Designing Open Vegetation Architecture for the Restoration of the Endangered Coastal Island Plant Iris dichotoma

Vegetation-mediated light environments and spatial structure constrain plant survival and distribution. These constraints strongly shape the persistence and range of habitat-specialist species. Iris dichotoma is a Class II Endangered Wildlife in South Korea, and occurs within restricted coastal habitats on few islands in the Yellow sea, including Daecheong Island and Baengnyeong Island. This study integrates field surveys across five natural habitats on Daecheong Island, two artificial habitats on Baengnyeong Island and greenhouse experiments to identify the structural drivers of its spatial restriction. Field surveys showed that population size and reproductive performance were tightly. Field surveys showed that population size and reproductive performance were tightly linked to light availability shaped by vegetation architecture. Open coastal habitats with high relative light intensity (RLI) supported reproductive organ formation in 91.67% of individuals, whereas vertically stratified sites with reduced RLI (31.14% and 18.92%) exhibited complete reproductive failure. Greenhouse experiments using open-top chambers (OTC) with controlled light conditions confirmed strong shade intolerance. Under 30% light reduction, plant height declined from 49.7 ± 4.6 cm to 33.8 ± 1.5 cm, chlorophyll index and leaf width decreased significantly, and the median number of flower stalks fell from 4 (IQR 2.5~6.5) to 2 (IQR 1~2), with markedly reduced cumulative flowering. These results show that structural light limitation, rather than abiotic stress, governs population persistence. We therefore propose vegetation-architecture based restoration, emphasizing the design and maintenance of open, light-secured habitats, as a practical ecological engineering strategy for endangered island plant conservation.