The Transition from Mojave Desert to Great
Basin Desert on the Nevada Test Site
D. J. Hansen
W. K. Ostler
D.B. Hall
Abstract—Plant species and associations on the Nevada Test Site
are located along elevation and precipitation gradients. Associations in the Great Basin Desert had the highest species diversity.
Rodent sign and productivity of annual plants are least in the lower
and higher elevations and most abundant in the mid-elevations.
Microbiotic crusts are most abundant in fine-textured soils and of
low abundance in soils with active erosional processes. Texture of
surface soils (0 to 5 centimeters [0 to 2 inches]) differs little among
all associations except for a few that are correlated with playas and
steep mountain slopes. However, differences among associations
are observed for deeper substrates comprised of limestone, basalt,
and tuff parent materials.
The Nevada Test Site (NTS) is located about 105 km (65
miles) northwest of Las Vegas in southern Nevada (see fig. 1
of companion paper, Ostler and others, 1999). The site was
created by a series of land withdrawals in the early 1950s for
nuclear weapons testing. It comprises a total land area of
350,000 ha (1,350 square miles). Despite nearly 1,000
atmospheric and below-ground nuclear tests, the area is
relatively undisturbed and offers an excellent location for
biological studies. The area has had limited or no livestock
grazing since the 1950s and is designated as a National
Environmental Research Park.
The NTS consists of three large valleys, Yucca, Frenchman, and Jackass Flats. It has two high mesas, Rainier and
Pahute. It has a rough elevational gradient from south to
north with the lowest point at 829 m (2,688 ft) in Jackass
Flats and the highest point at 2,340 m (7,679 ft) on Rainier
Mesa. The site straddles the Mojave and Great Basin Deserts
and provides an excellent site to observe the transition
between these two deserts. Numerous detailed studies have
been conducted on NTS biota focusing primarily on inventories and evaluating the effects of nuclear testing. Approximately 730 plant species occur on the NTS, of which several
are sensitive or protected.
Methods _______________________
Shrublands of the NTS were classified using methods described in a companion paper presented in these proceedings
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.
D. J. Hansen, W. K. Ostler, and D. B. Hall are Biologists with Bechtel
Nevada, P.O. Box 98521, M/S NLV-081, Las Vegas, NV 89193-8521.
148
(Ostler and others 1999). Approximately 1,500 ecological landform units (ELUs) were delineated using aerial photography
and satellite imagery to distinguish ecological mapping
types. The boundaries of each ELU were field verified.
Landforms were selected because they are highly correlated
with soil types in Nevada (Peterson 1981) and have been
used historically to help classify habitat types in the Mojave
Desert (Berry 1979). Vegetation and other site parameters
were sampled within representative areas of each ELU.
Data were analyzed using cluster analyses and descriptive
statistics to help classify vegetation into 10 alliances and
20 associations.
Results ________________________
Results of the vegetation classification on the NTS are
shown in table 1. About 23 percent and 37 percent of the
ELUs sampled on the NTS were located in the Mojave Desert
and Great Basin Desert, respectively (table 2). The remaining 36 percent of the ELUs were located in a Transition Zone
between these two deserts. About 4 percent of the 1,508
ELUs sampled were classified as “miscellaneous” because
they were unique vegetation types, burned, scraped, or
disturbed by nuclear testing. In the Mojave Desert the
Larrea tridentata/Ambrosia dumosa Shrubland was the
most numerous association representing about 19 percent of
the ELUs on the NTS (18 percent of the total area). No other
association in the Mojave Desert represented more than
4 percent of the total ELUs. In the Great Basin Desert the
Artemisia tridentata-Chrysothamnus viscidiflorus
Shrubland was the most numerous association representing
about 11 percent of the ELUs on the NTS (7.5 percent of the
total area). No other association in the Great Basin Desert
represented more than 7 percent of the total ELUs. In the
Transition Zone between these deserts, the Coleogyne
ramosissima-Ephedra nevadensis Shrubland was the most
numerous association representing about 22 percent of the
ELUs on the NTS (21.6 percent of the total area). No other
association in the Transition Zone represented more than
6 percent of the total ELUs.
Distribution of Plant Alliances on the
Nevada Test Site
Figure 1 shows the distribution of plant alliances on the
NTS. Also shown are the generalized boundaries for the
Mojave Desert, Great Basin Desert, and the Transition Zone
between these deserts. Plant species that dominate associations within the Transition Zone have been historically
USDA Forest Service Proceedings RMRS-P-11. 1999
Table 1—Classification of vegetation on the Nevada Test Site.
