Succulent Thicket Research Articles

Restoring South African subtropical succulent thicket using Portulacaria afra: root growth of cuttings differs depending on the harvest site during a drought.

The restoration of succulent thicket (the semi-arid components of the Albany Subtropical Thicket biome endemic to South Africa) has largely focused on the reintroduction of Portulacaria afra L. Jacq-a leaf- and stem-succulent shrub-through the planting of unrooted cuttings directly into field sites. However, there has been inconsistent establishment and survival rates, with low rates potentially due to a range of factors (e.g., post-planting drought, frost or herbivory), including the poor condition of source material used. Here we test the effect of parent-plant and harvesting site on the root development of P. afra cuttings in a common garden experiment. Ten sites were selected along a ∼110 km transect, with cuttings harvested from five parent-plants per site. Leaf moisture content was determined for each parent-plant at the time of harvesting as a proxy for plant condition. Root development-percentage of rooted cuttings and mean root dry weight-was recorded for a subset of cuttings from each parent-plant after 35, 42, 48, 56, and 103 days after planting in a common garden setting. We found evidence for cutting root development (rooting percentage and root dry mass) to be strongly associated with harvesting site across all sampling days (p<0.005 for all tests). These differences are likely a consequence of underlying physiological factors; this was supported by the significant but weak correlation (r 2=0.10-0.26) between the leaf moisture content of the parent-plant (at the time of harvesting) and dry root mass of the cuttings (at each of the sampling days). Our findings demonstrate that varying plant condition across sites can significantly influence root development during dry phases (i.e., intra- and inter-annual droughts) and that this may be a critical component that needs to be understood as part of any restoration programme. Further work is required to identify the environmental conditions that promote or impede root development in P. afra cuttings.

Restoring South African subtropical succulent thicket using Portulacaria afra: exploring the rooting window hypothesis.

Drought prone, arid and semi-arid ecosystems are challenging to restore once degraded due to low levels of natural recruitment and survival of reintroduced plants. This is evident in the restoration of degraded succulent thicket habitats in the Albany Subtropical Thicket Biome located in South Africa. The current restoration practice for this ecosystem focuses predominantly on reintroducing Portulacaria afra L. Jacq., which is naturally dominant in terms of cover and biomass, but largely absent in regions degraded by domestic livestock. This has been achieved by planting unrooted cuttings with limited consideration of soil water availability in a drought-prone ecosystem. This study tests the effects of the timing of water availability after planting on the root development of P. afra cuttings. Cuttings were harvested from seven individual plants and grown in a glasshouse setting. Eighty four cuttings were taken from each individual, twelve for each of the seven watering treatments per individual plant. The treatments represented a time-staggered initial watering after planting, including: on the day of planting, 4 days, 7 days, 14 days, 21 days, and 28 days after planting. After 32 days, all treatments were watered on a bi-weekly basis for two weeks; a control treatment with no watering throughout the experiment was included. The proportion of rooted cuttings per treatment and dry root mass were determined at the end of the experimental period (day 42). The early onset of watering was associated with a higher percentage of rooting (X2(5) = 11.352, p=0.045) and had a weak, but non-significant, impact on the final dry root mass (F5,36 = 2.109, p=0.0631). Importantly, no clear rooting window within 28 days was detected as the majority of cuttings exhibited root development (greater than 50% of cuttings rooted for each individual parent-plant); this suggests that watering at the time of planting P. afra cuttings in-field for restoration may not be necessary. An unexpected, but important, result was that parent-plant identity had a strong interaction with the accumulation of root mass (F36,460 = 5.026, p<0.001; LR7 = 122.99, p<0.001). The control treatment, which had no water throughout the experiment, had no root development. These findings suggest that water availability is required for the onset of rooting in P. afra cutting. However, the duration of the experiment was insufficient to detect the point at which P. afra cuttings could no longer initiate rooting once exposed to soil moisture, and thus no rooting window could be defined. Despite harvesting material from the same source population, parent-plant identity strongly impacted root development. Further work is required to characterise the rooting window, and to explore the effect of parent-plant condition on in-field and experimental restoration results; we urge that experiments using P. afra closely track the parent-source at the individual level as this may be a factor that may have a major impact on results.

Regeneration dynamics of Portulacaria afra in restored succulent thicket of South Africa.

