The Guineo-Congolian Rain Forest Biome

Abstract

Tropical rain forests are represented in Angola by the narrow and fragmented southwards extension of the Guineo-Congolian rain forests of the Congo Basin and West Africa. This Chapter defines and characterises tropical rain forests, and compares the diversity of African forests with those of Central and South America and of South East Asia. The evolution and dynamics of African rain forests, and the role of human activity through the Holocene is discussed. Angola’s forest types are defined, their distribution, physical conditions, physiognomy and floristic and faunistic composition, plant-animal interactions, and forest gap-phase dynamics are detailed.

Key Concepts and Questions: This Chapter Explains

• What constitutes a rain forest.

• Why some perceptions of age, diversity, stability and intactness of rain forests have been misleading.

• What changes have occurred since the evolution of African rain forests that extended from the coasts of the Atlantic to the Indian Ocean.

• Why Africa is the ‘odd man out’ in terms of rain forest tree diversity.

• What vertebrate species characterise the Guineo-Congolian Rain Forests and how they separate their resource demands.

Context: Global and Continental Perspectives on Tropical Rain Forests

Tropical Rain Forests (TRF) represent the global peak of biological diversity. The tropical rain forest biome has more species and ecoregions than any other on Earth. Covering only 14% of the Earth’s land area, this biome supports at least 50% of the world’s known plant and animal species, with many more still awaiting discovery (Dinerstein et al., 2017). The current area of African rain forests is ca. 2.3 million km². The largest block of forest, covering 1.8 million km², lies in the Congo Basin (Fig. 12.1), with a similar vast area (2.5 million km²) of Forest/Savanna mosaics surrounding the closed forest block. The Congo Basin contains the second largest continuous expanse of tropical rain forest on the planet, after the slightly larger block of the Amazon Basin.

Fig. 12.1

Spatial distribution of African Rain Forests. From Mayaux et al. (2013) Philosophical Transactions of the Royal Society B 368: 1625. Creative Commons Attribution License 3.0

Definition

The classic definition of tropical rain forest is the brief diagnosis of Schimper (1903): “Evergreen, hygrophilous in character, at least 30 m high, but usually much taller, rich in thick-stemmed lianas and in woody as well as herbaceous epiphytes.” To this, one might add: “Woody species dominate all but the sparse undergrowth, and the richness of species of trees per ha exceeds that of any other biome. Trees are tall, straight-stemmed and unbranched for the first 20–30 m, often reaching 60 m as emergents from a closed canopy at 30–50 m” (Richards, 1952). Both these definitions are appropriate for the rain forests of northern Angola.

Whitmore (1998), in his authoritative and comprehensive volume on tropical rain forests, distinguishes between evergreen ever-wet (perhumid) forests, where over 100 mm rainfall is received during every month of the year, and semi-evergreen seasonally dry (monsoon) forests where regular annual periods of water stress are experienced. In addition to these two dryland forest (terra firme) types, extensive areas of swamp forests occupy one third of the Congo basin, and the narrow margins of some gallery forests of the Congo tributaries in northern Angola.

All three forest types are tall, closed canopy forests, found on all three tropical continents. Convergent evolution has produced similar responses to environmental conditions, with very similar life forms and physiognomy, despite very few floristic or faunistic links below family level. In Africa, tropical lowland evergreen ever-wet rain forest is found in Cameroon, Equatorial Guinea and Gabon. Most of Africa’s rain forests belong to the seasonally dry closed forest formations, which experience at least two months with little or no rain, but high humidity levels continue through much of the dry periods.

The exceptional diversity and productivity of these forests results from the coincidence of abundant, regular and reliable rainfall and high levels of solar radiation. The biological wealth of rain forests has, for centuries, attracted romantic descriptions and perceptions of the diversity, stability and pristine condition of the biome. However, the biological richness of the Tropical Rain Forest is not shared equally across the tropics. The age and stability of rain forests vary widely, and recent paleoecological studies demonstrate that the impact of human communities on TRF dynamics has been far greater, and over a much longer period, than previously thought—especially in Africa.

Comparative Diversity

The concepts of diversity and endemism are fundamental to understanding ecosystem structure and dynamics, as discussed in detail in Sect. 9.2. In terms of botanical diversity, the global inventory of vascular plants now stands at 384,000 species (Raven et al., 2020).

• The Afrotropical Region (Africa south of the Sahara, including Madagascar but excluding temperate South Africa), with an area of 22.6 million km², is home to 30,000 plant species.

• The Neotropical Region (South and Central America), extending over 19.2 million km², has 118,000 documented species.

• Southeast Asia and Australasia (the Indo-Pacific Region), with an area of 5.7 million km², is home to approximately 50,000 indigenous species of vascular plants.

The African Tropical Rain Forest, in terms of plant species richness, is therefore almost four times poorer than the Neotropics, a region of similar area. These disparities led the pioneer of modern tropical rain forest ecology, British botanist Paul Richards (1908–1995), to describe Africa as ‘the odd man out’ because of its low tree diversity compared with the rain forests of the Neotropics and the Indo-Pacific (Richards, 1973).

Recent surveys support Richards’ description. A global count of tropical forest tree species has estimated a total of between 40,000 and 53,000 species, with between 19,000 and 25,000 species in both the Neotropics and Indo-Pacific, but only 4500–6000 in tropical Africa (Slik et al., 2015). Here the contrast between the tropics and the northern temperate floras is worth noting. The tree diversity of the TRF is eight-fold greater than that of the combined numbers for the boreal, temperate and conifer forest biomes of the Palearctic Region of Eurasia, which covers an area twice that of the TRF biome. Temperate Europe has only 124 species of trees—less than the average number of tree species per hectare in most tropical rain forest ecosystems.

In terms of lizards and snakes, the low species diversity of the Afrotropics is similar to that of flowering plants, but mammals, especially primates, are more species rich in Africa than elsewhere. Despite the relative species paucity of the African TRF compared with the two other TRF regions, it is nevertheless extremely rich in species when compared with other African biomes and ecoregions, with the exception of the two extreme global botanical hotspots—the Cape Fynbos and Succulent Karoo Biomes.

