African Savanna Woodlands Research Articles

Modifications induced in soil physicochemical properties by repeated fire for different fuel load treatment in a West African savanna-woodland

Fire is known to stimulate growth of savanna vegetation, promote species’ diversity, and regulate tree and grass balance. However, the timing and frequency at which savannas are burned and the fuel properties can affect residual soil nutrient content and, ultimately, productivity. The objective of this study is to characterize changes in soil physical and chemical properties in a savanna-woodland subject to different fuel load treatments of burning. To characterize the soil physical and chemical variables, particle size distribution, total carbon, pH, phosphorous content and nitrogen were analyzed for three different soil layers. The results indicated no difference in soil texture between the different topsoil profiles before and after the burning event. Further, there was greater nutrient enrichment of the upper soil layer (p=0.014) due to ash deposition. The spatial patterns of soil temperature during the studied experimental fires could affect soil properties, resulting in new spatial pattern of soil nutrients. The findings of the present study have practical implications for savanna management. The current implementation of prescribed early fires should be continued with due consideration of the burning and fuel properties to avoid detrimental effects of intense fire on soil layers. Key words: Early fire, fire behavior, fire temperature, soil properties, Savanna ecosystem.

Mining the Microbiome of Key Species from African Savanna Woodlands: Potential for Soil Health Improvement and Plant Growth Promotion.

(1) Aims: Assessing bacterial diversity and plant-growth-promoting functions in the rhizosphere of the native African trees Colophospermum mopane and Combretum apiculatum in three landscapes of the Limpopo National Park (Mozambique), subjected to two fire regimes. (2) Methods: Bacterial communities were identified through Illumina Miseq sequencing of the 16S rRNA gene amplicons, followed by culture dependent methods to isolate plant growth-promoting bacteria (PGPB). Plant growth-promoting traits of the cultivable bacterial fraction were further analyzed. To screen for the presence of nitrogen-fixing bacteria, the promiscuous tropical legume Vigna unguiculata was used as a trap host. The taxonomy of all purified isolates was genetically verified by 16S rRNA gene Sanger sequencing. (3) Results: Bacterial community results indicated that fire did not drive major changes in bacterial abundance. However, culture-dependent methods allowed the differentiation of bacterial communities between the sampled sites, which were particularly enriched in Proteobacteria with a wide range of plant-beneficial traits, such as plant protection, plant nutrition, and plant growth. Bradyrhizobium was the most frequent symbiotic bacteria trapped in cowpea nodules coexisting with other endophytic bacteria. (4) Conclusion: Although the global analysis did not show significant differences between landscapes or sites with different fire regimes, probably due to the fast recovery of bacterial communities, the isolation of PGPB suggests that the rhizosphere bacteria are driven by the plant species, soil type, and fire regime, and are potentially associated with a wide range of agricultural, environmental, and industrial applications. Thus, the rhizosphere of African savannah ecosystems seems to be an untapped source of bacterial species and strains that should be further exploited for bio-based solutions.

Herbaceous seeds dominates the soil seed bank after long-term prescribed fire, grazing and selective tree cutting in savanna-woodlands of West Africa

The soil seed bank (SSB) is an important source of resilience for plant communities subjected to disturbances. This study aimed to identify characteristics of the SSB, its relationship with aboveground vegetation (AGV), and its significance for restoration of vegetation in West African savanna-woodlands. Data were collected from two long-term factorial experiments established in 1992 in Burkina Faso to examine the long-term ecological effects of grazing, early fire, selective tree cutting on savanna-woodland ecosystem. A total of 1920 soil samples were taken from three soil layers (i.e. 0-3 cm, 3–6 cm and 6–9 cm) and the SSB was assessed using the seedling emergence technique. The emerged seedlings were dominated by non-woody taxa. Only two woody species that were Flueggea virosa and Mitragyna inermis emerged, showing that woody species feature little in the SSB. The SSB density and richness decreased significantly with soil depth. Non-metric multidimensional scaling (NMDS) ordination indicated a lack of correspondence between the AGV and SSB. The results suggest site-specific responses to disturbance also following long-term disturbance, the SSB will be ineffective in regenerating the remnant vegetation of West African savanna-woodlands. Hence, ensuring the resilience of West African savanna woodland ecosystems will require targeted introductions (i.e. planting and seeding) of indigenous species and a reduction in on-going disturbances.

