Competitive interactions between grass and succulentshrubs at the ecotone between an arid grasslandand succulent shrubland in the Karoo
Ndafuda Shiponeni • Nicky Allsopp •
Peter J. Carrick • M. Timm Hoffman
Received: 26 January 2010 / Accepted: 9 November 2010 / Published online: 25 November 2010
� Springer Science+Business Media B.V. 2010
Abstract Nearest-neighbour analysis was used to
examine the competitive interactions between Stipa-
grostis brevifolia, a C4 perennial grass, and two leaf
succulent shrubs, Ruschia robusta and Leipoldtia
pauciflora, at the ecotone between semi-arid grass-
land and succulent shrubland in the Karoo. The root
distribution in the soil was also compared to assess
the degree of overlap in the potential use of soil
resources. Regressions between the combined sizes of
interspecific, nearest-neighbour species and the dis-
tance between them showed significant positive
correlations for S. brevifolia and R. robusta, which
suggest the presence of competition. We infer from
individual species regressions that the grass exerted a
stronger competitive force on the shrub R. robusta
than R. robusta on the grass. There was also evidence
for strong intraspecific competitive relationships
within S. brevifolia and R. robusta. There was no
evidence of competition between S. brevifolia and
L. pauciflora or among L. pauciflora individuals.
S. brevifolia had the deepest root system, and was
recorded at depths of 70 cm. Most of this root mass
occurred between 10 and 40 cm. Ruschia robusta
roots were recorded as deep as 55 cm, but more than
90% was found in the top 20 cm of the soil, creating a
degree of overlap with the vertical root distribution of
S. brevifolia. A clear separation in rooting depths
occurred between S. brevifolia, and L. pauciflora
which had only 3% of the total root mass below
10 cm. The partial overlap in the vertical root
distribution between S. brevifolia and R. robusta
may account for the observed competitive relation-
ship, but each species dominates in a different layer,
potentially minimising the net competition between
S. brevifolia and R. robusta. Our findings demonstrate
the possibility of a two-layer water-obtaining strategy
in a semi-desert ecosystem, where the succulent
shrubs seem to be playing the typical ‘‘grass’’ role
described in most models of water partitioning
between grass and woody plants. The stronger
competitive effect of S. brevifolia on R. robusta at
all the sites is of significance to species dynamics,
and might be related to winter/summer rainfall
dynamics at the climatic transition.
N. Shiponeni � P. J. Carrick � M. T. Hoffman
Plant Conservation Unit, Botany Department, University
of Cape Town, PO Box X3, Rondebosch 7701,
South Africa
Present Address:N. Shiponeni (&)
Department of Biological Sciences, University
of Namibia, Private Bag 13301, Windhoek, Namibia
e-mail: [email protected]
N. Allsopp
ARC-Livestock Business Division, Private Bag X17,
Bellville 7535, South Africa
Present Address:N. Allsopp
South African Environmental Observation Network,
SANBI, Private Bag X7, Claremont 7735, South Africa
123
Plant Ecol (2011) 212:795–808
DOI 10.1007/s11258-010-9864-0
Keywords Climatic ecotone � Nearest-neighbour
analysis � Plant competition � Resource partitioning �Root system � Vertical separation in rooting depth
Introduction
Species may segregate into different communities or
they may co-exist, according, in part to ccompetition
among them (Rosenzweig 1981; Scholes and Archer
1997; Gordon 2000). The principle of niche differ-
entiation through resource partitioning, both in space
and time is at the root of many competition theories
explaining co-existence between plants (Yeaton et al.
1977; Walker and Noy-Meir 1982; Knoop and
Walker 1985; Cody 1986; Sala et al. 1989). The
vegetation along the ecotone between Bushmanland
grassland and the succulent shrubland of Namaqua-
land, in the Karoo region of South Africa is
characterised by segregated grassland and leaf
succulent shrubland communities, but also commu-
nities in which grasses and succulent shrubs co-occur
(Fig. 1). While the distribution of perennial C4 grass
in the Karoo is generally associated with deeper and
sandy soils (Lloyd 1989; Carrick 2001), the grass
species S. brevifolia and certain leaf succulent shrub
species appear to break this segregation, forming
inter-mingled communities with no clear distinction
in soil properties (Shiponeni 2007), and creating
tension zones of possible competitive interactions at
the ecotone.
There is enough evidence in the Karoo to support
the occurrence of competition in arid and semi-arid
environments as observed among perennial shrubs
(Yeaton and Esler 1990; Esler and Cowling 1993;
Carrick 2003) and between perennial shrubs and
annuals (Cunliffe et al. 1990; Yeaton et al. 1993).