Stable or Unstable?

A popular perception is that the African rain forest is an ancient biome that has remained unchanged for millions of years. However, palaeoecological studies have shown that the African TRF experienced severe aridification during the cold/dry periods of the Pleistocene. Over its long history, the African TRF has existed both as a continuous band of closed forest, extending from the west to east coasts of the continent during warm/wet periods, and as fragmented blocks during cool/dry periods which experienced accentuated seasonality. This could account for the relative species paucity of the Guineo-Congolian forests compared with those of the Neotropics and Indo-Pacific forests, that did not suffer from such acute arid episodes. Nevertheless, centres of high species richness and endemism are found today in widely separated areas of the African rain forest. Numerous explanations have been presented to explain current patterns. These have been reviewed by Moritz et al. (2000), Weber et al. (2001), Hardy et al. (2013), Maley et al. (2018) and Couveur et al. (2020) and are summarised here.

• Allopatric speciation (where populations of the same species become geographically isolated) took place in isolated fragments, thus increasing species richness and endemism within the environment of surviving forests (increasing alpha diversity of each patch, and thus increasing beta diversity across the region). The forests served as important centres of diversification (evolutionary cradles) and which resemble islands in a sea of expanding savanna (Pleistocene refugia hypothesis, Fig. 12.2).

  Fig. 12.2

  

  Hypothetical range shifts in the Late Pleistocene indicating main refugia (black). From Hardy et al. (2013) Comptes Rendus Geoscience 345: 284–296

• An alternative explanation for the centres of richness and endemism is that they represent static sites where species accumulated due to low extinction rates and moderate speciation rates, accumulating both old (palaeo-) and new (neoendemics). These species survived as relict populations (evolutionary museum hypothesis) during cool/arid episodes (Fjeldså & Lovett, 1997; Huntley et al., 2019).

• In areas of high environmental diversity such as escarpments, gradient (parapatric) speciation dominated (even in the absence of geographic isolation) where ecological and behavioural factors facilitated speciation (Smith et al., 1997).

Today, Africa’s TRF is distributed as three main areas: the largest in Central Africa, with fragmented patches in West and East Africa. The West and Central blocks, home to ca. 8000 plant species of which ca. 80% are endemic, are defined as the Guineo-Congolian Regional Centre of Endemism by White (1979, 1983). It includes three sub-centres: Upper Guinea, Lower Guinea, and Congolia, each with distinctive endemic floras (Fig. 12.3). Both Upper and Lower Guinea are notable for high endemism. Lower Guinea (in which the Maiombe forests of Cabinda fall), is recognised for its high species richness, with Gabon alone having over 4700 and possibly even more than 6000 plant species.

Fig. 12.3

Guineo-Congolian forest subdivisions and topography. From Hardy et al. (2013) Comptes Rendus Geoscience 345: 284–296

The Upper Guinea TRF of West Africa is separated from the other two Guineo-Congolian sub-centres by an arid corridor known as the Dahomey Gap, extending northwards from the Gulf of Guinea coast through Benin and Togo. The gap has existed intermittently as an open savanna belt for many millions of years, closing during several warm/wet periods. It is important to recognise that the entire African TRF has been subject to repeated and extensive climatic changes and tectonic events throughout its history. Pulses of forest expansion and contraction continue to the present, with the Dahomey Gap having been open during the past 3000 years. During the periods of forest contraction, centres of forest diversity and endemism survived within refugia.

Pristine Status or Transformed?

The long-held perception that much of the African TRF remains intact, in its original primal structure and floristic composition, free of human impacts, has been challenged by research in many West and Central African countries. Evidence from palaeoecological, archeological and plant phylogenetic and demographic studies, and the analysis of charcoal deposits, has demonstrated a long history of forest transformation through agricultural activities (Maley et al., 2018). It is thus not only climatic oscillations that account for the dynamics of African rain forests. The Guineo-Congolian forests have been occupied by humans for millennia, as hunter gatherers and, in the past 10,000 years (the Holocene), as agriculturists. The impact of these early farmers and iron-smelters was locally concentrated, but the sites of former villages are dispersed over a wide region. Disturbed forests eventually returned to the original physical structure, even if the species composition was dominated for many hundreds of years by pioneer species such as members of the Fabaceae and Combretaceae. Fast-growing, sun-tolerant pioneer species of genera such as Cynometra, Erythrophleum, Gilbertiodendron and Julbernardia form monospecific populations that persist for many centuries, marking the position of former human settlements (Hart, 2001). The controlled use of fire by Homo sapiens, dating from 150,000 years ago, has become an increasingly important factor in ecosystem dynamics, not only in savannas, but also in changing the patterns of rain forest distribution in Africa.

During modern times, particularly over the past century, the rain forests of Angola have suffered from successive waves of disturbance, first by the introduction of commercial coffee Coffea canephora farming during the colonial era, then by commercial timber extraction, and more recently by deforestation for growing bananas, manioc and other cash crops. Hunting for bushmeat has also resulted in a skewed faunal composition of certain mammal, bird and reptile species. As a consequence, many Angolan forests have lost their typical faunistic and physiognomic structure. The concept of empty forests (Redford, 1992) applies increasingly to Angolan forests, where a wide diversity of animal species are trapped or shot for local markets. Tall, multi-strata forests have been transformed into short, dense regenerating thickets, dominated by pioneer species and in many areas, by invasive alien species such as the fast-growing tree Inga vera and the dense scrambling shrub Chromolaena odorata.

As we will see, tropical rain forests are highly diverse, vibrantly dynamic and with much yet to be discovered.

Angola’s Guineo-Congolian Rain Forests (Ecoregions 1, 2, 3)

Key Concepts and Questions: This Chapter Explains

• What characterises the five different groups of Guineo-Congolian rain forests of Angola.

• How topography interacts with oceanic and atmospheric processes to produce areas of high rainfall suitable for the growth of rain forest trees.

• How the rich floristic composition of Angola’s rain forests is influenced by access to light and position within the ground, understorey and canopy strata.

• What processes create the small scale patterning and dynamics of forest structure and composition.

• How a rich diversity of animals and plants use the multiple strata of the rain forest.

