Coastal restoration entails carbon‐biodiversity trade‐offs where upslope invasive plants harm mangrove trees but benefit their seedlings

Abstract Introduction Mangroves and their services face threats from sea level rise and invasive species. Although few invasive plants tolerate mangrove habitats, many more grow just upslope, with impacts on carbon cycling and biodiversity in either habitat. How ecosystem services change with coastal restoration may depend on how upland invasive trees compete with adjacent mangroves. Objectives To determine the nature and extent of upslope invasive trees' competitive effects on mangroves and compare restoration impacts on biodiversity and carbon storage, we pursued three objectives. First, we evaluated whether upslope invasive vegetation suppresses adjacent mangroves. Second, we tested for mangrove competitive release with experimental invasive removal. Third, we quantified the impacts of different restoration techniques on carbon stocks across the coastal ecotone. Methods We measured how mangrove trees, seedlings, and detritus varied with upslope invasive tree biomass and responded after a controlled, replicated restoration experiment. Results Before restoration, mangroves had less litter and lower soil carbon near large invasives, while after restoration, mangroves grew faster and gained more soil carbon where invasive trees were removed, supporting competition across the coastal ecotone. However, mangrove seedlings responded differently, with higher abundance near invasives before restoration and significant declines thereafter. Shortly following restoration, mangroves had gained some carbon, but much less than had been lost with invasive removal, especially where removal techniques significantly increased upslope seedling biodiversity. Conclusions Coastal invasive trees affect mangrove trees and seedlings differently so that restoration generates short‐term carbon‐biodiversity trade‐offs with implications for coastal sea level rise resilience.