However, studies on competitive interactions
between grass and shrubs in general, and succulent
shrubs in particular are lacking in the Karoo. In the
eastern Karoo, where perennial grasses and dwarf
shrubs co-exist, Hoffman et al. (1990) have proposed
that the abundance of grasses regulates the abundance
of dwarf shrubs through competition. Species or
growth forms are also characterised by several
different patterns of root distribution resulting in
separation in the use of soil resources sufficient to
permit co-existence (see Fowler 1986). In the
Succulent Karoo, Carrick (2003) ascribed evidence
of competition between the two leaf succulent
Aizoaceae shrubs, Leipoldtia schultzei and Ruschia
robusta, and the absence of competition between
these shrubs and a non-succulent Asteraceae shrub
Hirpicium alienatum, to vertical separation in rooting
morphologies. The shallow nature of the root systems
of leaf succulent shrubs is well documented (Esler
and Rundel 1999; Midgley and van der Heyden 1999;
Carrick 2003), but the root systems of perennial grass
in the broader Karoo region have not been studied. It
is thus not clear how species within this growth form
interact with other growth forms such as succulent
shrubs, which together with non-succulent shrubs are
the most abundant components of these communities
(Cowling et al. 1994). Midgley and van der Heyden
(1999) have proposed that perennial grasses and
succulent shrubs might be competing for water in the
same vertical zone, due to the shallow-rooted nature
of succulent shrubs. Understanding how species or
growth forms interact can be useful in understanding
and predicting vegetation dynamics or responses
particularly at climatic transitions and under the
influence of grazing. In this study, we investigate the
nature of competitive interactions between semi-arid
Fig. 1 Subset of grass and shrubs distribution at the
transition zone between Bushmanland grasslands and Nam-
aqualand shrublands, showing tension zones of mixed grass/
shrubs communities (extracted from Shiponeni (2007), based
on nonparametric decision-tree classification using ASTER
images)
796 Plant Ecol (2011) 212:795–808
123
perennial grass S. brevifolia, and two leaf-succulent
shrubs, Ruschia robusta and Leipoldtia pauciflora
(Aizoaceae) as a way of understanding co-existence
and potential vegetation change across the grassland/
shrubland ecotone. Vertical distribution in root sys-
tems was also studied to explain the nature of
possible belowground interactions between the two
growth forms.
The occurrence of plant competition is often
inferred from studies of spatial patterns (Fowler
1986). In this study, evidence of competition was
inferred from the relationship between distance and
size of neighbouring plants. This technique, the
nearest-neighbour analysis, first described by Pielou
(1960), has been widely used to study plant interac-
tions in arid and semi-arid areas (Yeaton and Cody
1976; Yeaton et al. 1977; Welden et al. 1988; Yeaton
1990; Esler and Cowling 1993; Carrick 2003). It is
based on the premise that significant positive linear
correlations between the sum of neigouring plant
sizes, and the distance that separates them, indicates
competition, while the lack of such correlations is
interpreted as the absence of competition between
the two plants. Many studies have also compared the
distribution of roots in the soil in order to explain the
degree to which species are likely to interact (Cable
1969; Pelaez et al. 1994; Briones et al. 1996; Casper
and Jackson 1997; Nobel 1997; Carrick 2003).
Methods
Study area
The study was carried out at four sites (Burdensputs,
Kougoedvlakte, Vaalputs and Goegap Nature Reserve)
stretching a distance of approximately 80 km from the
farm, Burdensputs (30� 240S; 18� 340E) near Kliprand
in the southeast, up to Goegap Nature Reserve (29�420S; 17� 590E) near Springbok in the northwest along
the ecotonal border between Bushmanland and Nam-
aqualand, in the Northern Cape Province of South
Africa. The area forms part of a zone separating the
predominantly winter rainfall Succulent Karoo and the
predominantly summer rainfall Nama-Karoo biomes.
Winter rainfall is a consequence of coastal low
pressure systems and summer rain comes as convective
storms (Cannon 1924; Desmet and Cowling 1999).
Rainfall patterns at the sites are depicted in Fig. 2.
Goegap Nature Reserve to the most north-western part
of the study area has a rainfall peak from April to
September, and it forms part of the winter rainfall
Namaqualand (Rosch 2001), whereas the other sites
have no marked seasonal pattern in rainfall and appear
to receive both winter and summer rainfall.
