Biology Reference
In-Depth Information
maximum heights to 21 m (Williams et al. 2008). From the mid to late Mio-
cene onward, the boreal forests moved episodically across the high northern
latitudes of the world. They were also spreading south into montane areas
of the west.
Alnus
(dominant) and some thermophilous (warmth-requiring)
angiosperms persisted in the Kenai lowlands at nearby coastal Homer in
southern Alaska:
Carya
,
Corylus
,
Ilex
,
Juglans
,
Myrica
,
Ostrya/Corylus
,
Ptero-
carya
(Asian),
Quercus
,
Ulmus/Zelkova
(Reinink-Smith and Leopold 2005).
Later in Alaska, the cold trend was augmented by blockage of warm air
from the south and from off the Pacifi c Ocean by rise of the Alaska Range.
Before around 5-4 Ma, southern Alaska was relatively fl at, but at about
this time the mountains began a period of signifi cant uplift that contrib-
uted to cooling and drying of the adjacent and interior lands. Arctic mam-
mals preserved circa 5.5-4.5 Ma in a beaver (
Arctomeles
,
Dipoides
) pond
on Ellesmere Island (78°33
′
N) show a boreal forest with larch, alnus, and
birch, together with sixteen species of beetles, rabbit (
Hypolagus
), shrew
(
Arctisorex
), bear (
Urus
), wolverine (
Plesiogulo
), an Asiatic horse (
Plesiohip-
parion
), and a deerlet (
Moschus
) that lives today in Siberia (Tedford and
Harrington 2003). Winter temperatures are estimated to have been about
15°C higher than today, and summer temperatures 10°C higher. A study of
tree ring width and isotopic composition in larch give a comparable MAT
of
1.9°C, or about 14°C warmer than at present (Ballantyne et al.
2006). After 5-4 Ma temperatures cooled rapidly.
Other events in the modernization of the northern fl oras during this
time include the disappearance of most Asian exotics (e.g.,
Glyptostrobus
,
Pterocarya
), and many thermophilous deciduous angiosperm trees (e.g.,
Carya
,
Fa g us
,
Liquidambar
,
Nyssa
,
Tilia
,
Ulmus
) from the high north in the
Mio-Pliocene. The sequence in the Seldovia Point fl ora is from a decidu-
ous forest, to mixed conifer and deciduous forest, to coniferous forest. The
estimated trend in MAT, based on modern analog and CLAMP methods, is
from 9°C to 3°C. The present MAT at Fort Yukon is
-
5.5°C
±
-
6.4°C. In the Porcu-
pine River region (fi g. 7.1), fossil woods of the middle Miocene are Taxo-
diaceae and
Pinus
with growth rings more than 1 cm wide, and in the late
Pliocene they are
Picea
with growth rings less than 1 mm wide. Elsewhere,
on the Seward Peninsula, the Pliocene Lava Camp fl ora, 75 km south of the
Arctic Circle, is a boreal forest with cold-temperate deciduous trees (
Alnus
,
Betula
,
Salix
), along with herbaceous elements that appear around 7-6 Ma
and are characteristic of tundra and near-tundra communities (the sedges
Carex
and
Cyperus
,
Epilobium
,
Oenothera
,
Symphoricarpos
,
Vaccinium
). These
began to spread by 3-2 Ma as a recognizable local shrub tundra of dwarf
Alnus
,
Betula
,
Salix
, and herbaceous tundra at the coldest and highest sites
