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Gradient Analysis of a Sonoran Desert Wash 1
2
Peter L. Warren and L. Susan Anderson
Abstract.--Vegetation was sampled along two parallel
environmental gradients in the Sonoran Desert, one in
upland bajada sites and one in xeroriparian wash sites.
The wash gradient was found to be more complex than the
upland gradient, with three areas of plant species turnover
compared to no turnover along the upland gradient.
The
more complex pattern of the wash gradient is likely due to
the interactions of three major limiting environmental
factors related to watershed area acting in different
portions of the wash gradient, whereas the upland gradient
is controlled by one overriding environmental factor.
in turn support animals that are rare or absent in
the upland habitats (Lowe, 1964).
For these
reasons xeroriparian habitats have long
been
recognized as contributing to the biotic diversity
of desert environments in disproportion to their
area (Shreve, 1951).
INTRODUCTION
Vegetation in the deserts of southwestern
North America has been studied systematically
since the opening of the Carnegie Desert Botanical
Laboratory in 1902.
Plant ecology of this desert
region is probably better known than any similar
arid region in the world.
Much of this research
has
concentrated on patterns
of
vegetation
distribution along desert bajadas, the gently
sloping
plains
of coalesced
alluvial
fans
extending from the mountain foothills to the flat
valley floors.
Despite
their
importance
in
desert
ecosystems, xeroriparian systems have been the
subject of remarkably little research.
There are
several
reasons for this.
First,
riparian
biologists have focused most of their atten~ion on
systems with permanent water to the excluslon of
desert washes (Campbell and Green, 1968; Brown et
al., 1981; Minckley and Brown, 1984).
Second,
studies of desert vegetation distribution patterns
have focused on the upland bajada gradient and
expressly avoided xeroriparian corridors, treating
them only as a factor that might throw confusion
into the general pattern of bajada vegetation.
Thus, plant distribution patterns along desert
washes have been largely ignored by riparian
ecologists on one side and desert plant ecologists
on the other.
Desert washes dissecting the bajada plain are
easily recognizable by the corridor of vegetation
along their channels that contrasts strongly with
the sparse vegetation of adjacent uplands.
A
transect across a typical Sonoran Desert bajada
intercepts an average of approximately 14 desert
washes per mile.
At an average riparian corridor
width of 25 feet, between six and seven percent of
total bajada surface area is covered by desert
wash vegetation. The intermittant nature of these
watercourses results in less luxuriant vegetation
than is found along streams with permanent flow,
and they have been termed xeroriparian systems
(Johnson et aI, 1981).
Runoff from surrounding
slopes increases the available water in and near
the wash permitting growth of plant species not
found in the surrounding desertscrub.
The plants
In this paper we attempt to answer three
questions:
1) What are the patterns of vegetation
distribution in a xeroriparian drainage
system?
2)
How does the pattern of riparian
vegetation compare to the surrounding
bajada
vegetation
gradient?
3)
What environmental
controlling factors might account for xeroriparian
vegetation distribution?
1
Paper presented at the First North American
Riparian Conference [University of Arizona,
Tucson, April 16-18,1985].
2
Peter L. Warren is a Research Assistant at
the Arizona Remote Sensing Center, Office of Arid
Lands Studies, University of Arizona, Tucson,
Arizona.
L.
Susan Anderson is a
Research
Ecologist with the Cooperative Park Studies Unit,
University of Arizona, Tucson, Arizona.
METHODS
We examined a simple, dendritic, xeroriparian
drainage system flowing from the wes~ slopes of
the Ajo Mountains west across the AJo Va~ley in
Organ Pipe Cactus National Monument.
ThlS area
receives an average of 6-8 inches of rain per
150
year. At this level of precipitation washes carry
water only a few days per year, and in some years
may not flow at all.
Physical features recorded for each site
included elevation, watershed area, stream order,
channel width, and corridor width.
These were
determined from aerial photographs and topographic
maps, depending upon the scale appropriate to the
site.
A series of paired upland and riparian sites
were. sampled along wash and adjacent bajada
gradlents.
Elevation of sample sites ranged from
2300 feet at the foot of the Ajo Mountains at
Alamo Canyon to 1400 feet in Growler Valley.
A
total of 33 riparian and 22 upland sites were
sampled.
