An annotated checklist of the vascular plants of Aberdare Ranges Forest, a part of Eastern Afromontane Biodiversity Hotspot

Abstract The Aberdare Ranges Forest, located in the Central highlands of Kenya, is an isolated volcanic mountain in the East African Rift Valley with unique flora. Despite its refugial importance to rare and endemic plant species, the diversity of plants in the Aberdare Ranges Forest remains poorly understood. The checklist presented here is a collation of data obtained from multiple floristic surveys and from herbarium specimen collections from the forest. A total of 1260 vascular plants taxa representing 136 families, 613 genera, 67 subspecies and 63 varieties are documented. The ferns comprised 84 species, lycophytes seven, gymnosperms six and angiosperms were 1163 taxa. This represents 17.9% of the Kenyan taxa, 1.7% of the African taxa and 0.3% of all the vascular plants known in the world. A total of 18 taxa were endemic and 14 taxa were found to be threatened globally. The life form, voucher specimen(s), habitat and distribution range of each taxon and a brief analysis of taxa diversity is presented in this checklist. This is the first comprehensive inventory of vascular plants in the entire Aberdare Ranges, providing a solid basis for more sustainable management and improved conservation of this montane forest. The checklist is also an important contribution to the world checklist of plants required by the Global Strategy for Plant Conservation.


Introduction
Tropical montane forests have been classified as biodiversity hotspots (Clark et al. 1999;Myers et al. 2000;Murphy and Bowman 2012) and they represent the Earth's most biologically rich, yet threatened areas that need to be prioritised for conservation and other biodiversity investments (Brooks et al. 2002;CEPF 2012). The Eastern Afromontane Biodiversity Hotspot (EABH) is amongst the eight known hotspots in Africa with globally significant diversity and endemism (Myers et al. 2000;CEPF 2012). This region stretches over a curving arc of widely scattered, but biogeographically similar mountains, including the East African mountains, that have attracted the interest of plants scientists due to the complex processes shaping the species richness and the accumulation of local plant diversity (Burgess et al. 2007;Chen et al. 2015;Liu et al. 2016). The East African mountains are of relatively young geological age and share similar tropical climates, but they tend to be isolated from each other (Liu et al. 2016).
Floristic explorations, with subsequent inventorying and digitisation of plants species in many parts of the world, are far from complete (Schmitt et al. 2010). The species-rich biodiversity regions in tropical Africa remain poorly sampled to date, underlining the need for thorough explorations and documentation especially with the current unprecedented rate of species extinction Stropp et al. 2016;Sosef et al. 2017). Indeed, the partial and uneven species-occurrence data have created a taxonomic impediment that have substantially impacted the effective conservation and sustainable utilisation of biodiversity (Callmander et al. 2005;Küper et al. 2006). In order to halt the continuing loss of plant diversity, the Convention on Biological Diversity (CBD), through the Global Strategy for Plant Conservation (GSPC), has over the past two decades advocated for intense exploration and documentation of plants species with the aim of achieving a complete world checklist of flora in the near future (COP 2002;Paton et al. 2008;Joppa et al. 2013;Sharrock et al. 2014).
The AR Forest is the only East African mountain situated at the Equator and flanks the Gregory Rift Valley to the east (Muiruri 1978;Bennun and Njoroge 1999). The rugged terrain, deep ravines, undulating hills and permanent streams coupled with tropical climates and broad elevation gradient provides diverse microhabitats for unique and diverse vegetation communities (Schmitt 1991). Despite the enormous socio-economic values attached to the AR ecosystem (Lambrechts et al. 2003;Louppe et al. 2009;Ark and Group 2011), the future of biodiversity in this forest is increasingly threatened by adverse land-use changes and exploitation of natural resources within the forest (KWS 2010; Kipkoech et al. 2019). Significant patches of indigenous forest have been cleared to pave way for subsistence agriculture or degraded through unsustainable extraction of forest products (Butynski 1999;Lambrechts et al. 2003).
The goal of this study was to provide a broad checklist of vascular plants in the entire AR Forest. Knowledge of plant diversity in the AR is critically important in designing appropriate conservation and management interventions to curb further biodiversity loss in this ecosystem. Specific objectives were to (i) document the vascular plants of the entire AR Forest, (ii) document the endemic and threatened vascular plants in the AR Forest and (iii) document the life forms and habitats of all the vascular plants in the AR Forest.

