Great Basin Naturalist
Volume 57 | Number 2
Article 9
5-7-1997
Biotic and abiotic factors influencing the
distribution of Coleogyne communities in southern
Nevada
Simon A. Lei
University of Nevada, Las Vegas
Lawrence R. Walker
University of Nevada, Las Vegas
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Recommended Citation
Lei, Simon A. and Walker, Lawrence R. (1997) "Biotic and abiotic factors influencing the distribution of Coleogyne communities in
southern Nevada," Great Basin Naturalist: Vol. 57: No. 2, Article 9.
Available at: http://scholarsarchive.byu.edu/gbn/vol57/iss2/9
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Creat Basin Naturalist 57(2), © 1997, pp. 163-171
BIOTIC AND ABIOTIC FACfORS INFLUENCING THE DISTRIBUTION
OF COLEOGYNE COMMUNITIES IN SOUTHERN NEVADA
Simon A. Lei 1 and Lawrence R. Walkerl
ABSTRACT.-Cnleogyne ramosissima is a desert shruh that forms a nearly monuspecific shrubland and occupies a
well·defined e1cvational band between approximately 1050 and 2150 m on mountain muges in southern Nevada. The
Coleogyne shrubland shares relatively broad upper and [ower ecotones with Pinw-]umpen18 and Larrea·AmbmsiLl plant
communities, respectively. We characterized the extent that environmental factors correlate with C<Jleogyne density and
examined variation in biotic and abiotic factors along the lower elevational boundary of COleogYUB in Lucky Stri'ke Canyon
near Las Vegas, Nevada. Coleogyne density was positively correlated with gravimetric soil moisture, soil organic matter,
C.oleogyne waler potential, Coleogyne stem and leaf phosphoms, and Coleogyne leaf biomass. However, Coleogyne density
wa... negatively com~Lated with soil temperatures, soJ comp..'1ction, and Colecgyne stem <Iud leaf nitrogen. Coleogyne stem
biomass and elongation were generally negatively correlated with Coleogyne density. Coleogyne density was weakly correlated with .~oil pH, soil depth, soil nitrogen. soil and leaf phosphorus; these variables did not exhibit a consistent. pattern
with increasing elevation. Edaphic factors, particularly soil moisture and soil organic mattel; appear to playa major role
in determining the distribution of QJleogyne shrublunds in southern Nevada.
Key wor-ds: Coleogyne rnmosissima, soils, lower Coleogyne ecotone, LucJ.:y Strike Canyon, Mojave Desert.
Coleogyne ramosi-ssima Torr. (blackbrush) is
a member of the Rosaceae family and is the
only species in the genus Coleogyne. Coleogyn£
primarily establishes along the upper and central portions of the Colorado River drainage
and is an endemic yet Widespread species in
the southwestern United States (Bowns 1973).
Previous studies have shown that Larrea tridentata-Ambrosia d,urwsa plaot communities
appear to dominate between 900 and 1200 m
elevation and are replaced by Coleogyne communities from 1200 to 1500 m elevation in the
Mojave Desert (Beatley 1969, 1976). Coleogyne
corrununities are replaced by Pinw; mOTIIJphyllaJuniperus osteospemw woodlands at elevations
of 1500-1800 m. Coleogyne forms nearly monospecific stands in much of its range with relatively broad upper and lower ecotones that
overlap adjacent communities by as much as
> 100 m in elevation in southern Nevada (Lei
and Walker 1995). Coleogyne generally grows
well on sand, sandy loam, and loam, less well
on gravel and clay loam, and poorly on dense
clay (Korthuis 1988). A negative correlation
exists between the abundance of grdsses and
shrubs within the Coleogyne shrublands. For instance, grasses, forbs, and cryptogamic crusts
are more common on deeper soils where clay,
silt, and mineral nutrients are mare abundant
(Callison and Brotherson 1985). Sbrubs, on the
other hand, are mOre common on shallow and
sandy soils. Bowns (1973) suggested that low
soil moisture limits the lower elevational
boundary of Coleogyne communities in Utah.
