An annotated checklist of the coastal forests of Kenya, East Africa

Abstract The inadequacy of information impedes society’s competence to find out the cause or degree of a problem or even to avoid further losses in an ecosystem. It becomes even harder to identify all the biological resources at risk because there is no exhaustive inventory of either fauna or flora of a particular region. Coastal forests of Kenya are located in the southeast part of Kenya and are distributed mainly in four counties: Kwale, Kilifi, Lamu, and Tana River County. They are a stretch of fragmented forests ca. 30−120 km away from the Indian Ocean, and they have existed for millions of years. Diversity of both fauna and flora is very high in these relicts and the coastal forests of Eastern Africa, extending along the coast from Somalia through Kenya and Tanzania to Mozambique, are ranked among the priority biodiversity hotspot in the world. In spite of the high plant species richness and their importance towards supporting the livelihoods of the communities that live around them, floristic studies in these forests have remained poorly investigated. Hence, based on numerous field investigations, plant lists from published monograph/literature, and data from BRAHMS (Botanical Records and Herbarium Management System) database at East African herbarium (EA), we present a detailed checklist of vascular plants recorded in this region. Our results show that Kenyan coastal forests play an essential role in the flora of Kenya and the plant diversity of the coastal forests of East Africa. The checklist represents 176 families, 981 genera, 2489 species, 100 infraspecific taxa, 90 endemic plants species, 72 exotic species, and 120 species that are included in the current IUCN Red List of Threatened Species as species of major concern. We also discovered three new species to the world from these relicts. Thus, Kenyan coastal forests present a remarkable and significant center of plant diversity.


Introduction
Africa is among the world's major centers of endemism and species-rich biodiversity regions (Burgess and Clarke 2000;Linder 2001). It hosts several centers of diversity with the eastern moist forests, which include coastal forests of East Africa and Eastern Arc forests as one of the significant centers of diversity (Lovett 1998). Another important center is the Congo Basin rainforest, which forms the second largest extent of continuous rainforest in the world (Linder 2014). Hamilton (1974Hamilton ( , 1982 presented four centers of endemism in Africa, which include Kivu, the East African coast, Cameroon Ga-bon area, and possibly small refugia in West Africa. Brenan (1978), working with data arranged according to country, identified the same regions as being rich in endemism.
Coastal forests of Eastern Africa are ranked among the 35 identified world's biodiversity hotspots due to the concentration of many endemic species and habitat loss (Mittermeier et al. 1998(Mittermeier et al. , 2004Olson and Dinerstein 1998). Together with the Eastern Arc mountains, they contain approximately 2000 endemic plant species (WWF-US 2003b ). These forests have lived for millions of years, of which they existed as a continuous belt of forest between the East and West coast referred to as the ancient Pan-African forest. However, fluctuation of climate, during the last 2 million years, caused their fragmentation, leading to loss of some lowland dry forests in Africa (Hawthorne et al. 1981). Today, these forests have lost considerable amounts of their wilderness due to anthropogenic pressures leading to small fragments up to less than 5 km 2 , with the most extensive patches recorded in the Kenya coastal forests (Wass 1995;Habel et al. 2017).
Loss of habitat due to human activities is the most severe threat to biodiversity and has become a major global environmental problem (Collinge 1996), hence, plant diversity protection has attracted more attention. Coastal forests of East Africa are ranked among the world's ten most threatened forest hotspots because they have lost more than 90% of their original habitat. Similarly, Brooks et al. (2002) and Azeria et al. (2007) pointed out that Northern Kenya and Southern Tanzania coastal forests should be highly prioritized because they are in danger of losing most of their biodiversity in coming times.
Floristic studies, however, provide a basic outline for plant conservation. Based on these studies, it is possible to determine the condition of an ecosystem, the primary relationships of species with each other or with the environment, and the identity of rare species or widespread species (Ivanauskas et al. 2007). Therefore, they must be carried out not only in a particular area but also over time (Mota et al. 2017). In spite the coastal forests of Kenya acting as significant reservoirs of carbon and biodiversity, and supporting the livelihoods of rural people, plant species diversity remains poorly studied. Despite a substantial amount of floristic research having been performed over the last decades on some of the Kenyan coastal relicts, (Robertson and Luke 1993;Lehmann and Kioko 2005;Luke 2005), a comprehensive study of the whole coastal region of Kenya is still extremely urgent due to its vast area and large number of threatened and endemic taxa. In addition, these forests are vital in providing ecological services at local, national, and global levels; it is therefore, crucial to understand their composition. Here we aim at presenting a detailed checklist of vascular plants recorded in the whole coastal region of Kenya. We achieved this through numerous field investigations carried out from 2015 to 2018, checking of plant species data from published monographs or literature (Beentje 1994;Luke 2005; Flora of Tropical East Africa (FTEA) 1952(FTEA) -2012, and obtaining data documented in the BRAHMS (Botanical Research and Herbarium Management System) database at the East African Herbarium (EA). We also document endemic and threatened species found in this region.