Mojave Desert
Lycium spp. Shrubland Alliance
Lycium shockleyi-Lycium pallidum Shrubland
Larrea tridentata/Ambrosia dumosa Shrubland Alliance
Larrea tridentata /Ambrosia dumosa -Shrubland
Atriplex confertifolia - Ambrosia dumosa Shrubland Alliance
Atriplex confertifolia - Ambrosia dumosa Shrubland
Transition Zone
Hymenoclea -Lycium Shrubland Alliance
Lycium andersonii - Hymenoclea salsola Shrubland
Hymenoclea salsola - Ephedra nevadensis Shrubland
Ephedra nevadensis Shrubland Alliance
Menodora spinescens - Ephedra nevadensis Shrubland
Krascheninnikovia lanata - Ephedra nevadensis Shrubland
Eriogonum fasciculatum - Ephedra nevadensis Shrubland
Ephedra nevadensis - Grayia spinosa Shrubland
Coleogyne ramosissima Shrubland Alliance
Coleogyne ramosissima - Ephedra nevadensis Shrubland
Great Basin Desert
Atriplex spp. Shrubland Alliance
Atriplex confertifolia - Kochia americana Shrubland
Atriplex canescens - Krascheninnikovia lanata Shrubland
Chrysothamnus-Ericameria Shrubland Alliance
Chrysothamnus viscidiflorus - Ephedra nevadensis Shrubland
Ericameria nauseosa Shrubland Alliance
Ericameria nauseosa - Ephedra nevadensis Shrubland
Artemisia spp. Shrubland Alliance
Ephedra viridis - Artemisia tridentata Shrubland
Artemisia tridentata - Chrysothamnus viscidiflorus Shrubland
Artemisia nova - Chrysothamnus viscidiflorus Shrubland
Artemisia nova - Artemisia tridentata Shrubland
Pinus monophylla/Artemisia spp. Woodland Alliance
Pinus monophylla/Artemisia nova Woodland
Pinus monophylla/Artemisia tridentata Woodland
deciduous (e.g., blackbrush) or have essentially leafless,
photosynthetic stems (e.g., Ephedra spp.), while the most
abundant dominant shrubs from associations in the Mojave
Desert and Great Basin Desert are evergreen in habit (e.g.,
creosote bush, big sagebrush, singleleaf pinyon, and Utah
juniper). The evolutionary adaptation of leaf reduction or
abscission during drought and stress-induced dormancy
may help maintain the abundance of blackbrush and Mormon tea (Ephedra nevadensis) in these ecotones. While the
abundance of species other than blackbrush are relatively
low in the Coleogyne ramosissima - Ephedra nevadensis
Shrubland, they are frequently present in small numbers
being found in small patches where animals have disturbed
the soil horizons or fire has reduced competition with
blackbrush.
Blackbrush occurs at intermediate elevations. At these
elevations lightning strikes, associated with storms blowing
from the south, are common during the summer and occasionally fuel loading reaches levels high enough to support
wildfires. Once burned these communities reestablish very
slowly; this phenomenon is also reported by Brown (1982).
Species Diversity
Species diversity (richness or the number of species) of
perennial trees and shrubs was greatest in the Great Basin
Desert associations (mean of 56 species) compared to associations in the Transition Zone (mean of 49 species) and the
Mojave Desert (mean of 36 species). Similar species diversity patterns were also observed for all combined perennial
species on the NTS (table 2) (e.g., Great Basin Desert: 21.7
species per ELU, Transition Zone: 17.4 species per ELU, and
Mojave Desert: 12.7 species per ELU).
Elevation and Precipitation
associated with either desert, and in some cases, listed as a
minor species in both deserts.
Associations that were considered typical or characteristic of the Mojave Desert were those that contained a presence
of Shockley’s desertthorn (Lycium shockleyi), rabbit thorn
(Lycium pallidum), creosote bush (Larrea tridentata), or
white bursage (Ambrosia dumosa), but lacked species characteristic of the Great Basin Desert (table 3). The distribution of creosote bush on the NTS (fig. 2) approximates the
boundaries for the Mojave Desert on the NTS. Associations
that were considered typical of the Great Basin Desert were
those that contained a presence of sagebrush (Artemisia
spp.), singleleaf pinyon (Pinus monophylla), or Utah juniper
(Juniperus osteosperma), but lacked species characteristic
of the Mojave Desert. The distribution of big sagebrush
(Artemisia tridentata) on the NTS (fig. 3) approximates the
boundaries of the Great Basin on the NTS. Associations
within the Transition Zone were considered to be those that
had a mixture of species, many species occurring in both the
Mojave and Great Basin Desert (table 3), such as Ephedra
(Ephedra) spp. and blackbrush, (Coleogyne ramosissima).