Hunter and McAuliffe (1994) proposed that an
increase in precipitation is the paramount factor permitting the downslope establishment of
Coleogyne. In the vicinity of Searchlight in
southern Nevada, Coleog[llle estahlishes as low
as 1080 m, whereas around Death Valley of
southern California, Coleogyne is absent below elevations of 1350 m (Hunter and McAuliffe
1994). Current geographic variation in precipitation appears to control the Coleogyne distribution at its lower elevational boundary. We
examined abiotic I'tetors that might limit Coleogyne at its lower elevationallimit in southern
Nevada.
HISTORICAL BIOGEOGRAPHY
Packrat (Neowma spp.) midden records from
the state of Nevada reveal that woodland vegelalion persisted to tbe end of the early Holocene at elevations as low as 1250 m in the
Mojave Desert (Betancourt et aJ. 1990). As
annual temperatures rose, Pinus-Juniperus
lDepartment or Biological Scie1tce>. Unilo"l::nit)' of Nevada-w ~~s, I...aJl Vegas. NV H91S.-4004.
163
164
[Volume 57
GREAT BASIN NATURALIST
trees migrated to higher elevations on desert
mountain slopes and were replaced by desert
shrubs (Betancourt et aJ. 1990). Cowogyne first
appears in the fossil record in 3 late-Wisconsin
Neotoma deposits in the Frenchman Flat area
of southern Nevada (approximately 10,000 yr
ago) and should have extended theu to about
700 m (Wells and Berger 1967). The entire
Cowogyne zone and at least the upper part of
the Larrea-Ambrosia zone around Frenchman
Flat were occupied by evergreen ]unipertts
woodlands during the late Pleistocene (Wells
and Jorgensen 1964). The arrival of xerophytic
shrubs and the extinction of woodlands in low
elevations in the Mojave Desert occurred about
8000 yr ago, and vegetation zones migrated
upslope significantly around 1000 yr ago and
continue through the present time (Betancourt
et aJ. 1990).
Coleogyne can be regarded as a paleoendemic species, exhibiting little variability and
perhaps on its way to extinction (Bowns 1973).
The shrubs are considered relict because they
were probably once more widespread than at
present, and their current distribution represents a restriction in their range with time
(Bowns 1973). However, paleoecological evidence reveals the Larrea-Coleogyne ecotone
has undergone frequent elevational migrations
during the Quaternary period (Pendleton et al.
1995). Evidence from packrat middens also
shows Cowogyne has repeatedly moved up
and down elevational gradients in response to
the last pluvial period (20,000-10,000 yr B.P;
Bradley 1964).
Coleogyne shrublands generally share relatively broad lower ecotones with LarreaAmbrosia in southern Nevada (Lei and Walker
1995). Lucky Strike Canyon represents the
typical vegetation and landscape conditions
preVailing in the region. The aim of this study
was to determine the elevational range and
Coleogyne density at its lower ecotone. This
study also involved an investigation of biotic
and abiotic factors along the lower Coleogyne
elevational boundary in Lucky Strike Canyon.
STUDY AREA
Lucky Strike Canyon is located approximately 65 km northwest of Las Vegas (36° 10'N,
115° lO'W) between Lee and Kyle canyons on
the east-facing slopes of the Spring Mountains
in southern Nevada. The Cowogyne community
in Lucky Strike Canyon is characterized by
hot summers above 40 °C and cold winters
below _10°C. Temperature means and extremes
are negatively correlated with elevation. Sum-
mer rainfall usually occurs during thunderstorms in July and August and comes from the
Gulf of California, drawn into the desert by
strong convectional currents (Rowlands et a1.
1977). Summer storms are often local aud intense. Winter rains, on the contrmy, are mild and
widespread and come from the Pacific Ocean.
They may last up to several days. Snow is frequent at high elevations in southern Nevada,
particularly at the upper Coleogyne ecotone
with Pinus-Juniperus woodlands. A relative
climatic shifts (Phillips and Van Devender 1974,
Cole and Webb 1985, Pendleton et aJ. 1995).
Pendleton et aJ. (1995) hypothesize that Cowogyne is currently migrating into areas where
it is adapted to edaphic and climatic conditions.
