The Mesic Savanna Biome

Abstract

The Savanna Biomes (Mesic/Dystrophic and Arid/Eutrophic) of Angola cover over 90% of the country. This Chapter presents the case for the recognition, across Africa, of these two distinctive savanna biomes, both defined by the co-dominance of fire-tolerant trees and C₄ grasses, falling within seasonal climates of warm, wet summers and mild, dry winters. The Mesic Savanna Biome occurs predominantly on the dystrophic (nutrient-poor) soils of the old peneplains of the Central African Plateau, mostly between 900 and 1400 m above sea level, and where rainfall ranges from 650 to 1400 mm per year. Fire is the main consumer of the grasslands, and is the key determinant of tree/grass dynamics. The biome covers 68% of Angola, and includes six ecoregions, to which may be added two ecoregions with a mosaic of tallgrass mesic savanna and forest patches occupying a further 16%, where Guineo-congolian forests and Zambezian mesic savannas interdigitate in northern Angola. Trees of the genera Brachystegia and Julbernardia characterise what is regionally know as miombo woodland. The physical, physiognomic, floristic and faunistic characteristics and unique natural phenomena of the biome are described in detail. The importance of the catena as a landscape feature determining vegetation pattern, and the role of termitaria in providing nutrient-rich islands in a sea of nutrient-poor soils, plus the abundance of ‘underground forests’ created by woody geoxyles, represent some of the special features of the biome.

Key Concepts and Questions: This Chapter Will Explain

• What the most extensive biome in Angola is, why it is called miombo, and what its main characteristics are.

• Why landscape physiography and dynamics are so important in shaping the patterns of ecosystems across the Angolan planalto and peneplains, how they function and how to recognise them.

• What mammal species are typical of the different savanna types and why.

• What behavioural responses miombo trees have to climate seasonality.

• What accounts for the presence of 'underground forests' in miombo landscapes.

Context: Angola’s Tropical Savannas

Savannas, in their diverse forms, occupy 20% of the Earth’s land surface and ca. 50% of the African continent. They cover over 80% of Angola (94% if the tallgrass savanna component of the Congolian Forest/Savanna Mosaics is included). They are represented by nine Angolan ecoregions in two biomes—the Mesic/Dystrophic savannas and Arid/Eutrophic savannas (Fig. 14.1). These two biomes equate with the ‘Panda’ (Brachystegia) and ‘Adansonia’ phytogeographic zones described by Gossweiler and Mendonça (1939). Burgess et al. (2004), in their classification of Africa’s terrestrial biomes, placed 24 ecoregions in what they call the Tropical and Subtropical Grasslands, Savannas, Shrublands and Woodlands Biome. These ecoregions cover the diversity of ecosystems, from the Sahel to the Kalahari, that Huntley and Walker (1982) defined globally as tropical savannas, a concept that is now widely adopted (Archibald et al., 2013; Bond, 2019; Frost et al., 1986; Sankaran et al., 2005).

Fig. 14.1

Southern African savannas. Vertical stripes—Mesic/Dystrophic savannas; Stippled—Arid/Eutrophic savannas. From Huntley (1982) Ecology of Tropical Savannas. Springer. Berlin

This chapter will cover the ecology of mesic/dystrophic savannas, focusing on the miombo ecosystems that characterise the Angolan planalto and eastern peneplains, but also including the dry Baikiaea open woodlands of the south. The arid/eutrophic savannas of the southwest and the coastal belt are described in Chap. 15.

Despite the importance of miombo across central Africa, relatively little research has been undertaken on the ecology and population biology of miombo ecosystems, when compared with that of fynbos, grassland, karoo and savanna ecosystems in southern Africa (Cowling et al., 1997). Barbosa (1970) and Monteiro (1970a, 1970b) provide detailed descriptions of the plant composition and structure of Angola’s miombo, with the latter author giving the first systematic quantitative floristic analysis of Angolan miombo communities and the morphological characteristics of the tree species. In the past decade, studies on Giant Sable (Vaz Pinto, 2019) and aspects of the structure and dynamics of miombo communities (Finckh et al., 2021; Gomes et al., 2021; Reverman & Finckh 2019; Zigelski et al., 2019; and included references) have introduced a new wave of miombo research in the country. Beyond Angola, the extensive work in savannas by French ecologists in the Ivory Coast (Abbadie et al., 2005), Zambian ecologists Chidumayo and Frost (1996) in Zambia and Zimbabwe, Belgian ecologist François Malaisse (1983) in Shaba, DRC, and South African ecologists Scholes and Walker (1993) and the research team at Kruger National Park (Du Toit et al., 2003) provide a very sound base on savanna structure and functioning. More recently, the work of William Bond (2019), Sally Archibald (2017) and colleagues has brought a vibrant surge of new perspectives on savanna ecology, in both arid and mesic savannas. Much of what follows draws on the work of these leading scholars.

