SHRUB ROOTING CHARACTERISTICS
AND WATER ACQUISITION ON XERIC
SITES IN THE WESTERN GREAT BASIN
Sara J. Manning
David P. Groeneveld
has data to support his hypothesis. In arid lands where
plants are often widely spaced, light is rarely a limiting
resource while water, due to low precipitation, is. It can
thus be hypothesized that within arid environments the
two most important resources that plants compete for are
water and nutrients. Since water is typically responsible
for mobilization and uptake of nutrients, the low water
availability on xeric sites we have examined may directly
affect nutrient acquisition. Therefore, in this paper, we
have focused on water as a primary limiting resource. It
should be recognized that both water and nutrients are
resources acquired by plant root systems, and it is the
paucity of knowledge ofbelowground features for desert
plants that severely limits differentiating between the
effects of nutrients and water.
Data on root systems are difficult to obtain. Excavations are labor intensive and costly, and reporting of findings in a meaningful manner has yet to be standardized.
Many studies report root morphologies using sketches
drawn to scale (see, for example: Cannon 1911; Cody
1986a; Spence 1937). Data on maximum rooting depth,
length of roots per volume soil, changes in root density
as depth and distance from the plant's main axis increases, and degree of suberization of root tissue are just
some useful pieces of information necessary to complete a
picture of belowground phenomena.
Here we report our observations on root systems of a
number of shrubs occurring on alluvial fans of the Owens
Valley, CA. Next, we report on a specific study of competition for water between two frequently co-occurring
shrubs with very different root systems. Finally, we summarize our findings by proposing a correlation between
root system morphology and shrub phenology and by
speculating on the role of roots in community dynamics.
ABSTRACT
Competition for limited soil water and nutrients may
be hypothesized to give rise to root morphologies adapted
to survival on xeric sites. To test this hypothesis, root systems were excavated for a number of shrubs occurring on
the alluvial fans in the Owens Valley, CA, including
Haplopappus cooperi (Cooper goldenbush), Chrysothamnus teretifolius (needleleafrabbitbrush), Tetradymia axillaris (longspine horsebrush), Artemisia tridentata
(big sagebrush), Purshia glandulosa (desert bitterbrush),
Hymenoclea salsola (white burrobrush), and Ephedra
nevadensis (Nevada ephedra). Root system morphologies
were species specific and predictable. Two examples of
divergent rooting strategies, H. cooperi, and C. teretifolius,
representing shallow, highly branched versus deeper taproot systems, respectively, were chosen for more intensive
ecophysiological investigation. Phenologic timing and
response to selective removal were consistent with a hypothesis that deeper rooting provides buffering against
water deficit. Both species initiated growth contemporally, but in H. cooperi, flowering rapidly proceeded,
while C. teretifolius did not flower until fall. Water potential~ ofH. cooperi were shown to be affected by neighboring shrubs, but under similar densities C. teretifolius
water potentials showed no effect.
A correlation has been observed between root morphology and flowering time for each of the other co-occurring
species excavated. The authors, therefore, propose that the
root systems of these species are fitted to a particular ecological stratagem. On numerous Great Basin sites this
suite of species can be found in associations of variable
composition. Set rooting patterns that are unique to each
species, such as those exhibited by H. cooperi and C. teretifolius, may permit these shrubs to avoid direct competition
and to coexist under the limiting conditions imposed by
their arid environment.
SITE DESCRIPTION
All excavations were carried out on east-facing alluvial
fans, at the foot of the Sierra Nevada and on the west
side of the Owens Valley, CA. The average elevation was
1,300m.
Precipitation is low on these sites; the average annual
precipitation, as recorded at the nearest weather station
in Bishop, CA, is 142 mm per year, and three-quarters of
this falls between October and April (NOAA 1988). It is
uncommon for the water table on the alluvial fans to be
high enough to contact the root zone, and by comparison
to many locations that are affected by shallow groundwater, we saw no evidence of a water table. Therefore,
we believe these shrubs rely solely on water from
precipitation.
INTRODUCTION
Rooting characteristics are critical to our understanding
of arid shrub communities. Tilman (1988) claims that the
two most important resources for which plants compete
are light and nutrients, and for mesic environments he
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.
Sara J. Manning and David P. Groeneveld are with the lnyo County
Water Department, 301 W. Line Street, Bishop, CA 93514.
