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Pacific Poison Oak

Toxicodendron diversilobum (Torr. & Gray) Greene

Brief Summary

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Western Poison-oak (Toxicodendron diversilobum) is found at elevations below around 1650 m in westernmost North America from British Columbia (Canada) to Baja California (Mexico), including the U.S. states of Washington, Oregon, California, and Nevada. It may grow as a shrub (sometimes tree-like), 0.5 to 4 m tall, or (when externally supported) as a vine more than 30 m long--or any form in between (Gartner 1991a,b). The (typically) 3-leafleted leaves turn bright red in autumn. Poison-oak and its close relatives are well-known for possessing skin-irritating oil (urushiol), which can cause severe allergic reactions in humans.

The taxonomy and nomenclature of North American Toxicodendron has been in flux for over a century, largely due to confusing within-species variation in growth form, leaf and leaflet shape, and other features (e.g., Gillis 1971; Gartner 1991). This has resulted in an abundance of synonyms, but five species are now generally recognized: Common Poison-ivy (T. radicans), Western Poison-ivy (T. rydbergii), Eastern Poison-oak (T. pubescens), Western Poison-oak (T. diversilobum), and Poison-sumac (T. vernix) (Senchina 2006).

Senchina (2008) reviewed the literature on animal and fungal associates of Toxicodendron in North America with a particular eye toward identifying potential biological control agents. Interest in finding new ways to control poison-oak and its relatives may increase in coming years given data suggesting that these plants may become more abundant and more ‘‘toxic’’ in the future, potentially affecting global forest dynamics and human health (Mohan et al. 2006).

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Comprehensive Description

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Pacific poison oak, Toxicodendron diversilobum (also known as Rhus diversiloba), is an angiosperm of the Anacardiaceae (sumac) family that is confined to western North America and is native to British Columbia, Washington, Oregon, northwestern Nevada, and California, where it grows most prolifically (USDA 2016). It is not a true oak and is closely related to poison ivy (Toxicodendron radicans), its counterpart in eastern North America (NPIN 2007). It is a many stemmed, deciduous shrub or woody vine that can survive and flourish in conditions from part shade to full sun. As a shrub it grows 2-6 ft. tall, and as a vine it climbs trees or other support by adventitious roots and/or wedging stems within grooves or crevices of the support. Vine stems typically grow 10-30 ft., but may be as long as 100 ft. (Gartner 1991). The leaves are bright green (often red in fall) with three or five round to ovate, diversely lobed or toothed leaflets that usually resemble oak leaves (Munz 1974). Leaves are usually glossy and both they and the other parts of the plant are covered with a toxic oil (urushiol) that causes a sometimes severe skin rash, at least in humans, although deer and livestock often browse the plant with impunity (NPIN 2007). Poison oak flowers in the summertime, with clusters of white flowers blooming from leaf axils. The fruits are small green or white berries (Munz 1974). As a perennial, T. diversilobum regrows and expands on its previous branches each year, and it is a hardy plant that is one of the first to regrow after forest fires.

Poison oak thrives in mixed coniferous forests, shaded woodlands, and chaparral, as well as in open fields, growing most thickly near streams or springs where it has unlimited access to water. It is common in most warm-climate woodlands and generally cannot survive at elevations above 5000 feet. Often, poison oak dominates the undergrowth, smothering surrounding trees and ground level vegetation with its sprawling vines. Its hardiness and ability to proliferate, coupled with its danger to humans, contributes to the almost universal perception of poison oak as a pest plant (Howard 1994).

References

  • Gartner, Barbara L. 1991. Relative growth rates of vines and shrubs of western poison oak, Toxicodendron diversilobum (Anacardiaceae). American Journal of Botany 78(10): 1345-1353.
  • Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). http://www.fs.fed.us/database/feis/. Accessed: May 17, 2016.
  • Munz, Philip A. 1974. A Flora of Southern California. Berkeley, CA: University of California Press.
  • NPIN. 2007. Toxicodendron diversilobum (Pacific poison oak). Native Plant Information Network, Lady Bird Johnson Wildflower Center. http://www.wildflower.org/plants/result.php?id_plant=LOCI3. Accessed May 15, 2016.
  • USDA, NRCS. 2016. Plant profile for Toxicodendron diversilobum (Pacific poison oak). The PLANTS Database (http://plants.usda.gov, accessed: May 16, 2016). National Plant Data Team, Greensboro, NC 27401-4901 USA.

