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A Field Assessment of Above- and Below-Ground Factors
Affecting Phreatophyte Transpiration
in the Owens Valley, California 1
D. P. Groeneveld, D. L. Grate, P. J. Hubbard, D. S. Munk,
P. J. Novak, B. Til1emans, D. C. Warren, and I. Yamashita 2
Abstract. Factors influencing the water balance physiology and transpiration of five Great Basin shrub and grass
phreatophytes are being investigated in shallow groundwater
zones of the arid Owens Valley, California. Measurements of
transpiration, atmospheric potential, canopy factors, root
density, soil moisture and xylem potential are presented and
discussed.
INTRODUCTION
Shallow groundwater/arid climate ecosystems
have received little detailed study. In an effort
to gain sufficient knowledge to manage such an ecosystem, Inyo County, California and the City of
Los Angeles have jointly funded a field investigation in the Owens Valley. The study is structured
to determine the transpiration rates, physiology
and morphology of five important plant species inhabiting shallow groundwater zones toward an ultimate management goal to preserve the shallow groundwater habitats during pumping and export of groundwater. This paper attempts to list the methods and
initial results of the first of three years of
field study. In essence, the study is attempting
to ask both "how much do shallow groundwater plants
transpire" and "how is transpiration controlled."
The accumulating data base is being used to provide
input and interpretive information bases for research conducted by the United States Geological
Survey to determine phreatophyte survival following pumping and to model vadose zone moisture
extraction by roots.
Owens River
~
~
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~ \ ~:;'. White Mountains
/-~.
Sierra Nevada ~ \
I
~~\.
/ \ /I A "
I
~J\
'7\A1'\
~nyo Mountains
~
; \
:::'
Owens Lake
/
Los Angeles Aqueduct
Figure 1.--Location Map of the Study Area.
The Valley floor precipitation is highly variable and averages from 10 to 15 cm depending on location. Low precipitation is augmented by copius
runoff from the Sierra Nevada combining to create a unique ecology of shallow groundwater and arid' climate.
Vegetation
Five plant species are ubiquitous to shallow
groundwater sites in the Owens Valley and are the
subject of this study: Nevada saltbush (Atriplex
torreyi), greasewood (Sarcobatus vermicu1atus), rabbitbrush (Chrysothamnusnauseosus ssp. viridu1us),
sa1tgrass (Distichlisspicata) and alkali sacaton
(Sporobo1us airoides) (Munz and Keck 1959). These
species are both phreatophytic, requiring or thriving on groundwater and halophytic, tolerating or
requiring high concentrations of soil salts.
Study Area
The Owens Valley is located in eastern California between the Sierra Nevada on the west and
the White and Inyo Mountains to the east (fig. 1).
The 25 to 30 km wide valley floor slopes gently
from the northeast to the southwest. The study
area comprises the valley floor lying between the
Inyo-Mono County line and the Owens Dry Lake and
ranges between 1250 m and 1160 m in elevation. The
crests of the Sierra Nevada and Inyo and White Mountains rise precipitously from the valley floor to
an average of 3,700 and 3,100 meters respectively.
METHODS
Ten study sites are developed to examine both
above and below ground factors to determine the
coupling of groundwater capillary movement and extraction by plants. Two of the sites are developed
for more intensive measurement of plant response to
artificially lowered water tables. A suite of data
including atmospheric potential, canopy leaf area,
IPaper presented at the North American Riparian
Conference [Tucson, Arizona, Apri116-18, 1985].
2Members of the Cooperative Vegetation Study
Team, funded by Inyo County and the City of Los
Angeles, Bishop, California.
166
transpiration and soil moisture is measured at least
monthly during the April through October growth
cycle. These records are kept for eight grass plots
and 32 individual shrubs. Cores for determining root
density are taken quarterly to conform to the seasons
of the year on a subsample of six grass plots and
18 shrubs.
active radiation (0.4 to 0.7 urn wavelength) at the
position of the in situ branches or leaves. Shrub
branches are placed within a cylindrical split and
hinged polycarbonate chamber for the series of measurements. Five branches per shrub are monitored
through a diurnal period for statistical representation of the canopy. Leaf areas of the shrub samples are estimated for correction of transpiration
and diffusive resistance data. The cuvette designed
for the grasses measures only one surface at a time,
so both abaxial and adaxial surfaces of the grass
blades are monitored.
Soil moi~ture content is measured periodically
by neutron probe accessed through aluminum tubes
fitted with welded closures. These tubes were placed
as deeply as possible into the soil profile at canopy
driplines of shrubs, through grass plots and into
ground with vegetation cleared for 3 meters. Three
master calibration curves with correlation coefficients of about 90 percent were calculated for use
on all access tubes at the study sites. These curves
are parallel and have intercepts which vary according
to depth at 15 and 30 centimeters due to neutron escape from the soil surface. The water table surface
is monitored at each site by a shallow well outfitted
with a recorder to compare to the observed soil moisture responses.