238
This file was created by scanning the printed publication.
Errors identified by the software have been corrected;
however, some errors may remain.
Temperatures are hot in the summer and cold in the
winter, with mean high and low daily temperatures for
July and January of 36.4 oc and 13.5 °C, and 11.6 °C and
-5.9 °C, respectively (NOAA 1988).
Soils on the sites have poorly developed profiles, are
rocky, and have low nutrient contents (Manning and
Barbour 1988).
ROOT SYSTEMS AND WATER
ACQUISITION IN TWO FAN SPECIES
We hypothesize that fan shrubs have different niches,
which can be delineated in part by root system morphology. Though quantitative traits of roots, such as maximum depth and number oflaterals, will vary among
members of the same species, the overall morphology
proves to be consistent within a species. The alluvial
fan, thus, proves to be a habitat in which many species
of shrubs with similar aboveground characteristics have
belowground features enabling them to exploit environmental resources differently.
Community dynamics on the alluvial fan could also be
influenced by belowground phenomena. Cody (1986b), for
example, presented data on spatial arrangement of many
perennials in the Mojave Desert. Among both plants of
the same species and of different species, he found positive associations to be more common than negative associations and random assemblages to be least common.
He concluded that the frequent positive associations he
observed could develop because the plants involved had
root systems that did not overlap and therefore were compatible with each other. Competition for water between
two species with different root morphologies may be negligible; therefore, their roots would exploit different soil
layers where water availability may not be the same.
Two of the species excavated in our study, H. cooperi
and C. teretifolius, were examined for evidence of both
interspecific and intraspecific competition for water.
These two relatively small composites commonly co-occur
on the alluvial fans of the Owens Valley. Similar timing
ofleader growth in the spring, similarities in general
aboveground morphology, and close systematic affinities
suggested the possibility of competition for resources,
such as water. However, differences in flowering time
and in root morphology implied a low to insignificant
degree of interference between species. We performed
a shrub-removal experiment to determine the presence
of competition between these two species.
ROOT EXCAVATIONS AND
OBSERVATIONS
Methods
Pits were dug adjacent to 10 fan shrub species using a
backhoe. Once the pits were opened, water was sprayed
onto the roots to remove adhering soil. Root systems of
each shrub excavated were identified and photographed.
Sketches were then made from 35-mm slide transparencies by camera lucida technique.
Results
Root system sketches of the 10 shrubs excavated appear
in figures 1 through 10. All are drawn at the same scale.
Ephedra nevadensis (Nevada ephedra) (fig. 1) has thick,
woody roots which do not grow much deeper than 0.5 m,
but which do spread laterally and produce clones.
Roots of Grayia spinosa (spiny hopsage) (fig. 2) are
shallow and diffuse, and there is no obvious taproot. This
particular individual was growing through the center of a
Chrysothamnus teretifolius (needleleafrabbitbrush)
shrub. Grayia spinosa frequently utilizes other shrub
species as nurse plants, particularly in areas of heavy
grazing.
Artemisia spinescens (bud sagebrush) also has a shallow, diffuse root system (fig. 3).
Coleogyne ramosissima (blackbrush) roots grow deeper
than those of the previous shrubs, but again the root system is diffuse (fig. 4). The individuals shown appear to be
clones which could have arisen by mechanisms of stem
splitting as described by Ginzburg (1963).
The root system of Haplopappus cooperi (Cooper goldenbush) (fig. 5) is similar to that of C. ramosissima: relatively shallow and diffuse.
Hymenoclea salsola (white burrobrush) possesses a
relatively short taproot with prominent laterals (fig. 6).
Tetradymia axillaris (longspine horsebrush) (fig. 7) also
has a taproot. Laterals emerging from the taproot show
strong downward growth.
Purshia glandulosa (desert bitterbrush) has a thick
taproot (fig. 8). Near-surface laterals were not evident.
The root system of our specimen of Artemisia tridentata
(big sagebrush) (fig. 9) began as a tap, but spread laterally
quite near the soil surface. The thick lateral roots then
turned downward some distance from the center of the
shrub.
Chrysothamnus teretifolius (needleleafrabbitbrush)
displays a thick taproot with prominent laterals (fig. 10).