Dissimilar effects of human and elephant disturbance on woodland structure and functional bird diversity in the mopane woodlands of Zambia

ContextHumans and elephants are major disturbance agents in the African savanna woodlands. While both species remove trees, humans selectively harvest larger stems, which are less vulnerable to elephants. Increasing human pressures raise the question of how the altered disturbance regime will modify woodland structure, and in turn biodiversity and ecosystem function.ObjectivesHere we investigate this process in the mopane woodlands of Zambia by examining relationships between woodland structure, species and functional bird diversity, and human and elephant disturbance intensity.MethodsWe conducted a single-season comparison of 178 plots from 45 sites using Bayesian mixed models.ResultsThe effect of elephants on tree density (− 7.7 ± 1.6%; deviation from intercept) and bird species richness (− 15 ± 6%) was greater than that of humans (density: − 3.5 ± 1.5%; bird richness: − 11.6 ± 4.7%). Despite this, elephants did not significantly affect woody biomass or functional bird diversity, whereas humans had a negative effect on both (woody biomass: − 9.3 ± 2.3%; functional bird diversity: − 5 ± 2%). Elephants were associated with reductions in species and functional turnover (5.3 ± 2.5% and 6 ± 3%, respectively).ConclusionsReplacement of elephants by humans is likely to reduce woody biomass and functional bird diversity affecting the woodland structure, sustainability, and functioning. Concentrated elephant disturbance could lead to spatial homogeneity in bird species and functional compositions, potentially reducing the spatial resilience of bird communities. This is the first study to highlight how the difference between elephant and human disturbances leads to dissimilar effects on biodiversity.

Allometric equations for below-ground biomass of four key woody species in West African savanna-woodlands

Allometric equations for below-ground biomass of four key woody species in West African savanna-woodlands

Identification of corilagin, punicalagin and ellagic acid derivatives from antibacterial and antioxidative extracts of African savanna woodland tree Terminalia laxiflora

Identification of corilagin, punicalagin and ellagic acid derivatives from antibacterial and antioxidative extracts of African savanna woodland tree Terminalia laxiflora

Effects of burning on soil macrofauna in a savanna-woodland under different experimental fuel load treatments

In West African savanna-woodland, the use of prescribed burning as a management tool has ecological implications for the soil biota. Yet, the effects of fire on soil inhabiting organisms are poorly understood. The aim of this study was to examine the responses of soil macro-invertebrates to early fires in a Sudanian savanna-woodland on a set of experimental plots subject to different fuel load treatments. The abundance of major macro-invertebrate taxa and functional groups, and taxon richness were quantified in soil cores collected from three different soil layers before and immediately after burning. The results indicated that, overall, there was substantial spatial and temporal variation in the composition of macro-invertebrate assemblages. The immediate effects of fire were to reduce total invertebrate numbers and numbers of many invertebrate groups dramatically. This is probably due to the fact that many of the surface-dwelling macrofauna perished as a result of less favorable microclimate due to fire, diminished resources, or migrate to safer environments. Fuel load treatment did not affect the community taxonomic richness or abundance of the soil-dwelling fauna. Furthermore, annual changes in community composition were more pronounced at the burnt site than in the control. This could be related to the inter-annual difference in precipitation pattern recorded during the two-year study period at our site. Since soil macrofauna population declines in fire-disturbed areas, increasing fire prevalence may jeopardize the long-term conservation of fire sensitive macrofauna groups. Special fire management attention is therefore recommended with due consideration to the type of burning and fuel properties to avoid the detrimental effects of intense fire affecting the resilience of savanna soil macrofauna species.

Locomotor Anatomy and Behavior of Patas Monkeys (Erythrocebus patas) with Comparison to Vervet Monkeys (Cercopithecus aethiops).

