Structure and Dynamics of a Pennsylvanian-Age Lepidodendron Forest: Colonizers of a Disturbed Swamp Habitat in the Herrin (No. 6) Coal of Illinois

Tree stumps of arborescent lycopods were discovered in situ at the contact of the Herrin (No. 6) Coal and the Energy Shale Members, near the Walshville paleochannel, the major river channel running through and draining the Herrin coal swamp. The stumps are associated with, and in one case attached to, fallen trunks identified as a Lepidodendron similar to L. aculeatum. The stumps and trunks occur in a lower carbonaceous fades of the Energy Shale. The trees were rooted in the peat and represent the final vegetation at the site. Various lines of evidence suggest that the trees were of approximately the same age and formed a single stand that died as a consequence of completing a determinate life cycle, not as a result of mud accumulation or drowning. Distribution of the tree stumps is random. Consideration of tree habit and development, edaphic (soil) quality of this and similar swamp sites, and colonization dynamics suggests that these trees were opportunists within a milieu of stressful habitats. Vegetation structure probably was influenced only slightly or not at all by density-dependent (competitive) factors. Lycopods of this type are characteristic of clasticcompression (mineral-substrate) swamp vegetation, and occur in some coal-swamp (peat-substrate) communities, possibly in relatively higher nutrient areas. Their presence above the Herrin Coal appears restricted to near-channel sites, which they may have colonized as depositional and/or edaphic conditions changed from those typical of a peat-substrate swamp to those more similar to mineral substrates. There are a number of similarities between modern and ancient swamps in the ecological strategies of the dominant plants and the structure of the communities; however, there is little morphological "convergence" among the dominant species. INTRODUCTION