Factors Influencing Recruitment of
Forage Plants in Arid Karoo Shrublands,
South Africa
Suzanne J. Milton
W. R. J. Dean
Abstract—We recorded mortality and natality in populations of
long-lived shrubs over five years in arid rangelands of the southern Karoo, South Africa and studied natural regeneration and survival of sown seeds in cleared plots protected or exposed to grazing.
Very little recruitment occurred in undisturbed shrub populations.
Germination was a function of seed availability and of autumn
rainfall, whereas recruitment was influenced by proximity of seedlings to established shrubs and by follow-up rainfall in spring and
summer. On the basis of these observations, we propose a simple
model to predict the circumstances under which rangeland forage
plants may be restored by resting, reseeding, clearing or combinations of these procedures.
800 m above sea level) in the arid shrublands of the southern Karoo, South Africa (Milton and others 1992). The area
receives a variable rainfall (167 mm p.a., range 50-400 mm
over 92 years) that peaks in autumn (March-April). Soils
are fine and alkaline and the vegetation is low growing
(<0.8 m) and clumped with a projected canopy cover <25%
(Figs 1a & 1b).
Rangeland at TKRC was in good condition (20,900 forage
plants/ha). Adjoining ranch 1 was in good condition and
moderately grazed (about 6 ha/sheep). Ranch 2 was in poor
condition, and although moderately stocked during the
study period (7.5 ha/sheep), was still carrying too many
There is a perceived need to increase the productivity
and species diversity of overgrazed arid shrublands in the
Karoo region of South Africa. Experience has shown that
this goal cannot be achieved simply by resting or intensive
browsing of range, because overgrazed shrublands are often
dominated by long-lived shrubs that are distasteful to domestic livestock. Overgrazed shrubland may remain unsuitable for ranching for many years because the component
species are long-lived and do not necessarily facilitate succession to alternative states (Westoby and others 1989).
Partial clearing of existing vegetation to alter water and
nutrient availability (Luken 1990), or re-seeding to establish
indigenous forage species, could possibly increase carrying
capacity or nature conservation value of such shrublands.
This paper reports on the demography of shrub populations of the arid Karoo and on effects of weather, neighboring plants, microsites and grazing animals on the survival
of seedlings. These preliminary observations provide a
basis for assessing the feasibility of rehabilitating Karoo
rangeland.
Study Site and Methods
The study was carried out at Tierberg Karoo Research
Center (TKRC), from which domestic livestock were excluded, and on adjacent sheep ranches (33°10'S, 22°17'E,
In: Roundy, Bruce A.; McArthur, E. Durant; Haley, Jennifer S.; Mann,
David K., comps. 1994. Proceedings: wildland shrub and arid land restoration symposium; 1993 October 19-21; Las Vegas, NV. Gen. Tech. Rep.
INT-GTR-315. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Intermountain Research Station.
Sue Milton and Richard Dean are Senior Research Officers with the
Percy FitzPatrick Institute of African Ornithology, University of Cape
Town, Rondebosch 7700, South Africa.
Figure 1—(a) Arid shrubland at Tierberg Karoo Research Centre in the southern Karoo, South Africa.
(b) Mixed species clumps of dwarf shrubs surrounded by bare ground.
216
sheep for the available forage (4,200 forage plants/ha). Indigenous steenbok (Raphicerus campestris: Bovidae) and
hares (Lepus capensis: Leporidae) were present at all three
sites at low densities (total biomass of mammalian herbi2
vores 135 kg/km ).
left on the remainder of the plots. All seedlings on these
plots were counted in April 1991, 1992 and 1993.
Sowing Experiments
A total of 700 seeds of each of the 3 shrub species
and 700 seeds of a winter annual (Tetragonia echinata:
Aizoaceae) were sown in March 1990 in the 50 vegetated
monitoring plots and in the 20 cleared plots. Treatments
were replicated as shown in Fig. 2. Germination and survival of seedlings emerging from the planted seeds were
made 5 days after every rain event, and once monthly in
the absence of rain. It was assumed that seedlings emerging in demarcated rows originated from sown seed.
A further trial, using only O. sinuatum seeds, was initiated in April 1991. Approximately 100 seeds (1 g) were
planted in and adjacent to each of 6 cleared plots on the
heavily grazed sheep ranch. Three of these plots were in
exclosures and three were in the surrounding area that
was grazed by sheep.
