Arches National Park

Ecology
Gorp.com

The word desert often conjures up the idea of a barren, desolate land void of life, with high temperatures and no water. This is not completely without foundation. In fact, the word desert originates from the Latin word "to desert, or abandon." Deserts, however, are not lifeless; certain plants have adapted to the extreme conditions.

The Desert Ecosystem

Deserts form where global weather patterns and geographic landforms create a climate characterized by less than ten 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 contribute to the arid environment of Arches.

Arches receives an average of nine inches of precipitation a year, most of it from melting winter snows. The elevation of the park (4,000 to 5,600 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 degrees 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 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 percent of their captured solar energy back to the sky. Without clouds to hold the heat in, the air rapidly cools.

Surface temperatures in direct sunlight are commonly 25 to 50 degrees 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.

Adaptations

Plants 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 each year.

Perennials, 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 that 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 soil crust.

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.

The 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 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 decades.


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