Effects of Severe Drought on Biodiversity
and Productivity in a Creosote BushBlackbrush Ecotone of Southern Nevada
Simon A. Lei
Abstract—Plant responses to severe drought was investigated in a
creosote bush-blackbrush (Larrea tridentata-Coleogyne ramosissima)
ecotone in Cold Creek Canyon of the Spring Mountains in southern
Nevada. Initial vegetation data were collected in June 1993, with
data comparably collected on approximately the same dates in 1997.
Species richness, density, frequency, percent vegetation cover, and
above-ground biomass of annuals and biomass of herbaceous perennials were significantly reduced during the 1997 drought compared
to the moist 1993 year. Many annual plants simply escaped drought
as seeds. However, species richness, density, frequency, and percent vegetation cover of woody plants were not significantly different, and were less prone to drought. Xerophytic plants of various
life-forms utilized different adaptive strategies to reduce and cope
with water stress. A severe drought in spring 1997 limited local
biodiversity and productivity, especially the winter ephemerals in
the creosote bush-blackbrush ecotone of southern Nevada.
Drought refers to a period with low precipitation in which
the water content of the soil greatly reduces and the plants
suffer from lack of water. Substantial rainfall variability is
a major factor in the occurrence of drought. Tilman and
Haddi (1992) state that the instability caused by environmental fluctuations may limit species richness, density, and
above-ground biomass (productivity) in Minnesota. Climatically extreme conditions, such as drought, may periodically
lower population densities (Tilman and Haddi 1992). They
(1997) further suggest that rapid local and regional climatic
change may be occurring, and may impact the biodiversity of
otherwise undisturbed environments.
The dominant plant life-form in southern Nevada is the
shrub, although subshrub, herbaceous perennials, and annual (ephemeral) life-forms constitute a significant fraction
of the total vegetation (Lei 1994). In warm, lowland deserts,
large trees are generally absent, except on flood-plains along
riparian corridors. Many shrubs lose all or a portion of their
leaves during droughts (Lei 1994). Herbaceous perennials,
which die back to the ground surface during drought periods,
are also common (Lei, personal observations). Woody species
can be classified into drought-tolerant, drought-deciduous,
and succulent.
Creosote bush (Larrea tridentata), white bursage (Ambrosia dumosa), and blackbrush (Coleogyne ramosissima) are
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.
Simon A. Lei is a Professor of Biology and Ecology, Department of Biology,
Community College of Southern Nevada, 6375 West Charleston Boulevard,
Las Vegas, NV 89146-1139.
USDA Forest Service Proceedings RMRS-P-11. 1999
three dominant shrub species in southern Nevada. Creosote
bush-blackbrush shrublands often form relatively broad
ecotones (Lei and Walker 1997). Species density and diversity are generally higher in ecotonal areas than in the
adjacent creosote bush-white bursage and nearly monospecific blackbrush shrublands. Stable vegetation assemblages,
dominated by creosote bush, white bursage, and blackbrush
occur on the oldest, least disturbed geomorphic surfaces
(Webb and others 1987). West (1983) states that composition
and productivity of annuals vary considerably from year to
year because they rely heavily on the timing and amount of
precipitation.
The droughts in southern Nevada are unpredictable in
terms of their duration and time of initiation. Previous
vegetation studies in southern Nevada have documented
that climatic patterns strongly influence the distribution of
plant communities, and weather patterns influence the
germination of annual plants (Beatley 1966, 1967, 1969,
1974, 1976). The aim of this study was to quantitatively
investigate how plants of various life-forms responded to
severe drought at a creosote bush-blackbrush ecotone in
southern Nevada.
Methods _______________________
Field Surveys
Vegetation analysis was conducted on approximately the
same dates in June of 1993 and 1997 at the creosote bushblackbrush ecotone in Cold Creek Canyon (36°15'N, 115°15'W;
elevation 1,350-1,550 m; fig. 1) of the Spring Mountains,
located approximately 60 km northwest of Las Vegas, Nevada. Data on species richness, density, frequency, and
percent cover of woody taxa were compared between 1993
and 1997. Density, frequency, percent vegetation cover, and
above-ground biomass of annual and herbaceous perennial
plants were determined and compared between these two
years. Prior to harvesting the herbaceous vegetation, two
sets of 9-month weather data, from October 1992 through
June 1993 and from October 1996 through June 1997, were
included because winter precipitation is an important source
of moisture during the early growing season (table 1). However, weather data after June were excluded because woody
vegetation were surveyed and herbaceous vegetation were
harvested by the end of June, and precipitation after this
month would not influence the results for that year.
