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Diversity with Successional Status in the
Pinyon-Juniper/Mountain Mahogany/
Bluebunch Wheatgrass Community Type
Near Dutch John, Utah
Allen Huber
Sherel Goodrich
Kim Anderson
Abstract-Alpha and beta diversity and vegetative cover for
Colorado pinyon (Pinus edulis Engeltn.) and Utah juniper
(Juniperus osteosperma [Torr.] Little) understories of northerly
exposures are compared at varying successional stages before and
after disturbance. Plant diversity and species richness are highest
in seral communities of alder-leaf mountain mahogany and
bluebunch wheatgrass where pinyon and juniper canopy cover does
not exceed 20 percent. Following disturbance at these sites, the
response of native understory species was rapid and vigorous.
Timely disturbance within the pinyon-juniper woodland sere appears necessary in order to maintain a responsive, productive, and
diverse native understory. These studies indicate that pinyonjuniper canopy cover of about 20 percent is a critical point for the
maintenance of native understory species.
In the Great Basin, Everett (1987) noted that as pinyonjuniper crown cover increases, cover, productivity, and density of understory species decrease. In the Green River
corridor of Daggett County, UT, similar relationships are
evident. On many northerly exposures in the area, plant
diversity and species richness are highest in seral communities where alder-leaf mountain mahogany (Cercocarpus
montanus Raf.) and bluebunch wheatgrass (Elymus spicatus
[Pursh] Gould) are commonly associated with approximately
50 other vascular plants. Where crown cover of pinyonjuniper is less than 20 to 25 percent, response of these
native understory species is rapid and vigorous following
fire. When crown cover exceeds 30 percent, the understory
trends toward depletion and the initial response following
fire is slower and less vigorous. At 40 percent or more crown
cover, many of the understory plant species have been
purged from the community. Established stands of closed
pinyon-juniper severely deplete understory seed reserves.
Succession following fire in closed stands where crown cover
of pinyon-juniper exceeds 40 percent is largely dependent
In: Monsen, Stephen B.; Stevens, Richard, comps. 1999. Proceedings:
ecology and management of pinyon-juniper communities within the Interior
West; 1997 September 15-18; Provo, UT. Proc. RMRS-P-9. Ogden, UT: U.S.
Department of Agriculture, Forest Service, Rocky Mountain Research
Station.
Allen Huber is Range Technician, Duchesne District, Ashley National
Forest, Duchesne, UT 8402l. Sherel Goodrich is Forest Ecologist, Ashley
National Forest, Vernal, UT 84078. Kim Anderson is a Doctorate Student at
Brigham Young University, Provo, UT 84602.
114
on seed migration to the site, creating ideal conditions
for cheatgrass (Bromus sps.) and other invasive exotic species. Fire intervals frequent enough to maintain alder-leaf
mountain mahogany and bluebunch wheatgrass communities in the Green River corridor are indicated to maintain
native plant communities of high diversity and vigor.
Eventual disturbance in closed stands of pinyon-juniper
where understory communities have been depleted or
purged will likely result in site occupation by invasive
weeds or species seeded by land managers.
Study Sites _ _ _ _ _ _ _ __
The study sites are located within the Green River corridor in Daggett County of northeastern Utah. The corridor is
within the Uinta Mountain Section defined by McNab and
Avers (1994). The sites are within a land type composed ofa
series of ridges and ravines formed by an alternate underlay of resistant Precambrian quartzite and highly erosive
shales. Within the Green River corridor, continuous and
many closed stands of Colorado pinyon (Pinus edulis Engelm.)
and Utah juniper (Juniperus osteosperma [Torr.] Little)
cover approximately 20,000 acres (8,100 ha) on the Ashley
National Forest, and these stands extend eastward well
beyond the National Forest boundary. Historically, seral
communities were maintained by wildfire. However, fire
suppression since the early 1900's has maintained areas
already supporting mature pinyon and juniper trees and
has allowed pinyon and juniper to invade other areas dominated by herbaceous plants and shrubs. Prescribed fire has
been used in the Green River corridor during the 1980's and
1990's to maintain viable and productive seral pinyonjuniper communities and to improve overall forage and
habitat for big game animals. Livestock grazing was discontinued at the study sites in the mid-1960's.
