mirrored file at http://SaturnianCosmology.Org/ 
For complete access to all the files of this collection
	see http://SaturnianCosmology.org/search.php 
==========================================================

*NORTH AMERICA DURING THE LAST 150,000 YEARS*

Compiled by Jonathan Adams, Environmental Sciences Division, Oak
Ridge National Laboratory, Oak Ridge, TN 37831, USA

Summary

From around 150,000 to 130,000 years ago, North America experienced
colder and generally more arid than present conditions. About 130,000
years ago, a warm phase slightly moister than the present began, and
conditions at least as warm as the present lasted until about 115,000
years ago. Subsequent cooling and drying of the climate led to a cold,
arid maximum about 70,000 years ago, followed by a slight moderation of
climate with a second aridity maximum around 22,000-13,000 14C years
ago. Conditions then quickly became warmer and moister, though with an
interruption by cold and aridity in many areas around 11,000 14C years ago.

------------------------------------------------------------------------

+Present-potential vegetation <NAprsnt.gif>

*This, roughly speaking, is the vegetation cover that existed 500 years
ago as Europeans began to arrive in the Americas. In the eastern USA,
forest was predominant with warmth-adapted temperate forest in the
south-east. To the west, decreased rainfall and the resulting fires
prevented trees from growing, and a wide grassland belt (prairie)
existed. In the mountains and plains of the far west, sparse rainfall
gave semi-desert, open scrub and open conifer woodlands. To the north,
various conifer forest belts extended across Canada, opening out into
the meadow-like tundra where climates were too cold for trees to grow.*

------------------------------------------------------------------------

*130,000-125,000 y.a. Eemian 'optimum', Isotope Stage 5e.* Data for this
period are in short supply. Frenzel (1992) and Velichko et al. (1992)
have only three data points for eastern USA; these suggest warmer (2-3
degC warmer) winter temperatures north of about 30 deg.N, and warmer
summer temperatures north of about 45 deg.N. The annual rainfall is also
suggested as being higher than present across the eastern USA. Whitlock
& Bartlein (1997) suggest slightly drier and warmer conditions than
present in the Cascade Range (45 deg.N) of the western Cordillera, with
dry woodland at lower elevations but a strong steppe element.

------------------------------------------------------------------------

*115,000-100,000 y.a. Late Eemian cool stage, Isotope Stage 5c.* From a
site in the Cascade Range (Whitlock & Bartlein 1997), open /Pinus/
forest predominated locally, with steppic elements also important. The
climate seems to have been cooler than present, and perhaps slightly
drier than present too.

------------------------------------------------------------------------

*40,000 ^14 C y.a.; a fairly cold phase, in the 'middling' period
between the two main glacial maxima__*_. A map summary for the eastern
USA has been compiled by Delcourt & Delcourt (1981), on the basis of a
number of pollen sites and other data such as ice margin positions.
Delcourt & Delcourt suggest that ice extent over eastern Canada was only
slightly less that its maximum extent around 20,000 y.a., and that it
extended just south of the Great Lakes. Spruce and jack pine forest
seems to have covered most of the eastern USA, with mixed cool temperate
forest in Tennessee and North Carolina. Southern pine forest with oak
and hickory existed south of this, extending around into easternmost
Texas. Open scrub is reconstructed for the Florida Peninsula._

------------------------------------------------------------------------
_*28,000-25,000 ^14 C y.a.; __*_shortly before Last Glacial Maximum. _In
the eastern USA, conditions may have been generally drier than today. A
xeric scrub cover existed in Florida at this time, instead of the
present forest (Watts & Stuiver 1980). In Maryland (38N, 75W) pollen
evidence indicates pine-birch barrens or spruce parkland dominating
after 30,000 14C years ago (Wells 1992 p.612), and it is possible that
most of the eastern USA had an open wooded vegetation cover at this time._

_Delcourt & Delcourt (1981) give a summary map for the eastern USA at
25,000 y.a., showing spruce and jack pine forest extending south to
about the latitude of Washington DC, and thinning out into forest-steppe
west of the Mississippi River. A mixed cool temperate forest belt seems
to have existed across the south Appalachian region. From South Carolina
southwards to northern Florida, oak-hickory and southern pine forests
are suggested as having survived._

_In the western cordillera of the USA, lake levels were higher than
today (indicating moister conditions) but not generally as high as they
would become at the LGM. In northern Arizona, altitudinal zones of
vegetation had not yet declined to their LGM positions (which were
reached by about 24,000 y.a.), but were lower than today. In the NW
Cordillera of the USA, in eastern Cascades Range at 45 deg.N, 120 deg.E,
forest cover was considerably less than today, with more steppe and
cold-tolerant species (Whitlock & Bartlein 1997). This followed an
earlier stage, 30,000-40,000 y.a., when there had been slightly warmer
conditions in the Cascades with more forest cover (though still
considerably less than today)._

