Vegetational zonation in a swamp forest, Middle Pennsylvanian, Illinois Basin, U.S.A., indicates niche differentiation in a wetland plant community

A rich fossil flora occurs in the roof shale of the Middle Pennsylvanian (Moscovian) age, Springfield Coal flanking the Galatia channel, a river deposit approximately 1 km in width and 150 km long, partially contemporaneous with the Springfield peat swamp. Siltstone and sandstone, > 30 m thick, were deposited in the channel during the final estuarine phase. The river formed under seasonal subhumid climate during early deglaciation as marine waters retreated from the land surface. It existed continuously through glacial maximum, under increasingly humid climate, as a low-sediment or black-water river with peat along its flanks. During early deglaciation, under subhumid climate and sea-level rise, the river became a sediment-laden estuary, drowning the peat swamp and burying vegetation in place under extensive mudflats. This buried flora was studied in three underground mines in southern Illinois, permitting vegetation to be characterized spatially. Three transects, comprising 100 sites, a total linear distance of ~ 12 km, reveal distinct zonation perpendicular to the channel margin. An innermost belt, 0.5 to 2.5 km wide, consists almost exclusively of the pteridosperm Neuropteris flexuosa. North of a river bend, three belts succeed the N. flexuosa zone: mixed pteridosperms and marattialean tree ferns; mixed arborescent lycopsids and marattialians; lycopsids with rare marattialians. South and inside the same bend, N. flexuosa is succeeded by mixed lycopsids, particularly Sigillaria, and marattialean ferns. Within the transects, marattialian ferns have wider distribution than other groups, suggesting broader environmental tolerances. Neuropteris flexuosa channel-margin mono-dominance may reflect high sedimentation rates, elevated nutrient conditions, or floating-seed dispersal. Lycopsid dominance in interior areas conforms with tolerance of deep, standing water. The spatial scale examined affects our perception of community assembly. Overall, distinct vegetational zonation suggests niche-assembly; however, within vegetational bands, there is considerable small-scale spatial variation suggesting stochastic properties.