Study site
The Aberdare Mountain, formerly known as Nyandarua Range, is the third highest mountain in Kenya, located at the equator and forms part of the easternmost wall of the Gregory Rift Valley (Muiruri 1978;Lambrechts et al. 2003;Georgiadis et al. 2007). The AR, together with Mount Kenya, constitute the Central Highlands of Kenya. It extends approximately 120 km southwards from the equator through Nyeri, Nyandarua, Muranga and Kiambu Counties to the Kikuyu escarpment, between 36°30'E, 0°05'S and 36°55'E, 0°45'S (Schmitt 1991;KFS 2010) (Fig. 1). The altitude ranges between 1800-4001 m a.s.l. The two highest peaks, Oldonyo Lesatima (4,001 m a.s.l.) on the northern side and Il Kinangop (3,906 m a.s.l.) on the southern side of the AR, are co-joined by a flat ramp slightly tilted to the east above 3000 m elevation (Bennun and Njoroge 1999). The AR covers an area of 2,162 km 2 with a boundary perimeter of 565 km (Butynski 1999).
The general topography of the AR is diverse and comprises numerous undulating hills formed through volcanism and faulting of the earth's surface from the early Tertiary to the Pleistocene periods (Peltorinne 2004). On the eastern side, the slopes are gradual, while on the western side, the slopes drop dramatically to Kinangop plateau and finally to the Gregory Rift Valley. The AR Forest is governed by two institutions, Kenya Wildlife Service (KWS), which manages the Aberdare National Park with an area of ca. 76,700 ha and the Kenya Forest Service (KFS) which governs the Aberdare Forest Reserve measuring ca. 139,500 ha. The National Park encompasses the summit of the AR above 3000 m a.s.l. and a narrow salient extending to the east to 1900 m a.s.l. (Chuah-Petiot 1997;Butynski 1999). The regions below and around the Park constitute the Forest Reserve, composed of three major forest blocks; Aberdare block, Kikuyu Escarpment block and Kipipiri Forest block (Butynski 1999;KWS 2010).
The AR harbours a tropical montane forest characterised by discontinuous vegetation structures along altitudinal and climatic gradients (Hedberg 1951;Lovett 1996). Four broad vegetation zones have been described, including montane humid forest at lower altitudes, sub-montane forest at mid elevations, sub-alpine vegetation at the moorlands and dry xeromorphic evergreen forest at the northern parts of the forest (Butynski 1999). The climate in the AR Forest is influenced by the Inter-Tropical Convergence Zone north and south during its annual cycle, producing a bimodal rainfall distribution (CEPF 2012). Long rainfall periods are experienced from March to May, while short rainfall periods are received from October to November. Annual rainfall varies with altitude and exposure to the dominant winds from the Indian Ocean, ranging from 700 mm on the drier north-western slopes to 3000 mm on the wetter south-eastern slopes (Schmitt 1991;Chuah-Petiot 1997;Lambrechts et al. 2003). The average temperature in the AR decreases with increasing altitude. The mean daily temperature varies between 10.3° to 25.8 °C, with the lowest temperatures experienced between July and August (KFS 2010;CEPF 2012).