Phenology and Leaf Area
A non-destructive method was developed for the
project which estimates leaf area and biomass on an
experimental plant or plot by point frame (Goodall
1952). The technique uses an empirical relationship developed between leaf area and leaf biomass
and a total of the interceptions of the point frame
pins with leaves on the shrub canopy or grass plot.
Each experimental plant is measured by this technique to follow phenology and leaf area for transpiration calculations.
Root system morphology and soil horizon are
accessed by trenching to the water table with a
structure emplaced to guard against collapse of the
trench walls. Root systems are exposed for viewing
and sampling with a pressurized stream of water
from a pickup truck-mounted tank and pump.
RESULTS
The results from one year of study are presented for the intensive study site located near Warm
Springs and south of Bishop in the northern Owens
Valley. Pertinent observations from several other
sites are included to help illustrate the ecology
of the five plant species.
Samples obtained in one liter volume cores to
assess root density are centered at 30.5 cm. increments starting at 15.3 cm. These cores are obtained
by sawtooth bit mounted on a 7.6 cm outside diameter barrel. The fine absorptive roots are separated from the soil volume by elutriation. The
system uses a water stream and turbulent agitation
to process eleven samples at a time. Root length
estimation by a statistical method modified after
Newman (1966) uses the empirical relationship between root length and the root/grid intersections
counted under a binocular stereomicroscope. A set
of criteria for judging "live" versus "dead" roots
was adopted based upon preliminary observation.
Soil Moisture
The statistical distribution of root density
is not normal (St. John and Hunt 1983). This is
due to t~e tendency for the deciduous fine roots
to be organized into cells according to proximity
to the more permanent roots. The transformation
log (X + 1) suggested by Anscombe (1949) for such
distributions normalizes the Owens Valley root data.
Plant Water Physiology
The Scholander type pressure chamber is used
to assess plant moisture status by measuring xylem
pressure potential (Ritchie and Hinckley 1975).
The pressure chamber technique is used on five samples per experimental shrub or grass plot. Predawn and mid-day measurements are collected to compare to a per plant suite of soil moisture, transpiration and stomatal conductance data.
Transpiration and stomatal conductance are measured with a null-balan~e porometer.(Beardsell, et
a1. 1972) manuf'actured by Li-Cor, Inc., of Lincoln,
Nebraska. Also obtained are time, ambient relative
humidity and temperature and photosynthetically
167
The soil at the Warm Springs site is predominantly coarse sandy loam to loamy sand textures
with bulk density ranging from 1.4 g/cm at the surface to 1.7 g/cm at 180 cm. deep. These factors
permit rapid infiltration but tend to limit soil
porosity and capillary movement of water with depth.
The water table naturally fluctuates with season between a high of about 150 cm. in March to a low of
180 cm. in September responding to evapotranspirative draft. Water may enter the root zones downward either by precipitation, or upward by capillarity from the water table surface. These influxes can be traced by sequential monitoring by neutron probe and through isocontouring of the volumetric soil moisture content by depth (fig. 2).
The top graph in figure 2 represents the calculated
volumetric soil moisture averaged for 10 shrubs,
three each of Nevada saltbush and greasewood and
four of the rubber rabbitbrush accessed at the
canopy drip lines. Alkali sacaton and saltgrass
are accessed through plots and presented in the
center graph as averages. The lowermost plot represents soil moisture beneath a microsite with
vegetation cleared for a radius of three meters.
The water table, indicated by dotted line, was
lowered by pumping in October to initiate soil
moisture drainage prior to measurements of the
artificially stressed system scheduled for the
following summer. The capillary recharge evident
in October beneath the shrubs was due to temporary
breakdown of the pumping equipment.
dent beneath the cleared micro site and the shrubs,
may mark a limit for dov,rnward percolation and upward
capillarity. In general, horizontal isocontours indicate that the soil moisture remains fairly constant through time and this trend can be seen until
early July beneath the cleared site. Preliminary
comparison of transpiration measurements on the
canopies of the shrubs to the soil moisture depletion observed beneath the shrub drip lines strongly
indicate that the earliest extraction occurs near
each shrub. The zone of soil moisture extraction
may then move outward to tap the zones between each
shrub. If so~ the depletion of the water content
beneath the cleared site indicates extraction by
more remote shrub roots.