_ _

_+ Eastern USA 28,000-25,000 14C y.a. <NAm28-25kyr.gif>_

_ _

_*This vegetation map showing the eastern USA during the period
28,000-25,000 14C y.a. has been compiled by Paul & Hazel Delcourt. An
ice sheet already covered most of Canada and extended south of the Great
Lakes. Boreal conifer woodlands and forests predominated in what is now
the cool temperate forest zone, and the cool and warm temperate forest
belts were compressed southwards. * _

------------------------------------------------------------------------

_Also see main QEN section on North America <new_na.html>_

_Quick-time movies of the spread of pollen types in North America
<http://darcy.geo.umn.edu/students/lensyl/scivis.html>over time._

_*18,000-15,000 ^14 C y.a. (about 21,000-17,000 calendar years ago). The
full glacial.*At this earliest mapped time slice, North America was
still in the grip of full glacial conditions. The ice sheet extent did
not remain static through this period; by 15,000 14C y.a., some retreat
of the major North American ice mass had occurred relative to the
'global' glacial maximum position some 3,000 radiocarbon years
previously (Dyke & Prest 1987), it still covered most of the continent
and was even more extensive in some places. The ends of what was later
to become an ice-free corridor had begun to open by 15,000 14C y.a., but
about 1000 km of ice still remained blocking its course._

_Lower sea levels had exposed a land bridge extending across from Alaska
to the eastern tip of Siberia. Known as Beringia, this exposed land
seems to have had localised areas of moist, peaty tundra (though not
true 'peatland') in stream and river valleys, but other pollen cores and
geomorphological indicators show that the higher slopes and knolls, and
areas towards the north and away from the Pacific, would have had a
large proportion of open ground (Elias _et al._ 1996, Colinvaux 1996).
In the interior of Alaska, conditions seem to have been particularly dry
and cold, and probably resembled polar desert. See the main QEN section
on North America <new_na.html> for a more detailed discussion of Beringia._

_ The Beringian Atlas Project
<http://instaar.colorado.edu/~duvall/develop.html> is focussing on
broadscale reconstruction of the environments of Beringia over the last
17,000 years (however, their maps are not yet fully available online). _

_The Cordilleran ice sheet (curving round across southern Alaska and the
Canadian Rockies) covered all of the islands along the west coast of
Canada, except possibly the Queen Charlotte Islands. The linked
Laurentide ice sheet covered most of eastern and central Canada. South
of these combined ice masses, open woody vegetation seems to have
prevailed towards the east and south of the USA._

_Much of the southeastern USA was wooded or forested, almost as far
north as the edge of the ice sheet. In general this forest cover seems
to have been more open than today, especially on the Atlantic coastal
plain, and it also thinned out westward, giving way to forest-steppe and
then open dry steppe in the mid-west (present-day prairie region). This
link <http://www.museum.state.il.us/exhibits/larson/environments.html>
shows what the landscape of much of Illinois may have looked like during
the last glacial period. Probably, the land eastward of Illinois would
have been more densely wooded than this, but still with significant open
areas between the trees. To the west, the tree element would have
declined to leave a dry and virtually treeless steppe._

_To the east of the Appalchians, there was a high proportion of cool
climate pines and often the presence of prairie herbs amongst the trees
(e.g. Brook & Nickmann 1996), and on the Atlantic coastal plain there
are indications of shifting sand dune activity which - together with the
predominance of jack pine - suggests that the woodland was fairly open
(Wells 1992). Apparent blowout structures formed along the coast from
the Carolinas to Geogia ('the Carolina Bays'), due to dry conditions
with rather open vegetation. The generally rather dry nature of the
climate may also be reflected by the occurrence of a 'blue clay' layer
(probably wind-deposited) in the Carolina Bays. See this link on the
blue clays <http://abob.libs.uga.edu/bobk/bayclay.html> _

_See the QEN section on North America <new_na.html> for references and
more detailed discussion of the evidence. _

_In the south-east, the Florida Peninsula may have been virtually a
desert of shifting dunes at some stages during the last glacial (Wells
1992), and at other times it seems to have been covered by a sparse
scrub vegetation. The Gulf coastal plain and northernmost Florida seems
to have been forested, with some open woodland (Watts & Stuiver 1980).
The relative proportion of deciduous trees (mainly oaks) in the
vegetation seems to have increased southwards. Some authors (e.g.Watts &
Stuiver 1980) appear to incline more towards the view of a mixture of
conifer forest and other more open types of conifer woodland dominating
in all except a few localities in the southern USA. Paul & Hazel
Delcourt (pers. comm., January 1998) suggest that the climate on the
Gulf coastal plain would have been seasonally more dry than today, but
with a more winter-wet and summer-dry pattern. The Delcourts note that
summer-dry conditions would favor drought tolerant species of oaks and
hickories at the expense of southern pines, so oak and hickory may have
been quite common within the forests and woodlands. The Delcourts
suggest that in northernmost Florida and on most of the Gulf coastal
plain, the variety of full-glacial habitats would have permitted a
mixture of both xeric and mesic forest communities to persist, more
mixed-mesic deciduous forests in fire protected sites along river
valleys and stands of temperate oaks and southern pines across sandy
interfluves._

