Photo by ASDM

Approximately half of the Monument’s vegetation (35,000 hectares, 86,500 acres, 45%) is Arizona Upland subdivision of the Sonoran Desert. Most of the rest is Lower Colorado River Valley subdivision. There is an anomalous vegetation type near the summit of Silver Bell Peak in which jojoba is the dominant plant and forms nearly continuous stands. This vegetation has the physiognomy of chaparral. The jojoba chaparral covers 648 hectares (1600 acres), 0.8% of the Monument area.

IFNM spans the transition between Arizona Upland (AZU) and Lower Colorado River Valley (LCV). AZU is characterized by saguaros and legume trees growing on mountain slopes and bajadas. Intervening valleys are relatively narrow and also densely vegetated for a desert, but ironwood trees and saguaros are uncommon because of killing winter frosts and excessively wet soils in rainy years. AZU is well developed in the Roskruge, Silver Bell, Ragged Top, and Waterman mountain ranges. Vegetation density and floral diversity are lower in the West Silver Bell Mountains and still lower in the Sawtooth Mountains. Some vegetation maps place the Sawtooths in LCV, but they are more accurately marginal AZU because of the sparse cover of trees and saguaros on the slopes.

The hotter more arid LCV is characterized by low rocky mountain ranges separated by broad valleys. The slopes of the ranges are usually devoid of both trees and saguaros because of the extreme aridity. Saguaros and ironwood trees grow on the lower bajadas and valley floors that, because of their lower elevation, are warmer than those in AZU. An additional reason is that the fine-grained soils hold more moisture than the rocky slopes. The trees in this hyperarid habitat are mostly restricted to drainages that concentrate runoff from watersheds. Avra Valley is the easternmost extension of LCV and is atypical. The valley floor is higher in elevation and is therefore both wetter and colder than other LCV valleys. Ironwood trees and saguaros are uncommon here; the washes are dominated by velvet mesquites and often contain blue palo verde trees. The Aguirre Valley and Tat Momoli Valley on the west side of IFNM and the Santa Cruz Flats on the northern boundary are more typical LCV. Trees, including ironwoods, are concentrated in washes, and saguaros are sparsely distributed on the intervening flats.

Photo by ASDM

The Monument presents a distinctive view of the Sonoran Desert with ancient legume and cactus forests. The geologic and topographic variability of the monument contributes to the area's high biological diversity. Ironwood trees, which can live in excess of 800 years, generate a chain of influences on associated understory plants, affecting their dispersal, germination, establishment, and rates of growth. Ironwood is the dominant nurse plant in this region, and the Silver Bell Mountains support the highest density of ironwood trees found in the Sonoran Desert. These trees provide, among other things, roosting sites for hawks and owls, forage for desert bighorn sheep, protection for saguaros against freezing, burrows for tortoises, flowers for native bees, dense canopy for nesting of white-winged doves and other birds, and protection against sunburn for night blooming cereus. 

The ironwood-bursage habitat in the Silver Bell Mountains is associated with more than 674 species, including 64 mammalian and 57 bird species. Within the Monument, the Ragged Top Mountains contain the greatest richness of species. The area is home to species federally listed as threatened or endangered, including the Nichols Turk's head cactus and the lesser long-nosed bat, and contains historic and potential habitat for the cactus ferruginous pygmy owl. The desert bighorn sheep in IFNM may be the last viable population indigenous to the Tucson basin.

Photo by ASDM

Documented flora in the Monument stands at 560 species in 308 genera and 80 families. The genera with the most examples are Euphorbia (spurges/succulents), Cylindropuntia and Opuntia (cacti), Eriogonum (buckwheats), Boerhavia (spiderlings), and Ambrosia (ragweeds). The families with the most examples are Asteraceae (aster/daisy/sunflower), Poaceae (grasses/cereals), Fabaceae (legumes/pulses), Cactaceae (cacti), Malvaceae (mallow), and Euphorbiaceae (spurges). Compared with the flora of the Tucson Mountains 20 miles to the west, IFNM has more species of cacti, but a lower density. Fifty-four exotic species occur in the Monument, comprising 9.6% of the IFNM flora. Some of these are native-exotics, plants that occur naturally nearby, but have been introduced to the Monument. Others are from distant places, and some are invasive and pose serious threats to the ecological integrity of the biological community.