Cowogyne shrublands appear to be migrating
toward higher elevations on desert mountain
humidity of < 20% is common in summer seasons. High evaporation and low precipitation
create a typical arid environment with an
slopes in southern Nevada and northward into
METHODS
southern parts of the Great Basin Desert (B.
Pendleton personal communication 1994). The
ecology of Cowogyne seedlings at higher elevations of the Mojave Desert and in the Great
Basin Desert may have different dynamics than
lower elevation populations in the Mojave
Desert. A global climatic shift toward lowered
annual precipitation and more extreme fluctua-
tions of temperature and rainfall began in the
Tertiary and continued through the Quaternary
to the present (Bowns 1973). The current composition and distribution of plant communities in southern Nevada has developed since
average annual rainfall of <200 mm.
Measurements were made in 1993 and
1994 across the lower ecotone of Coleogyne in
Lucky Strike Canyon. We established six 100m 2 circular plots (5.65-m radius) along a transect in the center of Lucky Strike Canyon at
each of 6 elevations (1160--1310 m; 2.9 km in
distance) at 30-m intervals for a total range of
150 m in elevation and a total of 36 plots
across the lower Coleogyne ecotone. The lowest
of the 6 elevations represents Larrea-Ambrosia
stands, below the range of Coleogyne shrubs.
The remaining elevations represent increasing
1997]
COLEOGYNE DISTRIBUTION IN SOUTHERN NEVADA
165
levels of Coleogyne density, bnt the Coleogyne
community extends to over 1600 ill elevation
in Lucky Strike Canyon (Lei and Walker
1995).
Soil samples at 2 depths (0-7 and 7-15 em)
were collected from random locations within
each of the 36 plots with core widths of 12 em.
All soils were passed through a 2-mm sieve
was measured from March to June 1993. Stems
and leaves of Coleogyne were collected and
before analysis. Soil and air temperatures and
soil moisture were measured every 6 wk from
May to August 1993; soil moisture was also
then oven-dried for 72 bat 60"C before weighing and analYZing for total nitrogen and phosphorus following acid digestion (Page 1982).
measured every 6 wk from May to August 1994.
Biomass of current~year stems and leaves was
We took soil temperature readings using a soil
thennometer with the probe at the soil surface
determined after oven-drying for 72 h at 60"C.
and at 5, 10, and 15 em below the surface in
open areas and beneath shrub canopies. Air
temperatures at 1.6 III were recorded concurrently with soil temperatures. We weighed soil
samples, dried them for 72 h at 1l0"C, and then
reweighed them to determine gravimetric soil
moisture. Oven-dried soils were then used for
soil pH, organic mattel; and nutrient analyses.
Soil organic matter and water potentials of
Coleogyne shrubs were measured in May, July,
and August 1994. Soil organic matter was
determined from ash remaining after soils
were heated to 550°C for 4 h. We measured soil
compaction in open areas in August 1994 using
a penetrometer that was inserted into the soil
after we had removed stony surface pavements.
Total Kjeldahl nitrogen (TKN), total Kjeldahl
phosphorus (TKP), soil depth, and soil pH
were measured in August 1993. Total nitrogen
and phosphorus were determined on ovendried soils following acid digestion using a
modified Kjeldalll method (Page 1982) for
rapid flow-through analysis of soil extracts.
Soil depth was estimated by determining the
depth to which a steel rod could be pounded
chamber and nitrogen gas. Twenty terminal
twigs from 1 Coleogyne bush (approximately
45-60 em canopy diameter) were harvested in
June 1994 for analysis of growth. The growth
of Coleogyne normally begins in March and
ceases in June (West 1983). Twig elongation
Statistical Analyses
One-way analysis of variance (ANOVA), followed by a Tukey's multiple comparison test
(Analytical Software 1994), was used to detect
differences among elevations in the lower elevational limits of Coleogyne and to compare
site means when a signiHcant elevation effect
was detected. Pearsons correlation analysis
was performed to correlate biotic and abiotic
factors with Coleogyne density. Soil moisture,
soil organic matter, and soil and plant nutrient
percentages were arcsine-transformed prior to
statistical analysis. Multiple analysis of variance
(MANOVA) was used to detect significant
effects of elevation, depth, month, time, and
year on soil moisture, soil organic matter, soil
temperature, and water potcntial of Coleogyne.