Fewer upland samples were taken because
some upland samples were associated with more than
one wash sample if they occupied an interfluvial
s~te
between two wash tributary samples
of
dlfferent size.
Cluster
analysis
was used
to
compare
floristic similarity between sites and determine
degree of association between groups of sites.
The similarity value is based on the euclidian
distance, the square root of the sums-of-squares
of the differences between species prominence
values for each pair of sites.
A larger value
indicates a greater degree of floristic difference. Sites with identical species composition and
prominence values would show zero difference.
A
combination of statistical and graphical analysis
was used to examine patterns of diversity and
species turnover along both the riparian and
upland gradients.
Sites were selected with the aid of aerial
photographs approximately every 1 to 2 miles along
the length of the major tributary and represented
a range of stream orders and watershed areas for
the drainage system (Johnson et al., 1984). Sites
were selected to avoid problems such anastomozing
cha~nels.
Every site sampled was characterized by
a slngle channel that carried all of the flow from
the watershed area upstream.
RESULTS AND DISCUSSION
The upland gradient follows a pattern similar
to that documented by other studies of desert
bajadas:
species diversity declines continuously
from higher to lower sites with very little
species turnover (Fig. 1A).
Species present on
higher sites decline in abundance at decreasing
elevation and disappear, but are not replaced by
new
species at lower elevations.
Only two
species, creosotebush and white-bursage, show an
increase in abundance at the lowest sites, while
the other nine common upland species all decline
(Table 1).
Few species have higher prominence at
Vegetation sampling involved compiling
a
complete plant species list for each site and
assigning each species a prominence value from 1
to 5 based on its abundance at the site. Wash
samp~es included the entire width of the
riparian
corrldor and extended approximately 100 meters
along the stream channel.
This resulted in a
greater sampling area for larger washes because
they have a wider channel and riparian corridor.
Adjacent upland bajada samples covered an area of
approximately one-half hectare.
Table 1.--Average prominence and species frequency (percent of sites
encountered) in three upland bajada elevation zones along the AlamoCherioni-Growler wash system at Organ Pipe Cactus National Monument.
These elevation zones correspond to vegetation assemblages of the
species rich upper bajada,
transitional middle bajada, and the
depauperate lower bajada.
Species
Ambrosia deltoidea
Larrea tridentata
FOUqUieria splendens
Carnegiea gigantea
Cercidium microphyllum
Opuntia fulgida
Krameria ~
Opuntia acanthocarpa
Olneya tesota
Ambrosia dumosa
Lycium an~ii
PrQSOPis glandulosa
2300-1900'
n=10
1900-1600'
n=7
1600-1300'
n=5
prom. freq.
prom. freq.
prom. freq.
4.0
3.7
2.8
3.1
2.5
2.9
1.7
1.9
1.1
0.6
0.8
0.2
100
100
100
100
100
100
80
90
50
30
50
10
151
1.6
4.6
1.3
1.7
1.4
0.9
1.6
0.3
0.9
2.0
0.3
57
100
57
71
57
43
71
14
43
71
29
1.2
5.0
0.4
1.4
0.4
0.2
0.6
0.6
0.4
2.2
40
100
20
60
20
20
60
40
40
80
BAJADA
5
~ 4 ...............................................................................:::::;> ...........................
.~ 3
E
oLa..
.•.....
....
2
.••.........
...................................................................
.
"
...... ,,'.~--- .... ...............................................
1,1,,;1 .......::'::.,.::......\....:~ ..::::::::~~::.:.:.:.::.::.........
I
. ".
. . "-.... . ....
",
-+---""""""'----.-------,----.-----'"-,-'--.--------.---\
2300
2200
2100
2000
1900
~~~
1800
1700
i···········
""- : ...... "'\"
1600
1500
1400
1300
Elevation (feet)
RIPARIAN
5
-
..... ---- .......~~ ...................... .
4
Q)
u
c
/
/
"
.".".....
--- --.....-:: ..... - - ~~..:-........
- -...
..'.
.,'
t·
-
...,.-.......
'::'.~
_-
.............. ,.::.......
..
....
.........
".
~ 3
-
".
'E
o
La...
.
e~~~L:::;;.~~~~~~~~~~~······-1.0
-2.5
-2.0
o
1.0
'.