Patas monkeys (Erythrocebus patas) living in African savanna woodlands and grassland habitats have a locomotor system that allows them to run fast, presumably to avoid predators. Long fore- and hindlimbs, long foot bones, short toes, and a digitigrade foot posture were proposed as anatomical correlates with speed. In addition to skeletal proportions, soft tissue and whole body proportions are important components of the locomotor system. To further distinguish patas anatomy from other Old World monkeys, a comparative study based on dissection of skin, muscle, and bone from complete individuals of patas and vervet monkeys (Cercopithecus aethiops) was undertaken. Analysis reveals that small adjustments in patas skeletal proportions, relative mass of limbs and tail, and specific muscle groups promote efficient sagittal limb motion. The ability to run fast is based on a locomotor system adapted for long distance walking. The patas' larger home range and longer daily range than those of vervets give them access to highly dispersed, nutritious foods, water, and sleeping trees. Furthermore, patas monkeys have physiological adaptations that enable them to tolerate and dissipate heat. These features all contribute to the distinct adaptation that is the patas monkeys' basis for survival in grassland and savanna woodland areas.

Structural relationships between form factor, wood density, and biomass in African savanna woodlands

Variation in tree biomass among African savanna species of equal size is driven by a wide inter-specific variation in wood specific gravity. Tree form and taper is a fundamental component of tree structure and has been used for over a century in forestry to estimate timber yields and in ecological theories of scaling laws. Here, we investigate variation in form factor in the context of biomass in African savannas. Biomass is a fundamental metric of vegetation state, yet in African savannas it remains unclear whether variation in form factor F (taper) or wood specific gravity (G) is a more dominant driver of biomass differences between tree species of equal stem diameter and height. Improving our knowledge of vertical mass distribution in savanna trees provides insight into differences in life strategies, such as tradeoffs between production, disturbance avoidance, and water storage. Here, we destructively harvested 782 stems in a savanna woodland near Kruger National Park, South Africa, and measured whole tree wet mass, wood specific gravity, water content, and form factor. We found that three of four dominant species can vary in mass by over twofold, yet inter-specific variation in taper was low and taper did not vary significantly between common species (P > 0.05) (species-mean form factors ranged from F = 0.57 to 0.77, where cone F = $$0.\bar{3}$$ , quadratic paraboloid F = 0.5, cylinder F = 1.0). Comparison of a general biomass allometry model to species-specific models supported the conclusion that the large difference in biomass between species of the same size was explained almost entirely (R 2 = 0.97) by including species-mean G with D and H in a general allometric equation, where F was constant. Our results suggest that inter-specific variation in wood density, not form factor, is the primary driver of biomass differences between species of the same size. We also determined that a simple analytical volume-filling model accurately relates wood specific gravity of these species to their water and gas content (R 2 = 0.68). These results indicate which species use a wide spectrum of water storage strategies in savanna woodlands, adhering to a trade-off between the benefits of denser wood or increased water storage.

Effects of seed burial and fire on seedling and sapling recruitment, survival and growth of African savanna woody plant species

Seed predation and seedling mortality can act as strong demographic “bottlenecks” to sapling recruitment in African savanna woodlands. Fire also limits tree recruitment from saplings by suppressing their growth. I conducted field experiments with 13 woody plant species to assess the effects of seed burial on seedling emergence rates and effects of fire on seedling and sapling survival and growth rates over a period of 8 years at a savanna plot in central Zambia, southern Africa. Seed removal rates by small rodents varied among years and buried seeds had significantly higher emergence rates than seeds exposed to predators in most but not all the species. Annual burning reduced sapling growth in some species but in other species saplings experienced successive shoot die back even in the absence of fire. The findings show that for some woody species, seed predation is an important constraint to seedling recruitment but not for others and annual fires are important hindrances to demography and growth for some species but not others. Thus, demographic “bottlenecks” occur at different life history stages in different savanna woody species and these have the potential to alter woody tree competitive relationships and ultimately savanna structure.

