Diversity of Ultramafic Vegetation on Gag Island

Abstract

Abstract The global distribution of ultramafic rocks spans many regions worldwide. Within the Indonesian archipelago, ultramafic formations are concentrated mainly in Sulawesi and extend eastward to West Papua, including Gag Island. A commonly cited characteristic of ultramafic vegetation is its high level of endemism. In this study, land-cover classification was performed using Sentinel-2A imagery and subsequently verified in the field using a sampling-plocot approach. The results show that species composition and overall diversity levels across land-cover types were relatively similar. On Gag Island, ultramafic vegetation is broadly divided into two principal land-cover types: shrubland and secondary forest. Shrubland typically occurs on ridges and hilltops, whereas secondary forest is found in valleys, particularly along channels and riversides. Shrubland was dominated by Parastemon versteeghii, Ploiarium sessile , and Calophyllum inophyllum , while secondary forest was primarily dominated by Dillenia ovalifolia, Calophyllum inophyllum , and Syzygium stipulare . Marked differences were observed in species richness and dominance between these two land-cover types. Additionally, several variants of the genus Calophyllum were recorded in the secondary forest and appear to play a significant ecological role. Nevertheless, species specificity within the ultramafic vegetation was not strongly pronounced. Several species commonly found in non-ultramafic habitats were also present here and, in some cases, were dominant. This indicates that, although ultramafic substrates impose distinct edaphic and ecological constraints, they do not completely restrict the occurrence or proliferation of more widespread, adaptable species.

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