The distribution of blackbrush on the NTS (fig. 4) approximates the boundaries of the Transition Zone on the NTS.
Associations that occur in the Transition Zone appear to
be comprised of shrubs that are predominantly drought
USDA Forest Service Proceedings RMRS-P-11. 1999
Plant associations within the Mojave Desert and Great
Basin Desert were ordered according to increasing elevation
and precipitation (fig. 5). Mean annual precipitation was
2
determined to be positively correlated (r = 0.85) with elevation on the NTS, based on correlation modeling and actual
weather recording data (French 1986; other detailed meteorological data are presented by Fransioli and Ambos 1997).
The importance of elevation, slope, and substrate in accounting for statistical variance in shrub species cover was also
described for the Nellis Air Force Range consisting of
2
1,228,355 ha (7,432 mi ) of shrubland adjacent to the NTS
(Pritchett and others 1997).
Soils and Parent Materials
Texture of surface soils (0 to 5 centimeters [0 to 2 inches])
appeared to differ little among all associations except for
two that were correlated with playas and steep mountain
slopes. The plant associations associated with playas were
the Lycium shockleyi - Lycium pallidum Shrubland and the
Atriplex confertifolia - Kochia americana Shrubland. Soil
texture of these associations had a greater percentage of
clay than other associations. The most abundant soil textures for surface soils of the associations were comprised of
three textural types: sandy loam (30 percent), loamy sand
149
Number of stands
Number of shrub species
Mean number of perennial species/ELU
Mean elevation (m)
Mean precipitation (cm)
Mean slope (degrees)
Mean rock pavement (percent)
Parameters
Lycium shockleyi-Lycium pallidum
4
11
8.8
941
14.8
0.8
32.5
Larrea tridentata/Ambrosia dumosa
287
49
13.2
1,080
16
5.7
63.2
Atriplex confertifolia-Ambrosia dumosa
51
49
16.2
1,153
16.9
13.6
57.4
Menodora spinescens-Ephedra nevadensis
42
50
19.4
1,198
17.5
8
65
Krascheninnikovia lanata-Ephedra nevadensis
29
34
17.6
1,258
18.2
3.6
58.1
11
23
12.8
1,263
18.3
2.3
55.5
Lycium andersonii-Hymenoclea salsola
44
50
15.2
1,263
18.3
3.8
47.6
Eriogonum fasciculatum - Ephedra nevadensis
14
36
20.4
1,292
18.7
36.3
17.5
Transition zone
Hymenoclea salsola-Ephedra nevadensis
Mojave Desert
Ephedra nevadensis-Grayia spinosa
93
66
19.7
1,413
20.2
8.8
57
Coleogyne ramosissima-Ephedra nevadensis
325
84
16.9
1,385
19.8
8.5
68.2
Atriplex confertifolia-Kochia americana
17
53
9.2
1,208
17.6
2.7
34.1
38
87
11.9
1,237
18
3.5
41.1
Atriplex canescens-Krascheninnikovia lanata
Table 2—Parameters associated with associations in the Mojave and Great Basin Deserts and Transition Zone on the Nevada Test Site.
76
61
21.4
1,485
21.1
7.1
57.2
Chrysothamnus viscidiflorus-Ephedra Nevadensis
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43
17.1
1,563
22.1
6.9
62.8
42
27
1,719
24
18.2
49.8
73
20.8
1,776
24.7
5.9
52.5
56
24.7
1,805
25.1
6.4
66.7
Great Basin Desert
47
29.1
1,831
25.4
8.6
47.8
54
27
1,985
27.3
8.1
53.5
47
28.6
2,054
28.2
14.3
47.8
Figure 1—Vegetation alliances on the Nevada Test Site. (1 = Pinus monophylla/Artemisia spp. woodland; 2 = Artemisia
spp. shrubland; 3 = Atriplex spp. shrubland; 4 = Chrysothamnus-Ericameria shrubland; 5 = Coleogyne ramosissima
shrubland; 6 = Ephedra nevadensis shrubland; 7 = Lycium spp. shrubland; 8 = Hymenoclea-Lycium shrubland; 9 = Atriplex
confertifolia-Ambrosia dumosa shrubland; 10 = Larrea tridentata/Ambrosia dumosa shrubland; 11 = Miscellaneous;
12 = Playa/Disturbances).