The Burdensputs and Kougoedvlakte farms are
privately owned and are grazed by domestic livestock
using commercial management practices. Vaalputs
Radioactive Waste Disposal Facility comprises three
commercial farms which were taken out of produc-
tion when these became a nuclear waste disposal site
from 1983. It is currently stocked with small indig-
enous antelope species. The Goegap Nature Reserve
area has been under conservation since 1960, and is
also stocked with indigenous antelope species. As
with other semi-arid and arid regions, lack of
moisture results in less weathering and leaching
giving rise to soils that have weakly developed
structure with freely drained soils and little organic
matter (Lloyd 1985a, b; Watkeys 1999).
We focused on ecotonal communities, where both
grass and shrubs co-occur. The grass component is
represented by one of the Bushmanland perennial C4
grass species, S. brevifolia. The shrub species co-
occuring with grass at the ecotone belong to a group
of leaf succulent plants in the family Aiozaceae,
which formerly constituted a separate family, the
0
5
10
15
20
25
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Rai
nfal
l (m
m)
De Riet GoegapBurdensputs Vaalputs
*
Fig. 2 The average monthly rainfall at four sites where mixed
grass-succulent communities occur: Burdensputs (134 mm;
38%; 1993–2003), De Riet (116 mm; 41%; 1984–2006),
Vaalputs (131 mm; 53%; 1987–2006), and Goegap (150 mm;
33%; 1977–2001). The names of the sites are followed by the
mean annual rainfall, coefficient of variation (%CV) and period
of length of record, respectively. *De Riet is the farm adjacent
to and in close proximy to the Kougoedvlakte site
Plant Ecol (2011) 212:795–808 797
123
Mesembryanthemaceae, and which are colloquially
referred to as ‘mesembs’. At each of the four sites,
a community co-dominated by S. brevifolia and
R. robusta was selected for sampling, but at Goegap
Nature Reserve Leipoldtia pauciflora also co-existed
with S. brevifolia and a community of this combina-
tion was sampled.
Vegetation cover and the nearest neighbours
Vegetation composition and projected canopy cover
in each of the communities were estimated using a
line intercept method (Canfield 1941), to obtain the
percentage cover and the relative abundance. Skeletons
of dead shrubs were frequent in these communities, and
thus were also recorded. For the nearest-neighbour
sampling, we walked along a 1 m-wide transect and
whenever one of the studied species was encountered
and its nearest neighbour was either a conspecific or one
of the other study species, then the distance between
them, two perpendicular diameters (one of which was
the longest possible) and height of each individual were
recorded. At least 50 pairs of each interspecific and
intraspecific combination of plants were sampled within
each community.
Roots excavation
The root systems of three isolated adult plants of
S. brevifolia and R. robusta were excavated at
Kougoedvlakte, and three L. pauciflora plants were
excavated at Goegap Nature Reserve. Excavation was
done by hand using a pointed wooden stick to loosen
the soil which was then hand removed, leaving the
root system intact. Excavation progressed by extract-
ing the soils, and recording the root architecture and
lateral extent. Diagrams were made and photographs
were taken as excavation progressed. At the recorded
depths (see below), roots were collected in separate
paper bags, washed of excess sand, then dried and
weighed. In the grass community, it was particularly
difficult to find isolated adult grass plants because the
grass density was high, and young adults were thus
excavated. The root data for S. brevifolia were
collected at the following depth categories: 0–5,
5–10, 10–20, 20–30, 30–40, 40–50 and 50–60 cm.
Excavation continued up to 65 or 70 cm where a
hardpan develops and further excavation was diffi-
cult; but roots from below 60 cm were only used for
total root mass value. All roots of S. brevifolia were
classified as fine roots (diameter \2 mm). Ruschia
robusta and L. pauciflora root data were collected at
these depths: 0–5, 5–10, 10–20, 20–30, 30–40 and
40–50 cm, and were grouped into fine roots (with
diameter \2 mm) and coarse roots (with diameter
[2 mm). The taproot of R. robusta extended below
50 cm, but it was cut at this depth because bedrock
prevented further excavation. The taproot of one
plant was successfully excavated in its entirety, but
this extra data was not included in the analysis. To
determine whether the root depths of S. brevifolia
and R. robusta vary along the ecotone, confirmatory
rapid excavations of plants were also done at the
northern part of the study area, next to Goegap Nature
Reserve.
Data analysis
Variation in representing the size of the plants is
determined from the morphology and shape or the
growth form of the individual species (Welden et al.
1988). In this study, the sizes of both the grass and
the succulent shrub individuals were considered as
the area of an elipse calculated from the canopy
diameters using the formula for the area of an elipse:
A ¼ p ab=4ð Þ2;
where A is the area of the plant, a and b are the two
diameters (Welden et al. 1988).
