Taxonomic Notes
For more taxonomic information see Thulin (2020).
Justification
Boswellia popoviana is a frankincense tree species endemic to Socotra Island (Yemen). This tree roots on rock only, and it is distributed predominantly along sea-facing steep limestone cliffs around the island and found on slopes in several valleys penetrating towards the granite mountains of the island. The species is mainly present in the eastern half of Socotra Island besides two relictual small stands in the west, and absent it is in the northwestern and southwestern coastal cliffs.
Boswellia popoviana has a restricted geographic range occurring only in Socotra Island, with an extent of occurrence (EOO) of 1,879 km², and an area of occupancy (AOO) of 204 km². Its distribution is relatively wide but fragmented. 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), leading to an inventory of 2,094 trees of B. popoviana in Socotra. Growing in several inaccessible rocky slopes at altitudes of c. 10–1,000 m asl, the total global population in the wild can be currently estimated at 4,000 mature individuals.
Boswellia popoviana is currently (2020–2023) found in about 20 different sites on Socotra (Maděra et al. 2024), corresponding to five locations and as many subpopulations, however many of these stands are extremely isolated and consist of only a few trees. Several relictual stands have strongly different morphologies (some are likely hybrids), which are now being examined in detail (e.g., in the southwest and southeast). The largest stand of B. popoviana is along the cliffs in Ayhaft Valley, where likely up to half of the total of individuals on the island, is present.
Threats for this species were examined in detail in 2020–2023 (Maděra et al. 2024), and consist of overgrazing (62.5% of the area where the species occurs), infrastructure (next to roads) (30%), and to a lesser extent the cutting of branches for fodder (16.5%), and resin harvesting (22.7%). The entire population is affected also by recent climate change effects in Socotra (e.g., cyclones in 2015 and 2018) causing flash floods, landslides, storms and droughts in the habitats. This is visible for example in Wadi Derhor (Firmihin), where landslides occurred in 2018–2019, affecting local B. popoviana populations. It is not a species that is commonly used for resin extraction, as the resin is considered of low quality locally (Miller and Morris 2004). More info on distribution and threats in Maděra et al. (2024).
Conservation efforts started in 2020 for B. popoviana, resulting in increased awareness, local seed germination and reforestation (in Qatariyah) and are ongoing.
The species is listed as Endangered due to its restricted distribution in five location, continuing decline is caused by threatened by climate change, development, overgrazing, collection for fodder and illegal collection.
Geographic Range Information
Boswellia popoviana is endemic to Socotra Island. It is locally widely distributed on the island yet stands are highly fragmented, mostly found in sea-facing steep limestone cliffs and along several valleys where it penetrates towards the granite mountains. Boswellia popoviana is absent in the northwestern and southwestern coastal cliffs, and there are several highly isolated small (relictual) over-mature stands in the extreme southeast and southwest, consisting of only a few trees. Boswellia popoviana has a restricted geographic range in Socotra with an extent of occurrence of 1,879 km2, and an area of occupancy of 204 km2. It is found in five locations, some of the sites have only a few individuals left and are rapidly declining.
Population Information
Boswellia popoviana is relatively locally widespread in Socotra but with a strongly fragmented distribution and several stands are strongly isolated and relictual (showing also morphological differences). Due to its ecology (cliff-rooting), this tree is difficult to inventory in detail, therefore our current knowledge on the exact distribution and size of subpopulations is based on estimates (detailed field surveys 2020–2023 by the Franklinia Project Team; Maděra et al. 2022, Maděra et al. 2024). There are about 20 sites on the island where this species occurs, corresponding to five major areas, but the exact number and structure of subpopulations and potential hybridisation is currently unclear (genetic studies are being carried out). The largest core population of B. popoviana is on the cliffs in the beginning of Ayhaft valley where recent surveys counted about 1,184 trees, more than one half of all currently known individuals, and the total here is estimated at about 2,000 individuals. In total, the entire global population of B. popoviana contains about 2,094 trees counted in recent field surveys (Franklinia Project Team, 2020–2023; Maděra et al. 2024). The inaccessible habitats where this species occurs, makes mapping difficult, and the total population size may well be about 3,000 to 4,000 individuals (Maděra et al. 2024).
