EFFECT OF MANIPULATING SOIL
WATER AND NITROGEN REGIMES ON
CLIPPING PRODUCTION AND WATER
RELATIONS OF CREOSOTE BUSH
M. R. Sharifi
F. C. Meinzer
P. W. Rundel
E. T. Nilsen
Seasonal growth patterns, clipping production (biomass
distribution into leaves, current twigs, and fruits), and
water relations responses of creosote bush (Larrea tridentata) to nitrogen and water amendments were studied over
a 2-year period in a Sonoran Desert wash woodland community. Leaf water potential varied in both irrigated and
nonirrigated individuals, with lowest values (greatest
water stress) observed in the nonirrigated individuals
at both predawn and midday, except when measurements
had been preceded by significant rainfall. Both irrigation
alone and irrigation plus nitrogen addition resulted in
significant increases in leaf and twig production rates over
control plants and those with only nitrogen added. Nitrogen addition alone had no signifzcant effect on vegetative
production rates. The effect of nitrogen addition was more
marked in the irrigated treatments, resulting in significantly higher leaf and shoot weights in the treatment
with both water and nitrogen additions compared with
the irrigation only treatment in the second year of the
experiment. Reproductive allocation was higher in the
nonirrigated than in the irrigated treatments, with the
lowest reproductive activity noted in the irrigation-only
treatment.
Skujins 1978), and may play a major role in determining
productivity under conditions of adequate water supplies.
Ettershank and others (1978) and James and Jurinak
(1978) found significant responses of arid vegetation to
nitrogen fertilizer without additional water. James and
Jurinak (1978) and Romney and others (1978) found that
the combination of nitrogen and water produced a synergistic production response, increasing plant growth much
more than the sum of the individual responses to water
or nitrogen alone.
Creosote bush (Larrea tridentata (DC) Cov.), an evergreen xerophytic member of the Zygophyllaceae, is one
of the most abundant perennial plants in the Sonoran,
Mojave, and Chihuahuan Deserts of southwestern North
America (Barbour 1969). Creosote bush has a broad ecological amplitude, and occurs on a wide variety of sites
in this area. Few manipulation experiments have examined the role varying nitrogen and water supplies play in
determining the ecological versatility observed in this
species (Ettershank and others 1978; Cunningham and
others 1979). In the present study, the research objectives were to assess the effect of additional water and
nitrogen supply on the growth dynamics and productivity
of twigs, leaves, and reproductive tissue of creosote bush
during a 2-year field manipulation experiment.
INTRODUCTION
MATERIALS AND METHODS
Unpredictable and highly variable amounts of precipitation in arid ecosystems are limiting to primary plant
productivity (Noy-Meir 1973; Fisher and Turner 1978).
When water is available, the productivity may be influenced or limited lzy other factors such as soil and air temperature, herbivory, microflora activity, and soil nutrient
availability. Nitrogen is generally considered to be the
second most important factor limiting growth in warm
desert ecosystems (Ettershank and others 1978; West and
The study site was a sandy wash woodland located in
Living Desert Reserve near Palm Desert, CA (33°44'N,
116°23'W, elevation 60 m). Most of the 149-mm mean
annual precipitation is of frontal origin and falls between
December and March. Late summer precipitation occurs
July through September as localized thunderstorms.
Summer precipitation is highly variable from year to year.
The average July maximum temperature exceeds 40 oc
and summer maximum temperatures greater than 4 7 oc
are not uncommon. Species codominant with creosote
bush in this wash woodland are palo verde (Cercidium
floridum) and smoke tree (Psorothamnus spinosus).
Scattered cat-claw acacia (Acacia gregii) and desert
willow (Chilopsis linearis) are also present. Among the
shrubs, cheesebush (Hymenoclea salsola) and sandpaper
plant (Petalonyx thurberi) are abundant. The experimental design included three treatments and a control.
Twelve mature creosote bush individuals (three per
ABSTRACT
Paper presented at the Symposium on Cheatgrass Invasion, Shrub DieOff, and other Aspects of Shrub Biology and Management, Las Vegas, NV,
April 5-7, 1989.
M. R. Sharifi is Assistant Research Ecologist, Laboratory of Biomedical
and Environmental Sciences, University of California, Los Angeles, CA
90024. F. C. Meinzer is Associate Agronomist, Hawaiian Sugar Planters'
Association, P.O. Box 1057, Aiea, HI 96701. P. W. Rundel is Professor of
Botany, Laboratory of Biomedical and Environmental Sciences, University
of California, Los Angeles, CA 90024. E. T. Nilsen is Professor of Biology,
Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.