Study area
The study was carried out in the Kenyan coastal region located in the southeast part of Kenya (Fig. 1). Coastal forests are mainly distributed in four counties in the coastal province of Kenya, lying within the latitude of 1°40' to about 4°40' south and between 0-ca. 840 m a.s.l. especially at around Kilibasi (Moomaw 1960;Robertson and Luke 1993). They are part of the larger coastal forests of East Africa which cover an area of approximately 3170 km 2 (Azeria et al. 2007;Wegner et al. 2009) of which, Kenyan coastal forests cover about 787 km 2 (Younge et al. 2002;Burgess et al. 2003). They contain various forest patches with Arabuko Sokoke, and Shimba hills National Reserves forests being the largest remaining forests patches (Burgess et al. 1998(Burgess et al. , 2003Younge et al. 2002;Matiku et al. 2013). Younge et al. (2002), estimated a total area covered by the coastal woodland/bushland to be 120 000 ha. Of this, 114 460 ha are in national reserves, 50 790 ha is in forest reserves, 16 000 ha remains ungazetted, and  ha are in the sacred Kaya Forests. The coastal region of Kenya supports a mosaic of different vegetation types. Much of the area supports bushland or thicket habitats, transition woodland, swampy forests, edaphic grasslands habitats, moist forests, and dry forests. The littoral vegetation includes mangrove vegetation along some parts of the coast and shoreline vegetation (Githitho 2004).

Plant collection and nomenclature
Plant species data for the checklist of Kenya coastal forests were obtained from three primary sources. Firstly, collection records collected during floristic surveys carried out between 2015 and 2018. Different forest patches were visited and surveyed at different seasons to mainly cover the area where there had been few or no previous collections. The investigations included random walks while collecting fertile plant specimens. Details of collected specimens were recorded in a notebook. Information such as habit, habitat, location, and collector details were recorded as well. All collected materials were then processed (pressed and dried) and later deposited at the East Africa herbarium (EA). Duplicates were stored at Wuhan Botanical Garden (HIB). Secondly, plant species data were obtained from the BRAHMS (Botanical Research and Herbarium Management System) database in the East Africa herbarium (EA). Lastly, plant species data for coastal forests of Kenya were assembled from monographs and published literature (Beentje 1994;Luke 2005;FTEA 1952FTEA -2012. Further information about the species recorded at the coastal forests was checked on specimens found in the Global Biodiversity Information Facility (GBIF) (www. gbif.org). Identification of collected specimens was made using taxonomic monographs/floras (FTEA) also through checking and comparing with identified specimens at the East African Herbarium (EA). Family and species circumscription, as well as spelling, authorities, and synonyms of scientific names, were updated based on online databases such as African plant database (www.villege.ch/musinfo/bd/cjb/africa), Catalogue of life (Roskov et al. 2019), and Tropicos (www.tropicos.org). Finally, we identified threatened and near threatened species based on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species (IUCN 2019).