Over-grazing by livestock has resulted in the widespread degradation of South Africa's succulent thicket ecosystems. This is characterised by a significant reduction in the cover of the dominant succulent shrub, Portulacaria afra. Because this species is unable to regenerate naturally in degraded habitat, active reintroduction is required to restore ecosystem function. However, reintroduction success is relatively low, and the recruitment barriers for this species are poorly understood. By conducting pairwise plot surveys in actively restored and adjacent degraded succulent thicket habitats, the extent of P. afra seedling abundance in these contrasting ecosystem conditions is quantified. Seedling abundance was significantly greater in restored ecosystems (W=23,p=0.03225). Additionally, seedlings found in restored habitats were strongly associated with open habitat, whereas seedlings in degraded ecosystems were more restricted to nurse sites (X2 = 122.84, df = 2, p-value < 2.2e-16). A weak (R2 = 0,237), but significant (p=0,0295) correlation between P. afra cover and seedling abundance was recorded. Active restoration of succulent thicket habitat through P. afra reintroduction appears to overcome recruitment barriers. This may suggest that, despite the poor survival of introduced individuals, natural recruitment could contribute to the regeneration of restored succulent thicket ecosystems.

The soil microbiomics of intact, degraded and partially-restored semi-arid succulent thicket (Albany Subtropical Thicket).

This study examines the soil bacterial diversity in the Portulacaria afra-dominated succulent thicket vegetation of the Albany Subtropical Thicket biome; this biome is endemic to South Africa. The aim of the study was to compare the soil microbiomes between intact and degraded zones in the succulent thicket and identify environmental factors which could explain the community compositions. Bacterial diversity, using 16S amplicon sequencing, and soil physicochemistry were compared across three zones: intact (undisturbed and vegetated), degraded (near complete removal of vegetation due to browsing) and restored (a previously degraded area which was replanted approximately 11 years before sampling). Amplicon Sequence Variant (ASV) richness was similar across the three zones, however, the bacterial community composition and soil physicochemistry differed across the intact and degraded zones. We identified, via correlation, the potential drivers of microbial community composition as soil density, pH and the ratio of Ca to Mg. The restored zone was intermediate between the intact and degraded zones. The differences in the microbial communities appeared to be driven by the presence of plants, with plant-associated taxa more common in the intact zone. The dominant taxa in the degraded zone were cosmopolitan organisms, that have been reported globally in a wide variety of habitats. This study provides baseline information on the changes of the soil bacterial community of a spatially restricted and threatened biome. It also provides a starting point for further studies on community composition and function concerning the restoration of degraded succulent thicket ecosystems.

Herbivory and misidentification of target habitat constrain region-wide restoration success of spekboom (Portulacaria afra) in South African subtropical succulent thicket.

Restoration of degraded subtropical succulent thicket, via the planting of Portulacaria afra (spekboom) truncheons, is the focus of a public works programme funded by the South African government. The goals of the programme, which started in 2004, are to create jobs, sequester carbon, restore biodiversity, reduce erosion, improve soil water holding capacity and catalyse private sector investment for upscaling of restoration. Here we report on a region-wide experiment to identify factors that can improve project success. Measures of success were survivorship and annual aboveground biomass carbon sequestration (ABCsr) of spekboom truncheons some 33–57 months after planting—starting in March 2008—into 173 fenced plots (0.25 ha) located throughout the global extent of spekboom thicket vegetation. We also collected data for 18 explanatory variables under the control of managers, and an additional 39 variables reflecting soil physical and chemical characteristics and rainfall patterns post restoration, all beyond the influence of managers. Since the latter covariates were available for only 83 plots, we analysed the two data sets separately. We used a prediction rule ensemble to determine the most important predictors of restoration success. There was great variation in percentage survivorship (median = 24, range = 0–100%) and ABCsr (median = 0.009, range = 0–0.38 t C ha−1 yr−1). The model using management variables explained less variance (53%) in survivorship than the model incorporating additional soil and rainfall covariates (62%). ABCsr models were better fits (78 and 88% variance explained, respectively). All model configurations identified browse intensity as a highly influential predictor of restoration success. Predicted success was highest for plots located in target habitat; however, only 45% were thus located, suggesting the need for expert input and habitat modelling for improving target habitat identification. Frost exposure was another important predictor influencing all models but was likely a consequence of locating sites off target habitat. Sites planted on equatorward slopes during the warm season showed reduced carbon sequestration, possibly due to elevated soil moisture stress associated with high radiation loads. Physiographic factors associated with improved restoration success were location on sloping ground (reduced frost exposure), increasing longitude (more warm-season rainfall) and increasing latitude (less frost coastwards). Few trends were evident among post-restoration climatic factors beyond the control of managers. Higher rainfall during the year post restoration had a negative impact on carbon sequestration while higher rain during the early months post restoration had a positive effect on both carbon sequestration and survivorship. Soil factors showed little importance for the survivorship model, whereas silt content, % K and Mg CEC emerged as predictors of carbon sequestration. Our results have direct relevance for improving the success of landscape-scale restoration projects envisioned for the ca. 8,930 km2 of degraded spekboom thicket.

