Forest thinning in the seaward fringe speeds up surface elevation increment and carbon accumulation in managed mangrove forests.
Mangroves are significant carbon (C) sinks and ecological engineers as they accumulate sediments and increase soil surface elevation. Thus, the forest management practice of thinning may not only alter forest structure, but also facilitate new biogeomorphological processes that affect soil development. Thinning may create additional opportunity for understorey species, such as the light-demanding Acanthus ilicifolius, to become a more prominent vegetation component of mangroves, which may further alter soil surface elevation trajectories. Forest structure and soil surface elevation change (SEC) were monitored along transects from the landward edge (upper intertidal) to seaward fringe before and after thinning of non-native Sonneratia apetala plantations along the seaward edge of a mangrove site in China. Soil C accumulation was also evaluated. Acanthus individuals colonized the forest gaps created by Sonneratia thinning in the seaward forest edge of the intertidal zones. In the absence of other pioneer species or other mangrove propagules/seeds, Acanthus pioneers occupied the areas rapidly (from 33.6 to 72.6 stems/m2), and were successful opportunists. Newly colonizing Acanthus vegetation significantly increased SEC from 25.1 mm/year before Sonneratia was thinned to 46.5 mm/year after thinning-induced Acanthus occupation. Furthermore, Acanthus occupation enhanced soil C accumulation at the seaward edge to 49.9 MgC ha-1 year-1; a rate twofold higher than in the Sonneratia plantation before thinning. Synthesis and applications. Prolific Acanthus growth formed small elevation mounds on the seaward forest edge, which was suspected as a positive effect induced by thinning Sonneratia plantations that further facilitated colonization of Acanthus clones. Thinning non-native Sonneratia disturbed the established zonation, altered SEC, and is facilitating further succession of these mangrove forests into un-occupied aquatic areas. Higher elevations created by Acanthus expansion (vertically and horizontally) may further promote a greater percentage of landward edge mangrove species to colonize at these more favourable intertidal elevations yielding at least short-term gains in soil carbon accumulation by altering ecosystem function. Silvicultural activity should consider unintended influences on sedimentation patterns both in situ and in adjacent habitats in tidal forests, where small elevation changes affect mangrove species habitat preferences.