Flowering, Natality and Mortality
Three common species of dwarf shrubs (all Asteraceae)
were selected for study: deciduous broad-leaved Osteospermum sinuatum preferred by sheep, evergreen microphyllous
Pteronia empetrifolia palatable to sheep, and evergreen
microphyllous P. pallens which is toxic to sheep. All individuals in 50 permanent 5 x 5 m quadrats were measured
(height, canopy diameter, basal stem diameter) and labelled
in November 1988. They were subsequently monitored annually until December 1991. Throughout this time the plots
were subjected to one of four different grazing treatments
as shown in Fig. 2. Despite the partly pseudo-replicated
sampling design, inferential statistics were used to test for
site and treatment effects on growth, survival and natality.
Weather Records and Soil Moisture
Seedling Emergence and Survival in
Natural Vegetation
Rainfall, mean daily temperature maxima and minima,
relative humidity and soil moisture data were collected by
a data logger at TKRC. Relative soil moisture was measured using calibrated nylon sensors buried at 50 and 150 mm
below the soil surface in one vegetated and one cleared plot.
Emergence and survival of seedlings in natural vegetation was monitored in 100 wire hoops (160 mm diameter),
two in each of 50 plots used for demographic monitoring.
All seedlings in the hoops were recorded five days after each
major rain event (>10 mm) and in dry periods, at 4-6 weekly
intervals.
Results
Seedling Survival in Cleared Plots
Flowering, Natality and Mortality
Twenty plots in TKRC, 10 in exclosures and 10 open to
grazing by indigenous mammals, were cleared of all vegetation in July 1989 by cutting plants at soil level. Cut
plants were removed from 5 plots in each treatment and
First flowering occurred at 2-3 years (5 mm b.d.) in O.
sinuatum and at 3-5 years (>10 mm b.d.) in Pteronia spp.
Sheep reduced flower production by 63% in O. sinuatum
and by 68% in P. empetrifolia but had no effect on toxic
P. pallens. The numbers of seedlings of a given species
that emerged in a plot were related to the numbers of flowers (or seeds) of that species in the plot (Fig. 3). There was
therefore little regeneration of palatable species where
sheep removed most of the flowers. Over the 3 yr period
(1988-1991) the annual turnover was <6% in all three species. Grazing had no effect on the natality : mortality ratio,
and 88% of the 104 recorded fatalities were among seedlings.
Background Seedling Emergence and
Survival
Seedlings emerged in early winter (April-June) when
relative humidity was high and temperatures were low
(Figs 4a & 4b). Although the emergence density of seedlings was related to pre-emergence rain, their survival
was correlated (P < 0.001) with post-emergence rainfall
(July-October).
More seedlings emerged on the overgrazed ranch than
elsewhere (P < 0.001) but the percentage that survived
did not differ between sites or treatments (Fig. 4b). Survival averaged (mean ± SD) 3.0 ± 2.5% in 1989, 3.3 ± 2.0
in 1990 and 26.1 ± 5.2 in 1991. Most seedlings (86%) were
of small (<0.5 mm) seeded Aizoaceae, the seeds of which
Figure 2—Layout of exclosures, grazing and clearing treatments at Tierberg Karoo Research Centre
(TKRC) and on adjacent sheep ranches. Exclo–
sures on Argentina ranch were used only in the
second seeding trial.
217
were dispersed from hygroscopic capsules during rain
showers. However, on littered or vegetated microsites,
19% (247/1320) of emergent seedlings were species with
large (>2 mm) wind dispersed seeds, compared with 7%
(78/1056) of seedlings that emerged on bare soil.
Clearing Experiments
Clearing of vegetation reduced the rate at which moisture
was lost from the upper 15 cm of the soil (Fig. 5).
The species composition of seedlings emerging from
naturally dispersed seeds on cleared plots was correlated
(P < 0.001) with the cover composition of the vegetation
on the plots prior to clearing. The distance between a
seedling and the perimeter of the cleared plot influenced
its chances of survival. Seedlings emerging 2-3 m from
established plants survived at higher densities (P < 0.001),
and reproduced earlier (P < 0.05) than seedlings that
emerged closer to neighbours (Fig. 6).