According to Kolmogorov–Smirnov and Liliefors
tests, size data were not normally distributed, and,
therefore, were square root transformed. Size data
were then standardised by dividing the size of each
plant by the mean size for that species, in the
particular pair combination, in order to minimise the
effect of different sizes of plants in each pair. Linear
correlations between the sums of transformed and
standardised size data for each pair and the distances
between pairs were calculated. In inter-specific
combinations, the relative contribution of each spe-
cies to the interaction was tested by regressing the
individual sizes of plants of the two species against
distance. Significant positive correlations were inter-
preted as an indication of competition among plants.
The proportions of roots in different depth classes
were described using graphs and schematic presen-
tations. The ratio of mean rooting diameter to mean
798 Plant Ecol (2011) 212:795–808
123
canopy diameter, and the root mass fraction (root dry
mass/total dry mass) between the three species were
compared using a one-way ANOVA followed by a
Scheffe’s test for post hoc comparison where
necessary.
Results
Vegetation cover and composition
The total vegetation cover in the communities ranged
between 36% at Burdensputs and 72% at Goegap
Nature Reserve (Table 1). The co-dominance between
the grass S. brevifolia and the succulent shrubs
R. robusta or L. pauciflora varied between the sites,
but the grass and succulent shrub comprised more than
80% of the total vegetation cover in each community.
Skeletons of dead shrubs made up the third most
abundant cover class at all sites.
The nearest neighbours
Intra- and inter-specific nearest-neighbour pairings
of S. brevifolia and R. robusta in all communities
showed significant positive linear correlations
between nearest-neighbour distances and the sums
of sizes of the two neighbours (Figs. 3, 4). Intra-
specific comparisons yielded stronger correlations
than inter-specific comparison. For inter-specific
pairs, the correlation between the size of R. robusta
and the distance from S. brevifolia was stronger,
than the correlation between S. brevifolia and the
distance from R. robusta. There was no relationship
between the sizes of L. pauciflora plants and the
distances between them (r = -0.001; P = 0.99), or
between L. pauciflora and S. brevifolia (r = 0.031;
P = 0.80).
Root distribution
The vertical distribution of roots and root architecture
of each species are schematically represented in
Figs. 5, 6 and 7. S. brevifolia had the deepest roots,
extending below 60 cm in the soil. The top 10 cm of
the root system of S. brevifolia was made up of a
dense tuft of thick and tough non-fibrous roots. These
roots contributed 30% to the total root mass, but they
did not spread horizontally (Fig. 5). The root system
then became more fine and fibrous below 10 cm,
spreading both horizontally and vertically throughout
the excavation depth. More than half of the total root
mass for S. brevifolia was found between 10 and
40 cm deep. Rapid comparative excavations near
Goegap Nature Reserve and Dabeep farm confirmed
this general root architecture for S. brevifolia.
Ruschia robusta at Kougoedvlakte had a taproot,
recorded as deep as 55 cm. The roots of R. robusta
were primarily coarse, with about 70% of the root
mass greater than 2 mm in diameter. More than 90%
of the total root mass occurred in the top 20 cm in the
form of secondary roots that spread horizontally,
often extending more than a meter from the stem
(Fig. 6). In all three, R. robusta plants excavated at
Kougoedvlakte, a coarse lateral root split from the
taproot at 30 cm. The diameter of the taproot then
reduced and continued down vertically into the
cracks in the bedrock. Since the ground hardened at
about 40 cm, excavation was difficult beyond this
depth and was discontinued at 50 cm in all but one
excavation (taproots were cut at this depth). Only one
of the three excavated R. robusta plants was followed
up to the end of the root sytem, measured at 55 cm.
