Hurricanes help mangroves move northward in a changing climate

The rising average temperatures and erratic weather conditions brought on by climate change are shifting the potential ranges of many tree species. Yet the migration of trees into newly available areas lags behind. Extreme storms such as hurricanes, however, can overcome these limitations for some coastal species, moving propagules well into new territory. Thus far, we have a poor understanding of how far and with what degree of success this happens, and how it affects intraspecific genetic variation in the trees involved.

In a new article published in Molecular Ecology, lead author John Paul Kennedy and colleagues studied propagules of black mangrove transported beyond their northern range limit during Hurricane Irma in 2017, genotyping them to trace their sources. They then compared this data to propagules transported in a non-hurricane year, as well as studying so-called ‘vagrant’ trees that had already established themselves beyond the species’ typical range.

The researchers found large numbers of propagules transported by Hurricane Irma versus very few in a non-hurricane year. Nearly all the transported propagules and vagrant trees came from the nearest possible sources, at the margins of the natural range. Because the range-margin comprises less genetic variation than the range-core, propagules being dispersed represent a fraction of the total variation within the species.

On one hand, the decrease in genetic variation of a northwardly migrating population could “constrain evolutionary responses and reduce fitness in these expanding populations,” write the authors. But on the other, “[e]xpansion of range-margin genotypes, that are presumably better adapted to climatic extremes experienced beyond the current distribution, may facilitate species range expansion with climate change. Limited immigration from range-core sources may also expedite adaptation to these marginal environments.” They note that the vagrant trees they studied were not only surviving, but thriving in their new environment.“

[W]hile forecast migration of these terrestrial trees for the next 100 years is mostly restricted to 10-20 km, we found dispersal to beaches >100 km from range-margin sources after a single storm event and vagrant trees 80 km from their assigned source. This contrast is consistent with longer transport potential for coastal species and highlights that coastal range expansions have the potential to occur rapidly over large spatial scales,” the authors write, noting that black mangrove distribution could expand northward by as much as 160 km over the next half-century.

“Atlantic Florida mangrove expansion will presumably not be a progressive march poleward, and instead, this process will likely occur via a series of starts and stops driven by propagule dispersal out of estuaries and over longer distances following extreme storm events.”