Characterizing safe sites for seedling establishment of endangered Arctomecon humilis Coville in a warm desert gypsum environment

Abstract

Plant species endemic to gypsum occur in arid and semiarid regions worldwide and are an important component of biodiversity conservation, but gypsophile habitat selection within more arid gypsum environments remains largely unexplored. Our goal in this study was to inform efforts to introduce new populations of the endangered Mojave Desert gypsophile Arctomecon humilis Coville (dwarf bear poppy) through direct seeding at more remote locations that are less impacted by threats associated with urban expansion. We characterized approximately 600 ‘safe sites’ where seedlings of this species had recently established and examined distribution of these safe sites across habitats on the gypsum cuesta landscape. Using drone imagery at the landscape scale, we found that juveniles showed an even stronger preference for the white surface cover class than adults and were also markedly under-represented in the dark crust cover class. Two-thirds of the juveniles were found on knoll tops and upper back slopes, while only 3% were found on bottoms. At the plot scale, most juveniles were associated with surfaces where lichen crust and white gravel occurred in a fine-scale mosaic. At the emergence microsite scale, two-thirds of the seedlings had emerged from narrow gaps between two or more contact substrates, most often lichen crust and gravel. Even when seedlings emerged from lichen crust alone, emergence was usually from a gap and not from the surface of intact lichen thallus. On bottoms, toe slopes, and lower back slopes where dark lichen crust is often heavy and continuous, very few juveniles were found. Fewer than 7% of juveniles were found in direct association with living adults, likely because of adaptations for both enhanced seed dispersal and a persistent seed bank. There was also no evidence for establishment facilitation by nurse plants of other species. These results are contrasted with gypsophile establishment ecology in less arid environments. They also enable drone-based screening at the landscape scale as an efficient first step in selection of introduction sites.