Initial vegetation data were collected in 1993, with data
comparably collected in 1997. Fifteen elevational transects at
five elevational sites were established. A total of 75, 100-m2
circular plots were placed across the entire creosote bushblackbrush ecotone. Plots within each transect were placed at
217
biomass. Individuals of annual plants were counted within a
randomly selected 1-m2 in each plot for density measurement before harvesting to determine above-ground biomass.
Percent cover of woody and herbaceous (annuals and perennials) vegetation was visually estimated on 10% increments. Herbaceous perennial plants were harvested within
a randomly selected 16-m2 in each plot. The harvested
annuals and herbaceous perennials was then transported to
a laboratory and oven-dried at 50 °C for 72 h, and weighted
to determine above-ground living plant biomass.
Statistical Analyses
Vegetation data from all five elevations within 1 year
(1993 or 1997) were pooled. One-way analysis of variance
(ANOVA), followed by Tukey’s Multiple Comparison Test
(Analytical Software 1994) were performed to detect significant differences in mean species richness, and to compare
site means when a significant drought effect was detected,
respectively. Student t-tests (Analytical Software 1994) were
used to detect significant differences in woody and herbaceous vegetation between the moist 1993 and arid 1997
years. Mean values are presented with standard errors, and
statistical significance is determined at p < 0.05.
Figure 1—Sketch map showing the location of Cold
Creek Canyon in the Spring Mountains of southern
Nevada. Las Vegas Valley lies to the southeast of the
Cold Creek Canyon.
Results ________________________
a fixed elevational interval of 30 m with directions from
evident landmarks, and topographic maps were used to
ensure that plots could be easily found in subsequent years
to facilitate repeated vegetation surveys. Brightly colored
flagging tapes were tagged on shrubs that occurred at the
center of each plot for ease of visualization. Altimeter was
used to detect elevation. Within each plot, all plant species
were identified using Munz (1974). Woody taxa were quantified to determine species richness, densities, and frequencies. Annual and herbaceous perennial taxa were quantified
to determine species richness, densities, frequencies,
percent vegetation cover, and above-ground living plant
Winter and spring 1993 were considerably more moist
than the winter and spring 1997 (table 1). Abundant rainfall
occurred in December 1992, and in January and February
1993, with a total amount of 148.8 mm (table 1), which
clearly exceeded the mean monthly rainfall from December
through February (35.6 mm) and mean total annual rainfall
(101.6 mm) in southern Nevada. However, mean monthly
air temperatures did not change considerably between these
two 9-month periods (table 1).
Field surveys revealed a significant reduction in density,
frequency, percent vegetation cover, and above-ground biomass (p < 0.001; table 2) in local annual plants in spring 1997
Table 1—Mean monthly precipitation and air temperature of Las Vegas Valley in October
1992 through June 1993, and in October 1996 through June 1997. Official
weather data were obtained from McCarran Airport in Las Vegas, near Cold
Creek Canyon. Actual weather data in the creosote bush-blackbrush ecotone
were not available. The letter “T” indicates trace precipitation, an amount
greater than zero but less than the lowest reportable value. Mean values are
based on the 1936-1997 period.
Month
October
November
December
January
February
March
April
May
June
218
1992-1993
Temperature Precipitation
1996-1997
Temperature Precipitation
°C
mm
°C
mm
21.6
11.5
6.4
7.6
10.1
16.1
19.7
25.0
28.1
31.0
0.0
43.4
41.4
64.0
3.6
0.3
0.3
2.0
19.3
13.6
7.7
9.3
10.9
17.1
18.6
27.6
29.1
2.8
2.1
4.6
7.6
T
0.0
1.0
T
T
USDA Forest Service Proceedings RMRS-P-11. 1999
compared to spring 1993 across the creosote bush-blackbrush
ecotone. Mean annual species richness declined significantly
(p < 0.01; fig. 2) during Spring 1997. Among the annual
species, red brome (Bromus rubens) by far experienced the
greatest decline in 1997 than in 1993. Biomass and density
of desert trumpet (Eriogonum inflatum), desert marigold
(Baileya pleniradiata), and fiddleneck (Amsinckia tessellata)
also showed a significant reduction (p < 0.001). Locally
uncommon annual species, including New Mexico thistle
(Cirsium neomexicanum) and evening primrose (Oenothera
spp.), were nearly absent during the 1997 drought period.