Angola’s Mesic Savannas (Ecoregions 6–11)

1 Definition and Distribution

The key diagnostic criteria of tropical mesic savanna ecosystems are the co-dominance of trees and grasses, climatic seasonality of warm, wet summers, mild dry winters (between 650 and 1400 mm rainfall per year with a dry season of four to eight months), the dominance of the herbaceous stratum by C₄ grasses, the prevalence of infertile, dystrophic soils, and the role of fire as the main consumer of plant biomass.

The Mesic Savanna Biome occupies 68% of Angola’s land area (Fig. 2.3, Table 2.3):

• Ecoregion 6—Escarpment Savannas: 5.2%

• Ecoregions 7 and 8—Miombo woodlands and grasslands: 45.1%

• Ecoregion 9—Baikaiea woodlands: 13.5%

• Ecoregion 10—Cryptosepalum dry forests: 0.05%

• Ecoregion 11—Flooded grasslands: 4.3%.

To this may be added the tall grassland components of the Congolian Forest/Savanna Mosaics of northern Angola (Ecoregions 2 and 3) which occupy 16% of Angola’s land area.

Brief descriptions of these Ecoregions are given in Chap. 2, with maps pf their distribution provided in Figs. 3.11, 3.12, 3.13, 3.19, 3.20, 3.21. Here we will focus on the miombo, the typical mesic savanna ecoregion which occupies over 2.7 million km² of Africa, and is the dominant vegetation formation of Angola.

Miombo is a term initially adopted by English-speaking ecologists, from the Zambian name (muombo) for Julbernardia paniculata or alternatively muuyombo for Brachystegia boehmii. Gossweiler and Mendonça (1939) referred to the miombo formation as the mata de panda. In some parts of Angola, J. paniculata is known as umpanda, while one of the many local names for Brachystegia spiciformis is mupanda (Figueiredo & Smith, 2012). Miombo is now used internationally to describe the central, southern and eastern African woodlands, savannas and included grasslands that are dominated by the woody genera Brachystegia, Julbernardia and Isoberlinia. Figures 14.2 and 14.3 present examples of typical Angolan miombo. The interdigitation of often treeless grasslands with woodlands is a characteristic feature of miombo. Fire and soil moisture relations maintain sharp boundaries between grassland and woodland, reflected in the typical catenal sequence found in miombo landcapes (Box 14.1).

Fig. 14.2

Typical mature open miombo woodland of Brachystegia and Julbernardia, near Caconda

Fig. 14.3

Mixed miombo in a site protected from excessive fires. Barbosa Nature Reserve, Chianga Research Station, Huambo. Note continuous grass cover below the woodland canopy

2 Landscapes, Soils, the Catena Concept, Termitaria and Underground Forests

Miombo systems typically occur on the vast peneplains of the geologically old ‘African’ and ‘Post-African’ planation surfaces of the Central African Plateau, at 900–1400 m. In the moister western half of Angola, the underlying geology comprises crystalline Precambrian rocks (formed before 550 million years ago) which weather to produce ferralsols. In the eastern half of the country, Kalahari sands (arenosols) have been deposited by wind and water over the past five million years. Both these soil types have been strongly leached for millions of years and as a result are of low fertility (dystrophic). In the west, large granite domes are frequent, but to the east, one can travel nearly 800 km across featureless, unbroken rolling plains and shallow valleys between Huambo and Cazombo.

An important ecological feature of the miombo is the regular, repeated sequence of gently undulating rises and falls in the landscape, with associated soil and vegetation patterns—a feature known as the catena (Box 14.1). Catenal sequences are not limited to the mesic savannas, but also occur in many arid savanna ecosystems on granitic sands and basaltic clays, where sodic soils might develop at the base of hillslopes (Venter et al., 2003) (Box 15.1). Miombo soils are mostly highly leached, acidic, with low cation exchange capacities, and with low available nitrogen and phosphorus. The large woodland trees of the upper catena capture most of the available nutrients. As a consequence of fires and termite activity, plant litter build-up is slow and soil organic matter levels are low. The ferralsols of the west have high levels of aluminium, which place limits on crop production.