Rapid root excavations of R. robusta in the northeast
of the study area showed similar structure in the first
20 cm, but lacked the taproot, and thus the rooting
system was shallower. Leipoldtia pauciflora had the
Table 1 Vegetation canopy
cover (%) at the five
communities sampled for
nearest-neighbour analysis,
and percentage cover for the
most abundant species
a Dead shrubs made up the
third most abundant cover
and thus was included
Site Total S. brevifolia R. robusta L. pauciflora Dead shrubsa
Burdensputs 36 16 15 – 5
Kougoedvlakte 53 39 9 – 5
Vaalputs 48 16 23 – 10
Goegap
Ruschia 70 46 22 – 5
Leipoldtia 72 24 – 42 3
Plant Ecol (2011) 212:795–808 799
123
A
0
1
2
3
4r = 0.58***; y = 1.0 + 0.02x
Burdensputs n = 80
0
1
2
3
4r = 0.53***; y = 1.15 + 0.02*x
Kougedvlakte n = 52
0
1
2
3
4 r = 0.71***; y = 1.13 + 0.02x
Vaalputs n = 52
0
1
2
3
4r = 0.69***; y = 1.19 + 0.02x
Goegap n = 70
B
0
1
2
3S. brevifolia: r = 0.23*; y = 0.74 + 0.01xR. robusta: r = 0.46***; y = 0.36 + 0.01x
Burdensputs
0
1
2
3
S. brevifolia: r = 0.38*; y = 0.63 + 0.01xR. robusta: r = 0.46***; y = 0.49 + 0.01x
Kougoedvlakte
0
1
2
3S. brevi fo l ia : r = 0 .42**; y = 0.72 + 0.01xR. robusta : r = 0.60***; y = 0.41 + 0.02x
Vaalputs
0 20 40 60 80 100
0 20 40 60 80 100
0 20 40 60 80 100
0 20 40 60 80 100
0 20 40 60 80 100
0 10 20 30 40 50 60 70 80 90 100
0 20 40 60 80 100
0 20 40 60 80 1000
1
2
3S. brevi fo l ia : r = 0.40***; y = 0.77 + 0.01xR. robusta : r = 0.65***; y = 0.42 + 0.02x
Goegap
Distance (cm) between nearest neighbours
Squ
are
root
tran
sfor
med
and
sta
ndar
dize
d si
zes
(are
a) o
f nea
rest
nei
ghbo
urs
Fig. 3 Correlations between the sizes and the distance
between nearest neighbour plants in inter-specific comparisons
using a combined sizes, and b individual species sizes where
each line shows the effects of one species on the other species.
*P \ 0.05, **P \ 0.001, ***P \ 0.0001. In b straight line,
Stipagrostis brevifolia; dashed line, Ruschia robusta
800 Plant Ecol (2011) 212:795–808
123
A B
0
1
2
3
4
r = 0.73***; y = 0.57 + 0.03x
Burdensputs n = 52
0
1
2
3
4r = 0.62***; y = 0.46 + 0.03x
Kougoedvlakte n = 59
0
1
2
3
4
r = 0.53***; y = 1.30 + 0.02x Vaalputs n = 52
0 20 40 60 80 1000
1
2
3
4
r = 0.70***; y = 0.84 + 0.02xGoega p n = 5 2
0
1
2
3
4
5r = 0.64***; y = 0.65 + 0.03x
Burdensputs n = 51
0
1
2
3
4
5
r = 0.62***; y = 0.46 + 0.03xKougoedvlakte n = 50
0 20 40 60 80 100
0 10 20 30 40 50 60 70 80 90 100
0 20 40 60 80 100
0 20 40 60 80 100
0 20 40 60 80 100
0 20 40 60 80 1000
1
2
3
4
5r = 0.72***; y = 0.77 + 0.03x
Vaalputs n = 52
0 20 40 60 80 1000
1
2
3
4
5 r = 0.78***; y = 0.87 + 0.037xGoegap n = 50
Distance (cm) between nearest neighbours
Squ
are
root
tran
sfor
med
and
sta
ndar
dize
d si
zes
(are
a) o
f nea
rest
nei
ghbo
urs
Fig. 4 Correlations between the size and the distance between
nearest neighbour plants between intra-specific comparisons
using combined sizes between a Stipagrostis brevifolia pairs,
and b Ruschia robusta pairs. ***P \ 0.0001
Plant Ecol (2011) 212:795–808 801
123
shallowest root system and only 3% of the total root
mass was found below 10 cm, and none below 20 cm
(Fig. 7). The roots were also predominantly fine, with
about 80% of the total dry root mass less than 2 mm
in diameter.
The ratios of mean rooting diameter to mean canopy
diameter were significantly different among the three
species F(2,6) = 67.65, P \ 0.0001. Ruschia robusta
had a dramatically greater relative root diameter
(5.77 ± 0.36, mean ± 1 SE) than S. brevifolia
(2.19 ± 0.21) P = 0.0002, and L. pauciflora (2.08 ±
0.24) P = 0.0001. The root mass fraction (root dry
mass/total dry mass) on the other hand was not
significantly different F(2,6) = 2.38, P = 0.17 among
R. robusta (0.26 ± 0.01), S. brevifolia (0.19 ± 0.023,
mean ± SE), and L. pauciflora (0.32 ± 0.07).
