While much attention has been paid to the climatic controls of species’ range limits, other factors such as dispersal limitation are also important. Temperature is an important control of the distribution of coastal mangrove forests, and mangrove expansion at multiple poleward range limits has been linked to increasing temperatures. However, mangrove abundances at other poleward range limits have been surprisingly insensitive to climate change, indicating other drivers of range limitation. For example, along the west coast of North America, the poleward mangrove range limits are found on the Baja California and mainland coasts of Mexico, between 26.8 - 30.3°N. Non-climatic factors may play an important role in setting these range limits as 1) the abundance of range limit populations has been relatively insensitive to climate variability and 2) an introduced population of mangroves has persisted hundreds of kilometers north of the natural range limits. We combined a species distribution model with a high-resolution oceanographic transport model to identify the roles of climate and dispersal limitation in controlling mangrove distributions. We identified estuarine habitat that is likely climatically suitable for mangroves north of the current range limits. However, propagules from current mangrove populations are unlikely to reach these suitable locations due to prevailing ocean currents and geomorphic factors that create a patchy distribution of estuarine habitat with large between-patch distances. Thus, although climate change is driving range shifts of mangroves in multiple regions around the world, dispersal is currently limiting poleward mangrove expansion at several range limits, including the west coast of North America.

Particle transport simulation

Propagule trajectories were computed using surface-ocean current data from a mesoscale and tide-resolving configuration of the Massachusetts Institute of Technology general circulation model (MITgcm). The simulation was carried out in a latitude-longitude-polar cap (LLC) configuration with a polar cap that has 4320 grid cells on each side (hereafter referred to as LLC4320). There are 243 million horizontal grid cells and 90 vertical (depth) levels for a total grid count of 2.2 ^ 10¹⁰. The model has a nominal horizontal grid resolution of 1/48°, which ranges from 0.75 km near Antarctica to 2.2 km at the Equator, and a vertical grid spacing of 1 m near the surface to better resolve surface currents and diurnal cycles.

A Lagrangian approach was used to compute particle propagation by linearly interpolating the zonal and meridional LLC4320 surface-ocean velocities and using a first-order Euler time-stepping method. Vertical motion was neglected, which is a reasonable assumption for mangrove propagules that are buoyant and generally remain on the ocean surface. Release locations were generated from the mangrove extent data described above. Release locations were shifted to the closest ocean grid cell (hereafter called ‘coastal grid cells’) and potential duplicate locations removed. Particles were released hourly at 589 coastal grid cells from 13 September 2011 to 13 November 2011  (i.e., 1,448 time steps in total), which coincides with the period of reported propagule availability in the region. Simulated propagules were tracked over 12 months, which was the maximum floating period considered.

From the Lagrangian particle trajectories, dispersal trajectory density maps were generated by aggregating all modeled particle trajectories on a 1/48° resolution grid (i.e., the native grid resolution). We then tracked all the particles that stranded, i.e., reached an ocean cell adjacent to a land grid cell within the respective floating period. To account for the role of habitat suitability in our assessment of mangrove range limit dynamics in the region, we combined dispersal simulations with our coastal wetland layer. More specifically, we susbetted the strandings that were in grid cells potentially suitable for mangrove establishment and growth, i.e., either a saltmarsh or mangrove cell.