Impervious horizons in miombo soils, such as laterite layers, can cause waterlogging over poorly aerated perched water tables, with stunted growth of trees. Over much of Angola’s moist miombo, seasonally waterlogged sites have short hygrophilous grasslands with scattered trees and shrubs and in many areas, an abundance of termitaria. The termitaria play a prominent role in the fine-scale patterning and nutrient relations of miombo landscapes (Box 14.2). A further remarkable feature of the miombo is the presence of ‘underground forests’—unique to the mesic/dystrophic savannas of African miombo and Brazilian cerrado (Box 14.3). These three special features of mesic savanna landscapes—the catena, the diversity of termitaria, and the extensive ‘underground forests’ demand detailed description and are presented in three Boxes.

Box 14.1 The Catena: a Classic Ecological Feature of African Savannas

Much of what one needs to understand about African landscapes can best be learned from the air. Neither an aircraft nor a drone is needed. With an internet connection, a remarkable aid to the ecologist is Google Earth. This is particularly the case for researchers exploring the many remote corners of Angola, where access can be difficult if not impossible. At the click of button, a bird’s eye-view can be accessed for any point in the country or the world. What becomes immediately visible from space, over much of the Angolan planalto, is a dense pattern of dark and light shades, representing the reflectance (albedo) of different vegetation densities. The patterns are often repeated in a regular sequence, dark shades on higher ground, light shades in valleys.

A good example of pattern is found in Bicuar National Park in Huíla (Fig. 14.4). The border of Bicuar is clearly delineated along its northern boundary, where deforestation has left a treeless belt of pale sands, contrasting with the dark, thickly wooded landscapes protected within the park. A dendritic pattern, like lines on a fingerprint, results from alternating wooded and grassy belts. This is a sequence of distinct soils and vegetation, consistently repeated on specific facets of valleys (mulola) and rises (tunda). The drainage-line (mulola) grasslands occur throughout central and southern African mesic savannas where they are known as dambos in many countries. The pattern results from the slow erosion of the planalto and peneplains, and the downslope movement of water, soil particles and nutrients. The patterns reflect a chain (catena) of topographical/soil/vegetation sequences. In his description of the vegetation of Bicuar, Teixeira (1968) provides a map and a profile diagram of vegetation structure that illustrates the sequence of soils and vegetation (Figs. 14.5, 14.6). It is a model repeated across the Central African Plateau, from Malange in Angola, across Zambia and Malawi, to Gorongosa in Mozambique.

Fig. 14.4

Google Earth (Landsat/Copernicus) view of Bicuar National Park. Note deforested margins (pale to white) around its borders. Also, note the light lines of the grassy mulolas and the dark bands of miombo and Baikiaea/Brachystegia woodland on the higher ground of the tundas. The centre of the Google Earth image, at 1242 m altitude, is located at 15° 17′ 23.02’’ S; 14° 44′ 15.13’’ E

Fig. 14.5

Vegetation map of Bicuar National Park. From Teixeira (1968), which mirrors the catenal sequences seen on the 2018 satellite image

Fig. 14.6

Profile diagram of the catena across a mulola in Bicuar. 1 Loudetia grasslands with scattered Parinari and Pygmaeothamus geoxyles on colluvial and alluvial soil. 2 Open savanna of Burkea, Terminalia, Tristachya and Eragrostis on colluvial and illuvial soil. 3 Woodland of Brachystegia, Julbernardia, Hyparrhenia and Andropogon on eluvial and colluvial soil. 4 Thicket of Hippocratia, Baphia, Croton, Combretum and Baikiaea on eluvial soil. From Teixeira (1968) Parque Nacional do Bicuar. Instituto de Investigação Agronómica de Angola, Nova Lisboa

The catena follows a sequence of both soil moisture and nutrient conditions.

• The higher ground is well-drained, losing fine particles and basic cations (calcium, magnesium, potassium and sodium) through surface and subsurface runoff, resulting in a leaching and acidification of these coarse elluvial soils that have formed by weathering in-situ.

• The colluvial soils of the concave slope are also subject to leaching, receiving material by downslope creep. Drainage transports material further downslope and vertically to the deeper, grey illuvial soils at the bottom of the slope, which also receive cations from upslope with a resultant increase in their pH.

• In larger valleys, the bottomland soils might be added to by streamflow carrying loose, unconsolidated, silty alluvial sediments.

Catenas typically occur in mesic savannas where the seasonal climate provides enough precipitation to drive the translocation processes without flushing the cations and sediments out of the landscape.