Floristic surveys and checklist collation
A botanical team from the National Museums of Kenya (NMK) and Sino-Africa Joint Research Center (SAJOREC) carried out field investigations from 2016 to 2019. Floristic surveys were done during both wet and dry seasons to capture a wide range of phenological cycles of taxa, especially flowering and fruiting. General walk-over surveys were used in specimen collection and habitat characterisation (Gonçalves and Goyder 2016 and references therein). Field investigations targeted pristine areas not surveyed before, as well as thorough re-surveys of previously investigated areas. Plant specimens were collected in quadruplicates and each specimen was tagged with labels containing its scientific name, collection date, location and herbarium specimen number. Fertile voucher specimens with either flower, fruit or both were collected and identified with the names of taxa recorded in the field and confirmation done at the East African herbarium (EA). Standard botanical references were used in identification of specimens, i.e. Flora of Tropical East Africa (FTEA 1952(FTEA -2012, Blundell (1992), Agnew and Agnew (1994), Beentje et al. (1994) and Agnew (2013). Specimens were preserved by pressing and drying. A pair of each of the taxon collected was deposited in the EA herbarium in Nairobi and another in the Wuhan Botanical Garden herbarium (HIB) in China. Herbarium acronyms follow Thiers (2020 onward: http://sweetgum. nybg.org/science/ih/). Vascular plants specimens, previously collected for varied purposes from the AR and deposited in the EA, were compiled with our collections to develop a checklist of vascular plants. Habitat(s) and relative distribution range for each taxon were determined using our collections, herbarium specimens in the EA and published bibliographies. The broadest altitude ranges, that is between the minimum and maximum altitude a taxon is known to occur, were searched and recorded. Thence, the distribution ranges of plants' taxa were not restricted to the AR as some taxa had wide distribution ranges extending to the sea level. Life forms of taxa collected were categorised as herbs (plants less than 50 cm high or less than 100 cm, but annual and without persistent woody stems), shrubs (plants between 50 cm to 5 m high with woody stems branching at or near the ground), climbers (plants with twining herbaceous or woody stems) and trees (plants taller than 5 m with a clear main trunk) (Schmitt 1991;Beentje 2016). Standard bibliographies, for example, (FTEA 1952(FTEA -2012, Blundell (1992), Beentje et al. (1994) and Agnew (2013), were also used to define life forms, particularly from herbarium specimens. Endemic taxa were obtained by searching all the vascular plants recorded, including existing endemics cited in literature, in an on-line occurrence database in the Global Biodiversity Information Facility (GBIF) (https://www.gbif.org). Moreover, the conservation status of all the vascular plants recorded were assessed in the IUCN Red List of Threatened Species (https://www.iucnredlist.org) and categorised as Critically Endangered (CR), Endangered (EN), Vulnerable (VU) and Near Threatened (NT). The current taxonomic circumscription of each taxon recorded was checked in the Tropicos database (http://www.tropicos.org/), African Plant Database (http://www.ville-ge.ch/musinfo/bd/cjb/africa/recherche.php?langue=an) and the Catalogue of Life, 2019 Annual Checklist (http://www.catalogueoflife.org/). Finally, the recorded plants' taxa were grouped into their respective classes and families and presented alphabetically.

Taxa diversity
A total of 1260 vascular plants representing 136 families, 613 genera, 67 subspecies and 63 varieties were recorded, which represented indigenous, naturalised, exotic or introduced plants in the AR. Ferns and fern-allies were 91 in total, with 1169 taxa of seed plants. The most diverse class was Magnoliopsida (72.9% of the total species recorded), followed by Liliopsida (19.4%), then Polypodiopsida (6.6%), Lycopodiopsida (0.6%) and the least diverse was Pinopsida (0.5%) of the total taxa recorded (Fig. 2). The top taxa-rich families were Asteraceae (11.1%), Poaceae (8.2%), Fabaceae (6.6%) and Lamiaceae (3.9%) of the total vascular plants recorded (Table 1). The most diverse genera were Cyperus (20), Helichrysum (19), Senecio (17), Asplenium (17), Crotalaria (15) and Solanum (15), while other genera had less than 14 taxa (Table 1).  A total of 54 exotic plants' taxa from varied origins were recorded (Appendix I). The majority of the exotic taxa were from Australia (14), followed by South America (12), Mexico (six), Europe (five), North America (three), while the other regions had less than three taxa. Taxa in the genus Eucalyptus were all from Australia. Herbs were the majority with 24 taxa, followed by trees 18 then shrubs and climbers with 11 and three taxa, respectively.