_The north-western USA was dominated by dry alpine tundra and polar
desert (trees were almost absent except perhaps very scattered remanants
of spruce; Whitlock & Bartlein 1987), with scattered areas of
cold-tolerant conifer woodland in the central Rockies (mixed with
semi-desert vegetation) (See Davis' reconstruction, below). Further
south in the Cordilleran region of the USA, a mixture of semi-desert
scrub and sparse conifer woodland existed in a mosaic that depended on
altitude (Thompson et al. 1993), with the open-conifer woodland
dominating in most areas (see Davis' reconstruction). _

_A new (1998) map reconstruction of the western USA
<http://geo.arizona.edu/palynology/nam_p18k.gif> by Owen Davis shows
temperate open woodland covering most of the southwest, and also parts
of the High Plains. The view of the High Plains as woodland-covered is
perhaps somewhat controversial (and he is thinking of changing it to
semi-desert!), an alternative view being that it is a result of
selective conifer pollen transport/preservation, in an environment which
was mainly grassland at the LGM. Another divergence in opinion in the
literature is Wells (1992) suggests a desert area in the northern High
Plains (based on dunes etc.), in the area which Davis suggests was
tundra. In the northern-most area of the NW plains, close to the
Canadian border, a pollen site at Mitchell Lake suggests semi-arid
conditions; regional vegetation from c. 17 960 to 16 100 BP is
interpreted as an extremely cold semi-arid /Artemisia/ steppe. (J
Paleolimnology 16: 37-57, 1996). The vegetation c. 16,100 to 11,900
seems to have been an /Artemisia-Betula/ shrubland._

_ This link <http://web.ngdc.noaa.gov/paleo/image/gsafinal.gif>shows
palaeovegetation reconstructions by T. Webb & J. Overpeck based on
pollen data only, from the eastern USA. Note however that the
reconstruction of deciduous forest for Florida seems unlikely given the
various sources of evidence for aridity in that area. _

_In general it seems from the pollen profiles that the climate in the
eastern USA was drier than most boreal forest in Canada or Siberia
exists under at present (Jackson et al. 1997). Spruce (/Picea/; possibly
including an extinct warm temperate species) was common and may have
formed dense stands in places, including the Gulf of Mexico coastal
plain. Mixed in with the mainly coniferous forests, and extending quite
far north, were temperate tree taxa (e.g. /Juglans, Acer, Quercus,
Fagus/) occurring at low densities but perhaps locally abundant on moist
soils (Delcourt et al. 1980, Jackson et al. 1997)._

_ _

_The presence of abundant spruce pollen in the mid-western USA used to
be thought to indicate a major westward expansion of forest or woodland,
but this is now considered to be just the effect of long-distance
transport and selective preservation of tree pollen. It is now thought
that densely wooded vegetation did not proceed much further west than it
does at present, though open forest steppe did apparently extend over
the east of the present prairie zone._

_	_

_In Mexico, conditions in the highlands may have been quite similar to
those existing today (Markgraf 1993), although somewhat colder and with
more dry grassland (Lozano-Garcia et al. 1993). However, the tropical
forests of central America seem to have been much reduced in area.
Savanna predominated in the Yucatan Peninsula (Markgraf 1993), and
although rainforest survived in upland parts of Panama it may have been
broken up by dry scrub or deciduous forest in the drier lowlands._

_ _

_+ <NAL2215.gif>_

_ _

_* At the most extreme stage of the last glaciation, most of Canada and
much of the northern USA were covered by an ice sheet thousands of
metres in thickness. Colder and often drier than present conditions
predominated across most of the USA. The eastern deciduous and conifer
forests were replaced by more open conifer woodlands with cooler-climate
species of pines and a large component of spruce. The open spruce
woodland and parkland extended somewhat further west than present, into
what is now the prairie zone. As a result of aridity and lowering of sea
level (which lowered inland water tables), much of Florida was covered
by drifting sand dunes. Notably moister than present conditions occurred
across much of the south-west, with open conifer woodlands and scrub
common in areas that are now semi-desert. * _

_J. Adams' reconstruction of North America during last the glacial
period - 18,000-15,000 ^14 C ya <./NEW_MAPS/northam.gif>_

------------------------------------------------------------------------

_*Link to Illinois State Museum site showing retreat of Laurentide ice
sheet <http://www.museum.state.il.us/exhibits/larson/glaciers.html>
(note that this does not include details of the retreat of the
Cordilleran ice sheet which formed the western wall of the ice-free
corridor after 13,000 14C y.a.)

