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Annals of Botany 80 : 731–739, 1997
Frequencies, Microclimate and Root Properties for Three Codominant Perennials
in the Northwestern Sonoran Desert on North- vs. South-facing Slopes
P A R K S. N O B E L* and M A T T H E W J. L I N T O N
Department of Biology, UniŠersity of California, Los Angeles, CA 90095-1606, USA
Received : 1 April 1997
Accepted : 25 July 1997
At a site in the northwestern Sonoran Desert the percent ground cover for the C subshrub Encelia farinosa was eight$
times higher on more arid 20° south-facing slopes than on 20° north-facing slopes at 820 m elevation, and was sixtimes higher on north-facing slopes at a 300-m-lower elevation, also the more arid condition. The ground cover of
the C bunchgrass Pleuraphis rigida decreased over 50 % from 20° north-facing slopes to the more arid conditions of
%
a 36° north-facing slope, a 20° south-facing slope and a 20° north-facing slope at a 300-m-lower elevation. The CAM
leaf succulent AgaŠe deserti also had greater ground cover for the 20° north-facing slopes at 820 m compared with
520 m. For these three codominants that averaged 58 % of the total ground cover, the key for the relative frequency
of E. farinosa was apparently its greater root growth on the warmer slopes during the winter. The key for the other
two species was most likely soil water availability, especially during the seedling stage for A. deserti. The wetter soil
conditions on 20° north-facing slopes at 820 m apparently led to individual plants of P. rigida that were twice as large
as on south-facing slopes. Thus root properties may exert the primary influence on relative plant frequency in this
desert ecosystem for which soil temperature and water availability are crucial.
# 1997 Annals of Botany Company
Key words : AgaŠe deserti, Encelia farinosa, Pleuraphis rigida, rooting patterns, soil temperature, Sonoran Desert,
water availability.
INTRODUCTION
Ground cover and biomass productivity in deserts, which
occupy approximately a quarter of the earth’s land area, are
generally low compared with other ecosystems because of
limited, and often sporadic, rainfall (McGinnies, Goldman
and Paylore, 1968 ; Noy-Meir, 1973 ; Leith and Whittaker,
1975 ; Beadle et al., 1985 ; Walter, 1985). As a result, the
interplay of environmental factors such as air temperature
and soil moisture on relative plant performance can be more
apparent in deserts with their extreme conditions than in
ecosystems where rainfall is more plentiful and temperatures
are more moderate (Beatley, 1974). These environmental
factors can affect desert plants by influencing seedling
establishment, net CO uptake and biomass productivity of
#
the shoots, or seasonal growth and deployment of roots
(Rundel and Nobel, 1991). Desert plants exhibit a wide
range of growth forms (Smith and Nobel, 1986 ; Gibson,
1996), although the influence of growth form on relative
species frequencies has rarely been related to local environmental conditions accompanying different slopes or
elevations. Microclimatic conditions associated with global
climate change (Adams et al., 1990) may also have a major
impact on species composition in deserts, as certain species
may currently be on the verge of elimination based on water
availability or temperature. Analysing effects of microclimate on plant distribution may therefore enable predictions of future species composition of deserts to be made.
Approximately 92 % of the nearly 300 000 species of
* For correspondence.
Fax ­310 825-9433, e-mail psnobel!biology.ucla.edu
0305-7364}97}060731­09 $25.00}0
vascular plants use the C pathway (Nobel, 1991 a ; Winter
$
and Smith, 1996). Approximately 1 % of vascular plants use
the C pathway, most of which are grasses that have evolved
%
in tropical or subtropical regions with high temperatures
and often limited water availability (Monson, 1989). C
%
species tend to replace C species as aridity increases in
$
various ecosystems (Ehleringer and Monson, 1993) ; in a
mixed prairie in the Northern Great Plains, the relative
biomass of the C species increases in midsummer con%
comitant with higher irradiation, higher temperature and
water stress, and then decreases during the less arid autumn
(Ode, Tieszen and Lerman, 1980). Crassulacean acid
metabolism (CAM) plants, which constitute about 6 % of
vascular plant species (Nobel, 1991 a ; Winter and Smith,
1996), tend to open their stomates at night when temperatures and hence evaporative demand are lower, leading to
a high water-use efficiency (CO fixed per unit water
#
transpired). Hence CAM plants are favoured in waterlimited environments, such as in deserts and as epiphytes in
tropical and subtropical regions (Nobel, 1988 ; Andrade and
Nobel, 1996).
