Litter dynamics across browsing-induced fenceline contrasts in succulent thicket, South Africa

Article · November 2008with100 Reads
DOI: 10.1016/j.sajb.2008.04.002
Abstract
Semi-arid succulent thicket in South Africa has experienced extensive livestock-induced transformation, reflected in extensive structural changes and loss of biodiversity, biomass and soil carbon. The ecological mechanisms contributing to this transformation are not fully understood but are believed to include the breakdown of ecosystem processes including litter production and decomposition, which are rate-limiting steps in nutrient cycling and incorporation of organic matter into the soil. In this study we investigated the effect of transformation on litter production and decomposition in succulent thicket. We measured litter production and decomposition of four dominant perennial woody plants (Euclea undulata, Pappea capensis, Portulacaria afra and Rhus longispina) across replicated fenceline contrasts. Litter production was measured over 14 months using mesh traps. Decomposition was measured over 15 months using a combination of litterbags and leaf packs. Litter production in succulent thicket was very high for a semi-arid system (approaching that of temperate forests), with the leaf- and stem-succulent P. afra contributing the largest component. Transformation caused a significant reduction in litter production at a landscape scale (4126 vs 2881 kg/ha/yr), primarily due to reduced cover of P. afra. Surprisingly, transformation had few significant effects on the rate of decomposition of litter, possibly due to a switch from biotic to abiotic decomposition processes. The perennial vegetation in succulent thicket, particularly P. afra, appears to play a critical role in the maintenance of the ecosystem by facilitating the incorporation of organic matter into soil. Transformation of succulent thicket leads to a disruption of the carbon cycle, ultimately resulting in degradation of the ecosystem. Successful restoration is likely to depend on increasing the rates of organic matter return to soils. P. afra is a potential carbon restoration pump as it is both drought-resistant and easily propagated from cuttings.
2 Figures
Litter dynamics across browsing-induced fenceline contrasts in succulent
thicket, South Africa
R.G. Lechmere-Oertel
a
, G.I.H. Kerley
a
, A.J. Mills
b,
, R.M. Cowling
c
a
Centre for African Conservation Ecology, Department of Zoology, Nelson Mandela Metropolitan University, Box 77000, Port Elizabeth 6031, South Africa
b
Department of Soil Science, Stellenbosch University, Private Bag X1, Matieland, 7602, South Africa
c
Department of Botany, Nelson Mandela Metropolitan University, Box 77000, Port Elizabeth 6031, South Africa
Received 3 September 2007; received in revised form 27 March 2008; accepted 16 April 2008
Abstract
Semi-arid succulent thicket in South Africa has experienced extensive livestock-induced transformation, reflected in extensive structural changes
and loss of biodiversity, biomass and soil carbon. The ecological mechanisms contributing to this transformation are not fully understood but are
believed to include the breakdown of ecosystem processes including litter production and decomposition, which are rate-limiting steps in nutrient
cycling and incorporation of organic matter into the soil. In this study we investigated the effect of transformation on litter production and
decomposition in succulent thicket. We measured litter production and decomposition of four dominant perennial woody plants (Euclea undulata,
Pappea capensis,Portulacaria afra and Rhus longispina) across replicated fenceline contrasts. Litter production was measured over 14 months
using mesh traps. Decomposition was measured over 15 months using a combination of litterbags and leaf packs. Litter production in succulent
thicket was very high for a semi-arid system (approaching that of temperate forests), with the leaf- and stem-succulent P. afra contributing the largest
component. Transformation caused a significant reduction in litter production at a landscape scale (4126 vs 2881 kg/ha/yr), primarily due to reduced
cover of P. afra. Surprisingly, transformation had few significant effects on the rate of decomposition of litter, possibly due to a switch from biotic to
abiotic decomposition processes. The perennial vegetation in succulent thicket, particularly P. afra, appears to play a critical role in the maintenance
of the ecosystem by facilitating the incorporation of organic matter into soil. Transformation of succulent thicket leads to a disruption of the carbon
cycle, ultimately resulting in degradation of the ecosystem. Successful restoration is likely to depend on increasing the rates of organic matter return to
soils. P. afra is a potential carbon restoration pump as it is both drought-resistant and easily propagated from cuttings.
© 2008 SAAB. Published by Elsevier B.V. All rights reserved.