Litterfall and nutrient return in a semi-arid southern African savanna woodland dominated by Colophospermum mopane

The objective of this study was to investigate litter production, litter standing crop and nutrient return to soil in a semi-arid southern African savanna in Bulawayo, Zimbabwe. We used a randomized block design with five blocks of 100 × 100 m demarcated in a 10-ha pocket of Colophospermum mopane-dominated open woodland protected from grazing and fire. Litter traps were installed beneath large (8.3 m crown diameter) and small mopane trees (2.7 m crown diameter) and in the intercanopy area, representing 27, 3 and 62% of the woodland area, respectively. Mean annual total litterfall over 2 years of observations was 197, 83 and 35 g m−2 yr−1 beneath large and small trees as well as in the intercanopy area, respectively. Leaf proportions of total litterfall beneath large and small trees and in the intercanopy area were 68.6, 73.0 and 75.3%, respectively. Litterfall followed a uni-modal distribution pattern and was much higher during the period of May–September (dry period) compared to other months. The total potential annual element inputs via litterfall beneath large trees were 2 and 5 times greater than beneath small trees and in the intercanopy area, respectively. Total litter standing crop was 405, 177 and 67 g m−2 beneath large and small trees and in the intercanopy area, respectively. Concentrations of N, P and K in litterfall and surface soil were closely correlated with each other. At all sampling sites, element accession to soil through litterfall followed the decreasing sequence C > Ca > N > Mg > K > P. These results suggest that litterfall is a major process responsible for soil organic matter and nutrient enrichment beneath isolated trees in semi-arid savannas.

Effects of tree cover and season on soil nitrogen dynamics and microbial biomass in an African savanna woodland dominated by Colophospermum mopane

The objective of this study was to investigate the effects of tree cover and season on soil N dynamics and microbial biomass in a semi-arid southern African savanna in Zimbabwe. We used a randomized complete block design with five blocks of 100 × 100 m, demarcated in a 10-ha pocket of Colophospermum mopane-dominated woodland protected from grazing and fire. In each block, we randomly selected three mopane trees with large canopies (8.3 m crown diameter) and another three with small canopies (2.7 m crown diameter). We determined soil organic carbon and nutrient concentrations, litterfall N and C inputs, microbial biomass and N transformations beneath large and small mopane trees as well as in the intercanopy areas. Soil organic carbon, microbial biomass, N, P and K were more than twice those beneath large trees than in the intercanopy areas. Rainy-season net mineral N accumulation rate in the surface soil (0–10 cm) ranged from 3.71 μg g−1 mo−1 in the intercanopy areas to 8.80 μg g−1 mo−1 beneath large trees; correspondingly, net nitrate accumulation rate ranged from 1.33 to 3.60 μg g−1 mo−1. Dry-season net mineral N and net nitrate accumulation rates were similar across sampling sites and did not exceed 2 and 0.4 μg g−1 mo−1, respectively. Litterfall N inputs were positively and significantly correlated with soil N availability, microbial biomass N and N transformations. At all sampling sites, microbial biomass and mineral N pools in the dry season were maximum when soil moisture (∼5%) and N transformations were minimum. In contrast, when soil moisture (9–13%) and N transformations were maximum in the rainy season, microbial biomass and mineral N pools were minimum. It is concluded that the improved soil conditions beneath isolated trees in semi-arid savannas may enhance herbaceous biomass yield especially of canopy shade-tolerant species.

Fuel and fire characteristics in savanna - woodland of West Africa in relation to grazing and dominant grass type

Fuel characteristics, fire behaviour and temperature were studied in relation to grazing, dominant grass type and wind direction in West African savanna–woodland by lighting 32 prescribed early fires. Grazing significantly reduced the vegetation height, total fuel load, and dead and live fuel fractions whereas plots dominated by perennial grasses had higher values for vegetation height, total fuel load and the quantity of live fuel load. Although fire intensity remained insensitive (P > 0.05) to any of these factors, fuel consumption was significantly (P = 0.021) reduced by grazing, rate of spread was faster in head fire (P = 0.012), and flame length was shorter in head fire than back fire (P = 0.044). The average maximum temperature was higher (P < 0.05) on non-grazed plots, on plots dominated by annual grasses, on plots subjected to head fire, and at the soil surface. Lethal temperature residence time showed a nearly similar trend to fire temperature. Wind speed and total fuel load were best predictors of fire behaviour parameters (R2 ranging from 0.557 to 0.862). It can be concluded that grazing could be used as a management tool to modify fire behaviour, back fire should be carried out during prescribed burning to lower fire severity, and the fire behaviour models can be employed to guide prescribed early fire in the study area.

