White Mountains Online

Petrified Forest National Park
Shortgrass Prairie -

The Desert's Grassland
by Lorna McNeil-Cox

Petrified Forest National Park Arizona. A place of badlands, the Painted Desert, and only nine inches of annual rainfall. Based on that description, images of a barren desert come to mind. In reality, it is not truly a desert at all! It is actually a mosaic of plant communities and ecosystems - called the shortgrass prairie. That does not mean, however, that you should expect to see the Midwestern prairies of Nebraska growing happily in Arizona. A shortgrass prairie is an area technically referred to as an "extreme tension zone." It is not a desert, and not quite a prairie. The shortgrass prairie of Petrified Forest is actually one of the farthest westward extensions of this zone type.

The Petrified Forest area is located on the Colorado Plateau, only 50 miles from the Mogollon Rim, and at an elevation of over 5,000 feet. Due to the rain shadow effect of the mountains blocking moisture, the climate quickly changes from the wet pine forests of the higher elevations of the Mogollon Rim, down to pinyon-juniper woodlands, and then to the drier shortgrass prairie of Petrified Forest at elevations of 5,000 to 6,500 feet. Much of the prairies's existence in Petrified Forest is marginal. The best example of the grassland community is found on the loamier soils of the higher mesas in the park that receive more precipitation.

Overall, the climate here effectively favors grass and shrub growth over tree growth. The grasses and shrubs of Petrified Forest are well adapted to survive the windy conditions, limited water, and soil types that exist here.

The loamy soils hold moisture well, but prevent deep percolation of water. The soils are in part responsible for badland formations. Water tends to run off and cause erosion rather than to soak in. With low annual rainfall amounts distributed through the summer and winter months, growth of the shallow root systems of grasses and shrubs is favored over the deeper root systems of trees. Blue Gramma is the dominant grass species, with a root system specifically adapted to effectively capture surface water. This grass forms a thick shallow root mass (called "sod") so dense that water is prevented from percolating below 24 inches down into the soil The sod of the prairie grasses deprives the deeper rooted plants of water and assures the dominance of this plant community in areas of low rainfall or drought.

Plants of the shortgrass prairie have many other adaptations for survival. Though the heavier clay soils hold more moisture, those conditions allow for the build up of salts in the soil. Most plants cannot tolerate the concentrates of salts found in the heavier soils. The four winged saltbush and greasewood, both found in Petrified Forest, thrive in such adverse conditions. They are lovers. They have adapted to absorb salts and store them in special structures on their leaves, or to secrete salts through pores called stomata as they transpire (breathe).

The four-winged saltbush is named for its parchment-like wings on each seed, and has additional adaptations to defend itself from harsh conditions. Powdery, whitish scales cover its stems and protect the plant from dryness, shade it from profuse heat and light, and reduce evaporation from the leaf surfaces.

Many grasses protect themselves from dry windy conditions with a layer of silicon oxides in the outer covering of the stem. This allows them to remain tough and upright, yet flexible enough to bend.

Many sagebrush, shrubs, and some grasses possess a special adaptation for photosynthesis. They are called C4 plants, referring to the number of carbon compounds initially produced in the process of photosynthesis. Most plants have only three carbon compounds and are referred to as C3 plants. C4 plants can fix twice as much carbon dioxide as C3 plants do during photosynthesis. This allows C4 plants to develop more sugars, grow faster, and function better in a hot, dry climate.

Competition between plants for the available water and habitable soils has resulted in additional adaptations for survival. Many plants can produce toxins in their root systems that prevent other plants from encroaching on the occupied space. This results in the often evenly spaced appearance of plants in arid and semi-arid areas. Many forms of wildlife are also well adapted to the shortgrass prairie environment. The kangaroo rat, common in Petrified Forest, can live its entire life without ever drinking water! The kangaroo rat is perfectly adapted to dry living conditions and is able to obtain all necessary moisture from the plants and seeds it eats.

Over 60 species of mammals have been identified within the park, though the most commonly seen mammals are the pronghorn, cottontail, jackrabbit and prairie dog. There are over 20 species of reptiles, and even a few amphibians. At least 203 different bird species have been observed inhabiting or passing through Petrified Forest.

Though Petrified Forest appears a lifeless desert, the shortgrass prairie community offers a lively work of wonder to those who take the time to look. Though most of us come to Petrified Forest for a glimpse of the Triassic past, we also find that Petrified Forest is a place with an interesting present. We see a place of changes and diversity. Today the National Park Service protects Petrified Forest's past and present for the future. We invite you to explore Petrified Forest, but leave it undisturbed to allow nature to take its course. We have the unique opportunity to preserve not only the past, but also the intriguing natural systems of today!

Printed Courtesy Petrified Forest National Park, Michele Hellickson - Superintendent

Petrified Forest National Park, Home Page
Behind the Scenery - Research at the Petrified Forest National Park

Prehistoric Man In the Petrified Forest

Holbrook Home Page