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Distribution and Abundance of Plants in
Colorado Plateau Hanging Gardens
James F. Fowler and Nancy L. Stanton 1
Abstract - Hanging gardens are rare, mesophyticlhydrophytic habitats
associated with springs on xeric canyon walls of the Colorado Plateau. We
surveyed regional distribution and community importance of vascular plant
taxa in 48 hanging gardens of Zion and Capitol Reef National Parks, and
Glen Canyon National Recreation Area (GLCA). Regional metapopulation
distributions of 144 plant species are not bimodally distributed as predicted
by Hanski's- (1982) core-satellite hypothesis but instead have a log series
distribution (Chi square = 12.31, df = 143, P = 1.0). Distribution and
abundance of ten species endemic to the plateau ranged from one plant at
one site for Sphaeromeria ruthiae to a mean of 22% canopy coverage at
18 sites for Cirsium rydbergii. Species area curves for ZION hanging garden
plant assemblages are significantly different from GLCA's (t(.05)3,36=2.12,
P=.04S) due to Zion's higher species richness; however, the slope values
(z= 0.17-0.18) are not significantly different. A higher number of endemic
taxa and lower species richness indicates that GLCA's hanging gardens are
more isolated biogeographically than ZION's. Since these habitats are rarely
directly disturbed, management objectives in maintaining biodiversity at
these sites should be directed toward groundwater recharge area and long
term monitoring of plant distribution and abundance.
One can pass from the stark reflection of heat and glare off
slickrock into a shaded, cool, moist habitat of trickling water,
maidenhair fern, columbine, orchids, and lush glaSses and
sedges. The source of water is small springs and seeps that issue
from sandstone aquifers intersected by the canyon system of the
plateau. Most hanging gardens are afforded at least some
protection from wind and sun by their location in depressions
or larger alcoves on canyon sidewalls or deep within canyon
headwalls. Seveml Colorado Plateau endemic plant species have
been found in these habitats (Welsh 1984, 1989a, 1989b). Our
research objectives were to compare the distribution and
abundance of vascular plants in these hetbaceous hanging garden
plant communities which develop around seeps and springs in
Zion and Capitol Reef National Pruks (ZION and CARE) and
Glen Canyon National Recreation Area (GLCA).
INTRODUCTION
The Colorado Plateau geologic province haIbors isolated
hanging garden habitats within its entrenched drainage system
(Welsh 1989a, Welsh and Toft, 1981). A suite of sedimentary
geologic fonnations consisting of alternating sandstones and
silty-sand to mudstones is exposed over much of the Colorado
Plateau. This characteristic stratigraphy, exposed through
dissection by the incised drainages of the plateau, creates the
world-famous aesthetics of the canyon country. Hanging gardens
have been described as part of canyon country since the earliest
recorded explomtions of the region; e.g., Glen Canyon was
named by Powell for the "oak glens" surroWlding hanging
gardens on the Colomdo River canyon walls. The contrast
between a hanging garden and the surrounding xeric habitat or
plantless slickrock can be striking, not only in terms of
vegetation, but also in ambient air tempemture and humidity.
METHODS
James F. Fowler is Biology Instructor at State Fair Community
College, Sedalia, MO and Nancy L. Stanton is Professor of Zoology,
University of ~oming, Laramie, WY.
1
We define hanging gardens by the predominance of
mesophytic and hydrophytic hetbaceous vegetation (Tuhy and
MacMahon 1988, Stanton et al. 1992) growing on wet rock
315
length to support linear hydrophytic plant communities in cracks
and in narrow strips of wet, saturated soils. The vegetation is
intennixed with thin sheet flow of water over wet rock surfaces.
Ledge-soil complexes are composed of wet colluvial soils
located just up slope of stabilizing ledges, including wet soil
underneath an alcove. Since these soils develop directly from
the weathering of sandstone, they are obviously very sandy and
may be virtually saturated near seeps. Much of the characteristic
herbaceous hanging garden vegetation is found on these wet
colluvial soils. Seeplines are drier, linear microhabitats that
develop at fractures in the sandstone or on impervious bedding
planes on canyon walls and at the back of drier alcoves. Vrrtually
all water in this microhabitat is used by the vegetation or lost
to evaporation. Excess water for sheet flow on rock surfaces is
not available. The distribution and areal extent of wet rock and
colluvial soils on a garden depend on the physical attributes of
the site including relative discharge of the seep or spring, facies
characteristics, and location within the drainage. The resulting
microhabitats fall into four major categories as listed in the
methods section: wet wall, ledge, soil-ledge complex, and
seepline.