* _

_*14,000 radiocarbon years ago. *Indicators of a significant warming and
moistening of climate begin to appear at around this time, but only in
some areas. In Alaska, a widespread change from herb-dominated to moist
shrub-dominated tundra occurred at around this time, suggesting moister
and slightly warmer conditions (Andrews & Brubaker 1993.). A similar
trend towards moister and warmer conditions is seen in the changing
tundra flora and insect fauna on the eastern part of the Beringian land
bridge (Elias _et al._ 1996). _

_Further retreat of the Laurentide ice sheet had occurred, but the
future ice-free corridor still remained blocked by around 800 km of ice. _

_	_

_In the central Mexican uplands, an increase in woodlands and a
reduction in grasslands after 15,000 ^14 C y.a. suggests a switch to
more humid - but still cooler - conditions than had occurred previously
(Lozano-Garcia _et al_. 1993). Cooler conditions may have lasted up
until about 9,000 y.a. (Lozano-Garcia _et al_. 1993). _

_	_

_Elsewhere in North America, conditions seem to have remained in the
same dry and cold mode as before. In parts of the High Plains of the
MidWest, desert conditions might have been widespread, with continuing
dune activity and the absence of mammal fossils from this time (Wells
1992). However, this view is disputed by others, who suggest that
instead a dry tundra environment might have been predominant towards the
north (e.g. Beaudoin pers. comm. 1998). _

_The nature of environments in the northern High Plains/southern end of
the ice-free corridor before about 12,000 14C y.a. remains uncertain,
with conflicting evidence but perhaps the balance of it (from such
pollen sites at Mitchell Lake) suggesting rather arid conditions,
resembling semi-desert. After this stage, conditions may have become
substantially moister although there are still some signs of
drier-than-present conditions. _

_Further west, across Washington State, pollen diagrams suggest that dry
and very cold conditions dominated the Cordilleran region, with a sparse
parkland existing on the coastal plain and polar desert inland and at
higher altitudes (Thompson et al. 1993). _

_Owen Davis' reconstruction for the the western USA at 14,000 14C y.a.
<http://geo.arizona.edu/palynology/nam_p14k.gif>, based on plant fossil
data, emphasises the more traditional (now controversial) view that
woodland cover was very widespread in the High Plains areas to the east
of the Rockies _

_J. Adams' (1996) reconstruction of North America at the time of initial
warming - 14,000 ^14 C ya <./NEW_MAPS/northam0.gif>_

------------------------------------------------------------------------

_**_

_*13,000 radiocarbon years ago.* The vegetation distribution seems to
have been much the same as at 14,000 14C years ago, though with
continuing glacial retreat exposing new surfaces in North America. At
around this time, according to the mapping chronology of Dyke and Prest
(1987), a continuous ice-free corridor opened for the first time beween
Alaska and the contiguous USA. However, for a considerable part of its
length (about 750 km) it would have been less than 50 km wide, and
further obstructed in several places by large meltwater lakes. The
chronology of the first appearance of the ice free corridor is not
completely settled however; Bobrowsky & Rutter (1992) conclude that
although the southern and northern ends were open by this time, it is
quite possible that the central region was still closed. _

_However, conditions at the southern end of the ice-free corridor (e.g.
in the area around 50-52 deg.N and 110-115 deg.W, reviewed in detail by
Beaudoin et al. 1996) still seem to have been fairly arid. Burns et al
(1993) note the absence of radiocarbon dates on faunal remains between
about 21,300 and 11,600 14C y.a. in the Edmonton area, suggesting that
the landscape was incapable of supporting fauna during this interval.
Beaudoin (pers. comm., January 1998) suggests - on the basis of various
pollen sites - that the vegetation in southern Alberta between about
16,100 to 11,900 14C y.a. was a dry /Artemisia-Betula/ shrubland._

_ _

_+ 13,000 14C y.a. <NAm13kyr.gif>_

_ _

_ _

_* At around 13,000 14C y.a., retreat of the the western and eastern
North American ice sheets exposed an 'ice free' corridor linking Alaska
to the land to the south. The Bering Straits at this time also remained
dry land. Cold climate conifer forest began to colonize areas exposed by
the ice sheet in the eastern USA, and cool temperate forest vegetation
started to spread northwards.*_

_* * _

_See Owen Davis' reconstruction (1998) of the western USA at 13,000 14C
y.a. <http://geo.arizona.edu/palynology/nam_p13k.gif>_

_J. Adams' (1996) reconstruction of North America during deglaciation -
13,000 ^14 C ya <./NEW_MAPS/northam1.gif>_

_Beaudoin, Davis, Declourt, Delcourt, Richard & Adams' (1998)
reconstruction of North America during deglaciation - 13,000 ^14 C ya
<del13.gif>_

_Key to the vegetation classification system used in the paleovegetation
maps <adams3.html>_

------------------------------------------------------------------------

_**_

_*12,000 radiocarbon years ago. *By around 12,000 radiocarbon years ago
(Or possibly even by 13,000 14C y.a., see Davis' map above), boreal
forest had spread through the western lowlands of Washington State
(north-west USA), indicating the continued effects of warming and
moistening of climate as the ice sheets retreated (Thompson _et al_.
1993, & ShowTime database). On the eastern part of the Beringian land
bridge, insect communities suggest that present-day temperatures had
been reached (Elias _et al._ 1996)._

