Why Casa Grande Grows Only Creosotebush: Vegetation and Soils, 2008–2016

Every landscape tells a story, and National Park Service ecologists return to the same ground over the years to read it. Plants underpin every process on land — and managers can steer vegetation toward their goals — so they need to know which plants are there and whether that is changing.

The monument

Casa Grande Ruins National Monument, 472 acres in south-central Arizona, became the nation's first archeological reserve in 1892. It protects an ancient Hohokam farming community and its "Great House", a multistory earthen-walled building surrounded by smaller ruins and a compound wall. It is rich in archeology — and strangely poor in plants.

How it was monitored

The Sonoran Desert Network set up permanent 20 × 50-meter plots, recording every plant species and its height along six transects per plot, and sampling the soil for stability and biological soil crusts. Six plots were monitored from 2008 to 2016, three of them near archeological sites recently stabilized.

A near-monoculture

Creosotebush at Casa Grande Ruins National Monument. NPS/A. Wondrak Biel

• Creosotebush (Larrea tridentata) was the only common perennial on every plot.
• Fremont's desert-thorn (Lycium fremontii) turned up on one plot.
• No other perennial species appeared on any transect in any year — no saguaros, the Sonoran Desert's icon, and no invasive exotic plants either.
• Plant cover did not change over the years for any species.

Soils: good news

Biological soil crusts help keep soils in place. NPS/A. Wondrak Biel

What the monument does have is biological soil crust, which holds soil in place against erosion. Crusts cover nearly 60% of the available soil — among the highest the network has seen in any Sonoran Desert park or refuge. Bare soil is low (about 15%), soil stability is high, and the land is nearly flat: little risk of erosion, which is good for archeological sites near the surface.

The exception is the recently stabilized plots, where crust cover was lower and about half the ground was bare. The network's assessment points — thresholds that serve as an early warning — flagged erosion there, and saguaro recruitment, as potential concerns.

How the woodland disappeared

In the mid-to-late 1800s, travelers were struck by the tall mesquite. An 1879 account said mesquite hid the Great House until you were almost upon it; an 1869 survey recorded mesquite, greasewood and grass — which drew cattle ranchers by the early 1870s, and ranching prospered for about 50 years. In 1902, custodian Frank Pinkley reported that cattle had eaten all the forage within 100 yards of the Great House. Still, early records describe velvet mesquite woodland mixed with desert shrubland — until, by the end of the 1930s, accounts of dying mesquite were common. Recent vegetation mapping and this monitoring show where that change ended: low-diversity desert shrubland.

Vegetation map of the monument, mostly creosotebush, from Buckley and others, 2009. NPS

The likely driver: groundwater. When centrifugal pumps spread through the valley in the mid-20th century, the local groundwater was rapidly drawn down — in effect mined — so much that the ground subsided by as much as a meter by 1978.

Mesquite roots draw water up from deep soil, moistening the ground and creating microclimates where other plants grow. Drawing by NPS/Sarah Studd

Mature mesquite had been tapping the shallow water table of the Gila River floodplain. Through "hydraulic lift", their roots bring water up from deep soil into shallow layers, and their shade cools the air and soil — cool, moist islands where clusters of other plants grow. But old mesquite can't reshape their roots fast enough to follow a falling water table. Today's mesquite are scattered and shrubby, probably living on recent rain alone. As the big trees died, their microclimates vanished: the result has been desertification of the monument's vegetation since it was established.

The Casa Grande and Compound A, about 1926–1927. NPS photo

An older disturbance, too. The main soil, Coolidge sandy loam, drains well and was probably heavily disturbed by the Hohokam: digging canals, working irrigated fields, and building dense clusters of structures meant moving enormous amounts of soil, much as modern development does. Quarrying caliche — a hard carbonate layer in desert soils — for the Great House was likely intense disturbance. That may have changed how water soaks in and is held around roots. Many perennials that grow in similar climates elsewhere seem unable to survive here — perhaps for lack of soil water.

Research that would help

• A detailed soil survey, for erosion risk and other resource questions.
• A small experiment growing expected-but-absent native plants in the park's own surface soils and crusts, under different microclimates, to find why so few perennials grow.
• A species-level inventory of biological soil crusts, which do an outsized share of nutrient cycling and erosion control here.

Monitoring continues at Casa Grande and nine other parks in southern Arizona and New Mexico.

Sources

Based on "Status and Trends of Vegetation and Soils at Casa Grande Ruins National Monument, 2008–2016," Sonoran Desert Network, National Park Service, summarizing C. L. McIntyre, J. A. Hubbard and S. E. Studd, Status and trends of terrestrial vegetation and soils at Casa Grande Ruins National Monument, 2008–2016, Natural Resource Report NPS/SODN/NRR—2018/1800; a work of the United States government in the public domain.