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Authors: Spencer Trombley and Luke Ehrmann; Gordon L. Miller, Ph.D. Seattle University EVST 2100 - Natural History: Theory and Practice
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Broad-scale Impacts of Fire

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More info for the terms: ground fire, prescribed fire, woodland

See this Fire Study in FEIS for further information on prescribed fire and postfire responses of many plant community species including Pacific poison-oak: Damage and recovery in southern Sierra Nevada foothill oak woodland after a severe ground fire (blue oak-interior live oak/annual grass community)
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Broad-scale Impacts of Plant Response to Fire

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Fire response is probably related to Poison oak's successional role in the plant community.  Dense Pacific poison-oak thickets may develop in chaparral that is control burned several times [12].  Pacific poison-oak may become locally extinct in Douglas-fir forest, however, that is burned every 4 years for 20 years or more [53].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Common Names

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Pacific poison-oak
Pacific poison oak
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Cover Value

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More info for the terms: density, woodland

The federally endangered least Bell's vireo uses Pacific poison-oak for nest sites in oak woodlands [25].  Fremont cottonwood (Populus fremontii)/Pacific poison-oak woodlands contribute to bird diversity and density in California [28].  A rare colony of ringtail was found inhabiting a Fremont cottonwood/Pacific poison-oak woodland on the Sacramento River [3].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Description

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More info for the terms: adventitious, shrub, shrubs, vine

Pacific poison-oak is a many-stemmed, deciduous, native shrub or woody vine. Shrubs are erect with stems from 2 to 6 feet (1-2 m) tall.  Vine stems commonly reach 10 to 30 feet (3-10 m), but may be as long as 100 feet (30 m) [22].  As a vine, Pacific poison-oak climbs trees or other support by adventitious roots and/or wedging stems within grooves or crevices of the support [7,22,62].  The bright green leaves have three (sometimes five) round to ovate, diversely lobed or toothed leaflets that usually resemble oak leaves [45,62].  Small flowers occur in leaf axils, with male and female flowers on separate plants [38,53].  The fruits are white drupes [45].  Rhizomes are at or just below the soil surface, and are extensive [46].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Distribution

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Pacific poison-oak is distributed from Baja California north to British Columbia [31,45,58].  It occurs west of the Cascade Range in Washington, Oregon, and California [32] and is ubiquitous in California west of the Sierra Nevada and the Mojave Desert [38].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Fire Ecology

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More info for the terms: charate, fire regime, fresh, root crown, seed

Pacific poison-oak's primary POSTFIRE REGENERATION STRATEGY is vigorous sprouting from the root crown and/or rhizomes [16,46,68]. Fire is not required for Pacific poison-oak seed germination.  Keeley [37], however, reported a significant (p less than 0.001) increase in germination when seeds were exposed to charate.  Postfire seedlings probably originate from both soil-stored seed and fresh seed dispersed by birds. FIRE REGIMES: Find fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Fire Management Considerations

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More info for the terms: cover, forest, fuel, fuel continuity, prescribed fire, shrub, vines