_	_

_The ice-free corridor through Canada had by this time widened
considerably, though it remained wholly or partly obstructed by large
meltwater lakes (Dyke & Prest 1987). To the north, birch shrub-tundra
had suddenly became widespread thoughout deglaciated areas of Alaska and
north-west Canada. _

_	The putative desert area which had occupied much of the the High Plains
of the Midwestern USA began to show signs of moister conditions, with
dune stabilization and the spread of shortgrass prairie vegetation and
its associated grazing mammals (Wells 1992). At the southern and western
fringes of the High Plains, pine and spruce woodland seems to have begun
to spread in at around this time (Wells 1992). Davis (see map
reconstruction) suggests more extensive woodland vegetation, throughout
the High Plains area._

_	Temperate tree species began to spread and increase in abundance through
the forests and open woodlands of the eastern USA, although the
vegetation remained dominated by boreal conifers (Overpeck _et al_.
1992, ShowTime database). A map of vegetation distribution for 12,000
y.a. <http://web.ngdc.noaa.gov/paleo/image/gsafinal.gif> has been
assembled by R. Webb & J. Overpeck for the eastern USA, using pollen data._

_	Through much of the southern and central Cordilleran area of the USA,
conditions may have been slightly moister than at present (although
generally semi-arid), with greater woodland and scrub cover than at
present. The same appears to have been the case for the lowland American
and Mexican deserts to the south (Thompson _et al_. 1993, Benson et al.
1997)._

_See Owen Davis' reconstruction (1998) of the western USA at 12,000 14C
y.a. <http://geo.arizona.edu/palynology/nam_p12k.gif>, suggesting only
very restricted areas of semi-desert vegetation._

_J. Adams' (1996) reconstruction of North America during deglaciation -
12,000 ^14 C ya <./NEW_MAPS/northam2.gif>_

_Beaudoin, Davis, Declourt, Delcourt, Richard & Adams' (1998)
reconstruction of North America during deglaciation - 12,000 ^14 C ya
<del12.gif>_

_Key to the vegetation classification system used in the paleovegetation
maps <adams3.html>_

------------------------------------------------------------------------

_*	*_

_*11,000 Radiocarbon years ago. *The picture of climate and vegetation
change at around 11,000 radiocarbon years ago is complicated by the fact
that in some areas there was a cold and arid event that correlates with
the 'Younger Dryas' cold event in Europe. Changes in pollen spectra from
the eastern seaboard (see below), the midwest and the nw Pacific coast
suggest colder, drier conditions, with a mean annual temperature decline
of about 3-4 degC across these areas (Anderson 1997). A relatively dry
phase in the Great Basin (comparable in aridity with the present-day)-
following on from the previous initial moist phase - also corresponds in
age to the Younger Dryas (Benson et al. 1997)._

_A significant change in forest composition occurs in the north-eastern
USA (Connecticut, New York, Maine) at this time. The abundant pollen
records from this area indicate a change from a mixed hardwood deciduous
forest (oak, ash, spruce, fir) to a boreal assemblage (spruce, fir,
larch, birch, alder). The forest canopy seems to have opened up
somewhat, perhaps due to the severe winters, allowing an increase in
birch and alder. In general the cooling seems to have been around 3 to 4
deg.C in mean annual temperature. See web page by Dorothy Peteet.
<http://neonet.nlr.nl/providers/neonet.nlr.nl/local/ceos/datasets/EARTH_LAND_GISS_Paleocl_Dryas1.html>_

_However in most regions of North America the Younger Dryas interval was
apparently a time of continuing colonization by interglacial-type
vegetation, quite unlike the major setback that it represented for
European and Eurasian forests. In fact, Elias suggests that insect
assemblages from the Beringian land bridge indicate summer temperatures
several degrees warmer than todays' at around 11,000 14C y.a. (Elias _et
al._ 1996), though this might hypothetically have been just before a
cooling event. In contrast, preliminary results from the nearby Kodiak
Island, Alaska suggest substantially colder than present vegetation
conditions during the 11,000-10,000 14 y.a. interval (Peteet; see URL
above). _

_Rising sea levels seem to have finally cut off the land bridge of
Beringia sometime after 11,000 14C y.a. (rather later than was
previously thought), according to recent studies by Elias _et al._
(1996). According to the most recent data correlation and mapping effort
by the PALE group, a fairly broad swathe of land still existed across
the Bering Straits around 11,000 14C y.a.
<http://www.ngdc.noaa.gov:80/paleo/pale/bmaps.html> Given that sea level
barely rose at all during the subsequent Younger Dryas period, this
landbridge would presumably have remained in place until at least 10,000
14C y.a., acting as a continuing potential conduit for human groups
crossing from Siberia. _

