Characteristics of Mountain Mahogany
(Cercocarpus) Species and Hybrids in Utah
Hybrid Zone
Scott C. Walker
Deborah Turley
Abstract—Species within the genus Cercocarpus, commonly called
mountain mahogany, are valuable browse species for wildlife and
livestock. Utah is the primary zone of overlap of the three species
common to the Intermountain area. Hybridization occurs between
curlleaf mountain mahogany (Cercocarpus ledifolius Nutt.), true
mountain mahogany (C. montanus Raf.), and little-leaf mountain
mahogany (C. intricatus Wats.) throughout central and northern
Utah where the species come in contact. Hybrid frequency and
characteristics are dependant upon parental varieties of each
species.
Cercocarpus species are vital components in native ecosystems. They increase diversity, maintain soil stability,
and provide good quality habitat for many species of wildlife
and domestic livestock. Three species of Cercocarpus are
common in the Intermountain area of the Western United
States: Cercocarpus montanus Raf. (true mountain mahogany), C. intricatus Wats. (little-leaf mountain mahogany),
and C. ledifolius Nutt. (curlleaf mountain mahogany) represented by two distinct subspecies; var ledifolius and var
intermontanus N. Holmgren. As shown in figure 1, Utah is
the primary zone of overlap for these shrubs (Pyrah 1964;
Davis 1990). The highest concentration of C. intricatus lies
directly and almost entirely within the zone of overlap.
Cercocarpus species are associated with desert shrub, sagebrush, pinyon-juniper, mountain brush, ponderosa pine,
and mixed aspen-conifer zones (Davis 1990).
Where the species of Cercocarpus come in contact with
each other hybridization nearly always occurs. This report is
not intended to be a comprehensive review of the hybridization of species within the genus Cercocarpus. It is more an
overview of the occurrence of hybridization, a brief review of
the literature, and a report of observations recorded by the
authors.
shrub is evergreen, intricately branched, and usually less
than 1 m but can grow up to 2.5 m in height (Blauer and
others 1975). Stutz (1974) suggests that C. intricatus is a
dry, harsh site segregant of curlleaf. C. intricatus is distinguished from C. ledifolius solely on the basis of leaf size and
plant stature. This morphology is not environmentally induced, but rather a genetic assimilation of adaptive characteristics from C. ledifolius var ledifolius. This becomes
apparent when the two species are growing in a common
garden and maintain their individuality, suggesting that
these two taxa are genetically distinct (Pyrah 1964).
C. ledifolius is an erect evergreen shrub that often grows
into small trees, 2 to 8 m tall, with stiff sharp branches. C.
ledifolius occurs on mountain slopes, often in pure stands as
groves surrounded by open sagebrush slopes or mixed with
mountain brush, pinyon-juniper, ponderosa pine, Douglasfir, or white fir. The best developed stands are routinely
Discussion _____________________
Throughout their range of overlap each of the three species occupy rather distinct habitats. The more xeric C.
intricatus occurs on harsh sites that are exposed to high
temperature and drought (Blauer and others 1975). This
In: McArthur, E. Durant; Ostler, W. Kent; Wambolt, Carl L., comps. 1999.
Proceedings: shrubland ecotones; 1998 August 12–14; Ephraim, UT. Proc.
RMRS-P-11. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Rocky Mountain Research Station.
Scott C. Walker and Deborah Turley are Research Biologists, Utah Division of Wildlife Resources, Great Basin Research Center, 540 N. Main 32-7,
Ephraim, UT 84627.
32
Figure 1—Distribution of curlleaf mountain mahogany,
true mountain mahogany, and little-leaf mountain
mahogany (from Davis 1990).
USDA Forest Service Proceedings RMRS-P-11. 1999
found on all exposures of warm dry slopes, typically growing
in shallow soils or rocky ridges on slopes averaging around
50 percent (Davis 1990; Blauer and others 1975; Holmgren
1987). The most distinguishing characteristic between C.
ledifolius var ledifolius and C. ledifolius var intermontanus
is leaf size and degree of pubescence on the leaf. C. ledifolius
var intermontanus leaves are (ob)lanceolate or elliptic-lanceolate, 5 to 8 (10) mm wide, sparsely hairy, the midrib and
lateral veins conspicuously visible. C. ledifolius var ledifolius leaves are narrowly lanceolate to linear, 1.5 to 4 (6.5)
mm wide, densely white-hairy beneath, the pubescence
sometimes obscuring the midrib and certainly the lateral
veins (Holmgren 1987). C. montanus grows 1 to 2 m in
height, less commonly, a small tree up to 4 m. C. montanus
communities can usually be found at lower elevations than
C. Ledifolius, occurring on similar slopes but usually with
deeper soils (Davis 1990). Leaves are deciduous, short petiolate, the blade obovate to (ob)lanceolate or orbicular, 6 to 44
mm long, 5 to 23 mm wide, crenate-serrate (Welsh and
others 1987; Blauer and others 1975).
