Journal of the Arkansas Academy of Science
Volume 72 Article 15
2018
Winter Surveys of Cotinus obovatus (American
smoketree) in the Ozark Mountains
Gary R. Graves
National Museum of Natural History; University of Copenhagen, gravesg@si.edu
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Recommended Citation
Graves, Gary R. (2018) "Winter Surveys of Cotinus obovatus (American smoketree) in the Ozark Mountains," Journal of the Arkansas
Academy of Science: Vol. 72 , Article 15.
Available at: https://scholarworks.uark.edu/jaas/vol72/iss1/15
Winter Surveys of Cotinus obovatus (American smoketree) in the Ozark
Mountains
Cover Page Footnote
I thank the Ozark National Forest (USDA) for issuing research permits (OZF-FW-FY17-03) and two
anonymous reviewers for comments on the manuscript. I acknowledge the continuing support of the
Alexander Wetmore Fund (Smithsonian Institution) and the Smoketree Trust.
This article is available in Journal of the Arkansas Academy of Science: https://scholarworks.uark.edu/jaas/vol72/iss1/15
Journal of the Arkansas Academy of Science, Vol. 72, 2018
81
Winter surveys of Cotinus obovatus (American smoketree) in the Ozark Mountains
G.R. Graves
Department of Vertebrate Zoology, MRC-116, National Museum of Natural History, Smithsonian Institution, P.O. Box 37012,
Washington, D.C. 20013
Center for Macroecology, Evolution and Climate, University of Copenhagen,2100 Copenhagen Ø, Denmark
Correspondence: gravesg@si.edu
Running Title: Winter Smoketree Surveys
Abstract
Cotinus obovatus (American smoketree) is a rare
deciduous tree with a relictual distribution in
southeastern North America. Efforts to map its fine-
scale geographic distribution in the Ozark Mountains
have been limited to the growing season when the
distinctive blooming panicles and foliage facilitate
detection in hardwood-cedar woodlands. I describe the
physiognomic traits of leafless C. obovatus that permit
effective population mapping in winter landscapes.
Clumped growth and diagonally leaning stems facilitate
detection at a distance. Bark texture, twig morphology,
and sap odor confirm the identity of the tree at close
range.
Introduction
Cotinus obovatus Raf. (Anacardiaceae: American
smoketree), the sole member of the genus in the Western
Hemisphere, has intrigued naturalists since Thomas
Nuttall discovered it in 1819 on a bluff above the Grand
River in eastern Oklahoma (Buckley 1881; Little 1942;
Long 1970; Mohr 1882; Nuttall 1821; Palmer 1921;
Sargent 1885, 1892; Weber 1990). Natural populations
of this suffusively rare deciduous tree have been
documented in 41 counties scattered in three disjunct
regions: (1) Ozark Plateau in Arkansas and Missouri
with a few scattered stations along bluffs of the
Arkansas River and its tributaries in eastern Oklahoma,
(2) southern Cumberland Plateau in northeastern
Alabama and adjacent Tennessee and Georgia, and (3)
the Edwards Plateau in west-central Texas. Cotinus
obovatus is currently considered globally secure (G4)
owing to protected populations on national forest lands
in the Ozark Mountains of Arkansas and Missouri, but
it is listed as threatened in Alabama (S2), Oklahoma
(S2), and Tennessee (S2) and as critically imperiled (S1)
in Georgia (Natureserve, 2017).
The center of abundance of C. obovatus on the
Ozark Plateau occurs in four counties that border the
Arkansas-Missouri state line (Baxter, Marion, Taney,
Ozark). Factors that restrict the tree’s geographic
distribution are unknown, but all well-documented sites
occur on soils derived from calcareous bedrock. An
effort to map its fine-scale geographic distribution in
relation to the soil chemistry and geology of the Ozark
Plateau has been ongoing for the past decade.
Population mapping has traditionally been conducted
during the growing season when the distinctive
blooming panicles (late April through mid-May) and
foliage (April-September) are easily detectable in
roadside surveys (Davis and Graves 2016). However,
dense understory foliage may conceal isolated
specimens of C. obovatus in the interior of hardwood-
cedar woodlands. If the tree can be identified at modest
distances (~10–40 m) in leafless winter landscapes,
detection efficiency during walking surveys may
actually increase relative to growing season efforts.
