The Paleocene/Eocene boundary (c. 55.2 ma) represents transient greenhouse warming of <220 ky duration that had a critical impact upon North American mammals but an apparently limited impact upon subtropical plants. The effect of enhanced warming on biomes already tolerant of paratropical/tropical climate conditions is essentially unknown at the Paleocene/Eocene boundary because most research has centered on high latitude changes in plant turnover. Fossil pollen and spores from the US Gulf Coast allow an assessment of the impact that Paleocene/Eocene climate events had on a Paleo—paratropical/tropical vegetation type. Pollen data from two marginal marine sections either side of the boundary in Alabama, USA, demonstrate secular but subdued changes in composition that are manifest primarily as a restructuring of the vegetation type. Taxa found in the Paleocene remain dominant in the early Eocene (≥89% of taxonomic groups), and both extinction and immigration rates are moderate. Immigrants probably came from at least two different continents, Europe and South America, which implies a highly individualistic response of plant species from different Paleocene biomes to greenhouse warming. Diversity changes are not pronounced across the boundary, but within-sample diversity changes reflect a more heterogeneous, or possibly more successional, early Eocene vegetation type than the late Paleocene. This does not lead to greater between-sample diversity because the Paleocene palynofloras are moderately more diverse, if less heterogeneous at the within-sample level. Results imply that on time-scales of 105 years, Paleocene/Eocene warming is correlative with only minor compositional and diversity changes in paratropical vegetation types.

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