The importance of the catena is that it is found across 60% of Angola, most typically in the miombo woodlands and savannas of the planalto and peneplains. The sequence follows a regular pattern. In Bicuar, the well-drained reddish sandy arenosols of the higher ground are occupied by Baikiaea woodland and Combretum thickets. Towards the margins of the higher ground, deep-rooted Brachystegia woodland transitions into a narrow fringe of Burkea woodland followed by a belt of shallow-rooted Terminalia savanna. These are found on the pale white sands at the margin of the convex slopes (Fig. 14.6). The lateral spread and shallow rooting of Terminalia in the oxygenated topsoil possibly accounts for its ability to grow in the seasonally waterlogged sands. During the rains, a perched water table results in a seasonal seepline below the Terminalia belt, with a fine flow of water from these white sands. This is followed by open grasslands on the seasonally high water table of the concave slope which ends in the dark poorly drained, heavier clayey fluvisols of the valley bottom. Like the grasslands, the valley soils are seasonally waterlogged, preventing tree growth. Seasonal waterlogging is revealed in the subsoil by blue and orange mottling and gleying. In some catenas, the valley bottom might include a small pool (tala) or a narrow line of forest trees (muxito), where erosion has created better-drained conditions suited to tree growth (Fig. 14.7).

Fig. 14.7

Miombo in Bicuar National Park, Huíla. The catena progresses from Baikiaea woodland and thicket on the top of the plateau, through Brachystegia/Julbernardia woodland, to a fringe of Burkea savanna on the margin of the bottomland, where geoxyles dot the grassland

Box 14.2 Pattern, Nutrient Hotspots and the Role of Termites in Mesic Savannas

To understand is to perceive patterns

Isaiah Berlin (1997)

The detection and understanding of spatial patterns in nature is one of the most fascinating activities for the inquisitive ecologist. Patterns appear at a wide range of scales, from continental to local. The distribution of savanna biomes is centred on the peneplains of the African Plateau (mesic/dystrophic) and the more recent erosional surfaces of the hot, arid river basins (arid/eutrophic savannas). At landscape scale, the soil/vegetation sequences of catenas are a feature of both mesic and arid savannas. At a finer scale, one of the most widespread patterns across the savannas of Africa are those created by termite colonies. Their importance as nutrient 'hotspots' in mesic/dystrophic savannas deserves special mention, as does their role as refugia for fire-intolerant savanna species (Joseph et al., 2013). The flora of termitaria in the miombo systems is extraordinarily rich, with over 700 species of woody plants found on the termitaria of Zambia alone (Fanshawe, 1969).

Termitaria are visible both on the ground and from satellite images. Termites and their colonies come in many forms and densities (Abe et al., 2000). They are eusocial insects, with clearly defined roles for workers and soldiers, for dispersing winged adults, and for the king and queen, responsible for reproduction. Two main groups are of special interest: humivorous termites that ingest organic matter and soil, and ligniferous termites that consume intact plant material. Both are critical agents in the mineralization of dead plant material. Here discussion will focus on the tall mound-forming termitaria of the fungus-culturing Macrotermes and the small, often spheroid nests or sharp spikes of the non-fungus-culturing Cubitermes and Trinervitermes.

The great mound termitaria of woody plant- and litter-feeding Macrotermes falciger are particularly prominent on deep, well-drained soils in Wet Miombo. Here they average 3–5 mounds ha⁻¹, of up to 5 m in height and with a basal area of ca. 30 m² (Erens et al., 2015a). They cut and harvest woody material, transforming it into sawdust and feeding the masticated and regurgitated wood or grass to cellulose-decomposing fungi, with which they have a mutualistic relationship (Fig. 10.12). The termites do not have the enzymes needed to digest the cellulose and lignin of the plant material, but thenitrogen-rich biomass of the fungi provide the termites with most of their food. Macrotermes cultivate gardens of fungal mycelia in chambers within the upper section of the mounds, where the moist microclimate is ideal for fungal growth. In summer, the sporophores emerge, providing delicious mushrooms for local communities (Fig. 10.11).

