Pacific Poison Oak

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)

More info for the term: forest

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].

Pacific poison-oak
Pacific poison oak

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].

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].

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].

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".

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].

More info on this topic.

More info for the terms: hemicryptophyte, phanerophyte

   Phanerophyte    Hemicryptophyte

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].

More info on this topic.

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

More info on this topic.

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

More info on this topic.

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

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.

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].

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].

More info for the terms: shrub, vine

Vine, Shrub

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].

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

     CA  OR  WA  MEXICO  BC

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].

Pacific poison-oak palatability is rated good to fair for horses and deer; and
fair to poor for cattle, sheep, and goats [53].

More info on this topic.

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].

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.

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

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].

More info on this topic.

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

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].

Rhus diversiloba Torr. & A. Gray [46]

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].

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.