Toxicodendron diversilobum

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).

     CA  OR  WA  MEXICO  BC

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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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

Rhus diversiloba Torr. & A. Gray:
United States (North America)

Note: This information is based on publications available through Tropicos and may not represent the entire distribution. Tropicos does not categorize distributions as native or non-native.

Toxicodendron diversilobum (Torr. & A. Gray) Greene:
China (Asia)
United States (North America)

Note: This information is based on publications available through Tropicos and may not represent the entire distribution. Tropicos does not categorize distributions as native or non-native.

More info for the terms: shrub, 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].

More info for the terms: association, shrub

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

More info for the term: 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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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

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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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 for the terms: cover, 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].

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].

More info for the terms: backfire, density, fire use, prescribed fire, root crown, wildfire

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 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.

More info for the terms: ground residual colonizer, rhizome, 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: charate, fresh, root crown

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.

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More info for the terms: climax, cover

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].

More info for the terms: layering, 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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More info for the terms: hemicryptophyte, phanerophyte

   Phanerophyte
   Hemicryptophyte

More info for the terms: shrub, vine

Vine, Shrub

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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].

Canada

Rounded National Status Rank: N2 - Imperiled

United States

Rounded National Status Rank: N5 - Secure

Rounded Global Status Rank: G5 - Secure

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].