Here I describe the physiognomic traits that facilitate
winter surveys of C. obovatus. These can be grouped
into characters that form the search image necessary to
detect trees at distance (growth form) and those that
confirm identity at close range (bark, twigs, sap).
Methods
Study area
Winter surveys of C. obovatus were conducted 28
February-14 March 2017 and 4-20 February 2018 in the
Sylamore Ranger District of the Ozark National Forest,
centered on the Norfork and Norfork SE topographic
quadrangles (7.5-minute series, U.S. Geological
Survey) in Baxter County, Arkansas. This region is
topographically complex (100–429 m), dissected by
deep valleys that drain into the White River and its
major tributary, the Buffalo River. Current land cover in
the national forest is a patchwork of secondary
hardwood forest, Juniperus virginiana (eastern redcedar),
and Pinus echinata (shortleaf pine). Glades are relatively
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Figure 1. Typical growth form of Cotinus obovatus (American
smoketree) in the Ozark National Forest, Baxter County, Arkansas.
Multiple stems emerge from a single root crown in many specimens.
rare and those that do occur are relatively small. Old-
growth trees and elevational bands of timber that exhibit
no evidence of logging are found on steep, rocky slopes.
Surface rocks over most of the study area are composed
of interbedded limestone, dolostone, and sandy
dolostone of the Everton Formation (middle
Ordovician) which is overlain by thin beds of St. Peter
sandstone. The highest ridges and knobs are capped by
cherty limestones of the Boone formation, (Haley et al.,
1993). Data reported in this study were obtained on off-
road surveys aimed at documenting the geographic
coordinates of witness specimens of C. obovatus at a
200 m grain size.
Data collection
I photographed specimens of C. obovatus (n = 177)
to document growth form and stem count. Selected
trees met the following criteria: (i) the base of the tree
was unobscured by undergrowth; (ii) the number of
stems (> 5 cm DBH) emerging from the root crown
could be counted in the photograph; and (iii) the stems
were mature enough to have attained the flaky bark
characteristic of older trees. I walked around each tree
until the principal axis of a focal stem, usually the
thickest for multi-stemmed specimens, was
perpendicular to the plane of the camera. From enlarged
photographs, I measured the angle of the focal stem
(from ground level) along its lower side with a
protractor. Landmarks for angle measurement were
stem base at ground level and a point on the stem ~1.5
m from the base. Only a single stem per specimen was
measured. Summary statistics (mean + standard
deviation) are presented for stem angle and stem count.
Results
Growth form
Sargent (1892:3) offered a brief description of C.
obovatus physiognomy: “A small tree, twenty-five to
thirty-five feet in height, with a straight trunk
occasionally twelve or fourteen inches in diameter,
usually dividing, twelve or fourteen feet from the
ground, into several erect stems which separate into
wide-spreading, often slightly pendulous branches.”
Individuals of this diameter are rare in the Ozark
Mountains, but more importantly, a majority of C.
obovatus trees are composed of multiple stems that
emerge from the root crown (Fig. 1).
In the sample of photographed trees (Fig. 2), the
number of stems per root crown varied from 1 to 15 (xˉ =
2.6 + 2.5). Stems have a tendency to lean or sprawl
diagonally from the base. Stem angle (from horizontal)
varied from 9° to 90° (xˉ = 62.7 + 18.6°; n = 177).
Solitary stems tend to be more vertical (xˉ = 68.2 + 16.6°;
n = 77) than stems randomly chosen from multi-
stemmed specimens (xˉ = 58.4 + 18.9°; n =100). In any
event, diagonally leaning stems and clumped growth are
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Figure 2. Histogram of stem lean angle (90o = vertical) and number
of stems per root crown of Cotinus obovatus (American smoketree)
in the Ozark National Forest, Baxter Country, Arkansas. Y-axis
represents the number of specimens in each bin.
hallmarks of mature C. obovatus in open and closed
canopy hardwood-cedar woodland in the Ozark
Mountains.
Bark
Standard floras (Mohr 1901; Steyermark 1963) of
states that harbor populations of C. obovatus omit
descriptions of bark, and the accounts and illustrations
in field guides (Leopold et al. 1998; Samuelson and
Hogan 2003; Sibley 2009; Simpson 1988) are too brief
or idiosyncratic to be useful for distinguishing the bark
of C. obovatus from that of other small forest trees with
rough bark. Buckley (1881: 125) observed, “…trunks
and larger limbs [of C. obovatus] coated with light gray
and deeply-furrowed bark resembling the bark of the
larger trees of the common sassafras (S. officinale).”