At a site in the northwestern Sonoran Desert where the
annually bimodal rainfall pattern leads to winter and
summer wet periods, representatives of each of the three
photosynthetic pathways constitute the three codominant
species among the 33 common perennials (Drennan and
Nobel, 1997), an unusual photosynthetic diversity. Specifically, the deciduous C brittle-bush Encelia farinosa Torrey
$
and A. Gray (Asteraceae), the perennial C bunchgrass
%
Pleuraphis rigida Thurber (Poaceae), and the monocarpic
CAM leaf succulent AgaŠe deserti Engelm. (Agavaceae)
bo970508
# 1997 Annals of Botany Company
732
Nobel and Linton—Frequencies on North- vs. South-facing Slopes
together constitute over half of the local ground cover
(Nobel and Jordan, 1983 ; Ehleringer, 1985 ; Nobel, 1988 ;
Hickman, 1993 ; Drennan and Nobel, 1996, 1997). These
three perennials all have relatively short shoots, minimizing
interspecific shading, and have relatively shallow roots,
facilitating examination of root deployment. Although
similar in stature, the three species have different growth
forms. In particular, E. farinosa has seasonally dimorphic
leaves that are relatively large and dark green in winter,
but greyish and smaller in summer (Ehleringer and
Bjo$ rkman, 1978), P. rigida has short-lived, thin leaves with
photosynthetically active sheaths (Nobel, 1980) and A.
deserti has massive succulent leaves that remain photosynthetically active for many years (Nobel, 1976, 1988).
Although similar in rooting depth, the optimal temperatures
for root growth are 25–27 °C for E. farinosa, about 5 °C
higher for A. deserti, and an additional 5 °C higher for P.
rigida (Drennan and Nobel, 1996).
This study investigated whether the photosynthetic
pathway, growth form, or other morphological and}or
physiological property could explain the preliminary observations of different frequencies among the three codominants on various slopes in the northwestern Sonoran
Desert. The populations studied were found on north-facing
and south-facing slopes, whose microclimates were quantified at the two solstices and an equinox. To help interpret
influences of air temperature and water availability, relative
ground cover was additionally examined for north-facing
slopes of greater inclination at 820 m and at a 300-m-lower
elevation. Leaf temperatures, light levels in the planes of the
leaf surfaces, shoot size and the locations of roots of all
three species at various depths in the soil were determined in
the late winter, a time following the seasonally most reliable
wet period (Nobel, 1988). Production of new roots was also
examined for all three species in the late winter. Seasonal
variations in soil temperature can affect root growth and
hence water uptake from various soil layers, which in turn
influences plant productivity and thereby possibly ecological
success. An explanation was thus developed as to why the
more arid south-facing slopes favoured E. farinosa over P.
rigida compared with north-facing slopes. Optimal temperatures for root growth coupled with the effect of different
slopes on water availability were also used to interpret the
distribution of A. deserti, leading to the conclusion that
environmental effects on root properties can be the primary
factor determining the relative frequency of these three
species in a desert ecosystem.
MATERIALS AND METHODS
Sites
The main field site was a series of small hills with different
slopes centred on 33° 38« N, 116° 24« W at a mean elevation
of 820 m in the University of California Philip L. Boyd
Deep Canyon Desert Research Center about 8 km south of
Palm Desert, California (a site known as Agave Hill).
Additional line transects were made about 2 km north of the
main site at a lower elevation of 520 m. The entire region is
characterized by small hills and numerous granitic outcroppings, with a relatively low plant ground cover
averaging about 30 % and a loamy-sand soil texture (Nobel
1976, 1980, 1988 ; Drennan and Nobel, 1997). The three
codominant species with respect to ground cover are AgaŠe
deserti Engelm. (Agavaceae), Encelia farinosa Torrey and A.
Gray (Asteraceae), and Pleuraphis rigida Thurber (Poaceae)
[formerly Hilaria rigida (Thurber) Scribner] (Hickman,
1993). Measurements at 820 m were made on 20³2° northfacing slopes, 36³3° north-facing slopes (the latter for line
transects only) and 20³2° south-facing slopes. At the 520m site, line transects were examined on 20³2° north-facing
slopes. Angles for slopes were judged along approx. 3-m
ground intervals using an inclinometer consisting of a large
protractor and a plumb bob. The aspects of all slopes were
within 12° of the indicated compass direction [readings with
a Brunton 37215 Pocket Transit (Forestry Suppliers,
Jackson, Mississippi, USA) were corrected for the 13° local
difference between true and magnetic north]. The mean
annually-bimodal precipitation at the main site is 266 mm,
with winter-early spring precipitation averaging 20 % more
than summer-early autumn precipitation and being less
variable from year to year (Young and Nobel, 1986 ; Nobel,
1988).