Keywords: Browsing impacts; Euclea undulata; Nutrient cycling; Pappea capensis;Portulacaria afra;Rhus longispina; Succulent thicket
1. Introduction
In the semi-arid rangelands of the world, transformation as a
result of unsustainable stocking rates is primarily recognised
through structural changes in the vegetation. This is particularly
evident in the arid succulent thickets (Vlok et al., 2003) of South
Africa. Up to 70% of this vegetation has been transformed
(Lloyd et al., 2002), in the sense that there have been significant
losses in biomass. Intact succulent thicket has unusually high
biomass for a semi-arid vegetation (Mills et al., 2005a) and is
loosely organised into a two-phase mosaic of perennial
vegetation patches (550 m across) and bare ground (Fabricius,
1997; Kerley et al., 1999;Lechmere-Oertel et al., 2005). These
patches comprise evergreen to weakly deciduous trees (b5m
tall), emergent from a matrix of woody and succulent shrubs,
often dominated by the evergreen leaf- and stem-succulent shrub
Portulacaria afra (L.) Jacq. (Didiereaceae; the common name is
spekboom, which translates from Afrikaans as fat tree). The
P. afra matrix is inter-woven with a variety of multi-stemmed
deciduous and spinescent shrubs. The nutrient- and clay-rich soil
(derived from shales and mudstones) beneath the vegetated
patch is covered by a thick (up to c. 10 cm) layer of plant litter.
A
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doi:10.1016/j.sajb.2008.04.002
P. afra is unusual in that it is able to switch between C3 and
CAM photosynthetic pathways, depending on soil moisture
(Guralnick et al., 1984; Guralnick and Ting, 1987). This enables
P. afra plants to assimilate carbon even in times of drought.
This is an appropriate strategy given the semi-arid climate
(mean annual precipitation [MAP] of 25400 mm distributed
throughout the year, with spring and autumn maxima). Rainfall
reliability is moderate (CV of 35% for MAP) (South African
Weather Bureau, 2002), although droughts of several months do
occur frequently, mainly when little rain is recorded in one of
the equinoctial seasons. Temperatures range from hot (mean
daily temperature of hottest month: 39 °C, highest recorded:
46 °C) to cool (mean daily temperature of coldest month: 15 °C,
lowest recorded: 9 °C) with a mean daily fluctuation of 14 °C.
Unsustainable browsing of succulent thicket, mainly by
goats, leads to the loss of P. afra and other succulents and multi-
stemmed shrubs, resulting in a pseudo-savannadominated
by a field layer of ephemeral or weakly perennial grasses and
dwarf karroid shrubs and scattered, umbrella shaped (owing
to browsing) individuals of canopy trees, namely Pappea
capensis, Euclea undulata and Schotia afra (Lechmere-Oertel
et al., 2005). During this process, the canopy of the perennial
patches is opened by livestock, exposing the litter layer and
soil surface to increased solar radiation and raindrop impact.
This goat-induced process of transformation may take several
years to occur, depending on stocking regimes. Restoration of
transformed succulent thicket does not occur spontaneously
(Vlok et al., 2003; Sigwela, 2004), and transformation ulti-
mately leads to degradation of the system measured as struc-
tural simplification (Hoffman and Cowling, 1991), loss of
biomass (Lechmere-Oertel et al., 2004; Mills et al., 2005a),
loss of soil organic matter (SOM) (Mills and Fey, 2004a,b;
Lechmere-Oertel et al., 2005) and soil erosion (Lechmere-
Oertel, 2003). In a state of extreme transformation, a depleted
and dying canopy tree layer is the only remnant of the original
perennial vegetation (Fig. 1).
Leaf litter and SOM play a key role in the maintenance of
productivity in semi-arid ecosystems. Leaf litter modifies the
local physical environment, influences germination and estab-
lishment success (Molofsky et al., 2000; Boeken and Orenstein,
2002), controls the distribution and activity of soil organisms
(Steinberger et al., 1984; Whitford, 2002), increases soil water
Fig. 1. (a) A fenceline contrast of intact and degraded thicket (photo: M. Powell), and (b) remnant Pappea capensis trees in a degraded thicket (photo: A. Mills). Both
photographs were taken west of Steytlerville in the foothills of the Groot Winterhoek Mountains.
652 R.G. Lechmere-Oertel et al. / South African Journal of Botany 74 (2008) 651659
availability and changes the soil microclimate (West, 1979;
Whitford, 2002). Soil organic matter influences key soil properties
such as water retention, bulk density, erodability, infiltration
(Mills and Fey, 2003, 2004c) an d the distribution and abundance of
organisms (Whitford, 2002). The rate at which litter is produced,
decomposed and incorporated into the soil, together with other
factors such as Al content (Percival et al., 2000), have a strong
influence on SOM content. Changes in litter dynamics are con-
sequently likely to have a cascading effect across many ecosystem
processes.
Although the patterns of transformation in succulent thicket
have been well documented (Hoffman and Cowling, 1990;
Hoffman and Cowling, 1991; Stuart-Hill, 1992; Kerley et al.,
1995; Lloyd et al., 2002; Lechmere-Oertel et al., 2004), no
research has been done on understanding the mechanisms
underpinning the transformation process. In this study, we
compared the rates of litter production and decomposition in
relatively intact and transformed thicket across replicated
fenceline contrasts, i.e. a snapshot natural experiment.
We predicted that perennial plants in intact thicket would be
less stressed in terms of water and nutrient supply, and would
consequently replace their leaves more often than plants in
degraded thicket, leading to greater rates of litter production.
