A method for determining the altitudes of Tertiary floras
Authors: Axelrod Daniel I. 
Published: September 2022

Effective temperature (ET) at lowland stations can be used to determine accurately the altitude of ET at stations in upland forests today. The ET of a Tertiary flora near sea level appears to provide a reliable basis for determining the altitude indicated by the ET of a contemporaneous upland flora because there was a more regular vertical distribution to ET zones in the Tertiary. This resulted from climates which were more broadly zoned and less diverse in type; from relief which was lower and which had less influence on the local distribution of temperature; and from temperateness (M) which was high over wide regions.

Since Tertiary climate was characterized by pronounced temperateness (M). the effective temperature (ET) of a Tertiary flora can be determined most accurately if inferences are drawn from analogous modern forests in areas of high temperateness (M 60 +), the regions where forests have persisted with least change since the Tertiary (i.e., Szechuan Yunnan, Puebla-Oaxaca, New Zealand, southern Chile, Canary Islands).

The method devised to determine altitudes of stations in modern upland forests is applied to Tertiary floras with the following results. (1) A late Eocene subalpine forest (Bull Run flora) in north-eastern Nevada probably lived near 4,300 feet, contemporaneously with road-leafed evergreen forest near sea level to the west (Montgomery Creek-Moonlight, Comstock floras). (2) Mixed deciduous hardwood forest, inferred by botanists to have occupied the ancestral Great Smoky Mountains during Eocene, probably had its lower margin near 2,500 feet, situated above the warm temperate broad-leafed evergreen forest that dominated the lowlands (Wilcox flora). (3) Middle Miocene floras of the Columbia Plateau and  adjacent region display a gradient from a marginal ecotone to warm temperate broad-leafed evergreen forest (Wishkaw, Grand Coulee, Latah, Whitebird floras) extending up to about 500 feet, to deciduous hardwood forest (Succor Creek, Horseshoe Bend, Upper Cedarville, Mascall floras) from 500-1.500 feet, to montane conifer-deciduous hardwood forest  (Blue Mountains, Trout Creek floras) ranging from 1,500-3,000 feet, to montane conifer forest (Trapper  Creek flora) above 3,000 feet. (4) Analysis of Eocene to early Oligocene floras along the Pacific Coast (Lat. 40-65°) reveals a vertical zonation of forests and climates, with the cooler zones rising to higher altitudes at lower latitudes.

The data support the principles that (1) altitudinal zonation of Tertiary climate exerted a primary control on the distribution and composition of forests much as it does today, that (2) a rise in altitude during the Tertiary corresponds to time transgression because floras of younger aspect lived in the cooler uplands, and that (3) owing to altitudinal zonation of climate, analysis of the secular trend of Tertiary climate should be based on sequences of floras in local areas at or close to sea level, and not on fossil floras in widely separated regions for they will display pseudoclimatic fluctuations since they occur in different climate zone.