Kasewe Forest

A(i)Site contains one or more globally threatened species

The Kasewe Forest IPA qualifies as an important Plant Area under criterion A(i) as it holds populations of 11 globally threatened plant taxa, of which one is assessed as Endangered and ten are assessed as Vulnerable. Given the very limited extent of remnant closed canopy forest in Sierra Leone, this site will also likely qualify under IPA criterion C.

The Kasewe Forest IPA is situated directly to the east of Moyamba Junction in central Sierra Leone. This IPA straddles multiple political divisions, namely the Tonkolili and Moyamba Districts which lie on the border of the Northern and Southern Provinces, respectively. The site is bounded by roads, with the Bo-Freetown highway to the southwest and unnamed roads to the north and east. Flat plains surround the IPA, which is contrasted by hills that lie centrally within the forest and rise to a height of c 470m asl. This IPA incorporates the Kasewe Forest Reserve which is designated as a forest reserve for utilization. Much of the forest that once occurred at Kasewe has largely been fragmented, and the remaining vegetation is outlined by the ‘core’ boundary. Some areas of the ‘buffer’ zone of this IPA are utilised by local communities, with a nursery for highland coffee (Coffea stenophylla), plantations of oil palm and rice, as well as planted restored forest also occurring within this area. Please note that Google products have incorrectly placed ‘Kasewe Reserve’ to the west of Moyamba Junction.

The Kasewe IPA is one of the few remaining patches of moist closed forest in Sierra Leone. The remnant native vegetation at the site holds high species diversity, including eleven globally threatened species. Notably, the Kasewe IPA is the most significant reserve for Coffea stenophylla (VU) wild genetic diversity in Sierra Leone (and globally), having the largest subpopulation of the only two sites where this species is known to occur in the wild.
Coffea stenophylla, or highland coffee, is a rare wild species of considerable interest to future coffee crop development due to its superior taste, disease resistance and climate resilience in comparison to other major commercial species (C. arabica, C. canephora and C. liberica) (Davis et al. 2020). Safeguarding the full genetic diversity of this threatened species in the wild is vital, especially whilst favorable traits are assessed (Davis et al. 2021). It should be noted that C. stenophylla and C. affinis are now thought to be a single taxon (A. Davis pers. comms. 2024). Additionally, the species concept of another wild coffee species historically recorded at the site, Coffea liberica, is currently being revised, greatly reducing the known distribution of this species (A. Davis pers. comms. 2024). C. liberica is now restricted to West Africa, while it was formerly thought to be found throughout West Africa and Central Africa (A. Davis pers. comms. 2024). C. liberica is of high conservation importance for Sierra Leone, and globally significant populations may be contained within this site (Davis et al. 2022). There is a historic collection of C. liberica from the site (in 1913) however recent survey did not relocate this species (Lebbie et al. 2024).
Most botanical survey efforts at the site have focused on forest areas on the hills (Faiyah et al. 2018, Johnny et al. 2022a, 2022b, Johnny et al. 2023, Lebbie et al. 2023). Recent botanical survey has confirmed the presence of a new threatened habitat at the site: Low-altitude lateritic bowal grassland, which is likely to contain rare species (Lebbie et al. 2024). Bowal vegetation is highly seasonal, and will be monitored over time to fully assess rare species occurance in this habitat. Given the extent of transformation and fragmentation of vegetation on the plains at the IPA, records of botanically significant species are scarce in these areas.
There have been a few historical collections of threatened species within the IPA, however not all of these species have been re-recorded in recent survey efforts. Dissotis leonensis (EN) was recorded at Kasewe in 1913 (Lane Poole #123) but has not been recollected in recent targeted survey (Lebbie et al. 2024). This species is a narrow range West African endemic that is associated with rocky habitat in open, low forest and wooded grassland (X. van der Burgt pers. comms. 2024), usually at altitudes of over 900 m asl however, it has also been recorded at low alt. in the Sula Mts (Tonkolili Observations #838 at 320 m alt. in 2014). D. leonensis is highly vulnerable to fire, thus it may be locally extinct due to the prevalence of anthropogenic fires within the IPA. Reassuringly, the threatened tree Nesogordonia papaverifera (VU) was re-recorded in recent surveys (Johnny et al. 2023, Lebbie et al. 2024).
The Kasewe IPA is also a nationally significant site for the threatened and useful tree Guibourtia copallifera (VU), holding the largest population in Sierra Leone (Johnny et al. 2023). Other notable narrow-range West African endemic plants that may be of high conservation importance at the site include the herb Aeschynomene deightonii (NT), as well as the unassessed climber Vahadenia caillei and shrub Psychotria albicaulis. Milicia regia (VU) has also been recorded at the site (Fayiah et al. 2018), however this has not been included in the assessment as there is no herbarium voucher to evidence this record. Further studies will likely uncover other conservation important species at this site.