Effects of transformation on rates of decomposition were more
difficult to predict. On the one hand, removal of the plant
canopy is likely to increase the rate of decomposition of surface
litter due to: i) greater exposure to UV light (Moorhead and
Callaghan, 1994); ii) greater exposure to light rainfall events,
and iii) warmer soil surface temperatures. On the other hand,
lower rates of decomposition may be expected due to reduced
soil water availability as a result of greater rates of evaporation
in transformed thicket.
2. Methods
2.1. Study area
The study area was located in the moderately steep (1525°),
north-facing foothills of the Groot Winterhoek Mountains
near Port Elizabeth, Eastern Cape Province, South Africa. Five
sites, each comprising relatively intact and transformed thicket
separated by a fence were identified. The vegetation at the sites
is an arid form of Sundays Thicket termed Sundays Spek-
boomveld (Vlok et al., 2003; Hoare et al., 2006). Transforma-
tion status of the sites was subjectively assessed based on
the biomass of the woody thicket component. Above-ground
biomass (dry matter) estimates for intact thicket were 67,000
97,000 kg
1
ha
1
, and 950023,900 kg
1
ha
1
for transformed
thicket (Lechmere-Oertel, 2003).
Our study was conducted over a 14-month period. Although
this is a relatively short period for observing litter and decom-
position processes, our focus was on the comparison between
transformed and untransformed states.
2.2. Litter production
Litter production was measured for a dominant perennial
species representing each of the four main growth forms
(Table 1). Litt er traps (0.5 mm mesh, 0.5 m × 0.5 m square bags
suspended between four metal rods) were placed directly
beneath the canopies of three individuals of each species
investigated (Table 1) on either side of the fence line contrast
(24 traps per site). The traps were adjacent to the main stem,
near the centre of the canopy, and were not exposed to litterfall
from other species. Total litter (leaves, twigs, small branches
and seeds) was collected from the traps every 6090 days for
14 months (May 2001October 2002), dried at 40 °C and
weighed. The data for each trap were pooled for the total
sampling period (506 days) and transformed using a natural log.
The mean monthly rainfall at Adolphskraal and Tygerhoek
(weather stations located within the study region) for the period
May 2001September 2002 was 26 and 36 mm, respectively
(South African Weather Bureau, 2002). This equates to approxi-
mately 310430 mm MAP, hence a relatively wet period for the
study site region.
Annual litter production (kg m
2
yr
1
) was compared across
the fenceline contrasts using a separate factorial ANOVA
for each species. A landscape estimate of litter production
(kg ha
1
yr
1
) was calculated by extrapolating the litter
production (kg m
2
yr
1
) of each growth form representative,
and multiplying it by the proportional cover (Lechmere-Oertel
et al., 2005) of that growth form. The annual litter production of
the non-perennial grass and forb field layer was estimated as
50% (West 1979) of the standing biomass (Lechmere-Oertel
et al., 2005).
2.3. Litter decomposition
The measurement of mass loss of litter placed in 1 mm mesh
bags is widely used to estimate litter decomposition rates in
Table 1
Some biological and physiognomic characteristics of the species used in the experiments
Species Family Growth form
a
Foliage % Cover
b
Biomass (kg ha
1
)
Euclea undulata Ebenaceae Small emergent tree, b3 m Evergreen, sclerophyllous 8.2 (6.5)
Pappea capensis Sapindaceae Canopy tree, b5 m Semi-deciduous, sclerophyllous 16.3 (10.0) 9 190
c
Portulacaria afra Didiereaceae Multi-stemmed leaf-succulent woody shrub, b3 m Evergreen, succulent 57.7 (27.1) 97 978
d
Rhus longispina Anacardiaceae Multi-stemmed spinescent woody shrub, b4 m Evergreen, mesophyllous 4.1 (4.7)
a
The heights in the growth forms are based on individuals measured in the field, not the maximum potential under more mesic conditions.
b
Percentage cover (standard deviation) is the average of five 100 m transects in each of the five intact sites.
c
Biomass was estimated by weighing ten dead trees and extrapolating to area using proportional cover and tree density data.
d
Biomass was estimated by harvesting all above-ground material in ten 1 m
2
quadrats at each of the five sites, and extrapolating to area using proportional cover.
653R.G. Lechmere-Oertel et al. / South African Journal of Botany 74 (2008) 651659
field studies (Swift et al., 1979; Huang and Schoenau, 1997;
Guo and Sims, 1999; Joshi et al., 1999); notwithstanding the
fact that detritivores larger than 1 mm do not have access to
the litter. A combination of litterbags and unmeshed leaf packs
was used to estimate rates of decomposition in this study. A
leaf litter mix was made from freshly harvested leaf material
of E. undulata, P. afra and P. capensis in a ratio that approxi-
mately reflected their proportional abundance at the sites.