The land type on which the studies are located consists of
two phases that are primarily influenced by exposure. On
southerly exposures, native grass-forb communities are
common in the early seral stage and are later succeeded by
rubber rabbitbrush (Chrysothamnus nauseosus [Pallas]
Britt.) and mountain big sagebrush (Artemisia tridentata
var. pauciflora Winward and Goodrich). However, the potential for cheatgrass (Bromus tectorum L.) invasion and
dominance on these exposures following disturbance is
high, especially if pinyon-juniper crown closure existed
prior to disturbance (Goodrich and Gale, these proceedings,
Goodrich and Rooks, these proceedings).
USDA Forest Service Proceedings RMRS-P-9. 1999
Table 1-Site information for three study sites treated with
prescribed fire.
Seral status a
Stand age (years)a
Year burned
6-1
Study site number
6-2
6-21
Late
300
1989
Mid
80
1993
Mid
N/A
1985
aSeral status and approximate stand age prior to prescribed burning.
Methods and Results
Table 2-Site information for five study sites not treated with
prescribed fire.
6-24B
Seral status
Stand age (yrs)
Mid
70
Study site number
6-24C
6-24E
6-24G
Mid
150
Late
215
Late
140
Division of Wildlife Resources and the Ashley National
Forest. Both northerly and southerly exposures were burned.
The intensity and spread of fire was sufficient to achieve
essentially 100 percent mortality of pinyon and juniper
within the perimeter of each burn. Ring counts of pinyon and
juniper trees at most ofthe study sites were used to approximate stand age (tables 1 and 2).
6-24H
Mid
N/A
On northerly exposures, early seral native grasses and
forbs are succeeded by a diverse shrub component consisting of alder-leaf mountain mahogany, low rabbitbrush
(Chrysothamnus viscidiflorus [Hook.] Nutt.), snowberry
(Symphoricarpos oreophilus Gray), serviceberry CAmelanchier alnifolia N utt.), and mountain big sagebrush. Approximately 50 native vascular plants are associated with
this community. Bluebunch wheatgrass is the principal
understory species. Cheatgrass is also present but generally at a much lower frequency than found on southerly
exposures.
This study includes eight study sites that are within
3.5 miles (north-northwest) of Dutch John, UT and are no
further than 2 miles apart. All are located on northerly
exposures with gradients between 20 and 35 percent.
Elevations for the sites range between 6,600 and 6,800 ft
(2,012 and 2,073 m). Mean annual precipitation for the
Dutch John area as indicated by the Flaming Gorge
Weather Station is 12.50 inches (31.75 cm), of which 63
percent is from April through September (Ashcroft and
others 1992).
Three of the eight study sites were burned by helitorch
and aerial seeded in a cooperative project between the Utah
A total of one hundred 19.69 by 19.69 inch (50 by 50 cm)
sample plots were established along five permanently
marked 30.5 m (100 ft) beltlines at each of the study sites.
Quadrat frequency was determined for all plant species at
most of the study sites as outlined by the U.S. Department
of Agriculture, Forest Service (1993). Alpha (number of
species per quadrat) and beta (total number of species) plant
diversity was determined from quadrat frequency. Four
hundred point samples were read at each study site to
determine ground cover. Crown cover of woody species was
also measured by line intercept along each of the five
beltlines. Pre-burn and post-burn data was obtained from
study sites 6-1 and 6-2 (tables 3 and 4).
Data was taken from study site 6-2 before burning (1993)
and 2 and 4 years after burning (1995 and 1997). Initial
response of native understory species following fire at
study site 6-2 was rapid and vigorous. Nearly all native
species present before the burn were present after the burn.
Of the shrubs, only mountain big sagebrush was absent
following fire. A noticeable increase in alpha and beta was
observed 4 years after burning. Some ofthe increase in beta,
and possibly alpha, is due to the presence of seeded and
annual species (table 4). All but two of the grass species
increased in frequency from 1995 to 1997. Bluebunch wheatgrass was found with the highest frequency. Seeded grasses
are present but are not dominant components at the site.
