Deserts form where global weather patterns and geographic
land forms create a climate characterized by less than
10 inches of accumulated moisture annually, and where
potential evapotranspiration exceeds precipitation. Arches
National Park lies at a latitude north of the equator
where dry air masses constantly descend toward the surface
of the earth. The area is also in the interior of a large
continent away from marine moisture and in the rain shadow
of the Sierra Nevada Mountains to the west. All of these
factors act to produce the arid environment of Arches.
receives an average of 9 inches of precipitation a year,
most of it from melting winter snows. The elevation of
the park (4,000 to 5600 ft.) and the snow create what
is called a cold or high desert. Low moisture in the air
allows more sunlight to reach the ground, raising daytime
temperatures, another distinguishing feature of a desert.
The average maximum summer temperature at Arches is 100
F. As a result of these unusual conditions, the plants
found here are a unique blend not found in other deserts
of the world.
Desert plants must be able to deal with extreme variations
in temperature and water availability, as well as intense
sunlight. In this high desert environment, temperatures
fluctuate greatly, both daily and annually. In summer,
highs climb well over 100 degrees F, while winter temperatures
often drop below zero. On a hot summer day the temperature
may fall 30 to 50 degrees F as night approaches, because
of the low humidity and lack of cloud cover. As the sun
sets, rock and sand, which do not hold heat well, release
almost 90% of their captured solar energy back to the
sky. Without clouds to hold the heat in, the air rapidly
Surface temperatures in direct sunlight are commonly 25
to 50 degrees F warmer than the air temperature six feet
above. Temperatures in the shade may be cooler by 20 or
more degrees. Winter snow and violent thunderstorms fall
on thin, sandy soils that do not retain much moisture.
use a variety of techniques to survive desert extremes.
Some plants, referred to as "drought escapers,"
make use of ideal growing conditions found in the spring
when temperatures are cooler and water more abundant.
These annual plants have a short life cycle and include
the spring wildflowers that occur in showy abundance early
plants that live longer than one year, must deal with
desert extremes in other ways. "Drought resistors"
are plants that have made adaptations to get them through
lean times. Cacti store water within their bodies, blackbrush
drop their tiny, leathery leaves in dry weather, and yucca
have tap roots up to 30 feet long which are able to reach
water deep underground. Many desert plants have lightly
colored, highly reflective leaves.
"Drought evaders" have even more radical adaptations.
Moss, a plant not commonly associated with deserts, thrives
because it can survive long periods of drought. When water
is unavailable, it literally dries up. When water is suddenly
plentiful, the plant readily soaks it up and becomes moist
and green almost immediately. Mosses are usually found
growing in the shade of larger plants or in cryptobiotic
Another interesting adaptation is that of the Utah juniper,
one of the most common trees in the southwest. During
a drought, the juniper will shut off water flow to one
or more branches, killing them in order to preserve the
rest of the tree.
Other desert plants may grow only in specialized habitats.
Moisture dependent monkey flower, easter flower and ferns
all can be found in well-shaded alcoves with dripping
springs. Cottonwood, willows and cattail, which require
lots of water, can be found on river banks.
Living Soil. A
unique desert plant community that you are sure to see
during your travels in Arches is cryptobiotic soil.
This crumbly, black soil crust is made up of fungi,
lichen, algae, moss and bacteria all living together
in a symbiotic relationship, one in which all the members
benefit from their communal coexistence. Cryptobiotic
crusts are very important to the desert community because
they stabilize the soil, prevent erosion, retain water,
and provide important nutrients such as nitrogen to
plants. A plant seed that lands in cryptobiotic crust
has a greater chance of survival than one that lands
in loose, dry sand. Unfortunately, cryptbiotic crusts
are very fragile. One misplaced footstep can quickly
turn crust to dust, and recovery and regrowth may take