The Kasewe Forest IPA occurs within the Western Guinean lowland forest biome and biodiversity hotspot (Olson et al. 2001). The original vegetation thought to occur at the site is moist closed forest, however human activities have significantly modified vegetation structure and composition (Cole 1968). The dominant ‘natural’ vegetation extant today is secondary tropical forest, which contains a mosaic of moist semi-deciduous forest, evergreen forest, savanna to grassland (Koroma et al. 2021, Johnny et al. 2022a). In forest, the dominant tree species are Monodora undulata (Annonaceae) and Parinari excelsa (Chrysobalanaceae) (Lebbie et al. 2023). Where the forest has been cleared, vegetation at various successional stages occurs, from ‘farmbush’ to thicket and secondary forest (Cole 1968). Other intensive disturbance has led to the collapse of forest ecosystems, with such land use categories including palm oil plantations and rice crops. Low altitude lateritic bowal grasslands occur at the site, and botanical survey is underway to characterize these areas (Lebbie et al. 2024).
The Kasewe Hills are locally unique and have a complex geology with extensive karst systems (Bowden 1980). The hills are composed of metamorphosed volcanic rock, such as andesitic to dacitic lava and tuff from the Proterozoic age (Bowden 1980, Goodenough et al. 2018). These hills rise to c 470 m above sea level above the surrounding plains of meta-sedimentary rock (Bowden 1980). The extensive foot slopes that have developed around the base of the hills have complex features characterized by an assemblage of subterranean and surficial landforms such as pipes, caves and collapse gorges (Bowden 1980). The foot slopes have a thick (up to 5m) crust of laterite. This hard lateritic rock is undermined by the development of subterranean cave systems by softer rock that contributes to the karst erosion. The meta-volcanic and meta-sedimentary derived soils present at the site are moderately fertile.
The climate is seasonal with a strongly diurnal rainfall pattern. There is a six-month rainy season (May to October) and six month dry season (November to April). The area is characterized by high temperatures with an average monthly temperature of between 26-28oC from June to October with a maximum temperature of 32oC from November May, and the average annual rainfall ranges between 2500 - 2800 mm (L’Hôte & Mahé 1995).