The explanatory power of silent comics: An assessment in the context of knowledge transfer and agricultural extension to rural communities in southwestern Madagascar.

The distribution of silent comic illustrations can facilitate the communication and transfer of scientific recommendations about sustainable land management (SLM) to local communities in countries where many people are illiterate. However, since there are cross-cultural differences in “visual languages”, visualization styles need to be carefully selected as well as locals’ comprehension of the illustrated recommendation evaluated systematically. Three agricultural recommendations were chosen for comic-style illustrations, distributed to six communities in the Mahafaly region of southwestern Madagascar and evaluated using a three-step, interdependent approach. The silent comics illustrated (i) composting of manure and its application to improve soil fertility; (ii) cautious utilization of succulent silver thicket as supplementary forage; and (iii) sustainable harvesting practices of wild yam. Results revealed that general understandability strongly depended on the community that was surveyed and on the environmental subject that was illustrated. We found a strong relationship between the general understandability of comics and the divergence that exist in communities’ socio-economic structure. Education level was an important factor that explained a better understanding of respondents for the comic illustrating compost production, but not for comics that illustrated sustainable usage of silver thicket and wild yam harvest. Willingness to follow the recommended practice was impaired when respondents valued no change to the improved technique compared to the common one. Effects of respondents’ socio-economic characteristics on the implementation of the recommended practice could not be clarified within this study due to the small subset of data. Based on the evaluation of recurring comments made by respondents and interviewers, we conclude that comics can be a useful communication tool to increase locals’ awareness and comprehension for SLM practices. This, however, requires that drawing details used to facilitate farmers’ ability to adopt a point-of-view inside the comic story are used thoughtfully as they might interfere with the central message.

Settling the browser–grazer debate for African buffalo in grass-limited Eastern Cape thicket, South Africa

Despite extensive evidence that African buffalo Syncerus caffer are grazers, De Graaff et al. using rumen content analysis of animals that had starved to death proposed that buffalo in grass-limited Eastern Cape thicket should be considered browsers. Although these anomalous findings were initially accepted, but later challenged, the browse-dominated diet continues to be used as a foundation for hypotheses on the diet of healthy animals. Consequently, the debate around buffalo as browsers or grazers in thicket has not yet been settled. We describe the diet of buffalo in the Addo Elephant National Park and include data from other published work from the region to test the importance of grass in buffalo diet. We show that the diet is dominated by grasses, even in grass-limited thicket, and that browse species are seldom dominant foods. Thus, there is no empirical evidence to corroborate the notion that buffalo switch their diet to browse when grass availability is low. In an attempt to advance our understanding of buffalo foraging in thicket, we reiterate that De Graaff’s work is not a valid measure of buffalo diet in succulent thicket and that additional testing of the browser–grazer hypothesis is not needed.Conservation implications: Our results confirm that buffalo are grazers, rather than browsers, in grass-limited Eastern Cape thicket. Thus, additional testing of the browser–grazer hypothesis for buffalo in the region is not needed.