1 Definitition and Distribution

In Angola, the largest block of tropical evergreen rain forest—the pluviisilva of Gossweiler and Mendonça (1939)—is found in the highest and wettest west-facing slopes of the Alto Maiombe, Cabinda. For most of Angola, the rain forests have been reduced by deforestation to small blocks of semi-evergreen and semi-deciduous forests on west-facing hills and escarpments, strongly influenced by orographic clouds, and seldom more than 200 km from the coast. Ranging in altitude from sites at 150 m on the coastal hills of Cabinda, rain forests are found at up to 1200 m. The distribution of Ecoregions 1–3 is indicated in Figs. 2.3, 2.4 and 2.5.

The Guineo-Congolian Rain Forests of Angola fall into five groups:

• First are the Maiombe forests of Cabinda (Ecoregion 1; Figs. 2.1, 12.4). These are surrounded by the sharp transition from rain forests to the tallgrass mesic savannas of the Congo Basin (Ecoregion 2).

  Fig. 12.4

  

  The canopy of Guineo-Congolian rain forest in the Alto Maiombe National Park, Cabinda. Note the straight trunk of a forest emergent at left, and the broken canopy in the foreground

• Second are the forests of the mountains and escarpments between the Congo and Cuanza rivers (the Northern Escarpment forests). These include the forests of the Serras da Canda and Mucaba in Zaire, Serra Pingano in Uíge, and the Dembos and Cazengo forests in Cuanza-Norte. These forests belong floristically to Ecoregion 1, and occur within the extensive matrix of the Western Congolian Forest/Savanna Mosaic (Ecoregion 2).

• Third are the forests of the Central Escarpment between the Cuanza and Coporolo rivers and include the moderately-sized Amboim/Gabela and Seles/Cumbira forests in Cuanza-Sul. These forests, also belonging to Ecoregion 1 and fall within a matrix of grasslands, savannas and woodlands—the Angolan Escarpment Savannas, Ecoregion 6).

• Fourth, southwards from the Coporolo to the Cunene rivers, only isolated fragments of moist forest with Guineo-Congolian elements are found along the Southern Escarpment near Chongoroi and at the base of the Serra da Chela. Nearly all of these rain forests have been extensively transformed and fragmented, with few undisturbed forest communities remaining. They fall within Ecoregion 6, and await study.

• Finally, across northern Angola, hygrophilous gallery forests, dominated by Guineo-Congolian species, follow the upper reaches of tributaries of the Congo Basin in Malange and Lunda-Norte, from 600 to 1200 m (Figs. 12.5, 12.6). They lie within the Western and Southern Congolian Forest/Savanna Mosaics (Ecoregions 2 and 3).

  Fig. 12.5

  

  Guineo-Congolian gallery forest penetrating Zambezian Brachystegia woodlands in the Southern Congolian Forest/Savanna Mosaic of Lunda-Norte. Note the sharp boundary between forest and savanna

  Fig. 12.6

  

  Mitragyna swamp forest on the margin of the Luele River, Lunda-Norte

Ecoregions 2, 3 and 6 are outlined in Chap. 2. Here we focus on the characteristics and dynamics of Ecoregion 1: the Guineo-Congolian Rain Forests of Angola.

2 Climate and Microclimate: The Roles of Fog (Cacimbo) and of Shade

The climate of Angolan rain forests is typical of the drier margins of seasonal or monsoon tropical rain forests around the globe. Local topography has a strong influence on rainfall received. In general, rainfall increases with altitude along the Angolan Escarpment, but decreases from north to south. The air temperature is hot but not excessively so (Table 12.1). It is never cold, with mean monthly temperatures typically above 18 °C. The forests of Cabinda experience two dry seasons, as illustrated in Fig. 5.9. There are short rainless periods during the hot months of December to February and an almost rainless cool period during June to August. Further south, the forests of Pingano, Gabela and Cumbira have a single but longer dry period, from June to the middle of September. The impacts of the dry periods differ between the hot/dry and the cool/dry seasons and from one year to the next, and are not effectively reflected in data averaged over many years as used in climatic data and diagrams. It is important to note that in ecology, extremes are more important than averages. The number of deciduous species increases southwards in proportion to the intensity of the dry seasons.

Table 12.1 Climatic data for stations within the Guineo-Congolian rain forest

The main factor determining the difference between the hot/dry and the cool/dry periods is the exposure to direct solar radiation and hence evapotranspiration. During mid-summer, the frequent cloudless periods between thunderstorms cause high evaporation rates off soil and plant surfaces. Air humidity decreases markedly. During the cool/dry winter, the rates of evaporation are reduced because of the almost continuous presence of the low stratiform cloud cover, the misty ‘cacimbo’ which characterises the coastal belt and escarpment of Angola. Despite the absence of rain, the air humidity is higher in the ‘dry’ months of July and August than it is during mid-summer. The frequency of misty, stratus clouds, especially at more elevated areas of the escarpment, results in the term ‘cloud forests’ being applied to them by Barbosa (1970). These clouds do not precipitate rain, but the evergreen trees can capture the high humidity of the mists within their canopies. This process was especially important during the arid periods of the Pleistocene as it allowed forests to persist along the Escarpment, acting as refugia for lowland forest species. The oceanic and atmospheric interactions of the Benguela Current and the South Atlantic Anticyclone in the formation of the stratus clouds, especially during winter, are described in Sect. 16.3.