Methods
A site dominated by Haplopappus and Chrysothamnus
was chosen on an alluvial fan. Circular quadrats (r = 1 m)
were randomly located and data on frequency, density,
and cover were recorded. A Poisson distribution for the
data was determin~d and a chi-square goodness of fit test
was run to assess association among members of the same
species. Association between the two species was determined with a contingency table (Manning and Barbour
1988).
Twenty-four shrubs of each species were selected as the
experimental or "target" shrubs for the removal experiment. Criteria for selection included large enough size
so as not to be harmed significantly by repeated sampling
and the presence of at least three members of each species
surrounding the target shrub within a 1.8-m radius. One
of four treatments was then applied to each target shrub:
I. All neighbors removed,
II. Chrysothamnus neighbors removed,
239
Scale: 3.5 em
=1 m
Figure 1-Ephedra nevadensis.
Figure 2-Grayia spinosa.
Figure 3-Artemisia spinescens.
Figure 4-Co/eogyne ramosissima.
Figure 5-Hap/opappus cooperi.
Figure 6-Hymenoc/ea sa/sola.
240
Scale: 3.5 em
=1m
Figure 7-Tetradymia axillaris.
Figure 8-Purshia glandulosa.
/
Figure 1~Chrysothamnus teretifo/ius.
Figure 9-Artemisia tridentata.
241
III. Haplopappus neighbors removed, and
IV. Control: no neighbors removed.
0
-I
Shrubs were removed in early spring, 1986. Predawn
water potential of all target shrubs was then measured
throughout the growing seasons of 1986, 1987, and 1988.
Leader growth, leaf senescence, and extent of flowering
were also monitored in these years by recording length,
number ofleaves, and number of flowers on 30 randomly
located branches of each target shrub.
....
•
t:
..,•
~
~
~
Ia
~
-2
~
-3
~
...
...
-4
-s
-6
HACO I
HACO II
HACO II!
HACO! V
CWE
CI-4TE
CHTE
CHTE
I
II
Ill
!V
-7
Results and Discussion
-I
Haplopappus proved to have a random distribution on
the site, while Chrysothamnus exhibited a clumped distribution. The two species were randomly associated with
each other (Manning and Barbour 1988). A review of the
literature shows that it is difficult to draw conclusions on
the interactions between species from aboveground vegetation sampling alone. Cody (1986b) suggested that a
clumped distribution indicated compatibility among the
plants, but he did not present physiological data on plant
interactions. Fonteyn and Mahall (1981) found no measurable competition for water among their clumped Ambrosia (ragweed) shrubs; however, Ehleringer (1984) and
Robberecht and others (1983) found significant interaction
for water among the clumped species in their studies. Age
of the plants involved most likely accounted for the mixed
results of these studies: young plants could be actively
interacting with neighbors, while older plants may have
already established dominance on a site at the expense of
some neighbors.
From early spring to late summer, predawn water potentials of both shrub species became progressively lower
(fig.11). Haplopappus water potentials fell much lower
than Chrysothamnus water potentials. Haplopappus
water potential also was affected by presence of neighbors,
though neighbor effects appeared to become less pronounced with time. In 1986, Haplopappus control shrubs
had significantly lower water potentials than Haplopappus shrubs around which all neighbors were removed,
while water potentials of Haplopappus shrubs in the partial-removal treatments (II and III) remained intermediate between the other two treatments. Chrysothamnus
shrubs showed no effect of neighbors on either its own or
the other species in the years examined.
Leader growth and degree of senescence paralleled the
predawn water potential results (see table 1). At the end
of the 1986 season, the leaf senescence rate for control
Haplopappus shrubs was higher than that of any treatment, and by 1987, branches on Haplopappus control
shrubs had the least average leader growth of any of the
experimental shrubs.
Evidence for competition is consistent with root morphology of these two species. A review of figure 5 shows
Haplopappus to have a shallow, diffuse root system. The
shrubs excavated had roots growing no deeper than 1.2 m.
There is no obvious taproot, and laterals begin proliferating into the soil at 10-cm depth. Chrysothamnus, as seen
in figure 10, has a thick taproot which, in the shrubs excavated, was still 14 mm in diameter at 1.8-m depth. Most
of the laterals branch from the tap at approximately
50-cm depth, and laterals continue to emanate from the
taproot at lower depths.
....
•
~
-•
-2
-3
~
~
-4
~
-5
Ia
~
-6
-7
-I
....
-2
.....
