II.II.III. Commiphora socotrana (Balf.f.) Engl.

Commiphora socotrana is a tree species endemic to Socotra Island. Phylogenetically, the species is likely affiliated with the Spinescens clade as defined by Gostel et al. (2016).

The species is a small tree reaching up to 5 m in height, characterised by intricately branched, spreading crowns with spinescent branch apices. The trunk is covered by a dark grey bark, sometimes with a plum-like hue, which is often obscured by the growth of lichens. The bark is smooth and non-exfoliating. It exudes an aromatic, amber-coloured resin. Leaves are alternate and clustered on short spur branches; they are either simple or pinnately compound with three leaflets. Leaflets measure 3–20 mm in length, are obovate to oblong, and have entire to slightly dentate-crenate or shallowly lobed margins. Flowers are small (c. 4 mm), solitary, initially yellowish-green and becoming reddish with age; the species is dioecious. Fruits are ovoid to globose, measuring 6–8 × 8–10 mm, green when immature and turning dark reddish at maturity.

The species occurs on Socotra Island and is primarily distributed in the north-central to north-western parts of the island, with smaller, more isolated occurrences in the south-west towards Shu-ʿUb. Commiphora socotrana is typically found in Croton socotranus shrubland, open succulent shrubland, and drought-deciduous woodland. While the species does not show a strict substrate preference, it is most commonly associated with limestone substrates and only rarely occurs on granite. The altitudinal range of the species extends from approximately 40 m to 950 m a.s.l.

The species is grouped into 10 threat-defined locations, considering severe cyclones as the most plausible rapid threat. Subpopulations are defined based on spatial clustering of suitable habitat patches, resulting in the recognition of eight subpopulations. A total of 511 individuals have been directly recorded. Considering the extent of suitable but incompletely surveyed habitat and the likelihood of undetected individuals in remote or inaccessible areas, the total population size is conservatively estimated to be approximately 4,000–5,000 individuals, with most individuals assumed to be mature due to widespread regeneration failure caused by overgrazing. Regeneration is scarce to absent across most of the species’ range, primarily as a consequence of intensive overgrazing by free-ranging goats. The area of occupancy (AOO) is estimated at 176 km². The extent of occurrence (EOO) is estimated at 1,372.7 km².

The principal threat affecting C. socotrana is intensive grazing by goats, which prevents regeneration by consuming seedlings and young plants and progressively restricts the species to increasingly limited areas. Resin collection does not currently pose a significant threat. Branch cutting for fodder occurs locally during the dry season. Natural threats include cyclones and associated landslides or rockfalls, which can cause localised mortality, particularly in exposed habitats.

A continuing decline in area of occupancy is inferred due to widespread regeneration failure caused by overgrazing, which restricts the species to a decreasing number of suitable sites. Habitat quality is also inferred to be in continuing decline as a result of chronic grazing pressure, compounded by increasing aridity and the intensification of extreme climatic events. Species distribution modelling under a low-emissions climate scenario (SSP1-2.6, 2081–2100) projects a severe future population reduction of approximately 97.2%, driven by a marked decline in climatically suitable area and a consequent reduction in both area of occupancy and habitat quality. This projection is based solely on climatic suitability and does not incorporate ongoing anthropogenic pressures such as overgrazing, and is therefore considered conservative.

According to the current IUCN Red List assessment, Commiphora socotrana is classified as Near Threatened (NT). However, based on newly collected data indicating continuing declines in habitat quality, near-complete regeneration failure, and an extreme projected future population reduction, the species would qualify as Critically Endangered (CR) under criterion A3.

Figure: Commiphora socotrana (Balf.f.) Engl.

Figure: AOO and EOO of Commiphora socotrana showing the area of distribution of this endemic species