Discussion
Competitive relationships
Results of nearest-neighbour analyses in this study
suggest that competitive interactions occur between
adjacent individuals within R. robusta–S. brevifolia
communities, as indicated by positive significant
correlations in both intra-specific and inter-specific
nearest-neighbour comparisons. The relative strengths
A % root mass
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70
Stipagrostis brevifolia
B
Soil depth (cm
)
0-55-10
10-2020-30
30-4040-50 50-60
Soil depth (cm
)
30 80 90 110 120 130 150 16010 20 40 50 100 1400
Horizontal distance (cm)
010
2030
4050
6070
70
Fig. 5 a Mean (±1 SE)
root mass at different soil
depths and b the plant
architecture showing the
root system for Stipagrostisbrevifolia, at
Kougoedvlakte, n = 3
802 Plant Ecol (2011) 212:795–808
123
of intra- and inter-specific competition are relevant to
species co-existence and community stability (Scholes
and Archer 1997). Thus, the higher ratios of intra-
specific to inter-specific competitive relationships
reported in this study between R. robusta and S. brev-
ifolia facilitate co-existence between the two species
along this ecotone. Conspecific individuals are expected
to compete strongly with each other, owing to the
similarities in the individual’s resource requirements
(Yeaton and Cody 1976; Turkington and Harper 1979).
Both intra- and inter-specific competitive relationships
have been reported previously among species of
Stipagrostis genus in the Namib desert (Yeaton 1990).
What brings about lack of intra-specific interactions
among L. pauciflora in the S. brevifolia–Leipoldtia
pauciflora community, on the other hand is not clear and
might require further investigations. Previous studies on
other leaf succulent shrub species like R. robusta,
L. schultzei (Carrick 2003), and L. constricta (Cunliffe
et al. 1990), in the Succulent Karoo have reported
evidence of intra-specific competition.
The distribution of S. brevifolia roots recorded in
the soil profile in this study is comparable to the
root distributions recorded for other grass species
from other deserts. Two grass species in semi-arid
Argentina have root systems mainly distributed in
the top 10–40 cm of the soil profile (Pelaez et al.
1994). In the Chihuahuan desert, the perennial
bunch grass, Hilaria mutica, had roots through the
entire excavated profile of 75 cm, and 70% of these
occurred between 5 and 30 cm (Briones et al.
1996). Ruschia robusta in this study has displayed
A % root mass
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70
coarse roots fine roots
Ruschia robusta
B
Soil depth (cm
) 0-5
5-1010-20
20-3030-40
40-50 50-60
30 10 20 40 50 80 90 110 120 130 150 160100 1400
Horizontal distance (cm)
0 1 0 20
3040
5060 70
Soil depth (cm
)
Fig. 6 a Mean (±1 SE)
root mass at different soil
depths and b the plant
architecture showing the
root system for Ruschiarobusta, at Kougoedvlakte,
n = 3. In a: coarse roots
[2 mm, fine roots \2 mm
Plant Ecol (2011) 212:795–808 803
123
differential rooting depth, with a taproot at
Kougoedvlakte and the absence of a taproot near
Goegap Nature Reserve. However, even where a
taproot is present, most of the roots of R. robusta
occurred in the first 20 cm of the soil. Carrick
(2003), observing similar distribution at Paulshoek
in Namaqualand, described the small proportion of
the root mass extending beneath 20 cm soil layer
in R. robusta, as unusually deep among the leaf
succulent Aizoaceae (‘mesembs’).
The shallow root structure for both R. robusta and
L. pauciflora recorded in this study confirms the
perception of succulents in the Succulent Karoo as
being extremely shallow rooted (Esler and Rundel
1999; Midgley and van der Heyden 1999; Carrick
2003). Shallow roots and succulent leaf tissue are
reported as the two common morphological adapta-
tions of succulent perennial shrubs in a winter rainfall
Namaqualand area enabling them to compete for
water uptake and store water (Von Willert et al. 1992;
Cowling et al. 1999). Shallow roots enable the
‘mesembs’ to take advantage of the frequent small
winter rainfall events which only penetrate the top
few centimetres of the soil (Cowling et al. 1994; Esler
and Rundel 1999; Carrick 2001). Leipoldtia paucifl-
ora with the most shallow rooting system in the
% root mass
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70
Leipoldtia pauciflora
coarse roots fine roots
B
A
Soil depth (cm
)
0-55-10 10- 20 20-30
30-40 40-5050-60
Horizontal distance (cm)
010
2030
4050
6070
Soil depth (cm
)
30 80 90 110 120 10 20 40 50 130 150 160100 1400 70
Fig. 7 A Mean (± SE) root
mass at different soil depths
and b plant architecture
showing the root system for
Leipoldtia pauciflora, at
Goegab Nature Reserve,
n = 3. In a: coarse roots
[2 mm, fine roots \2 mm
804 Plant Ecol (2011) 212:795–808
123
current study is confined to the north western part
of the study area, where the rainfall is typical of
the winter rainfall Succulent Karoo biome. The
morphological plasticity of R. robusta roots may
thus be an adaptation that has contributed to its wider
distribution at the ecotone, where rainfall is less
seasonal and less predictable relative to the main
Succulent Karoo biome. Another mesemb species that
displays a taproot is Ruschia spinosa, which is
widespread in the summer rainfall Nama-Karoo biome
(P. J. Grubb and P. J. Carrick, unpublished data).