Average volumetric water content beneath driplines of shrubs In = 10)
20.0
25.0
15.0
10.0
5.0
1
I
I_~_'_;~--'--;II~Iuu.tI~I
'--:'-':---:-:------:-_-:-:-_~-___:"":"III"""-'-,-:-,-,1
Feb
Mar
Apr
May
Jun
Jul
Aug
O~t
Sep
Nov
Dec
1.0
2.0
.... water table
Average volumetric water content beneath grass plots In = 2)
;~:gj
15.0
10.0
5.0
Soil moisture changes within the profiles with
vegetation cover coincides with leafing. Shrubs
leaf earlier than grasses and this trend can be
seen iu the data. Changes in soil moisture content
are evidently very low in late October conforming
with grass senescence and low measured shrub transpiration rates.
I
I
II, I
I
I
II
I
Ii1
Depletion of soil moisture beneath grasses
occurs at a faster rate than beneath shrubs during
July and August due, possibly, to extraction from
the restricted soil volume explored by grasses.
As a general rule, the two grasses senesce before
the three shrubs which may be due to low soil moisture.
~F~e~b--~M~a-r~~A-pr--~M~a-y---J~~-n---J~u~I~~A~uU9~-S~e-p-U~O~~t--~N~ov~~D--ec
1.0'
Rooting Relationships
2.0
25.0
20.0
15.0
10.0
Root system morphology is unique for each of
the five species. Alkali sacaton is a bunch grass
and saltgrass is rhizomatous. Nevada saltbush
characteristically has several large lateral roots
which are initiated while the shrub is in the seedling stage. Subordinate roots arise from these laterals to explore the remainder of the soil column.
Greasewood roots arise from few tap roots with lateral branching near the surface. The root system
of rabbitbrush is variable but tends to consist of
a number of dichotomizing taproots which, in turn,
give rise to lateral roots.
table
Volumetric water content beneath a point with vegetation cleared
5.0
c
.c
1.0
c.
<ll
'0
2.0
water table
The highly absorptive roots of each species are
probably ephemeral and function for less than one year.
These roots are also quite small, on the order of O. .'i'
mm diameter. and are seldom visible in the soil matrix.
Tetrazolium tests on dead roots extracted by hand indicated active microbial respiration which confirms
that breakdown of sher1 roots is rapid. 3
Oxygen is obviously an important factor for rooting in high groundwater sites. Excavation and pumping of trenches below the water table have shown'thqt
laature root systems of the five species are incapable
of withstanding prolonged waterlogging. By contrast,
histologic sections of the near surface roots of Nevada
saltbush and rabbitbrush from a site flooded continuously for six months confirmed that the predominantly primary roots contained aerenchyma, cortical air spaces
which decrease oxygen diffusion resistance to root tips
(Coutts and Armstrong 1978). Greasewood failed to
survive this flooding.
Figure 2.--Isocontours of soil moisture content
under three treatments. The shrub and grass
measurements are averaged.
The July and August precipitation apparently
only recharged the soil profile near the surface.
The November precipitation has more effectively recharged the soil. Capillary recharge from rising
water tables is evident in March through May. A
boundary to soil moisture movement consisting of
an approximately 10 cm. horizon with weak to strong
calcium carbonate and silica cementation is found
at a depth of about one meter. This horizon, evi-
3Tetrazolium chloride provided courtesy
of the California Crop Improvement Association.
Davis, California.
168
The results of root density and statistical analysis of the transformed data indicate that root density decreases with depth. is equal among individuals
or species of shrubs and varies with season. These
relationships are significant above the 0.99 level.
Root density versus depth describes a decay function
suggestive of the relationship for nitrogen in arid
ecosystems (West and Klernrnedson 1978). Further analyses are concentrating on random root density under
mixed species cover and on soil moisture as a determinant for rooting density. A detailed study at a
site of predominantly Nevada saltbush and devoid of
herbaceous cover indicated that per depth root density is equal up to ten meters from canopies.
Transpiration and Xylem Pressure Potential
Diurnal curves of transpiration rates for the
five species are roughly parabolic in response to
the combined diurnal progression of atmospheric potential and photosynthetically active radiation
(fig. 3). The curves of each species approach zero
at sunrise and sunset which indicates that stomatal
control is light sensitive.
-250
~-200
o~---
0
~
..