Plants life forms
Herbs were the commonest life form with 723 taxa, followed by shrubs with 148, trees with 137 taxa, while other intermediate life forms had fewer taxa (Table 2). Epiphytes accounted for 2.9% while aquatic herbs were 0.5% of total taxa recorded.

Endemic and threatened taxa
Our checklist contains a total of six endemic taxa in the AR Forest (Appendix II). However, there are additional 12 taxa that are endemic in both the AR Forest and Mt. Kenya Forest. The two montane forests are in close proximity and form the Central Highlands of Kenya with enormous socio-economic value (Muiruri 1978;Schmitt 1991). The majority of endemic taxa were in the family Asteraceae (nine) and Apiaceae (three) (Appendix II). The herbs (nine taxa) were the dominant life form amongst endemic plants, followed by shrubs (six), then woody herbs (two taxa) and a single tree.
A total of 13 taxa were found to be threatened or near-threatened globally in the AR Forest (Appendix III). The taxa-rich families were Asteraceae (four) and Cyperaceae (three taxa). The majority of the threatened taxa were herbs (nine), then trees (three) and the remaining were shrubs (two taxa).

Discussion
The AR Forest is a significant regional centre of plant diversity. With a total of 1,260 taxa recorded, it represents 17.9% of the total 7,004 vascular plants in Kenya,10.2% of the 12,317 vascular plants in East Africa, 1.7% of the estimated 74,000 taxa in Africa and 0.3% of the estimated world flora of 422,127 taxa (Govaerts 2001;MEWNR 2015). Globally, lycophytes were 0.5% of the total 1,290 lycophytes, ferns were 0.8% of the total 10,560 ferns, gymnosperms were 0.6% of the total 1,079 gymnosperms, while angiosperms were 0.4% of the total 295,383 angiosperms in the world (Christenhusz and Byng 2016). The ferns correspond to 12.9% of the 706 ferns recorded in Africa, gymnosperms represent 13.6% of 44 African species and angiosperms corresponded to 3.6% of 32,424 and 2.3% of 50,136 angiosperms in Tropical Africa and Sub-Saharan Africa, respectively (Klopper et al. 2007;MEWNR 2015). However, there could be changes in this richness as new taxa are being described, other taxa are being synonymised, exotic or invasive taxa are being introduced, while other taxa become extinct (Pimm and Joppa 2015;Christenhusz and Byng 2016).
The taxa-rich families in the AR, Asteraceae, Fabaceae and Poaceae, were consistent with studies done in other regions in Kenya, such as Kakamega Forest (Fischer et al. 2010), North and South Nandi Forests (Girma et al. 2015), Mount Kenya Forest (Zhou 2017) and Cherangani Hills Forest (Mbuni et al. 2019). These families are the most diverse and widespread in Kenyan vegetation, especially in upland forests. They are also amongst the largest families of plants in Sub-Saharan Africa, Tropical Africa and in the world flora (Klopper et al. 2007;Christenhusz and Byng 2016). Herbs were the dominant life form in the AR which supports the widely reported observation of the herbaceous layer as the most diverse community in forest ecosystems (Albrecht et al. 1997;Bhattarai and Vetaas 2003;Whigham 2004;Zhao and Fang 2006;Lee et al. 2013;Trigas et al. 2013). The herbaceous layer, as an understorey community, plays a fundamental role in carbon dynamics and energy flow, nutrient cycling and influences seedling growth for the overstorey community (Roberts 2004;Whigham 2004;Gilliam 2007). Thus, the high richness of herbaceous taxa in the AR promotes the self-productivity and viability of this Afromontane ecosystem. Endemic taxa in the Central Highlands of Kenya, comprising the AR and Mt. Kenya Forests, represent 3.1% of the total 577 endemics in Kenya and 0.8% of the total 2,350 endemics in the EABH (CEPF 2012;MEWNR 2015;Zhou et al. 2017). This signifies the critical role the AR plays in harbouring restricted taxa in unique habitats, particularly at higher elevations. Ancient isolation of the East African mountains after their formation (Chorowicz 2005) and the harsh climatic conditions associated with high elevations are thought to have facilitated taxa adaptation and speciation, resulting in the high endemic richness that was observed in this study (Lovett and Friis 1994;Lovett et al. 2000Lovett et al. , 2005. The threatened taxa represent 1.1% of the total vascular plants in the AR and 3.4% of the total 356 threatened plants' taxa known in Kenya (MEWNR 2015). This further exemplifies the regional and global importance of this forest, underlining the need of efficient conservation strategies. Plant species such as Warbugia ugandensis Sprague, Lactuca inermis Forssk, Carissa spinarum L., Prunus africana (Hook. Fil.) Kalkm and others, are traditionally utilised by the local community as remedies for various illnesses (Kokwaro 2009;Kamau et al. 2016a, b). With the rising popularity of herbal therapy in Kenya (Kokwaro 2009), such medicinal plants are over-exploited, hence the urgent need for close monitoring and protection to conserve their already shrinking populations. In addition, Pinus radiata D. Don, Prunus africana (Hook.f.) Kalkm and Croton alienus Pax are faced with logging and charcoal burning threats, thus, they should also be prioritised in conservation planning (Muiruri 1978;Lambrechts et al. 2003;KFS 2010). On the other hand, species such as Ficus sur Forssk., Ficus thonningii Blume and Indigofera erecta Thunb are considered sacred and they are strictly conserved by the local community living around the AR Forest (Muiruri 1978;Schmitt 1991;KWS 2010).

