Presence of hummock and hollow microtopography reflects shifting balances of shallow subsidence and root zone expansion along forested wetland river gradients

Abstract

Tidal freshwater forested wetlands (TFFWs) are in an active phase of transition to tidal marsh with sea level rise and salinity incursion along the Atlantic and Gulf Coasts of the United States (U.S.). A prominent feature of TFFWs is hummock/hollow microtopography where hollows represent the flat, base-elevation of the floodplain where inundation occurs relatively frequently, while hummocks provide elevated soil surfaces that often harbor relatively greater numbers and species of trees and shrubs. Hummocks appear at the landscape river boundary just seaward of bottomland hardwoods as tides reach those positions, persist for many years, and disappear as TFFWs eventually transition to marsh. We studied TFFW surface elevation processes along four Atlantic coastal landscape river gradients by using surface elevation tables and marker horizons. Shallow subsidence between trees, equating to as much as 5.5 mm/year, was an important process in hollow maintenance as roots held hummock elevations relatively more stable. However, hummocks were actively subsiding on all sites with little sign of root zone expansion within hummocks, despite hummock elevation gain on some sites. For down-river transitions, hollow infilling through increasing sediment accretion and root zone expansion were predominant processes driving loss of microtopography as marshes replaced TFFWs closer to the estuarine interface; hollows gained elevations to meet hummocks. While these results do not preclude the importance of healthy root zone processes to the maintenance (and formation) of hummocks, our results indicate that reductions in critical sediment supplies to offset natural shallow subsidence explain persistence and eventual loss of hummock and hollow microtopography in TFFWs.