The composition and complexity of the woody and succulent components of Albany thicket with and without elephants

African elephants (Loxodonta africana) are capable of visibly altering the structure of vegetation through their browsing habits, and such alteration of habitat may be magnified when the broader movements of elephants are restricted by fences. I assessed the structure and composition of the woody and succulent components of Albany thicket vegetation at 10 fenced sites in the Eastern Cape Province, South Africa – five with elephants present, paired with five where elephants were absent. Contrary to expectations, woody and succulent thicket vegetation was structurally similar across the 10 sites, despite the vegetation at the non-elephant sites being slightly taller, denser and more complex than the sites with elephants. Woody plant community composition was also similar across the 10 sites and elephant-induced damage to woody and succulent plants was generally low. Combined, these findings support the idea that at least the woody component of thicket vegetation is generally resilient to the browsing effects of indigenous browsers such as elephants. However, it is also possible that Albany thicket vegetation at the five elephant sites may not have been exposed to elephants for long enough for elephant browsing effects to manifest. Thus, I recommend continued monitoring of elephant browsing in fenced reserves within the Thicket Biome to establish how overall vegetation structure and composition may change over time, and with increases in elephant densities.

Quantitative classification and environmental interpretations for the structural differentiation of the plant communities in the dry valley of Jinshajiang River

The structural differentiation of plant communities are associated with species traits, and interspecific interactions in heterogeneous environment. The comprehensive analysis of spatial variation in species assemblages may help infer processes shaping ecological communities. Based on field investigation of 116 sites and 562 sampling points in the dry valley of Jinshajiang River, combined with vegetation classification by adaptive-affinity propagation, we used Moran’s Eigenvector Maps and variation partitioning to quantify the effects of spatial and environmental factors on the community structure. The results showed that: (1) the plant communities were divided into 30 groups by Adaptive-AP, and classified into 7 vegetation types, 23 formations. Savanna (30.0%) and warm deciduous broadleaved thicket (55.7%) were the main vegetation •研究报告• 408 生 物 多 样 性 Biodiversity Science 第 24 卷 types. (2) Mean annual temperature (MAT) and aridity index (k) are two dominant climate factors limiting the distribution of plant community types in the dry valley of Jinshajiang River. Savanna, succulent thicket and evergreen broadleaved thicket are dominant vegetation types in typical dry-hot valley. Warm deciduous broadleaved thicket and evergreen sclerophyllous forest are dominant in dry-warm valley. Warm needle-leaved forest and deciduous broadleaved forest are more adaptive to lower temperature. (3) The pure environmental fraction can explain 5.5% of the species composition variation, the pure broad-scale spatial fraction can explain 22.5% of the species composition variation, 6.6% can be explained by the fraction corresponding to broad-scale structured environment and the unexplainable part was 65.4%. Among all the factors, MAT and k indicated the critical difference among the community habitats, which has prominent impact on the change of community composition. The broad-scale spatial factors played an important role in shaping the community structure by geographic isolation.

Understanding Long-Term Variations in an Elephant Piosphere Effect to Manage Impacts

Surface water availability is a key driver of elephant impacts on biological diversity. Thus, understanding the spatio-temporal variations of these impacts in relation to water is critical to their management. However, elephant piosphere effects (i.e. the radial pattern of attenuating impact) are poorly described, with few long-term quantitative studies. Our understanding is further confounded by the complexity of systems with elephant (i.e. fenced, multiple water points, seasonal water availability, varying population densities) that likely limit the use of conceptual models to predict these impacts. Using 31 years of data on shrub structure in the succulent thickets of the Addo Elephant National Park, South Africa, we tested elephant effects at a single water point. Shrub structure showed a clear sigmoid response with distance from water, declining at both the upper and lower limits of sampling. Adjacent to water, this decline caused a roughly 300-m radial expansion of the grass-dominated habitats that replace shrub communities. Despite the clear relationship between shrub structure and ecological functioning in thicket, the extent of elephant effects varied between these features with distance from water. Moreover, these patterns co-varied with other confounding variables (e.g. the location of neighboring water points), which limits our ability to predict such effects in the absence of long-term data. We predict that elephant have the ability to cause severe transformation in succulent thicket habitats with abundant water supply and elevated elephant numbers. However, these piosphere effects are complex, suggesting that a more integrated understanding of elephant impacts on ecological heterogeneity may be required before water availability is used as a tool to manage impacts. We caution against the establishment of water points in novel succulent thicket habitats, and advocate a significant reduction in water provisioning at our study site, albeit with greater impacts at each water point.