245
This file was created by scanning the printed publication.
Errors identified by the software have been corrected;
however, some errors may remain.
50
treatment) were randomly assigned to one of four treatments: (1) no added water or nitrogen (control, C);
(2) water alone (W); (3) nitrogen added to the soil (SN);
(4) both water and nitrogen added (W+SN). Two 180°
spray Microjet sprinklers per shrub wetted a 2-m radius
under each shrub. In-line flow meters monitored delivery
rates.
Irrigation was applied to achieve field capacity in the
upper 2m of soil. Attainment of field capacity was assessed by monitoring neutron probe access tubes installed
1 m from the main stems. Irrigation was applied once
a month from April through November 1984 and March
through October 1985. For the added soil nitrogen treatment, NH4N03 was applied to a 4-m diameter circle
around each plant at a rate of 5 g N/m2 after the April
1984 and 1985 irrigations. An additional2.5 g/m 2 was applied after the September 1984 and 1985 irrigations. Predawn and midday leaf water potentials were measured in
the field with a pressure chamber (PMS Instrument
Corp., Corvallis, OR) every 4-6 weeks, using three shoots
from each individual. Measurements were timed to be
taken either right before, or more than a week after,
irrigation. Leaf conductance (g) was determined with
a LICOR steady state porometer (Model1600).
Depending upon canopy size, five to 10 branches per
individual were randomly selected in the four ordinal
compass directions for phenological measurements.
Branches of similar size (length and diameter) and age
were selected at random and tagged at the ninth to the
11th internode from the end before the onset of each
growing season. The following phenological measurements were taken at regular intervals: total shoot length;
number ofleaves in each developmental category (juvenile, mature, senescent, browsed); number of empty
nodes. The number of abscised leaves was calculated by
multiplying the number of empty nodes by two (each node
carried two leaves). Growth rate calculations were made
as described in Sharifi and others (1983) and Nilsen and
others (1981). During the period of peak biomass, five to
10 branches of different size classes were cut off 10 em
above the surface of the ground from each individual.
Before cutting the branches, the basal diameter of each
branch was measured to the nearest millimeter at the
level of the cut.
In the laboratory, branches were separated into wood,
current shoots, leaves, and fruits. The dry weight of each
component was measured after oven drying. Regression
analysis using a power function (Sharifi and others 1982)
was used to obtain equations relating the dry weight of
each component to the basal diameter of the stem. The
basal diameters of all the main stems of each experimental individual were then measured 10 em above the
ground surface. Using the regression equations obtained,
the biomass ofleaves, current shoots, and fruits of each
individual were estimated for the growing seasons 1984
and 1985. Canopy height (at the center) and diameter
(mean of two perpendicular measurements) were recorded
for each plant; canopy area and volume were computed
from these values. The canopy area was used as the denominator in biomass calculations.
~ 40
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1984
1985
Figure 1--Seasonal pattern of precipitation (monthly
total), temperature, and midday and predawn leaf
water potential during 1984-1985 at the Living
Desert Reserve.
RESULTS
Climate monitoring indicated that 1984 was characterized by relatively high levels of precipitation compared
with 1985 (fig. 1). In 1984 there were two periods of significant rainfall; one occurred in the summer and one in
the winter. In 1985, however, there was no significant
summer rain, and most of the precipitation fell in the
winter. Summer maximum temperatures typically averaged about 40 °C. In the winter, mean maximum temperatures dropped as low as 9 °C. Irrigation resulted
in higher predawn and midday shoot water potentials
both years of the experiment (fig. 1). Leaf water potentials were lowest in midsummer, and ranged as low as
-5.3 MPa in 1985 when there was no summer precipitation. The large seasonal fluctuations in leaf water
potential in irrigated individuals during 1984 resulted
from a combination of relatively long intervals between
irrigations (1 month), large seasonal changes in evaporative demand, and heavy precipitation, which occurred in
August and December 1984. The differences in leaf water
potential between irrigated and nonirrigated individuals
tended to diminish after significant precipitation events
and just prior to irrigation.