Similarly, local herbaceous perennial vegetation also
exhibited a significant difference (p < 0.05; table 2) in
above-ground biomass between the two years that differed
in timing and total amount of precipitation. Conversely,
Table 2—Frequency, mean density, percent vegetation cover, and
above-ground biomass of annuals (1 m2) and herbaceous
perennials (16 m2) at the creosote bush-blackbrush ecotone
in Cold Creek Canyon of southern Nevada during 1993 and
1997. Mean values are expressed with standard errors, and
mean values in rows followed by different letters are
significantly different at p ≤ 0.05 based on Student t-tests.
Life-form
Annual
Perennial
Parameter
1993
1997
Density
Frequency
Percent cover
Biomass
140.1 ± 5.7 a
100.0 %
26.3 ± 1.4 a
6.1 ± 0.3 a
20.3 ± 1.2 b
60.0 %
7.4 ± 0.7 b
1.5 ± 0.05 b
Density
Frequency
Percent cover
Biomass
4.7 ± 0.04 a
68.0 %
3.9 ± 0.001 a
1.9 ± 0.02 a
4.4 ± 0.02 a
65.3 %
2.1 ± 0.001 a
0.8 ± 0.003 b
15
a
Annuals
Mean number of species
12
H. Perennial
a
a
a
a
9
b
6
3
0
1993
1997
Year
Figure 2—Mean species richness of annual, herbaceous (H), and woody (W) perennial plants at the
creosote bush-blackbrush ecotone in Cold Creek
Canyon of southern Nevada. Mean values are expressed with standard errors, and columns within the
same life-form labeled with different letters are significantly different at p < 0.05 based on Tukey’s
Multiple Comparison Test.
USDA Forest Service Proceedings RMRS-P-11. 1999
W. Perennial
species richness (fig. 2), density, frequency, and vegetation cover (table 2) of herbaceous perennials were not
significantly different. Herbaceous vegetation was sparely
distributed in Cold Creek Canyon. In general, many species, such as Indian paintbrush (Castilleja chromosa),
locoweed (Astragalus spp.), speargrass (Stipa speciosa),
and desert globemallow (Sphaeralcea ambigua), largely
died back to the ground surface. Nevertheless, populations
of perennials were more stable than populations of annuals
despite the arid winter 1996 and spring 1997 seasons.
On the contrary, all local woody, subwoody (suffrutescent),
and succulent taxa remained nearly the same in species
density, frequency, and percent vegetation cover in both
1993 and 1997 years (table 3). Percent vegetation cover was
38.7 and 38.1% in 1993 and 1997, respectively. Creosote
bush-white bursage-Mojave yucca (Yucca schidigera) associations were commonly established at the lower half of the
creosote bush-blackbrush ecotone. Joshua-tree (Yucca
brevifolia) and blackbrush were commonly established at
the upper half of the creosote bush-blackbrush ecotone.
Individuals of snakeweed (Gutierrezia sarothrae) were frequently found on disturbed sites despite the occurrence of
severe drought. Among the three major plant life-forms,
woody taxa consistently had the most stable populations,
while annual taxa had the least stable populations at the
creosote bush-blackbrush ecotone.
Discussion _____________________
Extremely low annual species richness, density, frequency,
percent vegetation cover, and above-ground biomass were
detected during the severe drought in spring 1997 in southern Nevada. From casual observations, a number of seeds
were found in the seed bank to avoid such severe drought.
The drought began in 1995, and reached its greatest intensity during the first 6 months of 1997. In this study, exceptionally low precipitation in these years may explain the
major reduction in overall species diversity and productivity
of annual plants.
The life cycle of annual plants rarely experiences water
deficit and other types of environmental stresses. Plants
simply escape the harsh environment as seeds in this study.
After rain, seeds germinate and plants grow, flower, and
produce seeds rapidly before the water supply is exhausted
(Frietas 1997). Annuals have relatively high growth and
photosynthetic rate, and can complete their life cycle before
the advent of environmental stress. The seed bank at my
study site stores some winter ephemeral seeds, which can
germinate and complete their life cycle when soil moisture
is abundant during spring seasons preceded by winter
rainfall. Germination and mortality of desert ephemerals
are largely controlled by a combination of soil moisture and
temperature (Barbour and others 1987). Many winter
ephemerals in the Mojave Desert germinate after fall or
winter rains in excess of 10-15 mm; if falling below this
critical limit, germination is nearly absent (Barbour and
others 1987), which concurs with this study. Exceptionally
low density of annuals was found at my study sites because
rainfall in winter and spring 1997 fell below 15 mm. Beatley
(1967) proposes that density of annuals strongly correlates
with increasing rainfall between 15-45 mm. Beatley (1967)
suggests that winter annuals grow slowly through winter,
219
Table 3—Mean density (plants/100 m2) and frequency (number of plants present) of
woody, subwoody, and succulent taxa at the creosote bush-blackbrush ecotone
in Cold Creek Canyon of southern Nevada during 1993 and 1997. All taxa were
not significantly different (p > 0.05) in mean density and frequency.