One of the important emergent properties of forests is the microclimate that they collectively create below their closed canopies. Above the canopy, the microclimate does not differ much from an open field or large clearing. This is quite different from the microclimate of the forest floor. Outside the canopy, wind speeds are higher, air temperatures are warmer and relative humidity is lower. Below the canopy, not only is the air still, cool and humid, but the light climate is significantly different from that of the forest canopy or that of an open forest gap. The key difference is the amount of photosynthetically active radiation (PAR) that reaches the understorey plants. Only 2% of the PAR received at the top of the forest canopy reaches the ground, and most of this is received as ‘sunflecks’—the narrow beams of sunlight that shine down through small gaps in the canopy. The larger the gap, the greater the impact of sunflecks on the understorey plants and soil. Sunflects can account for 70–80% of the solar energy reaching the forest floor. Large gaps can result in the drying and warming of the soil, promoting the germination of the seeds of pioneer species. These pioneers are light demanders—needing full sunlight and high levels of PAR for germination, seedling establishment and the rapid growth of saplings, that soon out-compete any climax species that might germinate in the gaps. Some climax species such as Entandrophragma utile of the Maiombe can germinate in shade, but within a year need full light, so that the faster growing pioneers most often keep them shaded, ultimately killing them. Much of the surviving rain forest of Angola is today made up of pioneer species, the result of over a century of disturbances from forestry and agricultural practices. These regenerating forests are termed secondary forest.

3 Physiognomic Structure

From a distance, the tropical rain forest offers a dark, sombre, almost monotonous appearance. But rain forest is only visually monotonous. It is exceptionally diverse, both in its multi-layered vertical structure (stratification) and in the rich diversity of tree species that constitute its floristic composition. The stratification of rain forests determines both community structure and microclimatic conditions such as light, temperature and humidity. The vertical layers (strata) commence at the ground, comprising a sparse undergrowth of shrubs, herbaceous plants, a few grasses, and sapling trees of canopy species. Litter is sparse. Mature forest is easily penetrated on foot. It is only on the margins, along rivers, or where tree-fall or human disturbance has broken the canopy and allowed light to penetrate, that the undergrowth becomes dense and impenetrable. Here lianes are abundant, thick, and spiralling in great loops advancing to the forest crown. Epiphytes include shrubby or herbaceous forms, including orchids, ferns, abundant mosses, and many strangler figs that send roots down to the ground. Some large epiphytes, such as the ferns Asplenium and Platycerium, create humus and rain water traps that provide habitats for frogs and mosquitoes.

Rain forests typically have three tree strata, plus the sapling and ground layers of herbaceous species. The stratification of rain forest is best described by means of profile diagrams, which provide a sample of the forest cross-section (Fig. 12.7). Each successive stratum comprises species that seldom advance above the height of the stratum, forming layered canopies at successive heights above the ground. The progression is not uniform, and is not easily identified without drawing a series of profile diagrams. Above the uppermost canopy, emergent trees of great height rise. The crowns of the trees are frequently narrow, although emergent trees may have crowns of up to 40 m in diameter, such as in Entandrophragma utile and Piptadeniastrum africanum. The plants of each stratum have different microclimatic requirements (light, temperature, moisture) and serve as habitats for animal communities that are adapted to such special environmental conditions, food and shelter availability, and mobility.

Fig. 12.7

Profile diagram of primary forest in Shasha Forest Reserve, Nigeria. From Richards (1939) Journal of Ecology, 27: 1–61

The largest trees often have plank buttresses (Fig. 12.8), bark is usually thin and smooth, unlike the thick corky fire-resistant bark of mesic savanna trees. Leaves are uniformly large (whether simple or compound), dark green, leathery, with entire margins. Most have acuminate ‘drip-tips’—the extension of the leaf tip into a narrow tongue that facilitates rapid shedding of water (Fig. 12.9). It is suggested that the drip-tips reduce the growth of epiphylls—minute gardens of cyanobacteria, green algae, bryophytes, lichens and filmy ferns that grow on the surfaces of the leaves of understorey trees, reducing their photosynthetic efficiency. Flowers are generally small, inconspicuous, and held in the top-most branches of the canopy or in many cases, arising directly from the bark, a feature known as cauliflory.

Fig. 12.8

Cultivation of ‘robusta’ coffee Coffea canephora below the rain forest canopy at N’Dalatando, Cuanza-Sul. Note the large buttresses on Ceiba pentandra trees, indicated by the person standing at the base of the buttressed tree on the right of photo. These broad buttresses support trees in the shallow soils of rain forests

Fig. 12.9

‘Drip tips’ of rain forest tree leaves, Ngoye Forest Reserve, South Africa. From Huntley (1965) Journal of South African Botany, 31: 177–205

4 Forest Canopy Growth Cycles: Gap-Phase Dynamics

Forests are in a continuous state of flux. An understanding of forest dynamics helps explain forest history. Two terms used by forest ecologists can be confusing to students and need explanation from the outset:

• Primary forest refers to old, relatively undisturbed forests comprising slow-growing, shade-tolerant species. Primary forests are often referred to as climax or pristine forest.

• Secondary forest refers to relatively young, regenerating forests on sites that have been transformed by storm damage or felling for timber extraction or agriculture. Secondary forest comprises fast growing, shade-intolerant species.

Disturbance of mature primary forest triggers the development of sun-tolerant pioneer species of secondary forest, which might persist for decades, gradually being replaced by shade-tolerant climax species, which might then persist for centuries before human or natural events open the canopy and allow sun-tolerant pioneers once more to enter the community.

The long-term dynamics of Africa’s tropical rain forests, at continent-wide spatial scales and in geological time scales, were discussed in the introduction to this chapter. Here we will consider changes to forest structure and composition at temporal scales of a few years to centuries (the life spans of a few generations of climax tree species), and at spatial scales that range from treefall gaps to a few hectares of patch disturbance caused by cyclones, landslides or human activities. These processes constitute the forest canopy growth cycle, also known as forest gap-phase dynamics. The disturbance/regeneration process has three stages: the gap-phase, building-phase and the mature-phase of the growth cycle. Together, these result in the mosaic of different ages and composition that characterise rain forest.

Contrasts Between Pioneers and Climax Species’ Life Histories

Following disturbance to a forest, the open patch created is colonised by pioneer species which typically have wide geographic ranges and ecological amplitudes. Light demanding (heliophile) pioneer species have a group of characteristics (traits) that distinguish them from shade-tolerant climax species. These features include producing small but abundant seeds with efficient seed dispersal mechanisms that reach long distances from the parent trees. The seeds of pioneers are capable of dormancy and long-term storage as seedbanks in the soil. The seeds, which germinate only in sun-lit forest gaps, produce masses of seedlings with fast growth rates and short lifespans. Pioneer tree species have wide, open crowns, rapidly filling the forest gap. They reach maturity within a few years and soon produce further abundant seeds.

