Variation in the number of taxa within a community (alpha diversity) often is linked to environmental factors (e.g., climate, disturbance frequency) or biological innovations (e.g., arborescence, seeds, insect pollinators) that allow some organisms to subdivide niche space more finely and thus increase the carrying capacity of a habitat. Land plants in particular show marked variation in alpha diversity in modern communities, with higher values associated with tropical communities and lower latitudes. Previous work has suggested that alpha diversity in plant communities increased steadily through the mid-Paleozoic to a late Paleozoic – Mesozoic plateau, followed by a dramatic increase in mean alpha beginning in the late Cretaceous and continuing into the Cenozoic. This pattern parallels estimates of global diversity through the Phanerozoic, leading to suggestions that increases in global diversity were driven by increases in alpha. However, subsequent compilations have indicated that the observed alpha diversity pattern may be an artifact of temporal changes in preserved environments and spatial and stratigraphic sampling constraints. Here we analyze species lists drawn from over 1400 paleobotanical collections compiled in the Paleobiology Database (www.paleodb.org) from primary literature and personal field work. Analyses were restricted to North American and European macrofloras preserved in fluvial, deltaic, and lacustrine depositional environments where specimens were collected from geographically well-constrained (e.g., < 10 x 10 m) and stratigraphically well-constrained (e.g., a horizon or bed) sites. Preliminary results indicate that mean alpha increased in the Devonian –Carboniferous but did not change significantly through the remainder of the Phanerozoic. If alpha did not change in coordination with global diversity, these results suggest that increases in global may have been driven by increases in among-community diversity. Alternately, the diverging patterns could reflect the temporal variation in depositional environments sampled; we test this using factor analysis to assess the relative association of alpha diversity with paleoenvironment (based on lithological data), paleolatitude, climate, sampling intensity, and geological age.