............
2.0
Log watershed area (sq. miles)
Figure 1.--Comparison of plant species distributions along upland and
xeroriparian gradients in the Sonoran Desert.
The upland
gradient (A) exhibits a continuous decline in diversity with
decreasing elevation.
The riparian gradient (B) is more complex
with three major areas of species turnover.
152
2.5
intermediate
sites.
Growth form
diversity
parallels general species diversity. The numerous
growth forms present at the top of the gradient,
including trees, shrubs, and stem succulents,
decline to one dominant form at the bottom.
available soil moisture than the heavier siltyloam of the valley bottom by virtue of its higher
infiltration rate and lower soil water potential.
Water is likely to also be the primary limiting
factor along the xeroriparian
gradient,
however it probably does not control species distribution simply as a function of plant-available
soil
moisture throughout the gradient.
The
frequency, volume, and duration of flow along
washes of different size is a function of both
watershed area and the regional rainfall regime.
The most important and easily measured variable
affecting frequency and amount of runoff is watershed area. Therefore, we used watershed area as
our
primary axis for ordination of
species
distribution.
This pattern of decreasing vegetction diversity along bajada elevational gradients has been
documented
at numerous sites throughout
the
Sonoran Desert.
Near the southern edge of the
desert Felger (1966) studied bajadas along the
coast of Sonora. At the northeast margin of the
desert, bajada gradients have been documented in
at least three locations (Yang and Lowe, 1956;
Whittaker and Niering, 1965; Lowe et al., 1973).
Phillips and McMahon (1978) examined a bajada in
the north central Sonoran Desert, and Yeaton and
Cody (1979) studied a northwestern location at the
transition between the Sonoran and Mojave deserts.
In all of these studies the same basic pattern of
decreasing diversity with decreasing elevation was
observed.
The
riparian
vegetation gradient
shows
several important differences from the upland
gradient, suggesting that the controlling environmental factors for riparian vegetation operate
differently from those in the upland community.
The riparian gradient differs from the bajada
gradient in two major ways.
First, there is
higher species turnover and therefore
higher
between
site
diversity along
the
riparian
corridor.
Second, there is greater floristic
The primary environmental factor controlling
this pattern of decreasing diversity on desert
bajadas is available soil moisture
(Klikoff,
1967).
Under the same rainfall regime, the
coarse, rocky soil of the upper bajadas has more
Table 2.--Average prominence and species frequency (percent of sites
encountered) in four watershed area classes encountered along the
Alamo-Cherioni-Growler wash system at Organ Pipe Cactus National
Monument.
The four zones are defined by watershed area and species
distribution patterns:
Class 1 is 0.002-0.02 sq. mi., Class 2 is
0.02-0.8 sq. mi., Class 3 is 0.9-50 sq. mi., and Class 4 is 60-400 sq.
mi. For a more complete species list see Johnson et al. (1984).
Species
Class 1
n=9
Class 2
n=9
Class 3
n=11
Class 4
n=4
prom. freq. prom. freq. prom. freq. prom. freq.
Krameria ~
Cercidium microphyllum
Ambrosia deltoidea
Condalia spathulata
Acacia constricta
Sarcostemma cynanchoides
Sphaeralcea sp.
Prosopis glandulosa
Acacia greggii
Plumbago scandens
Ephedra nevadensis
Brickellia californica
Celtis pallida
Aloysia wrightii
Penstemon parryi
Zizyphus obtusifolia
Anisacanthus thurberi
Ambrosia ambrosioides
Cercidium floridum
Baccharis sarothroides
Nicotiana trigonophylla
Clematis drummondii
Hymenoclea salsola
1.7
1.3
4.0
0.8
0.9
0.1
0.1
0.2
0.3
56
67
100
44
44
11
11
11
22
153
0.8
2.8
3.2
1.0
2.9
0.1
0.3
1.2
1.8
0.4
0.8
1.6
1.2
1.0
0.4
0.3
0.3
0.3
0.3
44
89
89
56
89
11
22
44
89
44
33
67
67
56
33
33
11
22
11
0.1
0.7
0.9
0.9
0.7
0.4
1.4
3.2
3.4
9
27
54
45
36
36
73
100
100
0.4
0.4
0.3
0.9
0.9
1.1
0.9
2.6
1.5
1.0
1.1
0.3
27
27
27
67
67
54
36
82
82
54
46
27
0.5
25
0.2
1.2
1.8
3.3
3.5
25
75
75
100
100
0.5
0.5
2.2
1.2
1.5
1.2
2.0
3.5
50
50
75
50
75
75
75
100
the loss of many of the preferential and obligate
riparian shrubs that dominate the second class.