Regional distribution of the 144 species of vascular plants
found on 48 hanging gardens in ZION, GLCA, and CARE fit
a log-series distribution pattern (fig. 1) (Williains 1964 as cited
in Krebs 1989) (Chi square = 12.31, df = 143, P = 1.0). In
addition, the log-series pattern fits all within parle distributions
(Chi square = 12.41-GLCA, = 7.17-ZION, = 1.76-CARE, P =
1.0). Most vascular plant species were found on only one or
two sites; for example, 56 of the 144 species were found in only
one hanging garden and 15 were found on only two. At the
other extreme, only the maidenhair fern, Adiantum
capillus-veneris, was found on most hanging gardens (47 out of
48).
walls and on wet colluvial soils with moisture supplied by
aquifer water via wide seep(s) (Stanton et al. 1992). They are
apparently microciimatically isolated from the surrounding
slickrock and xeric vegetation.
Data collection on the physical pammeters of each hanging
garden included mean aspect, elevation, size, and UTM
coordinates. Measurements taken to detennine size were part of
the vegetation sampling grid. Abundance of plant species was
determined by Daubenmire (1959) canopy cover classes (1-6)
using a 20 cm x 50 cm sampling frame. A systematic sample
(n=10) with random starts (Manly 1989, Krebs 1989) was taken
on each hanging ganlen microhatiitat accessible by non-technical
climbing. Gardens were also searched for plant species which
may not have been detected in quadrat sampling. Voucher
specimens for each plant species present on each hanging ganlen
were collected and identified by the staff at Rocky Mountain
Herbarium in Laramie, WY.
RESULTS
Herbaceous hanging garden vegetation develops either
directly in cracks on moist rock swfaces, or on colluvial soils
supplied with subsoil moisture from the seep. Many seeps issue
from the transition between the Navajo sandstone and the
Kayenta Fonnation in these three parks. Others issue from
impenneable facies within the Navajo sandstone. Each hanging
garden was visually separated into the following microhabitats:
wet walls, ledges, ledge-soil complex, and seeplines. Wet walls
include slopes and floors covered with thin sheet flow of water
as well as vertical walls and are dominated by ferns, prokatyotic
and protistan communities. Ledges are of sufficient width and
144 species on 48 hanging gardens
60~----------------------------------------------------------~
F
50
r
e
q
40
U
30
e
n
20
c
y
10
o
10
30
20
47
Number of hanging gardens occupied
Figure 1. -
Histogram of hanging garden vascular plant species regional distribution in ZION and GLCA.
316
Table 1. -
Plant species richness and physical parameters of hanging gardens in Zion National Park: size in m 2, aspect In
degrees, elevation In ft., location in UTM coordinates. R Richness.
=
HanglngGarden
Pine Creek
Upper Emerald
Lower Emerald
Grotto
Menu Falls
Fall
Falling Weter
Narrows Trail
Trail's End
Canyon Overlook I
Canyon Overlook II
Court Patriarchs
Snail
Kaye's
Weeping Rock
Hailstone
Table 2. -
Size
1170
131
628
190
274
420
383
226
4
70
99
61
124
812
28
R
Aspect
344°
120°
1582
1SO
218°
31°
318°
259°
303°
238°
124-242°
1n°
270°
192°
196°
100°
6
33
12
28
23
14
21
26
13
14
26
20
13
29
28
13
Elevation
4200
4700
4300
4600
4500
4500
4600
4500
4500
5250
5250
4750
4500
4600
4500
4700
UTM X
UTM Y
325849.8
326061.4
326166.9
327434.5
324416.6
328178.1
326982.4
327489.1
327397.1
327979.7
327878.7
325531.3
327713.9
327134.3
328346.6
327052.6
4120485.0
4125225.0
4124882.0
4124914.0
4127546.0
4126268.0
4123753.0
4128541.0
4128715.0
4120384.0
4120338.0
4123556.0
4127987.0
4126287.0
4126609.0
4126318.0
Plant species richness and physical parameters of hanging gardens in Glen Canyon National Recreation Area:
size in m 2, aspect In degrees, elevation in feet, location in UTM coordinates. R Richness.