In the first sowing trial, more seedlings emerged on
cleared (7.0 ± SD 9.5%) than on vegetated plots (3.9 ± SD
8.6%). Some of these shrub seedlings survived on cleared
plots, but all shrub seedlings that emerged in vegetated
plots died during their first summer (Fig. 7). No further
shrub emergence occurred in the second autumn, but the
winter annual (T. echinata) had many innately dormant
seeds which emerged at higher densities in the second
autumn (Fig. 7). The resultant plants set more seed in
cleared than in vegetated plots (Table 1).
In the second sowing trial, on the overgrazed ranch, 25
O. sinuatum seedlings emerged in vegetated plots and 92
emerged in cleared plots. Survival, after 2 years, was similar in vegetated (24%) and cleared (23%) plots.
In the first sowing trial, emergence and survival of shrub
seedlings, but not winter annuals, was greater on overgrazed Ranch 2 than elsewhere (Fig. 7). In the second sowing trial on the overgrazed ranch, 59 O. sinuatum seedlings
emerged in grazed areas and 58 emerged in exclosures.
After 2 years, survival in grazed plots (10%) was lower
Figure 3—Relationships between densities of
emerging seedlings and the number of flowers or
seeds of three non-succulent shrubs and of succulent Mesembryanthema in the southern Karoo.
Figure 4—(a) Rainfall, temperature range and humidity at Tierberg Karoo Research Centre in the
southern Karoo, and (b) seedling emergence over
four years in exclosures at TKRC (no sheep) and
on adjoining sheep ranches.
Figure 5—Soil moisture fluctuations at 150 mm below soil surface in undisturbed and cleared vegetation. Rain in millimeters per day.
218
Table 1—Numbers of seeds produced by a winter annual (Tetragonia echinata) grown in cleared and vegetated plots in
exclosures and grazed rangeland in the southern Karoo.
Means with shared superscripts do not differ significantly
(ANOVA, P < 0.01).
Number
of plants
Treatment
Exclosure cleared
Rangeland cleared
Exclosure vegetated
Rangeland vegetated
11
24
24
28
Seeds per plant
Mean
± SD
6.36a
4.00ab
0.95b
1.21b
7.77
6.83
0.85
1.19
2
(X = 9.7, 1 df, P < 0.01) than in exclosures (36%). Seedlings in exclosures were larger (P < 0.001) than in plots
grazed by sheep (Table 2).
Discussion
Implications of Low Turnover Rates for
Management
Figure 6—Box and whisker plot showing median,
upper and lower quartiles and ranges of seedling
densities at three years in undisturbed vegetation
and in cleared plots at distances of 1, 2, and 3 m
from neighboring plants.
Karoo shrublands, in common with those in arid parts of
the United States (McAuliffe 1988) and Australia (Eldridge
and others 1990), are dominated by plants that live for decades or centuries. Population turnover rates are low. For
this reason, compositional losses caused by overgrazing or
resting will be very slow. Restoration of productivity or
diversity to over-exploited arid shrubland within a human
lifetime may require re-seeding and active management of
herbivory, competition and microsites.
Factors Influencing Forage Recruitment in
Karoo Shrublands
Defoliation and florivory reduces seedling recruitment
in many Karoo plants (Milton 1992; Milton and Dean 1988,
1990b, 1993; van Breda and Barnard 1991). Few species
of Karoo succulents (Esler and others 1992) and long-lived
shrubs have innately dormant seeds. Such species rely on
regular seed production for population perpetuation, and
are therefore lost from overgrazed rangeland (Milton 1992;
O’Connor 1991). By altering the growth rates of forage
Table 2—Heights (mean ± one standard deviation) of one- and
two-year-old Osteospermum sinuatum seedlings from
seeds sown in Trial 2 on an overgrazed Karoo shrubland.
Seeds were sown in cleared or vegetated exclosures and
in cleared or vegetated rangeland grazed by sheep.
Means with shared superscripts do not differ significantly
(ANOVA, P < 0.01).
Treatment
Exclosure cleared
Exclosure vegetated
Rangeland cleared
Rangeland vegetated
Figure 7—Emergence and survival of sown seeds
in exclosures, grazed vegetation, and cleared plots.
