The Desert Ecosystem. 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.

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 cools.

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.

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 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 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.