PNNL

Abstract

Coastal forests are increasingly vulnerable to climate change and sea-level rise, with flooding and salinity driving transitions to marsh-dominated systems. However, the extent to which elevated atmospheric CO2 and temperature modulate these transitions remains unclear. Here, we use a hydraulically enabled vegetation demographic model (FATES-Hydro) to simulate how physiological traits, demographic processes, and vegetation composition shape shoreline ecosystem responses to climate change. We conduct numerical experiments at two sites—freshwater Lake Erie and saline Chesapeake Bay—under historical and future climate scenarios involving elevated CO2 and temperature. Simulations show that elevated temperature intensifies vapor pressure deficit and hydraulic stress, increasing tree mortality across species. Elevated CO2 enhances carbon assimilation and leaf biomass but does not alleviate water stress or prevent mortality. While individual-level responses are broadly consistent across broadleaf and conifer species, differences in canopy structure and stand density lead to divergent ecosystem trajectories. In broadleaf systems, lower canopy density facilitates rapid marsh invasion, which compensates for tree loss and sustains productivity. In contrast, dense conifer stands experience higher mortality and slower marsh colonization, resulting in long-term declines in leaf area and gross primary production. These findings highlight the need to incorporate vegetation structure and mortality mechanisms into models to improve projections of coastal ecosystem function under climate change.