Urushiol volatilizes when burned, and human exposure to Pacific poison-oak smoke is extremely hazardous [40].  The smoke often poisons people who think they are immune to the plant [46]. Pacific poison-oak vines are a ladder fuel [61]. Goats can be used as an alternative to prescribed fire for fire hazard reduction at urban-wildland interfaces.  Near Oakland, California, goats were put on a Monterey pine-redgum (Eucalyptus camaldensis) forest with a heavy shrub understory and on an adjacent site where the forest was managed as a fuelbreak and had less shrub cover in the understory.  Goat utilization of Pacific poison-oak was in the fuelbreak 67 percent, somewhat lower than utilization of toyon, California blackberry, and coyote brush. Annual production of Pacific poison-oak biomass before goat browsing in the fuelbreak was 99 kilograms per hectare; it was 33 kilograms per hectare afterwards.  Total biomass of forage species was significantly (p less than 0.05) reduced [61]. A stocking rate of 600 goats per hectare on the Oakland site broke the vertical live fuel continuity in the dense shrub stand.  Initial goat browsing to reduce biomass and vertical fuel continuity could be followed up by prescribed fire [61].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Growth Form (according to Raunkiær Life-form classification)

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More info for the terms: hemicryptophyte, phanerophyte

   Phanerophyte    Hemicryptophyte
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Habitat characteristics

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Pacific poison-oak generally grows in acid soils, and is not limited to any particular soil texture or drainage pattern.  It occurs on well-drained slopes and in riparian zones [1,39,64].  It is found at elevations of less than 5,000 feet (1,524 m) west of the Sierra Nevada, growing on all aspects [45].  In the Siskiyou Mountains it is found at up to 4,400-foot (1,340-m) elevations on steep southern exposures [66].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Habitat: Cover Types

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This species is known to occur in association with the following cover types (as classified by the Society of American Foresters):

   213  Grand fir
   222  Black cottonwood - willow
   224  Western hemlock
   229  Pacific Douglas-fir
   230  Douglas-fir - western hemlock
   231  Port Orford-cedar
   232  Redwood
   233  Oregon white oak
   234  Douglas-fir - tanoak - Pacific madrone
   235  Cottonwood - willow
   243  Sierra Nevada mixed conifer
   244  Pacific ponderosa pine - Douglas-fir
   245  Pacific ponderosa pine
   246  California black oak
   247  Jeffrey pine
   248  Knobcone pine
   249  Canyon live oak
   250  Blue oak - Digger pine
   255  California coast live oak
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Habitat: Ecosystem

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This species is known to occur in the following ecosystem types (as named by the U.S. Forest Service in their Forest and Range Ecosystem [FRES] Type classification):

   FRES20  Douglas-fir
   FRES21  Ponderosa pine
   FRES24  Hemlock - Sitka spruce
   FRES27  Redwood
   FRES28  Western hardwoods
   FRES34  Chaparral - mountain shrub
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Habitat: Plant Associations

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This species is known to occur in association with the following plant community types (as classified by Küchler 1964):

More info for the terms: forest, shrub

   K001  Spruce - cedar - hemlock forest
   K002  Cedar - hemlock - Douglas-fir forest
   K005  Mixed conifer forest
   K006  Redwood forest
   K009  Pine - cypress forest
   K010  Ponderosa shrub forest
   K026  Oregon oakwoods
   K028  Mosaic of K002 and K026
   K029  California mixed evergreen forest
   K030  California oakwoods
   K033  Chaparral
   K034  Montane chaparral
   K035  Coastal sagebrush
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Immediate Effect of Fire

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More info for the term: wildfire

Fire top-kills Pacific poison-oak [13,16].  Wirtz [68] reported that an October, 1953, wildfire in a coastal sage scrub/grassland community near Berkeley, California, top-killed all Pacific poison-oak present, leaving only large branches and stumps. Rhizomes on the soil surface are probably killed by all but light-severity fire, and shallowly buried rhizomes are probably killed by moderate to severe fire.  More deeply buried rhizomes are probably not killed.
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Importance to Livestock and Wildlife

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Black-tailed deer and all classes of livestock browse Pacific poison-oak [53]. It is the most important black-tailed deer browse in some areas of California [5,6].  Birds eat Pacific poison-oak fruits [53].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Key Plant Community Associations

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More info for the terms: association, forest, shrub, woodland

Pacific poison-oak occurs in mixed evergreen forests [29,30,32,59], woodlands,
chaparral, [25,26,27], coastal sage scrub [39], and riparian zones
[25,26,27,39,58].  It is the most widespread shrub in California [7].
Holland [33] described a Pacific poison-oak chaparral community type that may be
maintained by frequent fire.  Because it is dominated by Pacific poison-oak,
little is known of its community composition.