Approximately 2 g of the dried litter mix was heat-sealed into
10 × 10 cm 1-mm-nylon mesh bags. Leaf packs were made by
threading fresh leaves of two dominant perennial species (P.
capensis and P. afra) onto a pre-weighed 10 cm section of thin
galvanised wire. Once threaded, the wire ends were twisted to
prevent the loss of material, and the packs were dried at 40 °C, and
weighed.
Quadrats laid out in a split-plot factorial experimental design
were used to evaluate the rates of litter decomposition from the
leaf litterbags and packs. Five each of the mixed bags, P.
capensis packs and P. afra packs were pinned onto the soil
surface within each 1 m ×1 m quadrat. Three such quadrats were
placed in each of the two dominant habitats (under a canopy tree
or in the matrix vegetation) on either side of the transformation
contrast. If there was litter present in the quadrat, then the bags
and packs were nestled into it until covered. The quadrat was
protected from curious domestic animals with 20 mm mesh
wire.
A mixed bag, P. capensis pack and P. afra pack were
randomly harvested from each quadrat approximately every
3 months for 15 months (May 2001October 2002), transported
to the laboratory in separate paper bags and dried at 40 °C. After
reweighing, the samples were ashed at 550 °C for 6 h to estimate
contamination by inorganic soil particles (Potthoff and Loft-
field, 1998). Percentage mass loss was calculated using Eq. (1).
Mass loss due to decomposition was calculated per kg of dry
litter as a function of time using Eq. (2). Annual decomposition
rate coefficients kwere calculated using the single negative
exponential decay function (Olson, 1963), reworked into Eq. (3).
This constant is useful for comparative purposes (Stamou et al.,
1994; Carnevale and Lewis, 2001).
Mass Loss kðÞ¼Mt=M0
ðÞ100 ð1Þ
Rate of Loss g kg1d1

¼M0Mt
ðÞ=M0=1000ðÞ½=tð2Þ
Decomposition constant k¼ln Mt=M0
ðÞ½=t=365ðÞ ð3Þ
where M
0
is the initial mass of litter, M
t
is the ash-free mass of
retrieved litter, and tis the number of days the litter was in the
field.
Control bags and packs were harvested immediately after
being set out, weighed and ashed to measure mass change due
to handling and inorganic material present in the fresh litter.
Significant differences within the percent litter remaining
(arcsin-transformed), rate of mass loss (ln-transformed) and
decomposition constant (ln-transformed) were identified for
impact and habitat effects (and interactions) using separate
generalised linear models for each litter type. Number of days
(t) was used as a continuous covariable. Tukey post-hoc HSD
tests were used to separate the means where there were
significant differences in the treatment effects.
Fig. 2. The percentage of original litter mass remaining as a function (solid line)
of days in the field for (a) Pappea capensis, (b) Portulacaria afra, and (c) mixed
litter bags. The function with the greatest r-squared value was chosen from
linear, logarithmic, power and exponential fits.
654 R.G. Lechmere-Oertel et al. / South African Journal of Botany 74 (2008) 651659
2.4. Soil temperature and incipient radiation
Soil temperature at 5 cm below the surface was measured
every hour for 373 days from 17/03/2001 to 26/03/2002 using
HOBO-H8 loggers with an external HA-6 temperature probe
(Onsetcomp Inc., 2001). Twenty loggers were distributed over
representative examples of the two habitats on either side of the
transformation contrast at each site: intact canopy, intact matrix,
transformed canopy and transformed matrix. As there were
recording problems with some of the loggers, the data were
cleaned to remove extreme outliers that were obviously incor-
rect, e.g. soil temperatures of 50 °C. The data were sum-
marised (min., max. and SD) by treatment at daily intervals. The
summary data were compared between treatments using a fac-
torial ANOVA.
The difference in incident radiation beneath P. capensis
canopies across the transformation contrast was determined
using a Licor LI 185A quantum light meter fitted with a flat
sensor. Light readings were taken at c. noon on a clear summer
day under ten canopies on either side of the transformation
contrast at each site, i.e. a total of 100 readings. A light reading
in the open was taken immediately after each canopy reading.
The percent change in incident radiation was compared across
the transformation contrast using a factorial ANOVA after
arcsin transformation of the data. All statistical analyses above
were performed in Statistica 6.1 (Statsoft Inc., 2001).
3. Results
3.1. Litter production
Exploratory data analysis of annual litter production showed
that the effects of transformation and habitat were growth form
specific (Fig. 2). The two canopy tree species, E. undulata and
P. capensis, produced c. 60% and 55%, respectively, less litter
in transformed than intact thicket (Table 2), representing a
significant transformation effect for these two species (Table 3).
There were no significant differences for the succulent shrub P.
afra and the spinescent multi-stemmed shrub Rhus longispina.