Species
Acamptopappus shockleyi
Ambrosia eriocentra
Ambrosia dumosa
Atriplex canescens
Atriplex confertifolia
Coleogyne ramosissima
Echinocactus polycephalus
Encelia virginensis
Ephedra nevadensis
Eriogonum fasciculatum
Eurotia lantata
Gaura coccina
Grayia spinosa
Hymenoclea salsola
Gutierrezia sarothrae
Krameria parvifolia
Larrea tridentata
Menodora spinescens
Opuntia acanthocarpa
Opuntia basilaris
Opuntia echinocarpa
Opuntia ramosissima
Psorothamnus fremontii
Salazaria mexicana
Stephanomeria pauciflora
Yucca brevifolia
Yucca schidigera
1993
Density
Frequency
10.7
0.05
24.5
0.6
0.5
9.4
0.02
3.4
5.5
1.7
2.7
0.01
0.01
0.9
0.7
3.9
4.5
0.01
0.3
0.2
0.3
0.2
0.2
0.7
0.1
0.9
1.9
then grow rapidly in spring as temperatures rise. Late
September is the earliest and early April is the latest time
that germination of winter annuals occur; a 7- to 8-month
growing season is the longest, and 6 to 10 weeks is the
shortest that appear possible in southern Nevada. However, even when the growing season is compressed into a
few weeks, and preceded by heavy rains, more than onethird of the seedlings fail to reach maturity (Beatley 1967).
Under the best conditions, density of ephemerals may be
1,000 m2, cover 30%, biomass 60 g/m2, but typically, density
is 100 m2 and biomass 10/g m2 (Barbour and others 1987).
In this study, density, biomass, and percent vegetation
cover of winter ephemerals were significantly reduced
during the 1997 drought compared to the moist spring
1993. Wide variations of germination and mortality exist in
desert annuals from region to region, site to site, and
species to species during each year in southern Nevada
(Beatley 1976).
Red brome was the dominant winter ephemeral at my
study site, and is a common member of many creosote bushblackbrush ecotones, which occur at elevations between
1,220 to 1,310 m in southern Nevada (Beatley 1966). Red
brome was most abundant under the shrubs, especially at
the periphery of shrub canopies in the creosote bush and
blackbrush shrublands. Red brome occupied the same
ecological niche as the native winter annuals in this study.
This grass has the ability to dominate other native winter
220
60
3
87
7
5
53
3
16
40
13
27
3
7
16
7
52
73
5
12
13
12
11
15
13
3
39
65
Density
10.2
0.03
24.7
0.5
0.4
9.1
0.02
3.2
5.4
1.4
2.7
0.01
0.01
1.2
0.9
3.8
4.5
0.01
0.3
0.2
0.3
0.2
0.3
0.7
0.2
1.2
2.1
1997
Frequency
60
1
87
7
5
53
3
16
40
13
27
1
7
16
7
52
73
5
12
13
12
11
15
13
1
39
65
ephemerals primarily due to its greater reproductive success, greater ability to survive to maturity and reproduce,
and higher survival percentages during the growing season
(Beatley 1967).
There are several potential explanations for the significant reduction in local annuals, but not woody perennials,
during a severe drought period in this study. First, annuals
usually avoid germination in the drought year, and usually
remain dormant in the seed bank of their original habitats.
Seeds of many annual species may remain dormant for
years, and may not germinate until the advent of next
moisture and nutrient pulse. Second, density, frequency,
biomass, and vegetation cover of annuals are strongly
associated with the timing and amount of precipitation,
which can vary considerably from year to year. Third,
annuals commence their life cycle each year from seeds,
which would experience a greater mortality rate than
established woody perennials. Fourth, established perennial plants may experience a more moist environment than
annuals since roots of perennials can obtain moisture from
deeper soils. Annual plants have a shallow root system, and
root biomass of annuals generally occurs at the upper 15 cm
of the soil (Lei, personal observation). Fifth, annuals in
southern Nevada are generally C3 winter species, and may
be more drought sensitive than native C3 and C4 perennials.
