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Wildlife, Animals, and Plants
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Introductory
SPECIES: Toxicodendron diversilobum | Poison-Oak
ABBREVIATION :
TOXDIV
SYNONYMS :
Rhus diversiloba Torr. & A. Gray [46]
SCS PLANT CODE :
RHDI
COMMON NAMES :
poison-oak
TAXONOMY :
The currently accepted scientific name of poison-oak is Toxicodendron
diversilobum (Torr. & Gray) E. Greene (Anacardiacae) [31]. Poison-oak
and western poison-ivy (Toxicodendron rydbergii) hybridize in the
Columbia River Gorge area [38].
LIFE FORM :
Vine, Shrub
FEDERAL LEGAL STATUS :
NO-ENTRY
OTHER STATUS :
NO-ENTRY
COMPILED BY AND DATE :
H. Harris, 1990
LAST REVISED BY AND DATE :
J. Howard, January 1994
AUTHORSHIP AND CITATION :
Howard, Janet L. 1994; Harris, Holly T. 1990. Toxicodendron
diversilobum. In: Remainder of Citation
DISTRIBUTION AND OCCURRENCE
SPECIES: Toxicodendron diversilobum | Poison-Oak
GENERAL DISTRIBUTION :
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].
ECOSYSTEMS :
FRES20 Douglas-fir
FRES21 Ponderosa pine
FRES24 Hemlock - Sitka spruce
FRES27 Redwood
FRES28 Western hardwoods
FRES34 Chaparral - mountain shrub
STATES :
CA OR WA MEXICO BC
ADMINISTRATIVE UNITS :
CHIS KICA LAVO OLYM PINN PORE
REDW SAMO SEQU WHIS YOSE
BLM PHYSIOGRAPHIC REGIONS :
1 Northern Pacific Border
2 Cascade Mountains
3 Southern Pacific Border
4 Sierra Mountains
KUCHLER PLANT ASSOCIATIONS :
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
SAF COVER TYPES :
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
SRM (RANGELAND) COVER TYPES :
NO-ENTRY
HABITAT TYPES AND PLANT COMMUNITIES :
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 poison-oak chaparral community type that may be
maintained by frequent fire. Because it is dominated by poison-oak,
little is known of its community composition.
Many of the plant species commonly associated with 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 (Artemesia
california), coyotebrush (Baccharis pilularis), and sugar sumac (Rhus
ovata) [6,26,27,47,65].
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 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].
VALUE AND USE
SPECIES: Toxicodendron diversilobum | Poison-Oak
WOOD PRODUCTS VALUE :
NO-ENTRY
IMPORTANCE TO LIVESTOCK AND WILDLIFE :
Black-tailed deer and all classes of livestock browse poison-oak [53].
It is the most important black-tailed deer browse in some areas of
California [5,6]. Birds eat poison-oak fruits [53].
PALATABILITY :
Poison-oak palatability is rated good to fair for horses and deer; and
fair to poor for cattle, sheep, and goats [53].
NUTRITIONAL VALUE :
Percent crude protein in 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]. 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
COVER VALUE :
The federally endangered least Bell's vireo uses poison-oak for nest
sites in oak woodlands [25].
Fremont cottonwood (Populus fremontii)/poison-oak woodlands contribute
to bird diversity and density in California [28]. A rare colony of
ringtail was found inhabiting a Fremont cottonwood/poison-oak woodland
on the Sacramento River [3].
VALUE FOR REHABILITATION OF DISTURBED SITES :
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 poison-oak for restoration.
OTHER USES AND VALUES :
Urushiol has been found to mediate DNA strand scission. This activiity
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 poison-oak sap to remove warts,
corns, and calluses; to cauterize sores; and to stop bleeding. They
drank a decoction made from poison-oak roots to treat dysentery [60].
MANAGEMENT CONSIDERATIONS :
Safety/Medical: The entire 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]. 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 poison-oak-fed goats bolsters
the immune system against 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 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: 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: Poison-oak vines sometimes kill their support plant by
smothering or breaking it [46].
Poison-oak iblossoms are a source of good quality honey [46].
BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Toxicodendron diversilobum | Poison-Oak
GENERAL BOTANICAL CHARACTERISTICS :
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, 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].
RAUNKIAER LIFE FORM :
Phanerophyte
Hemicryptophyte
REGENERATION PROCESSES :
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].
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].
Poison-oak is propagated by stem cuttings [23].
SITE CHARACTERISTICS :
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].
SUCCESSIONAL STATUS :
Facultative Seral Species
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
Willamatte Valley foothills, Oregon [51]. In climax oak woodland,
poison-oak cover may reach 25 to 50 percent [17].
SEASONAL DEVELOPMENT :
Poison-oak leaf buds open from February to March, and stems elongate
from March to April [37]. Flowering occurs from from March to
June [15]. Leaves drop from late July to early October [22],
and fruits disperse in summer and fall [37,53].
FIRE ECOLOGY
SPECIES: Toxicodendron diversilobum | Poison-Oak
FIRE ECOLOGY OR ADAPTATIONS :
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 poison-oak seed germination. Keeley [37],
however, reported a significant (p<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.
POSTFIRE REGENERATION STRATEGY :
Tall shrub, adventitious-bud root crown
Rhizomatous shrub, rhizome in soil
Ground residual colonizer (on-site, initial community)
Secondary colonizer - off-site seed
FIRE EFFECTS
SPECIES: Toxicodendron diversilobum | Poison-Oak
IMMEDIATE FIRE EFFECT ON PLANT :
Fire top-kills 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 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.
DISCUSSION AND QUALIFICATION OF FIRE EFFECT :
NO-ENTRY
PLANT RESPONSE TO FIRE :
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]. 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 poison-oak; prefire poison-oak density was
unknown. By postfire year 3, poison-oak sprouts dominated most burn
plots in the riparian zone [18].
Westman and others [67] estimated that 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.
Poison-oak also establishes from seed after fire, although this response
is not well documented in the literature. Poison-oak seedlings were
osberved following site preparation and prescribed burning of an
interior live oak-blue oak woodland in Madera County, California.
Prefire poison-oak seedling density was 0 percent; seedling density at
postfire year 1 was 42 per 8,712 square feet [20].
DISCUSSION AND QUALIFICATION OF PLANT RESPONSE :
Fire response is probably related to Poison oak's successional role in
the plant community. Dense poison-oak thickets may develop in chaparral
that is control burned several times [12]. Poison-oak may become
locally extinct in Douglas-fir forest, however, that is burned every 4
years for 20 years or more [53].
FIRE MANAGEMENT CONSIDERATIONS :
Urushiol volatilizes when burned, and human exposure to poison-oak smoke
is extremely hazardous [40]. The smoke often poisons people who think
they are immune to the plant [46].
Poison-oak vines are a ladder fuel [61].
Goats can be used as an alternative to precribed 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 poison-oak was in the fuelbreak 67 percent, somewhat
lower than utilization of toyon, California blackberry, and coyotebrush.
Annual production of poison-oak biomass before goat browsing in the
fuelbreak was 99 kilograms per hectare; it was 33 kilograms per hactare
afterwards. Total biomass of forage species was significantly (p<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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SPECIES: Toxicodendron diversilobum | Poison-Oak
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Index
Related categories for Species: Toxicodendron diversilobum
| Poison-Oak
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