Climax species have essentially the opposite characteristics, although a continuum of intermediates might be found as the process of plant succession advances, often over centuries, from disturbed forest to the composition of climax communities. But secondary forests are easily identified by a suite of indicator species, few in number, which form secondary forests of low species diversity. In Cabinda, and across much of the Angolan Escarpment, these include species of Anthocleista, Ceiba, Macaranga, Maesopsis, Milicia, Musanga, Ricinodendron, Spathodea, Terminalia and Trema.

Shade-demanding (sciophile) climax species have animal-dispersed seeds that germinate with seedlings establishing under the canopy of pioneer species. Here they persist as saplings until a small gap is created in the canopy, usually by branch fall or tree death, allowing sun flecks to enter. The saplings of climax species, led by an apical shoot, rise as spears in the shade of the lower strata, emerging into the sun and shading out competition from sun-demanding pioneer species. There is thus a continuous cyclic replacement of sun-dependent pioneer species and shade-tolerant climax species in the process of gap formation, seed germination, sapling growth and canopy occupation. The resulting pattern of the frequent opening and closing of patches, regenerating from pioneer to climax structure, is called a shifting mosaic steady state.

The seeds of climax species are usually fleshy and cannot be stored. They are desiccation sensitive and must germinate immediately and are called recalcitrant seeds. However, an exception to this general rule are the seeds of the many legume species that dominate some African rain forests. They are called orthodox seeds, which survive drying and can form part of the seed store of the forest floor for many years, even decades. They are poor dispersers. In the Congo basin, such as in the Ituri forest, the legume tree Gilbertiodendron dewevrei is monodominant over 75% of the area. It produces explosively dehiscent seed pods, most of which do not disperse very far from parent trees. The shade-tolerant juveniles persist under their parent trees, awaiting a break in the canopy that allows them to fill the gap. Extensive monospecific communities of G. dewevrei are found in the DRC, with smaller communities in Cabinda. These trees have slow growth, low dispersal capacity and low resilience to disturbance. The origin of these extensive monospecific forests of the Ituri are described by British/Gabonese ecologist Lee White (2001) as an enigma, possibly related to human activities as long ago as the Iron Age (2500–1500 BP).

5 Floristic Composition

Very few ecological studies have been undertaken in Angolan rain forests, but the early works of Gossweiler and Mendonça (1939), Monteiro (1962, 1965) and Barbosa (1970), as synthesised by White and Werger (1978), are important sources on their floristic composition. In the absence of quantitative information on the Angolan rain forest communities, it is not possible to provide more than indicative floristic lists of species, as recorded for the various forest blocks by Barbosa (1970).

The Guineo-Congolian rain forests of Angola occur as isolated blocks along the mountains and escarpments of western Angola (Ecoregion 1) and as rain forest fragments surrounded by a broad belt of tallgrass Zambezian mesic savannas (Ecoregions 2 & 3) that stretch across northern Angola. These forests represent the southward extension of decreasing forest area and species diversity from Cameroon, through Equatorial Guinea, Gabon and Republic of the Congo to Cabinda, and then to the last outliers of Guineo-Congolian elements such as Piptadeniastrum africanum and Newtonia buchananii in the ravines below the Chela escarpment. Guineo-Congolian mammals, birds, reptiles and amphibia follow a similar decrease of species richness from north to south.

Here we focus on the floristics of the main forest blocks found in Angolan ecoregions (1) Guineo-Congolian Rain Forests, (2) Western, and (3) Southern Congolian Forest/Savanna Mosaics that form part of the continuum of the Guineo-Congolian forest flora in Angola.

Maiombe Forests: Cabinda (Ecoregion 1: Guineo-Congolian Rain Forest, Figs. 2.1, 12.4).

In Angola, the flora of the Guineo-Congolian regional centre of endemism reaches its richest expression in the semi-evergreen and semi-deciduous Maiombe forests of Cabinda. The Alto Maiombe forests lie at between 350 and 800 m on the well-drained soils of hilly sea-facing slopes of the ridge of low mountains running from the northwest in Gabon to the southeast, along the Cabinda border with the Republic of the Congo. The forests cover about 3500 km² within Angolan territory. The best examples reach 60 m height, are multi-storeyed and evergreen to semi-evergreen. Rainfall ranges from 1400 to 1700 mm per year, reinforced by the influence of mist during the cooler, drier months.

• The canopy of mature forest, sometimes reaching 60 m, includes Gilbertiodendron ogoouense, Homalium viridiflorum, Julbernardia seretii, Librevillea klainei, Mammae africana, Pentadesma leptonema, Tetraberlinia bifoliata and Xylopia lenombe. Many of the canopy species are briefly deciduous during the short dry season.

• The mid-stratum, from 20 to 30 m, includes Anopyxis klaineana, Dialium pachyphyllum, Macaranga gilletii, Monodora angolensis, Piptostigma mayumbense and Xylopia staudtii. Most sub-canopy species are evergreen, but those exposed to direct sunlight by treefall, are briefly deciduous.

• Lianes include Combretum platypterum, C. racemosum, Gigasiphon gossweileri, Letestudoxa bella, Physedra heterophylla and Popowia klainii.

• In sites that have been disturbed by windfalls or logging, regeneration is led by Alstonia boonei, Daniellia klainei, Ongokea gore, Pentaclethra macrophylla, Piptadeniastrum africanum, Ricinodendron heudelotii, Symphonia globulifera, Trichilia gilgiana and Xylopia hypolampra.

• The ground strata of disturbed sites often have a dense population of Afromomum species.

Similar but less diverse forests (the Baixo Maiombe) lie below the Alto Maiombe, at 150–350 m, receiving lower rainfall (1200–1300 mm) and with fewer evergreen trees and a greater prevalence of semi-evergreen and semi-deciduous species.

• The dominants of canopies include Balanites mayumbensis, Dacryodes buettneri, Dialium dinklageri, Gossweileriodendron balsamiferum, Guibourtia arnoldiana, Irvingia grandiflora, Ongokea gore, Oxystigma oxyphyllum, Monodora myristica and Pentacletra eetveldeana.

