Changes in mangrove tree mortality, forest canopy, and aboveground biomass accumulation rates following the 2017 hurricane season in Puerto Rico and the role of urbanization

Mangrove ecosystem responses to tropical cyclones have been well documented over the last half a century, resulting in repeated measures of tree mortality, aboveground biomass reduction, and recovery by species, size, and geomorphology. However, no studies have investigated the role of urbanization in mangrove hurricane resistance and resilience, despite increasing urbanization of tropical shorelines. This study gauges the initial response and short-term recovery of Puerto Ricos mangroves along well defined and quantified urban gradients following the 2017 hurricane season. Survival probability of tagged trees decreased with time, and the mean mortality across all sites was 22% after eleven months. Mean canopy closure loss was 51% one month after the hurricanes, and closure rates also decreased with time following the storms. Aboveground biomass accumulation decreased by 3.5 kg yr-1 per tree, corresponding to a reduction of 4.5 Mg ha-1 yr-1 at the stand level. One year later, the mangroves have recovered to 72% canopy closure and to nearly 60% of their pre-storm growth rates. No connection to urbanization could be detected in the measured dynamics. Instead, species, size and geomorphology were found to play a role. Larger trees suffered 25% more mortality than smaller size classes, and Laguncularia racemosa suffered 11% less mortality than other species. Hydro-geomorphology was also found to play a role, with forests in tidally restricted canals experiencing more canopy loss but faster recovery than open embayment systems. These findings suggest size, species, and geomorphology are important in mangrove resistance and resilience to tropical storms, and that urbanization does not play a role. Managing mangrove ecosystems for optimal shoreline protection will depend upon knowing which forests are at greatest risk in a future of increasing urbanization.