Clades, ecological amplitudes, and ecomorphs: phylogenetic effects and persistence of primitive plant communities in the Pennsylvanian-age tropical wetlands

Abstract

Abstract Pennsylvanian-age wetland plant communities and landscape gradients exhibit persistent species composition and ecomorphic structure. Such patterns are attributable in large part to strong phylogenetic partitioning of ecological resource space at the level of higher taxa. Each of four major class-rank clades in tropical wetlands was centered in a physically distinct part of the lowland ecosystem. Once established in these physical settings clades tended to resist displacement until removed by physically driven extinction. Lycopsid trees were the principal dominants of the wettest habitats and had subpartitioned these environments along generic lines. Seed plants were the dominants on well to poorly drained clastic 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 physical stress. Ferns initially were interstitial opportunists and colonizers 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-Mississippian transition and established the major classes. Within the wetlands, the replacement of species by close relatives drawn from the same families or genera contributed significantly to persistence of communities and landscape gradients. Replacement was more likely to occur from within a clade where similar ecologies already existed, than from across major clades between which the basic ecologies were different. Possibly interacting with the phylogenetic factors were community and landscape level multispecies effects that may have placed limits on species replacement patterns. The existence of such emergent properties of multispecies assemblages is suggested by a breakdown of the system beginning with major, climatically induced extinctions at the Middle-Late Pennsylvanian transition. Following extinctions of the major Middle Pennsylvanian trees, opportunistic ferns gave rise to dominants in many parts of the wetlands, perhaps due to loss of some aspects of system self-regulation. As climatic drying continued seed plants began their largely passive rise to dominance in most kinds of habitats.