Seasonal variation in forages utilized by the African buffalo (Syncerus caffer) in the succulent thicket of South Africa

AbstractThe succulent thicket of the Eastern Cape, South Africa, is characterized by densely wooded vegetation that is dominated by succulents with little understory of ephemeral and weakly perennial grasses and forbs. Studies have developed around the question: how do bulk grazers such as the African buffalo (Syncerus caffer) survive in the succulent thicket? In this study, the diet of the African buffalo at the Great Fish River Reserve (GFRR) was studied in two seasons (wet and dry). The diet profile was assessed from faecal matter, using the micro‐histological analysis method. During the wet season, grass species contributed 72% to the diet while 28% was contributed by browse species. In the dry season there was a significant increase in the intake of browse by 5% (χ2 = 19.94, df = 11, P &lt; 0.05). There were species that were neglected in the wet season but became principal dietary items in the dry season, these included Setaria neglecta, Cymbopogon plurinodis, Capparis sepiaria and Portulacaria afra. Diet quality, as estimated from faecal samples, suggested that the buffalo were nutritionally stable; however, the presence of sarcoptic mange in the buffalo suggests nutritional stress.

Shearing at the end of summer affects body temperature of free-living Angora goats (Capra aegagrus) more than does shearing at the end of winter

Angora goats are known to be vulnerable to cold stress, especially after shearing, but their thermoregulatory responses to shearing have not been measured. We recorded activity, and abdominal and subcutaneous temperatures, for 10 days pre-shearing and post-shearing, in 10 Angora goats inhabiting the succulent thicket of the Eastern Cape, South Africa, in both March (late summer) and September (late winter). Within each season, environmental conditions were similar pre-shearing and post-shearing, but September was an average 5°C colder than March. Shearing resulted in a decreased mean (P < 0.0001), minimum (P < 0.0001) and maximum daily abdominal temperature (P < 0.0001). Paradoxically, the decrease in daily mean (P = 0.03) and maximum (P = 0.01) abdominal temperatures, from pre-shearing to post-shearing, was greater in March than in September. Daily amplitude of body temperature rhythm (P < 0.0001) and the maximum rate of abdominal temperature rise (P < 0.0001) increased from pre-shearing to post-shearing, resulting in an earlier diurnal peak in abdominal temperature (P = 0.001) post-shearing. These changes in amplitude, rate of abdominal temperature rise and time of diurnal peak in abdominal temperature suggest that the goats’ thermoregulatory system was more labile after shearing. Mean daily subcutaneous temperatures also decreased post-shearing (P < 0.0001), despite our index goat selecting more stable microclimates after shearing in March (P = 0.03). Following shearing, there was an increased difference between abdominal and subcutaneous temperatures (P < 0.0001) at night, suggesting that the goats used peripheral vasoconstriction to limit heat loss. In addition to these temperature changes, mean daily activity increased nearly two-fold after March shearing, but not September shearing. This increased activity after March shearing was likely the result of an increased foraging time, food intake and metabolic rate, as suggested by the increased water influx (P = 0.0008). Thus, Angora goats entered a heat conservation mode after shearing in both March and September. That the transition from the fleeced to the shorn state had greater thermoregulatory consequences in March than in September may provide a mechanistic explanation for Angora goats’ vulnerability to cold in summer.

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

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.

Coexistence of succulent tree aloes: partitioning of bird pollinators by floral traits and flowering phenology

Coexistence among species that lack genetic barriers to hybridization usually depends on pre‐mating isolating barriers. It has been difficult to explain coexistence among African Aloe species because they readily hybridize, often flower simultaneously and are mostly bird‐pollinated. Here we show that co‐flowering aloes in a succulent thicket community in South Africa partition the fauna of flower‐visiting birds. Aloe species with small amounts of concentrated nectar in long corolla tubes were pollinated primarily by long‐billed sunbirds. These species co‐flowered with species with large amounts of dilute nectar in short corolla tubes which were pollinated primarily by short‐billed, generalist nectarivores. Aloe species which share pollinators tend to have divergent flowering times and differences in pollen placement on birds. Without these isolating barriers, genetic dissolution of sympatric Aloe species would be likely.