1~o
:c: -150
*
&.\
0
5.0
7.0
2000
0 PAR
atmospheric
potential
•
1500
20
1000
' \
:
col
/0o
-50
3.0
•
\
i·~-100~·
~/
~
Transpiration rates remain somewhat constant
through the period between April and September and
then decrease in October (fig. 4). May measurements
for alkali sacaton and saltgrass show rapid transpiration rates characteristic of new growth. Rabbitbrush transpiration increased dramatically during the July rainy period. The greasewood were 75%
2500
[,~
Atmospheric Potential and
PAR vs Time
10
500
'6O~o
9.0
11.0
13.0
15.0
17.0
19.0
~
21.0
time (pst)
~
-1.0
-20
c. '"
~ ~ 1 - - - - - - - - - - - - - - - - - - - 1 -3.0
Transpiration vs Time
~.!!:
8.0
<>
g
'5'
~'~~~
5
10.0
~<I)
The two diurnal transpiration curve shapes correlate with the photosynthetic pathway inherent for
each of the five species. Photosynthetic pathways
were confirmed by review of lists of plants known
to use the C4 (dicarboxylic acid) pathway (Downton
1975. Raghavendra and Das 1978). Thin sections
were prepared to confirm the presence of "Kranz
anatomy" in leaf tissue known to indicate the C4
pathway (Huber and Sankhla 1976). Nevada saltbush,
alkali sacaton and saltgrass are C4 plants and exhibit diurnal transpiration curves which are parabolic. Rubber rabbitbrush and greasewood utilize
the C3 (Calvin-Benson) pathway and exhibit diurnal
transpiration curves with a mid-morning high and
steady decrease through the day. Leaf conductances
are initially much higher in the C3 plants but decrease below rates for C4 plants by mid-afternoon.
1~
Nevada saltbush
• rubber rabbltbrush
6.0
c
-1.0
-2.0
'---_ _ _ _ _ _ _ _ _ _ _
-""t::-_~
_ __' -
3.0
c
o
.~ 4.0
'I
-300
P',
2.0
'0------ 0
/1
'or;
3.0
5.0
7.0
9.0
11.0
13.0
15.0
17.0
19.0
j
-150
21.0
r-----------------~ 0
time (pst)
6.0
0.5
•
Leaf Conductance vs Time
'"
4.0
Q)
'"
'"
Q)
<>
2.0
Nevada saltbush
• rubber rabbitbrush
4.0
III
-
0.1
2.0
5.0
7.0
9.0
11.0
13.0
15.0
17.0
19.0
Apr
21.0
May
Jun
Jul
Aug
Sep
Oct
time (pst)
Figure 3.--Diurnal atmospheric potential, radiation (PAR) and the transpiration and stomatal
responses of two of the study plants at the
Warm Springs Site, 6-12 and 6-13, 1984.
Figure 4.--Seasonal variations in weather and
plant response.
(0) alkali sacaton,
(0) saltgrass, (.) Nevada saltbush,
(.) greasewood, (.) rubber rabbitbrush.
169
LITERATURE CITED
denuded by a blister beetle (Epicauta normalis) 4
after the July reading. The continued rise in
transpiration may reflect undepleted soil moisture
storage within the rhizosphere under this species.
Predawn xylem pressure potential measurements
showed an increase to the July measurement and then
decreased. The predawn measurements fail to show
the marked seasonal depression of xylem potential
characteristic of arid zone species (for example
see Branson et al. 1'976). The stable predawn
xylem potentials indicate a fairly abundant supply
of soil moisture in light of the transpiration
rates maintained through the season. The highest
potentials occur during midsummer, probably in response to the rainy period and low atmospheric potential. The shrub species have marked capacity
to adjust osmoticallyS which may account for both
the lower initial xylem potentials that occurred
with relatively abundant soil moisture (fig. 2)
and the reverse trend of rising transpiration
during a decreasing xylem potential evident for
greasewood during July through September.
SUMMARY
A study is being conducted in the arid Owens
Valley, California to correlate transpiration by
five native phreatophytes with soil, plant and
atmospheric variables. Measurements of soil
moisture demonstrate zones of root extraction by
du~ation and quantity.
Root density with depth
varies by season but is equal per depth among
individuals and species at each microsite.
Transpiration measurements correlate highly with
atmospheric potential and photosynthetically
active radiation. Measurements of xylem pressure
potential demonstrate that the shallow water
tables dampen the seasonal variation typical of
arid zone species.
4Derham Guiliani. 1984. Personal correspondence on file. Consulting entomologist,
Big Pine, California.
SPeter Dileanis. Pressure volume curves
and data on file. U.S.G.S. Botanist,
Sacramento, California.
170
Anscombe, F. J. 1949. The statistical analysis of
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Branson, F. A., R. F. Miller and I. S. Mcqueen.
1976. Moisture relationships in twelve
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Beardsell, M. F., P. G. Jarvis and B. Davidson.
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Coutts, M. P. and W. Armstrong. 1978. Role of
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F. T. Last (eds.) Tree physiology and yield
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Downton, W. J. S. 1975. The occurrence of C4
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Goodall, D. W. 1952. Some considerations in the
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late 1974 - mid 1977. Photosynthetica
12(2):200-208.
Ritchie, G. A. and T. M. Hinckley. 1975. The
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165-259.
St. John, I. V. and H. W. Hunt. 1983. Statistical treatment of VAM infection data.
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