The termitaria may be occupied for many centuries (Erens et al., 2015b) by successive generations of queens, which live for up to 20 years. Mound-building termites select clay particles rather than sand grains to build their nests, thus concentrating nutrients within the matrix of nutrient-poor soils. The termites forage widely. This activity concentrates nitrogen, phosphorus and exchangeable cations in the building material of the mounds, accentuated by the increased evaporation of moisture through the extensive ventilation channels in the termitarium, passing through the chimneys of these large, architecturally complex structures. As a mound grows, it develops a hard external surface, through which rain does not penetrate deep into the colony, resulting in mobile compounds (salts) concentrating in a central accumulation zone. Once exposed, it becomes a favoured salt-lick of many animal species. The salt licks of Cangandala National Park are the nutrient epicentres of animal life, favoured by all larger mammals, most especially by Giant Sable. Sodium levels in active termitaria were found to be 20 times higher than in the surrounding woodland soils (Baptista et al., 2012; Fig. 14.8). The nutrient richness of these hotspots is reflected in the specialised adaptations (bird dispersed fruit, spiny, browse-resistant branchlets, hard sclerophyll leaves) of the plant species that colonise the Macrotermes mounds. The termites thus create a nutrient-rich hotspot—an ecosystem of its own making which contributes to the wider range of plant species that occupy the rich soils of the mound, and to the many species of animals that are dependent on the system (Malaisse, 1978; Fig. 14.9).

Fig. 14.8

Giant Sable congregate at a termitarium salt-lick, Cangandala National Park. Photo Pedro Vaz Pinto

Fig. 14.9

The complex trophic web of a high termitarium in the miombo of Shaba, DRC, including 25 species representing different trophic levels. Producers include: grass Setaria (1); trees Balanites (5); succulents Sanseviera (11), Commiphora (15); shrub Grewia (18). Consumers include: flower-eaters Zonabris (3), leaf-eaters Bunaea (6), fruit-eaters Treron (7), Cricetomys (16). Carnivores include: Naja (23). Decomposers include: termites Macrotermes (12), mushrooms Termitomyces (20). Parasites include ticks Aponomma (24). From Malaisse (1978)

Far more abundant are the diverse structures, both above and below ground, of humus and soil-feeding Cubitermes and Trinervitermes, often built on seasonally waterlogged soils (Figs. 14.10, 14.11). These are especially abundant in Wet Miombo. In Shaba, DRC, they have a biomass twice that of the Macrotermes and build their nests with their faecal matter, comprising soil particles and digested humus, creating local concentrations of nutrients at a finer spatial scale than the Macrotermes mounds, depleting the organic matter of the zone around each nest. In the Dry Miombo-related Burkea-Ochna savanna in northern South Africa, the Cubitermes have up to 380 nests ha⁻¹, a density greatly exceeded in Wet Miombo. Goffinet (1976) found 1460 Cubitermes and Trinervitermes mounds ha⁻¹ in miombo savanna of Shaba. He calculated that 10.4 million termites ha⁻¹, represented a dry weight of 22.95 kg ha⁻¹, far higher than the vertebrate biomass for miombo savannas.

Fig. 14.10

Chimney structures of humivorous termites on the soils of Wet Miombo chanas in Luando Strict Nature Reserve, Malange

Fig. 14.11

Chimney structures of humivorous termites on the soils of Wet Miombo chanas in Luando Strict Nature Reserve, Malange

As we will see later, termites also play significant roles in the patterns of ‘fairy circles’ of the Namib Desert. Across Africa, from the mesic savannas to the hyper-arid desert, termites are remarkable ecological engineers.

Box 14.3 Angola’s Underground Forests

A classic feature of the Angolan miombo is one of its most puzzling phenomena. Most miombo tree species grow into broad-crowned specimens of 10–20 m in height, forming the typical open woodlands of the planalto. But some of their close relatives live separate lives as ‘underground trees’, often just a few metres away. The anharas do ongote of Huambo and Bié (on ferralitic soils) and the chanas da borracha of the Lundas (on arenosols) form part of specialised plant communities described by White (1976) as ‘underground forests’, dominated by geoxylic suffrutices—now better known as geoxyles.

Geoxyles are woody plants that have reduced their aboveground stems and branches to short shoots which produce leaves, flowers and fruits immediately after the passage of the frequent fires that sweep across miombo ecosystems (Fig. 14.12). Geoxyles invest their woody growth in underground rootstocks (lignotubers or xylopodia), often with a dense network of rhizomes and roots that can cover over 50 m² (Fig. 14.13). They thus place their regenerative organs underground for most of the year to protect them from damage by fire, frost or herbivory (Figs. 14.14, 14.15 and 14.16).

