**2. Floristic composition in the African savanna**

#### **2.1 Factors driving the structure of the African savanna vegetation**

The vegetation structure is coined as a three-dimensional distribution of plant biomass. The structure typically denotes perpendicular distribution despite having a flat configuration. More significantly, the species composition and fluctuations in temporal scales can be used to characterize the vegetation structure. According to Ref. [47] factors, including geologic substrate, terrain, human activity, fire, and large herbivores, particularly elephants, all influence the woody vegetative structure of savanna ecosystems.

Plant species' adaptations to the environment include both biotic and abiotic elements, resulting in the structure of the vegetation [48]. The vegetation structure studies mainly concentrate on density, canopy cover, and standing crops of various species [49]. Resource managers use the measurements derived from these factors to understand better how vegetation responds to different management approaches [49].

Several researchers (e.g., [50–52]) have investigated how woody canopy cover varies throughout African savannas in response to environmental conditions. Given that tropical savannas span roughly one-eighth of the land surface [53] and play a significant role in the global carbon cycle [54], it is critical to know how these differences in crown widths and tree densities are formed.

Vegetation structure and the composition of plant species play a trivial role in animal habitat suitability. This is due to the fact that the sensitivity of diverse animal species depends on the quantity of vegetation and bare patches in the landscape, making the condition of spatial heterogeneity a valuable indicator of the suitability of animal habitat [55]. Grass plains, tree savannas, woods, and thickets can all be distinguished by their vegetative structure. Besides, according to Ref. [56], structural vegetation differences exist within a particular habitat, like between short-grass and tall-grass plains.

Variations in the structure of savanna vegetation can be undergone seasonally due to the deciduous nature of woody plants. This may happen when the tall grasses are burnt so as to transform the area to a short-grass area or due to the impact of animals which may defoliate or trample on the vegetation [57–61]. Savanna trees are reported to have both positive and negative competition on the individual tree scale and, therefore, influence the growing grasses beneath their canopies in relation to grasses in inter-canopy areas. Hence, plant species composition in sub-canopy areas can be impacted by large trees, thereby modifying the vegetation structure of the nearby environment [62, 63].

However, this has a positive impact: rich soil nutrients and grass leave in subcanopy areas, an increase in the availability of soil water due to the hydraulic lift, a reduction in evapotranspiration, and an increase in the productivity of grass [62]. Furthermore, animals are supplied with shade, shelter from the elements, and huge woody plants to browse. Large woody plants also provide higher sub-habitats, a higher yield of highly palatable grass species, and many more rewards [63–65].

Holdo [44] denotes key determinant variables of vegetation structure, such as fire, browsing, and grazers. These were pointed out by Ref. [44] to determine the vegetation structure across rainfall and fertility gradient. Elephants were identified as keystone browsing species. Although the current elephant population does not control the woody vegetation on its own, [44] postulate that the population density has

an impact on the tree-size class distribution of woody vegetation because an increase in population density causes a shift in tree size distribution from mature to small height classes, as well as a shift in woodland to grasslands. Refs. [44, 66, 67] identified fire to have a detrimental impact on the type and structure of woody vegetation. It was also noted that grazers affect the woody vegetation by regulating the quantity of fuel suitable for fire and enhancing competition between grasses and woody plants, especially in the regeneration of nutrients and water. Besides, browsers were identified by Ref. [43] to have the greatest influence on woody species regeneration. Using *Euclea divinorum* seedlings, it was discovered that fire reduces seedling survival by 50% and browsers reduce it by 70%, with elephants having no effect.

#### **2.2 Diversity and richness in an African savanna**

African savanna ecosystems have experienced severe variations in their vegetative composition, diversity, and species richness due to the influence of human land use and changes in climate conditions. According to Ref. [68], there is a challenge in comprehending the factors affecting the variations in spatial patterns of composition, diversity, and structure of woody species.

#### **2.3 Factors influencing species diversity and composition**

Savanna ecosystems play a trivial role in the entirety of the biosphere's activities. Trees and grasses are two divergent life forms characterizing savanna ecosystems. The structure and purpose of savanna are said by Ref. [69] to be influenced by the availability of resources, such as rainfall and nutrients. This is also stated to be affected by natural and anthropogenic disturbances, such as fire and herbivory.

The supply of water is detected by rainfall through the quantity that is then consequently accessible by plants and is subjected to the drainage aspects and storage, which comprises topography, vegetation cover and losses, soil texture, and compaction as a result of evaporation and evapotranspiration. There is a high variation of rainfall in the spatial and temporal scales in the savanna ecosystem. This is exacerbated by aridity, with many places experiencing regular droughts, which according to Ref. [70], can be a chief influencer of the variations in the composition of the vegetation. Recruitment and growth of trees are preferred to grasses in years of high rainfall compared to the times experiencing droughts where growth and recruitment of trees are limited [69].

Species diversity and composition are also affected by soil nutrients. Mainly, there are limited nutrients in the soil since soils in most tropical savannas are obtained from old weathered acid crystalline igneous rock leading to leaching sandy soils with low fertility and CEC. Specifically, low nitrogen and phosphorous availability limit numerous savanna ecological systems [71]. In addition, the water in the soil also hinders the plants from accessing the nutrients because mineralization of nutrients, transport, and root uptake are all factors of soil water content.