Plant frequency and size
Non-overlapping line transects 60 m in length using the
line intercept method (Greig-Smith, 1983 ; Bonham, 1989)
were measured in the late winter (1–6 Mar. 1996) at random
locations and in random directions on a series of hills
having the designated slopes. The length of intersection of
any living plant of A. deserti, E. farinosa, P. rigida or all of
the other species combined was noted ; such distance
intervals were summed along each transect and then divided
by 60 m to estimate the percent ground cover for the three
species considered individually and for all the other species
combined. Ten m¬10 m quadrats were located randomly
on 20³2° north-facing or south-facing slopes at 820³10 m
and subdivided into four 5 m¬5 m subsections to facilitate
plant counting. All plants of the three species considered
were counted, including all individual rosettes of A. deserti,
which occurs predominantly in clonal groups. Canopy area
was also determined on 1–3 and 26–27 Mar. 1996 for
randomly selected plants of A. deserti, E. farinosa and P.
rigida in the same region as the quadrats ; areas of the
approximately elliptical canopies were defined as major
axis¬minor axis¬π}4. Only living (green) leaves were
considered ; this was the most difficult to ascertain for P.
rigida, many of whose leaves were brown.
Slope and leaf microclimate
To help characterize the microclimate of a slope, air and
soil temperatures, photosynthetic photon flux (PPF, wavelengths of 400 to 700 nm) and soil water potential were
measured at the centre of bare-ground patches approx. 1 m
in diameter for 20³2° north-facing and south-facing slopes
at 820³10 m near the winter solstice (16 Dec. 1995), the
spring equinox (2 Mar. 1996) and the summer solstice (15
Jun. 1996). Using copper-constantan thermocouples
0±51 mm in diameter, temperature was measured over 24-h
periods at 30 cm above the ground, which represents the
Nobel and Linton—Frequencies on North- vs. South-facing Slopes
733
T     1. Percent ground coŠer for Šarious slopes
Slope
Ground cover (%)
Direction
faced
Angle
(°)
Elevation
(m)
AgaŠe
deserti
Encelia
farinosa
Pleuraphis
rigida
Other
Total
North
South
North
North
18–22
18–22
33–39
18–22
810–830
810–830
800–830
510–530
8±6³0±5
7±7³1±0
7±2³0±9
3±1³1±6
1±5³0±4
11±9³1±4
1±1³0±3
8±9³1±0
8±8³0±9
3±7³0±6
4±0³0±6
0±9³0±4
13±1³1±9
11±4³1±7
13±6³1±7
9±8³1±3
32±2³1±3
34±7³1±9
25±9³1±4
22±6³1±2
Data are based on 60-m line transects measured on 1–6 Mar. 1996 and are presented as means³s.e. (n ¯ 10 transects).
upper part of the canopy where most leaf area occurs for the
three species considered (the thermocouple was shielded
from direct solar irradiation), at the soil surface and 10 cm
below the soil surface, which is approximately the centre of
the rooting zone for the three species (Nobel and Jordan,
1983 ; Drennan and Nobel, 1996). Daytime PPF in the plane
of the slopes was determined with an LI-191SB line quantum
sensor (Li-Cor, Lincoln, Nebraska, USA). Soil moisture
content was determined gravimetrically using 200–300 g
samples that were collected 9 to 11 cm below the soil surface
and immediately weighed, followed by drying at 80 °C until
no further weight change occurred (generally 3 d) ; water
content by mass was converted to water potential using a
moisture release curve for soil from the field site at 820 m
(Young and Nobel, 1986).
Leaf temperatures and PPF in the planes of the leaves
were also determined over a 24-h period at 820³10 m at the
end of the winter wet period (2 Mar. 1996). Measurements
were made on medium-sized plants based on their contribution to ground cover (approx. 18 leaves unfolded from
the central spike for A. deserti, 25 branches emanating from
the main stem near the plant base for E. farinosa and 60
living culms for P. rigida) using four leaves at mid-canopy
occurring in each of the four cardinal directions. Temperature was measured at mid-leaf with copper-constantan
thermocouples 0±51 mm in diameter, and PPF was measured
on both abaxial and adaxial leaf surfaces with a Li-Cor LI190S quantum sensor.
Rooting depth
Excavations of roots of the three species occurring on
20³2° north-facing or south-facing slopes at 820³10 m
were also performed in the late winter (1–3 Mar. 1996) by
carefully removing sequential 4-cm-thick layers of soil with
a geologist’s pick, trowel and various brushes ; roots were
cut by scissors and then collected. In the succeeding year (28
Feb.–1 Mar. 1997), the entire root system was excavated
without dividing into layers, and old roots were distinguished from new, whitish roots that grew in response
to winter rainfall (Nobel, 1997) ; the total rainfall for the
winter wet period preceding the excavations was 52 mm,
71 % occurring on 13 Jan. 1997. In all cases, multiple-yearold, but relatively small isolated plants of similar size
(approx. nine unfolded leaves for A. deserti, 13 main
branches for E. farinosa and 35 culms for P. rigida) were
selected. Excavations extended up to 50 cm radially from
the shoot and to a depth of 36 cm, thereby also ensuring
recovery of essentially the entire root system (Nobel and
Jordan, 1983 ; Nobel, 1988). The surface area of living roots
(verified by the presence of turgid cells within the stele for
representative roots) in each layer was the product of root
length, mean root diameter and π ; roots of the monocotyledonous A. deserti and P. rigida were fairly uniform in
diameter, whereas those for the dicotyledonous E. farinosa
differed in diameter and so were divided into central tap
root, primary lateral roots, and secondary or higher-order
lateral roots before such multiplication. The mean root
depth was the root surface area for each layer multiplied by
the depth at mid-layer summed over all layers and then
divided by the total root surface area. Data were analysed
using ANOVA and Student’s t-test.