Litter production of all species was significantly dependent on
site location (Table 3). Irrespective of transformation status, P.
afra produced more than three times the amount of litter than
the other three species (Table 2).
At a landscape scale in intact thicket, succulent shrubs and
canopy trees produced 60% and 17%, respectively, of the total
litter (Table 2). By contrast, the ephemeral field layer con-
tributed 90% of the litter produced in transformed thicket. The
total annual production of litter at a landscape scale was 30%
lower in transformed than intact thicket (Table 2). A comparison
restricted to the litter production of perennial plants only,
showed that transformed thicket produced 90% less litter, most
of that difference being associated with a decrease in the
proportional area of P. afra (Table 2).
3.2. Litter decomposition
Transformation had no significant effect on decomposition
for any of the litter types, and site location had the only
significant effect on all the decomposition variables (Table 4).
There was a trend of an increasing loss rate and decomposition
constant from site 1 to site 5, a reflection of decreasing aridity.
The only significant treatment effect was the interaction be-
tween site and transformation for P. afra litter (Table 4). The
main qualitative difference between the litter types was that P.
afra litter had a higher decomposition constant k(mean, stan-
dard deviation, range: 2.18, 1.08, 0.156.04) than P. capensis
(0.89, 0.56, 0.033.51) or the mixed litter (0.58, 0.25, 0.03
Table 2
Mean (standard deviation) litter production at a small patch (g m
2
yr
1
) and landscape scale (kg ha
1
yr
1
)
Growth form Ephemeral field layer Canopy trees Woody shrubs Succulent shrubs Total
Representative species 50% biomass Pappea capensis Euclea undulata Rhus longispina Portulacaria afra
Intact
Litter yield (g m
2
yr
1
) 0.300 335 (116) 338 (146) 120 (259) 464 (135)
Proportional cover
a
0.21 0.21 0.20 0.55
Landscape yield (kg ha
1
yr
1
)
b
630 704 240 2552 4126
Transformed
Litter yield (g m
2
yr
1
) 0.300 151 (99) 132 (83) 132 (132) 453 (154)
Proportional cover 0.86 0.06 0.09 0.02
Landscape yield (kg ha
1
yr
1
) 2580 91 119 91 2881
Interactions between the species, transformation effects and significant differences from the ANOVAs are shown in Table 3.
a
Totals of the proportions may exceed 1 due to overlapping of plants in a growth form.
b
Landscape scale data were calculated by extrapolating the patch scale data proportionally to percentage cover data (see methods for more details).
Table 3
Results of a factorial ANOVA of patterns of litter production (log transformed)
in relation to site and transformation status
Treatment effect df Euclea
undulata
Pappea
capensis
Portulacaria
afra
Rhus
longispina
Site 7 9.64** 1.34** 5.20* 1.79
ns
Transformation 1 91.31** 59.98** 0.56
ns
0.87
ns
Site * transformation 7 6.28** 3.29* 2.14
ns
60
ns
Error MS 32 0.164 0.166 0.059 0.247
Each species had a separate ANOVA.
Significance levels: * pb0.01, ** pb0.001,
ns
= non-significant.
Significant values are highlighted in bold.
655R.G. Lechmere-Oertel et al. / South African Journal of Botany 74 (2008) 651659
1.69) for all habitat and transformation treatments. Inspection of
the mass loss curves for the litter types (Fig. 2) showed that P.
afra litter had the steepest rate of mass loss of all litter types.
3.3. Soil temperature and incipient radiation
There were significant differences (F=54.0, df=4, pb
0.001) in soil temperatures between both the habitat and
transformation treatments (Table 5). The range and variance of
soil temperatures (including upper and lower extremes) were
significantly higher in transformed sites for both habitat types.
The average daily maximum soil temperature was 52% and
30% higher in transformed sites for matrix and canopy habitats,
respectively (Table 5). The average daily minima in matrix and
canopy habitats were 21% and 23% lower, respectively. Both
the highest (50.7 °C) and lowest (0.7 °C) temperatures recorded
were in the transformed matrix. The daily amplitude of soil
temperature was two to three times higher in transformed
thicket for the canopy and matrix habitats respectively (Table 5).
These patterns held true for both the canopy and matrix habitats,
although were more pronounced in the latter.
P. capensis canopies reduced incipient radiation by 85%
(SD = 5%). Transformation had a significant effect on the per-
cent reduction in light (F= 7.70, df =1, p=0.007). Canopies in
transformed and intact thicket reduced radiation by 84%
(SD = 2.7%) and 89% (S D = 1.4%), respectively.
