Diverse upland Eocene forests, western U.S.A.

Abstract

Mid-to Late Eocene floras in the eastern United States, preserved in thick piles of volcanic rocks, changed composition with increasing elevation. A deciduous hardwood forest above 455 m was replaced by conifer-deciduous hardwood forest near 1365 m, and it was supplanted by montane conifer forest at levels of about 1730 m. whereas a normal lapse rate (5.5°C/1000 m, usually indicates elevation of modern upland stations within 100-2000 m, a lapse rate of 3.0°C/1000 m, introduced recently to estimate palaeoelevation, yields levels for modern upland stations that are 1000-2000 m higher than actual elevations. Similar levels would be expected for Eocene-Oligocene floras and place treeline fully 1200 m higher than it is today. There is no geologic evidence for such high relief in this area in the Eocene. Furthermore, reasons for the decrease of treeline elevation fully 1000 m in the later Cenozoic-and in the face of increasing relief, are inexplicable.