Many of the plant species commonly associated with Pacific poison-oak were
previously listed under DISTRIBUTION AND OCCURRENCE information.  Other
common associates follow, listed by community type.

Associates in mixed evergreen forests include Pacific madrone (Arbutus
menziesii), sugar pine (Pinus lambertiana), bigleaf maple (Acer
macrophyllum), tanoak (Lithocarpus densiflorus), California bay
(Umbellularia californica), and chinquapin (Chrysolepsis chrysophylla)
[11,17,18,43].

Woodland associates include valley oak (Quercus lobata), interior live
oak (Q. wislizenii), Monterey pine (Pinus radiata) [42], Coulter pine
(P. coulteri) [9], bigcone Douglas-fir (Pseudotsuga macrocarpa) [8,64],
and California walnut (Juglans californica) [48].

Chaparral associates include toyon (Heteromeles arbutifolia), chamise
(Adenostoma fasciculatum), and California scrub oak (Quercus dumosa).
Coastal sage scrub associates include California sagebrush (Artemisia
californica), coyote brush (Baccharis pilularis), and sugar sumac (Rhus
ovata) [6,26,27,47,65].

Pacific poison-oak associates in riparian zones include bigleaf maple,
California sycamore (Plantus racemosa), white alder (Alnus rhombifolia),
[17], boxelder (Acer negundo), willow (Salix spp.), California
blackberry (Rubus vitifolius), toyon, and wild grape (Vitis spp.)  [69].

Published classifications naming Pacific poison-oak as a dominant part of the
vegetation are:

Description and classification of the forests of the upper Illinois
   River drainage of southwestern Oregon [1]
Preliminary plant associations of the Siskiyou Mountain Province [2]
Coast redwood ecological types of southern Monterey County, California [10]
Plant communities of Santa Rosa Island, Channel Islands National Park [14]
Plant association and management guide: Siuslaw National Forest [29]
Plant association and management guide: Willamette National Forest [30]
The community composition of California coastal sage scrub [39]
Plant associations within the Interior Valleys of the Umpqua River
   Basin, Oregon [55]
The vascular plant communities of California [59]
An introduction to the plant communities of the Santa Ana and San
   Jacinto Mountains [65].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Life Form

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More info for the terms: shrub, vine

Vine, Shrub
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Management considerations

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More info for the terms: fire management, vines

Safety/Medical:  The entire Pacific poison-oak plant is covered with oily resin.
Human dermatitis results when skin comes in direct contact with the oil,
either by touching the plant or touching something that has contacted
it, such as clothing or firewood.  Urushiol is the poison present in the
oil [46].  Pacific poison-oak does not cause dermatitis in wildlife or
livestock, but pets may react to it [53]. (See FIRE MANAGEMENT.) American
folklore holds that drinking the milk of Pacific poison-oak-fed goats bolsters
the immune system against Pacific poison-oak because the poison is present in
the milk in trace amounts.  Drinking the milk probably does not grant
immunity, however.  Analysis of milk from does fed a straight Pacific poison-oak
diet for 3 days showed no trace of urushiol.  Some urushiol was present
in the does' urine, but most was apparently catabolized [40].

Control:  Pacific poison-oak is controlled by glyphosate, triclopyr, or 2,4,5-T.
Used alone, 2,4-D is ineffective.  Goats are an effective biological
control [40,50].

Other:  Pacific poison-oak vines sometimes kill their support plant by
smothering or breaking it [46].