219
n
12
16
13
3
May 1992
height ± SD
11.6a
4.6b
3.2b
4.0b
3.7
1.7
1.3
2.0
n
15
6
6
0
August 1993
height ± SD
13.0a
3.2
6.7b
3.4
4.5b
1.5
no survivors
plant populations, herbivory can bring about changes in
vegetation composition.
Most shrubs and succulents germinate in autumn in the
southern Karoo (van Breda and Barnard 1991). Large germination events were related to seed availability (dependent
on vegetation composition, current herbivory and rainfall).
Few seedlings reach reproductive maturity in undisturbed,
arid shrublands (Eldridge and others 1990; Milton 1993;
Owens and Norton 1992). In the southern Karoo, seedling
survival was dependent on moisture availability in the six
months after emergence. Seedlings survived where competition from established plants had been reduced, prolonging
water availability after rain events.
Droughts (Danckwerts and Stuart-Hill 1988), hail storms
(Powrie 1993) and intensive trampling and grazing (Bosch
and Gauch 1991) also reduce competition and provide opportunities for seedling establishment. Insects and mammals that uproot plants, dig pits, or excavate nests ensure
continual seedlings recruitment by creating establishment
sites in stable vegetation (Dean and Milton 1991; Milton
and Dean 1990a; Dean and Yeaton 1992).
Seed traps influence the composition and arrangement of
Karoo vegetation (Fig. 8). Small seeds (mostly produced by
low-growing succulent mesembryanthemaceae) are trapped
by fine soil particles so that their seedlings occur mainly
in inter-shrub gaps. Winged or bristled seeds (Liliaceae,
Asteraceae, Aizoaceae) are tumbled by wind until trapped
in multi-stemmed plants, litter or mammal diggings (Dean
and Milton 1991; Hoffman and Cowling 1987; Milton 1993).
Non-succulent shrubs establish beneath low-growing,
succulent hosts which they later out-compete (Yeaton and
Esler 1990). The shrubs tolerate one another for decades,
forming mixed-species clumps. In this way Karoo vegetation is arranged in a mosaic of plant islands and bare
ground, much like the vegetation of the Chihuahuan Desert,
Mexico (Montaña 1992). Grazing, and other factors that
increase the proportion of bare ground to vegetated and littered microsites, influence the composition of vegetation.
Model for Change in Southern Karoo
Shrublands
On the basis of the foregoing discussion, improvement
in composition of Karoo rangeland could be achieved by:
a) removing herbivores, b) adding seed of forage species,
c) reducing competition from established plants, and d)
adding seed traps. In addition to this it may be necessary
to restore ecosystem functioning (Milton and others 1994).
This could involve soil amelioration (Dean 1992; Roux and
Opperman 1986; Schlesinger and others 1990; Snyman and
Fouché 1991), and reintroduction of animal or microbe species that move soil, facilitate nutrient uptake, pollination or
dispersal, or alter competitive interactions between plant
species (Bond 1993; MacMahon 1987).
The model (Fig. 9) presents hypothetical mechanisms by
which an overgrazed arid shrubland (1), could be rehabilitated to a more productive and diverse shrubland (2), or further degraded to distasteful (3) or ephemeral vegetation (6).
Figure 8—Recruitment opportunities for plant species with small, smooth seed and large,
winged seed.
220
There is little information on how transitions from ephemerals to perennials may be facilitated, but our experiments
suggest that seed traps should be provided, or small-seeded
species that can establish in the open should be selected
for initial re-seeding.
An important aspect for future research is the effect of
scale on vegetation rehabilitation. We have no information
on the effects of the size of an area of transformed Karoo
vegetation on its prospects for recovery, and annual dispersal distances of the component plant species are unknown.
Acknowledgments
This report is a contribution to the Desertification Programme of the FitzPatrick Institute, University of Cape
Town. The Programme is funded by the Foundation for
Research Development, the Department of Environment
Affairs and the Southern African Nature Foundation. Attendance at the Wildland Shrub and Arid Land Restoration
Symposium was funded by the Foundation for Research
Development and the FitzPatrick Institute. We thank
M.T. Hoffman, M.C. Rutherford, and W.R. Siegfried for
comments and suggestions on a draft of this paper.
Figure 9—Rehabilitation model for Karoo shrubland.
