Vegetation Classification and Mapping Project Report - USGS

Introduction
1853 included several scientists who
recorded information on the flora, fauna,
and landscape (relevant comments are
summarized in Thomas et al. 2003). Beale
(1857) proceeded westward from Zuni,
New Mexico, and crossed the Puerco
River, traveling west until he reached the
Little Colorado River near Holbrook;
Beale noted cottonwood trees on the river
bottoms, along with abundant grass.
These expeditions, as well as the
movement of people and livestock to gold
strikes in California, set the stage for new
settlers in the area. Cattle and sheep were
herded through northern Arizona, and
ranchers moved sheep and cattle into Little
Colorado drainage areas. The coming of
the railroad in 1881 further encouraged
ranching (Thomas et al. 2003).
1.5.4 Possible Human Impacts on the
Landscape
Early humans may have burned the
vegetation to attract or to drive big game
(Pyne 1982), although the few artifacts
from the Paleo period at PEFO suggest
that there were very few people in the area
during this time period. Archaic people
may have also burned or manipulated
vegetation. In Puebloan times, intensive
farming, using floodwater agriculture and
manipulation of intermittent streams,
may also have affected certain areas of the
park. Of all the human activities occurring
in the Petrified Forest region, livestock
grazing during the historic period probably
had the most significant effect on the
landscape.
Thomas et al. (2003) summarized the
changes in livestock numbers in Northern
Arizona during the 1700s and 1800s. On
Navajo lands, there were around 64,000
sheep by 1742 and perhaps 500,000 by the
1850s (Young 1968). The Navajo people
are said to have grazed sheep on lands
between the Little Colorado River and the
San Juan River, and from the San Francisco
Peaks to the Rio Puerco, thus possibly
covering lands that are now included
within the park. After the Navajo returned
from their exile at Bosque Redondo in
New Mexico in 1868, they rebuilt their
herds up to around 1,100,000 sheep,
400,000 goats, and 60,000 horses (Young
1968). These herds were likely kept within
reservation boundaries. However, grazing
on higher elevations and upstream of
drainages running into the park likely may
have affected parklands through erosion
and down cutting of stream channels.
The number of sheep held by ranchers
outside Navajo lands in Arizona increased
from about 800 in 1870 to 76,000 in 1880
(Sheridan 1995). This increased to 700,000
in the next decade. Most of Arizona’s
sheep were raised in the northern
counties. Barnes (1913) estimated
approximately 1,570,000 sheep in Arizona.
Cattle were not as abundant as sheep,
their numbers increased from around
50,000 in the late 1870s (Hamilton 1883)
to 812,000 in 1913 (Barnes 1913). There
were ranches in the Holbrook area in the
late 1800s and early 1900s, so grazing no
doubt had a direct effect on lands that
are now part of the park. In addition,
grazing in the Little Colorado and Puerco
drainages may also have affected the park
lands, through erosion and deposition, as
well as downcutting of stream channels.
A series of droughts in the late 1800s and
early 1900s, in association with intense
grazing, probably accelerated erosion,
downcutting, and decline of groundwater
levels (Thomas et al. 2003).
1.5.5 Climate
Petrified Forest National Park is within
Bailey’s (Bailey et al. 1994) Colorado
Plateau Semidesert Province, a region of
around 195,000 km 2 (75,300 mi 2 ) including
parts of Arizona, New Mexico, and
Utah. In general, the climate has the cold
winters typical of high elevations. Summer
days are often hot, and nights are cool.
Temperature at PEFO is highest in July and
lowest in January [July mean high = 92.3° F
with an extreme of 105° F in 1995; January
mean high = 47.5° F with an extreme of
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