HangingGarden
Dune
Crossbed
Rattle-sna ke
Hardwood
Pedestal
Zephyr
Graffiti
Upper Three
Lower Three
Surprise
Ivy
Baby
Baby Too
Zigy
Hook
Hawk
Swallow
Ice
Comer
Channel
Barbara
Marla
Stone Basin
Wrong
Boon-doggle
Camp
Pyro
=
Size
173
115
836
11n
269
81
44
825
162
150
70
35
38
1215
351
193
52
893
249
714
344
86
115
35
17
341
37
R
18
22
20
29
15
16
7
15
6
9
7
10
10
30
27
12
11
18
13
11
21
14
16
11
11
19
21
Aspect
90°
201°
356°
155°
160°
174°
175°
230°
230°
120°
233°
263°
205°
186°
212°
243°
7°
238°
112°
156°
60°
138°
109°
195°
5°
202°
317
Elevation
4000
4040
3800
3840
3800
3800
3800
4100
3840
3880
3840
3760
3920
3880
4160
4160
3880
4050
4050
3920
3980
3850
3950
3840
3800
3800
3800
UTM X
525718.3
525855.6
525142.8
526840.1
511965.1
512511.3
512332.5
510509.6
510372.5
527149.5
527418.3
527633.3
527706.3
514304.1
515549.0
516034.8
514926.3
526533.3
526583.3
526583.4
506610.6
505060.1
505925.5
504665.9
538226.2
537350.4
568255.1
UTM Y
4124789.0
4124822.0
4122410.0
4125133.0
4127387.0
4127486.0
4127398.0
4116716.0
4116613.0
4125485.0
4125696.0
4125514.0
4125909.0
4123519.0
4123938.0
4122757.0
4122935.0
4127594.0
4127625.0
4127412.0
4139277.0
4139649.0
4141012.0
4139096.0
4159463.0
4158251.0
4159430.0
distnbution records for the species and for GLCA. They are
isolated locations in Pictograph, Ribbon, and Knowles Canyons
respectively which show little, if any, sign of visitation Each of
the stands had many individual plants of this species and appear
to be vigorous. Disjunct populations of American spikenard,
Aralia racemosa, and cliff jamesia, Jamesia americana var.
zionis, were found on two ZION hanging gardens: Falling Water
and Grotto. American spikenard had an 8% average canopy
cover on ledge microhabitats as well as several individual plants
on both hanging gardens. Cliff jamesia had much fewer
individuals and did not show up in community samples. Zion
tansy, Sphaeromeria ruthiae, was collected only at Falling Water
HG. This rare species is endemic to Washington County and
previously noted only in ponderosa pine communities (Atwood
et aI. 1991). At the other extreme are a few hanging gardens in
GLCA such as Hawk HG (Table 3) in which the endemic
Rydberg thistle, Cirsium rydbergii, covered almost all of the
vegetated surface over large parts of the hanging garden.
Plant species richness varied from 6-30 per site while size
varied from 4 to 1215 m2 (Tables 1, 2). The area-species
relationships (MacArthur and WIlson, 1%7), S =cAZ where S
= number of ~ies, A = area, are S = 0.889A.l7 for ZION and
S = 0.733A· 1 for GLCA where S is species richness and A is
area of each hanging garden (fig. 2). The slopes (z) of the two
regression lines (.17 vs .19) are not significantly different
(t(.OS)2,39 = 2.02, P >.50). However, the constant values (0.889
vs .733) are significantly different (t(.OS)2,36 = 2.12, P = .04)
primarily due to the higher average number of species per
hanging garden in ZION. The area-species relationship for
CARE was not calculated due to small sample size (n = 5).