Perhaps the most accurate description was penned soon
after the rediscovery of the tree in Alabama in 1881
(Mohr 1882: 218), “The bark is rough, covered with a
whitish gray epidermis of a deep chestnut-brown
underneath, and exfoliating in oblong square scales of
uniform size.”
In the Ozark Mountains, the smooth gray bark of
young C. obovatus begins to crack and crenellate when
the stems reach a diameter of 4–7 cm, although some
sprouts as large as 10 cm have immature bark. The bark
of mature stems is relatively thin and fractured into
rounded rectangular or oval scales that are usually 1.8–
3.5 times longer than wide (Fig. 3). Scale tips are
frequently curled or curved away from the stem. Some
specimens possess imbricated bark scales with curved
tips pointing downward. Scale size and shape tend to be
relatively uniform within individuals but vary
significantly among trees. The bark is pale to medium
gray when dry and dark gray to black when wet. The
diagnostic combination of bark texture and color
distinguishes C. obovatus from other small trees with
craquelure bark in the Ozark Mountains (e.g., eastern
redbud, Cercis canadensis; flowering dogwood, Cornus
florida; sassafras, Sassafras albidum; and gum bumelia,
Sideroxylon lanuginosum). Large specimens of C.
obovatus frequently exhibit cambial dieback, which can
affect as much as 80% of the stem girth. Exposed wood
weathers to grayish black. The crown foliage of many
older specimens is nourished by a meandering band of
cambium and phloem winding up the stem. The cause
of cambial dieback is unknown.
Twigs
Sargent (1892: 3) provided a concise description of
the twigs of C. obovatus, “The young shoots are purple
at first, but soon become green; during the first winter
they are bright red-brown and are covered with small
white lenticular spots and marked by large prominent
leaf-scars…The winter-buds are acuminate and an
eighth of an inch long, and are covered with thin dark
red-brown scales.” During the late winter in the Ozark
Mountains, the reddish-brown color, moderate diameter
(3-5 mm), and absence of knobby or obliquely
projecting buds give twigs a smooth silhouette and
distinctive appearance (Fig. 4) that help confirm the
identity of C. obovatus at moderate distances.
Sap
Buckley (1881:3) noted that broken branches of C.
obovatus, “emitted a yellowish sap, the odor of which
was highly aromatic; to me very unpleasant.” Mohr
(1882: 218) observed that the bark, when bruised,
exuded “a resinous sap of a heavy, disagreeable
terebinthinous odor.”
I found the pungent odor to be reminiscent of
thesap of Rhus copallinum (winged sumac) and R.
glabra (smooth sumac), both of which are widespread
in the Ozarks, and nearly identical to that of Cotinus
coggygria (Eurasian smoketree), which is occasionally
used in urban landscaping in the Ozark region. Broken
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twigs of C. obovatus are odorous, even in cool or cold
weather (0–5° C). The odor persists for years in dead
twigs. The aromatic constituents of C. obovatus sap
Figure 3. Bark of mature stems of Cotinus obovatus (American
smoketree).
have not been analyzed but intensive analyses of C.
coggygria have revealed that the wood and bark are rich
in monoterpenes, sesquiterpenes, and phenols (Matić et 
al. 2016). As a matter of practice, all witness trees
encountered during winter surveys in areas where C.
obovatus trees occurred at low density were confirmed
through the examination of growth form, bark, twigs,
and sap odor.
Figure 4. Twigs of the American smoketree (Cotinus obovatus).
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Discussion
Winter survey efficiency has not been quantified
although it certainly varies with topography, understory
visibility, and the density of Cotinus obovatus trees.
Once the search image has been formed, observers
should be able to detect most if not all mature specimens
within 20 m of the survey path at slow walking speed
(1-3 km/hr). In relatively open understories, the
clustered, leaning stems of C. obovatus may be apparent
to the unaided eye at distances as great at 50 m.
Binoculars are indispensable for scanning prospective
specimens, and I never attempted a cross-country survey
without them.
Acknowledgments
I thank the Ozark National Forest (USDA) for
issuing research permits (OZF-FW-FY17-03) and two
anonymous reviewers for comments on the manuscript.
I acknowledge the continuing support of the Alexander
Wetmore Fund (Smithsonian Institution) and the
Smoketree Trust.
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