Invasive sweetclover (Melilotus alba) impacts native seedling recruitment along floodplains of interior Alaska.
Sweetclover (Melilotus alba) is a non-native legume that has formed dense and extensive patches along several rivers in Alaska. Our research objective was to determine if sweetclover impacts recruitment of native seedlings in floodplain habitats. To determine if sweetclover impacted recruitment, we conducted a removal experiment along two rivers in interior Alaska. When compared to areas where sweetclover was removed, areas with sweetclover had approximately 50% greater mortality of native seedlings, 25% less recruiting species, and a significant reduction in the quantity of light available to seedlings on floodplain surfaces. To determine if sweetclover shading was a mechanism that limited seedling recruitment, we grew eight common early-successional floodplain plant species in a greenhouse under a range of lighting conditions that were representative of shading under sweetclover. We observed that species restricted to the earliest seral stages of floodplain succession experienced greater reductions in biomass than species that persist into later stages of floodplain succession. Shading seedlings in a greenhouse did not lead to mortality during the growing season. However, when seedlings were over-wintered, we observed that greater shading during the growing season can result in higher seedling mortality. Our study indicates sweetclover invasions have created a novel shade environment in early seral floodplain plant communities, which has the potential to alter community composition. To preserve biodiversity and structure of plant communities, sweetclover should be actively managed to prevent its spread onto additional glacial rivers in Alaska.