The major plant life forms in IFNM are four ecologically important native trees (ironwood, foothill palo verde, blue palo verde, velvet mesquite) (0.7% of plants, but visually the second most dominant life form after columnar cacti), 322 perennials (57%), and 238 annuals (43%). This count does not include the rare instances of shrub live oak, western soapberry, cottonwood, willow, eucalyptus, and salt cedar. Legumes are the dominant family in the dry tropics. Composites are typically dominant in the temperate zone, as they are in IFNM. About half of the total Sonoran Desert flora is largely derived from thornscrub and tropical dry forest to the south. The dominance of composites in IFNM is likely a consequence of the fact that AZU is higher and cooler than the other Sonoran Desert subdivisions and therefore has fewer tropical elements. All three Southwestern plants commonly known as crucifixion thorns occur in the Monument. All are essentially leafless extremely thorny shrubs. They are unrelated to one another; each species belongs to a different family. Their superficial similarity is an example of convergent evolution.

Photo by Donna Pugh, Tucson

In addition to the biological and geological resources, the Monument holds abundant rock art sites and other archeological objects of scientific interest. Humans have inhabited the area for more than 6000 years. More than 200 sites from the prehistoric Hohokam period (600 AD to 1450 AD) have been recorded in the area. They include campsites used by Archaic hunter-gatherers about 6000 BC, villages, hamlets, and agricultural fields where Hohokam farmers lived and cultivated corn, beans, and squash; sites where proto-historic and historic Tohono O’Odham Indians, who are believed to be descendants of the Hohokam, camped, farmed, and harvested domesticated and wild plant crops; a small mission/vista constructed in the late 1700s by Tohono O’Odham laborers for Spanish Franciscan friars; and remnants of historic mining camps and cemeteries dating to the 1880s. Three areas within the IFNM have been listed on the National Register of Historic Places. The archeological artifacts found in the area include rhyolite and brown-chert chipped stone, plain and decorated ceramics, and worked shell from the Gulf of California. The area also contains the remnants of the Mission Santa Ana del Chiquiburitac, the last mission constructed in Pimeria Alta.

Photo by Donna Pugh, Tucson 

The proximity of IFNM to the nearly one million people in the Tucson area and to over four million in the Phoenix area has also made it attractive to off-road vehicle use. Unchecked cross-country travel destroys fragile desert soils, inhibiting vegetation recruitment and harming wildlife species. Recreational shooting takes its toll on the visual resources and harms vegetation. Livestock grazing may be having long-term ecological consequences, eroding soils, spreading invasive species, and impairing wildlife habitats. Encroaching development and the growing effects of cross-border smuggling activity are also real threats. National Monument designation renders IFNM archaeological sites vulnerable to increasing impacts associated with casual visitation, vandalism, looting, illegal collection of pottery shards and other surface artifacts, theft of boulders inscribed with prehistoric rock art, and site damages and theft of artifacts by people using metal detectors to search for “treasure troves” they believe to be buried in historic mining camps, ghost towns, and Spanish missions. Increasing sale of artifacts at local flea markets, antique stores, and also on the internet is contributing to an increase in local “shard harvesting” at these sites.

The IFNM area contains exceptional multiple-use opportunities and offers natural, cultural, scenic, and recreational resources worthy of additional protection as a national monument, and with respect to IFNM itself, as a component of the National Landscape Conservation System. A wise and well thought out resource management plan is needed to ensure that the area can be enjoyed by future generations while maintaining its unique characteristics. The Friends of Ironwood Forest is working diligently to protect the biological, geological, archaeological, and historical resources and values for which the Ironwood Forest National Monument was established.

Photo by Donna Pugh, Tucson