A simulation analysis of the detectability of understory burns in miombo woodlands

The miombo woodlands of southern Africa are one of the most extensively burned biomes in the tropics. The detectability of understory burns in these woodlands was assessed with a sensitivity analysis approach, based on a hybrid geometrical–optical radiative transfer model. Model input data were obtained from a variety of sources, including field biometry and spectroradiometry, and satellite data. The effects of variable tree percent cover, leaf area index, stand density, burn scar age, illumination and observation geometry, and spectral region, were taken into account. Detectability of understory burns was defined as the spectral separability of burned and unburned understory, measured with the Jeffries–Matusita distance, for all possible combinations of the green, red, and near-infrared channels of the Moderate Resolution Imaging Spectrometer (MODIS) sensor. Single channels, or pairwise combinations of channels perform poorly at detecting understory burns, but a large improvement in detectability is obtained for the combination of the three spectral domains. The detectability of understory burns is largely insensitive to the effects of stand structure and illumination/observation geometry, and depends primarily on burn scar age. Our results agree with those of previous satellite-based studies of burns scar detectability in African savanna woodlands.

Physio-climatic classification of South Africa's woodland biome

In an effort to develop more holistic ecosystem approaches to resource assessment and management, landscapes need to be stratified into homogeneous geographic regions. These regions can then be used in a monitoring framework to develop reliable estimates of ecosystem productivity. A regional characterization of the woodland biome has been developed for South Africa, delineated by satellite imagery and using environmental data and a rigorous statistical methodology. Distribution maps of key environmental variables are analyzed by factor analysis, an iterative clustering technique and maximum likelihood classification to quantify and identify homogeneous physio-climatic units. A spatial clustering technique was used to identify regions, which are statistically different with regard to five physiographic, climatic and edaphic variables deemed important within southern African savanna woodlands. The woodland biome of South Africa at 1km resolution was successively divided. Thirty year mean monthly temperature, total plant-available water balance of soil, elevation, landscape topographic position, and landscape soil fertility were used as input classification variables. The map data were submitted to a factor analysis and varimax axis rotation. The factor analysis removes correlations from the input variables, reduces the dimensionality, and normalizes the axis measurements. A cluster analysis was performed on the three principal factor scores using a modified iterative optimization clustering procedure to determine the finest level of classes statistically permitable. Twenty-seven identified unimodal cluster signatures were then submitted to a maximum likelihood classification where the statistical probability of the GIS cell assignment is carried out to determine class membership. The final map of custom physio-climatic regions is described, and these custom regions are compared with a vegetation potential map of the woodland types identified in the South African summer rainfall zone.

Long-term impacts of abundant perennial water provision for game on herbaceous vegetation in a semi-arid African savanna woodland

The objective of this study was to examine the impact of provision of perennial water supply, as a management strategy to enhance game viewing, on herbaceous vegetation. Changes in herbaceous species composition and functional characteristics were investigated in relation to (1) distance from perennial and ephemeral surface water (spatial component), and (2) change over a 16-year period (1980–1996; temporal component). Herbaceous functional characteristics revealed a gradient of herbivore utilization in relation to proximity to perennial and ephemeral water. Herbaceous composition change since 1980 revealed significant deterioration in range condition. It has been hypothesized that the implications of an abundance of perennial water for game are made manifest by drought conditions and that a history of heavy utilization by herbivores prevents or inhibits recovery of the herbaceous vegetation to its pre-drought state.

The importance of herbivore interactions for the dynamics of African savanna woodlands: an hypothesis

Current hypotheses to explain dynamic transitions between savanna grasslands and woodlands in Africa focus on grazing by elephant or the influence of fire. Using a simple mathematical model, this paper argues that interactions between small herbivores such as impala or buffalo and large herbivores such as elephant or giraffe may provide a plausible alternative hypothesis. The interplay of competition and facilitation between these types of herbivores could explain transitions between grassland and woodland and vice versa. A review of the literature is presented in support of this hypothesis.

