STRUCTURE OF THE NORTH AMERICAN VEGETATION GRADIENT DURING THE LATE PALEOCENE-EARLY EOCENE WARM CLIMATE

Late Paleocene–early Eocene pollen and spore data taken from the US Gulf Coast (paleo-latitude 32 °N), western interior basins (Wyoming, North Dakota; paleo-latitude 44–47 °N), and Canadian Arctic (paleo-latitude >68 °N) represent a vegetation proxy for ancient paratropical, subtropical, and temperate biomes. These data provide information on the latitudinal diversity gradient of plants during an ancient greenhouse climate with non-freezing winters at polar latitudes. Comparing pollen data from the early Paleogene with a pollen dataset compiled at the same latitudes from the late Holocene (3000 years BP to present), reveals that the diversity gradient between middle to high latitudes was steeper than today at the same sampling intensities. If 69 late Paleocene–early Eocene samples are taken from each region, the gradient manifests as a step-like decrease of c. 50% in taxonomic diversity from the US Gulf Coast (n=188 taxa) to the western interior (n=90) and again a c. 50% decrease in taxonomic diversity from the western interior to the Canadian Arctic (n=44). The diversity gradient is formed by the “spillover" of paratropical taxa into other regions of North America which reflects also the modern pattern of plant ranges. Taxa inhabiting the Arctic therefore had great geographic ranges with endemism greatest in the paratropical biome. Paleogene diversity gradients show that decreasing diversity with increasing latitude is ancient and not dependent on freezing temperatures.