Fever and sickness behavior during an opportunistic infection in a free-living antelope, the greater kudu (Tragelaphus strepsiceros)

To study their thermal responses to climatic stress, we implanted seven greater kudu (Tragelaphus strepsiceros) with intra-abdominal, brain, carotid, and subcutaneous temperature data loggers, as well as an activity logger. Each animal was also equipped with a collar holding a miniature black globe thermometer, which we used to assess thermoregulatory behavior. The kudu ranged freely within succulent thicket vegetation of the Eastern Cape Province, South Africa. The kudu spontaneously developed a bacterial pneumonia and consequent fever that lasted between 6 and 10 days. The fever was characterized by a significant increase in mean 24-h abdominal temperature from 38.9 +/- 0.2 degrees C to 40.2 +/- 0.4 degrees C (means +/- SD, t(6) = 11.01, P < 0.0001), although the amplitude of body temperature rhythm remained unchanged (t(6) = 1.18, P = 0.28). Six of the kudu chose warmer microclimates during the fever than when afebrile (P < 0.0001). Despite the selection of a warmer environment, on the first day of fever, the abdominal-subcutaneous temperature difference was significantly higher than on afebrile days (t(5) = 3.06, P = 0.028), indicating vasoconstriction. Some kudu displayed increased frequency of selective brain cooling during the fever, which would have inhibited evaporative heat loss and increased febrile body temperatures, without increasing the metabolic maintenance costs of high body temperatures. Average daily activity during the fever decreased to 60% of afebrile activity (t(6) = 3.46, P = 0.014). We therefore have recorded quantitative evidence for autonomic and behavioral fever, as well as sickness behavior, in the form of decreased activity, in a free-living ungulate species.

Demography and population dynamics in the elephants Loxodonta africana of Addo Elephant National Park, South Africa: is there evidence of density dependent regulation?

Density dependence of the Addo Elephant National Park (South Africa) elephants Loxodonta africana was assessed using a long-term data set. Estimated carrying capacity is 0.1–0.5 elephants km−2 but stocking rates have been up to 4 elephants km−2. Population growth rate was found to be positively correlated with increasing density. There was no relationship between birth rate, the age of first calving or calf sex ratio and elephant density but there was a positive relationship between birth rate and rainfall during conception year. Mortality rates, particularly for juveniles, were low, and mean inter-calf interval was 3.3 years. There is no evidence of density dependent regulation in this population, despite the population being consistently above the estimated sustainable carrying capacity and a loss of phytomass and biodiversity. This is interpreted in light of the characteristics of the aseasonal habitat, succulent thicket vegetation and the ability of elephants to utilize accumulated vegetation biomass. These findings indicate that density dependence should not be considered as an option in the control of elephant numbers in this Park, or where elephant resources are not seasonally limited.

Brushpiles and dung as rehabilitation patches: effect on soil resources in degraded succulent thicket, Eastern Cape, South Africa

A substantial portion of the Thicket Biome in the Eastern Cape has been transformed by overgrazing. An experimental rehabilitation study of degraded succulent thicket was initiated to determine whether increasing the patchiness on the landscape would result in an increase in soil resources. Five types of treatments were used for the rehabilitation trials, namely: piles of black rhinoceros (Diceros bicornis) dung placed so as to mimic rhino latrines; Portulacaria afra brushpiles; brushpiles of woody Thicket species; brushpiles of woody Thicket species over-sown with seed; and the control (area left unchanged). Measurement occurred before and three years after treatment and change was related to an untransformed reference site. All of the treatments did increase the litter in the patches, which resulted in an increase in soil organic content especially in the top 3cm of soil. All of the rehabilitation patches were successful in accumulating basic elements (pH and electrical conductivity results) as well as available phosphorus. The dung treatment had the greatest effect on the soil resources underneath and surrounding the patches. The dung treatment significantly increased the electrical conductivity as well as the available phosphorus and available organic nitrogen. The rehabilitation trial has shown that by increasing the patchiness of degraded areas in succulent thicket it is possible to initiate "islands of fertility".