Habitat Use by Sympatric Wild and Domestic Herbivores in an African Savanna Woodland: The Influence of Cattle Spatial Behaviour

The spatial distribution of two wild ungulates, impala and kudu, and of domestic cattle, was monitored across seasons in a 9400 ha ranch of the 'highveld' of Zimbabwe. Habitat use, preference, selectivity and overlap were estimated. Predation was negligible and the study concentrated on interspecific competition. Cattle were contained in paddocks, and were moved regularly, while wild ungulates moved freely. Wild ungulate habitat use, preference and selectivity between paddocks with and without cattle were compared, as was the variation in broad diet, group size and densities. All herbivores were selective, with a marked preference for the nutrient-rich Acacia/ Dichrostachys vegetation type. Terminalia communities were almost completely avoided. Selectivity increased as the dry season progressed. Habitat overlap was always high. Interspecific competition occurred between cattle and impala, especially during the wet season and the hot dry season. Impala showed a switch in habitat preference and an increase in selectivity and started to concentrate on 'refuge habitat'. Some also switched in their diet composition. During the hot dry season, when resources were at their lowest, most impala stopped using paddocks with cattle. Kudu seemed relatively unaffected by the presence of cattle: the variations observed in kudu spatial behaviour may have been caused by competition with impala.

Topo-edaphic, floristic and physiognomic gradients of woody plants in a semi-arid African savanna woodland

The variation in topo-edaphic conditions, woodland composition and physiognomic structure of a semi-arid savanna woodland, namely the 59 400 ha Klaserie Nature Reserve, was studied with multivariate techniques. Spatial variation in the texture and chemistry of both the A-and B-horizons was substantial. One gradient of the A-horizon was of increasing clay, silt, organic matter, pH and conductivity, while a second gradient was of increasing rockiness, slope and gravel content, associated in part with upper landscape positions. Physiognomic structure of the woodland also displayed substantial spatial variation. Variation in tree density had the greatest effect on total canopy volume, and the density of coppice and dead individuals were closely correlated. Topo-edaphic variation, after the effect of vegetation composition had been accounted for, had a direct effect on woodland structure, as clay content of soils or rockiness influenced shrub density, and catenal position together with silt-or gravel-content of soils influenced coppice density. Ordination analyses identified that the included topo-edaphic variables accounted for a significant amount of floristic variation, but also showed that important environmental variables were omitted. Floristic variation of woody elements was of the nature of a large number of relatively equivalent, independent gradients rather than of a few simple primary gradients. Ten woodland types were identified by TWINSPAN which differed in their topo-edaphic and physiognomic character. Distinctive woodland assemblages were found on rocky outcrops or alluvial areas, on deep, sandy soils or on clay-or silt-rich soils. Colophospermum mopane formed the densest woodlands. Characteristics of the B-horizon were often dissimilar to those of the A-horizon, and seemed important for woodland composition.

Stock assessment of large mammals in Nigerian wooded savanna and its management implications

Assessment of canopy and ground cover was carried out in two vegetation types at Yankari Game Reserve (YGR) using the rapid field technique of Riney (1963). Animal population was enumerated by two census methods, namely fixed transect belt (FTB) and King's census (KC) methods. The population of large mammals, enumerated by these methods, was compared for the upland savanna (US) and riparian woodland (RW) of the reserve. Morning and evening counts of animals were also compared between vegetation types. Percentages of perennial grass, litter and browse plants were higher in the RW than in the US. Consequently, RW had better soil protection and ecological stability than US. Total animal number in RW was also higher than in US, probably due to the fact that RW provided better shelter and contained more food and water than US. The FTB method of obtaining a census indicated that waterbuck, with a population of 5067, was the most abundant species during the evening count along the RW. The KC method estimated the population of this species to be 8815. Elephant and roan antelope appeared to be present during evening counts whereas the buffalo population was higher ( P < 0.01) in the morning than in the evening. The management implications of the two census methods are discussed, both in regard to their usefulness in long term ecological monitoring, and in determining the conservation status of habitats and animal population in African savanna woodlands.