The mechanisms for transitions between these states
of the vegetation include stock withdrawal, natural disturbances (grazing, trampling, drought), and active management of either the vegetation (selective clearing, re-seeding)
or of the environment. Although oversimplified, the model
provides a variety of testable hypotheses. Following Savory
(1991), many livestock ranchers believe that short-duration,
high-intensity grazing increases both productivity and
abundance of forage species. We have excluded this mechanism from our Karoo rehabilitation model because there
is no evidence that it increases forage plant populations
in arid Karoo shrubland (Hoffman 1988).
References
Bond, W.J. 1993. Keystone species. In: Schulze, E.D.;
Mooney, H.A. eds. Biodiversity and Ecosystem Function.
Springer, Berlin: 238-253.
Bosch, O.J.H.; Gauch, H.G. 1991. The use of defoliation
gradients for the assessment and ecological interpretation of range condition. Journal of the Grassland Society
of southern Africa 8: 138-146.
Danckwerts, J.E.; Marais, J.B. 1989 An evaluation of the
economic viability of commercial pastoralism in the
Smaldeel area of the eastern Cape. Journal of the Grassland Society of southern Africa 6: 1-7.
Danckwerts, J.E.; Stuart-Hill, G.C. 1988. The effect of severe drought and management after drought on the mortality and recovery of semi-arid grassveld. Journal of the
Grassland Society of southern Africa 5: 218-222.
Dean, W.R.J. 1992. Effects of animal activity on the absorption rate of soils in the southern Karoo, South Africa.
Journal of the Grassland Society of southern Africa 9:
178-180.
Dean, W.R.J.; Milton, S.J. 1991. Disturbances in semi-arid
shrubland and arid grassland in the Karoo, South Africa:
mammal diggings as germination sites. African Journal
of Ecology 29: 11-16.
Dean, W.R.J.; Yeaton, R 1992. The importance of harvester
ant Messor capensis nest-mounds as germination sites
in the southern Karoo. African Journal of Ecology 30:
335-345.
Eldridge, D.J.; Westoby, M.; Stanley, R.J. 1990. Population
dynamics of the perennial rangeland shrubs Atriplex
vesicaria, Maireana astrotricha and M. pyramidata under grazing. Journal of Applied Ecology 27: 502-512.
Esler, K.J.; Cowling, R.M.; Ivey, P. 1992. Seed biology of
three species of Mesembryanthema in the southern Karoo.
South African Journal of Botany 58: 343-349.
Hazards of Vegetation Manipulation
As indicated in Fig. 9, reversal of changes involving the
replacement of one long-lived plant species by another may
require active intervention by the land manager. There is
no universally correct way to manage rangeland (Noy-Meir
1993): all techniques should be critically evaluated by experiment for each type of range. Reseeding, with or without partial clearing, tilling or mulching have all been attempted in arid and semi-arid southern Africa (Roux and
Vorster 1983). All are costly, none is infallible, and some
may exacerbate existing rangeland problems.
Seed addition could increase forage plant abundance but
is unlikely to succeed without some reduction of established
plants and temporary protection from grazing. Clearing
should be approached with caution because it increases
runoff (Snyman and Fouché 1991) and because the costs
involved in vegetation manipulation are unlikely to be offset by short-term benefits in arid rangelands (Danckwerts
and Marais 1989).
Droughts and hail storms that kill many established
plants may provide windows of reduced competition when
supplementary seeding of forage plants could lead to recruitment. Walker and others (1986); and Westoby and
others (1989); suggested that opportunistic management
could capitalize on just such natural disturbances. Clearing prior to re-seeding appears to be unnecessary where
grazing has reduced above-ground perennial biomass below the normal range for the region.
221
Hoffman, M.T. 1988. The rationale for Karoo grazing systems: criticisms and research implications. South African
Journal of Science 84: 556-559.
Hoffman, M.T.; Cowling, R.M. 1987. Plant physiology, phenology and demography. In Cowling, R.M. & Roux, P.W.
(eds) The Karoo Biome: a preliminary synthesis. South
African Scientific Programmes Report 142: 1-34.
Luken, J.O. 1990. Directing ecological succession. Chapman
and Hall. London.
MacMahon, J.A. 1987. Disturbed land and ecological theory:
an essay about a mutualistic association. In: Jordan,
W.R.; Gilpin, M.E.; Aber, J.D. eds. Restoration ecology.
Cambridge Univ. Press, Cambridge: 221-237.