Fire has conventionally been utilized as an instrument for managing and conserving savanna ecosystems. This is mainly because there is an exposure of the woody meristems within the flame zone (< 5 m) to the fire damage compared to the grass meristems. Besides, according to Ref. [53], grass meristems can recuperate more efficiently in the short term. Hence, the frequency of fires overpowers the recruitment of mature woody plants. It is noted that there can be interactive impacts on the savanna ecosystem structure by fire and grazing where the high grass biomass buildup is permitted by low grazing, which can impact the biomass and population of the tree

*Contextualizing the Factors Affecting Species Diversity and Composition in the African Savanna DOI: http://dx.doi.org/10.5772/intechopen.108413*

by fueling intense fires. Ref. [53] denotes that woody plants are kept within the flame zone by heavy browsing; hence a combination of tree-grass is strongly affected by a strong grazer-browser fire relationship.

Fire has since time immemorial been a significant feature of the ecology in African savanna ecosystems. Fire, according to Refs. [39, 72, 73], has been used both as a selective and regulatory agent, as well as a destructive force. For instance, 25 to 50% of an area has been reported to be subjected to burning on an annual basis. Furthermore, the entire African zone has been estimated to undergo burning for about 2.5 years due to anthropogenic activities [74, 75].

Fire is noted to be vital for a lot of farmers in Africa and it is said to be a cheap hunting tool for clearing vegetation that is not wanted, maintenance of grasslands, and removal of dry vegetation and crop residues to promote the productivity of agricultural produces and permit better visibility [76, 77]. Most fires occur at the beginning of the dry spell when herbaceous biomass has dried out. Refs. [73, 78] reports fire to be known for shaping the savanna ecosystem. As a result, all savanna vegetation communities virtually display fire dependence or tolerance [79, 80].

Plant species respond differently to fire. Some are totally resilient, while the aboveground biomass of other plants may be damaged but can shoot from belowground structures after the fire, and still, other plant species rely on the seed to recover [78]. The frequency of fire consumes the accumulated production of grass and litter in most tropical savannas, and this favors the predominant grassland vegetation to develop and be maintained by decreasing the natural regeneration of trees and shrubs [81].

Fire can also inspire growth in the flowering of plants and, consequently, the production of seeds among shrubs and herbaceous species [82]. Fire is depended upon for the germination of seeds of a variety of plant species in that it provides one or more physical cues, such as light and temperature as well as chemical cues, including smoke, gases, and nutrients [83–85].

Fire suppression on a temporal basis is regarded as being induced by human disturbance [86]. This is because some woody and herbaceous species, which are generally resilient in burnt areas, often become feeble in protected areas. Lack of fire occurrence may disturb their productiveness either by sustaining a dense layer of standing dead material that hinders young suckers or by keeping alive buds on old axes and thereby wearying the plant [87].

Savanna structure and composition are also stated by Ref. [88] to be impacted by the physical effects of defoliation and selective feeding. The viability of some woody plant populations may be compromised due to the excessive pressure emanating from heavy browsing, such as the elephant. This may result in community fluctuations coupled with species diversity and structural diversity loss. Besides, herbivory plays a trivial role in the cycling of nutrients, dispersal of seeds, and creation of microsites and space, thereby improving and increasing the recruitment of shrubs [88].

Plant species diversity in African savanna is determined by some factors, including competitive exclusion, disturbance processes, and environmental heterogeneity [89–91]. The diversity in species is decreased by competitive exclusion since it is considered as strong competitors first overpower lesser competitors and later cause them to become extinct locally. Moreover, environmental heterogeneity can stimulate species diversity by promoting spatial niche apportioning and heterogeneity in plant community composition across environmental slopes [89]. Disturbances can diminish the diversity of plant species by eradicating disturbance-sensitive species. This can also be done by increasing the diversity of species by opening up growing space and

resources for utilization by the colonizing species, maintaining species richness by retarding or averting competitive exclusion, and modifying spatial heterogeneity in the composition of the plant community [91].

The competition was backed by Ref. [92] to influence the savanna ecosystem's vegetation types and structure. Their study exhibited that areas receiving less rainfall are dominated by grasslands, whereas those receiving rainfall of more than 650 mm annually are mostly composed of woodlands except for areas where moisture is transformed by a landscape factor, such as topography.

Sharam et al. & Pellew [43, 67] additionally view vegetative structure in the savanna ecosystem as being affected by demographic factors. Ref. [67] in his study of African savannas, postulates factors, such as elephants, giraffes, and fire, directly impact the type of structure of the woodland. He further states tree height affects woody vegetation structure. He sees trees surpassing 6 m as ecologically mature because they escape the influence of fire due to the fact that the fire threshold is 3 m and also browsing by giraffes, whose threshold is approximately 5.75 m. According to Ref. [67], fires influence the regeneration of trees, whereas elephants lead to mortality in trees of all size classes of regeneration (< 1 m), recruitment (1–5.75 m), and mature phases (> 5.75 m).
