CLADES, ECOLOGICAL AMPLITUDES, AND ECOMORPHS - PHYLOGENETIC EFFECTS AND PERSISTENCE OF PRIMITIVE PLANT-COMMUNITIES IN THE PENNSYLVANIAN-AGETROPICAL WETLANDS

Pennsylvanian-age wetland plant communities and landscape gradients ex hibit persistent species composition and ecomorphic structure. Such pa tterns are attributable in large part to strong phylogenetic partition ing of ecological resource space at the level of higher taxa. Each of four major class-rank clades in tropical wetlands was centered in a ph ysically distinct part of the lowland ecosystem. Once established in t hese physical settings clades tended to resist displacement until remo ved by physically driven extinction. Lycopsid trees were the principal dominants of the wettest habitats and had subpartitioned these enviro nments along generic lines. Seed plants were the dominants on well to poorly drained elastic substrates, a diverse set of habitats reflected in high diversity of species and architectures. Sphenopsids were most abundant in aggradational environments subject to high levels of phys ical stress. Ferns initially were interstitial opportunists and coloni zers of disturbed areas in a variety of environments. These ecological patterns were established in concert with the architectural radiation of the vascular plants, which occurred during the Devonian-Mississipp ian transition and established the major classes. Within the wetlands, the replacement of species by close relatives drawn from the same fam ilies or genera contributed significantly to persistence of communitie s and landscape gradients. Replacement was more likely to occur from w ithin a clade where similar ecologies already existed, than from acros s major clades between which the basic ecologies were different. Possi bly interacting with the phylogenetic factors were community and lands cape level multispecies effects that may have placed limits on species replacement patterns. The existence of such emergent properties of mu ltispecies assemblages is suggested by a breakdown of the system begin ning with major, climatically induced extinctions at the Middle-Late P ennsylvanian transition. Following extinctions of the major Middle Pen nsylvanian trees, opportunistic ferns gave rise to dominants in many p arts of the wetlands, perhaps due to loss of some aspects of system se lf-regulation. As climatic drying continued seed plants began their la rgely passive rise to dominance in most kinds of habitats.