McAuliffe, J.R. 1988. Markovian dynamics of simple and
complex desert plant communities. American Naturalist
131: 459-490.
Milton, S.J. 1992. Effects of rainfall, competition and grazing on flowering of Osteospermum sinuatum (Asteraceae)
in arid Karoo rangeland. Journal of the Grassland Society of southern Africa 9: 158-164.
Milton, S.J. 1993. Growth, flowering and recruitment of
shrubs in grazed and in protected rangeland in the arid
Karoo, South Africa. Vegetatio.
Milton, S.J.; Dean, W.R.J. 1988. Flower and fruit production of Rhigozum obovatum (Bignoniaceae) in road reserves and grazing land. South African Journal of Science 84: 798-799.
Milton, S.J.; Dean, W.R.J. 1990a. Mima-like mounds in the
southern and western Cape: are the origins so mysterious? South African Journal of Science 86: 207-208.
Milton, S.J.; Dean, W.R.J. 1990b. Seed production in rangelands of the southern Karoo. South African Journal of
Science 86: 231-233.
Milton, S.J.; Dean, W.R.J. 1993. Selection of seeds by harvester ants (Messor capensis) in relation to condition of
arid rangeland. Journal of Arid Environments 24: 63-74.
Milton, S.J.; Dean, W.R.J.; Kerley, G.I.H. 1992. Tierberg
Karoo Research Centre: history, physical environment,
flora and fauna. Transactions of the Royal Society of
South Africa 48: 5-46.
Milton, S.J.; Dean, W.R.J.; du Plessis, M.A.; Siegfried, W.R.
1994. A conceptual model of rangeland degradation: the
escalating cost of declining productivity. BioScience. 44:
70-76.
Montaña, C. 1992. The colonization of bare areas in twophase mosaics of an arid ecosystem. Journal of Ecology
80: 315-327.
222
Noy-Meir, I. 1993. Compensating growth of grazed plants
and its relevance to the use of rangelands. Ecological
Applications 3: 32-34.
O’Connor, T.G. 1991. Local extinction in perennial grasslands: a life-history approach. American Naturalist 137:
753-773.
Owens, M.K.; Norton, B.E. 1992. Interactions of grazing
and plant protection on basin big sagebush (Artemisia
tridentata ssp. tridentata) seedling survival. Journal of
Range Management 45: 257-262.
Powrie, L. 1993. Responses of Karoo plants to hail damage
near Williston, Cape Province. South African Journal of
Botany 59: 65-68.
Roux, P.W.; Opperman, D.P.J. 1986. Soil erosion. In:
Cowling, R.M.; Roux, P.W.; Pieterse, A.J.H. eds. The
Karoo Biome: A Preliminary Synthesis. Part 1 - Physical
environment. South African National Scientific Programmes Report 124: 92-111.
Roux, P.W.; Vorster, M. 1983. Development of veld management research in the Karoo region. Proceedings the
Grassland Society of southern Africa 18: 30-34.
Savory, A. 1991. Holistic resource management: a conceptual framework for ecologically sound economic modelling. Ecological Economics 3: 181-191.
Schlesinger, W.H.; Reynolds, J.F.; Cunningham, G.L.;
Huenneke, L.F.; Jarrell, W.M.; Virginia, R.A.; Whitford,
W.G. 1990. Biological feedbacks in global desertification.
Science 247: 1043-1048.
Snyman, H.A.; Fouché, H.J. 1991. Production and wateruse efficiency of semi-arid grasslands of South Africa as
affected by veld condition and rainfall. Water SA, 17:
263-268.
van Breda, P.A.B.; Barnard, S.A. 1991. 100 veld plants of
the winter rainfall region. Bulletin 422, Department of
Agricultural Development, Pretoria, South Africa.
Walker, B.H.; Matthews, D.A.; Dye, P.J. 1986. Management
of grazing systems - existing versus an event-orientated
approach. South African Journal of Science 82: 172.
Westoby, M.; Walker, B.; Noy-Meir, I. 1989. Opportunistic
management for rangelands not at equilibrium. Journal
of Range Management 42: 266-274.
Yeaton, R.I.; Esler, K.J. 1990. The dynamics of a succulent
Karoo vegetation. A study of species association and recruitment. Vegetatio 88: 103-113.