Climate change effects on the potential distribution of the endemic Commiphora species (Burseraceae) on the island of Socotra

Abstract

The Socotra Archipelago (Yemen) is an interesting biodiversity hotspot, with a significant proportion of endemic species that have evolved to survive in an arid subtropical environment, inscribed as a World Heritage Site by UNESCO. The terrestrial ecosystems of Socotra face several threats, including climate change, overgrazing and soil degradation. Socotra Island has four endemic species of the genus Commiphora (Burseraceae). Little is known about their local distribution and ecology, yet these trees could be useful indicator species. Our study focuses on the distribution and niche characterisation of the four endemic Commiphora species of Socotra and how climate change may affect them. The aim is to improve insights into their habitats and to provide an essential basis for future local management plans and ecological restoration. We compared the current distribution with the forecasted potential distribution under a CMIP6 (Coupled Model Intercomparison Project) climate scenario, allowing us to define target conservation areas and assess potential local extinction risks. To achieve this, we collected distribution data in the field throughout Socotra Island, covering the current distribution ranges of the four species. To assess the potential distribution of these species, we applied three models (GAM, MaxEnt, RandomForest) using bioclimatic, topographic and soil variables. Forecasts under a climate change scenario were made using bioclimatic variables from the CMCC-CESM2 climate model for two different socioeconomic pathways. The distribution of three endemic Socotran Commiphora is mainly correlated to clay content in the soil and winter precipitation, while C. socotrana is affected by seasonal precipitation and temperature. Under different potential future climate scenarios, the distribution of C. ornifolia is predicted to remain stable or increase, while C. parvifolia distribution could increase, yet C. planifrons and C. socotrana are predicted to undergo a strong reduction of suitable areas and an upward shift in the mountains. Our results highlight that it is essential to conserve the unique terrestrial ecosystems in Socotra and to preserve these endemic trees which have a wide range of ecosystem services. Updates on the predicted extinction risk assessment are fundamental to understand conservation priorities and strategize future actions to ensure the persistence of Socotran myrrh trees and other endangered endemic tree taxa on the island.

Overgrazing strongly impedes the natural regeneration of the endemic Boswellia species on Socotra Island

“…At the same time, the terrestrial biota of Socotra is under increased pressure from global warming, unsustainable use of resources (overgrazing), and other factors (Van Damme, Banfield 2011;Attorre, Van Damme 2020). The archipelago has eleven endemic species of Boswellia and five species of Commiphora (four of which are endemic), which represents the highest global diversity of Burseraceae species found in an area (Thulin 2020;Lvončík, Řepka 2020;La Montagna et al 2023). The frankincense trees found on the Socotra Archipelago are facing similar threats affecting the other Boswellia species worldwide, and several studies have mentioned overgrazing as an important factor in their decline (Attorre et al 2011;Lvončík et al 2013).…”