RESULTS
Distribution
The average ground cover for AgaŠe deserti, Encelia farinosa
and Pleuraphis rigida between 20³2° north-facing and
20³2° south-facing slopes at 820³10 m was 6–8 % for each
species (Table 1). Under these conditions the percent cover
did not vary significantly with the slope for A. deserti, was
eight-times higher on south-facing slopes for E. farinosa
(P ! 0±001), and was twice as high on north-facing slopes for
P. rigida (P ! 0±01). For north-facing slopes at this elevation
with a mean slope of 36°, the ground cover for A. deserti and
E. farinosa was not significantly different than for 20° northfacing slopes, but the ground cover for P. rigida was halved
(Table 1 ; P ! 0±01). For 20° north-facing slopes at an
elevation that was 300 m lower (520 m), the percent cover
for A. deserti and P. rigida was 64 to 90 % lower (P ! 0±01),
whereas the ground cover for E. farinosa was six-times
higher (P ! 0±001 ; Table 1).
Plant frequencies (Table 2) showed somewhat different
patterns than plant cover (Table 1) for the 20° north-facing
Šs. south-facing slopes at 820 m. In particular, A. deserti
was 40 % more frequent on north-facing slopes (P ! 0±05),
E. farinosa was seven-times more frequent on south-facing
slopes (P ! 0±001), and P. rigida was not statistically
different on the two slopes (P " 0±5). The mean canopy area
per plant (Table 2) was 26 % larger for A. deserti on southfacing slopes (P ! 0±05), 57 % larger for E. farinosa on
south-facing slopes (P ¯ 0±08) and 104 % larger for P. rigida
on north-facing slopes (P ! 0±001). The plant frequency
multiplied by mean plant canopy area (Table 2) predicted
734
Nobel and Linton—Frequencies on North- vs. South-facing Slopes
T     2. Plant frequency and canopy area on north-facing
and south-facing slopes
Frequency (number of plants per 100 m#)
Slope
A. deserti
E. farinosa
P. rigida
North
South
51±0³4±8
36±6³3±2
17±4³3±2
115±2³8±8
39±4³3±6
36±0³4±2
Microclimate
Plant canopy area (cm#)
North
South
A. deserti
E. farinosa
P. rigida
1774³63
2231³92
693³47
1085³77
2216³169
1088³107
Slope angles were 20³2° and the elevation was 820³10 m.
Frequency was based on 10 m¬10 m quadrats, and plant canopy area
(major axis¬minor axis¬π}4) was based on individual plants. Data
were obtained 1–3 and 26–27 Mar. 1996 and are presented as
means³s.e. (n ¯ 11 quadrats for frequency and 100 plants for area).
ground cover (Table 1) within 0±5 % of the measured cover
(also in %) for both slopes for all three species. Plant height
was not significantly affected by slope for A. deserti
16 Dec. 1995
40
Air temperatures 30 cm above the ground were generally
similar for 20° north-facing Šs. south-facing slopes near the
winter solstice (Fig. 1 A), the spring equinox (Fig. 1 B) and
the summer solstice (Fig. 1 C). Temperatures were higher for
south-facing compared with north-facing slopes at the soil
surface (P ! 0±05 for most times during the daytime) and
10 cm below the soil surface (P ! 0±05 for most times during
the day- and night-time) near the winter solstice and the
spring equinox, but were similar near the summer solstice
(Fig. 1 D–I). Near the centre of the rooting zone, 10 cm
below the soil surface, the temperatures for north-facing Šs.
south-facing slopes averaged over a 24-h period were 11±5
Šs. 18±0 °C for the winter solstice (Fig. 1 G), 13±2 Šs. 18±3 °C
for the spring equinox (Fig. 1 H) and 35±8 Šs. 36±0 °C for the
summer solstice (Fig. 1 I).
Differences in PPF in the planes of 20° north-facing Šs.
