Natural Resources Monitoring at Casa Grande Ruins National Monument

Overview

Created as the nation's first archeological reserve in 1892, Casa Grande Ruins National Monument preserves an ancient Hohokam Culture farming community and "Great House," a multi storied, earthen-walled structure surrounded by the remains of smaller buildings and a compound wall. The monument contains 61 documented prehistoric archeological sites on its 472.5 acres.

The desert biome is dominant in this low-elevation park. Major issues of concern relative to natural resources include adjacent land use, groundwater depletion, and invasive exotic plants. Conflicts between natural and cultural resources are another issue. Both native and non-native species have damaged and threatened cultural resources at the park since the early 1930s. Even mammals and birds that might not normally be considered pests, such as native round-tailed ground squirrels (Spermophilus tereticaudus), threaten archeological structures and sites by burrowing, nesting, feeding, and roosting on or near them.

The Sonoran Desert Network monitors air quality, invasive exotic plants, climate, landbirds, groundwater, and vegetation and soils at Casa Grande Ruins National Monument. The results of this work can be found in a variety of publications and other information. The network also maintains species lists for the park.

Park Setting and Key Resources

Size: 191 hectares
Elevation range: 431–436 meters

Biogeography and physiography

Casa Grande Ruins National Monument is located within the Basin and Range physiographic province, characterized by nearly level valley floors surrounded by mountain ranges. The monument is close to four mountain ranges: the San Tan, Sacaton, Picacho, and Casa Grande mountains. The monument drains into the McClellan Wash, a tributary of the intermittent Gila River, which flows 1.5 miles north of the monument. Casa Grande Ruins also lies within the Sonoran Desert ecoregion, spanning 55 million acres in Arizona, California, Baja California, and Sonora. Casa Grande Ruins National Monument is located in the desert biome. Average annual precipitation is 9 inches (230 mm).

Local geology and soils

The monument lies on an alluvial deposit composed of Quaternary-age gravel, sand, and silt. The alluvium thickness increases from 400 feet at the Gila River to more than 1,200 feet in Coolidge; alluvium at the monument is approximately 800–1,200 feet thick.

Soils at and near the monument are classified at Hyperthermic Arid soils, which have a mean annual soil temperature of greater than 72°F and receive less than 10 inches of annual precipitation on average. Coolidge sandy loam comprises over 80% of the soil. The rest is the Laveen loam soil type. Both soil types have less than 15% rock fragments by volume, are considered well-drained, and have a slight risk of water erosion. The soils at the proposed expansion areas adjacent to and near the current monument are also Laveen loams and Coolidge sandy loams. At the proposed Adamsville unit, there are four soil map units: Coolidge sandy loam; Denure sandy loam (1–3% slopes); Gunsight-Pinamt complex (–8% slopes); and Laveen loams. The Gunsight-Pinamt complex soils have 35–60% rock fragments by volume. Soil properties have important consequences for vegetation composition, persistence and productivity.

Biological soil crusts

Open spaces on the soils are covered by biological soil crusts, a community of cyanobacteria, algae, lichens and bryophytes. Biological soil crusts provide key ecosystem functions. They increase resistance to erosion by water and wind, contribute organic matter, and fix atmospheric nitrogen. Cyanobacteria weave through the upper few millimeters of soil, binding together soil particles.

In the Sonoran Desert, cyanobacteria dominate the crust community. Lichens are a composite, symbiotic organism composed of a fungus and either a cyanobacteria or a green algae, and occur on the soil surface. Bryophytes, which also occur on the soil surface, are small, non-vascular plants, including mosses and liverworts.

The distribution and species composition of biological soil crusts is influenced by soil chemistry and disturbance. The recovery of biological soil crusts from disturbance depends on factors such as the climatic regime and type of disturbance. Generally, crusts recover slowly in areas with high annual temperature and low annual precipitation, such as Casa Grande Ruins National Monument. Biological soil crusts follow a recovery sequence in which, typically, cyanobacteria first colonize a site, followed by cyanolichens, other lichens, and then moss.

Vegetation

Casa Grande Ruins National Monument is composed primarily of desert shrubland dominated by creosotebush (Larrea tridentata). In some areas, these plants are spaced about 2–3 m apart, with no other shrub species present. In other areas, shrubs, such as wolfberry (Lycium fremontii, Lycium andersonii), cattle saltbush (Atriplex polycarpa), triangle bur ragweed, (Ambrosia deltoidea), desertbroom (Baccharis sarothroides), or littleleaf ratany (Krameria erecta) grow in association with creosote. The ground between shrubs usually appears barren.