The flowering period for these species of Cercocarpus is
from mid May to late June. Flowering periods for C. ledifolius and C. intricatus tend to overlap in areas of common
occurrence. While C. montanus flowers nearly 2 weeks later
than the evergreen types. The flowering period may overlap
for plants on contrasting canyon slopes where aspect may
affect plant distribution and flowering phenology. Hybrid
plants generally begin flowering 2 weeks after the later
flowering C. montanus (Pyrah 1964).
Hybridization between the species is not a rare occurrence, suggesting genetic reproductive barriers between the
taxa are weak. Though hybrids form upon contact between
all of the taxa, frequency of F1 hybrids varies depending on
parental species combinations. Only those that involve C.
ledifolius var ledifolius as one of the parents produce hybrids
with significant segregant progeny (table 1) (Stutz 1990).
The duration of pollen viability is an important factor in
species isolation in nature. As shown in table 2, a portion of
the pollen of C. ledifolius and C. montanus remains viable for
more than 10 days. Abnormal pollen tubes become increasingly abundant during this period of time (Stutz 1990; Pyrah
1964). This extended pollen viability allows for the cross
pollination among these species.
Hybridization of species within the genus Cercocarpus
offers a unique perspective. At the ecotones where species of
Cercocarpus meet, the distribution of each population is
controlled by precipitation and soil characteristics. There is
Table 1—Summary of natural hybridization in Cercocarpus (from Stutz
1990).
Parentsa
from F1
Opportunity
for
hybridization
led led x led interm
led led x intricatus
led led x montanus
led interm x intricatus
led interm x montanus
intricatus x montanus
low
high
low
moderate
moderate
moderate
Abundance of
F1 hybrids in
contact zones
high
high
high
moderate
high
low
a
Progeny
hybrids
abundant
abundant
abundant
moderate
few
none
led led = C. ledifolius var ledifolius; led interm = C. ledifolius var
intermontanus.
USDA Forest Service Proceedings RMRS-P-11. 1999
often some overlap of the two populations at the ecotone.
Where hybridization occurs, there is generally not a distinct
band or zone that these hybrids occupy. For example, where
C. ledifolius and C. montanus come in contact, C. ledifolius
is generally growing upslope of C. montanus. When hybrids
form between the two, they occur almost without exception
within the bounds of the C. montanus population rather
than where the two populations interface. This suggests
that C. montanus is the maternal parent supported by what
has been observed for flowering dates and pollen durability
(Pyrah 1964). Since pollen from the earlier flowering C.
ledifolius can remain viable for more than 10 days, crosspollination onto C. montanus is possible. However, before
pollen is shed from C. montanus, the stigmas of C. ledifolius
have withered, and as a result reciprocal pollination is
usually not permitted. Generally it appears that pollination
is most common from C. ledifolius to C. montanus.
Noting that the flowering periods are similar for C. ledifolius and C. intricatus, one would expect similar trends for
both species in hybridization with C. montanus. Observations were made that demonstrated exceptions to this pattern for hybrids between C. intricatus and C. montanus. One
example occurs on the Chippean Rocks formation in the
Abajo mountains of southeastern Utah (elevation 2,500 m).
The spires of this weathered sandstone deposit extend up
from a forest of ponderosa pine with a dense shrub understory. The formation is only a few hundred meters in height.
C. montanus is scattered in the understory on lower slopes
of the formation, while C. intricatus is distributed across
upper slopes and on top. Hybrid plants are also located at the
top of the formation within the C. intricatus stand. This
representation of C. montanus not being the probable maternal parent suggests the possibility that reciprocal crossing
can occur.
Another site of interest is found near the Wind Caves
(elevation 2,100 m) located in Logan Canyon in northern
Utah. There is a variety of intermediate phenotypes between
C. ledifolius var intermontanus and C. intricatus, all growing among a C. montanus population. The variety of intermediate phenotypes of the individuals within the populations may be a result of the hybridization of C. intricatus X
C. ledifolius. Also present were hybrid products of C. ledifolius X C. montanus and plants suspected of being C. intricatus
X C. montanus hybrids.
Most of the dozen mahogany sites that were inspected for
this project contained hybrid plants within the population.
The Wind Cave site was unique in which hybrids occurred
where all three species were growing within close proximity,
and where there were hybrids derived from each combination of species. As this study was by no means a
comprehensive inventory, other sites that may exhibit
similar population dynamics are likely in the Intermountain area.
When C. ledifolius is in contact with C. intricatus and
C. montanus, hybridization nearly always occurs. However, each population, and progeny within populations,
display a different pattern of hybrid products. Plants
may show traits or characteristic of either parent. For
example, a single hybrid plant might be more upright,
have a tendency for evergreen leaves characteristics
associated with C. ledifolius, and have multiple stems,
a characteristic of C. montanus. One of the most telling
33
Table 2—Duration of pollen viabilitya for Cercocarpus montanus and C. Ledifolius var intermontanus
(modified from Pyrah 1964).
