The Late Oligocene Creede Flora, Colorado

Important changes have occurred in the composition of forest, woodland, and riparian vegetation in the Creede area since the Oligocene. The evidence for this is based on fossil samples from several sites in each of 4 areas in the northern part of the Creede basin. All of them are rich in plant remains deposited in a lake near the shore or on floodplains interfingering with lake sediments. The florules at the 2 lacustrine sites (Birdsey Gulch and Dry Gulch) differ considerably in composition owing to topographic setting and exposure, whereas those of the two floodplain environments (5-Mile Bridge and Wason Cliffs) are more similar. The general sample of the vegetation of the area is adequate and most probably represents typical vegetation of the region. The major changes these samples reflect are in the relative abundance of taxa, many of which are represented by similar species in the region today. Some foret trees (Abies, Picea, Pinus, Pseudotsuga) have become more dominant in the vegetation, whereas earlier they were rather rare. Woodland shows a similar relationship, as does the riparian community, in which new taxa have entered the region (Alnus, Populus cf. angustifola), or have become dominant (Betula, Populus cf. tremuloides). Grasslands are relatively new yet are being displaced by shrubs and woody perennials as the result of recent overgrazing by cattle and sheep. Some taxa that contributed to these vegetation zones disappeared as climate became less equable (Arbutus, Cercis, Cerocarpus, Pinus, Populus, Quercus) or as the imcidence of summer rainfall decreased (Crataegus, Mahonia, Picea, Prunus, Tsuga, etc.) While the dynamic nature of change in terms of species abundance within communities is apparent, most Creede species show that there has been relatively little morphologic change during the past 26 m.y.