We have analyzed the distribution and community importance
of endemic taxa, disjunct pop'ulations of more widespread
species, and possible endangered species (Table 3). Federal and
state status was taken from Atwood et al. (1991). Quantitative
measurement of hanging garden vegetation communities
indicates that maidenhair fem, Adiantum capillus-veneris, the
most widely distributed hanging garden species on a regional
basis, was also the most abundant species in wet wall and
seepline microhabitats in most hanging gardens. Grasses,
especially Jones reedgrass, Calamagrostis scopulorum, and
sedges, Carex, tended to be abundant on ledge and ledge/soil
complex microhabitats. Zion daisy, Erigeron sionis, was found
on only two hanging gardens in ZION as a vety small part of
the community canopy coverage. There are, however, many
individual plants on the wet wall at Canyon Overlook IT HG
within one meter of the trail on which thousands of tourists walk
each year. Alcove bog-orchid, Habenaria zothecina, was found
on wet ledge/soil complex microhabitats on four hanging
gardens in GLCA where it occupied 2-5% of the canopy. Three
hanging gardens have a north aspect (Table 3): Rattlesnake-356°,
0
Swallow-~, and Camp-5 • These sites represent new
Log (s)
DISCUSSION
Regional species distribution patterns have often been
descnbed as log series (Williams, 1964) or .lognormal (May,
1975) for many different taxa. A recent hypothesis fonnulated
by Hanski (1982) predicts a bimodal distribution pattern for
similar sites within a region: a mode of rare species found at
few sites, a mode of abundant species found at most sites, and
vety few species found at a moderate number of sites. Our
distribution patterns did not support the bimodal pattern of
Hanski's (1982) core-satellite hypothesis. Hanging garden
vascular plants have a log series distribution pattern with many
species found at only a few sites. Many of these are transients
=.171og (A) + .88
Log (s) = .19 log (A) + .73
1.6
Log
(species
richness)
o
Zion NP
...
Glen Canyon NRA
1.4
1.2
o
0.8
o
0.6-=-• • • • • • • • • • • • • • • • •. "
o
0.5
1.5
2
2.5
3
3.5
Log (area in sq. m 2)
Figure 2. -
Area-Species relationship linear regressions for ZION and GLCA hanging garden vascular plant species.
318
Table 3. - Protection status and average canopy coverage values for hanging garden vascular plants with narrow distributions: *
species endemic to Colorado Plateau, ( ) coverage range, T trace, [ ] sample size. Microhabitat symbols: ww wet
wall, I = ledge, Is = ledge/soil, sl = seepline. Note: Carex sp. values not separable by species but ranged from (T-.37).
=
=
=
=
=
Microhabitat
Status
Species
Jamesia americana zionis
Fed
St
C2
G5T1
ww
sl
T
[2]
151
*Habenaria zothecina
C2
G1
151
.05
[1 ]
*Erigeron sionis
C2
G2
152
.01
(T-.01 )
[2]
*Carex curatorum
Is
.10
(T-.26)
[3]
-
T
[1 ]
I
G5Q
[8]
151
*Carex haysii
[5]
Aralia racemosa
Dodecatheon pulchellum zionense
*Sphaeromeria ruthiae
.03
(.03-.04)
[3]
C2
G2
152
.08
(T-.15)
[3]
.04
.11
(.08-.13)
[3]
.20
(.06-.35)
[8]
[1]
T
[1 ]
*Zigadenus vagnatus
.13
[1 ]
Rubus neomexicanus
.04
[2]
*Cirsium rydbergii
.03
(T-.13)
.05
[1 ]
.22
(T-.79)
[14]
.037
[1 ]
.063
[1 ]
[7]
Cladium califomicum
*Primula specuicola
.01
(T-.08)
[8]
.07
(.03-.15)
[3]
.02
(.02-.03)
[5]
*Mimulus eastwoodiae
.08
(T-.2O)
[5]
T
T
*Aquilega micrantha
.10
(.01-.20)
[4]
T
[1 ]
.01
(T-.02)
[2]
.03
(T-.06)
[9]
(Houle, 1990), and 0.59 in Oklahoma (Uno and Collins, 1987).
