Shrubs as nurse species for plant communities in arid environments: a case study from Socotra Island (Yemen)

Question: Plant–plant facilitation is a critical ecological mechanism in arid environments, influencing biodiversity and ecosystemresilience globally. Shrubs often serve as nurse species, enhancing tree regeneration and sheltering plant communities, particularlyin overgrazed or degraded habitats. In this study, we examine the role of shrubs as nurse species in the Socotra Archipelago (Yemen),a biodiversity hotspot in which several endemic tree species, including the iconic frankincense (Boswellia spp.), myrrh (Commiphoraspp.), and Socotran dragon's blood (Dracaena cinnabari) trees, are threatened. This is largely due to a lack of natural regenerationcaused by the combined effects of overgrazing by goats, sheep, and climatic events such as extreme droughts and cyclones. To aidconservation of threatened trees in arid regions, nature-based solutions are urgently needed to help tree regeneration. Effective nurseplants have this potential, particularly in arid environments. We therefore examined the role of thorny, poisonous, and/or unpalat-able shrubs as nurse plants in sheltering threatened plant communities, with a focus on woody species in an arid insular context.Study Area: The Socotra Archipelago (Yemen) situated in the western Indian Ocean, east of the Horn of Africa. It is a biodiver-sity sanctuary and a UNESCO Natural World Heritage Site.Methods: Vegetation surveys were conducted in 144 paired plots under six common shrub species and adjacent open areas.Community data, environmental variables, and functional traits were analysed using RLQ and fourth-corner analyses, whileLinear Mixed Models (LMMs) evaluated the effects of environmental variables and nurse species on key functional traits basedon Community Weighted Means (CWMs). Facilitation effects were quantified using the Relative Interaction Index (RII).Results: Our analysis revealed significant variations in species composition and functional traits between under-canopy andopen-area plots. Certain shrubs, such as Cebatha balfourii, facilitated significantly higher species richness under its canopy com-pared to open areas. Elevation and grazing pressure influenced these interactions, with notable effects on functional traits like the occurrence of legumes and tree growth forms. Buxus hildebrandtii was less effective in supporting species richness, while C.balfourii, Lycium sokotranum, and two Cissus species exhibited significant positive facilitation. The LMMs confirmed the impor-tance of environmental variables and nurse shrub characteristics in shaping plant community dynamics.Conclusions: The results highlight differences in the facilitative potential of the studied species, with some showing a strongerability to act as nurse shelters through microhabitat amelioration and protection from herbivory. The presence of tree speciesunder shrubs is confirmed, and the role of these nurse species in supporting diverse plant communities provides critical insightsfor conservation strategies, supporting biodiversity resilience and sustainable management in degraded landscapes like SocotraIsland and other arid environments. Future efforts should focus on leveraging nurse shrubs to mitigate environmental pressuresand enhance ecological restoration, in particular to help conserve range-restricted and threatened plant species.