This site is one of the most severely threatened IPAs in Sierra Leone. Since it was first gazetted in 1919, the Kasewe Forest Reserve (which overlaps with 95% of this IPA) has been exploited for resources like timber, charcoal, gum copal, other non-timber forest products and bush meat (Johnny et al. 2023). The rebel war (1991-2001) and the Ebola outbreak (2014-2015) had detrimental impacts on the region’s natural resources, particularly affecting forest reserves such as Kasewe. During these periods of instability, population densities around forest reserves increased as communities sought safety and livelihoods. Over the years, these impacts have expanded and intensified to unsustainable levels. Activities such as farming, logging, charcoal burning, seedling harvesting and hunting have been observed to be current threats to the IPA, resulting in clearing, fragmentation and alteration of species composition and structure of vegetation at the site (Faiyah et al. 2018, Johnny et al. 2023, Lebbie et al. 2023, Lebbie et al. 2024).
Exploitation rates have increased at the IPA in recent years (Johnny et al. 2022a, Johnny et al. 2022b, Google Earth 2024). The Bo to Freetown highway which runs along the southern and southwestern border of this IPA is one of the busiest provincial roads in the country, providing easy access for exploitation and transport of resources from the reserve (Johnny et al. 2022a). Trunks from the tree species Margaritaria discoidea, Harungana madagascariensis and Anisophyllea laurina are preferentially cut for firewood (Koroma et al. 2021), though these taxa are not currently of global conservation concern, while the threatened tree Guibourtia copallifera (VU) is preferentially logged for charcoal production. These disturbances reduce tree cover and increase the availability of light, thereby facilitating the dominance of pioneer species and incursion of invasive species (Johnny et al. 2023).
Wild populations of the threatened plant, Coffea stenophylla, are under significant strain due to unsustainable harvesting at the site (Lebbie et al. 2024). Local production of highland coffee is reliant on extracting seedlings directly from the reserve, leading to a significant decline in the species’ recruitment. While this coffee species has only recently ‘rediscovered’ in the wild in 2018, there is significant interest in it’s potential to help futureproof the coffee industry against climate change, leading to pressure and decline of wild populations at the site (Chadburn & Davis 2017, Lebbie et al. 2023). Urgent action is needed to minimize harvesting pressure and ensure sustainable management of this species, which will hopefully result from a nursery is under construction at the site (A. Davis pers. comms. 2024). A population analysis of wild C. stenophylla was recently undertaken the site (Lebbie et al. 2023). This study found that while all forest plots contained C. stenophylla, population structure varied with disturbance intensity. Mature individuals were found in only 59% of plots, with fewer stems recorded in degraded forests than moderately intact forest. Sites devoid of this species were under intense charcoal production as well as some regenerating farm bush. C. stenophylla was most abundant in forest that was lightly disturbed due to selective logging in the past, thus intermediate disturbance regimes may be favored by this species (Lebbie et al. 2023
Another threat to the IPA is altered fire regimes due to the prevalence of anthropogenic fire. Activities such as slash-and-burn agriculture and charcoal production have significantly increased at the instance of fire within the IPA. As a result, novel and persistent non-forest vegetation communities such as savanna have been created, despite this IPA being climatically suitable for moist tropical forest (Dwomoh & Wimberly 2017). There have been some small-scale efforts to restore forest (Green Scenery 2020).
Active management of the site is recommended to mitigate risks to the remaining intact habitat, This includes clear demarcation and monitoring of conservation areas are conducted the forestry department (Abu-Bakarr 2015, Johnny et al. 2022a). Additionally, there is need for greater educational awareness of the site’s value and botanical significance, particularly targeting charcoal burners and Coffea stenophylla seedling collectors (Lebbie et al. 2023).

Ecosystem services are well documented at the site. The site serves as an important watershed for surrounding communities, with several unnamed watercourses and one seasonal lake originating from the hills. The Kasewe Hills promote an orographic rainfall effect for the area, and existing forest cover provides watershed protection services (Johnny et al. 2023). Additionally, the site provides resources and livelihoods for neighboring communities through harvesting resources such as charcoal, timber and highland coffee propagules (Koroma et al. 2021). This IPA is an important genetic reservoir for Coffea stenophylla (Davis et al. 2021). The ethnobotanical importance of the site to local communities has been well researched by Johnny et al. (2022a). This site holds 128 medicinal plant species belonging to 46 families, which are used to treat 42 human ailments (Johnny et al. 2022a). Due to the sparse availability of health services are sparse in the vicinity, local communities rely on the reserve as a source of bush medicines. Additionally, the site holds significant cultural value, with the gallery forest 1km N of the town of ‘Vaama’ cited as being used as sacred forest (Bowden 1980). This site is not known to be utilized for tourism.

Gabriella Hoban, Royal Botanic Gardens, Kew

Xander van der Burgt, Royal Botanic Gardens, Kew

Charlotte Couch, Royal Botanic Gardens Kew

Jonathan Johnny, Njala University College

Martin Cheek, Royal Botanic Gardens Kew

Aiah Lebbie, Fourah Bay College

Fanny Massally, Njala University College

Samuel Sokpo, Njala University College

Momoh Plato Sesay, Njala University College