• Here species that are not common in the Alto Maiombe appear, including Bombax reflexum, Canarium schweinfurthii, Newtonia buchananii, Milicia excelsa, Entandrophragma utile and Lannea welwitschii.

• Sun-tolerant pioneers, that grow to 45 m height, and of great economic value include Milicia excelsa (moreira), Piptadeniastrum africanum (mesinga) and Terminalia superba (limba).

Cazengo Forests: Northern Escarpment (Ecoregion 1: Guineo-Congolian Rain Forest; Ecoregion 2: Western Congolian Forest/Savanna Mosaic, Figs. 2.2, 12.8).

Barbosa (1970) described three sub-types of semi-deciduous forests south of the Congo—the Cazengo, Amboim and Seles forests. Typical of the Cazengo subtype are the ‘cloud forests’ of the mountains and escarpments of the Dembos in Cuanza-Norte, the Cazengo forest near Ndalatando and the Pingano forests near Uíge. These forests are surrounded by the tallgrass savannas of the Western Congolian Forest/Savanna Mosaic. The Serra do Pingano is one of several steep-sided mountains of sedimentary rocks of the West Congo System, stretching 60 km in length and 6 km breadth, from northwest to southeast. These forests are receiving increased research attention to record their biodiversity. They are currently under severe threat from deforestation (Lautenschlager & Neinhuis, 2019).

• The canopy of this subtype of forests is at 30–50 m height; taller trees include Ceiba pentandra, Celtis gomphophylla, C. zenkeri, Entandrophragma angolense, Gilbertiodendron kisantuense, Khaya anthotheca, Macaranga angolensis, Milicia excelsa, Piptadeniastrum africanum, Pterocarpus soyauxii, P. tinctorius, Ricinodendron heudelotii, Synsepalum cerasiferum, Treculia africana and Zanha golungensis.

• Ceiba pentandra, the kapok tree, is one of the few rain forest species found both in Africa and in the Neo-tropics. It is also one of the tallest indigenous trees in Africa, reaching 65 m, with massive buttresses (Fig. 12.8). While impressive by African forest standards, this height is, however, dwarfed by the world record trees of 107 m and 114 m for Eucalyptus regnans and Sequoia sempervirens of temperate rain forests of Tasmania and California respectively. Even in equatorial Amazon, trees do not match the heights reached by those of temperate rain forests.

• In disturbed forests, pioneer species in areas of regeneration include Alchornia cordifolia, Clausena anisata, Croton mubango, Harungana madagascariensis, Milletia versicola, Musanga cecropioides, Spathodea campanulata and Trema orientalis.

• A shrub layer, up to 8 m tall, includes Carapa procera, Cola welwitschii, Cyathea manniana, Fernadoa superba and Olax viridis.

• Lianes include Adenia lobata, Dalbergia altissima, Entada gigas, Hippocratea andongensis, Landolphia owarensis, Securidaca welwitschii and Urera thonneri.

• Epiphytes are abundant, including many orchids, the fern Platycerium elephantotis and the only African species of Cactaceae Rhipsalis baccifera.

• In sites receiving some sunlight, herbs of genera such as Acanthus, Aframomum, Asystasia and Desmodium, and grasses represented by Acrosceras, Isachne, Leptaspis, Olyra and Oplismenus occur.

Amboim and Seles Forests: Central Escarpment (Ecoregion 1: Guineo-Congolian Rain Forest; Ecoregion 6: Angolan Escarpment Woodlands; Figs. 12.10, 12.11).

Fig. 12.10

Cumbira Forest, Cuanza-Sul. Regenerating forest, including species of Albizia, Celtis and Ficus. Photo Francisco Gonçalves

Fig. 12.11

Cumbira Forest, Cuanza-Sul. Invasion of understorey by Inga vera. Photo Francisco Gonçalves

The Amboim subtype, identified by the former name of the town of Gabela, occurs on the Central Escarpment in Cuanza-Sul. The forests lie within the mosaic of grasslands, savannas and woodlands that constitute the Angolan Escarpment Woodlands. Most of these forests have been transformed for agricultural purposes. The species composition of the Amboim subtype is similar but less rich than the Cazengo flora. The southernmost forests, also mostly transformed into coffee plantations in colonial times, and now being cleared for cash crop farming, are the Seles forests. These include the Cumbira forest, lying at 680–1100 m on the western slopes of the Serra Njelo near Uku (formerly Vila Nova do Seles), Figs. 12.10, 12.11. The Cumbira forests are now very much reduced in diversity (Gonçalves & Goyder, 2016).

• Barbosa (1970) listed these forests as including Antiaris toxicaria, Albizia glabrescens, A. glaberima, A. adianthifolia, Blighia unijugata, Bosqueia angolensis, Celtis gomphophylla, C. mildbraedii, C. zenkeri, Croton mubango, Ficus capensis, F. mucuso, Maesopsis eminii, Monodora angolensis, Piptadeniastrum africanum and Ricinodendron heudelotii.

Gallery Forests: Malange and Lunda-Norte (Ecoregion 3: Southern Congolian Forest/Savanna Mosaic; Figs. 12.5, 12.6).

Across northern Angola, and including strong representation of the Guineo-Congolian flora and fauna, are the gallery forests of the Congo tributaries that extend southwards to within 100 km of the Congo-Zambezi divide that runs latitudinally through central Angola. In the lower reaches of the Congo tributaries, these dense evergreen gallery forests have closed canopies of up to 30 m height, including a mix of Guineo-Congolian, Zambezian and Afromontane elements.

• These mixed forests include Chrysophyllum magalismontanum, Prunus africana, Syzygium guineense, Treculia africana, Uapaca guineensis and Xylopia aethiopica. Palms include Elaeis guineensis, Raphia gossweileri, and the spiny climbing palm Eremospatha cuspidata.

• Swamp forests occur on poorly drained basins of the river valleys, with Mitragyna stipulosa, Nauclea diderichii, Pandanus candelabrum, Phoenix reclinata, Syzyguim guineense, Spondianthis preussii, Treculia africana and Uapaca guineensis.

