II.II.IV. Commiphora planifrons (Schweinf.) Engl.

Commiphora planifrons is a tree species endemic to Socotra Island. The species is phylogenetically affiliated with the Spinescens clade as defined by Gostel et al. (2016), as its DNA sequences were included in the molecular analyses supporting the circumscription of this clade.

The species is a small tree reaching up to 4 m in height, characterised by horizontally spreading branches that form a flat, umbrella-like crown. The trunk is covered by a light grey bark, often extensively colonised by lichens; the bark is rough and exfoliates in small sheets. When damaged, it exudes an amber-coloured resin with a strong aromatic scent. Leaves are alternate and clustered on short spur branches; they are pinnately compound, composed of 9–23 overlapping leaflets. The leaflets are elliptic to ovate, concave, thinly to densely hairy, with slightly crenate and revolute margins, and measure approximately 1–1.5 mm in length. 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, where it forms local concentrations with numerous individuals in the Haggeher Mountains in the central part of the island. It is also present, though more sparsely, in other parts of Socotra, including eastern, western, and southern regions, where it likely persists as a relict of a formerly more continuous woodland that once covered much of the island. Recent field surveys have documented isolated individuals at elevations considerably lower than previously reported in the literature. Commiphora planifrons primarily occurs in semi-deciduous woodland and Dracaena woodland in the central mountains, and is somewhat less common in shrubland habitats in the west (Ma’Aleh, Qalansiyah), east (Momi), and south on the Qatariyah Plateau. The species does not exhibit a strong substrate preference and occurs on both granite and limestone. Its altitudinal range extends from approximately 160 m to 1,250 m a.s.l.

The species is grouped into 10 threat-defined locations, considering severe cyclones as the main plausible rapid threat. Subpopulations are defined based on spatial clustering of suitable habitat patches, resulting in the recognition of 12 subpopulations. A total of 308 individuals have been directly recorded. Given the fragmented distribution, the presence of suitable but unsurveyed habitat in remote mountainous areas, and the likelihood of overlooked individuals, the total population size is conservatively estimated at approximately 5,000–6,000 individuals, predominantly mature individuals, as regeneration is scarce or locally absent. Regeneration is scarce to absent across most of the species’ range, primarily due to intensive overgrazing by free-ranging goats. The population is considered severely fragmented. The area of occupancy (AOO) is estimated at 172 km². The extent of occurrence (EOO) is estimated at 2,048.8 km².

The principal threat affecting C. planifrons is intensive grazing by goats, which prevents regeneration by consuming seedlings and young plants and restricts the species to increasingly limited refugia. Resin collection does not currently pose a significant threat. Branch cutting for fodder occurs during the dry season, particularly because livestock is moved to higher altitudes where this species is most abundant. Natural threats include cyclones and associated landslides or rockfalls, which can cause significant localised mortality in mountainous habitats.

A continuing decline in area of occupancy is inferred due to the widespread failure of regeneration, which progressively restricts the species to a low number of suitable microsites. Habitat quality is also inferred to be in continuing decline as a result of overgrazing, compounded by increasing aridity and the intensification of extreme climatic events. Species distribution modelling under a high-emissions climate scenario (SSP5-8.5, 2081–2100) projects a dramatic future population reduction of approximately 97.9%, driven by a severe contraction of climatically suitable area and a consequent decline in both area of occupancy and habitat quality. This projection is based solely on climatic suitability and does not account for ongoing anthropogenic pressures such as overgrazing, and is therefore considered conservative.

According to the current IUCN Red List assessment, Commiphora planifrons is classified as Near Threatened (NT). However, based on newly collected data indicating severe regeneration failure, continuing habitat degradation, population fragmentation, and a projected extreme future decline, the species would qualify as Critically Endangered (CR) under criterion A3.

Figure: Commiphora planifrons (Schweinf.) Engl.

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