Differential rates of vertical accretion and elevation change among aerial root types in Micronesian mangrove forests

Abstract

Root systems in mangrove swamps have captured the attention of scientists for decades. Among the postulated roles of root structures include a contribution to the geomorphological stability of mangrove soils through sediment trapping and binding. In this study, we used feldspar marker horizons and sediment pins to investigate the influence of three different functional root types—prop roots in Rhizophora spp., root knees in Bruguiera gymnorrhiza, and pneumatophores in Sonneratia alba—on vertical accretion and elevation change in three mangrove forests in the Federated States of Micronesia. Prop roots facilitated vertical accretion (11.0 mm year −1) more than pneumatophores or bare soil controls (mean, 8.3 mm year −1). Sediment elevation, on the other hand, increased at an average rate of only 1.3 mm year 1 across all root types, with rate differences by root type, ranging from −0.2 to 3.4 mm year −1, being detected within river basins. This investigation demonstrates that prop roots can assist in the settling of suspended sediments from estuarine waters, yet prop root structures are not as successful as pneumatophores in maintaining sediment elevation over 2.5 years. As root densities increase over time, an increase in turbulence-induced erosion and in shallow subsidence as organic peat layers form is expected in Micronesian mangrove forests.