Distribution of vertical root structure has been
widely advocated to explain co-existence and com-
petition between species (Walter 1971; Yeaton et al.
1977; Knoop and Walker 1985; Fowler 1986; Briones
et al. 1996). The relative dominance of roots in the
soil between the species is presented in a hypothetical
illustration in Fig. 8 to explain the observed compet-
itive relationships. Leipoldtia pauciflora with most of
its roots in the top 10 cm is expected to utilise water
in this layer, facilitating co-existence with S. brevi-
folia which has most of its absorbing roots between
10 and 40 cm. Lack of competition between
S. brevifolia and L. pauciflora can thus be ascribed
to this clear separation in the vertical root systems.
For S. brevifolia and R. robusta, the degree of partial
overlap in the vertical root distribution may account
for the observed evidence of competition between
them. Stronger competitive impacts of S. brevifolia
on R. robusta may be explained by the fact that
R. robusta has only about 6% of its root mass beneath
20 cm, while a greater proportion of the root system
of S. brevifolia extends deeper down the soil profile.
As each species dominates in a different layer, the net
competition by S. brevifolia on R. robusta in the
communities where they co-occur is minimised,
facilitating their co-existence over a wider area along
the ecotone. These results reveal the possibility of co-
existence between the succulent shrubs and grass,
which has been largely overlooked in the Karoo.
Although untested, it has been proposed that
succulent shrubs and grasses compete for water in
the same horizontal plane (Midgley and van der
Heyden 1999). Similarly, Desmet (2007) contends
that ‘mesembs’ and grasses never co-dominate in
plant communities, as both functional groups share a
similar, shallow root structure.
The competitive relationship for soil resources
between grass and succulent shrubs in these commu-
nities is unique and contrasts with savanna compe-
tition-based models where grass is proposed to obtain
water from the top layer and shrubs mainly from
deeper soil layers (Walter 1971; Walker and
Noy-Meir 1982; Knoop and Walker 1985; Sala
et al. 1989; Kochy and Wilson 2000; Snyman
2005). The succulent shrubs are shallower rooted
comparing to grass and are thus expected to utilise
water mainly from the top layer. Ruschia robusta,
with a high proportion of root mass in the first 20 cm
of the soil is expected to predominantly utilise water
in the upper layers of the soil, and this reduces the net
competition from the deeper rooting grass. A thor-
ough analysis of water relations between grass and
succulent shrubs is, however, required, to improve
our understanding of species co-existence between
the two growth forms. Furthermore, grasses are
generally hypothesised to have higher root mass
fraction compared to trees and non-succulent woody
plants (Waisel et al. 1991; Kochy and Wilson 2000;
A B
Predominantly R. robusta
Strong overlap
S. brevifolia
0
60
50
40
30
20
10
Predominantly S. brevifolia
Soil depth (cm
)
Predominantly L. pauciflora
Predominantly S. brevifolia
S. brevifolia
0
60
50
40
30
20
10
Fig. 8 Hypothetical model of relative dominance of roots in a
soil profile of Stipagrostis brevifolia and a Leipoldtia paucifl-ora and b Ruschia robusta. The clear vertical separation in root
proportion in a avoids competition, whereas the partial overlap
in b allows for competition between the species
Plant Ecol (2011) 212:795–808 805
123
Snyman 2005), but a contrasting observation indi-
cated that succulent shrubs have greater root mass
proportion than grass, as shown for L. pauciflora and,
R. robusta in this study, and L. schultzei in a previous
study (Carrick 2003). We demonstrate that succulent
shrubs do fit the simple two-layer hypothesis but they
operate in the inverse way to that proposed for
savanna grass–woody systems.
In addition to differential root distribution dis-
cussed in this study as a mechanism that facilitates
species co-existence at the ecotone, the grass and
succulent shrub species dominate biomes character-
ised by summer and winter rainfall, respectively.
This may suggest that a certain degree of temporal
segregation in plant growth and water uptake among
the species is likely, as previously hypothesised
(Midgley and van der Heyden 1999). Variations in
seasonal rainfall patterns have profound effects on the
abundance of grass and Karoo shrubs in the eastern
Karoo (Hoffman et al. 1990; Hoffman and Cowling
1990; Bousman and Scott 1994; O’Connor and Roux
1995). A detailed investigation of the phenological
patterns of grass and shrubs at the ecotone will yield
useful information on this.
