Biology Reference
In-Depth Information
Mountains had reached suffi cient heights to begin interfering with mois-
ture coming from the west. The Paleocene to early Eocene vegetation north
of about the Colorado-Wyoming border included
Acer
,
Betula
, cf.
Cornus
,
Corylus
, members of the Fagaceae and Juglandaceae, and the Asian temper-
ate tree
Zelkova
of the Ulmaceae, while toward the south the vegetation was
more tropical but still forested (chap. 5). From about the middle Eocene
through the middle Miocene, the deciduous forest was widespread. But by
the late Miocene, although there was deciduous forest in the uplands of the
Plains region, and in gallery forests along streams, the intervening land now
included woodland/savanna trending toward grassland. Thus, beginning in
about the middle Eocene, the deciduous forest was becoming reduced. later
it would essentially be eliminated from one part of its former range—the
midcontinent region of interior North America.
The second fragmenting event was the rise of the Sierra Nevada, Cas-
cade Mountains, and Coast Ranges, which began the trend toward deserti-
fi cation of low- to midelevation habitats between these mountains and the
western slopes of the Rocky Mountains. The rich deciduous forest of the
Columbia Plateau and the surrounding landscape was being replaced in a
second region of its former distribution during the late Miocene and es-
pecially in the Pliocene by the Basin and Range and other deserts, and by
shrubland/chaparral-woodland-savanna. Most of the present-day mesic
Asian trees and shrubs had disappeared.
The third fragmenting event was a result of topographic confi gurations
and climatic change. In North America, the principal mountain systems
run north and south; so when the Quaternary glaciations began in earnest
around 2.6 Ma, the plants and animals were able to move southward until
milder conditions returned during the interglacials. In Europe the moun-
tains run east and west, so the biota was trapped between the Fennoscan-
dian glaciers advancing from the north and the glaciers of the Pyrenees-
Alps-Carpathians to the south. The glaciers actually met at times, and the
biota was destroyed. There was regeneration to deciduous forest during each
of the interglacials, but succession during the last interglacial was halted by
agriculturalists moving westward from the Fertile Crescent between the Ti-
gris and Euphrates rivers of modern-day Iraq, Syria, and Turkey, who were
clearing the land with axe and fi re. With this event, the north temperate
deciduous forest was removed or reduced from a third region of its former
range, leaving segments isolated in eastern North America and eastern Asia,
with individual elements persisting in a few mesic sites of the American
west and the Middle East. As noted, in addition to the disruption of once
more continuous ranges, there were also east-west-east migrations into the