2 Mar. 1996
A
B
15 Jun. 1996
C
30 cm
North-facing
South-facing
30
(45±0³1±2 cm on 20° north-facing slopes and 46±9³1±3 cm
on 20° south-facing slopes ; means³s.e. for n ¯ 100 plants)
or P. rigida (40±2³1±2 cm on north-facing slopes and
38±3³1±1 cm on south-facing slopes), but E. farinosa was
taller on south-facing slopes (32±8³1±2 cm on north-facing
slopes and 40±1³1±4 cm on south-facing slopes ; P ! 0±01).
20
Air or soil temperature (°C)
10
0
D
E
F
0 cm
50
40
30
20
10
0
40
G
H
I
–10 cm
30
20
10
0
6
12
18
0
6
12
18
Solar time (h)
0
6
12
18
24
F. 1. Air and soil temperatures 30 cm above the ground (A–C), at the soil surface (D–F) and at 10 cm below the soil surface (G–I) for 20³2°
north-facing (D) and south-facing (^) slopes at 820³10 m in the northwestern Sonoran Desert. Measurements were made near the winter solstice
(A, D and G), the spring equinox (B, E and H) and the summer solstice (C, F and I). Data are means (n ¯ six slopes) ; s.e. averaged 0±3 °C at 30 cm,
0±5 °C at the soil surface and 0±3 °C 10 cm below the soil surface, essentially all within the symbol size.
Nobel and Linton—Frequencies on North- vs. South-facing Slopes
16 Dec. 1995
2000
2 Mar. 1996
A
735
15 Jun. 1996
B
C
North-facing
South-facing
PPF (µmol m–2 s–1)
1500
1000
500
0
6
9
12
15
6
9
12
15
6
9
12
15
18
Solar time (h)
F. 2. Photosynthetic photon flux (PPF) near the winter solstice (A), the spring equinox (B) and the summer solstice (C) in the planes of 20³2°
north-facing (D) and south-facing (^) slopes at 820³10 m. Data are means³s.e. (n ¯ six slopes).
Leaf temperature (°C)
30
Encelia farinosa
Pleuraphis rigida
B
C
20
10
North-facing
South-facing
0
Leaf PPF (µmol m–2 s–1)
Agave deserti
A
D
E
F
1500
1000
500
0
6
12
18
0
6
12
18
0
6
12
18
24
Solar time (h)
F. 3. Leaf temperatures (A–C) and photosynthetic photon flux (PPF ; D–F) in the late winter for AgaŠe deserti (A and D), Encelia farinosa (B
and E), and Pleuraphis rigida (C and F) on 20³2° north-facing (D) and south-facing (^) slopes at 820³10 m. Measurements on 2 Mar. 1996
are averages for four leaves at mid-canopy, one in each of the four cardinal directions, for medium-sized plants. Data for temperature are means
(n ¯ 4 plants) and s.e. averaged 0±4 °C ; data for PPF represent totals for both leaf surfaces and are means³s.e. (n ¯ six plants).
south-facing slopes were also greatest near the winter
solstice, intermediate near the spring equinox and least near
the summer solstice (Fig. 2). The total daily PPF integrated
over the daytime for north-facing Šs. south-facing slopes on
clear days was 10 Šs. 32 mol m−# d−", respectively, near the
winter solstice (Fig. 2 A), 27 Šs. 41 mol m−# d−" near the
spring equinox (Fig. 2 B) and 55 Šs. 57 mol m−# d−" near
the summer solstice (Fig. 2 C).
The water potential 10 cm below the soil surface at
various times during the spring was higher for 20° north-
736
Nobel and Linton—Frequencies on North- vs. South-facing Slopes
Encelia farinosa
Agave deserti
0
Pleuraphis rigida
B
A
C
4
Depth in soil (cm)
8
12
16
20
24
28
North-facing
South-facing
32
36
0
10
20
30
0
10
20
30
0
Root surface area per layer (% of total)
10
20
30
F. 4. Root distribution with depth for A. deserti (A), E. farinosa (B) and P. rigida (C) excavated in the late winter on 20³2° north-facing (D)
and south-facing (^) slopes at 820³10 m. Plants on the two slopes were similar in shoot height and diameter and relatively isolated from other
plants : A. deserti had eight–ten leaves unfolded from the central spike of folded leaves, E. farinosa had 12–14 main branches bearing leaves and
P. rigida had 33–36 living culms. Data obtained on 1–3 Mar. 1996 are plotted at the middle of the 4-cm-thick soil layers and are means³s.e.
(n ¯ five plants).
facing compared with south-facing slopes (P ! 0±01). In
particular, Ψsoil on north- Šs. south-facing slopes was
®0±41³0±11 and ®0±96³0±10 MPa, respectively, on 22
Apr. 1995, ®0±97³0±14 and ®2±32³0±33 MPa on 2 Mar.
1996 and ®1±56³0±23 and ®3±64³0±39 MPa on 26 Mar.