Species diversity
A total of 2489 vascular plant species and 100 infraspecific taxa (subspecies and varieties) belonging to 981 genera and 176 families are comprehensively listed and presented here. This represents 39.55%, 63.78%, and 78.67%, respectively, of the total number of taxa found in Kenya flora (FTEA 1952(FTEA -2012. Also, it represents more than half the total number of plant species found at the coastal forests of East Africa (WWF-US 2003b). 120, 89, and 72 entries represent threatened and near threatened, endemic, and exotic species of the coastal forests of Kenya, respectively (Tables 5,7,9). Three new species were discovered and described from these relicts; Adenia angulosa Passifloraceae (Ngumbau et al. 2017) recorded from the coastal forests of Kenya and Tanzania, Zehneria monocarpa, Curcubitaceae (Ngumbau et al. unpublished) from Kenyan coastal relicts, and Croton kinondoensis, Euphorbiaceae, (Ngumbau et al. in press). Morphology and ITS sequence of Croton kinondoensis placed it under the Adenophorus group, which is known to be endemic to Madagascar and Comoros Islands. This new entity represented an independent dispersal of Croton from Madagascar to Africa mainland (Ngumbau et al. in press).

Growth forms
The growth forms (trees, shrubs, lianas, and herbs) have widely different mechanical architectures, which can also vary phenotypically with the environment (Rowe and Speck 2005). In the coastal forests of Kenya, herbs represent the highest percentage of life form with 48.09%, followed by shrubs 19.96%, trees 15.79%, climbers 8.28%, lianas, 6.07%, and epiphytes 1.81% (Table 2).

Species in need of conservation attention
The Red List and Red Data species system is an approach developed by the IUCN for evaluating the conservation status of species, and in particular for identifying and documenting species in need of conservation attention (IUCN 2019). According to this system, 120 species from the coastal forests of Kenya (Table 5), belonging to 39 families and 98 genera were categorized as Extinct in the wild (EW), Critically Endangered (CR), Endangered (EN), Vulnerable (VU), and Near Threatened (NT). Of 120 species recorded, one taxon was recorded as extinct in the wild, 11 were critically endangered, 20 endangered, 52 vulnerable, and 26 near threatened ( Table 3). The top three families which had most of its members threatened and near threatened were   (Table 8).

New records
Several new species have been discovered recently from the coastal forests of Kenya, Adenia angulosa (Ngumbau et al. 2017), Dovyalis keniensis (Williams 2017), Zehneria monocarpa (Ngumbau et al. unpublished) and Croton kinondoensis (Ngumbau et al. in press) belonging to family Passifloraceae, Salicaceae, Cucurbitaceae and Euphorbiaceae respectively. In addition, one species from family Poaceae, Panicum peteri, was collected and recorded for the first time in Kenya.

Conclusion
Kenya coastal forests host a large number of species and provide a unique habitat for many species of special concern and endemic plant species. It harbors hundreds of ed for each species. Species without voucher numbers were not included in the checklist. Species preceded by "?" means that there is doubt over the correct identification, those preceded with "letter" means that they have not been described and finally vouchers documented as "sr" means that they were sight recorded. EA means the East African Herbarium, Nairobi, Kenya; HIB means the Herbarium of Wuhan Botanical Garden, Wuhan, China, and SAJIT means Sino Africa Joint Investigation Team. The families are divided into four groups, namely lycophytes, monilophytes, gymnosperms, and angiosperms. Families of lycophytes and monilophytes are organized based on the PPG I system (PPG I 2016), while those of gymnosperms are organized based on Christenhusz et al. (2011) and families of angiosperms are organized based on the APG IV system (APG IV 2016).             Our gratitude is expressed to Roy Gereau and anonymous reviewer for their incredible suggestions and comments towards improving the manuscript. We also express our gratitude to Jane Chilande for her assistance in extracting data from BRAHMS database. This work was financially supported by grants from the Backbone Talents Project of Wuhan Botanical Garden, CAS (Y655301M01), and from Sino-Africa Joint Research Center, CAS (SAJC201614).