4. Discussion
4.1. Impacts of transformation on litter production and
decomposition
Litter production in intact succulent thicket is comparable to
that of a number of other ecosystems in higher rainfall regimes,
such as temperate forests, dry tropical forests, and Mediterra-
nean-type shrublands (Table 6). This may be related to the
unusually high biomass of succulent thicket for a semi-arid
ecosystem (Mills et al., 2005b), particularly of the dominant P.
afra, emphasising the keystone role of this species in main-
taining carbon cycling. Transformation of succulent thicket
reduces litter production to a level more comparable with desert
and dry savanna systems (Table 6). Desert perennial shrublands
annually shed between 30% and 60% of their total above-
ground biomass as litter (West, 1979). In forests, the range is 1%
to 5% (West, 1979). Although the data presented here are for
one year only, intact succulent thicket shed 46% of its standing
above-ground biomass, and transformed succulent thicket 12
32%.
The reduced litter production of the canopy tree growth form
can be understood in terms of reduced canopy volumes owing to
browsing, combined with the drought-resistant nature of the
plants. We do not understand yet why the trees are more affected
by transformation and the shrubs less so. This will require more
information on the ecophysiology of the species in transformed
and untransformed sites. The absence of response of P. afra to
transformation suggests strong drought-resistance. P. afra has
Table 4
F-values and degrees of freedom (df) for the three decomposition response
variables from the split-plot nested ANOVA for the three litter types
Model effect df Loss rate
(g kg
1
d
1
)
Decomposition
constant (k)
Remaining
litter (%)
Mix bags
Site 4 1.75 2.64*4.73**
Transformation 1 0.63 1.15 2.97
Site * transformation 4 0.80 0.23 2.32
Habitat
(site * transformation)
1 0.66 0.90 1.29
Error MS 279 0.20 0.21 17.5
Pappea capensis packs
Site 4 7.56** 11.14** 9.77**
Transformation 1 0.71 0.88 0.53
Site * transformation 4 0.43 0.58 0.31
Habitat
(site * transformation)
1 1.37 1.44 0.60
Error MS 268 0.20 0.29 143.2
Portulacaria afra packs
Site 4 6.11** 20.95** 13.33**
Transformation 1 1.86 3.53 3.72
Site * transformation 4 0.74 2.53* 0.82
Habitat
(site * transformation)
1 0.50 1.48 0.56
Error MS 238 0.11 0.19 215.6
Significance levels: * pb0.05, ** pb0.01,
ns
= non-significant.
Loss rate and kwere natural log transformed and remaining litter (%) was
arcsin-transformed prior to analysis.
Table 5
Soil temperature (°C) data in canopy and matrix microhabitats in intact and transformed thicket
Habitat *
transformation
Lowest t
recorded
Highest t
recorded
Means of daily summary
1
(n= 375 days)
Minimum Mean Maximum Range Mean difference (transformed- Intactintact)
2
Canopy Canopy
Intact 7.8 42.9 15.9 (3.5)
a
19.0 (4.1)
a,b
24.2 (6.3)
a
8.3 (4.1)
a
Min: 3.3 ± 2.9
Transformed 0.7 48.0 12.2 (3.8)
b
19.5 (4.6)
b
31.1 (7.4)
b
18.7 (5.9)
b
Max: 12.0 ± 4.2
Matrix Matrix
Intact 5.4 36.6 15.6 (3.6)
a
18.7 (3.4)
a
23.2 (5.0)
a
7.6 (4.1)
a
Min: 3.7 ± 2.0
Transformed 0.7 50.7 12.3 (3.7)
b
22.0 (5.0)
c
35.1 (7.2)
c
22.8 (6.1)
c
Max: 6.9 ± 5.0
Different letters indicate significant post-hoc Tukey HSD tests (pb0.001) within a temperature variable.
1
Daily summaries were generated for each habitat at each site (n= 5 sites). Mean data are the averages (SD) over time (375 days).
2
The mean difference is the average of the daily difference (transformed-intact for each site) in minimum and maximum temperatures in each habitat.
656 R.G. Lechmere-Oertel et al. / South African Journal of Botany 74 (2008) 651659
several adaptations to cope with prolonged drought stress, such
as leaf succulence and the ability to switch between C3 and CAM
photosynthetic pathways (Guralnick et al., 1984; Guralnick and
Ting, 1987). It is important to note that P. afra produced the most
litter of all the species tested. This suggests that, at a landscape
scale, the significant reduction of litter production is due to the
loss of biomass of the component species, particularly of the
highly palatable P. afra.
Despite the very significant differences in the soil micro-
climate between the different habitats and across the transfor-
mation contrasts, there were surprisingly few significant
differences in the decomposition rates and constants (k). This
is potentially explained by different transformation effects that
can decrease as well as increase decomposition. Decreased
biotic activity on transformed sites could be expected as a result
of reduced litter cover (Steinberger et al., 1984), soil aridity and
extreme soil temperatures (Mackay et al., 1986; Cepeda-Pizarro
and Whitford, 1990). However, an increase in physical break-
down via raindrop impact (Strojan et al., 1987), and photo-
oxidation (Schaefer et al., 1985; Whitford, 2002) may also
occur. Indeed, Moorhead and Reynolds (2002) suggest that
abiotic forces may be the primary control of decomposition in
semi-arid climates, which is consistent with Noy-Meir's (1973)
hypothesis of increased abiotic controls in desert systems.