1996 (data are means³s.e. for n ¯ 6 slopes). At other times
(1 Jun. 1995, 16 Dec. 1995, 15 Jun. 1996), the soil was
extremely dry, with a water content of less than 1 % by
mass, corresponding to soil water potentials below
®12 MPa. Even in these dry cases, the water content was
always higher (average of 27 %) for the north-facing
compared with the south-facing slopes (P ! 0±05).
Leaf temperature and PPF
The daily variations for leaf temperatures at the end of
the winter wet period (Fig. 3) were generally similar to those
for air temperatures (Fig. 1 B). However, temperatures for
leaves of A. deserti (Fig.3 A) substantially exceeded those of
the air (P ! 0±01) during the daytime, when leaves of E.
farinosa (Fig. 3 B) on north-facing slopes were lower than
those on south-facing slopes (P ! 0±05). The relatively small
leaves of P. rigida were tightly coupled to air temperatures
(cf. Fig. 1 B) throughout the day on both slopes (Fig. 3 C).
At the end of the winter, slope had the greatest influence
on the PPF in the planes of the leaves for basal rosettes of
A. deserti (P ! 0±01 ; Fig. 3 D–F). In particular, the total
daily PPF summed for both leaf surfaces at mid-leaf and
mid-canopy for medium-sized plants on north-facing Šs.
south-facing slopes was 32 Šs. 43 mol m−# d−", respectively,
for A. deserti (Fig. 3 D), 37 Šs. 42 mol m−# d−" for E. farinosa
(Fig. 3 E) and 34 Šs. 36 mol m−# d−" for P. rigida, whose
culms were essentially vertical (Fig. 3 F).
T     3. Mean rooting depth and total root surface area of
relatiŠely small, isolated plants of similar size for 20³2°
north-facing and south-facing slopes at 820³10 m
Mean depth (cm)
Slope
A. deserti
E. farinosa
P. rigida
North
South
8±4³0±5
11±4³0±5
10±6³0±4
13±4³0±6
9±0³0±4
11±4³0±3
Root surface area (cm#)
North
South
A. deserti
E. farinosa
P. rigida
548³33
714³44
225³18
291³21
368³18
464³27
Data were obtained as for Fig. 4 and are means³s.e. (n ¯ 5 or 6
plants).
Rooting depth and new roots
Roots of all three species extended deeper into the soil for
small, similar-sized, isolated plants on 20° south-facing
slopes (28–32 or 32–36 cm soil layers) compared with 20°
north-facing slopes (20–24 or 24–28 cm soil layers ; Fig. 4).
The mean root depth for north-facing slopes was about
8 cm for A. deserti (Fig. 4 A), 11 cm for E. farinosa (Fig. 4 B)
and 9 cm for P. rigida (Fig. 4 C) but 35, 26 and 27 % greater,
respectively, for south-facing slopes (P ! 0±01 ; Table 3).
Root surface area was also greater for plants on southfacing compared with north-facing slopes, averaging 30 %
for A. deserti, 29 % for E. farinosa and 26 % for P. rigida
(P ! 0±05 ; Table 3).
Nobel and Linton—Frequencies on North- vs. South-facing Slopes
T     4. New root area on relatiŠely small, isolated plants of
similar size for 20³2° north-facing and south-facing slopes at
820³10 m
Area of new roots (% of total root surface area)
Slope
A. deserti
E. farinosa
P. rigida
North
South
0±20³0±06
2±16³0±37
1±84³0±29
4±88³0±42
0±00³0±00
0±18³0±04
Data are for plants of the size ranges in Fig. 4 and Table 3 were
obtained 28 Feb.–1 Mar. 1997 and are means³s.e. (n ¯ 9 or 10
plants).
The area of new roots produced in response to a winter
wet period ranged from none for P. rigida on north-facing
slopes to nearly 5 % of total root surface area for E. farinosa
on south-facing slopes (Table 4). For the two slopes, new
roots averaged 1±18 % of the total root surface area for A.
deserti, 3±36 % for E. farinosa and 0±09 % for P. rigida. In
all cases, the proportion of new roots was higher for southfacing compared with north-facing slopes (P ! 0±001 for A.
deserti and E. farinosa, P ! 0±01 for P. rigida ; Table 4).