6 Faunal Composition

The African tropical rain forest is biologically the richest biome on the African continent, exceeded only by the Cape flora in terms of plant species diversity, although it is much richer in all animal groups. Angola’s Guineo-Congolian Rain Forests (within ecoregions 1, 2 and 3) belong to the Atlantic Equatorial Coastal Forests ecoregion as described by Burgess et al. (2004), which occupies 189,000 km² of five Central African countries. The Atlantic Equatorial Coastal Forest ecoregion has 93 amphibian, 120 reptile, 484 bird, 169 mammal and over 6000 plant species (Burgess et al., 2004). The biodiversity of Angola’s rain forests has not been fully documented, but key animal species, which are restricted to or most commonly found in Angola’s Rain Forest Biome, are listed in Table 12.2.

Table 12.2 Vertebrate species typical of the Guineo-Congolian rain forests of Angola

7 Rain Forest Mammals

Tropical rain forests are fabulously rich in animal life with twice as many species of mammals, birds, amphibians and reptiles as temperate forests. This faunistic wealth is due to the three-dimensional space provided by a tropical rain forest canopy that can reach up to 60 m height. The partitioning of resources provides a rich diversity of living opportunities, across different strata of the canopy, different ecological niches and feeding guilds, and through diurnal, crepuscular and nocturnal activity patterns. The forest trees themselves create microclimates and microhabitats for climbers, epiphytes, parasites, amphibians, reptiles, birds and mammals and a great diversity of invertebrates. Different feeding guilds follow the phenology of flower, pollen, fruit and leaf development, with many complex coevolved adaptations. The perennial humid and hot climate of the tropical evergreen forests provide a food source throughout the year. A seasonal succession of species come into flower and fruit through the year. Animal pollinators partition their services and resource-use across the varied phenological patterns of the different tree species. However, in seasonal semi-evergreen and semi-deciduous forests such as those in Angola, fruiting goes through a bottleneck during the dry season. This places a limit on the total biomass of frugivorous animal species that can be sustained. Some trees produce a mass of fruit during a narrow period, satiating consumers and thus ensuring that at least some seeds escape consumption, germinate and establish. The phenomenon is known as mast fruiting.

Forest mammals are mostly frugivores, but feeding guilds also include folivores, nectarivores, insectivores and carnivores. Primates, squirrels and antelope typically select different strata of the forest, from the forest floor, understorey, mid-stratum, to canopy feeders. As many as 17 sympatric species of primate and nine squirrel species occur in some of the rain forests of Gabon. The diversity of primates is also unusually high in the small enclave of Cabinda, where 14 species have been recorded, including five sympatric species of nocturnal lorises. These include the frugivorous Potto, three bush babies (the undergrowth frugivorous Thomas’s Galago, the canopy insectivorous Demidov’s Galago and the plant gum-eater Southern Needle-clawed Galago) and the insectivorous Golden Potto. This coexistence of similar species is possible due to partitioning of food, hunting technique and space. The diversity of life in rain forests is illustrated graphically in Fig. 12.12.

Fig. 12.12

Rain Forest animals occupy different strata of the forest, from the ground layer to the uppermost canopy, and from early morning to night. This partitioning in space and time of mammals in Maiombe Rain Forest, Cabinda has been illustrated by Angolan artists Fernando (Hugo) and Fatima Fernandes, adapted from MacKinnon (1972) The behaviour and ecology of the Orang-Utan (Pongo pygmaeus). PhD thesis, University of Oxford.. Ground Stratum: a Black Guineafowl, b Forest Buffalo, c Sitatunga, d Civet, e Red River Hog, f Blue Duiker, g Forest elephant, h Water Chevrotain, i Leopard, j Golden cat. Mid Stratum: k Great Blue Turaco, l Beecroft Scaly-scaled Squirrel, m Western Lowland Gorilla, n African Palm Civet, o Tree Pangolin, p Demidoff’s Dwarf Galago. Canopy and Upper Stratum: q Moustached Monkey, r Black-casqued Hornbill, s Northern Talapoin, t Central Chimpanzee, u Golden potto, v Fishing Owl

Another diverse group within the rain forests of Africa are the ungulates, of which 29 species are known, compared with 43 species found in the savannas. Whereas savanna ungulates often form large herds, forest ungulates are mostly solitary. However, like other mammal groups, many species occupy the same forest landscape, with up to 15 ungulate species being found in a single Gabon forest type (Hart, 2001). These include seven fruit and seed eaters, seven folivores and one mixed feeder. Ungulate density in mixed forests was found to be twice that found in the monodominant forests.

The largest mammals of the rain forests of Angola include Forest Elephant, Forest Buffalo Central Chimpanzee and Western Lowland Gorilla, the latter possibly surviving in isolated populations in the Maiombe forests of Cabinda now extinct in Angola. The Forest Elephant is both genetically and behaviourally distinct from the Savanna Elephant. Given its secretive habits and isolated populations, ecological and behavioural characteristics are far less well-researched than those of the Savanna Elephant. However, studies in Gabon, Central African Republic and Republic of the Congo indicate that the species forms small groups of two to three individuals. They do not form batchelor herds, single adult males instead keeping to themselves. These are features that differ markedly from the savanna species. In Gabon, the elephants prefer disturbed forest with a high biomass of tall, pithy herbs of Marantaceae and Zingiberaceae genera (Turkalo & Fay, 2001).

Of the 24 species of carnivore known from African rain forest, only two—Golden Cat and Palm Civet—are found exclusively in forests. Both occur in the rain forests of Angola. Ansorge’s Cusimanse, known only from the type specimen and a recent record from Cumbira (Vaz Pinto et al., 2020), is a gregarious forest carnivore that is probably more common in Angola than published records show, as the species constituted 83% of carnivore meat surveyed in a market in the DRC (Ray, 2001).

