Mangrove seedling freeze tolerance depends on salt marsh presence, species, salinity, and age

With increasing global average temperatures reducing the frequency and severity of freeze events, species are undergoing range expansions. As a result, mangroves are expanding into higher latitudes previously dominated by salt marsh. The success of mangroves in these new habitats may vary based on seedling temperature tolerances, seedling age, interactions with existing salt marsh habitat, and other environmental stressors. To test this, we conducted three experiments on interactions between mangrove seedlings (Rhizophora mangle, Avicennia germinans, and Laguncularia racemosa) (1 and 6 months old) and salt marsh species (Distichlis spicata and Salicornia virginica) under varying freeze occurrence and duration (0, 1, and 4 h at −5°C) across varying salinity gradients (5, 20, and 40 PSU). Laguncularia racemosa was the most susceptible to freezing conditions, followed by R. mangle and A. germinans at 100, 40, and 33% mortality, respectively. Salt marsh excluded A. germinans under freeze conditions, whereas L. racemosa only survived when salt marsh was present (reduced mortality by 60%). Increasing salinity showed a trend where the effects of salt marsh on A. germinans and L. racemosa were reduced and replaced, respectively, under freeze events. Age of seedlings also influenced susceptibility, where newly established seedlings were less susceptible to freeze conditions.