These shrubs are replaced by a group of shadetolerant
shrubs which grow under or in the
canopies of the overstory trees.
The primary
limiting factor controlling the second area of
species turnover appears to be shading by trees
and consequent loss of the shade-intolerant shrubs
that dominate the second floristic class.
differentiation between different portions of the
riparian gradient.
Plant species distribution along the continuum of watershed area shows three relatively
distinct areas of species turnover (Fig. 1B).
These turnover points, characterized by coincident
decline and loss of some species and addition and
increase of others,
occur at approximately 0.02
square miles, just under 1.0 square miles,
and
40-50 square miles.
The presence of multiple
turnover points along the xeroriparian gradient
suggests that several environmental variables are
operating to limit species distribution and that
they operate in a reciprocal fashion with different factors operating to a greater or lesser
degree in different portions of the continuum of
watershed area (Whittaker, 1967).
The fourth floristic class is characterized
by two distinct vegetated areas, the tree dominated bank~ and floodplain, and a shrub dominated
channel.
The trees that became dominant in the
third zone remain so here.
The channel itself
supports stands of scour resistant species including Hymenoclea salsola, Ambrosia ambrosioides, and
Baccharis sarothroides.
The factors limiting
species distribution in washes with the largest
watershed areas appears to be a combination of
scouring in the open channels and shading along
the heavily vegetated banks and floodplains.
A somewhat unexpected observation is that a
number of species share similar distribution patterns along the gradient, increasing, decreasing,
and reaching maximum prominence over the same
portion of the watershed area continuum.
The
result is the formation of four species assemblages,
demarcated by the areas of turnover
discussed above,
along washes with different
watershed area.
The environmental factors controlling the species composition of each of the
floristic
classes can be inferred from
the
ecological distribution of the dominant species in
each assemblage (Table 2).
The
xeroriparian gradient is much
more
complex than the upland gradient.
The average
floristic similarity index among sites in the
riparian gradient is almost twice the average for
the upland gradient, indicating a higher degree of
florisitic differentiation between wash sites. In
addition, species distributions along the xeroriparian gradient are controlled by at least three
environmental factors in contrast to the single
overriding factor along the upland
gradient.
These results suggest that ecological response
and/or recovery of xeroriparian vegetation following disturbance may be different from upland
vegetation, and generalizations drawn from studies
of upland vegetation should not be extended to
xeroriparian habitats.
The first floristic class is associated with
small washes with watershed area under 0.02 square
miles. In these small washes the dominant species
are those with predominately non-riparian distributions such as Cercidium microphyllum, Ambrosia
deltoidea, and Krameria~.
Although a few
preferential riparian species are found in these
washes, the watersheds are too small to substantially increase available moisture above that
supplied by ambient precipitation. These drainages
probably do not flow every year and cannot support
many preferential or obligate riparian species.
CONCLUSIONS
The gradient of xeroriparian vegetaiion along
a Sonoran Desert wash is more complex than the
adjacent upland gradient because it is controlled
by
several interacting environmental
factors
related to watershed area,
while the upland
gradient is controlled by one overriding factor.
The riparian corridor shows three areas of species
turnover and four distinct
vegetation classes
controlled by frequency and amount of runoff,
shading,
and
channel scouring mitigated
by
watershed area.
The second floristic class is dominated by
preferential and obligate riparian shrubs such as
Acacia constricta,
Celtis pallida, Brickellia
californica, and Aloysia wrightii.
This first
major species turnover is probably controlled by
the watershed area threshold at which increased
moisture is reliably greater than that supplied by
ambient precipitation.
These larger washes, with
watershed areas greater than 0.02 square miles,
probably flow almost every year. The increased
runoff Rllows faster-growing riparian shrubs to
replace the non-riparian species which dominated
the smallest washes.
For these reasons, riparian vegetation will
likely respond differently to disturbance than
surrounding
uplands.