Pacific poison-oak blossoms are a source of good honey [46].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Nutritional Value

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Percent crude protein in Pacific poison-oak foliage collected throughout
California averaged 24.2 in March, 20.6 in May, 10.1 in July, and 6.5 in
September [5].  Pacific poison-oak is relatively high in phosphorus, sulfur,
and calcium as compared to other browse species [24].  The following
mineral content (percentage basis) was reported for the foliage [54]:

            Ca     P     K     Mg     S
          1.00   0.23  1.13  0.59   0.19
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Occurrence in North America

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     CA  OR  WA  MEXICO  BC
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Other uses and values

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Urushiol has been found to mediate DNA strand scission.  This activity may have application in DNA sequence studies [70]. Native Americans used the stems to make baskets and the sap to cure ringworm [15,60].  Chumash Indians used Pacific poison-oak sap to remove warts, corns, and calluses; to cauterize sores; and to stop bleeding.  They drank a decoction made from Pacific poison-oak roots to treat dysentery [60].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Palatability

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Pacific poison-oak palatability is rated good to fair for horses and deer; and
fair to poor for cattle, sheep, and goats [53].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Phenology

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Pacific poison-oak leaf buds open from February to March, and stems elongate from March to April [37].  Flowering occurs from March to June [15].  Leaves drop from late July to early October [22], and fruits disperse in summer and fall [37,53].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Plant Response to Fire

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More info for the terms: backfire, density, fire use, forest, prescribed fire, reaction intensity, root crown, seed, wildfire, woodland

Pacific poison-oak sprouts vigorously from the root crown and/or rhizomes after fire [13,15,43,46,52].  It sprouts in the first postfire growing season, and for several years thereafter [13,16,52].  Pacific poison-oak sprouts were noted the September following the July, 1985, Wheeler Fire on the Los Padres National Forest, California.  The wildfire had spread into a riparian zone containing Pacific poison-oak; prefire Pacific poison-oak density was unknown.  By postfire year 3, Pacific poison-oak sprouts dominated most burn plots in the riparian zone [18]. Westman and others [67] estimated that Pacific poison-oak fails to sprout when fire reaction intensity exceeds 200 kcal/sec/sq m.  Their estimate was derived by modelling fire behavior of a backfire set in coastal sage scrub in the Santa Monica Mountains of California, and observing sprouting the following year.  The coastal sage scrub had not burned for 20 to 22 years. Pacific poison-oak also establishes from seed after fire, although this response is not well documented in the literature.  Pacific poison-oak seedlings were observed following site preparation and prescribed burning of an interior live oak-blue oak woodland in Madera County, California. Prefire Pacific poison-oak seedling density was 0 percent; seedling density at postfire year 1 was 42 per 8,712 square feet [20]. Response of vegetation to prescribed burning in a Jeffrey pine-California black oak woodland and a deergrass meadow at Cuyamaca State Park, California provides information on prescribed fire use and postfire response of many mixed-conifer woodland species including Pacific poison-oak.
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Post-fire Regeneration

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More info for the terms: ground residual colonizer, rhizome, root crown, secondary colonizer, shrub

   Tall shrub, adventitious-bud root crown    Rhizomatous shrub, rhizome in soil    Ground residual colonizer (on-site, initial community)    Secondary colonizer - off-site seed
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Regeneration Processes

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More info for the terms: layering, root crown, scarification, vine

Pacific poison-oak reproduces vegetatively by sprouting from the rhizomes and root crown after disturbance such as fire or browsing has removed topgrowth [15,44,53].  It also reproduces by layering when vine stems contact the ground [46]. Pacific poison-oak seeds are dispersed by birds [53].  Seedlings occur both before and after fire, suggesting that the seeds do not depend upon fire for scarification.  The seeds have a gummy seedcoat which leaches off very slowly, resulting in delayed germination [37]. Pacific poison-oak is propagated by stem cuttings [23].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Regional Distribution in the Western United States

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This species can be found in the following regions of the western United States (according to the Bureau of Land Management classification of Physiographic Regions of the western United States):

    1  Northern Pacific Border
    2  Cascade Mountains
    3  Southern Pacific Border
    4  Sierra Mountains
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Successional Status

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More info on this topic.