DISCUSSION
The frequency of the C subshrub Encelia farinosa sub$
stantially increased from 20° north-facing to 20° southfacing slopes at 820 m in the northwestern Sonoran Desert,
whereas the C bunchgrass Pleuraphis rigida and the CAM
%
leaf succulent AgaŠe deserti changed relatively little in
frequency. The south-facing slopes had similar air temperatures but higher soil temperatures and lower soil water
availability than the north-facing slopes. The change from a
20° to a 36° north-facing slope, which should lead to less
water availability due to greater runoff and slightly lower
soil temperatures, was accompanied by a major decrease in
the ground cover only for P. rigida. Comparing 20° northfacing slopes at 820 m Šs. 520 m, the approx. 2±6 °C higher
temperature and 30 % lower annual rainfall at the lower
elevation (Nobel and Hartsock, 1986) were accompanied by
major reductions in the frequency of A. deserti and P. rigida
but increases in E. farinosa for this more arid location. The
average relative growth of new roots was consistent with
observations for plants in flat ground, when a winter wet
period with more rainfall (169 mm) led to new roots
averaging 3±4 % of the total root surface area for A. deserti,
9±0 % for E. farinosa and 0±9 % for P. rigida (Nobel, 1997).
The large size of the leaves of A. deserti and its nocturnal
stomatal opening caused its leaf temperatures to be below
ambient air temperatures at night and to be above near
midday, consistent with previous results (Nobel, 1988). The
higher soil water potential for 20° north-facing compared
with south-facing slopes, and the resulting differences in
transpiration, apparently accounted for the lower midday
leaf temperatures for E. farinosa on north-facing Šs. southfacing slopes. Yet because the plants were relatively far
apart, leading to a total ground cover averaging only 29 %,
737
interspecific competition for soil water and for light was
probably less than for most other ecosystems (Gates, 1980 ;
Monson et al., 1992). The increases in frequency, canopy
size, plant height and ground cover for E. farinosa in going
from a 20° north-facing slope at 820 m to a 20° south-facing
slope or to a 20° north-facing slope at a lower elevation
(520 m) were presumably not caused by less soil water or by
slight changes in PPF. Because its leaf temperatures at midcanopy and the air temperatures 30 cm above the ground
were not appreciably affected by the aspect of the 20° slopes,
attention is focused on soil temperatures.
Because few roots occurred near the soil surface except
for approximately vertical ones directly under the plants,
the soil surface temperature should be less crucial for root
deployment than the soil temperature 10 cm below the
surface, which is near the centre of the rooting zone for
all three species. Near the beginning and the end of the
winter wet period of 16 Dec. 1995 to 2 Mar. 1996, soil
temperatures at this depth averaged 12 °C on north-facing
slopes and 18 °C on south-facing slopes. Compared with
root growth at the optimal temperature of 25 °C for E.
farinosa under winter conditions, growth at 12 °C is only
11 % of maximal while that at 18 °C is 58 % of maximal
(Drennan and Nobel, 1996). In this regard, approximately
three-fold more new roots of E. farinosa were observed for
south-facing compared with north-facing slopes at the end
of the winter wet period in 1997. For the 2±6 °C higher
ambient temperatures at the lower elevation, winter root
growth at 10 cm would be 31 % of optimal, so plant success
measured by ground cover—if dictated by winter root
growth—would be expected to be intermediate to that for
the two slopes at higher elevation ; this was only the case for
E. farinosa.
Using a model to predict soil temperature on 15 September
as a function of depth and soil surface temperatures (Nobel,
1991 b), along with recorded air temperatures at 820 m, the
properties of the local soil (Nobel and Geller, 1987) together
with soil temperatures measured on 15 Jun. 1996, the soil
temperature 10 cm below the soil surface during a typical
summer wet period averaged 34±1 °C on 20° north-facing
slopes and 36±0 °C on 20° south-facing slopes. Under
summertime conditions, root growth of E. farinosa at this
depth would be 43 % of maximal on the north-facing slopes
and 38 % on the south-facing slopes (Drennan and Nobel,
1996) ; the mean duration of soil water availability and
hence root growth is less during the summer (Young and
Nobel, 1986 ; Nobel, 1988). Indeed, production of new roots
per plant is five-fold greater in the winter than in the
summer for E. farinosa in relatively flat ground at 820 m
(Nobel, 1997). On the other hand, optimal temperatures for
root growth are higher for the other two species, averaging
31 °C for A. deserti and 36 °C for P. rigida (Drennan and
Nobel, 1996), so they can have greater root growth during
summer wet periods than E. farinosa. In particular, for
plants in flat ground A. deserti has approximately equal
production of new roots in the winter and the summer and
P. rigida has six-fold greater root production in the summer
(Nobel, 1997).
For plants of the same shoot size on the two slopes at
820 m, roots for A. deserti, E. farinosa and P. rigida
738
Nobel and Linton—Frequencies on North- vs. South-facing Slopes
averaged 28 % greater in area and were deeper on 20° southfacing compared with 20° north-facing slopes. Root systems
of Lupinus albus (Rodrigues, Pacheco and Chaves, 1995),
Picea abies (Persson et al., 1995) and other species (Nye and
Tinker, 1977) also tend to be more extensive vertically and
larger in surface area when the plants are droughted. Thus
the deeper roots with more surface area per unit shoot area
for plants on south-facing slopes for the three species are
consistent with the more arid conditions there than on
north-facing slopes.