Landscape dysfunction and reduced spatial heterogeneity in soil resources and fertility in semi‐arid succulent thicket, South Africa

Abstract Succulent thicket in the Eastern Cape of South Africa is one of many rangeland ecosystems in the world that displays evidence of unsustainable grazing pressure. Widespread transformation of succulent thicket has resulted in the replacement of the typical two‐phase perennial vegetation patches with a structurally simple field layer of ephemeral and weakly perennial grasses and forbs. We hypothesized that (i) transformation of succulent thicket leads to a switch from a spatially heterogeneous landscape to a homogenous and relatively infertile state; and (ii) that this loss of fertility is associated with a breakdown in the processes that conserve resources and promote water use efficiency. We tested these hypotheses at five fenceline contrast sites in Sundays River Thicket, an arid form of succulent thicket. We compared soil fertility (organic carbon, available nitrogen and phosphorus), texture, matric potential, and surface microtopography in the two main microhabitats on either side of the fencelines using a nested anova. Our results show that intact Sundays River thicket has a distinct spatial pattern of soil fertility where nutrients and organic carbon are concentrated under the patches of perennial shrubs, compared with under canopy trees. Transformation results in a significant homogenization of this pattern and an overall reduction in the fertility of the landscape. The proportion of the landscape surface that promotes infiltration due to a distinct litter layer decreases from 60% to 0.6%. Soil moisture retention (matric potential) also decreases with transformation. We interpret these patterns within the framework of semi‐arid landscape functionality.

Patterns and implications of transformation in semi-arid succulent thicket, South Africa

Transformation of South African succulent thicket in response to grazing is widespread and characterized by the extensive replacement of dense closed-canopy thicket with a ‘pseudo-savanna’ of remnant trees with an ephemeral field layer. In this study, we examined the patterns of transformation in a semi-arid succulent thicket ecosystem using replicated fence line contrasts. We quantified the changes in plant species and functional diversity, physiognomy and biomass. We tested whether the remnant tree guild of the transformed thicket is in a stable state, using aerial photographs and ground-truthing to track the survivorship of canopy trees over 60 years in transformed landscapes. We investigated the impacts of transformation on recruitment into the canopy tree guild by measuring seedling establishment across the fence line contrasts. Transformation results in a significant loss of plant and functional diversity. There is a significant reduction in the biomass and structural complexity of the vegetation, both vertically and horizontally. The canopy tree guild in transformed thicket is not stable owing to ongoing adult mortality and little successful recruitment. These results are interpreted in a framework of ecosystem functioning and long-term stability. We suggest that the pseudo-savanna typical of transformed succulent thicket is not a stable alternative state to intact thicket, but rather an intermediate stage in a trajectory towards a highly desertified state where only the ephemeral grasses and forbs persist.

Transformation of thicket to savanna reduces soil quality in the Eastern Cape, South Africa

Xeric succulent thicket in the Eastern Cape, South Africa has been used for farming goats since the early 1900s. This habitat is characterised by a dense cover of the succulent bush Portulacaria afra and by a warm, semi-arid climate with evenly distributed annual rainfall of 250–400 mm. Heavy browsing by goats results in the loss of P. afra and transforms the thicket to an open savanna dominated by annual grasses. Eight fence-line comparisons between thicket and savanna were used to investigate differences in soil quality associated with the vegetation change. Composite soil samples were taken to a depth of 10 cm from 1 ha plots on either side of the fence-line. Associated with the change from thicket to savanna, a significant decrease (paired t-test, P < 0.05) was found in total C (respective means of 5.6 vs. 3.0%), total N (0.33 vs. 0.24%), labile C (2.8 vs. 1.5%), CO2 flux (1.9 vs. 0.5 µmol m−2 s−1), soil respiration in the laboratory (144 vs. 79 ng C kg−1 s−1), (NH4)OAc-extractable Mg (55 vs. 28 mmolc kg−1), and laboratory infiltration rate (51 vs. 19 mm h−1). In the same direction there was a similarly significant increase in modulus of rupture (16 vs. 34 kPa), water-soluble Ca (2.3 vs. 3.4 mmolc kg−1) and pH (6.7 vs. 7.7). The soil C content of 5.6% in thicket is surprisingly high in this warm, semi-arid climate and suggests that the dense P. afra bush strongly regulates soil organic matter through microclimate, erosion control, litter quantity and perhaps chemistry. Savanna soils had a greater tendency to crust (as evident in a lower rate of laboratory infiltration and greater modulus of rupture) than thicket soils. This was attributed to their lower organic matter content, which probably reduced aggregate stability. Savannas are likely to be more prone to runoff and erosion not only because of sparser vegetation but also because of a decline in soil quality.