Fig. 14.12

A brightly coloured orange and yellow ‘lawn’ of the geoxyle Cryptosepalum maraviense on the margins of a mulola near Chitembo, Bié. The white sands exposed on the margin of the Cryptosepalum community probably indicates the position of the seepline on the soil catena. Photo John Mendelsohn

Fig. 14.13

Exposed underground rhizomes of a geoxyle, Ochna arenaria. Photo Amândio Gomes

Fig. 14.14

Above-ground leaves of Ochna arenaria. Photo Amândio Gomes

Fig. 14.15

Flowering Cryptosepalum maraviense—the dominant geoxyle of the anharas do ongote. Photo John Mendelsohn

Fig. 14.16

Fruiting Ficus sp. geoxyles on the Bulozi Floodplain. Photo John Mendelsohn

The first scientist to describe the geoxyle habit was pioneer Danish ecologist Eugen Warming. Working in the cerrado of Brazil (the South American equivalent of miombo) in 1865, Warming illustrated the underground root systems of the Brazilian geoxyle Andira laurifolia (Warming, 1908). He described its “curious mode of growth, for this is a clear example of a tree which due to fires was forced to live a life under the ground.” The pioneer of Angolan plant ecology, Swiss-born John Gossweiler, provided a detailed description of the geoxyles of anharas and chanas, and noted that: “With the appearance of grasses, the community turns pyrophilic and it is precisely the fire that stimulates the dominants to develop flowers eight days later. The maturation of fruits occurs in less than three months, before the first rains of September, with the result that the seeds are ready to germinate in exactly this season” (Gossweiler & Mendonça, 1939). [“Com o aparecimento das gramineas a comunidade torna-se pirofitica, e e precisamente a queimada que estimulando o desenvolvimento os dominantes provoca a sua floracao oito dieas depois. A maturacao do fruto realiza-se em menos de tres meses, antes das primeiras chuvas de Setembro, de tal sorte que as sementes estao aptas a germinar precisamente nesta epoca.”]

The growth cycle of grasses follows that of geoxyles, growing and flowering in mid-summer, by which time they cover the geoxyle aerial organs. The geoxyles effectively disappear from sight. However, Gossweiler was ambivalent about fire being the key driver of geoxyle evolution, mostly because of the low fuel load of the sparse chana grasslands. He also pointed to the very sandy and nutrient-poor soils as a constraint to woody plant growth. This opinion was supported by forest botanist Romero Monteiro (1970a, 1970b), in his detailed study of the miombo of the Bié Plateau. He considered edaphic factors more important than fire. White (1976) also placed emphasis on the highly leached and very poor nutrient status of miombo soils, and on the summer season waterlogging of the sites on which they abound, with dry winters, as key constraints on their growth into trees. White, like Gossweiler and Monteiro, did not mention frost as a factor.

Until recent decades, the study of miombo and cerrado ecosystems had been rather neglected by ecologists. But the evolution of geoxyles is now a hotly debated topic. Researchers in Angola (Finckh et al., 2016) have supported an early proposal from South Africa (Burtt Davy, 1922) that frost is the prime causal factor. However, frost is rare or absent over most of the geographic range of geoxyles in southern Africa and Brazil. While direct field observation of geoxyles has proved inconclusive, phylogenetic research on the evolutionary history of geoxyles has provided new insights.

South African savanna ecologist William Bond and colleagues have used modern phylogenetic analyses of over 1400 woody species to test whether the geoxyle habit evolved simultaneously in Africa and South America. They also sought to date the first appearance of tropical humid savannas (miombo and cerrado). They found that the geoxyle life-form evolved independently multiple times during the Pliocene (from ca. 5.3 Ma), in both miombo and cerrado, always associated with the mesic savanna biome in which fires are a regular and ecologically significant feature (Maurin et al., 2014). In nutrient-poor, seasonally waterlogged soils, the slow growth rates of juvenile trees reduces their probability of rising above the kill-zone (‘fire-trap’) of fires. Selection for plants with adaptations to protect regenerative organs from fire by hiding them below the ground would be a selective advantage (Bond, 2019). A further selective driver is against herbivory of these slow-growing plants. Many geoxyles are defended against herbivory by accumulating secondary compounds, including poisons, that retard digestion or in some cases, can be lethal (Sect. 11.4).

German ecologist Paulina Zigelski (2019) and Angolan colleague Amandio Gomes found the geoxyle life-form in 198 species in 40 families within the miombo—a perfect example of convergent evolution of an adaptive strategy. Amandio Gomes et al. (2019) found that related pairs of tree and geoxyle life-forms are found within sub-species, between species of the same genus, and between species of different genera. They concluded that although trees and geoxyles differ considerably in their growth form, they are remarkably similar in other morphological traits. Studies of the eco-physiology and plant functional traits are beginning to explain some features of the life of underground trees, but as Gossweiler and Mendonça (1939) observed nearly a century ago: “Not only the ecology but also the genetic dynamics of the ‘Chana da Borracha’ offers a wide field for future work.”