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151
Lycium shockleyi
Lycium pallidum
Ambrosia dumosa
Larrea tridentata
Krameria erecta
Menodora spinescens
Grayia spinosa
Krascheninnikovia lanata
Lycium andersonii
Hymenoclea salsola
Eriogonum fasciculatum
Coleogyne ramosissima
Ericameria cooperii
Ephedra nevadensis
Artemisia spinescens
Atriplex confertifolia
Kochia americana
Atriplex canescens
Chrysothamnus viscidiflorus spp.
Ericameria nauseosa
Ephedra viridis
Purshia stansburiana
Purshia glandulosa
Artemisia tridentata
Artemisia nova
Chrysothamnus viscidiflorus spp.
Leptodactylon pungens
Juniperus osteosperma
Purshia tridentata
Pinus monophylla
Parameters
Lycium shockleyi-Lycium pallidum
1.0
7.5
0.5
9.0
0.0
3.5
47.5
17.0
12.5
Larrea tridentata/Ambrosia dumosa
0.0
0.0
0.4
0.1
0.5
5.5
43.4
12.4
6.6
1.3
2.1
3.3
3.0
1.7
0.4
0.6
0.6
7.1
0.1
2.7
Atriplex confertifolia-Ambrosia dumosa
0.0
0.0
0.0
15.6
4.2
3.8
1.8
0.6
4.4
6.4
1.2
0.8
3.6
0.2
8.9
0.7
31.3
0.1
0.3
0.5
Menodora spinescens-Ephedra nevadensis
0.0
0.2
0.8
2.7
10.3
3.4
4.5
13.9
1.1
3.6
4.5
2.7
0.5
4.7
0.5
11.8
0.6
8.2
Krascheninnikovia lanata-Ephedra nevadensis
0.1
2.5
2.2
4.0
2.4
0.8
2.2
8.3
28.5
8.0
3.7
0.1
1.9
1.2
12.3
5.2
4.7
Lycium andersonii-Hymenoclea salsola
6.4
0.6
2.6
0.4
6.4
1.5
1.1
2.4
0.4
0.4
5.3
7.3
51.8
9.6
0.2
4.6
5.4
0.5
0.5
0.6
8.5
0.2
0.5
1.9
2.5
4.5
51.5
0.1
1.6
0.0
0.4
2.2
1.6
0.3
Hymenoclea salsola-Ephedra nevadensis
Transition zone
0.3
0.2
0.4
2.2
0.3
3.0
3.0
5.3
0.3
5.0
0.0
3.7
3.7
3.4
1.3
28.1
0.7
2.3
16.4
Eriogonum fasciculatum - Ephedra nevadensis
Mojave Desert
Ephedra nevadensis-Grayia spinosa
0.1
3.5
1.9
1.0
1.6
2.1
1.4
3.2
2.6
0.8
56.3
0.7
9.6
0.2
1.4
0.0
0.8
1.5
0.2
1.0
0.6
0.1
1.3
0.3
0.2
0.0
0.1
0.0
0.0
8.0
5.3
0.5
1.5
0.3
0.4
1.4
0.9
0.4
0.0
0.0
0.5
0.6
0.8
0.5
8.1
3.3
5.9
7.9
1.7
4.1
4.9
22.8
0.9
0.7
Coleogyne ramosissima-Ephedra nevadensis
0.9
0.1
6.6
49.8
22.1
2.4
0.8
0.1
0.6
0.1
0.0
5.8
0.2
2.8
5.4
0.2
0.0
1.0
0.4
0.9
0.2
2.6
3.2
10.8
5.0
3.9
0.1
1.8
0.0
2.9
0.8
3.3
0.9
57.8
1.0
0.3
0.4
0.2
0.7
0.3
0.0
0.1
0.6
0.1
0.0
0.4
5.1
5.3
4.7
1.6
0.4
1.8
0.8
11.4
2.6
3.9
0.3
6.6
36.7
0.3
1.5
0.4
0.2
5.0
1.0
0.2
0.3
0.2
0.4
0.2
0.1
0.0
0.5
0.2
0.1
0.1
0.2
0.1
5.2
4.4
51.7
2.3
1.3
0.8
6.0
0.5
0.1
0.7
4.5
1.5
0.6
3.5
1.9
0.2
21.0
8.9
15.0
15.7
6.0
1.8
0.0
2.1
0.8
0.1
0.1
1.9
8.1
0.9
2.0
0.8
0.6
56.1
1.9
3.3
2.9
1.4
0.6
2.2
0.1
0.0
0.1
0.0
1.9
1.0
0.7
0.0
0.4
0.5
0.2
7.1
0.3
0.2
0.0
2.0
0.6
0.6
0.0
0.0
0.2
0.1
4.3
0.1
0.8
0.0
1.4
5.2
0.3
1.7
0.3
0.3
14.3
67.9
2.7
0.7
0.6
0.2
0.9
Great Basin Desert
5.3
0.7
11.1
Table 3—Percent abundance of dominant species found in associations in the Mojave and Great Basin Deserts on the Nevada Test Site.
