Negative indirect effects of hurricanes on recruitment of range-expanding mangroves

Disturbances often have positive, direct effects on invasions by dispersing propagules or creating environmental conditions that favor invasive species. However, disturbances that alter interactions between resident and invading species could also affect invasion success. In northeast Florida, the black mangrove Avicennia germinans is expanding into salt marshes, where it interacts with the dead litter (wrack) of the native marsh cordgrass Spartina alterniflora. From 2015–2017, we performed monthly surveys before and after 2 hurricanes in 3 marsh microhabitats (bare sediment, vegetation, wrack) to quantify mangrove propagule and seedling densities. Wrack increased propagule retention up to 10 times relative to other microhabitats. Hurricanes did not directly harm mangrove propagules or seedlings. However, storm surge relocated wrack to upland environments, which indirectly inhibited mangroves by temporarily disrupting the facilitative effects of wrack on propagule recruitment and exposing intertidal bare patches that decreased propagule retention and seedling establishment. Wrack remained absent from intertidal areas for 1–3 mo. Because hurricane season overlaps with propagule recruitment, hurricane timing and wrack return time to intertidal areas influence the degree that hurricanes disrupt wrack−mangrove interactions. We demonstrate that large-scale disturbances can negatively and indirectly affect invader recruitment by altering interactions with resident species.