3 Climate and Seasonality

Climate is the primary determinant of the potential distribution of tropical savannas, regionally moderated by soils, fire and herbivory. The mesic savannas of Angola, lying mostly between 900 and 1400 m above sea level, but ranging from sites at 800–2400 m, have warm to hot summers and mild winters, with rainfall from 650 to 1400 mm per year (Table 14.1). To the north, the Congolian Forest/Savanna Mosaic occupies higher rainfall areas with precipitation of 1400–1700 mm per year.

Table 14.1 Climatic data for stations within the Mesic Savannas

Seasonality of temperature and rainfall is one of the key characteristics of tropical savannas, and accounts for the limited availability of soil moisture for plant growth during the dry season. In the Angolan mesic savannas, moisture availability in the upper 30 cm of the soil drops below permanent wilting point for up to eight months of the year. Soil water availability increases with depth, and below 90 cm water is available to deeply rooted plants throughout the year.

Rainfall seasonality results in marked deciduous properties in most tree and shrub species, and the regular drying out of the grass cover, with consequent frequent natural fires. Deciduous trees translocate about half their leaf potassium and phosphorus back into their branches before leaf fall. This pattern of translocation does not take place in the trees of arid/eutrophic savannas. Mesic savanna grasses translocate nutrients from foliage to roots at the end of the growing season, further reducing their nutritional value to herbivores. Fire and herbivory have resulted in adaptations to repeated defoliation. Some highland valleys have frequent frosts in winter, but over most of the mesic savannas, frosts are of rare occurence.

4 Floristic Composition, Physiognomic Structure and Phenology

The key feature of miombo woodlands, which distinguishes them from all other African ecosystems, is the dominance, or at least strong prominence, of species of two genera—Brachystegia and Julbernardia. These and several other miombo trees belong to the legume family Fabaceae, sub-family Detarioideae. The miombo flora is rich, and is at the heart of the Zambezian regional centre of endemism, which has an estimated 8 400 species within its range across southern, central and eastern Africa (White, 1983). Barbosa (1970) describes in some detail seven types of miombo in Angola. These fall into two ecoregions: Angolan Wet Miombo—(Ecoregion 7) and Angolan Dry Miombo (Ecoregion 8). To the south, these transistion into Zambezian Baikiaea Woodlands—(Ecoregion 9). To the north of the main miombo woodlands, two further ecoregions, the Western and Southern Congolian Forest/Savanna mosaics (Ecoregions 2 and 3), form a transition from the Zambezian centre of endemism to the Guineo-Congolian centre of endemism as defined by White (1983). A narrow belt of mixed mesic savanna follows the Central Escarpment (Ecoregion 6). Furthermore, two ecoregions, Flooded Grasslands (Ecoregion 11) and Cryptosepalum Dry Forests (Ecoregion 10), fall fully within the mesic savanna biome. All of these ecoregions are outlined in Chap. 2. The present chapter focuses on the characteristics of miombo ecosystems of Ecoregions 7 and 8.

Across the vast expanse of the miombo, a remarkable uniformity of structure prevails, varying in detail but not in the repeated catena structure of woodlands interspersed with open savannas and grasslands. Smaller pockets of dry evergreen/semi-deciduous closed forest might occur on deeper soils and as gallery or riverine forest along watercourses.

The broad crowns of the dominant trees reach up to 25 m in height in mature Wet Miombo on deep soils, but the canopy is usually lower, down to 8 m in height, in Dry Miombo. The understorey comprises shrubs and saplings, and the woodland floor carries a sparse cover of grasses and forbs. Climbers and epiphytes are rare, except in the Cryptosepalum/Marquesia dry forests, where lichens and mosses might cover tree trunks and soil.

Over 90% of the dominant species of miombo woodland trees are deciduous, but do not normally drop all their leaves every year. Annual leaf loss is about 60% in miombo trees, compared with 90% in trees of arid savannas. Shrubs of the miombo woodland understorey are less deciduous than trees. Shallow-rooted miombo shrubs such as Vangueria infausta and Lannea discolor shed leaves early in the dry season and do not flush new foliage until after the rains.

