Landscape gradients and patchiness in riparian vegetation on a Middle Pennsylvanian braided-river plain prone to flood disturbance (Nýřany Member, Central and Western Bohemian Basin, Czech Republic)

Abstract

We reconstruct the spatial heterogeneity and community ecology of riparian vegetation preserved in fluvial deposits of the upper Asturian to middle(?) Cantabrian (upper Moscovian) Nýřany Member, Central and Western Bohemian Basin, Czech Republic. Poorly fossiliferous channel sandstone and conglomerate dominate at the four localities studied, but fine-grained intercalations, representing abandoned channels, floodplains, and shallow lakes, yielded rich megafloral and palynological assemblages. Sedimentological evidence indicates that high-energy flow characterized a braided-river plain prone to avulsion and flooding. Taphonomic observations and multivariate analysis of 41 quadrats containing mostly (par)autochthonous megafloral assemblages reveal that riparian vegetation comprised a collage of monospecific to low-diversity communities, with patchiness prevalent at local and regional scales. The vertical arrangement of megafloral assemblages provides evidence for ecological succession at some localities, with pioneering sphenopsid-dominated seres being replaced by marattialean tree ferns and then by medullosalean pteridosperms as ponded waters shallowed and exposure increased. At other localities, the composition and structure of megafloral assemblages remains unchanged through thin stratigraphic intervals, indicating short-term persistence and resilience of communities. Non-metric multi-dimensional scaling exposes habitat partitioning between different plant groups, which were organized along ecological gradients controlled by the drainage and stability of substrates. Better-drained and stable channel margins supported long-lived site-occupiers, such as cordaitaleans and monospecific stands of medullosalean pteridosperms. Application of the Patch Dynamics Concept, which predicts patterns of community recovery after disturbance, indicates that these species assemblages were ‘dominance-controlled’ (i.e., contained one or more K-selected species that were competitively superior to all others). Pioneering vegetation comprising fast-growing, opportunistic taxa, particularly ferns and sphenopsids, was most common on wetter, shifting substrates of frequently flooded abandoned channels, low-lying floodplains, and lake margins. In these ‘founder-controlled’ communities, several r-selected species had similar competitive abilities. Ferns, sphenopsids, and various lycopsids dominated swamps, marshes, and ephemeral mires on distal floodplains. The heterogeneous distribution of riparian plants is consistent with that of modern, disturbance-dominated fluvial environments, although Pennsylvanian species richness was much lower. A review of floristic patterns in latest Middle Pennsylvanian vegetation of Euramerica confirms that arborescent ferns proliferated across clastic wetlands during the late Asturian, although medullosalean pteridosperms remained the most important component of most communities. The Nýřany Member seems to be an exception, as pteridosperm relative abundance may have declined dramatically at a level coincident with the fern radiation.