Taxonomic Notes
See Thulin (2020); listed as "Boswellia bullata with red flowers" in Miller and Morris (2004). Species status is being investigated (molecular investigation ongoing), there are some similarities between B. scopulorum and B. "popoviana" with pinnate leaves; B. scopulorum could be potentially be a hybrid as well.
Justification
Boswellia scopulorum is a rare rock-dwelling frankincense tree, restricted in its global distribution to a single mountain in Socotra Island (Yemen). The only population is known from Bitgobihir Mountain in the northeastern part Socotra Island. Recent surveys of current distribution, number of trees and assessment of threats were carried out in 2020–2023 by local Socotri and external specialists (Mendel University, La Sapienza, Ghent University; Franklinia-funded Boswellia project; Maděra et al. 2022, Maděra et al. 2024) in the entire Socotra Archipelago, leading to an inventory of Boswellia scopulorum of 68 trees (in 2020–2023). The total global population of B. scopulorum can be estimated currently at c. 70 trees. The species has a restricted geographic range with an extent of occurrence of 12 km2 (3.58 km2), and an area of occupancy of 12 km2 found in a single location.
Because of its very limited distribution, this species is highly vulnerable to landslides, drought and other effects of climate change, as well as to overgrazing. The most recent assessment (2020–2023) indicates overgrazing affects 91.2% of the entire population, and to a lesser extent resin harvesting (7.4%) and branch cutting for fodder (10.3%), see Maděra et al. (2024). Individuals are over-mature, i.e., virtually no young trees are found in these areas due to the strong grazing pressure (the only locality is near busy settlements and close to the city of Hadiboh).
The very small distribution area and low abundance of trees (c. 70 in the wild), threatened by climate change and overgrazing, indicate that this Socotran endemic has a high extinction risk. The species is not known for its local ethnobotanical uses. Conservation efforts started in 2020 for B. scopulorum, resulting in increased awareness and inventories.
The very small distribution area and low abundance of the trees in a single subpopulation and location, threatened by climate change and overgrazing causing continuing decline, are the reasons why this insular endemic is listed as Critically Endangered.
Geographic Range Information
Boswellia scopulorum is endemic to Socotra Island. The only population is known from Bitgobihir Mountain in the northeastern part of island. The species has a restricted geographic range with an extent of occurrence of 12 km2 (3.58 km2), and an area of occupancy of 12 km2.
Population Information
One small population of Boswellia scopulorum is known from Bitgobihir Mountain in Socotra Island, counting 68 trees in total. There are potentially more individuals in this mountain, however, it is a difficult terrain to access for detailed monitoring. Originally this population was considered as "B. bullata with red flowers” in Miller and Morris (2004), but it is also similar to B. popoviana with pinnate leaves. The population trend is unclear, because this species was only recently described (Thulin 2020). The adult population is probably decreasing as seedlings are absent which indicate goat grazing in all more accessible areas below these rock-dwelling trees, but due to its relictual occurrence in very inaccessible habitats where direct pressure by goat grazing is reduced it could be less affected.
Habitat and Ecology Information
Boswellia scopulorum is a small tree up to 6.5 m in height and up to 45 cm in diameter at breast height (dbh). This species is very rare, restricted to Socotra Island where it occurs only in one area (Bitgobihir) on limestone cliffs in dry, semi-deciduous woodland at an altitude of 81–425 m.
It is an exclusively rock- and cliff-dwelling species distinguished by its pinnate leaves with 9–13 deeply crenate-serrate leaflets (Thulin 2020). Blooms from March to April (less in October/November) with red flowers in long panicles. Fruits ripen mainly in May. Recent
in situ research showed a rate of empty and fully developed seeds of 85% and seed germination rates of up to 15% (Hamdiah
et al. 2022). Although there are many empty seeds, the ability for natural regeneration is retained.
Threats Information
The endemic Boswellia species of the Socotra Archipelago undergo general challenges to the islands' terrestrial biodiversity as discussed in Van Damme and Banfield (2011), Attorre and Van Damme (2020) and Madĕra and Van Damme (2022). The challenges specifically to the endemic Boswellia in Socotra, including B. scopulorum, are reviewed in Maděra et al. (2024).
The specific threats to B. scopulorum have been assessed in detail in situ in 2020–2023 (Franklinia Project team, Madĕra et al. 2022, Madĕra et al. 2024). The species is sensitive to overgrazing and only survives currently as a relict stand in a single location; surviving (mature) trees are largely inaccessible to goat browsing, yet seedlings are absent which indicates strong grazing pressure. This species is threatened by grazing in the area where it occurs (91.2% of the entire population, 48.5% high, 11.8% middle and 30.9% low intensity), by resin harvesting (7.4% of the population) and by direct branch cutting for fodder (10.3% of the population affected, with low intensity).
Its vicinity to the largest city in Socotra Island (Hadiboh), the busy northern roads, and high density of settlements near its location add to the pressure of (over)grazing. Climate change effects are likely also to have a strong impact in the future through extreme weather events (causing landslides, flash floods, cyclones, droughts) which affect all Boswellia in Socotra (Maděra et al. 2024).
Use and Trade Information
No local uses are known for this species except sporadic use for fodder and some indications of sporadic resin collection; the fairly inaccessible and very limited area where it occurs reduces its practical uses.
Conservation Actions Information
Boswellia scopulorum is an extremely rare species occurring in only one mountain on Socotra Island (Bitgobihir), with only c. 70 trees currently known in the wild. The taxonomy of this species is not clear, it could be a potential hybrid (DNA analysis is ongoing). Conservation efforts for this threatened tree species (and other endemic Boswellia in the Socotra Archipelago) started in 2020, including local awareness and inventory work, within the Boswellia project supported by the Franklinia Foundation (implemented by local teams in coordination with a consortium of Mendel, Sapienza and Ghent Universities and in cooperation with the local authority EPA). Replantation and reforestation efforts have not started yet for B. scopulorum. Further conservation action should include the protection of the one area in Socotra where it occurs (Bitgobihir), further local awareness and direct replantation efforts.