Additional effects requiring further research include: i) reduced
rainfall interception by the thicket canopy and therefore pe-
riodically greater soil water availability in transformed sites;
and ii) warmer soil temperatures potentially promoting micro-
bial activity.
4.2. Implications for ecosystem functioning and productivity
The high litter production by P. afra may explain the
unusually high levels of SOM found in intact succulent thicket
(510%, Lechmere-Oertel et al., 2004; Mills and Fey, 2004a)
compared to other semi-arid systems; suggesting that it is a
keystone species. The combination of the reduction in the
quantity of litter produced in transformed succulent thicket and
the switch from perennial to ephemeral growth forms (and
hence the type of litter) will reduce the incorporation of organic
matter into the soil (Whitford et al., 1998; Knoepp et al., 2000;
Whitford, 2002). This is evident in the near absence of any
accumulated litter in transformed thicket compared to up to 90%
litter cover (N5 cm deep) in intact thicket (Lechmere-Oertel
et al., 2004). Soil quality (in terms of structure, water holding
content and nutrient cycling/supply) is likely to be compro-
mised by the reduction in SOM (Mills and Fey, 2003) which in
turn impairs ecosystem functioning. The endpoint of this pro-
cess is desertification, which has already occurred over large
areas of succulent thicket (Lloyd et al., 2002).
4.3. Restoring biomass and ecosystem functioning
At a landscape scale, P. afra produces most of the litter in
succulent thicket (c. 2500 kg ha
1
yr
1
). Our data shows
that this litter will be incorporated into SOM relatively
quickly, with most litter decomposing in less than one year.
P. afra appears to be the keystone species in this ecosystem,
acting as a carbon pump, and incorporating carbon into soils
at a rate incongruous with the prevailing rainfall regime. Once
lost from the system, P. afra does not re-establish, even if
livestock and game stocking densities are reduced. Fortu-
nately, however, this plant propagates vegetatively and
planting of truncheons can consequently be used to restore
degraded thicket landscapes (Swart and Hobson, 1994; Mills
and Cowling, 2006). Such restoration results in considerable
return of carbon in biomass and soils. This sequestered
carbon has a market value and can potentially be used to fund
large-scale restoration across the thicket biome (Mills et al.,
2007).
Table 6
Litter production in transformed and intact Portulacaria afra thicket (shown in bold) in comparison with a range of ecosystems worldwide
Ecosystem Additional information Total litterfall (kg ha
1
yr
1
) Source
Desert Larrea shrubland, Nevada 194530 Strojan et al. (1979)
Desert Haloxylon shrubland, Russia 440 West (1979)
Fynbos Protea and Erica shrubland, South Africa 700 Witkowski (1989)
Semi-arid woodland Eucalyptus crebra, Australia 720 McIvor (2001)
Desert Eurotia ceratoides, Russia 920 West (1979)
Semi-arid rangeland Atriplex vesicaria, Australia 1094 West (1979)
Semi-arid woodland Eucalyptus drepanophylla, Australia 1270 McIvor (2001)
Pine woodland Pinus pinaster, Spain 1728 Santa Regina (2001)
Oak woodland Quercus rotundifolia, Spain 2320 Santa Regina (2001)
Desert Artemisia tridentata, Russia 2500 West (1979)
Transformed thicket Pseudo-savanna2880 This study
Dry woodland Russia 2900 West (1979)
Cool temperate forest Global average value 3100 Bray and Gorham (1964)
Chaparral Mixed community, California 3550 Mooney et al. (1977)
Intact thicket Portulacaria afra dominant 4100 This study
Tropical seasonal forest Average value, Ivory Coast 4440 West (1979)
Warm temperate forest Global average value 4900 Bray and Gorham (1964)
Temperate oak forest Average value, Greece 5003 Stamou et al. (1994)
Equatorial forest Global average value 9700 Bray and Gorham (1964)
657R.G. Lechmere-Oertel et al. / South African Journal of Botany 74 (2008) 651659
Acknowledgements
The World Bank Global Environment Facility funded this
research through the South African National Biodiversity
Institute's Conservation Farmingproject. Additional funding
was provided through the National Research Foundation and
the Department of Water Affairs and Forestry Working for
Woodlands programme. The Mazda Wildlife Fund sponsored
the vehicles. Thanks to the following people for various forms
of assistance: Louise Visagie (field and laboratory assistance);
Arthur Rudman and Chris Bosch (field accommodation and
access to sites); Desmond Slater, Ron Watson, Charlie Bolton
and Gered Vermaak (access to sites). Andrew Knight and
Ayanda Sigwela provided much insight through discussion.
