PALEOECOLOGY: APPLICATION TO PENNSYLVANIAN AND MODERN MIRES BY EXAMINING LEAF AND ROOT DECOMPOSITION

In modern mires, leaf mats record the initial stages of land-plant decomposition at the peat surface. Leaf taphonomy (fragmentation, soil microarthropods fecal pellets and fungal hyphae) indicates active decomposition and nutrient recycling. Roots and rootlets in the leaf mat indicate oxic conditions and sufficient soil moisture to prevent desiccation. Uncompressed leaf mats also occur in permineralized peat from Pennsylvanian mires, enabling us to compare leaf decomposition in modern and ancient mires as an indicator of terrestrial decomposition pathways and rates.

We studied leaf mats from three sites: Corkscrew Swamp Sanctuary (modern freshwater cypress mire in southwest Florida), Barnes Sound (basin mangrove mire with minimal tidal export in southwest Florida), and permineralized leaf mats from coal balls of the Kalo Formation that accumulated in either freshwater or brackish Pennsylvanian mires. Within the peat from Corkscrew Swamp, the position of nutrient gathering rootlets in the leaf mat indicates oxic conditions and suggests that decomposing leaves are a nutrient source for wetland plants. Corkscrew Swamp leaf mats had no horizontally disposed woody roots in the leaf litter. Instead, vertically oriented woody roots produced living horizontal rootlets 2-2.5 cm below the surface of the peat. No rootlets occurred in water-logged peat, possibly due to anoxic conditions. Desiccated dead rootlets at the surface of the peat suggest that water availability limits rootlets in the dry, surface peat.

Mangrove peat from Barnes Sound does not accumulate above the tidal range, and the mire surface is continually waterlogged. At this site, vertically oriented Rhizophora (red mangrove) and Avicennia (black mangrove) aerial roots sent living rootlets into waterlogged peat. Due to tidal exchange, water-logged mangrove peat seldom becomes anoxic at the surface.

In the Pennsylvanian peat, both Cordaitean roots and rootlets grew horizontally through the leaf mat. The presence of horizontal woody roots indicates oxic conditions in the surficial leaf mat and suggests that individual leaves within the leaf mat persisted long enough for tiny horizontal rootlets to become woody. Accumulation in salt water or above the water table in freshwater mires could explain the presence of rootlets in Pennsylvanian leaf mats.