Mean values arc expressed with standard errors,
and significance was determined at P < 0.05.
RESULTS
The mean density of Coleogyne increased
significantly (P < 0.0001) as elevation increased
across the lower ecotone (Fig. 1). Soil temper-
atures (Figs. 2A, 2B) deelined significantly
into undisturbed soils. Soil pH was detennined
from a slurry consisting of equal parts of soil
with increasing elevation across the lower eco-
and distilled water. No edaphic measurements
tone by month and depth (P < 0.0001) and
were made during winter months due to the
presence of snow on the ground at the upper
portions of the lower Coleo~yne ecotone.
were cooler under shrubs than in the open (P
In June 1993, within each of the 27 plots
that contained Coleogyne, we measured the
following cbaracteristics of Coleogyne: water
potential, stem growth, and nutrient status
(TKN, TKP) and biomass production of stems
and leaves. We measured water potentials on
10- to 20-cm-Iong terminal branches of Coleogyne shrubs at predawn and midday in May
and August 1994 using a portahle pressure
< 0.05). All possible interactions (open spacesbrub canopy, soil depth, month, and elevation) were also significant (P < 0.01). 1emperatures were usually cooler at higher elevations
and under shrub canopies.
Coleogyne density was negatively correlated
with air temperatures (r = -0.95, P < 0.001)
across the lower boundary in Lucky Strike
Canyon. Air temperatures were also signifi-
cantly different (P < 0.0001) among the 3
summer months (data not shown).
166
[Volume 57
GREAT BASIN NATURALIST
70
OPEN SPACE
d
60
60
~
g
A.
-
em
a
a
50
50
lZ"2J a
b
b
15 em
e
e
~
"~
;;
~
~
w
~
~
u
40
w
40
'""
~
30
w
3D
0.
"w
20
~
8"
~
6
~
10
a
a
10
oL-----1100
20
1150
1200
1250
1300
1350
o L-_--""
ELEVATION (m)
11001150
Fig. 1. Density of Coleogyne along its lower ecotone (n
= 6; mean ± 8;:.). Narrow vertical bars denote standard
errors of the means. Columns labeled with different letters are significantly different at P < 0.05.
1200
1300
1250
ELEVATION
1350
(m)
UNDER SHRUB CANOPY
60
Soil moisture (Table 2; ()....7 em soil depth)
increased witb elevation (P < 0.0001) but also
varied by month (May, July, and August; P <
0.0001) and year (1993, 1994; P < 0.0001).
Significant interactions for soil moisture were
detected between soil deptb (0-7 and 7-15
em) and year (P < 0.05), soil depth and month
(P < 0.0001), and between soil depth, month,
and elevation (P < 0.05).
A similar pattern of soil moisture was seen
at depths of 7-15 cm as well as among the
summer months and between the 1993 and
1994 years. Soil moisture percentage was the
highest in the month of August 1993 (data not
shown) because of summer storms that occurred
from late July through mid-August.
Coleogyne shrubs were significantly more
water stressed (more negative water potential
values; Fig. 3) in August of 1994 at lower elevations at both predawn (P < 0.001) and midday (P < 0.0001). Water potentials were positively correlated with Coleogyne density at
predawn and midday (r = 0.95, P = 0.001; r =
0.97, P < 0.001, respectively). Significant interactions were detected between water potential
and month (May and August 1994; P < 0.0001).
Coleogyne shrubs experienced more water stress
in May than in Angust, although a similar pattern was detected between these months. Bowns
(1973) proposed that shedding of leaves is an
adaptive feature for conserving water, which is
B.
[ZZl 0 em
_
15 em
ab
c
c
50
~
u
w
"'"
""
'0.w"
40
a
be
c
3D
w
~
~
0
20
~
10
o L-_--""
1100
1150
1200
1250
1300
1350
ELEVATION (m)
Fig. 2. Soil temperatures at the soil surface and at
depths of 15 cm in open spaces (A) and under shrub
canopies (B) in August 1993 (n = 36 in each treatment;
mean ± Sy). Narrow vertical bars denote standard errors of
the means. Columns labeled with different letters are significantly different at P < 0.05.
the factor limiting growth and development of
Coleogyne. All possible interactions (month, elevation, and time) were also significant (P < 0.05).
