Expansion of many tree species lags behind climate-change projections. Extreme storms can rapidly overcome this lag, especially for coastal species, but how will storm-driven expansion shape intraspecific genetic variation? Do storms provide recruits only from the nearest sources, or from more distant sources? Answers to these questions have ecological and evolutionary implications, but empirical evidence is absent from the literature. Hurricane Irma provided an opportunity to address this knowledge gap at the northern range limit of the neotropical black mangrove (Avicennia germinans) on the Atlantic coast of Florida, USA. We observed massive post-hurricane increases in beach-stranded A. germinans propagules at, and past, this species’ present-day range margin when compared to a previously-surveyed, non-hurricane year. Yet, propagule dispersal does not guarantee subsequent establishment and reproductive success (i.e., effective dispersal). We also evaluated prior effective dispersal along this coastline with isolated A. germinans trees identified beyond the most northern established population. We used 12 nuclear microsatellite loci to genotype hurricane-driven drift propagules from nine sites (n = 896 propagules) and isolated trees from four sites (n = 10 trees), determined their sources of origin, and estimated dispersal distances. Almost all drift propagules and all isolated trees came from the nearest sources. This research suggests that hurricanes are a prerequisite for poleward range expansion of a coastal tree species and that storms can shape the expanding gene pool by providing almost exclusively range-margin genotypes. These insights and empirical estimates of hurricane-driven dispersal distances should improve our ability to forecast distributional shifts of coastal species.

Genotype data (drift propagules) – 896 Avicennia germinans (black mangrove) drift propagules collected at nine beach survey sites along the Atlantic coast of Florida, USA on 14-16 October, 2017 (five weeks after Hurricane Irma made landfall in Florida).

Genotype data (vagrant trees) - 10 isolated A. germinans trees from four locations beyond the most northern established population of this species along the Atlantic coast of Florida. Samples were collected between July 2016 and June 2017.

Genotype data (reference source populations) - Reference data set of 12 A. germinans source populations that encompass the entire Florida distribution of this species. This data set used inter-population FST = 0.1 to reduce the 32 Florida collection sites in the Kennedy et al (2020) data set: https://doi.org/10.5061/dryad.69p8cz8xh

Genotype data (drift propagules) – column 1: site (identification code for collection site); column 2: ID (identification code for sampled propagule); column 3: lat (latitude at central point for each survey site); column 4-27: multi-locus microsatellite genotype (12-loci). *NOTE: blank cells are missing data

Genotype data (vagrant trees) – column 1: site (identification code for collection site); column 2: ID (identification code for sampled tree); column 3: lat (latitude for sampled tree); column 4-27: multi-locus microsatellite genotype (12-loci)

Genotype data (reference source populations) – column 1: site (identification code for collection site); column 2: ID (identification code for sampled tree); column 3-26: multi-locus microsatellite genotype (12-loci). *NOTE: blank cells are missing data