The composition and ecology of Late Carboniferous tropical, dryland plant communities are described for the first time. The study focuses on a 700-m-thick succession through the Langsettian Joggins Formation. Red bed units (5–110 m thick) bearing isolated pedogenic carbonate nodules occur at fourteen intervals and are interpreted as originating in a seasonally dry, well-drained, alluvial-plain setting characterized, in places, by an anastomosing fluvial geometry. A quantitative quadrat analysis of red bed floral assemblages preserved as compressions, impressions, calcareous permineralizations, and charcoal was undertaken. Cordaites, represented by woody trunks, branches, pith casts, leaves, and seeds, comprise 74% of the red bed floral thanatomass, together with minor pteridosperms and sphenopsids, and rare sigillarian lycopsids. Taphonomic data demonstrate that while fire-prone cordaite-pteridosperm vegetation dominated nearly all seasonally dry floodplain niches, hydrophilic lycopsids and sphenopsids were restricted to riparian settings where water availability was greatest. Calculation of standard diversity indices indicates that growing conditions were generally stressful, consistent with a seasonal environment. The composition of these dryland communities differs markedly from lycopsid-dominated wetland communities known from gray, coal-bearing successions at other intervals in the Joggins Formation. Sequence stratigraphic analysis of the Joggins Formation suggests that dryland communities represent the vegetation of seasonal continental-interior environments in contrast to wetland communities that grew in humid coastal settings. Repeated alternations between continental and coastal settings were caused by tectonically and/or eustatically driven base-level fluctuations that resulted in marine transgressive-regressive rhythms minimally hundreds of kilometers long.