_	_

_The retreating North American ice sheets had by now exposed an ice-free
corridor that was some 500 km or more wide along much of its length
(Dyke & Prest 1987). Animal remains start to appear at the southern end
of the corridor about this time. Tree species began to colonize
deglaciated parts of Canada, with conifer parkland appearing in the
Rocky Mountains at about 11,000 years ago (Thompson _et al_. 1993). A
number of pollen sites have been obtained from this general area, and
the summarized 'ShowTime' pollen abundances appear to suggest that
spruce (/Picea/) parkland was also widespread at the southern end of the
ice-free corridor. However, this picture may be based on poor
radiocarbon dates. In a review of the environmental history of the
southern end of the corridor, Beaudoin et al. (1996) suggest that most
of the vegetation was in fact non-arboreal, with birch shrubs. They note
that earlier studies suggested spruce woodland already important in the
landscape by this time, but that this is now in doubt because the
radiocarbon-dated materials were contaminated by coal and carbonates.
However, Beaudoin (pers. comm, January 1998) cites recently published
evidence that vegetation in this area c. 11 900-10 200 BP was mainly or
partly /Picea woodland/, in an environment characterized by consistently
arid and windy conditions" (J Paleolimnology 16: 37-57, 1996). _

_Shrub tundra, and not parkland, also seems to have remained the main
vegetation type across most of Alaska and in the northern part of the
deglaciated corridor. _

_	In southeastern Alaska, a short-term climate cooling between about
10,600 and 9,900 14C years ago was marked by expansion of tundra
elements and deposition of inorganic sediments (Hansen & Engstrom 1996).
To the south of the southern entrance to the ice-free corridor, both
dune and zoological evidence suggests a temporary return of desert
conditions to the High Plains of the American Midwest (Wells 1992),
although the timing and relative severity of this event remains highly
uncertain. The parkland and conifer woodland that had covered the
eastern prairie zone during the full glacial period had by now mainly
disappeared, being replaced by treeless grassland, although from the
high conifer pollen percentages it seems that a band of conifer parkland
may still have existed across the northern part of the prairie zone
(ShowTime database)._

_	In the eastern USA generally, deciduous forest species continued to
increase in abundance throughout the 'Younger Dryas' period. In the
newly deglaciated areas of the north-east, however, the predominant
vegetation remained tundra and spruce parkland (ShowTime database). _

_ _

_+ Shortly before the onset of the 'Younger Dryas' cold phase (about
11,000 14C y.a.) <NAmpreDryas11kyr.gif>_

_ _

_* Just before the beginning of the Younger Dryas cold phase, continuing
retreat of the ice sheets and recolonization of vegetation allowed
conifer forests to return across the Great Lakes region. In the 'ice
free corridor' of western Canada, spruce parkland (and, locally, spruce
forest) had colonized. Alaska remained as tundra, and areas most
recently exposed by the retreating ice were also still tundra. In the
eastern USA, the cool temperate forest belt was returning, but it was
generally somewhat further south than its present northern limit. Cool
temperate forest had in fact reached as far as Maine in the
north-eastern USA, but in this area it was eliminated and replaced by
boreal forest during the subsequent Younger Dryas cold period.*_

_* * _

_See Owen Davis' reconstruction (1998) of the western USA at 11,000 14C
y.a. <http://geo.arizona.edu/palynology/nam_p11k.gif>, just before the
Younger Dryas cooling_

_J. Adams' (1996) reconstruction of North America during deglaciation-
11,000 ^14 C ya <./NEW_MAPS/northam3.gif>_

_Beaudoin, Davis, Declourt, Delcourt, Richard & Adams' (1998)
reconstruction of North America during deglaciation - 11,000 ^14 C ya
<del11.gif>_

_Key to the vegetation classification system used in the paleovegetation
maps <adams3.html>_

------------------------------------------------------------------------

_* 10,000 radiocarbon years ago. *The colder, relatively arid phase
associated with the Younger Dryas seems to have ended shortly before
this time slice (around 10,200 radiocarbon years ago), in those areas
which it had affected. At the end of the Younger Dryas in the
north-eastern USA (New York, Maine, Conneticut) a change in forest
composition occurred, from boreal trees to a warmer white pine and oak
forest. See web page by Dorothy Peteet.
<http://neonet.nlr.nl/providers/neonet.nlr.nl/local/ceos/datasets/EARTH_LAND_GISS_Paleocl_Dryas1.html>_

_From the macrofossil changes, it is evident that trees such as spruce,
fir, and paper birch became locally extinct at the time of this warming
event, and have remained absent from this region for the past 10,000
years. In many other areas of North America, forest vegetation continued
to spread in, although it had not yet attained its full Holocene
coverage and remained an open parkland or forest-grassland mosaic in
many areas, especially in Canada (ShowTime database). _