Coleogyne density was positively correlated
with soil organic matter (r ~ 0.93, P < 0.001).
Soil organic matter at 0-7 cm depth was significantly greater as elevation increased (Table
1997]
-.
-
167
COLEOGYNE DISTRlBUnOK IN S01.;THERK NEVADA
~
-2
TABLE 1. Pearsons correlation coefficients of Coleogyne
ramosissima densitv to various biotic and abiotic factors of
Coleogyne. All factors were measured during 1993 except
soil compaction (August 1994). Nitrogen and phosphorus
values were determined by Kjeldahl digestion.
OJ
-3
Factor
0
~ -1
-'
z
f-
0
"-
'"
-4
OJ
f- -5
~
OJ
z
>-
"
-6
•
-7
0
OJ
-'
0 -8
c:::::J
Predawn
<.l
1100
1150
1200
•
b
1250
Midday
1300
1350
ELEVATION (m)
Fig. 3. Predawn and midday water potentials of Coleogyne shrubs across the lower ecotone in August 1994 (n =
15 in each treabnent; mean ± $,r.). Narrow vertical bars
denote standard errors of the means. Columns labeled
with different letters are si.gni£cantly different at P <
0.05.
Soil organic matter
Q-7em
7-15 em
Soil compaction
Soil nitrogen
0-7 em
7-15 em
Coleogyne nitrogen
stems
leaves
Soil phosphorus
0-7 em
7-15 em
Coleogyne phosphorus
stems
leaves
Coleogylte biomass
stem
leaves
Coleogyne stem elongation
Soil depth
Soil pH
0-7cm
7-15 em
2; P < 0.0001). A similar pattern was seen at
depths of 7-15 em (data not shown). There
was also a significant interaction (P < 0.05)
between elevation and soil depth.
Coleogyne density was negatively correlated
,,~th soil compaction (Table 1). Greatest soil
compaction occurred at Lan'en-Ambrosia stands
at lower elevations, and least compaction occurred in nearly monospecific Coleogyne stands
at higber elevations (Table 2; P < 0.0001).
Total soil Kjeldahl nitrogen \fKN) at 2 depths
did not exhibit a consistent pattern with elevation (P > 0.05), and Coleogyne density was
weakly positively correlated with total soil
nitrogen at 0-7 em or 7-15 em \fable 1). The
upper 7 em generally had higher TKN than
the soils 7-15 em deep. Total soil phosphorus
(TKP) in the upper 7 em of the soil did not
vary with elevation (P > 0.05), and Coleogyne
density was not significantly correlated with
total soil phosphorus (Table 1). The highest
TKP values occurred at an elevation of 1280
m, which was just below the nearly pure
Coleogyne stands; the lowest TKP values were
in Larrea-Ambrosia stands (1160 m).
Total nitrogen content in Coleogyne stems
declined significantly (Table 3; P < 0.05) as
elevation increased, and Coleogyne density
was negatively correlated ,,~th total Coleogyne
r
0.93*"'*
0.89**'"
-0.89***
0.37*
0.39*
-0.86***
-0.87"'**
0.26NS
0.68***
0.99***
0.80***
-O.42r-iS
-O.62 NS
.p < 0,05
up < 0,01
u~p < 0.001
NS ::. nonsignificant
stem nitrogen (Table 1). ColeogYlle leaf nitrogen content, consistently higher than stem
nitrogen, did not differ significantly along the
lower elevationa! limit of Coleogyne \fable 3;
P> 0.05).
Total phosphorus content in Coleogyne stems
did not increase Significantly (Table 3; P >
0.05) with elevation along the lower ecotone,
but Coleogyne leaf phosphorus content did
increase significantly (Table 3; P < 0.05) with
eIevatioo.
Mean soil depth did not differ significantly
with increasing elevation (P > 0.05) but was
weakly negatively conelated \\~th the density
of Coleogyne \fable 1), although it declined
slightly with elevation. The deepest soil was
found at an elevation of 1190 m, the lowest
elevation at which Coleogyne occurred.