Riparian vegetation is
subject to high levels of natural disturbance and
may recover more rapidly than upland vegetation.
However, it may be more difficult to evaluate the
extent of recovery of riparian vegetation because
the expected natural diversity is the result of a
complex interaction of watershed area above a
specific site, and the regional precipitation
regime.
Recovery of vegetation structure in
xeroriparian habitats may be rapid compared to
recovery of species diversity.
The third floristic class includes moderate
to large watersheds with areas from just under 1
square mile up to approximately 50 square miles.
The riparian corridor of these washes is dominated
by trees, mostly Prosopis glandulosa, Cercidium
floridum,
and Acacia greggii.
Two of these
species, Prosopis and Acacia, are present in shrub
form in smaller washes, ~ut only achieve tree size
in washes with watershed areas greater than 1
square mile. Species turnover associated with the
shift to dominance by trees is characterized by
154
Lowe, C.H. 1964. Arizona landscapes and habitats.
In:
C.H.
Lowe, ed. The vertebrates of
Arizona. Univ. Ariz. Press, Tucson.
ACKNOWLEDGEMENTS
Several of the ideas presented here were
developed during discussions with R.R. Johnson and
C.H. Lowe. R.R. Johnson participated in the field
work and the Organ Pipe staff, especially Dick
Anderson and Bill Mikus,
provided logistical
assistance.
This research was supported by the
Cooperative National Park Resources Studies Unit
at University of Arizona.
Lowe, C.H., J.H. Morello, G. Goldstein, J. Cross
and R. Newman. 1973. Analis comparativo de
la, vegetacion de los desiertos subtropicales
de
Norte y Sud America (Monte-Sonoran).
Asociacion Argentina de Ecologia, Ecologia.
1:35-43.
Minckley, W.L. and D.E. Brown. 1984. Wetlands.
In: D.E. Brown. ed. Biotic communities of the
American Southwest-United States and Mexico.
Desert Plants. 4:223-287.
LITERATURE CITED
Brown, D.E., N.B. Carmony and R.M. Turner. 1981.
Drainage map of Arizona showing perennial
streams and some important wetlands.
Ariz.
Game and Fish Dept., map scale 1:1,000,000.
Phillips, D. and J.A. McMahon. 1978. Gradient
Southanalysis of a Sonoran Desert bajada.
west Naturalist. 23:669-680.
Campbell, C.J. and W. Green.
1968. Perpetual
succession of stream-channel vegetation in a
semiarid region.
Jour. Ariz. Acad. Sci.
5:86-98.
Shreve,
F.
Desert.
192 p.
1951.
Vegetation of the Sonoran
Carnegie Inst. Washington, Pub. 591,
Whittaker,
R.H.
1967.
Gradient analysis
vegetation. BioI. Rev. 42:207-264.
Felger, R.S. 1966. Ecology of the gulf coast and
islands of Sonora.
Ph.D.
Dissertation.
Univ. Ariz., 460 pp.
of
Whittaker, R.H. and W.A. Niering. 1965. Vegetation of the Santa Catalina mountains, Arizona
(II).
A gradient analysis of the south
slope. Ecology. 46:429-452.
Johnson, R.R., S.W. Carothers and J.M. Simpson.
1981. A riparian classification system. In:
Warner and Hendrix. eds. California riparian
systems:
A conference on their ecology,
conservation
and
productive
management.
Univ. Calif., Davis. Sept. 17-19, 1981.
Yang, T.W. and C.H. Lowe. 1956. Correlation of
major vegetation climaxes with soil characteristics in the Sonoran Desert.
Science
6:41-45
Johnson, R.R., P.L. Warren, L.S. Anderson and C.H.
Lowe.
1984.
Stream order in ephemeral
watercourses: a preliminary analysis from the
Sonoran Desert. Hydrology and water resources
in Arizona and the Southwest--Proceedings of
the American Water Resources Association,
Arizona section. 14:89-100.
Yeaton, R.I. and M.L. Cody. 1979. The distribution of cacti along environmental gradients
in the Sonoran and Mojave deserts.
J.
Ecology. 65:529-541.
Klikoff, L.G. 1967. Moisture stress in a vegetational continuum in the Sonoran
Desert.
Amer. Midland. Natur. 77:129-137.
155