Table 1 shows the predawn and midday leaf water
potentials, maximum conductance, and vapor pressure
deficit (VPD) from the diurnal measurements made on
three dates. With the exception of the August 1984 date,
predawn and midday leaf water potentials were lower
in nonirrigated individuals than in irrigated ones. The
diurnal cycle for August 1984 was measured 2 days after
a thunderstorm with 31 mm precipitation, and showed
insignificant differences in leaf water potential between
the treatments. Maximum leaf conductance differences
between irrigated and nonirrigated individuals were
noted in the peak summer diurnal cycle of July 1984.
246
Similar differences, though not so marked, were observed
in the late spring cycle in 1985. Following the storm in
August 1984, again, no significant differences between
treatments were observed. The greatest fluctuations in
VPD were noted during August 1985, with a peak of 9.0
kPa. Because there were only minimal differences in leaf
water potential and conductance between fertilized and
unfertilized individuals, the data are not separated here.
Leaf production and shoot elongation rates are represented as a percent of the maximum of the three treatments and control for both years in figure 2. Two flushes
of leafing and shoot growth were observed, one in the
spring and one in the summer. Leaf production and
shoot elongation were significantly higher in the irrigated
(W and W+SN) treatments than in the nonirrigated
(C and SN) treatments in 1984. Addition of soil nitrogen
alone caused no increase in vegetative growth rates in
1984, the first year of the experiment.
In 1985, a drier year, there was only minimal growth
during the spring and summer growth periods in the
nonirrigated treatments, while theW and W+SN treatments resulted in significantly higher leaf and shoot
growth rates. Water augmentation resulted in a lower
production of fruits in both years.
Figure 3 represents the absolute clipping biomass (sum
ofleaves, current shoots, and fruits) for each treatment
in 1984 and 1985. While the clipping biomass of individuals in the control (C) and soil nitrogen (SN) treatments
were similar, the irrigated plants (Wand W+SN) produced a consiaerably higher-biomass than the nonirrigated (C and SN) ones. In 1985, which was characterized
by lower precipitation than 1984, the total clipping biomass of all individuals in all treatments was lower than
in 1984. Overall, the low precipitation level in 1985
caused about a 50 percent reduction in productivity in
the nonirrigated treatments, compared with 1984, while
the reduction in the irrigated treatments was only about
10 percent.
Biomass allocation to leaves, current shoots, and fruits
are shown in figures 4 and 5. Biomass allocation patterns
were similar for both years. The proportion of clipping
biomass allocated to the reproductive tissue component
was significantly higher in the'nonirrigated treatments
Table 1-Predawn and midday leaf water potential, maximum leaf
conductance, and vapor pressure deficit for creosote bush
in Living Desert Reserve
Date
Maximum
--=-P..::....;re=d=a:..:.::w=n'--- -~M~id,_,d=a'!Sy__ conductance Maximum
11
Nl2
I
Nl
Nl
I
VPD
mmollrrfls
kPa
7/20/84 -2.35 -3.56 -3.70 -4.70
(±0.10) (±0.14) (±0.16) (±0.19)
480 75
(±49) (±8)
5.1
8/30/84 -2.30 -2.05 -3.65 -3.53
(±0.09) (±0.09) (±0.17) (±0.16)
300 242
(±53) (±43)
9.0
5/2185
167
67
(±30) (±12)
7.1
MPa
MPa
-2.30 -3.05 -2.88 -4.00
(±0.11) (±0.12) (±0.16) (±0.13)
1=irrigated individuals.
NI = nonirrigated individuals.
1
2
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100
2000
r-1-
CJ 1984
(/)
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....... 1500
~
1985
~
0
iD c:
Figure 2-Effect of water and nitrogen addition on
growth rates of current shoots, leaves, and fruit in
1984 and 1985. Values are expressed as the percent
of the maximum for both years and all treatments.
C)
z
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::J
(.)
N8 1000
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01
500
0
t
~
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c
n
SN
w
Figure 3-Effect of water and nitrogen addition
on total clipping production (sum of leaves, current shoots, and fruit) in 1984 and 1985.
247
W+SN
1984
during both years of the experiment. This trend was more
pronounced during 1985, the drier year. The lowest fruit
production levels were seen in the irrigation only treatment (W). While this trend was consistent both years
of the experiment, the drier conditions of 1985 did appear
to result in some stimulation of fruit production in this
treatment.