Velvet mesquite (Prosopis velutina) and barrel cactus (Ferocactus wislizenii) are scattered throughout the shrubland, with the barrel cactus usually growing singly and the mesquite frequently in clumps of a few to several individuals. Perennial herbaceous vegetation is notably sparse in the monument, with purple threeawn grass (Aristida purpurea) and desert globemallow (Sphaeralcea ambigua) found only occasionally. A vegetation mapping report and plant guide for the park provide more detail.

Climate

The monument experiences climate typical of the Sonoran Desert ecoregion: highly variable, bimodal precipitation with a considerable range in daily and seasonal air temperature and relatively high potential evapotranspiration rates. Approximately 40% of the annual precipitation falls during thunderstorms from July through September, when maximum air temperatures can exceed 104°F and lead to violent (and often localized) rainstorms. The bulk of the remaining annual precipitation falls in relatively gentle events of broad extent from November through March.

Weather and climate data for Casa Grande Ruins National Monument and all other Sonoran Desert Network parks can be found at The Climate Analyzer, an interactive website that allows users to create custom graphs and tables from historical and current weather-station data. A weather and climate inventory was created for the Sonoran Desert Network in 2007.

Key Issues

Surrounding land use

Agricultural fields near the monument are potential vectors for non-native invasive plants, such as red brome (Bromus rubens) and Johnsongrass (Sorghum halepense). In addition, periodic dredging of the Pima Lateral Canal results in sediment deposition, likely containing non-native plant seeds, along the monument boundary.

To maintain high yields, farmers typically use pesticides and herbicides. Because the insects killed by these chemicals are the primary food source for many animals, the loss or contamination of insects may cause mortality, impaired health, or abandonment of the area by animals. Drift, or overspray, from the aerial application of herbicides and pesticides can adversely affect the monument's vegetation.

Residential and commercial developments may increase non-native plants, trash, and runoff; decrease the water quality of runoff due to toxins from vehicles; disrupt animal movement patterns; and increase mortality of native animals due to free-roaming pets. Residential development and housing density are expected to continue to increase in the area surrounding the monument.

Groundwater depletion

A century of groundwater use for agricultural, residential, and commercial uses has lowered the groundwater table around Casa Grande Ruins National Monument. In some areas, earth fissures and land subsidence have formed due to the compaction of alluvium following groundwater removal. The nearest fissure is approximately 10 miles from the monument. The nearest documented land subsidence is five miles south of the monument. The potential for subsidence at the monument, which could threaten the Great House, is a concern.

Invasive exotic plants

Biological invasions into new regions, whether accidental or deliberate, have increased at unprecedented rates in the past few hundred years. In the American Southwest, historic and current land-use practices, such as livestock grazing and fire suppression, are thought to have helped make arid lands more susceptible to invasion and subsequent loss of native species, as well as decreased biodiversity.

During a survey effort from 1999 to 2001, 29 non-native, introduced plant species were recorded at Casa Grande Ruins National Monument, 12 of which were grasses. Most of the other species were forbs, with two trees/shrubs: tamarisk (Tamarix spp.) and tree tobacco (Nicotiana glauca). During 2006, 17 non-native species were recorded. Several of these non-native species were introduced to the park as a direct result of human activities, such as past settlement, grazing, farming, excavation, and construction activities.

Natural/cultural resource conflicts

Native and non-native species have damaged and threatened cultural resources at Casa Grande Ruins National Monument since the early 1930s. Even mammals and birds that might not normally be considered pests threaten archeological sites by burrowing, nesting, feeding, and roosting on or near the sites. Species that particularly threaten cultural resources and human health and safety include round-tailed ground squirrels (Spermophilus tereticaudus), house finches (Carpodacus mexicanus), common pigeons (Columba livia), and European starlings (Sturnus vulgaris).

The acidic urine and fecal matter of birds damages the monument's archeological sites by reacting with their alkaline walls. This is especially a concern at areas of animal concentration, such as nesting and roosting sites. Burrowing mammals dig up archeological resources, in some cases exposing them to water, wind, and theft, and disrupt the artifact layering. Rodent burrows can also undermine the bases of prehistoric structures. In some cases, rodents have burrowed beneath modern encapsulation materials to get to the softer prehistoric walls. The burrows can be up to 18 inches below the surface and 10 feet long. This undermines the soil surface stability and can result in cave-ins when people walk across them.

The number of birds roosting and nesting in the Great House increased dramatically during the 1990s, when five gallons of bird debris fell onto the ruin floors every week. Burrowing round-tailed ground squirrels began to become problematic in the mid-twentieth century. Efforts to control the round-tailed ground squirrels, a native species, illuminate a conflict between management objectives: preserving the ruins versus protecting the native species and ecosystem processes.