Data for: Plant thresholds and community composition of coastal marsh-forest ecotones in the US Northeast

Sea level rise is causing coastal salt marshes to migrate upslope into
coastal forests and other terrestrial ecosystems.  However, the
factors that control marsh migration rates are not well understood,
particularly in the US Northeast, where this phenomenon has received
little attention. To determine the relationship between environmental
variables and plant species composition in marsh migration zones, we
examined plant coverage and environmental data for three sites
experiencing marsh upslope migration in New Jersey, New York, and
Massachusetts that varied in slope from 1-3%. We found that only 10% of
the variation in plant community composition was explained by inundation
time, while models containing multiple predictor variables, including
edaphic variables, explained much greater levels of variance. Random
forest models predicting native halophyte presence /had accuracies of 69 –
84%, with salinity, flooding duration, and light availability as key
predictors. The most accurate model for mature tree presence (84%
accuracy) highlighted salinity and flooding time as the most important
variables. Threshold Indicator Taxa ANalysis (TITAN) identified plant
community changepoints at soil salinities of 0.8 and 7.6 PSU, reflecting
the lower and upper boundaries of the marsh-forest ecotone. Treelines
varied in elevation by site, suggesting that the amount of tidal flooding
that trees can withstand varies. These results highlight the importance of
multiple variables interacting to determine species distributions and
community composition in marsh-forest ecotones.