Conclusions
The AR Forest harbours a notably diverse flora with endemic taxa. This study provides a floristic checklist with up-to-date nomenclature that will underpin future research into the diversity and conservation of the AR ecosystem. Varied environmental aspects can be researched within the AR, particularly with its mosaic of microhabitats, broad elevation gradient and associated temperature and rainfall variations within a short span of land. Our study provides the baseline data for such studies. In general, the checklist is a major step towards building a holistic biodiversity knowledge of the AR ecosystem.

Checklist
An annotated checklist of the vascular plants of the AR Forest is presented below. Vascular plants are grouped into five classes and arranged alphabetically in their respective families. Families in Lycopodiopsida and Polypodiopsida are arranged, based on the Pteridophyte Phylogeny Group I system (PPG I 2016), Pinopsida is arranged according to Christenhusz et al. (2011), while Liliopsida and Magnoliopsida are arranged, based on the APG IV 2016 system (Chase et al. 2016). Taxa preceded by ( E ) are endemic while those with (*) are exotic and/or naturalised in the AR Forest. For each taxon recorded, full authority is given, life form, brief notes on habitat and distribution range, voucher specimen number and the herbarium where it was deposited. The broadest altitude range of each taxon is indicated in metres (m) which mostly extends beyond the elevation of the AR Forest. EA refers to the East African herbarium in Nairobi, Kenya, while HIB refers to Wuhan Botanical Garden herbarium in Wuhan, China. The collectors are abbreviated as follows: SK means Solomon Kipkoech, SAJIT refers to Sino-Africa Joint Investigation Team, FOKP means Flora of Kenya Project, KEFRI refers to Kenya Forestry Research Institute and EANHS stands for East Africa Natural History Society.

F52. Begoniaceae
Begonia meyeri-johannis Engl. -Life form: Woody climber. Habitat: Moist forest, 1350-2800 m. Vouchers: SK 0183, SK 0187 (EA, HIB).              Technology and Innovation, Kenya Forest Service and Kenya Wildlife Service for granting research permits and providing security during floristic surveys. We would also like to thank Prof. Thaís Elias Almeida and anonymous reviewers for their invaluable suggestions to this manuscript. This study was supported by grants from the Backbone Talents Project of Wuhan Botanical Garden, CAS (Y655301M01) and from Sino-Africa Joint Research Center, CAS (SAJC201614).