Experimental Evidence for Edge-Mediated Microclimatic Refuge in a Southern Range Margin Population of Silene regia

Due to widespread grassland loss, native perennial grassland species such as Royal catchfly (Silene regia) are today restricted to fragmented habitats such as roadsides and powerline rights-of-way. In Georgia (United States), the species is listed as critically imperiled, and reintroduction efforts have had limited success, highlighting the need to identify environmental factors supporting its persistence at the southern margin of its range. This study investigated whether microclimatic and edaphic factors associated with a forest-grassland edge influence the abundance, growth, and fecundity of S. regia in Georgia's (US) last confirmed wild population. We monitored 59 and 81 individual plants across the 2023 and 2024 growing seasons, respectively, in a managed powerline right-of-way where an open grassland transitions sharply into a forest. Data were collected on spatial distribution, plant height, flowering, seed capsule production, soil nutrients, and microclimate (air temperature and soil moisture). The 2024 season followed a right-of-way widening, which shifted the forest-grassland edge by approximately 2.8 m. Individuals within a 2 m wide band along the forest-grassland edge exhibited significantly greater height, flowering, and seed capsule production compared with those in the open grassland or forest interior. The location of maximum growth and reproduction shifted between years in accordance with the physical movement of the edge, providing experimental evidence for an edge effect. Soil nutrient composition did not differ significantly across the site, but daily maximum air temperature was significantly higher in the open grassland than along the edge or within the forest. Our results suggest that partial shading along forest-grassland boundaries mitigates heat stress and enhances resource allocation to growth and reproduction in S. regia, providing a climatic and competitive refuge at the southern boundary of its range.