In this study, woody, suffrutescent, and succulent taxa in
the creosote bush-blackbrush ecotone exhibited little change
USDA Forest Service Proceedings RMRS-P-11. 1999
in community composition, with vegetation cover and species
density and frequency remaining nearly the same throughout
the severe drought period. Woody taxa establishing at desert
mountain slopes were characterized as drought-tolerant
(evergreen), drought-deciduous, and succulent plants in this
study. Drought-tolerant plants, such as creosote bush and
saltbush, are true xerophytes because they grow and transpire throughout periods of water stress and have a high
degree of tolerance for desiccation (Frietas 1997). Net photosynthesis and root growth are possible, and transpiration
rates are very low when moisture is limiting (Barbour and
others 1987). The advantage of being evergreen is that when
water becomes available again, no lag exists while new
tissues are produced. This adaptive mode is a more competitive strategy than other life-form strategies in a stressful
environment with limited resource availability. The cost of
maintaining evergreen leaves that can withstand episodic
environmental stress is less than that of producing new
photosynthetic leaves annually. Many evergreen species,
however, shed a fraction of their leaves during severe drought
periods, presumably to reduce surface area and water loss
through transpiration (Barbour and others 1987).
Although the evergreen life-form solves certain environmental problems, it creates others. When water stress intensifies, stomatal control can provide temporal drought-escape
mechanisms (Frietas 1997). Reducing water loss by partial
stomatal closure restricts the C02 influx and lowers the
photosynthetic production. Such water loss also results in
the reduction of evaporative cooling and can overheat the
leaf (Frietas 1997). Drought-tolerant plants respond to seasonal drought by adjusting the amount of leaf area exposed
and by reducing gas exchange via partial stomatal closure
mechanisms (Frietas 1997). Midday stomatal closure may
contribute to drought avoidance, decrease water loss, and
increase water-use efficiency.
However, drought-deciduous species, such as white
bursage, blackbrush, and goldhead (Acamptopappus
shockleyi), avoided water stress by becoming temporarily
dormant and shedding some of the older leaves as stress
intensified during the dry season in this study. Drought can
be so severe in southern Nevada that perennial plants have
to survive for several months without much photosynthetically active lamina. Most drought-deciduous species produce only one crop of leaves a year and enter a long summer
dormancy following leaf drop (Barbour and others 1987).
Their leaves are energetically inexpensive to manufacture
compared to conifer and other evergreen leaves, and their
photosynthetic rates are generally 2-3 times higher than
evergreens (Barbour and others 1987). These plants appear
to exhibit maximum photosynthetic activity in the cool,
moist winter and spring months.
Succulent plants, including cacti, have a very shallow root
system that can absorb water even from light rains. Cacti are
uncommonly distributed in my study area because they
cannot tolerate water stress at the cellular level. Internal
water stress rarely exceeds –0.5 MPa (Barbour and others
1987). In wet periods, water is stored in massive parenchyma cells, swelling the stem; in dry periods, water is used
and the stem shrinks (Barbour and others 1987). Cacti
USDA Forest Service Proceedings RMRS-P-11. 1999
utilize the CAM photosynthetic pathway, with stomates
open at night when under mild air temperatures and
relative humidity. The assimilated CO2 is stored in the form
of organic acids and later, during daytime, the stored CO2 is
utilized to form carbohydrates while the stomata remain
closed. Among all plant life-forms, cacti clearly exhibit the
highest water use efficiency, even higher than true xerophytes. Yet, cacti have extremely slow growth rates because
they allocate much of their energy in tolerating adverse and
stressful environments.
Plants of various life-forms utilized different adaptive
strategies to reduce and cope with water stress at the
creosote bush-blackbrush ecotone in Cold Creek Canyon of
southern Nevada. The severe drought in 1997 limited the
overall local biodiversity and productivity of local annual
plants because they simply avoided the drought year as
seeds. Hence, a change to a harsher, drier climate in southern Nevada would increase the chance of annual species to
remain dormant in the seed bank of their original habitats.
Acknowledgments ______________
I express my gratitude to Steven Lei, David Valenzuela,
and Shevaum Valenzuela for their valuable field assistance
during the course of study. I sincerely appreciate Wesley
Niles for identifying the unknown plant species, and appreciate David Charlet for critically reviewing the manuscript.
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