Age
days
Nb
montanus germination
#c
%
N
var intermontanus germination
#
%
1
2
5
10
14
362
331
305
332
381
278
91
65
82
81
76.8
27.5
21.3
24.7
21.2
374
——
222
308
228
152
——
42
38
20
40.7
——
18.9
12.3
8.8
a
Pollen was germinated on sterile nutrient agar.
Number in sample.
c
Number germinated.
b
and variable characteristics of hybrids is leaf morphology.
Variability is high, both within species as well as among
hybrids. Leaves taken from adjacent hybrid plants can
demonstrate varying parental traits. For example, leaves
may display strong C. ledifolius traits with more linear
slightly toothed, and strongly enrolled margins, while leaves
from other hybrids maintain stronger C. montanus traits
with a more spatulate shape, stronger toothed and slightly
enrolled margins.
The seeds of hybrids are usually highly inviable, which
suggests the presence of chromosomal or genetic sterility
within the hybrid. Such sterility is probably due to differences in parental chromosomes or to incompatible gene
interaction (Stutz 1990).
For the three mahogany species, seed production always
occurs on second-year branches or from second-year buds.
The flowering phenology of a plant is synchronized and seed
is produced in a single flush. However, C. montanus plants
have been observed to demonstrate a unique partitioning of
resources. One branch will put energy into copious seed
production and will produce very little or no annual leader
growth, while an adjacent branch, on that same plant, will
produce very little or no seed, but instead will produce large
amounts of annual growth. The trigger mechanisms for
seed-versus-growth production within this genera are poorly
understood. Among hybrid plants a unique flowering phenology also occurs. The triggering mechanism for bud and
seed production in hybrids seems to be confused. Hybrid
plants produce maturing seed, new flowers, and developing
buds concurrently on the same branch of current-year’s
growth, as compared to the synchronized flowering on second-year’s growth of nonhybrids. These branches may continue to produce buds and flowers well beyond the normal
flowering period of both parent species.
Hybridization or genetic assimilation is often the key to
long-term survival, adaptation, or evolution of a species. The
unique traits demonstrated by Cercocarpus hybrids include
adaptation of parental characteristics. Hybrids exhibit characteristics that demonstrate a unique relationship to either
parent. When hybrids become established in adaptive habitats, individuals tend to be larger than either parent. This is
probably an expression of hybrid vigor and can be accounted
for by the wide genetic diversity between these two species.
34
Hybridization among the three species of mountain mahogany is a common occurrence. This leaves one to surmise
that the possibility that a stable hybrid could be produced;
one that will produce viable seed, have desirable characteristics of both parents (such as being evergreen, and have the
ability to resprout), and would be of benefit to the rangelands
they occupied.
Each parental species can produce unique products or
perform different functions. Hybrids may have the ability to
produce all products produced by both parents or perform
both functions. This could explain the continual flowering
process that occurs on hybrids and the ability to produce
seed on current years’ growth. An interesting point is the
idea that these hybrids may be able to synthesize new
products not found in either parent. Gas chromatographic
work done by Pyrah (1964) showed there are indeed unique
compounds produced by the hybrids that are not present in
either parent. Though not identified, these compounds may
have additional value not yet understood.
References _____________________
Blauer, A. C.; Plummer, A. P.; McArthur, E. D.; Stevens, R.; Guinta,
B. C. 1975. Characteristics and hybridization of important Intermountain shrubs. I. Rose family. Res. Pap. INT-169. Ogden, UT:
U.S. Department of Agriculture, Forest Service, Intermountain
Forest and Range Experiment Station. 35 p.
Davis, J. N. 1990. General ecology, wildlife use, and management of
the mountain mahoganies in the Intermountain West. In: Johnson,
Kendall L., ed. Proceedings of the fifth Utah Shrub Ecology
Workshop; 1988 July 13-14; Logan, UT. Logan, UT: Utah State
University: 1-13.
Holmgren, N. H. 1987. Cercocarpus ledifolius var intermontanus
(Rosaceae), a new varietal name for the Intermountain curlleaf
mountain-mahogany. Brittonia. 39: 423-427.
Pyrah, G. L. 1964. Cytogenetic studies of Cercocarpus in Utah.
Provo, UT: Brigham Young University. 44 p. Thesis.
Stutz, H. C. 1974. Rapid evolution in Western shrubs. Utah Sci. 35:
16-20, 33.
Stutz, H. C. 1990. Taxonomy and evolution of Cercocarpus in the
Western United States. In: Johnson, Kendall L., ed. Proceedings
of the fifth Utah Shrub Ecology Workshop; 1989 July 13-14;
Logan, UT. Logan, UT: Utah State University: 15-25.
Welsh, S. L.; Atwood, N. D.; Goodrich, S.; Higgins, L. C. 1987. A
Utah flora. Great Basin Natur. Mem. 9. 894 p.
USDA Forest Service Proceedings RMRS-P-11. 1999