-3
•
~
~
L
•Ia
Ia
~
-4
-5
-6
-7
-8
90
120
ISO
180
Day
210
240
270
300
or year
Figure 11-Results of predawn xylem potential
(XPP, in MPa) measurements for all target shrubs
of the removal experiments, 1986-88. Ordinate is
day of year. Open symbols correspond to Hap/apappus cooperi (HACO) and closed symbols
represent Chrysothamnus teretifolius (CHTE).
Roman numerals signify the removal treatment
applied: I =all neighbors removed, II = CHTE
neighbors removed, Ill = HACO neighbors removed, IV =control.
Removal experiment results clarify the ecological role
of the root systems of each shrub species. Haplopappus
has a shallow root system capable of taking up water and
nutrients from the upper layers of soil. Since most of the
soil water is replenished in the winter months, Haplopappus adds new growth and flowers in the spring. By the
end of the summer, upper soil layers are often quite dry
(data presented in Manning and Barbour 1988) and water
potentials of plants surviving in this soil tend to reflect
the soil water potential and are extremely low. Water in
upper soil layers may become limiting before growth and
flowering are complete, and thus presence of neighbors
around Haplopappus reduces an individual's ability to
grow and speeds annual senescence.
Chrysothamnus root morphology enables it to exploit
water, not only in the region of Haplopappus roots, but
242
Table 1-Hap/opappus cooperi (HACO) and Chrysothamnus teretifolius (CHTE) growth and senescence responses to removal
SUMMARY
treatments. Values shown are averages of the six shrubs
per treatment plus or minus standard deviation. Letters in
common (a,b,c) denote no significant difference among
treatments for that species
Treat·
ment
1987growth
HACO
CHTE
The Owens Valley is a transition area. Moving northward through the Owens Valley, vegetation changes from
that more characteristic of the Mojave Desert, a warm
desert, to vegetation more commonly associated with the
Great Basin, a cool desert. Precipitation in both deserts
falls mainly in the winter months; and, in the Great
Basin, a majority of this precipitation is snow. The xeric
fan shrubs we have excavated in the Owens Valley are of
both Mojave and Great Basin origin.
In the Owens Valley, winter temperatures are cold,
and growth does not usually occur before late February.
About the time that growth begins, the period of maximal
likelihood for precipitation is completed, and shrubs must
then rely on water already absorbed by the soil during the
winter to complete their annual growth and reproduction.
The alluvial fan soils are sandy and contain numerous
cobbles and rocks; thus they have a low field capacity.
Precipitation falling on these coarse soils during the
period of lowest evapotranspiration readily percolates
to increase soil water storage.
Since xeric shrubs depend on winter-precipitationderived soil water storage to carry out their growth andreproduction cycles, those with shallow root systems must
complete their cycles when both the near-surface soil water
is available and temperatures are conducive to growth. In
the Owens Valley, these coincide during the spring.
All the alluvial fan shrubs, with the exception of,
perhaps, P. glandulosa, have a portion of their roots in
the uppermost 0.5 m of the soil. Nitrogen in desert soils
19861eafsenescence
HACO
CHTE
- - - - - - Millimeters - - - -
- - - - - - - - Percent - - - - - - -
5.62±3.00a 6.74±7.26a
38.83±8.46a
35.78±10.39a
39.08±6.21 a
II
2.01±0.97b 4.01±3.02a
46.00±16.59a
Ill
2.08±0.58b 1.60±1.53a
51.52±16.08ab 30.90±14.19a
IV
1.50±0.67b 2.06±2.33a
67.66±13. 75bc 34.15±16.88a
also in deeper soil layers. Therefore, Chrysothamnus can
initiate growth at approximately the same time as Haplopappus, grow, maintain this spring growth into late summer, and then flower in early fall. If soil water is not
limiting at the depths to which Chrysothamnus roots
grow, or if the density of roots at these depths is low, then
removal experiments would not show competition for
water between Chrysothamnus shrubs. Chrysothamnus
may take water away from Haplopappus since there is
some overlap of the root systems, but Chrysothamnus still
has access to other water while Haplopappus does not.