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DISCUSSION
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While water availability was the major factor controlling phenological events and productivity in this study,
nitrogen supply apparently played a limiting role in the
irrigated treatments during the second study year. It
is possible that nitrogen released to the soil from organic
matter under the creosote bush canopies was utilized by
irrigated plants during a prolonged growth period in the
first study year, resulting in smaller differences in growth
rates between individuals in the irrigated treatments
(Wand W+SN) during that year. This may have resulted
in soil nitrogen depletion under irrigated individuals that
received no nitrogen supplementation (W), which manifested itself in the second year of the study.
Release of nitrogen from "fertile islands" under creosote bush canopies has been described by Romney and
others (1978) and Charley and West (1975). Organic
matter contents of 2 percent and greater in soils under
canopies of desert plants have been reported by Romney
and others (1973).
Our finding that increased soil water availability increased the ratio of vegetative to reproductive growth
is consistent with the study reported by Cunningham
and others (1979). In that experiment, high soil moisture
content increased the ratio of vegetative to reproductive
tissue. In another field study in the Sonoran Desert of
California, it was observed that increased amounts of
spring rain resulted in a shift in the ratio of vegetative
to reproductive tissue in 1981 compared with 1982 (Nilsen
and others 1987). Our observation that creosote bush is
capable of undergoing a variable number of growth periods each year indicates that maximizing resource utilization by synchronization of growth activity with resource
availability may be an important survival strategy for
this species.
This flexibility in biomass allocation represents an additional mechanism of adapting growth patterns to environmental conditions. The success of this species in the
desert environment is clearly linked to a marked degree
of phenotypic plasticity in its response to changing environmental conditions.
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62.115
Control
Soil nitrogen
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67.415
47.7$
Water
Water + Soil nitrogen
Figure 4-Biomass allocation (percent) to vegetative
and reproductive tissue under differing regimens of
nitrogen and water supplementation in 1984.
1985
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ACKNOWLEDGMENTS
ehoola
We thank the Living Desert Reserve Foundation for
the use of the research site and the staff of the Philip
Boyd Deep Canyon Research Center for their help. In
addition, thanks are also due to Don Herman, who helped
collect data, and Peter Clark, who assisted in irrigation
and fertilization. This research was supported by the
61.815
Water + Soil nitrogen
Figure 5-Biomass allocation (percent) to vegetative
and reproductive tissue under differing regimens of
nitrogen and water supplementation in 1985.
248
National Science Foundation (Grant No. BSR 82-16814)
and the Department of Energy (Grant No. DE-AC03-76SF00012 from the Ecological Research division of the
Office of Health and Environmental Research).
Nilsen, E. T.; Sharifi, M. R.; Virginia, R. A.; Rundel,
P. W. 1987. Phenology of warm desert phreatophytes:
seasonal growth and herbivory in Prosopis glandulosa
var. torreyana (honey mesquite). Journal of Arid Environments. 13: 217-231.
Noy-Meir, I. 1973. Desert ecosystems: environment and
producers. In: Johnston, R. F., ed. Annual review of
ecology and systematics. Palo Alto, CA: Annual Review,
Inc.: 25-52.
Romney, E. M.; Hale, V. Q.; Wallace, A.; [and others].
1973. Some characteristics of soil and perennial vegetation in northern Mojave Desert areas of the Nevada
Test Site. Springfield, VA: U. S. Atomic Energy Commission Reports, UCLA 12-916, Office of Information
Service. 340 p.
Romney, E. M.; Wallace, A.; Hunter, R. B. 1978. Plant response to nitrogen fertilization in the northern Mojave
Desert and its relationship to water manipulation. In:
West, N. E.; Skujins, J., eds. Nitrogen in desert ecosystems. Stroudsburg, PA: Dowden, Hutchison and Ross:
232-253.
Sharifi, M. R.; Nilsen, E. T.; Rundel, P. W. 1982. Biomass
and net primary production of Prosopis glandulosa
(Fabaceae) in the Sonoran Desert of California. ,American Journal of Botany. 69: 760-767.
Sharifi, M. R.; Nilsen, E. T.; Virginia, R. A.; [and others].
1983. Phenological patterns of current season shoots
of Prosopis glandulosa var. torreyana in the Sonoran
Desert of Southern California. Flora. 173: 265-277.
West, N. E.; Skujins, J. 1978. Nitrogen in desert ecosystems. Stroudsburg, PA: Dowden, Hutchison and Ross.
307 p.
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249