A phenomenon peculiar to miombo trees is the flush of brightly coloured new leaves immediately after leaf fall in August/September, weeks before the first rains. The colouration is due to the presence of anthocyanin pigments initially predominating over chlorophyll, which might protect the soft young leaves from herbivores (Figs. 14.17, 14.18). The early flush is made possible by the internal recycling of nutrients, especially nitrogen, before the previous season's leaves fall in late winter; a unique feature of miombo trees. Furthermore, water, carbohydrate and nutrient reserves held in the trunk, branches and roots stored from the previous growing season, and access to soil water from deep root systems of the miombo trees, contribute to this eco-physiological characteristic. Heating of the soil surface might be a trigger for the early flush of miombo leaves (Malaisse et al., 1975). Or perhaps the early leaf emergence might be triggered by the lengthening photoperiod following the spring equinox (Frost, 1996; Ryan et al., 2017). While some arid savanna trees flush before the first rains, few display the richly coloured pigmentation of miombo species.

Fig. 14.17

Spring colours in the pre-rain flush of foliage of miombo trees, typical of mesic savanna

Fig. 14.18

Spring colours in the pre-rain flush of foliage of miombo trees, typical of mesic savanna

For most canopy trees, flowering occurs immediately following leaf flush, except for Julbernardia paniculata which flowers in mid-summer. Fruits take at least six months to mature, with the dry pods of the dominant genera Brachystegia, Julbernardia and Isoberlinia dispersing seeds by explosive dehiscence. This dispersal mechanism is effective for only a short distance from the tree (within 5–20 m), placing limitations on re-population of miombo in sites clear-felled and converted to crops.

There is no dormancy in miombo seeds, which germinate soon after they fall. Seed establishment is low, mortality is high (due to low soil moisture availability and heat stress), and growth is slow. Most seedlings die before reaching the sapling stage, which can take eight years, during which period they are vulnerable to damage by fires. The main strategy for regeneration after disturbance (such as fire or felling) is through regrowth by coppicing from stems or branches, or from root suckers. In this respect, miombo is very resilient to disturbance if the tree stumps are not killed, and recovery back to woodland physiognomy and floristic composition can be achieved in 30–50 years. However, the rate of total transformation of woodlands for charcoal or agricultural production, currently accelerating over most of the Angolan miombo, is resulting in the long-term degradation of these ecosystems.

5 Faunal Composition of Mesic Savannas

The mesic savannas of Angola are rich in most vertebrate groups, while lower in overall vertebrate biomass when compared with untransformed arid savannas (Sect. 10.5). Table 14.2 provides a shortlist of vertebrate species of miombo woodlands and grasslands, which are the most extensive mesic savanna ecoregions of Angola.

Table 14.2 Vertebrate species typical of Angolan Miombo Woodlands, Savannas and Grasslands

6 Mammals of the Angolan Mesic Savannas

The reliable summer rainfall and abundance of perennial rivers and streams that characterise Angola’s mesic savannas account for most herbivores found in these woodlands, savannas and grasslands being water-dependent grazers. These include Giant Sable Antelope, Roan Antelope, Lichtenstein’s Hartebeest, Defassa Waterbuck, Red Lechwe, Puku, Oribi, Southern Reedbuck and Common Eland. All of these are low-density species, which are either roughage feeders or are very selective of higher nutrition herbage in a generally poor-nutrient environment. Red Lechwe form large herds on the open grasslands of river floodplains, once numbering over 1500 along the Luando River. Oribi also prefer open shortgrass habitats. Nomadic Eland, once numbering over 1000 in Quiçama, and 500 in Bicuar, made seasonal movements over home-ranges of several hundred km². All the other species mentioned are rather sedentary, with home-ranges in tens of km². While Africa’s largest roughage feeder, the Savanna Elephant, is widespread across most African savannas, the species is absent or very rare in Angola’s mesic savannas, even according to historic records. Similarly, neither Cape nor Forest Buffalo has been known to occur in more than small populations in Angola’s mesic savannas. Forest Buffalo were known from records across Northern Angola, from Quiçama to Luando and to Lunda Norte. Few if any viable populations remain.

As may be expected in ecosystems with low biomasses of prey species, predatory carnivores are also scarce in the mesic savannas. While African Lion and Southern African Cheetah occurred across the savannas from the Cunene to the Cassai in the mid-twentieth century, they are extinct or nearly so over the mesic savannas today. Rare sightings of lion have been made in Luando, while cheetah remain in low numbers in the arid savanna of Iona. African Leopard were once ubiquitous across all biomes in Angola. Medium-sized carnivores that occur in small populations throughout Angola’s mesic savannas include Side-striped Jackal, Caracal, Serval and African Wild Dog.