Competition and community dynamics
The asymmetric competitive relationship between
S. brevifolia and R. robusta is consistent in all the
communities studied. While a small competitive
difference can be sufficient to cause rapid extermina-
tion of the less competitive species (Hardin 1960), the
persistence, and therefore co-existence, of species in
any community depends on the sum of its fitness and
abundance throughout its life phases. The very high
number of seeds produced by R. robusta, and the
extreme drought tolerance of its seedlings (Hoffman
et al. 2009) may lead to far higher numbers of
R. robusta seedlings establishing successfully, than
those of S. brevifolia, and this may compensate for its
relatively poor competitive ability as an adult, and
hence mitigate for co-existence in the community.
Unfortunately, the nearest-neighbour method, as used
here, primarily measures the impact of competition on
the growth of neighbouring adult plants, which is
distinct from the impact of competition on the
survival, or from the impact of competition of adults
on seedlings, both of which influence persistence and
co-existence more directly than growth.
From the observed nature of the competitive
interactions, and patterns in rooting depths, it is
inferred that the stronger competition from grass
indicates reduced soil water availability to R. robusta
individuals. In the presence of S. brevifolia, the results
likely indicate a reduction in water availability in the
upper soil layer, where most of the root mass of
R. robusta is concentrated. This competitive dynamic
will interact with rainfall patterns across the ecotone,
such that changes in winter- or summer-rainfall
patterns, associated with climate change, may influ-
ence the overall fitness of the two ecotonal growth
forms in the study area by influencing their establish-
ment, survival and mortality. The winter-rainfall
shrublands in the west receives primarily small frontal
rainfall events that do not percolate deeply, while the
summer-rainfall grasslands in the east receive primar-
ily larger convective rainfall events that penetrate the
soil more deeply (Carrick 2001). While there is
conjecture concerning trends, and future predictions,
of a shift in the magnitude or distribution of the two
rainfall systems, as a result of anthropogenic climate
change (MacKellar et al. 2007; Hoffman et al. 2009), a
concomitant shift in the co-existence and distribution
of the dominant grass and leaf succulent shrubs with
such a shift in rainfall patterns is consistent with the
results of this study. This interpretation is in line with
previous non-mechanistic postulations, e.g. that of
Esler and Rundel (1999) who suggest that an increased
summer rainfall could lead to a rapid increase in grass
cover and thus an increased competitive pressure from
grasses.
This study indicated that both intra- and inter-
specific competition are important for co-existence
between R. robusta and S. brevifolia in these com-
munities. Competitive relationships (observed in
R. robusta–S. brevifolia combinations) as well as
lack thereof (in L. pauciflora–S. brevifolia combina-
tion) were explained by the vertical distribution of the
root systems in the soil. The root distribution between
the grass and the shrubs described in this study
demonstrate the possibility of co-existence of two
growth forms according to the two-layer hypothesis
for water acquisition among grasses and trees
proposed in the savanna. However, in this arid
environment, the upper layer is occupied by the roots
of a succulent shrub which appear to have specialised
in capitalising on extremely small rainfall events, and
the lower layers occupied by grass roots. Our findings
806 Plant Ecol (2011) 212:795–808
123
make a contribution to arid ecology in general, and
specifically to community ecology in the Karoo
where the possibility of co-esistence between the
succulent shrubs and grass has been largely over-
looked or misinterpreted. The results also show how
understanding the nature of competitive interactions
at a climate ecotone can be used to predict how the
vegetation may respond to climate change and
provide ecological insights in understanding observed
or expected changes in species distribution along the
ecotone.
Acknowledgments This study was funded under the
umbrella of the BIOTA Southern Africa project (www.
biotaafrica.org), by the German Federal Ministry of Educa-
tion and Research under promotion number 01 LC 0024A. We
thank the Mazda Wildlife Vehicle Fund for the use of a
courtesy vehicle during the course of the study. Rainfall data
for Goegap Nature Reserve and Vaalputs Radioactive Waste
Disposal Facility were supplied by their respective offices,
while Burdensputs and De Riet rainfall data were obtained
from the landowners. We gratefully acknowledge the land-
owners in Namaqualand for kindness and permission to work
on their land, the management of Vaalputs Radioactive Waste
Disposal Facility for permission to work on Vaalputs farm, and
the Northern Cape Department of Nature Conservation for
permission to undertake research on Goegap Nature Reserve.
Special thanks to Mariana Lot, the para-ecologist at Paulshoek,
for field assistance. The following people are thanked for
assistance in one way or the other during field trips, Lee
Simons, Azieb Woldensae, Sebataolo Rahlao, Elizabeth Cla-
assen and Vonkie Claassen.
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