Coleogyne density showed a weak negative
correlation with soil pH at depths of 0-7 em
(Table 1) and did nol vary with increasing elevation (P > 0.05). Soils near the center of the
ecotone (1210 m) tended to bave higher pH
values than soils at other elevations.
168
[Volume 57
GREAT BASIN NATIJRALIST
TABLE 2. Soil moisture and soil organic matter at depths of 0-7 em, soil compaction (kgIcm 2). and soil nutrients (TKN
and TKP at 0-7 and 7-15 em depths) along the lower Coleogyne ecotone. Mean values of soil moisture, organic matter,
and nutrients are expressed in percentages and were arcsine-transformed. Columns labeled with different letters are
significantly different at P < 0.05.
Elevation
(m)
Soil moisture
Soil OM
(%)
(%)
1160
1280
0.900
1.000b
1.43abe
1.70be
1.79bc
4.3ab
4.5ab
5.7ab
1310
2.0Oc
6.8b
BOO
1220
1250
2.9a
Soil compaction
(kg/em')
7.6a
6.5ab
6.9ab
5.8bc
6.4ab
5.Oe
Mean stem biomass of Coleogyne tended
(nonsignificantly; P = 0.0552) to decrease with
elevation (Table 3) and to be negatively correlated (Table 1) with Cowogyne density. Mean
leaf biomass of Cowogyne (Table 3) showed no
statistical significance across the elevational
boundary, but Coleogyne density showed a
weak positive correlation with Cowogyne leaf
biomass (Table 1). Unlike stem biomass, leaf
biomass tended to increase as elevation increased up to 1300 m.
Mean stem elongation of Coleogyne (3.87 +
0.43 em; Table 3) from March to June did not
display a definite pattern across the lower ecotone of Coleogyne (Table 1; P > 0.05). The
lowest mean stem elongation took place near
the center of the ecotone, whereas the highest
rate occurred in Larrea-Ambrosia stands. Cowogyne density was not significantly correlated
with stem elongation (Table 1).
DISCUSSION
Abiotic factors appeared to limit the distribution of Coleogyne at its lower elevational
boundary in Lucky Strike Canyon in southern
Nevada. Previous studies have shown that low
soil water content influences the distribution
of Cowogyne at its lower ecotone in Utah
(Bowns 1973). The appearance and persistence
of Cowogyne at its lower elevational limit in
Searchlight in southern Nevada is probably related to an increase in precipitation (Hunter and
McAuliffe 1994). Precipitation is the major control of soil water content. However, no one has
tested whicb edaphic factor may be most important in determining the distribution and density
of Cowogyne in southern Nevada.
Soil temperatures were negatively correlated
with Coleogyne density. Soil moisture reaches
a minimum while soil temperature reaches a
Soil TKN
Soil TKP
0-7
7-15
0-7
7-15
0.07a
0.08n
0.000
O.04a
O.03a
0.03a
0.820
0.71a
O.06a
0.020
0.740
O.IOa
0.06a
O.09a
0.05a
O.69a
0.81a
O.63a
O.65ab
0.73ab
0.72ob
l.00b
0.82ob
0.67a
maximum in early summer in southwestern
Utah (Bowns 1973). This pattern was more
prominent in Larrea-Ambrosia stands (1160 m
in elevation) where Cowogyne shrubs were
ahsent and apparently could not tolerate a
combination of relatively high soil temperatures and low soil moisture. Soil temperatures
in open areas displayed a greater fluctuation
and were consistently and significantly warmer
than soil temperatures under shrub canopies.
Shreve (1924) suggested that insolation is of
more importance than air temperature in determining soil temperature. Little variation was
observed between temperatures at 15-cm depth
across the ecotone beneath canopies and in
open spaces. Much of the fluctuation that
occurs during late sPling and summer can be
attributed to changes in soil moisture content
(BOWDS 1973). BawDS and West (1976) reported
that soil temperatures in Cokogyne communities start to increase in February and March)
peak in June or July, but decline in August due
to wet soils. Soil temperatures in our study did
not decline in August despite moderately wet
soils during the thunderstorm season. Soil moisture and temperatures at the Nevada Test Site
in Mercury in southern Nevada do not always
correspond to the lower temperature and increased precipitation associated with increases
in elevation, but they are influenced by physiography and soil properties (Rickard and
Murdock 1963).