Atriplex confertifolia-Kochia americana
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2.0
4.8
0.8
2.2
0.1
0.9
25.1
33.8
7.5
3.6
1.2
0.2
1.3
0.0
0.3
1.8
0.1
3.1
1.9
0.5
0.3
0.5
3.7
0.0
2.3
1.1
0.8
2.6
55.4
1.7
2.3
5.9
1.7
14.1
0.8
0.1
0.0
0.0
0.1
0.5
0.0
0.0
0.0
2.8
0.3
1.1
1.4
0.3
22.8
5.4
1.5
4.1
7.3
7.9
27.5
0.7
1.0
1.8
0.3
0.1
0.0
Figure 2—Distribution and percent abundance of creosote bush on the Nevada Test Site.
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Figure 3—Distribution and percent abundance of big sagebrush on the Nevada Test Site.
154
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Figure 4—Distribution and percent abundance of blackbrush on the Nevada Test Site.
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156
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Figure 5—Average elevation and precipitation of vegetation associations on the Nevada Test Site.
(18 percent), and loam (14 percent). Soil samples from the
remaining 38 percent of associations were classified into
more than 30 different textural types. The Eriogonum
fasiculatum - Ephedra nevadensis Shrubland was located on
the steepest slopes (36 degrees) with all other associations
averaging slopes less than 18 degrees, with a mean slope of
7 degrees.
Despite little difference in soil texture near the soil surface, associations were observed to differ in the type of
substrate upon which they most commonly appeared. The
Coleogyne ramosissima - Ephedra nevadensis Shrubland
and Atriplex confertifolia - Ambrosia dumosa Shrubland
appeared frequently on shallow soils of limestone-derived
parent materials, while most of the associations in the Great
Basin Desert occurred in the basalt or tuff formations.
Remaining associations occurred in alluvial soils. Creosote
bush appeared to be limited by shallow soils or the presence
of a caliche layer. Lunt and others (1973) suggest that big
sagebrush and creosote bush have unusually high oxygen
requirements, while roots of white bursage appear to require
less oxygen.
Microbiotic Crusts
The presence of microbiotic crusts on the soil surface (i.e.,
nonvascular microorganisms such as algae, fungi or lichens
that are frequently important for enhancing soil fertility and
surface stabilization) was noted during the field surveys.
Within the NTS 58 percent of the ELUs sampled had no
visual evidence of microbiotic crusts, 28 percent had low
visual evidence of crusts, 11 percent had moderate visual
evidence of crusts, and only 4 percent had high visual
evidence of microbiotic crusts. Visual evidence of crusts were
observed to decrease with increases in elevation. For example, no microbiotic crusts were observed in 31 percent of
sites within the Mojave Desert, 43 percent of sites within the
Transition Zone and 50 percent of sites within the Great
Basin Desert. Associations with high abundance of crusts
also had higher percentages of soil fines (clays or silts). Low
presence of crusts or their absence was associated with
active soil erosional processes (e.g., along washes and steeper
unstable slopes).
Productivity of Annual Vegetation
The presence of annual vegetation and its relative abundance were not randomly distributed across the three areas
(Mojave Desert, Transition Zone, and Great Basin Desert)
based on Chi-square analyses (58.6, P < 0.001, df = 4).
Associations in the Mojave Desert and the Transition Zone
tended to be over-represented in the moderate category (41
and 35 percent respectively) and underrepresented in the
low-very low category (43 and 43 percent respectively)
while associations in the Great Basin Desert tended to be
overrepresented in the low-very low category (60 percent)
and underrepresented in the moderate category (22 percent). Levels within the high-very high category were similar for all three areas. This is consistent with regional floras
which show a greater number of annual species in drier and
hotter deserts of the Southwest compared to the colder
Great Basin Desert.