Four years after burning, ground cover reached 85 percent
of pre-burn potential and alder-leaf mountain mahogany
recovered to nearly 85 percent of its pre-burn crown cover.
At study site 6-1, where crown cover of pinyon-juniper
was about 60 percent before burning, the understory community had been reduced by pinyon and juniper compe~ition.
Initial response of native species after fire was sluggIsh at
best. The increase in plant diversity is due to the presence
Table 3-Comparison of diversity, ground cover, and woody species crown cover pre-fire and post-fire at study sites 6-2 and 6-1.
Study
6-2
6-2
6-2
6-1
6-1
Diversity
Year
Alpha
1993
1995
1997
1989
1993
7.9
7.6
8.3
1.4
4.7
Beta
51
55
68 b
17
38 c
Ground
cover
P-J
Woody species crown coverS
ARTR
SYOR
AMAL
CEMO
CHVI
- - - - - - - - - - - - - - - - - - - - - - - - - - Percent - - - - - - - - - - - - - - - - - - - - - - 10.9
4.4
8.8
11.4
95
11.2
0.0
1.6
8.4
7.5
48
0.0
0.0
10.5
2.6
9.5
74
0.0
0.0
0.0
0.0
1.0
84
62.3
0.0
0.0
0.0
0.0
57
0.0
--0.6
3.4
2.0
0.0
0.0
ap-J = pinyon-juniper, CEMO = Cercocarpus montanus, AMAL = Amelanchier alnifolia, SYOR = Symphoricarpos oreophilus, ARTR = Artemisia
tridentata, CHVI = Chrysothamnus viscidiflorus.
bSixteen of the 68 species were either seeded or were annuals.
CEighteen of the 38 species were either seeded or were annuals.
USDA Forest Service Proceedings RMRS-P-9. 1999
115
Table 4-Comparison of number of species pre-fire and post-fire at study sites 6-2 and 6-1.
Study
Year
Graminoids
Forbs
6-2
6-2
6-2
6-1
6-1
1993
1995
1997
1989
1993
10
13
18a
4
11 b
31
36
43 c
10
25 d
Number of species
Shrubs
8
6
7
2
3
Seeded
Annuals
0
6
6
0
6
5
9
10
0
12
·Ot the 18 graminoid species,
four were seeded and two were annuals.
bOf the 11 graminoid species, five were seeded and one was an annual.
cOt the 43 torbs species, two were seeded.
dOt the 25 forb species, one was seeded.
of seeded and annual species. Seeded grasses and annuals
recorded high frequencies in 1993. AIder-leaf mountain
mahogany and bluebunch wheatgrass are represented (frequency values of 2 and 1) at the study site but are minor
components in the present community. Ground cover was
nearly 70 percent of pre-burn potential.
Data obtained from eight study sites indicate that as
crown cover of pinyon-juniper increases, alpha and beta
diversity and crown cover of alder-leaf mountain mahogany
decrease (table 5). Alpha and beta values were high and
remained stable to 20 to 25 percent pinyon-juniper crown
cover. Under these conditions, alder-leaf mountain mahogany increased in crown cover. Alpha and alder-leaf
mountain mahogany crown cover showed decline at about
30 percent pinyon-juniper crown cover. At 50 percent crown
cover, alpha and beta were significantly reduced, and alderleaf mountain mahogany had been essentially purged from
the community.
Discussion and Management
Implications _ _ _ _ _ _ _ __
Initial response of native understory species following fire
correlates closely with the per'Cent crown cover of pinyon
and juniper trees. Our studies indicate that the understory
is most productive, diverse, and responsive to disturbance
when pinyon-juniper crown cover is at or below 20 percent.
Initial response following fire at study site 6-2 was rapid
and vigorous. The annual stage described by Barney and
Frischknecht (1974) was essentially bypassed, and the perennial grass-forb stage was relatively short (approximately.
2 years). Alpha and beta had recovered to pre-burn levels.
Most of the shrubs present at the site sprouted after fire,
and alder-leaf mountain mahogany had nearly reached
pre-burn crown cover in only 4 years.
