C. Michael Hogan PhD
October 15, 2008
Western poison-oak is a widespread shrub in far western North America, with a variable growth form ranging from erect shrub to thick sprawling vine. An inhabitant of both forests and chaparral, the species has diverse propagation strategies based upon seed dispersal, aggressive creeping roots and post-fire root crown regeneration. It is best known for its production of oily toxic chemicals that discourage herbivory and also cause severe skin rash reactions.
DISTRIBUTION AND ASSOCIATES
Western poison-oak is distributed from British Columbia to Baja California. The species occurs in forest understories and in chaparral systems along the Pacific Coast Ranges and inland to the western slopes of the Sierra Nevada. It is also found in the Mojave Desert and in parts of the state of Nevada. Common plant associations are oak woodlands, mixed conifer forests, montane chaparral, coastal sage, bishop pine coastal forest, pine-cypress forest and cedar/hemlock/douglas-fir forest. Alliant understory species are toyon, manzanita, California coffeeberry, (Hogan) salal and coyote brush. Most occurrences of Western poison-oak are found at elevations of less than 1600 meters.
T.diversilobum is a deciduous multi-stemmed shrub or woody vine. In shrub form a height of one to three meters is attained, while vines may reach a length of ten to twenty meters. In vine form the plant commonly entwines itself around an oak or other support tree, but may also clamber across a dense understory of associate shrubs. An extensive rhizome network comprises the subsurface root architecture. Leaves in leaflets of three are typically ovate and variably lobed, with length ranging from three to nine cm. New leaf growth appears glossy green or occasionally bronze; fall foliage turns a deep red. Diminutive flowers occur on leaf axils, with male and female flowers found on separate plants. Fruits consist of greenish-white drupes.
Western poison-oak is one of the most common shrubs of the Pacific west coast and often occurs in ravines and north facing slopes, (Quinn) but may occur in a variety of microhabitats. This species is shade tolerant, but is also found in rather sunny exposures of open canopy forest floors and intermixed with other chaparral species in high sunlight environments. It may account for one fourth to one half of the understory cover in some Coast Range California climax oak woodlands, such as parts of Annadel State Park. Berries are a valuable food source for a gamut of birds and wildlife, (Storer) but this plant also produces phenolic oils from its leaves and stems that discourage herbivory. (Shoenherr) In spite of this strategy, a number of animals, including Black-tailed deer, browse the leaves of T.diversilobum, (Bissell) foliage being quite high in phosphorus, calcium and sulfur.
Propagation occurs both by bird carried seed dispersal and by aggressive rhizome colonization; moreover, regeneration after fire or aggressive browse can occur from the root crown. The seeds have a thick gummy coating which causes a time delay in seedling growth, and the seeds do not require fire scarification for germination, unlike some chaparral species. Flowering occurs from March to June, while fruit dispersal takes place in the late summer and autumn. Birds including the endangered Bell's vireo utilizes T.diversilobum vines and shrubs for nesting locations within oak woodlands.
All elements of T.diversilobum are coated with an oily resin, the principal active ingredient being urushiol. Dermititus is caused by human skin contact with urushiol, although wildlife and livestock do not appear similarly affected. Urushiol is a mixture of a number of organic compounds, each a catechol substituted with an alkyl chain that has 15 or 17 carbon atoms; moreover, Western poison-oak has a predominance of 17 carbon alkyls. The severity of irritation caused by urushiol is largely dependent upon the degree of unsaturation of the alkyl chains, with two double bonds implying a much greater reaction than lesser degrees of saturation.
The mechanism of attack begins with entry of urushiol long chains into the dermis or epidermis to bind with surface proteins, forming complete antigens. These affected cells then transmit the antigens to T4-inducer leucocytes. A second exposure to urushiol, often in rapid succession, may then create an allergic reaction characterized by skin blistering, irritation, redness and edema.
* C.Michael Hogan (2008) Rhamnus Californica, GlobalTwitcher, ed. Nicklas Strömberg
* Ronald D. Quinn and Sterling C. Keeley (2006) Introduction to California Chaparral, University of California Press, 322 pages ISBN 0520245660
* Tracy Irwin Storer, Robert Leslie Usinger and David Lukas (2004) Sierra Nevada Natural History, University of California Press, 439 pages ISBN 0520240960
* Allan A. Schoenherr (1992) A Natural History of California, University of California Press, ISBN 0520069226
* Harold D.Bissell and Helen Strong (1955) The crude protein variations in the browse diet of California deer, California Fish and Game. 41(2): 145-155.
* Peter D. Bryson (1996) Comprehensive Review in Toxicology for Emergency Clinicians, CRC Press, 849 pages, ISBN 1560326123