Biology Reference
In-Depth Information
To the far northwest at Quartz Creek in the Yukon part of Beringia at the
LGM (around 24 kyr), plant fossils have been recovered from a rodent nest,
the stomach contents of an extinct horse, and an alluvial peat (Zazula et al.
2003). The vegetation included prairie sage (
Artemisia frigida
), grasses (
Poa
,
Elymus
), sedges, rushes (
Juncus/Luzula
), and other plants that formed a
herb-tundra ecosystem (with some small, scattered trees) on which grazed
mammoths, horses, and bison in a cold dry climate.
In the southwest, a study of stalagmites in caves of the Guadalupe
Mountains of Texas and New Mexico show a mid-Holocene dry period with
modern conditions established some 4000 years ago (Polyak and Asmerom
2001). In the broader region of the western United States, the Medieval
Warm Period (900-1300 CE) is evident (Cook et al. 2004). In the Mojave
Desert,
Neotoma
(packrat) middens spanning the past 24,000 years reveal
that the present arid desert of
Larrea tridentata
(creosote bush) was pre-
ceded above 1000 m by a moister pluvial vegetation of
Pinus monophylla
(piñon pine),
Juniperus osteosperma
(Utah juniper),
Purshia mexicana
(bit-
terbush),
Cercocarpus ledifolius
(mountain mahogany), and
Prunus fascicu-
lata
(desert almond; Koehler et al. 2005).
Studies of two rare peat bogs in central Texas provide an interesting
scenario of how accounts of Quaternary biogeography have improved over
time. When John Potzger and B. C. Tharp (1943, 1947, 1954) made the
fi rst investigations of these sites, Potzger identifi ed pollen of
Abies
and
Picea
in combined amounts of 11 percent in the lowermost levels. Neither fi r
nor spruce grows within several hundred miles of central Texas (the pres-
ent vegetation is oak-hickory woodland and oak savanna). Earlier, Brown
(1938) had reported cones, twigs, and wood of the boreal
Larix
(larch)
,
Picea
, and
Thuja
(northern white cedar) from Pleistocene deposits in Loui-
siana. The combined records were included in the monumental “Biogeogra-
phy of the Pleistocene,” a 1949 review by E. S. Deevey running more than
a hundred pages. With the data then available, it was plausible to conclude
that at the LGM the boreal forest, represented by
Abies
,
Larix
,
Picea
, and
Thuja
, extended all the way to the Gulf Coast and remained there until
about 9 kyr (the age of the lowermost sediments in the Texas bogs). It was
thus also reasonable to assume that the deciduous forest had been forced
even farther south. Today there are about fi fty species of plants from east-
ern North America disjunct in eastern Mexico, separated by the Chihua-
huan Desert of south Texas and northern Mexico, in an altitudinal zone be-
tween 1000-2000 m along the eastern slopes of the Sierra Madre Oriental,
including
Acer
,
Alnus
,
Carpinus
,
Carya
,
Cornus
,
Diospyros
,
Fagus
,
Fraxinus
,
Juglans
,
Liquidambar
,
Magnolia
,
Ostrya
,
Platanus
,
Populus
,
Tilia
, and
Ulmus
.
