In order to map the full habitat of S. mascatense in Oman, herbarium records from the Royal Botanic Garden Edinburgh (E), the Oman Botanic Gardens (OBG), the Oman National Herbarium (ON), and the Sultan Qaboos University College of Science Herbarium (SQUH) were added to records from previous studies, unpublished work, and the quadrat locations from the current study (Fig. 3 and Appendix A). The bulk of the population distribution is in the Western Hajar Mountains between Jebel Akhdhar in the east and Jebel Shams to the west. Small isolated populations exist on Jebel Aswad and are scattered elsewhere throughout the Eastern Hajar Mountains. In Dhofar, southern Oman, several scattered populations have been documented on Jabal Al Qamar and Jabal Samhan. No specimens are known to exist in the Western Hajar Mountains from Jebel Misht north to Musandam.
The surveys took place between June of 2018 and August of 2019. Quadrats were surveyed in June, July, and August as this is the optimal fruiting period of S. mascatense in the Western Hajar Mountains. However, one lower elevation quadrat was visited in April of 2019 in an area known for early fruiting. There were 6 quadrat locations that were impossible to physically access, so three accessible locations located in the appropriate strata were chosen to ensure that a minimum of 17 quadrats were surveyed in each stratum not including the quadrats of yellow-brown morphotype specimens. Actual elevations of the 114 quadrats surveyed in this study ranged in altitude between 1134 and 2419 m a.s.l. There were some specimens of S. mascatense found at elevations as low as 800 m a.s.l.; however, they were rare and located in protected areas with northern aspects. Throughout the study, 14 specimens of the yellow-brown morphotype were located in nine quadrats.
During the survey, the quadrats with the largest populations of S. mascatense were found at elevations of 1360–2350 m a.s.l. in the eastern half of the study area on Jebel Akhdhar (Fig. 4). Overall, a total of 513 specimens of S. mascatense were located. Of these, only 34% (172) had mature fruit. Of those 172 fruiting specimens, 92% (158) were the purple-black morphotype, and 8% (14) were the yellow-brown morphotype (Fig. 5). S. mascatense specimens ranged from 0.5-5 m in height and with driplines from 0.25-4 m in diameter.
Throughout the survey, 54 perennial non-cultivated species from 32 families were observed growing with S. mascatense. The three most species-abundant families were Asteraceae, Fabaceae, and Lamiaceae with 13.6%, 11.4% and 11.4% representation respectively. Of the species associated with S. mascatense, eight are regionally endemic, three are endemic to Oman, and two are considered rare and threatened [2]. The five most common species found were Dodonaea viscosa (L.) Jacq., Olea europaea subsp. cuspidata (Wall. & G.Don) Cif., Euryops arabicus Steud. ex Jaub. & Spach, Sageretia thea (Osbeck) M.C. Johnst., and Grewia erythraea Schweinf which were found in 80%, 63%, 56%, 51%, and 48% of the quadrats respectively. Average species richness across all quadrats of the study was 7.4. The average species richness had a weak yet highly significant negative correlation with elevation (r = -0.27) with a p-value of 0.003. Shannon’s diversity index followed a similar trend, with an average of H’ = 1.192 and a weak yet highly significant negative correlation with elevation (r = -0.23) with a p-value of 0.013. Plant names follow that of The Plant List [37], and plant family delineation came from the Angiosperm Phylogeny Group (APG IV) [38]. Phytosociological nomenclature was not used to describe the plant communities.
The first division of the TWCA was utilized for the community study. The first division produced two groups accounting for 87.5% of the variation in the data (Appendix B). With these two groups the ISA gave the most significant indicator species with the lowest average significance level. These two groups are the two main plant communities associated with S. mascatense, and fall along an altitudinal gradient. The communities were named after the two species with the highest IV in the group (Table 1). These were the Acridocarpus orientalis-Euphorbia larica community and the Olea europaea-Euryops arabicus community. Quadrats aligning with the Acridocarpus orientalis-Euphorbia larica community were found from 1134–1925 m a.s.l. where quadrats aligning with the Olea europaea-Euryops arabicus community were found between 1320–2419 m a.s.l. Both communities had instances of continuation along the altitudinal gradient due to other influencing environmental factors such as water availability, slope, aspect, and geological features present. TWINSPAN analysis corroborated the TWCA grouping. The top eight significant indicator species by IV according to ISA were the same species as those shown by the ISA of the TWCA. The only differences were the IVs and p-values for those eight significant indicator species. The dendrogram from the TWCA (Appendix B), a table of indicator species and average p-values (Table 1), and a table of indicator species from the first division of TWCA and TWINSPAN (Appendix C) are shown for reference. Using the groups indicated by TWCA, MRPP showed a highly significant difference between groups using the main matrix of presence-absence data (A = 0.20, T = 53.54, p < 0.00000001) and using the matrix of environment variables (A = 0.43, T = 55.73, p < 0.00000001). Ordination with NMDS also grouped the same two main plant communities along the altitudinal gradient (Appendix D). A two-dimensional solution was chosen for the final solution for NMDS because the mean stress in the Monte Carlo test with 250 runs was 16.91 at a p-value of 0.012. The best solution had final stress of 17.07 after 55 iterations.