The most iconic and charismatic of Africa’s rain forest mammals are the giant apes—Gorilla and Chimpanzee. Both Western Lowland Gorilla and Central Chimpanzee were once common in the Maiombe forests of Cabinda, but are now extinct or greatly reduced in number. Gorilla still occur in strong populations, totaling over 100,000 individuals, elsewhere in Central Africa, while chimpanzee populations are estimated at between 170,000 and 300,000 across their range. The potential of re-introducing these two species into the Maiombe National Park, once effective management programmes are instituted, holds promise. The availability of long-term studies on gorilla and chimpanzee ecology and behaviour from neighbouring countries such as Gabon and Equatorial Guinea (Tutin & Vedder, 2001, White & Tutin, 2001) can be drawn on to guide the process.

Gorillas are generalist herbivores, but, more specifically, are frugivores rather than folivores. Given their large body size and high energy demands, they occur at low densities. They are nomadic within their home ranges, which in Gabon average 400 km²—the area needed to maintain a viable population of family groups. Maiombe National Park, at 2074 km², would thus offer sufficient area for several populations. The Maiombe habitat has many areas of secondary regenerating forest comprising a dense understorey of Marantaceae and Zingiberaceae, including species of Haumania and Aframomum, both keystone plants for gorilla and chimpanzee in the dry season when fruits are in low supply. Chimpanzee are more ecologically adaptable than gorilla and their home range of 15–20 km² would allow appreciable populations to be developed and conserved in the Maiombe National Park.

The species richness of rain forest is high, but abundance is low, with many rare species occupying distinct niches and feeding guilds. Biomass too is low when compared with savanna species, which often occur in large antelope herds or bird flocks. Yet primate biomass alone may reach 2 tonne/km² in the rain forests of Gabon, compared with an average of 12 tonne/km² biomass in mammals of African savannas. Biomass of animals (or the carrying capacity of a habitat) is determined by the seasonal availability of food resources. In the seasonal (monsoon) rain forests such as those of Angola, the carrying capacity baseline for community maintenance is set by the food available during the lowest ebb of the dry season. Food resources might decline to 10% of the wet season peak, and the fauna depends on certain keystone foods, such as perennially fruiting Ficus species. Where these are removed through deforestation for agriculture, dependent frugivores will suffer. The extirpation and extinction of bird species in many Afromontane and Escarpment forest fragments of Angola is probably more the result of reduction of food, nesting sites and shelter resources than due to hunting.

8 Interconnections Between Plants and Animals

The close relationships between plants and animals have fascinated biologists since Darwin’s pioneer studies. Not all relationships are positive. Ehrlich and Raven (1964) introduced the term coevolution, and the concept of endless ‘arms races’ between plants and animals. Coevolution is the process in which two species undergo reciprocal evolutionary change through natural selection. The ‘arms race’ describes the development of increasingly efficient poisons and defensive architecture required to prevent predation. Counter-measures, to detoxify or overcome such defence mechanisms, evolved in insects and vertebrates. More recently it has been suggested that such ‘arms race’ arguments are less convincing than more general mutual dependencies between animals and plants, where organisms interact to their mutual benefit. A classic example of such mutualisms and symbiotic relationships is found in the Congo rain forest tree Barteria fistulosa. Stinging ants Tetraponera aethiops live in the tree’s hollow twigs and branches, and protect the host plant against herbivory. The ant’s sting is said to be able to penetrate the hide of Forest Elephant. Ants benefit from starch and sugar produced by the tree’s extrafloral nectaries. Such insect-plant mutualisms are common in rain forests.

More than in any other biome, rain forest plants depend on animals as pollinators of flowers and dispersers of seeds. Most rain forest plants are pollinated by insects, while 75% of forest plants in Africa rely on animals for dispersal (White, 2001). The most complex insect-plant symbiotic relationship is that of the pollination of figs by fig wasps, that are host-specific to each of the 600 species of Ficus. This relationship developed multiple times over a period of 90 million years, among several unrelated wasp lineages. The wasp’s eggs are laid and hatch within the specialised inverted flower inflorescence of the fig. The adult offspring mate inside the flower, where the male dies. The female exits via the mouth of the fruit, collecting pollen as she departs and flies to a young fig flower, where on laying her eggs, she pollinates the styles of the flowers of the young fig. The wasp’s eggs hatch within the fig, the next generation mates and the cycle commences once again.

In the shadowy, gloomy interior of the forest, plants use nectar and pollen to attract animals. Strongly scented flowers attract bats, brightly coloured flowers attract sunbirds, and juicy fruit attracts frugivorous birds, primates and the mini-ungulates of the forest floor. Long-tubed flowers of the Rubiaceae require long-tongued moths to reach their nectar and disperse their pollen. The flowers of Adansonia, Musa and Parkia have very long peduncles, with large flowers hanging free of branches and leaves, and easily accessible to bats. Orchids have highly elaborate and coevolved pollination mechanisms, while some forest floor members of the Araceae have foul-smelling flowers that attract flies and beetles. In no other biome are animal-plant interactions more diverse and complex than in tropical rain forests.

Rain forest fruits vary in dispersal efficiency, from those that offer low qualities and quantities of reward for frugivores, to the specialist species that bear fruit that are high in fats and proteins. These are often brightly coloured, juicy drupes. In a study of fruit use by vertebrates in Gabon, Gautier-Hion and Michaloud (1989) found 39 species of frugivorous vertebrates, including seven large canopy birds, eight small and two large rodents, nine squirrels, seven ruminants and six monkeys. A similar diversity of fruit specialists might be expected in the Maiombe forests of Cabinda. An example of the close interaction between rain forest plants and animals is the ten-year study of gorilla ecology in the Lope Reserve, Gabon, conducted by Tutin & White (1998), which illustrated the role of gorilla in dispersing between 11,000 and 18,000 seeds per season of the dominant tree Cola lizae. Gorilla, both as seed dispersers and as disturbance agents of forest structure, create optimal conditions for C. lizae seed germination, growth and survival. These researchers also observed the positive influence of cool air temperature on the flowering behaviour of the tree, and postulated the potential vulnerability of this keystone species to climate warming. In a warmer world, the C. lizae flowers may not set seed, leading to the extinction of this narrow endemic species. The critical dependencies between threatened primate species, endemic plants, pollination systems, microclimate and global phenomena such as climate warming, illustrate the importance of ecological studies in the tropical rain forests of Africa.