Environmental Engineering Reference
In-Depth Information
45,000 to 11,000 years ago during the Wisconsin. 38 Under glacial climates, with cooler sum-
mers and greater winter rainfall, warm desertscrub communities dominated by creosote-
bush were restricted to the Bolson de Mapimi area in the southern Chihuahuan Desert 39 ;
using the climatic criteria of Schmidt 40 ; and below 984 ft elevation in the Lower Colorado
River Valley in the Sonoran Desert. 41
Packrat middens in rockshelters in the Hueco Mountains 35 km northeast of the
international boundary at 1,340-1,430 m elevation just west of El Paso, Texas, in the
northern Chihuahuan Desert yielded a 42,000 year series of packrat middens. 38,39 Samples
dated from 42,000 to 11,000 year B.P. (radiocarbon years before 1950) were dominated by
woodland species including pinyon pines ( Pinus edulis , Pinus remota ), juniper ( Juniperus
sp.), and shrub oak ( Quercus pungens ). 39 The middle Holocene (ca. 8000-4500 year B.P.)
vegetation was desert grassland with Chihuahuan desertscrub developing about 4500
years ago with the arrival of creosotebush, lechuguilla ( Agave lechuguilla ), and ocotillo
( Fouquieria splendens ).
Midden series from Maravillas Canyon and Río Grande Village on the Texas-Coahuila
boundary at 600-835 m elevation in the Big Bend of Texas provide vegetation records for
the last 45,600 years. 39 As in the Hueco Mountains, the late Wisconsin (22,000-11,000 year
B.P.) samples documented pinyon-juniper-oak woodlands in areas that now support
Chihuahuan desertscrub. However, the middle Wisconsin (45,600-22,000 year B.P.) was
more xeric with little or no papershell pinyon ( P. remota ) and increased desert grassland
elements. The Holocene vegetational history differed from the northern Chihuahuan
Desert as desertscrub developed by 10,500 year B.P. without the middle Holocene desert
grassland. The oak in the Wisconsin samples was Hinckley oak ( Quercus hinckleyi ), now
a rare endemic shrub only found in the Solitario and near Shafter north of Big Bend
National Park.
In Arizona, woodlands with single leaf pinyon ( Pinus monophylla ), junipers, shrub live oak
( Quercus turbinella ), and Joshua tree ( Y. b re v if oli a ) were widespread in the present Arizona
Upland subdivision of the Sonoran Desert. 41 Ice age climates with greater winter rainfall
from the Pacific and reduced summer monsoonal rainfall from the tropical oceans likely
favored woody cool-season shrubs with northern affinities42 42 rather than the summer-
rainfall trees, shrubs, and cacti of tropical forests and subtropical deserts.
In the Puerto Blanco Mountains of Organ Pipe Cactus National Monument (9 miles
north of the Sonora border), saguaro and brittlebush returned to Arizona soon after the
beginning of the Holocene about 11,000 years ago but were living in xeric woodlands.
Although Sonoran desertscrub vegetation formed about 9000 years ago when displaced
woodland species finally retreated upslope, the modern climatic regime and community
composition were not established until foothills paloverde, ironwood, and organpipe
cactus arrived about 4500 years ago. However, the plant communities never achieved an
equilibrium state because the climate continued to fluctuate, although on lesser scales.
Notable late Holocene climatic events were the wet period about 1000 years ago coincident
with the development of the sophisticated Anasazi and Hohokam Indian cultures, and the
relative aridity of this century.
Similar successional stages likely occurred during each interglacial. Although the
late Holocene desertscrub communities likely resembled the original late Miocene
Sonoran Desert, relatively modern desertscrub communities were developed for about
5%-10% of the 2.4 million years of the Pleistocene. Ice age climates with woodlands in
the desert lowlands typical of about 12,000 year B.P. persisted for about 80%-90% of this
period. 43,44
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