COMMUNITY STRUCTURE OF A PENNSYLVANIAN LYCOPSID-PTERIDOSPERM FOREST

The remains of a Late Carboniferous forest were preserved on a bedding surface covering an area of 600 square meters in the Bernice Basin (Sullivan County, northeastern Pennsylvania). The age of the beds is late Moscovian (Asturian). Sedimentologic and taphonomic analyses indicate that this forest was preserved in situ, after being blown down during a storm. This assemblage preserves a relatively diverse plant community growing on a clastic substrate. Analysis of the distribution and orientation of the different plant parts and taxa preserved on the surface and in the top centimeters of the bed permits reconstruction of the forest structure. The Bernice forest had three layers: an up to 40 m high upper canopy with two lycopsid species, a less than 15 m high understory of at least four pteridosperm species, and a monospecific, highly discontinuous Sphenophyllum dwarf shrub layer. Reproductively immature lycopsids were found at all heights within the canopy and belonged to the same species as the canopy dominants. Thus, it appears that the canopy species could replace themselves within this community and maintain site dominance. One pteridosperm species was a free-standing tree while the others formed thickets in which the plants had thin stems and supported each other. Sedimentological evidence of periodic flooding, abundant charcoal (fusain) in the former forest soil as an indicator of forest fires, and periodic plant growth retardation recognizable on lycopsid stems suggest climatic fluctuations and recurrent flood and drought conditions. On the basis of this evidence the climate in which this forest grew is interpreted to have been seasonally wet and dry as many tropical climates are today. The forest architecture and species diversity of the Bernice swamp forest is comparable to analogous modern tropical swamp environments as is the number of stems per hectare.