A brief botanical survey into Kumbira forest, an isolated patch of Guineo-Congolian biome

Abstract Kumbira forest is a discrete patch of moist forest of Guineo-Congolian biome in Western Angola central scarp and runs through Cuanza Norte and Cuanza Sul province. The project aimed to document the floristic diversity of the Angolan escarpment, a combination of general walk-over survey, plant specimen collection and sight observation was used to aid the characterization of the vegetation. Over 100 plant specimens in flower or fruit were collected within four identified vegetation types. The list of species includes two new records of Guineo-Congolian species in Angola, one new record for the country and one potential new species.


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 , White 1983. Th e Angolan escarpment is dominated by semi-deciduous forest and a mosaic of forest-savanna and gallery forest of Guineo-Congolian affi nity, 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. Th ese 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 "Coff ee forests" in Dembos, Cazengo and Gabela regions . 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 affi nities, 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. Th e 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 fl ora and avifauna , Olmos 2008unpub., Maiato 2009, Mills 2010. Th e 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 fl ora is mostly limited to the literature of preindependence 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 eff orts are being made in order to document and obtain baseline data of areas with high socio-ecological importance (for example Th e Future Okavango Projecthttp://www.future-okavango.org and the Okavango Wilderness Projecthttp://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. Th e 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 fi ll 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 affi nities of Kumbira Forest, the fi ndings are based primarily on a rapid botanical assessment conducted between 10 th and 18 th June 2014.

Study area
Th e 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 signifi cant 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 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. Th e 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.
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 2011 unpub.) is unfortunately far from being achieved.

Sampling strategy
Th e botanical team consisted of staff from the Royal Botanic Gardens Kew, UK and from the Herbarium of Lubango, Angola. Th e team used a combination of general walk-over survey (Filgueiras et al. 1994), plant specimen collection and sight observations to aid the characterization of the vegetation and the compiling of an inventory of the plant species present. Over 100 herbarium specimens were collected, mostly of fl owering and fruiting plants, but some sterile collections (lacking fl owers or fruits) were made of the more common or important species which were not fl owering or fruiting at the time of our visit.
Due to the limited time available, no plot-based surveys to quantify the composition of the diff erent vegetation types were conducted. Plant specimens were collected in duplicate or triplicate, with one set deposited in Lubango for incorporation into the main collection, the remaining set(s) being taken back to the UK where they were identifi ed by comparison with material housed in the Kew herbarium. A range of botanical specialists were consulted to help in the identifi cation of diffi cult plant groups.
Due to the paucity of useful literature on Angolan plants, and to the limited previous botanical work in the region, it was not possible to name all collections made, particularly sterile material, but we were able to name the large majority. Th ose named only to genus are nevertheless included within the checklist in Appendix 1. We have not included the additional species listed by Gossweiler and Mendonça (1939) and Barbosa (1970) from this forest subtype in the Appendix 1, as it is not possible to say which forest patch they are from.
Th e recent Angolan plant checklist by Figueiredo and Smith (2008) was used as the baseline for assessing species records and distributions in the country. Th e African Plants Database (http://www.ville-ge.ch/musinfo/bd/cjb/africa/recherche.php) and the World Checklist series (http://apps.kew.org/wcsp/home.do) for a number of plant families were used as the standards for up-to-date taxonomy of African plant species.

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

• Moist high forest (F)
Surveyed at former coff ee plantation at foot of Serra Njelo, c.7 km SW of Conda [centred on 11°09.26'S, 014°17.56'E]). Th e canopy trees in this part of the forest were retained as shade for the coff ee grown underneath (Coff ea canephora, C. robusta), and there has been considerable regrowth of forest understorey since the plantations were abandoned. Th e 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).Th e understorey was rich in Rubiaceae, and the herbaceous fl ora included many ferns and occasional epiphytes, Justicia paxiana is recorded from Angola for the fi rst time.
• Inga-dominated former coff ee plantation (Inga) (Surveyed at: mostly at Monte Belo estate, former coff ee plantation at foot of Serra Njelo, c. 11 km SW of Conda [centred on 11°10.68'S, 014°16.36'E]). Th e exotic, evergreen Inga trees (Inga vera from northern South America) planted initially as shade for the coff ee 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 fi g Ficus saussureana, the fi rst 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)
Th ere 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. Th e commonest subsistence crops grown in the region are manioc (cassava) and plantain. Th e 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). Th ey 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).

Phytogeographical context
Coastal regions of Angola are strongly infl uenced by the Benguela cold-water current which reduces surface evaporation and rainfall. Although the current moves off -shore north of Namibe, corresponding with the northern limit of the Namib Desert, coastal vegetation north of the desert remains dry, and is dominated by dry scrub woodland and succulents such as aloes, euphorbias and baobabs. Kumbira Forest is a very localized patch of forest between this dry coastal vegetation and the moist savannas of the plateau. Barbosa (1970) regarded the forest as typically Zambezian, the canopy dominated by Albizia species that lose their leaves in the dry season. Barbosa also quoted from Gossweiler and Mendonça (1939) who commented on the high percentage of Ficus species. However, we found that most of the species within the forest are Guineo-Congolian rather that Zambezian in fl oristic affi nity sensu White (1983) -the species recorded are generally widespread within the Guineo-Congolian phytogeographical region but often rare or previously unrecorded in Angola.
And it must be noted that Albizia adianthifolia which we encountered in the forest, is distributed widely across tropical Africa, and not just in the Zambezian region. Kumbira Forest must be one of the most southerly areas in western Angola with significant Guineo-Congolian vegetation. Th e upper limits of the forest contain occasional afromontane elements, and the forest merges into moist woodland characterized by widespread species.

Conclusions
We surveyed key sites in a small portion of Kumbira Forest in Cuanza Sul Province, reporting on diversity and fl oristic affi nities of the fl ora. Th e vegetation types were identifi ed: moist high forest, submontane forest & woodland, and Inga-dominated former coff ee 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. Th ere 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. Th ree species are newly recorded for Angola, Ficus saussureana, Justicia paxiana and Tarenna pavettoides. We also report a potential new species of Rytigynia.

Appendix 1
List of the vascular plants collected and observed in Kumbira Forest, Cuanza Sul, Angola. Habitat types in Kumbira forest relate to the phytochoria recognised by White (1983)