FLORAL DIVERSITY AND CLIMATE CHANGE IN CENTRAL COLORADO DURING THE EOCENE AND OLIGOCENE

Eocene-Oligocene climate change was accompanied by dramatic changes in floral composition and diversity worldwide. Central Colorado has several fossil floras and is ideal for studying the effects of climate change on floral diversity on a regional scale. In this study we examine the relationship between temperature and floral diversity. Based on modern patterns, floral diversity can be expected to decline as temperatures cool. We also examine floral composition to estimate timing of floral response to climate change. Diversity and composition of four floras ranging from late Eocene to late Oligocene in age--Florissant (34.1 Ma, n=943, 2 sites), Antero (33.8 Ma, n=29, 1 site), Pitch-Pinnacle (33.7-29 Ma, n=182, 1 site), and Creede (26.9-26.3 Ma, n=13,502, 14 sites)--were compiled from the literature and museum collections. Individual-based rarefaction curves were generated using genera to estimate and compare diversity. The number of shared taxa and Jaccard indices were calculated for all site comparisons. Mean annual temperature (MAT) is from the published literature, derived from each flora using leaf physiognomic techniques. Genus-level diversity declined with global temperature, with Florissant having the highest diversity levels (44 genera/180 specimen sample) and the warmest MAT (11-14 ºC) and Creede having the lowest diversity levels (12 genera/180 specimens) and coolest MAT (4.2 ºC). Although the Antero sample is small, the beginning of the rarefaction curve is steeper than that for Pitch-Pinnacle, suggesting that further sampling would yield higher diversity levels than Pitch Pinnacle and Creede. Pitch-Pinnacle and Creede had the most genera in common (11) and the highest level of similarity (J=0.367). Antero had more shared genera with Florissant (9) than Pitch-Pinnacle (4) or Creede (5). However, Jaccard values suggest that the Antero flora had the strongest affinity with Creede (J=0.167). This lack of clear affinity is likely a result of the small Antero sample size. It appears that as climate cools, floral diversity declines. The Pitch-Pinnacle flora has a greater affinity with Creede than with Florissant, suggesting that significant Eocene-Oligocene cooling occurred earlier. Affinity of the Antero flora remains unclear. Future work on the pollen and identification of new collecting sites should provide a more complete understanding of the Antero flora.