Abstract

Introduction

Angola lies almost wholly within the southern zone of tropical grassland, bounded by tropical rain forest of the Congo in the north and by the Kalahari Desert in the south (Shaw 1947). In general the country comprises four main physiographic components: a coastal plain from the Atlantic seaboard of approximately 200 m above sea level, and from 12 to 200 km wide; a narrow steep escarpment, from 200 to 1000 m altitude; an interior plateau, occupying nearly 80% of the country lying between 1000 and 1600 m, and a mountain belt rising above the escarpment and plateau of about 2620 m in central plateau (Huntley and Matos 1994).

Between the Karoo-Namib phytochorion of the coastal belt and the Brachystegia dominated Zambesian phytochorion of the interior plateau, a discontinuous series of moister vegetation type extends southwards from the Guinea forest and Congo savanna systems following the escarpment (Huntley 1974, White 1983). The Angolan escarpment is dominated by semi-deciduous forest and a mosaic of forest-savanna and gallery forest of Guineo-Congolian affinity, which is restricted to the interior of Cabinda province and large but discontinuous patches of forest in Zaire, Uíge, Cuanza Norte and Cuanza Sul provinces (Barbosa 1970, Huntley 1974).

These vegetation formations, as referred above, cover large areas of Cabinda with a tree strata of about 30 to 40 m height, while in the south are restricted to extensive “Coffee forests” in Dembos, Cazengo and Gabela regions (Huntley 1974). In this latter Kumbira forest, located in Conda, Cuanza Sul province is no doubt the most important and, probably the most southerly and most isolated patch of this biome, with various elements of Congo basin and West African affinities, dominated by genera Celtis, Morus, Albizia, Bombax and Pterocarpus (Barbosa 1970).

To these formations can be added the afro-montane forests, also of great biogeographic interest (Hall 1960a), restricted to small and isolated patches of forest in Benguela, Huambo and Huíla provinces. The total area of these forest patches was estimated to be approximately 200 ha, the best known of which is Mount Moco in Huambo province which provides habitat for its flora and avifauna (Huntley 1974, Olmos 2008 unpub., Maiato 2009, Mills 2010). The Angolan escarpment with approximately 1000 km of extent is unique, beautiful and constitutes the section of the Great Escarpment of Southern Africa poorly known in terms of its biodiversity, but high level of endemism (Huntley and Matos 1994, Figueiredo 2008, Clark 2011).

Despite the recent published checklist of Angolan vascular plants (Figueiredo and Smith 2008), data on the Angolan flora is mostly limited to the literature of pre-independence era, and current knowledge on Angolan plants is poor and restricted to isolated and focused studies carried out by individuals or institutions. Increasing interest and efforts are being made in order to document and obtain baseline data of areas with high socio-ecological importance (for example The Future Okavango Project – http://www.future-okavango.org and the Okavango Wilderness Project – http://www.wildbirdtrust.com/portfolio/okavango-widerness-project) which focus on the Cubango and Cuito river catchments respectively). However large parts of the country, such as the Angolan escarpment, remain to be studied biologically.

In terms of biodiversity, only the avifauna has been investigated in any detail (Hall 1960a, Cagan and Riley 2005, Mills 2010, Cáceres et al. 2015). Recommendations have being made to undertake botanical surveys in the Kumbira area (Barbosa 1970, Huntley 1974). The ecological importance of Kumbira forest in maintaining the highest number of Angolan endemic avifauna highlighted the area as important for conservation of biodiversity. But at the same time concerned due to the increasing human pressure (Huntley 2011 unpub.). To fill in the gap in terms of botanical diversity, we undertook a botanical survey into Kumbira forest. Here we document our current knowledge of the plant diversity and phytogeographical affinities of Kumbira Forest, the findings are based primarily on a rapid botanical assessment conducted between 10^th and 18^th June 2014.

Methods

Study area

The central escarpment of western Angola on which Kumbira forest is part of, runs through Cuanza Norte and Cuanza Sul provinces. Barbosa (1970) recognized three types of moist high forest along this escarpment occurring in discrete geographic and climatic zones. North to south, these are:

i. forests of subtype Cazengo to the north of the Cuanza River;

ii. forests of subtype Amboim, between the Cuanza and Keve (Queve) Rivers, of which the most significant is the area of forest around Gabela;

iii. forests of subtype Uku (Vila Nova do Seles) to the south of the Keve River;

Kumbira forest (11°07.00'S; 014°17.00'E) is a discrete patch of moist forest vegetation in this third zone, SW of Conda (Figure ​(Figure1),1), where cloud gathers under the knife-edge ridge of the Njelo mountain which reaches around 1500 m in elevation and prevents the cloud from moving further inland. The forest forms part of about 200 000 ha of semi-deciduous moist forest (Cagan and Riley 2005) and occurs on middle altitude slopes at around 700–900 m. Below this altitude dry open vegetation predominates, and at higher elevation the forest gives way to woodland and ultimately open rocky mountain slopes into rocky gorges in Njelo mountain.

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Figure 1.

Map of Angola and its provinces, Cuanza Sul province highlighted and Kumbira in Conda municipality (black dot). Kumbira forest (top), the forest with Njelo mountain in the background (bottom).

Kumbira forest is considered to be part of Gabela Important Bird Area (Dean 2001), and holds unique and threatened bird community (Cagan and Riley 2005). Although its ornithological importance the area is not protected by law, the proposed implementation of Gabela Natural Reserve, recently reinforced (Huntley 1974; Huntley 2011 unpub.) is unfortunately far from being achieved.