Air temperatures in winter may influence
the distribution of Larrea-Ambrosia at their
upper elevational boundary. Beatley (1974)
stated that the average extreme minimum air
temperatures on all Larrea-Ambrosia sites are
above _17°C. Variation in air temperatures
along the lower boundary of Cowogyne in this
study was I·G because the plots covered only
150 m in elevation. Nevertheless, Cowogyne
1997]
169
COLEOGYNE DISTRIBUTION IN SOUTHERN NEVADA
Table 3. Coleogyne stem elongation (em; n = 20 per bush), total Coleogyne stem and leaf nitrogen (TKN) and phosphorus (TKP), and Coleogyne biomass (g) along the lower Colrogyne ecotone (n = 27). Mean values of nutrient status
are expressed in percentages and were arcsine-transfonned. Columns labeled with different letters are significantly different at P < 0.05.
Stem (%)
Leaves (%)
Biomass (g)
Elevation
Stem elongation
(m)
<em)
TKN
TKP
TKN
TKP
Stems
Leaves
It90
1220
1250
1280
1310
4.230
4.05.
3.57,
3.85,
3.83,
0.S8ab
0.6Oa
O.50,b
O.40b
OAlb
O.43a
0.400
0.500
0.55.
0.62,
0.800
0.87,
0.76a
0.8la
0.75,
0.920
L2la
0.93,
l.35,b
2.17b
O.ISa
0.030
O.04a
O.04a
0.05,
0.03,
densities were negatively correlated with air
temperatures.
Rickard and Murdock (1963) stated that soils
of Coleogyne communities of Yucca and Frenchman flats in south central Nevada have more
available moisture than Lan·ea-Ambrosia soils
and that availahle moisture is greater in the
suhsurface (10-30 em) than in the surface
(0-10 em). The lower limits of ColeogYf1£ distrihution may be a result of low soil moisture
(Bowns and West 1976). Soil moisture in our
study was positively correlated with ColeogYf1£
density. ColeogYf1£ growth ceased in mid-June.
presumably due to limited soil water content.
Soil moisture is greatest prior to the beginning
of ColeogYf1£ growth in mid- March. Exhaustion
of soil moisture coincides with cessation of
growth of Coleogyn£ in mid-June (West 1983).
In fact. Coleogyne underwent summer dormancy, with no growth. around mid-June. No
water potential readings of ColeogYf1£ shrubs
were recorded in July 1994. Bowns and West
(1976) concluded that summer dormancy of
Coleogyn£ is a result of soil moisture depletion
rather than higb air temperatures, and no chilling treatment is required to break dormancy.
Plants exbibiting drought-induced summer
dormancy have evolved to take advantage of
water when it is present and to become dormant when water is scarce (Bowns 1973).
Soil organic matter increased with elevation
across the lower Coleogyne boundary. Deposition of older and outermost Coleogyne leaves
at the onset of summer dormancy and decom-
position of winter ephemerals can generate a
considerable amount of organic matter for the
soil in Coleogyn£ communities (Bowns 1973).
West (1983) stated that composition and productivity of annuals vary from year to year because they rely heavily on precipitation. Bromus
rube"" (red brome grass), a dominant winter
ephemeral of many Larrea-Coleogyn£ ecotones
0.18a
0.14a
D.ISa
0.133
between elevations of 1220 and 1310 m (Heatley
1966), tends to form c:npetlike vegetation among
shrubs in the spring season of wet years. It
was not abundant in Larrea-Ambr-osia stands.
Tbe presence of cryptogamic crusts can also
increase organic matter in soils (Loope and
Gifford 1972). Coleogyne zones in Lucky Strike
Canyon exhibited a higher organic content than
Larrea-Ambrosia communities presumably because of abundant Bromus ephemerals.
ColeogYf1£ density was generally negatively
correlated with soil compaction. Variations of
soil compaction may playa secondary role in
limiting the distribution of Coleogyne at its
lower elevational boundary. Soils at higher
elevations tend to have lower compaction primarily due to better inftltration; also. they are
more permeable to air and water required by
plant roots (Howns 1973).
