Grasslands

General Information

Grassland vegetation in Arizona, which include the mountain grassland type, the plains grasslands, and the desert grassland, covered approximately one-fourth of the state, 18,210,000 ac, in the early 1950s (Nichol 1952). Changes in management practices in the grassland vegetation zone over the years (alterations by grazing, both domestic and wildlife; exclusion of fire; and others) have resulted in an invasion of shrub and other woody plant species from adjacent vegetation zones on many sites. Thus, shrubs and other woody plant species form an intermixed overstory on many areas classified as grasslands at one time (Ffolliott and Throud 1975b). Occurring throughout a range of elevations, individual grassland types that comprise this zone exhibit differences in terms of water yield and other natural resource potentials.

Mountain Grasslands

Mountain grasslands, small in aggregate area, are scattered throughout the mixed conifer and ponderosa pine forest types (Judd 1962, Lowe 1964). While tree species are not usually part of these communities, isolated trees or shrubs occur through invasion from the adjacent forests on cinder cones and elevated places within the grasslands. The forest edge, where the two habitats come together, is a well delineated ecotone. An ecotone is the transition area between two adjacent ecological communities. Greatest development of the mountain grassland type in Arizona is reached in the White Mountains and on the Kaibab Plateau. It is also represented on some of the higher, isolated mountains ranges in southern Arizona (Ffolliott and Throud 1975b).

Plant species—Characteristic grasses are timothy (Phleum pratense), Arizona fescue (Festuca arizonica), mountain muhly (Muhlenbergia montana), pine dropseed (Blepharoneuron triholepis), black dropseed (Sporobolus interreptus), mountain brome (Bromus marginatus), and the introduced Kentucky bluegrass (Poa pratensis). Many of these species have high forage value for livestock and wildlife species.

Recreation is a valuable resource of these high elevation grasslands because they are intermixed with the relatively cool and esthetic mixed conifer and ponderosa pine forest types.

Precipitation—Annual precipitation ranges from 30 to 45 inches, with almost 50% occurring during the summer season. The general weather patterns of mountain grassland areas coincide with those in the adjacent forests types.

Soils—Soils are usually fine-textured alluviums that are frequently and easily compacted and often have poor drainage. Mountain grasslands occupy relatively level terrain compared to most of the adjacent forests.

Plains Grasslands

The plains grassland type occurs as continuous or nearly uninterrupted cover between 5,000 and 7,000 ft, essentially in the eastern half of Arizona (Lowe 1964) (Figure 10-1 from Ffolliott and Throud 1975). Plains grassland occasionally extends upward into the lower portion of the ponderosa pine vegetation type, and is often found intermixed with pinyon-juniper woodlands and sagebrush.

Precipitation—The average annual precipitation in the plains grasslands is about 17 inches, with extremes of approximately 10 and 20 inches (Lowe 1964). About 70% of the total annual precipitation falls between April and September (Judd 1962).

Soils—Soils of the plains grassland type are generally very shallow-to-shallow, and predominately medium-textured. Deep, medium-to-fine textured alluvial soils are found along many of the drainages on these ranges.

Desert Grasslands

The desert grassland type reaches its best development in southeastern Arizona, at elevations between 3,000 and 5,000 ft, although it also occurs in the northwestern quarter of the state (Judd 1962, Lowe 1964) (Figure 10-1 from Ffolliott and Throud 1975). Desert grassland is a transitional grass-dominated landscape community, often positioned between desert shrubland below and the chaparral vegetation type above.

The climate of the desert grasslands is generally characterized by warm to hot summers, and by mild and open winters (Martin 1973). This grassland type is the most arid of all North American grassland regions.

Plant species—The grasses and forbs that characterize desert grassland ranges tend to grow and set seed rapidly because the rainy seasons are relatively short and somewhat uncertain. Most of the growth takes place during July, August, and sometimes part of September. However, since temperature is usually not a limiting factor at this time of year, the growth period is determined primarily by the amount, distribution, and timing of precipitation (Humphrey 1958). By October, most of the grasses and forbs are dry, and commonly remain so until the next rainy season. Most of the plant growth occurs in the summer rainy period because of the less favorable temperatures during the winter growing season, the frequent desiccated condition of the soil in the spring, and because many forage species are warm season grasses.

Precipitation—Precipitation in the desert grasslands varies from 10 to 12 inches annually, averaging about 6 inches (Judd 1962, Lowe 1964). Most of the precipitation occurs as liquid rainfall during two seasons, summer and winter (Humphrey 1958). Summer rains are largely in the form of thunderstorms, with high-intensity events lasting for short periods. Winter storms are frontal, producing relatively low-intensity precipitation events, with rainfall occurring intermittently for several days. Normally, relative humidities are low throughout the year, except during the storm periods.

Temperatures and wind velocities are often high in the desert grasslands, and evaporation rates are high. For example, a 14-year pan evaporation record for the Santa Rita Experimental Range in south-central Arizona indicated an average annual evaporation rate of 109 inches (Humphrey 1958). The high temperature and wind velocities, coupled with the low humidity, account for the comparatively high evaporation rate.

Soils—Many of the soils of the desert grasslands have been developed under high temperatures and low rainfall, and are generally known for their reddish color (Buol 1965). These soils are classified into three great soil groups: The Red Desert, the Reddish Brown, and the Reddish Chestnut. The Red Desert soils, which received from 11 to 20 inches of precipitation annually, develop at elevations between 2,500 and 5,000 ft. The value of these soils for range depends almost entirely upon the amount of precipitation they receive. The Red Desert soils provide only seasonal grazing for domestic livestock, while the Reddish Brown and Reddish Chestnut soils normally provide somewhat more forage.

Images Available

There are currently over 90 images available in the image database illustrating various aspects and conditions found in grassland ecosystems in the southwestern United States. These can be accessed after getting into the database by using the key word grassland. Additional key words are available for searching the images in a drop down list within the database.