Results

From the vegetation survey carried out in Kumbira forest, four main vegetation types were identified. Here we provide a very brief overview of the forest types and their dominant species as recorded by the rapid botanical survey. The habitat types outlined here are also assigned to each of the species listed in (Appendix 1).

• Moist high forest (F)

Surveyed at former coffee plantation at foot of Serra Njelo, c.7 km SW of Conda [centred on 11°09.26'S, 014°17.56'E]). The canopy trees in this part of the forest were retained as shade for the coffee grown underneath (Coffea canephora, Coffea robusta), and there has been considerable regrowth of forest understorey since the plantations were abandoned. The canopy is c. 25-30 m high, and the commonest tree is the seasonally deciduous Albizia adianthifolia, Trema orientalis, Markhamia zanzibarica, Antidesma venosum and several species of Ficus are common elements. We also encountered Anthocleista schweinfurthii, Cola welwitschii, Pteleopsis diptera, Synsepalum cerasiferum, Turraea vogelii and Vitex welwitschii in some areas. An arborescent Dracaena and a species of Erythrina were also noted (sight records only).The understorey was rich in Rubiaceae, and the herbaceous flora included many ferns and occasional epiphytes, Justicia paxiana is recorded from Angola for the first time.

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Figure 2.

Turraea vogelii Hook.f. ex Benth, Cola welwitschii Exell & Mendonça ex R. Germ., Pancovia golungensis (Hiern) Exell & Mendonça, Cochlospermum angolense Welw. ex Oliv., Inga vera Willd. subsp. vera, Pavetta gossweileri Bremek, Pittosporum viridiflorum Sims, Clerodendrum poggei Gürke

• Submontane forest/woodland (W)

(Upper margins of Moist High Forest). The moist high forest gives way to deciduous woodland at higher elevations (above c. 900 m). Characteristic trees and shrubs include Harungana madagascariensis, Dombeya rotundifolia, Cochlospermum angolense, Grewia flavescens, Pittosporum viridiflorum and Hymenocardia acida. We also encountered Podocarpus milanjianus, an afromontane element, in the gullies at high altitude.

• Inga-dominated former coffee plantation (Inga)

(Surveyed at: mostly at Monte Belo estate, former coffee plantation at foot of Serra Njelo, c. 11 km SW of Conda [centred on 11°10.68'S, 014°16.36'E]). The exotic, evergreen Inga trees (Inga vera from northern South America) planted initially as shade for the coffee are now overgrown and let little light through the canopy, seriously reducing the development of a shrub or herb layer except in gaps. Inga vera was recently referred as one of the most impressive and worrying invasive species of western Angola, forming dense stands in localized sites as observed in Kumbira forest (Rajmánek et al. 2016). Nevertheless, we did record a variety of ferns and other herbaceous plants in this area, in addition to the very large fig Ficus saussureana, the first record of this Guineo-Congolian species from Angola. We observed some regeneration of Inga from seed, demonstrating its potential as an invasive species. A second exotic mimosoid legume tree was encountered in this area – Leucaena leucocephala – also native to the New World.

• Ruderal and secondary habitats (Sec)

There is extensive encroachment of agriculture in the vicinity of villages, roads and tracks, and extraction of timber trees in some areas of forest. We did not survey this beyond making occasional observations. The commonest subsistence crops grown in the region are manioc (cassava) and plantain. The oil palm Elaeis guineensis is widely planted. We also encountered dense stands of the pigeon pea Cajanus cajan in one area. Disturbed areas of forest, recently cleared, had large patches of the invasive shrub Solanum mauritianum.

• Plant diversity

Our records are a brief snapshot, based on a visit of just six collecting days, at just one time of the year (June). They are by no means comprehensive. Nevertheless, we recorded 92 species from the forest.

• New plant records for Angola

We report two new records of Guineo-Congolian forest species for Angola – the tree Ficus saussureana and the herb Justicia paxiana. In addition, Tarenna pavettoides is newly recorded for the country, and may represent an undescribed subspecies. We also report a potential new species in Rytigynia sp. nov. (Appendix 2).

Discussion

Conclusions

We surveyed key sites in a small portion of Kumbira Forest in Cuanza Sul Province, reporting on diversity and floristic affinities of the flora. The vegetation types were identified: moist high forest, submontane forest & woodland, and Inga-dominated former coffee plantation. In addition, ruderal or disturbed areas also occur.

Species composition is overwhelmingly Guineo-Congolian, and this forest represents one of the most southerly areas of such vegetation along the Angolan escarpment. Most species have a wide geographic distribution, but some such as Pavetta gossweileri are more restricted, and are reported from just Cuanza Sul and Cuanza Norte. Deinbollia laurifolia is restricted to riverine lowland habitats from Cuanza Sul to Cameroon.

Much of the area surveyed had good regeneration of the understorey since coffee cultivation ceased, but we observed large trees being taken out of the forest, and evidence of village agriculture encroaching in a number of places. Areas formerly planted with New World legume species, particularly Inga vera, were in less favourable condition as the overgrown Inga shades out the understorey. There is some evidence of this species reproducing and spreading naturally, and we observed other potentially invasive species such as Solanum mauritianum. In addition, some coffee estates are being reestablished, which will reduce the extent of undisturbed or recovering forest further. Three species are newly recorded for Angola, Ficus saussureana, Justicia paxiana and Tarenna pavettoides. We also report a potential new species of Rytigynia.

Acknowledgements

Appendix 1

Appendix 2

Notes

References