_	The ice sheets of North America were by this time greatly reduced in
size. Large lake systems had formed around the fringes of the Laurentide
ice sheet but the retreat of the western Cordilleran ice sheet had left
a broad area of lowland which was gradually becoming colonized by
conifer parkland or open forest (ShowTime database). Beaudoin et al.
(1996) suggest that conifer forest was present in the south-eastern
foothills of the Rocky Mountains in Canada by around this time. Note
that the parkland was probably much sparser towards the ice sheet
margins, with a treeless zone of several hundred km resulting from lags
in tree colonization in the areas exposed by the rapidly-retreating ice
sheets. For example, pollen ecological studies by Ritchie in southern
Canada suggest a lag of 2000 ^14 C years at many sites between
deglaciation of a site and subsequent colonization by spruce and other
boreal trees. _

_In the western USA, such as the Owens Lake site in the Great Basin,
earliest Holocene conditions may have been moister than present (with
open juniper scrub vegetation replacing the open semi-desert) between
about 10,000 and 9,000 14C y.a., followed by dessication to conditions
similar to those of today (Benson et al. 1997)._

_See Owen Davis' reconstruction (1998) of the western USA at 10,000 14C
y.a. <http://geo.arizona.edu/palynology/nam_p10k.gif>. He suggests that
grassland vegetation became predominant in the High Plains of the
western USA about this time, replacing open conifer woodland. He
suggests that up until this stage, however, open conifer vegetation may
have been present across the northern High Plains_

_	_

_In the north-eastern USA/south-eastern Canada, colonization by trees
had progressed up to the stage of giving boreal conifer forest in many
areas. Deciduous tree species continued to increase in range and
abundance through the eastern USA (ShowTime database). _

_At the north-western end of the Plains region, in Alberta, conditions
may have been more slightly arid than present up until about 9,000 14C
y.a. (Beaudoin pers. comm. 1998). Schweger and Hickman infer early
Holocene high salinity and aridity from lakes in central Alberta and
correlate these with the GCM simulations (e.g. by Kutzbach). _

_J. Adams' (1996) reconstruction of North America during the early
Holocene- 10,000 ^14 C ya <./NEW_MAPS/northam4.gif>_

_Beaudoin, Davis, Declourt, Delcourt, Richard & Adams' (1998)
reconstruction of North America during deglaciation - 10,000 ^14 C ya
<del10.gif>_

_Key to the vegetation classification system used in the paleovegetation
maps <adams3.html>_

------------------------------------------------------------------------

_**_

_*9,000 radiocarbon years ago. *By 9,000 years ago, forest cover seems
to have returned to many areas of North America that had been barren or
covered with ice during the last glacial period. In the eastern USA, for
example, the temperate forest cover had reached approximately its
maximum Holocene extent (Overpeck et al., ShowTime database, Jackson et
al. 1997). A map of vegetation distribution for 9,000 y.a.
<http://web.ngdc.noaa.gov/paleo/image/gsafinal.gif> has been assembled
by R. Webb & J. Overpeck for the eastern USA, using pollen data._

_ In deglaciated parts of Canada, however, open parkland remained the
predominant vegetation cover (but with substantial areas of conifer
forest existing within this), with relatively low tree pollen influxes
and a large proportion of the pollen of steppe plants (ShowTime
database). In Alaska, tundra was still the predominant vegetation type,
with relatively restricted areas of spruce parkland, possibly due to
relative aridity. _

_ In the Midwest of the USA, prairie vegetation had approximately the
same position as at present, through extending somewhat further north
and east than during the late Holocene. Beaudoin et al. (1997) suggest
that prairie extended about 80 km further north between 9,000 and 6,000
14C y.a., with records suggesting greater aridity, higher temperatures
and greater salinity._

_Main QEN section on North America <new_na.html>_

_See Owen Davis' reconstruction (1998) of the western USA at 9,000 14C
y.a. <http://geo.arizona.edu/palynology/nam_p09k.gif> Semi-desert
vegetation seems to have become much more widespread in the
south-western USA at about this time._

_J. Adams' (1996) reconstruction of North America during the early
Holocene- 9,000 ^14 C ya <./NEW_MAPS/northam5.gif>_

_Beaudoin, Davis, Declourt, Delcourt, Richard & Adams' (1998)
reconstruction of North America during deglaciation - 9,000 ^14 C ya
<del9.gif>_

_Key to the vegetation classification system used in the paleovegetation
maps <adams3.html>_

------------------------------------------------------------------------

_*North America around 8,000-5,000 14C years ago.* Most of the Holocene
was slightly warmer than at present, with a greater northward extent of
temperate forest in the east, and tree limits lying slightly further
north in Canada and Alaska._

_A number of pollen sites show that the prairie (grassland) zone of the
central United States was expanded relatively far eastwards between
9,000 and 6,000 14C y.a. (Webb et al. 1993 Ref.). In the NE Prairie zone
of Minnesota, pollen records suggest that there was a considerable
expansion of the prairie around 8,800-7,800 14C y.a. (7,800-6,800 cal.
y.a.) (reviewed by Dean et al. 1996). For example at the Elk Lake site,
pollen-based reconstructions suggest that annual precipitation was 100mm
less than today, and the mean annual temperature was warmer by up to 2
deg.C. (though ostracodes - lake invertebrates - confusingly suggest
that conditions were /cooler/ than today up until around 7,800 14C y.a.
at this same site). _