Algae, mosses. and licbens that form cryptogamic crusts appear to fix nitrogen in Coleogyne communities (Loope and Gifford 1972) and
may increase fertility of the associated soils
(Callison and Brotherson 1985). Total soil nitrogen did not exhibit a definite pattern across
the lower Coleogyn£ ecotone. Total phosphorus
remained fairly constant. Coleogyne leaves had
a higher content of nitrogen and phosphorus
than stems. which corresponds with Bowns
(1973). Coleogyne leaves consistently stored
more nutrients than stems. Coleogyne stems
and leaves were relatively low in phosphorus.
Nevertheless, Provenza (1978) proposed that
phosphorus content is highest during the growing period but appears to decrease as the Coleogyn£ growth period continues through June.
Coleogyn£ stem biomass (g) per branch declined as Coleogyne became denser with elevation. On the other band, Coleogyne leaf biomass (g) per branch increased slightly as the
species became denser with elevation. This
phenomenon may indicate a production of more
170
GREAT BASIN NATURALIST
[Volume 57
current-season leaves as stem biomass declines Plateau in southern Utah (Pendleton et al. 1995).
with elevation.
Seeds are generated only in wet years when
The total rate of current-season stem elon- rains arrive during the winter or during spring
gation of Co/eogyne did not vary significantly growth (Bowns 1973). Survival of seedlings
during the growing season throughout the lower is exceedingly poor: young seedlings usually
elevational boundary. Nevertheless, relatively die as a result of inadequate soil moisture, and
high air and soil temperatures may determine older seedlings are eaten and uprooted by
the rate of stem elongation in Co/eogyne; peri- rodents and rabbits (Bowns and West 1976).
ods of warm weather result in abundant growth, On the contrary, seedling survival can be relawhile cool periods retard the growth rate bvely high at high-e1evational sites of the Col(Bowns 1973).
orado Plateau in southwestern Utah.
Co/eogyne density and disrnbution appear
Cowogyne density was weakly negatively
correlated with soil depth across the lower to be influenced by edapbic factors, particuboundary at Lucky Strike Canyon. Soil depth larly soil moisture and soil organic matter.
declined slightly from the ftrst appearance of Coleogyne also exhibits variations in stem elonCo/eogyne to the nearly pure Cowogyne stands. gation, as well as nurnent content (N and P)
However, Callison and Brotherson (1985) sug- and biomass production of stems and leaves
gest that shallowness of soils is an important across its lower elevational boundary. Lucky
feature of Co/eogyne communities and may par- Srnke Canyon is representative of the vegetatially determine the abundance and distribu- tion and landscape conditions prevailing in
tion of Co/eogyne. Rooting patterns of Cowog- southern Nevada. Experiments and establishyne enable the plant to extract water more ment of long-term plots at various sites are
efficiently from shallow and sandy soils than necessary to further understand the complex
from deep soils (Korthuis 1988). The majority relationships between the distribution of Coleof Co/eogyne roots are found at depths of 10-30 ogyne and associated biotic and abiotic factors
em, and caliche layers are considered a major in southern Nevada.
obstacle to many roots in southwestern Utah
ACKNOWLEDGMENTS
(Bowns 1973). Hence, roots of many shrubs run
horizontally and contribute to a general deWe thank Yin-Chin Lei and Steven Lei for
crease in root biomass moving from plants
toward open areas between plants (BaWDS collecting soil samples. John Bolling assisted
1973). Soil depth is a Significant factor associ- with soil and plant nutrient analyses (TKN,
ated with Co/eogyne density and distribution TKP). Helpful comments by Richard Hunter
only when considering the entire mountain and Burton Pendleton greatly improved the
range, but not across its lower ecotone in manuscript. The UNLV motor pool and Department of Biological Sciences provided logistical
southern Nevada (Lei 1995).
Soil pH did not differ significantly between support, and the UNLV Graduate College proLarrea-Ambrosia stands and the lower Co/eog- vided partial ftnancial support.
yne ecotone. Soil pH in COWogyne communities
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