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Edited by J Van Staden
659R.G. Lechmere-Oertel et al. / South African Journal of Botany 74 (2008) 651659
    • Most biodiversity surveys have predominantly focussed on vegetation surveys, specifically plant species diversity and community compositions (e.g. Lechmere-Oertel et al., 2008; Hanke et al., 2014) primarily in protected areas (Fazey et al., 2005 cited in Chazdon et al., 2009) and as fence-line contrasts between protected areas and inhabited areas (Todd and Hoffman, 1999; Lechmere-Oertel et al., 2008 ). Therefore, the various potential integrated conservation options in these areas can be better evaluated when the biodiversity status is fully investigated and understood to make informed conservation decisions (Hirsch et al., 2011; Miller et al., 2011; Salafsky, 2011).
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    Full-text · Article · Jul 2016
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    • Spekboom-dominated ecosystems store carbon in excess of 200 t/ha, a remarkable feature for a xeric ecosystem and comparable to that of mesic forest ecosystems (Mills et al. 2005a). Spekboom contributes most of the above-ground carbon (Mills & Cowling 2006; Lechmere-Oertel et al. 2008) and, with its dense canopy, provides the relatively cool and dry conditions necessary for the accumulation of large levels of soil carbon (Lechmere-Oertel et al. 2005a). Comparisons of degraded and intact stands reveal carbon losses of more than 80 t C ha −1 (Mills et al. 2005b).
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    • This would happen because, although the dense spekboom canopy of intact thicket would intercept a large portion of the gross rainfall (Cowling and Mills, 2011), the vegetation cover would increase infiltration of water in the soil. The latter trend would be owing to decreased raindrop energy and a greater soil aggregate stability of the more organic-rich soil beneath the canopy compared to soil properties outside the canopy (Lechmere-Oertel et al., 2008; Mills and Fey, 2004). Consequently runoff would be lower, and moisture retention in the soil profile higher in intact compared to degraded vegetation.
    [Show abstract] [Hide abstract] ABSTRACT: a b s t r a c t Almost half of the 16,942 km 2 of South Africa's subtropical thicket with a substantial Portulacaria afra (spekboom) component has been heavily degraded by domestic herbivores. The subtropical thicket biome is a drought-prone and water-stressed area, and many of the region's watersheds comprise of eroded landscapes clothed in degraded spekboom thicket. The objective of this study was to determine the impact of degradation of spekboom thicket on hydrological processes. We hypothesised that degradation of spekboom thicket would reduce infiltration and, hence, reduce soil moisture retention and increase run-off and erosion. We tested this hypothesis by collecting data on rainfall, infiltration, soil moisture retention and run-off in degraded thicket, and e as a reference site e in an adjacent stand of relatively intact thicket. The results showed clear trends in the impacts of spekboom thicket degradation on hydrological processes. The more than hundred-fold lower infiltration in soils associated with degraded thicket relative to the soils beneath the intact, spekboom canopy, resulted in lower levels and less retention of soil moisture, almost double the amount of runoff, and an almost six-fold increase in sediment load. Thus, restoring degraded thicket will reduce erosion and likely improve baseflows, in addition to sequestering carbon.
    Full-text · Article · Apr 2013
    • Biomass is high, with mean dry aboveground mass around 80 000 kg ha À1 (Lechmere-Oertel et al., 2005a). Litter production is comparable with that in higher rainfall regimes such as temperate forests (Lechmere-Oertel et al., 2008). Thicket soil is relatively rich in carbon (Mills and Fey, 2004a), in contrast to that in most other soils of South Africa (du Preez et al., 2011).
    [Show abstract] [Hide abstract] ABSTRACT: Investigations were made of plant and soil responses to severe degradation through heavy grazing and browsing in arid, succulent, subtropical thicket. Severe degradation of thicket is of major concern in terms of threatened biodiversity, unsustainable utilization and collapse of other ecosystem services. We used a natural, field contrast, case-study approach, sampling within plots under lightly and heavily stocked conditions. Mean plant species diversity and richness did not change significantly at sample plot level although there was a 27 per cent decline in richness with degradation at the scale of the study site. On degraded plots, there was a high species turnover and high beta diversity, which created a replacement zone rather than an impoverished zone. Replacement species were confirmed as mainly, but not exclusively, weedy annual grasses and alien forbs. The few persisting perennials were small trees that survived above the browse line, and hardy shrub species. Perennial persisting and replacement species below the browse line may suggest potential candidates for restoration. Nitrogen was the only measured nutrient that showed a significant decrease with degradation. Phosphorous, potassium and magnesium increased significantly, with the first two mentioned reaching potentially excessive levels. Soil salinization occurred with an order of magnitude increase in sodium. Thus, the increased cation exchange capacity occurred together with development of a nutrient imbalance. The elevation in some nutrients and soluble salts is ascribed to wind-determined directional grazing that concentrate livestock in the degraded area. The implications of the above altered soil conditions for thicket restoration need to be further explored. Copyright © 2012 John Wiley & Sons, Ltd.
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