Biology Reference
In-Depth Information
in the far north. Inland around Fairbanks, the vegetation was boreal forest
at around 2 Ma (Péwé et al. 2009).
Thus, the broad and overlapping trends in ecosystem history in north-
western North America include the appearance of an early montane co-
niferous forest in the highlands of the northern Rocky Mountains at about
45 Ma; the early presence of elements and then versions of the boreal co-
niferous forest in the lowlands to the north between 10 and 6 Ma; and its
subsequent spread into other montane regions as shown in the Beaverhead
Basins fl oras in nearby Montana, and somewhat less strong representation
in the Florissant fl ora more distant in central Colorado, both of late Eocene
to early Oligocene age (35 Ma). The appearance of tundra in the lowlands
is diffi cult to date specifi cally because with the early formation of local
glaciers in the Eocene and afterward there were narrow fringing zones of
soil rubble, lichens, and various herbaceous plants that constituted tree-
less vegetation in a cold climate (i.e., tundra). However, there were more
kinds and widely distributed tundra elements, and local tundra/near-tundra
vegetation, in the coldest northern and high-altitude sites around 7-6 Ma;
and a recognizable and relatively widespread tundra ecosystem between 3
and 2 Ma. In the highlands, altitudes suitable for tundra conditions were
available at scattered sites along the cordilleras after about the late Eocene.
The climatic component was added after about 15 Ma, and this is taken as
the approximate beginning of the alpine tundra ecosystem (earliest in the
highlands to the north, later to the south).
In the northeast, vegetation changes were infl uenced by similar trends
in climate, but without the simultaneous extensive orogenic activity of the
west. For example, the Holmatindur Tuff fl oras of eastern Iceland (10.3-
9.5 Ma) show a sequence from temperate forest (
Taxodium
,
Carpinus-
Ostrya
,
Carya
,
Fa gus
,
Nyssa
) to a cool to cold-temperate forest (
Picea
, de-
crease in
Fagus
), to a cold-temperate forest with intermingled subarctic
woodland (increase in
Picea
,
Alnus
, and
Betula
). The same trends are evi-
dent toward the southeast in the middle Miocene Martha's Vineyard fl ora
of Massachusetts and the late Miocene Brandywine fl ora of southern Mary-
land (I, 246-50). However, without the high elevations, comparable rain
shadow effects, and the intensive landscape modifi cations from volcanism
that were present in the west, no dry grasslands, shrublands, or near-desert
vegetation developed in the east. There was also greater moisture brought
into the midcontinent region from low pressure systems in the Gulf of
Mexico. The Appalachian montane coniferous forest probably appeared
fi rst in the highlands to the north in the late Eocene, and was confi ned to
the highest altitudes toward the south. It extended downslope and further