_Following on from this phase, there are signs of moister conditions in
central-southern Canada around 6,000 14C y.a. Apparently due to moister
and cooler conditions, peat accumulation began in central Alberta about
6,000 14C y.a. (Vance et al. 1995). _

_The western USA shows signs of early-to-mid Holocene warmth and
dryness. Forest cover the Cascades Range of the western Cordillera of
the USA was slightly greater than today (with oaks and ponderosa pine),
with indicators of warmer drier conditions, between about 8,000 and
3,000 14C y.a. (9,100-3,900 cal. y.a.) (Whitlock & Bartlein 1997). After
this, a slight summer cooling and increase in rainfall occurred._

_ _

_+ <NA8kyr.gif>_

_ _

_*Although climates were as warm or slightly warmer than at present
around 8,000 14C y.a., large ice masses remained in eastern Canada
because there had not yet been enough time for the ice sheet to melt
away completely. Retreating rapidly and cooling the climate at their
immediate periphery, the ice masses exposed a rim of new land that took
time for the forest to colonize fully. In Alaska too, the present forest
cover had not yet returned fully, with ongoing ecological succession.
Most other areas, however, had somewhat similar vegetation cover to the
present. Drier conditions in the prairie zone caused greater eastward
and northward extent of the grasslands than at present.*_

_* * _

_Owen Davis' reconstruction (1998) of the western USA at 8,000 14C y.a.
<http://geo.arizona.edu/palynology/nam_p08k.gif>_

_Beaudoin, Davis, Declourt, Delcourt, Richard & Adams' (1998)
reconstruction of North America during the Holocene - 8,000 ^14 C ya
<del8.gif>_

------------------------------------------------------------------------

_* Since 5,000 radiocarbon years ago. *Various pollen sites suggest that
there was a relatively dry phase in the east-central prairie/grassland
zone in the mid-Holocene (about 6,000-4,000 14C y.a.) when prairie
vegetation extended perhaps 200 km further east into the eastern forest
zone in some places, and probably further north also. After 4,000 14C
y.a., conditions became cooler and moister. Some workers who have
studied this area suggest that different parts of the prairie zone
experienced warmest and driest temperatures at different times. In the
extreme north-east prairie zone, the temperature maximum seems to have
been around 6,000 14C y.a., whereas in a few hundred km further south it
may have occurred later (at around 4,000 14C y.a.) (Dean et al. 1996). _

_ Towards 3,000 14C y.a. the climate in the extreme NE of the USA/SE
Canada cooled slightly to the present conditions and boreal taxa
reinvaded some of their former territory in the Great Lakes and St
Lawrence regions (Richard et al. 1993, Jackson et al. 1997) and in the
northeastern mountains (Spear et al. 1994, Jackson et al. 1997). In the
extreme north of the grassland/prairie zone, rising lake levels and
decreased salinity, and a southward advance of boreal forest over
prairie, all suggest moister and cooler conditions (Beaudoin et al. 1997). _

_Maps of vegetation distribution for 6,000 and 3,000 14C y.a.
<http://web.ngdc.noaa.gov/paleo/image/gsafinal.gif> have been assembled
by R. Webb & J. Overpeck for the eastern USA, using pollen data._

_Phases of aridity in the Mid-West grassland/prairie zones seem to have
occurred sporadically during the Holocene, resulting in reactivation of
dunes which had been active during the Last Glacial, for example the
Sand Hills of Nebraska (Muhs et al. 1997). There seem to have been at
least two very intense prehistoric droughts resulting in remobilization
of the Sand Hills during the last 3,000 14C years (Muhs et al. 1997)._

_A rapid decline in the formerly abundant conifer known as hemlock
(/Tsuga/) occurred throughout the north-eastern USA at around 4,000 14C
y.a., with disease being a likely cause. This species has never achieved
its former levels of abundance, but has recovered substantially through
the north-east (Davis, l981). _

_A phase of cooler and wetter-than-present conditions at about 2,600 14C
y.a. (2,600 y.a.) has been found for southern Quebec, and the
cordillieran west of North America (evidence summarized by van Geel et
al. 1996)._

_ _

_+Present-potential vegetation <NAprsnt.gif>_

_ _

_See Owen Davis' reconstruction (1998) of the western USA at 5,000 14C
y.a. <http://geo.arizona.edu/palynology/nam_p05k.gif>_

_Beaudoin, Davis, Declourt, Delcourt, Richard & Adams' (1998)
reconstruction of North America during the Holocene - 6,000 ^14 C ya
<del6.gif>_

_Beaudoin, Davis, Declourt, Delcourt, Richard & Adams' (1998)
reconstruction of North America during the Holocene - 5,000 ^14 C ya
<del5.gif>_

_Key to the vegetation classification system used in the paleovegetation
maps <adams3.html>_

------------------------------------------------------------------------