Table 1
List of the 8 indicator species with the highest indicator values for the two plant communities associated with S. mascatense in the Western Hajar Mountains of Oman.
Species | IV | p* |
Group 1. Acridocarpus orientalis-Euphorbia larica community |
(17 significant indicators) |
Acridocarpus orientalis A.Juss. | 78.2 | 0.0002 |
Euphorbia larica Boiss. | 77.0 | 0.0002 |
Polygala mascatensis Boiss. | 58.9 | 0.0002 |
Grewia erythraea Schweinf. | 57.4 | 0.0002 |
Tephrosia apolina (Delile) DC. | 50.8 | 0.0002 |
Vachellia gerrardii Benth. subsp. negevensis | 50.6 | 0.0002 |
Fagonia indica Burm.f. | 48.0 | 0.0002 |
Ochradenus arabicus Chaudhary, Hillc. & A.G.Mill. | 45.5 | 0.0002 |
Group 2. Olea europaea-Euryops arabicus community |
(8 significant indicators) |
Olea europaea subsp. cuspidata (Wall. & G.Don) Cif. | 80.2 | 0.0002 |
Euryops arabicus Steud. ex Jaub. & Spach | 75.3 | 0.0002 |
Dodonaea viscosa (L.) Jacq. | 73.0 | 0.0002 |
Teucrium mascatense Boiss. | 50.6 | 0.0002 |
Sageretia thea (Osbeck) M.C. Johnst. | 48.8 | 0.0006 |
Helianthemum lippii (L.) Dum.Cours. | 32.9 | 0.0008 |
Juniperus polycarpos var. seravschanica (Kom.) Kitam. | 32.9 | 0.0014 |
Ephedra pachyclada Boiss. | 18.8 | 0.0238 |
* p is the significance level of the indicator species according to the Monte Carlo test. |
The Acridocarpus orientalis-Euphorbia larica (Ao-El) community describes most of the lower S. mascatense habitat range with an average elevation of 1345 m a.s.l. The habitat ranges from steep shale-covered hillsides, rolling hills and summits, as well as the occasional wadi (dry river bed). The soil structure of this community varies from very rocky in places to rocky-clay-silt packed in runnels between exposed rock slabs. Quadrats surveyed at the higher elevation limits of the Ao-El community were located on steep hillsides with little access to ground water. This was the most diverse community associated with S. mascatense with an average species richness of 8.7 and a Shannon’s diversity index of H’ = 2.08. It also held the most diverse quadrat surveyed in the study with a species richness of 17 and a Shannon’s diversity index of H’ = 2.83. The top five species in the Ao-El community are Grewia erythraea, Acridocarpus orientalis, Euphorbia larica, Polygala mascatensis, and Vachellia gerrardii subsp. negevensis which occurred in 83%, 79%, 79%, 69%, and 62% of the sample plots respectively.
The Olea europaea-Euryops arabicus (Oe-Ea) community, with an average elevation of 2067 m a.s.l., associates with S. mascatense at the higher elevation limits of its habitat. The habitat ranges from gently rolling hills, hillsides with moderate to steep slopes, to areas near or in wadis. Many of the quadrats at the lower elevation limits of the Oe-Ea community were located in wadis or on steep protected north-facing hillsides. The soil varied from very rocky, rocky gravel, to rocky-clay-silt packed in runnels between exposed rock slabs. The average species richness for the Oe-Ea community was 6.96 with a Shannon’s diversity index of H’ = 1.86. The top five species according to their relative frequency in the Oe-Ea community are Dodonaea viscosa, Olea europaea subsp. cuspidata, Euryops arabicus, Sageretia thea, and Teucrium mascatense which occurred in 96%, 84%, 75%, 62%, and 51% of the sample plots respectively.