Conservation of the rare Astragalus tragacantha on the French Mediterranean coast: A case study of factors leading to transplantation success

In the face of habitat degradation and subsequent species decline, translocation has become a popular management tool to increase persistence of threatened species. Astragalus tragacantha , a rare and endangered thorny cushion shrub mainly occurs, in southeastern France, along degraded coastal habitats, near a large Mediterranean conurbation. Most individuals (88.4% of 4919) are listed inside the French Calanques National Park (Cal-NP), south of the city of Marseille. Its peri-urban location combines challenges of protection under high anthropogenic pressure. This study was carried out as part of the European LIFE Habitats Calanques project, aiming to restore fragmented coastal habitats and vulnerable plant populations. Our objective was to implement translocations to reduce the extinction risk of A. tragacantha populations. Plantlets were transplanted in the Cal-NP, in seven reinforcement (augmentation) sites and three (re)introduction ones displaying contrasted microhabitats. A subset of plantlets was regularly monitored to look for correlations between survival rates and environmental factors such as soil type, nutrient contents, or herbaceous and rocks cover. After 53 months, the average survival rate of transplanted A. tragacantha was 26%, with high variability between sites. Contrasting effects of environmental factors on plant survival versus final plant size were observed. Indeed, survival rates were higher in rocky microhabitats compared to richer soils. On the latter, plant cushions were bigger, but those habitats were less favorable due to competition with herbaceous plants. This study highlights the need to consider the influence of microhabitat on plant survival, but also plant development and reproduction, when planning the translocation of threatened plants for conservation.