The changes in distribution of P. rigida with slope and
elevation may relate to the observation that most of its
leaves are brown and apparently dead during much of the
year at 820 m. Although it can exhibit an extremely high
photosynthetic rate of up to 67 µmol m−# s−" (Nobel, 1980),
it appears to be only marginally able to cope with the local
conditions. Also, its leaves take approx. 3 weeks to develop
a substantial surface area following a wet period (P. S.
Nobel, unpubl. res.), so its net CO uptake capability does
#
not respond as quickly to rainfall as that of E. farinosa or A.
deserti, the latter having evergreen leaves that persist for up
to 6 years, with plant water uptake occurring within 24 h of
rainfall (Nobel, 1976, 1988). Following light rainfalls, the
soil may therefore dry before leaves of P. rigida can develop.
During the winter wet period in 1996 Šs. the drier one in
1997 (169 Šs. 52 mm rainfall), P. rigida developed ten-fold
more new roots compared with less than three-fold more for
the other two species. Also, P. rigida has the lowest plant
water storage capacity of the three codominants (Nobel and
Jordan, 1983), so it may be the most vulnerable to low soil
water availability. In this regard, its ground cover at 820 m
decreased at least two-fold in going from a 20° north-facing
slope to a 20° south-facing slope, to a steeper (36°) northfacing slope, or to one 300-m-lower in elevation, all of which
represent drier soil conditions. Individual clumps of P.
rigida had over twice the canopy area for the wetter 20°
north-facing compared with south-facing slopes at 820 m,
again suggesting that small changes in soil water availability
may be crucial.
The percent ground cover for A. deserti was similar for
the 20° and 36° north-facing slopes and the 20° south-facing
slopes at 820 m but decreased for 20° north-facing slopes
at 300-m-lower elevation. Because the orientation of the
rosette of rigid massive leaves was dependent on the local
slope, leaves of A. deserti on 20° south-facing slopes
intercepted 34 % more PPF in the late winter than those on
20° north-facing slopes (PPF interception varied only 6 to
14 % for the two slopes for the other two species, whose
flexible leaves are not as spatially affected by the slope). This
may help to account for the larger mean shoot size for A.
deserti on south-facing slopes despite lower water availability, as net CO uptake by this species is PPF-limited,
#
even in the high radiation environment of a desert (Nobel,
1984, 1988), so increased CO uptake capability may lead to
#
larger plants. The decrease in frequency of A. deserti from
north-facing to south-facing slopes may be in response to
the more arid conditions on the south-facing slopes, with
the higher soil temperature playing a minor role. The lower
ground cover for A. deserti at the lower elevation may also
be in response to the more arid conditions. The effects of
soil water might be expected to be least for the CAM species
with its higher water-use efficiency than the C or C species,
$
%
but water is crucial for seedling establishment by A. deserti.
In particular, only 1 year in a span of 17 years at 820 m had
sufficient rainfall followed by short enough drought periods
for its seedlings to become established (Jordan and Nobel,
1979), and over 99 % of the reproduction of A. deserti is
vegetative, by means of ramets (Nobel, 1988). Thus its longterm success may be dictated by soil water availability at the
seedling stage, making its frequency and hence its percent
ground cover particularly vulnerable to local soil water
content.
In conclusion, E. farinosa at the sites considered was
apparently most affected by soil temperatures and root
growth during the winter, P. rigida by its inability to tolerate
more arid conditions, and A. deserti also by soil water
availability, probably during seedling establishment. Therefore, soil temperature and water availability are apparently
more crucial than photosynthetic pathway or growth form
for the relative success of the three codominants as measured
by percent ground cover for various slopes and elevations in
this desert ecosystem. Such responses emphasize the
importance of root properties and can have important
implications for predicting altered plant frequencies resulting from new environmental conditions, such as those
accompanying global climate change.
A C K N O W L E D G E M E N TS
Preliminary data were collected in 1995 by Xilian Yue for
microclimate and Dr Philippa Drennan for plant distribution. Much of the data from 1–3 Mar. 1996 was
collected by Aaron De Hart, Jane George, Reid Hamamoto,
Co Lai, Andrew Na, Dang Ngo, Deimkanh Nguyen,
Maurice Pitesky, Sasan Sanandaji and Kiki Zaharopoulos ;
Barry Greenhouse and Amir Lagstein collected data on that
trip and also during a subsequent one. Much of the data on
new roots was collected by Edward Bobich, Paul Chang and
Yong Chung on 28 Feb.–1 Mar. 1997. This research was
financially supported by the US Department of Energy
Office of Health and Environmental Research, Program for
Ecosystem Research, grant DE-FG03-93ER61686 and US
National Science Foundation grant IBN-94-19844.
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