Table 2-Root
system morphology and flowering time of the shrubs excavated on the Owens Valley alluvial fans
Species
Family
Root system
Flowering time
Ephedra
nevadensis
Gnetaceae
shallow
March-April
Grayia
spinosa
Chenopodiaceae
shallow, diffuse
March-June
Artemisia
spinescens
Asteraceae
shallow, diffuse
April-May
Co/eogyne
ramosissima
Rosaceae
rei. shallow,
diffuse
April-June
Haplopappus·
cooperi
Asteraceae
rei. shallow,
diffuse
March-June
Hymenoc/ea
sa/sola
Asteraceae
taproot
March-June
Tetradymia
axillaris
Asteraceae
taproot
April-May
Purshia
glandulosa
Rosaceae
taproot
April-June
Artemisia
tridentata
Asteraceae
taproot
August-October
Chrysothamnus
teretifolius
Asteraceae
taproot
September-November
243
has been shown to decrease exponentially with depth
(West and Klemmedson 1978), and Groeneveld (these
proceedings) has found root density to follow a similar
distribution. We believe that the near-surface roots are
essential for nutrient uptake. Thus, co-occurring plants
compete for the relatively nutrient-rich, near-surface
soil water. This resource, therefore, tends to be depleted
comparatively rapidly with the effect that shrubs with
shallow roots are usually forced into dormancy by early
summer. By contrast, deeply rooted shrubs growing in
the same habitat may have a longer period over which to
carry out the same physiological activities since they have
access to water stored at depth.
Our study of Haplopappus and Chrysothamnus provided a good example of adaptations provided by different
root system morphologies. Both species relied on the
near-surface water for leader and leaf growth. Presence
of neighbors near Haplopappus influenced its xylem water potential and its growth, while near neighbors around
Chrysothamnus appeared not to cause measurable responses in target Chrysothamnus shrubs. Haplopappus
leaves senesced by late summer, while Chrysothamnus
leaves remained viable until the end of summer when the
shrub flowered. The influence ofbelowground factors on
such phenologic characteristics demonstrates how two
shrubs of close taxonomic affinity and similar aboveground morphology have adapted differently to the same
environment.
For the shrubs in our study, there appears to be a qualitative correlation between type of root system and time of
flowering (see table 2). Neither of the late-season flowering shrubs have shallow, diffuse root systems; they have a
taproot and prominent laterals emerging at some depth in
the soil. Among the spring-flowering shrubs, most have
shallow root systems (E. nevadensis, G. spinosa, A
spinescens, C. ramosissima, Hymenoclea salsola, and
Haplopappus cooperi).
A deep root system does not preclude a shrub from
flowering in the spring. As an example, T. axillaris has a
relatively deep taproot, but flowers during April and May.
In the Owens Valley, this shrub is usually found low on
the fans where soils tend to be less rocky and quite sandy.
Though we do not have soil water data for sites occupied
by this species, it is possible that water is not held very
long in the soil even at the depths to which these root
systems penetrate. Another exception to the trend of deep
rooting/late season flowering is P. glandulosa, which flowers in spring, but has the deepest trending taproot system
that we observed. Witl}in the Owens Valley this species
has a more montane distribution. When it is found on the
alluvial fans, it tends to grow at higher elevations or in
washes, suggesting that it has a higher water requirement than some of the other fan shrubs. Furthermore,
the paucity of near-surface roots in P. glandulosa and its
association with a nitrogen-fixing actinomycete (Torrey
1978) may serve to reduce its dependency on nutrients in
upper soil layers. Including P. glandulosa in a comparison of xerophytic fan shrubs may be imprecise, since its
rooting ecology is quite different.
Root systems conceivably influence shrub distribution
as well as community dynamics on a site. As Cody
(1986b) hypothesized, deep-rooted shrubs may be more
compatible with members of their own species as well as
with other deep-rooted species, and thus may be found in
positive associations. This clumping could occur because
of nonlimiting soil water at depth and heterogeneous
terrain, upon which seedlings are variably successful.
Shallow-rooted species, with short time periods in which
to carry out their physiological activities, would tend to
compete for water and may exhibit negative distributions
on a site since such competition could act to eliminate
individuals with access to fewest resources.
A study of community dynamics cannot focus on a
single factor; root morphology alone cannot explain all
the complex phenomena occurring in a shrubland. Other
factors responsible for community composition include,
and are not limited to, sera} stage, seed dispersal processes, seed germination requirements, and herbivory.
Nevertheless, we believe that root systems must be examined for a thorough study of community dynamics in
aridland communities.
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