USDA Forest Service Proceedings RMRS-P-11. 1999
Rodent Activity
The absence of sign indicating burrowing rodents and the
low abundance of sign (e.g., burrows and excavated soils)
were found to be correlated with increasing elevation. For
example, the Mojave Desert was observed to have 45 percent
of the sites with none to low sign. The Transition Zone was
observed to have 59 percent of the sites with none to low sign,
and the Great Basin Desert was observed to have 80 percent
of the sites with none to low sign Both the moderate rodent
sign and the high to very high rodent sign were inversely
correlated with elevation. For example, the Mojave Desert
had 39 percent moderate sign and 16 percent high to very
high sign. The Transition Zone had 30 percent moderate sign
and 11 percent high to very high sign. The Great Basin
Desert had 14 percent moderate sign and 6 percent high to
very high sign. The Mojave Desert sites may have deeper
soils that are more conducive to burrowing, while the rockier
soils of the Transition Zone have fewer burrows. The Great
Basin Desert sites have shallow soils and more severe
winters that may also reduce burrowing or abundance of
animals likely to burrow.
Species Correlations
Pairwise comparisons of species abundance at a statistical
level of significance of α = 0.05 of the 718 species observed to
occur on the NTS indicate that there were 447 positive
interspecific associations between species and only 271
negative interspecific associations. This general trend is in
agreement with earlier studies conducted using 25 circular
plots of 30.5 m (100 ft) in diameter during the late 1960s at
the NTS (Wallace and Romney 1972). Using Chi-squared
analyses, Wallace and Romney reported a great many more
positive than negative measures of association.
Vegetation and Climate of the Last 45,000
Years
Vegetation patterns in response to changing climatic
conditions have been studied extensively on the NTS and
documented through the analyses of several packrat-midden
sites (Spaulding 1985). Vegetation patterns suggest that
about 45 thousand years before present (k yr BP), the NTS
had cooler and wetter conditions than currently; dominant
plants included littleleaf mountain mahogany (Cercocarpus
ledifolius), Utah juniper, sagebrush, and horsebrush
(Tetradymia spp). By 35 k yr BP, limber pine (Pinus flexilis)
and four-wing saltbush (Atriplex canescens) had established
at the site. By 25 k yr BP, shadscale (Atriplex confertifolia),
snowberry (Symphoricarpos longiflorus), and Utah
Fendlerbush (Fendlerella utahensis) became more common.
By 10 k yr BP, limber pine was no longer found in packratmidden samples and was replaced by the presence of singleleaf
pine and increase in the abundance of Utah Juniper.
Goldenweed (Haplopappus nanus), Dorr’s sage (Salvia
dorii), and grizzlybear pricklypear (Opuntia erinacea) became more prevalent.
During the past 5 k yr BP, creosote bush, white bursage,
and other species characteristic of the Mojave Desert
157
established within the NTS area as temperatures increased
and precipitation decreased. It was estimated that many
Great Basin Desert trees and shrubs were displaced about
457 to 610 m (1,500 to 2,000 ft) upward in elevation to what
they historically had been as the climate changed (Spaulding
1985). This climate change opened new niches for colonization by other species such as blackbrush. Ecotonal species
are often adapted to a wide change in climatic conditions
such as freezing temperatures as well as hot, droughty
climate.
Summary and Conclusions _______
Vegetation of the NTS is diverse and relatively protected
from the effects of livestock grazing. Plant communities
appear to respond to moisture and temperature gradients
associated with elevation gradients at the site. Other secondary patterns of plant distribution are associated with
adaptation to unique substrates such as playas, steep rocky
slopes, and novel substrates comprised of limestone, basalt,
tuff, and alluvium. Changes in temporal species patterns on
the NTS have been correlated with changing climate during
the past 45 thousand years. Position of species along the
elevation gradient on the NTS under current climatic conditions creates a Transition Zone characterized by a diverse
mixture of species. This assemblage of species is not recognized as a regional vegetation type, but appears to be shaped
by climatic extremes present in both the Mojave Desert and
Great Basin Desert. The width of this zone is dependent on
the steepness of the elevation gradient, slope aspect, and
moisture retention characteristics of the substrate. Individual species appear to be independently distributed across
the NTS within both deserts and the Transition Zone. This
distribution is based on species growth requirements, although assemblages of species that have similar biological
and physiological needs are found together consistently
enough to be considered as distinct plant communities (i.e.,
alliances and associations). A better knowledge of the vegetation and the environmental conditions within the Mojave
Desert and Great Basin Desert helps us better appreciate
the transition between these deserts with its varying temporal and spatial characteristics.
158
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