Biology Reference
In-Depth Information
juniper woodland, an oak-sagebrush shrubland, and fi nally to a western co-
niferous forest at the highest elevations. Asian exotics mostly disappeared
from the Rocky Mountains, High Plains, and the midcontinent region by
about 8 Ma, persisting somewhat later in the moister habitats to the west
and in the southeastern United States.
West of the Rocky Mountains, the change from Oligocene and Miocene
deciduous forest to Pliocene shrubland/chaparral-woodland-savanna and
near desert is recorded in the numerous fl oras and faunas around the Colo-
rado and Columbia plateaus. Recall that in volcanically active regions, lava
will dam streams, creating lakes. Volcanic ash will rapidly fi ll these lakes
with fi ne-grained sediments, providing ideal conditions for preservation of
the surrounding biota. One such assemblage is the middle Miocene Suc-
cor Creek fl ora of southeastern Oregon of 15-14 Ma (Graham 1963, 1965;
Fields 1992).
The Succor Creek fl ora (fi g. 2.13) is one of the largest Tertiary plant as-
semblages known for North America, and it has been studied for both mac-
rofossils and microfossils, yielding a total of about 160 species. The biodi-
versity was enhanced by the spatial heterogeneity and the varied paleosols
that developed on the surface of the Columbia River Basalts. The minimum
winter temperature is estimated at 1.7°C-4.4°C, the maximum summer
temperatures at 24.4°C-27°C, and the MAP at 1270-1520 mm based on
paleobotanical evidence. Paleosol analysis yield regional MAT estimates of
17°C and MAP of 1400-1500 mm (Takeuchi et al. 2007; to 17.7°C, Retal-
lack 2004). The leaves are 25-30 percent entire-margined (generally tem-
perate), with warm-temperate evergreen species in the lowlands ( Cedrela ,
Mahonia , Oreopanax , Quercus ), and cool-temperate deciduous species like
Acer on the moist slopes (see fi g. 2.14), along with Alnus , Betula , Carpi-
nus , Carya , Castanea , Cornus , Diospyros (persimmon), Fa g us , Fraxinus , Hy-
drangea , Juglans , Liquidambar , Magnolia , Nyssa , Ostrya , Platanus , Populus
(aspen), Prunus , other Quercus , Salix , Sassafras , Tilia , and Ulmus , including
a contingent of present-day Asian plants like Ginkgo , Glyptostrobus , Metase-
quoia , Ailanthus , Pachysandra , Pterocarya , and Zelkova . There were a few dry
elements ( Ephedra ), and other genera characteristic of habitats from damp
( Taxodium ), to wet ( Equisetum , Carex , Typha ), to aquatic ( Nymphaea , Pota-
mogeton ). At the highest elevations there was a western coniferous forest of
Abies , Picea , Thuja , and Tsuga .
Subtle shifts in climate, altitudes, and vegetation in the region are evi-
dent even within short spans of time. In the adjacent Trout Creek Flora
(13.8 Ma), present-day Mexican ( Cedrela , Oreopanax ) and Asian genera
( Ginkgo , Glyptostrobus , Pterocarya ) were not present, and cooler conditions
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