More info for the terms: climax, cover, forest, woodland

Facultative Seral Species Pacific poison-oak is a somewhat shade-tolerant species commonly occurring in seral woodland and mixed evergreen forest understories [51,56].  It is considered a climax species on south-slope Douglas-fir forests of the Willamette Valley foothills, Oregon [51].  In climax oak woodland, Pacific poison-oak cover may reach 25 to 50 percent [17].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Synonyms

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Rhus diversiloba Torr. & A. Gray [46]
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Taxonomy

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The currently accepted scientific name of Pacific poison-oak is Toxicodendron
diversilobum (Torr. & Gray) E. Greene (Anacardiacae) [31]. Pacific poison-oak
and western poison-ivy (Toxicodendron rydbergii) hybridize in the Columbia River
Gorge area [38].
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Value for rehabilitation of disturbed sites

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More info for the terms: restoration, shrubs

Pacific poison-oak has been recommended for use in restoration projects. Information on propagation and handling methods to "minimize risks" to planting crews is available [23].  Having worked on field crews in the Sierra Nevada foothills, however, this author recommends using native shrubs other than Pacific poison-oak for restoration.
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Howard, Janet L. 1994. Toxicodendron diversilobum. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/

Toxicodendron diversilobum

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Toxicodendron diversilobum (syn. Rhus diversiloba), commonly named Pacific poison oak[1] 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.[2] Peak flowering occurs in May.[3] 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.

Description

Toxicodendron diversilobum is extremely variable in growth habit and leaf appearance. It grows as a dense 0.5–4 metres (1+12–13 feet) tall shrub in open sunlight, a treelike vine 3–9 m (10–30 ft) and may be more than 30 m (100 ft) long with an 8–20 centimetres (3+147+34 inches) trunk, as dense thickets in shaded areas, or any form in between.[4][5] It reproduces by spreading rhizomes and by seeds.[2]

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, 3.5 to 10 cm (1+12 to 4 in) long, with scalloped, toothed, or lobed edges.[6] 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.[4]

White flowers form in the spring, from March to June.[4] If they are fertilized, they develop into greenish-white or tan drupes.[6]

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.[7]

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"[8]), the Baja California Peninsula, Nevada, Oregon, Washington, and British Columbia.[9] 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.[4]

Toxicodendron diversilobum is common in various habitats, from mesic riparian zones to xeric chaparral.[6] It thrives in shady and dappled light through full and direct sunlight conditions, at elevations below 1,500 m (5,000 ft).[4] 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.[4] 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.[10]

Ecology

Black-tailed deer, mule deer, California ground squirrels, western gray squirrels, and other indigenous fauna feed on the leaves of the plant.[4] It is rich in phosphorus, calcium, and sulfur.[4] Bird species use the berries for food, and utilize the plant structure for shelter.[4] Neither native animals nor horses, livestock, or dogs demonstrate reactions to urushiol.[2]

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.[11] It is usually removed by pruning, herbicides, digging out, or a combination.[12]

Toxicity

Toxicodendron diversilobum leaves and twigs have a surface oil, urushiol, which causes an allergic reaction.[2] It causes contact dermatitis – an immune-mediated skin inflammation – in four-fifths of humans.[13][14] 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.[15] In poison ivy, these components are unique in that they contain a -CH2CH2- group in an unbranched alkyl side chain.[16]

Exposure

Toxicodendron diversilobum skin contact first causes itching; then evolves into dermatitis with inflammation, colorless bumps, severe itching, and blistering.[17] 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.[4] The smoke can poison people who thought they were immune.[4] 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.[18][19]

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.[20] The juice or soot was used as a black dye for sedge basket elements, tattoos, and skin darkening.[20][21]

An infusion of dried roots, or buds eaten in the spring, were taken by some native peoples for an immunity from the plant poisons.[20]

Chumash peoples used T. diversilobum sap to remove warts, corns, and calluses; to cauterize sores; and to stop bleeding.[20] They drank a decoction made from the roots to treat dysentery.[22]

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.[4] It can be early stage succession where woodlands have been burned or removed, serving as a nurse plant for other species.