When pinyon-juniper crown cover exceeds 20 to 30 percent, thinning of the understory seems to accelerate. The
data indicate that beta remains stable but alpha begins to
decline. Beyond 30 percent, there is a rapid decline in
understory species and substantial decrease in alder-leaf
mountain mahogany crown cover. By the time pinyonjuniper crown cover reaches 50 percent, the understory and
soil seed reserves have been depleted, and many plant
species have been purged from the community. Crown cover
of alder-leaf mountain mahogany appears to be a positive
indicator for vascular plant diversity while crown cover of
pinyon and juniper appear to be negative indicators.
Our studies indicate that succession and management of
pinyon-juniper woodlands of the Green River corridor can
be correlated with percent crown cover of pinyon and
juniper trees. Age of stand appears to be a less reliable
indicator (compare stand ages and pinyon-juniper crown
cover percentages for study sites 6-24C, 6-24E, 6-24G, and 61). Fire intervals that keep pinyon-juniper crown cover
below 20 to 25 percent is indicated to maintain responsive,
productive, and diverse alder-leaf mountain mahogany
and bluebunch wheatgrass communities. Our findings
Table 5-Alpha and beta diversity and crown cover of shrubs and alder-leaf mountain mahogany (CEMO)
in relation to crown cover of pinyon and juniper (P-J).
116
Study site
Year
6-2
6-2
6-2
6-21
6-248
6-24H
6-24C
6-24E
6-24G
6-1
6-1
1993
1995
1997
1993
1997
1997
1997
1997
1997
1989
1993
Diversity
Alpha
Beta
7.9
7.6
8.3
9.1
7.2
N/A
4.5
3.2
2.7
1.4
4.7
51
55
68
49
62
N/A
50
37
30
17
38
Total shrub
crown cover
Crown cover
P-J
CEMO
- - - - - - - - - - - - Percent - - - - - - - - - - - 11.4
11.2
36.4
7.5
0.0
21.1
0.0
9.5
24.8
12.1
0.0
23.0
18.5
29.6
38.7
27.1
22.9
27.5
29.2
12.7
21.6
0.9
53.9
0.9
62.1
0.0
0.2
62.3
1.0
1.0
0.0
0.0
0.6
USDA Forest Service Proceedings RMRS-P-9. 1999
concur with Doughty (1987) who reported that understory
species begin to decline when trees reach one-third of their
climax potential (approximately 20 percent crown cover).
Where understory species have been depleted or purged
from the community, invasive species such as cheatgrass
can be expected to occupy these sites after disturbance if
seeding is not used to control their invasion.
References ---------------------------------Ashcroft, G. L.; Jensen, D. T.; Brown, J. L. 1992. Utah climate.
Logan, UT: Utah State University, Utah Climate Center. 125 p.
Barney, M. A.; Fischknecht, N. C. 1974. Vegetation changes following fire in the pinyon-juniper type of west-central Utah. Journal
of Range Management. 27(2): 91-96.
USDA Forest Service Proceedings RMRS-P-9. 1999
Doughty, J. W. 1987. The problems with custodial management of
pinyon-juniper woodlands. In: Everett, R. L., compo Proceedingspinyon-juniper conference; 1986 January 13-16; Reno, NV. Gen.
Tech. Rep. INT-215. Ogden, UT: U.S. Department of Agriculture,
Forest Service, Intermountain Research Station: 29-33.
Everett, R. L. 1987. Plant response to fire in the pinyon-juniper
zone. In: Everett, R. L., compo Proceedings-pinyon-juniper conference; 1986 January 13-16; Reno, NV. Gen. Tech. Rep. INT-215.
Ogden, UT: U.S. Department of Agriculture, Forest Service,
Intermountain Research Station: 152-157.
McNab, W. H.; Avers, P. E., comps. 1994. Ecological subregions of
the United States: Section descriptions. Administrative Pub!.
WO-WSA-5. Washington, DC: U.S. Department of Agriculture,
Forest Service. 267 p.
U.S. Department of Agriculture, Forest Service. 1993. Rangeland
ecosystem analysis and management handbook. FSH 2209-2l.
Ogden, UT: U.S. Department of Agriculture, Forest Service,
Intermountain Region.
117