Biological Survey of Ironwood Forest National Monument
About half of the Monument’s vegetation (35,000 hectares, 86,500 acres, 45%) is Arizona Upland subdivision (AZU) of the Sonoran Desert (Brown 1982). We classified most of the rest as Lower Colorado River Valley subdivision (LCV), though some botanists would classify much of it as AZU. 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. It also occurs in the Tucson Mountains, where Rondeau et al. (1996) seems to have included it in Arizona Upland. This vegetation has the physiognomy of chaparral and we tentatively classified it as such. Jojoba chaparral covers 648 ha (1600 acres), 0.8% of the monument area.
Several high-quality or rare vegetation types were mapped in detail. See links to image galleries at left and in thumbnails with the text below.
1. Dense Arizona upland: The heart of IFNM has large areas of well-developed Arizona Upland that includes dense groves of ironwood trees. These areas also usually support above-average saguaro and foothill palo verde densities. Additional dense saguaro stands occur in otherwise ordinary AZU vegetation.
2. Lower Colorado River Valley with large ironwoods: The largest ironwood trees occur in drainages in the lower elevation valleys. Some are in braided washes or sheet-flooded areas that do not appear to be channelized on the ground.
3. Jojoba chaparral: Whether this is truly chaparral or an odd association within Arizona Upland, it is a rare vegetation type in IFNM.
4. Xeroriparian woodlands: Desert washes are identical to wetter riparian areas as to ecological functions; the only difference is that washes only occasionally have surface water. Like rivers, desert washes harvest water, nutrients, and seeds from watersheds and support greater densities of life than adjacent land. The larger washes support dense ribbons of trees. LCV and AZU washes are mapped separately (items 2 and 5 respectively). In Arizona Upland the trees are mostly ironwood and blue palo verde with some mesquite. A minority of AZU washes had an abundance of large ironwood trees. In Lower Colorado River Valley Sonoran Desert the washes are more variable. In some almost all of the trees are mesquites. Others are dominated by either blue palo verde or ironwood, or a mixture of all three trees. Desert willow (Chilopsis linearis) is also often present in LCV washes, but this species is conspicuously absent from Ironwood Forest National Monument.
5. "Forests" of chollas and other cacti: There are several areas with dense stands of chollas. The largest is in the Pan Quemado Mountains, where an intermittent band of chainfruit cholla (Cylindropuntia fulgida) almost encircles the range. Other thick cactus patches are documented in the gallery, especially the saguaro forests.
6. Cactus dunes west of the Sawtooth Mountains: This small area is unique within Ironwood Forest National Monument and rare elsewhere. It consists of mostly flat, loose, pinkish sand densely vegetated with several opuntioid and other cactus species. Cacti are the codominant lifeform with foothill palo verde. Where a stabilized dune piles up against Wildcat Peak big galleta grass (Pleuraphis rigida) is also common. The only similar place we know of is the Cactus Plain near Bouse, AZ; that area has a much lower plant and cactus density and diversity.