodendron diversilobum – U.C. Photo gallery