MOUND AND RIDGE TREE-ISLANDS IN THE EVERGLADES PEATLAND

Low accretionary mounds and ridges of peat support ecologically important patches of forest called "tree-islands" in the broad sedge marshes of the Everglades peatland. Ridges are strongly elongated parallel to the predrainage direction of very slow overland flow and can reach over 1.5 km in length. In the northeastern area of deeper peat there is no apparent relationship to underlying topography on the mineral substrate and a purely peatland successional (biogeomorphic) origin is indicated for both mounds and ridges. Many ridges found elsewhere are focused upstream upon low mounds (and rarely deep depressions) in the marine limestone or sand below but stretch much farther downflow. They thus show strong but indirect bedrock controls. For islands investigated, the forest stage on mounds of deep peat and on the ridges in both areas appears to have arrived substantially after the establishment of the wide peat marshes they occupy. This is shown by peat stratigraphy (petrography) and pollen succession in the northern area and by more-complicated underlying gross stratigraphy (including layers of calcitic mud ["marl"], organic mud ["muck"], and some degraded organic sediments) in the southern area (Everglades National Park). This delay ran to greater than 3000 ¹⁴C years and the onset of forest occurred as much as ca. 1200 ¹⁴C years ago for several tree-islands in the deep-peat area. An unusual factor, floating peat-island formation and eventual reattachment, is strongly implicated in formation of the mound-type tree-islands on the deep peats. Ridges formed more gradually and the exact mode of origin and shaping may differ between the two main types (bedrock focused and unfocused).