See also

References

  1. ^ USDA, NRCS (n.d.). "Toxicodendron diversilobum". The PLANTS Database (plants.usda.gov). Greensboro, North Carolina: National Plant Data Team. Retrieved 11 December 2015.
  2. ^ a b c d C. Michael Hogan (2008); "Western poison-oak: Toxicodendron diversilobum" Archived 2009-07-21 at the Wayback Machine, GlobalTwitcher, ed. Nicklas Strömberg
  3. ^ iNaturalist: Toxicodendron diversilobum
  4. ^ a b c d e f g h i j k l U.S. Forest Service: Toxicodendron diversilobum
  5. ^ Integrated Taxonomic Information System (ITIS): Toxicodendron diversilobum (Western Poison-oak) - Overview
  6. ^ a b c Jepson
  7. ^ John Thomas Howell, Frank Almeda, Wilma Follette & Catherine Best (2007). Marin Flora. California Academy of Sciences; California Native Plant Society. p. 264. ISBN 978-0940228702.{{cite book}}: CS1 maint: multiple names: authors list (link)
  8. ^ Roots of native names, by Ron Sullivan, in the San Francisco Chronicle; published December 7, 2002; retrieved June 20, 2017
  9. ^ USDA, NRCS (n.d.). "Toxicodendron diversilobum". The PLANTS Database (plants.usda.gov). Greensboro, North Carolina: National Plant Data Team. Retrieved 2013-09-20.
  10. ^ Calflora
  11. ^ UC Integrated Pest Management Weed Photo Gallery and information (profile of this plant as an agricultural weed).
  12. ^ Sunset Western Garden Book [5th edition], (Menlo Park: Sunset Publishing, 1988), p. 506
  13. ^ R. S. Kalish, J. A. Wood & A. LaPorte (1994). "Processing of urushiol (poison ivy) hapten by both endogenous and exogenous pathways for presentation to T cells in vitro". Journal of Clinical Investigation. 93 (5): 2039–2047. doi:10.1172/jci117198. PMC 294319. PMID 7910172.
  14. ^ Mic-ro.com: Contact-Poisonous Plants of the World
  15. ^ Michael D. Corbett & Stephen Billets (1975). "Characterization of poison oak urushiol". Journal of Pharmaceutical Sciences. 64 (10): 1715–1718. doi:10.1002/jps.2600641032. PMID 1185545.
  16. ^ John C. Craig, Coy W. Waller, Stephen Billets & Mahmoud A. Elsohly (1978). "New GLC analysis of urushiol congeners in different plant parts of poison ivy, Toxicodendron radicans". Journal of Pharmaceutical Sciences. 67 (4): 483–485. doi:10.1002/jps.2600670411. PMID 641754.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  17. ^ Poison Oak/Poison Ivy Information Center
  18. ^ Hershko, Klilah; Weinberg, Ido; Ingber, Arieh (2005-01-01). "Exploring the mango-poison ivy connection: the riddle of discriminative plant dermatitis". Contact Dermatitis. 52 (1): 3–5. doi:10.1111/j.0105-1873.2005.00454.x. ISSN 0105-1873. PMID 15701120. S2CID 31162401.
  19. ^ Brody, Jane E. (June 16, 2014). "Steering Clear of Poison Ivy". New York Times. Retrieved 2016-01-11.
  20. ^ a b c d Univ. of Michigan, Dearborn – Native American Ethnobotany Database: Toxicodendron diversilobum
  21. ^ Conrad, C. Eugene (1987). Common shrubs of chaparral and associated ecosystems of southern California (Report). General Technical Report. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station. doi:10.2737/psw-gtr-99. hdl:2027/umn.31951d029772576. PSW-99.
  22. ^ Jan Timbrook (1990). "Ethnobotany of Chumash Indians, California, based on collections by John P. Harrington". Economic Botany. 44 (2): 236–253. doi:10.1007/BF02860489. JSTOR 4255231. S2CID 25807034.
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Toxicodendron diversilobum: Brief Summary

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Toxicodendron diversilobum (syn. Rhus diversiloba), commonly named Pacific poison oak 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. Peak flowering occurs in May. 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.

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