Generally, in the northern coastal cliffs of Socotra Island, more abundant populations occur with tree-like growth forms. In the southern cliffs, there are trees with different overhanging and more dwarf-like growth forms. In the southeastern valley of Kilisan, trees are morphologically different (under investigation). In general, there is a high morphological variability between the B. popoviana subpopulations, which is part of ongoing research to assess intraspecific variation. The population structure contains the highest number of young trees among all Boswellia species on Socotra Island (Attorre et al. 2011), so there is some natural regeneration. Due to its ecology, the population trend is likely decreasing, as climate change impacts and landslides could affect the species strongly in the future.
In places where populations of B. popoviana are in the proximity of other Boswellia species, hybrids occur (for example with B. socotrana subsp. aspleniifolia, B. dioscoridis and B. elongata).
Habitat and Ecology Information
Boswellia popoviana is a small tree or climbing shrub up to 7 m in height and up to 65 cm in dbh (diameter at breast height). The species is scattered on mostly sea-facing steep limestone cliffs and along several valleys penetrating far into the centre of the island. It is found in dry, semi-deciduous woodland, at an altitude of 13–987 m.
Boswellia popoviana is an exclusively rock and cliff-dwelling species, distinguished by its simple leaves (sometimes lobed at base), densely puberulous beneath, with regularly crenate margins, but also specimens with pinnate leaves are found. Blooms from March to April (less in October/November) with red flowers in long panicles. Fruits ripen mainly in May and June. Recent in situ research has shown that the rate of empty and fully developed seeds is highly variable between years and localities, in some cases almost 100% empty seeds were recorded; seed germination rates can reach over 50% (Hamdiah et al. 2022). Therefore, the ability for natural regeneration is retained and seedlings occur in rocky crevices, hardly accessible to goats. Notes on morphology, ecology and distribution in Miller (1992, 1997), Thulin and Al-Gifri (1998), Miller et al. (2004), Attorre et al. (2011), Lvončík et al. (2013) and Thulin (2020), now distribution and ecology are revised after detailed surveys in situ (Maděra et al. 2024).
Threats Information
The endemic Boswellia species of Socotra undergo general challenges to the archipelago’s terrestrial biodiversity as discussed in Van Damme and Banfield (2011), Attorre and Van Damme (2020), Madĕra and Van Damme (2022). The challenges specifically to the endemic Boswellia in Socotra, including B. popoviana, are reviewed in Maděra et al. (2024).
The specific threats to Boswellia popoviana have been assessed in detail in situ in 2020–2023 (Franklinia Project team; Madĕra et al. 2022, Madĕra et al. 2024). The species is threatened by grazing (by goats), the estimated proportion of the population affected is 62.5% (42.7% high, 13.6% middle and 6.2% low intensity). In addition, the species is threatened by road construction, (29.9% of the population is in the direct vicinity of roads or roadworks), by resin harvesting (22.7% of the population) and by branch cutting for fodder (16.5%, mostly with low intensity).
Manifestation of global climate change (heavy rains, long periods of drought) likely affects the population in a negative way but there is little direct evidence for this yet, except in Wadi Derhor/Esgego where recent landslides have occurred due to heavy rains. Natural regeneration is present in some stands, but not in all (e.g., not in very isolated stands). The species has also been illegally exported and sold in international markets by succulent collectors recently. More information in Maděra et al. (2024).
Use and Trade Information
This species is not intensively used locally (Miller and Morris 2004). Resin is collected from Boswellia popoviana only occasionally, as this is considered lower quality in comparison to several other local species of Boswellia. The inaccessible habitats where the species is growing also makes broader usage difficult. Boswellia popoviana has also been illegally exported recently and sold in international markets by succulent collectors.
Conservation Actions Information
Specific conservation efforts for this threatened tree species (and other endemic Boswellia in the Socotra Archipelago) started in 2020, including the establishment of local nurseries, awareness, local seed germination and reforestation. One reforestation area for B. popoviana has been supported in Qa’arah in 2020, 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) to produce seedlings from the adjacent cliffs in the southern part of the island.
The core population in Ayhaft valley, which may constitute half of the entire global population of B. popoviana, should be formally protected (this valley is one of the richest on biodiversity on the island and contains large populations of the other four Boswellia species, B. ameero, B. socotrana subsp. aspleniifolia, B. elongata, B. socotrana subsp. socotrana).
The inaccessible habitats partly conserve the species, but future conservation efforts need to take into account the high variability (and strong isolation) of subpopulations and potential cryptic diversity. A species action plan and monitoring of the existing populations and their threats are important. More information in Maděra et al. (2024).