Additionally, B~nd et al. (1988) found a Z value of 0.43 for
fynbos shrub lands surrounded by Afrotemperate evergreen
forests in South Africa In this context, our Z values are more
continental. The 56 species which occurred in only one hanging
garden (fig. 1) also supports MacArthur and Wilson's (1967)
hypothesis that habitat islands receive large numbers of
propagules from adjacent mainland areas.
Many Colorado Plateau endemic species found in hanging
gardens are widespread regionally within the hanging garden
habitat. In addition, Rydberg thistle and Zion shooting star can
be locally abundant within a hanging garden It is the habitat
from nemby xeric or riparian vegetation and probably not a
stable component of hanging garden species assemblage. Other
rare species such as Alcove bog orchid and Zion daisy are
restricted to hanging gardens and seeps and should be monitored
for population size changes.
The area-species relationship Z values for ZION (0.17) and
GLeA (0.19) are intennediate to MacArthur and Wilson's
(1967) predicted values for continental (0.l2-O.17) and island
(0.20-0.35) biota. While these intermediate values for insular
continental areas were also predicted by MacArthur and Wilson
(1967), empirical studies of plants on soil islands in granite
outcrops have found higher values: 0.566 in the Southeast U.S.
319
MacArthur, R. H.; and Wilson, E. O. 1967. The Theory of Island
Biogeography. Princeton Univ. Press, Princeton, NJ
Manly, B. F. 1. 1989. StIatified and systematic sampling. Unpub.
ms., ver 11-89.
May, R. M. 1975. Patterns of species abundance and diversity.
In: M. L. Cody and J. M. Diamond (eds.) Ecology and
Evolution of Communities, pp. 81-120. Belknap Press,
Harvard University, Cambridge, MA.
Stanton, N. L.; Buskirk, S. J.; am Fowler, J. F. 1992. Anrual
Report for Biogeography of Invertebrates of t:re Colorado
Plateau. unpub. ms. UW-NPS Research Center, Lanunie, WY.
Tuhy, 1.L.; and MacMahon J. A. 1988. Vegetation and Relict
Communities of Glen Canyon National Recreation Area,
Final Report for contract CXI200-6B076.
Uno, G. E.; and Collins, S. L. 1987. Primary succession on
granite outcrops in southwestern Oklahoma. Bull. Torrey Bot.
Club 114:387-392.
Welsh, S. L. 1984. Flora of Glen Canyon National Recreation
Area, unpub. ms.
_ _- - 1989a. On the Distribution of Utah's Hanging
Gardens. Gt. Basin Nat. 49:1-30.
_ _ _ _ 1989b. Hanging gardens of Zion National Park,
Final report NPS contract CXI590-7-0001.
_ _ _ _ and Toft, C. A. Biotic communities of hanging
gardens in southeastern Utah. Nat. Geo. Soc. Res. Report
13:663-681.
Williams, C. B. 1964. Patterns in the Balance of Nature.
Academic Press, London
that is rare and fmite. Except for Weeping Rock hanging garden
at ZION, these hanging garden habitats are rarely impacted by
human visitation Since the first requirement for hanging garden
existence is the presence of a perennial seep, conservation
priorities should be placed on managing the Navajo sandstone
aquifer and its recharge areas.
LITERATURE CITED
I
Atwood, D.; Holland, 1.; Bolander, R; Franklin, B; House, D;
Annstrong, L; Thorne, K; and England, L. 1991. Utah
Endangered, Threatened, and Sensitive Plant Field Guide.
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Bond, W. 1.; Midgley, J.; and Vlok, J. 1988. When is an island
not an island? Insular effects and their causes in fynbos
shrublands. Oecologia 77:515~521.
Daubenmire, R. 1959. A canopy-coverage method of
vegetational analysis. Northwest Science 33:43-64.
Hanski, I. 1982. Dynamics of regional distribution: the core and
satellite species hypothesis. Oikos 38:210-221.
Houle, G. 1990. Species-area relationships during primary
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Krebs, C. 1. 1989. Ecological Methodology. Harper & Row,
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320