Ironwood
Olneya tesota
Description: Ironwood is one of the largest and longest-lived
Sonoran Desert plants, reaching 45 feet in height and persisting
as long as 1,500 years. It is a single or multi-trunked evergreen
tree, and displays lavender to pink flowers starting in March.
By early summer, the pods mature. Each 2-inch pod contains one
to four shiny brown seeds that are relished by many Sonoran animals,
from small mammals and birds to humans. Its iron-like wood is
renowned as one of the world's densest woods. Unlike mesquite,
germination and establishment rates may be very infrequent.
The shaded sanctuary and richer soils created by ironwoods increase
plant diversity and provide benefits to wildlife. Some 160 species
of plants and 80 species of birds use ironwoods in some way. Ironwoods
are too hard to provide nesting cavities for birds, but the cacti
that grow beneath them provide such opportunities. Insects abound
within the ironwood complex, attracting birds and reptiles. As
with other legumes, the ironwood's leaf litter supplies nitrogen
to the soil and its seeds provide a protein-rich resource for
doves, quail, coyotes, and many small rodents.
Habitat: This tree is found only in the Sonoran Desert, in the dry locales below 2,500 feet, where freezing temperatures are uncommon. Ironwoods are most common in dry ephemeral washes.Ironwoods function as oases of fertile and sheltered habitat within a harsh and challenging desert landscape. As a tree becomes established, it tempers the physical environment beneath it, creating a micro-habitat with less direct sunlight, lower surface temperatures, more organic matter, higher water availability, and protection from herbivores. Air temperatures may be 15 degrees cooler under ironwoods than in the open desert sun five feet away. Ironwood also shelters frost-sensitive young saguaros, organ pipe cactus, and night-blooming cereus.
Distribution: Ironwoods grow in southwestern Arizona, southeastern California, and northwestern Mexico.
Status: The ironwood is widespread within its historical range in Arizona. Populations in Sonora are suffering from the conversion of Sonoran desertscrub to agricultural use, including the planting of the invasive, exotic species known as buffelgrass. Wood-cutting for fuel and charcoal has further contributed to ironwood losses. The Seri, a small tribe on the coast of Sonora, and other artisans depend on ironwood for crafting stylized figurines to sell to tourists. They now find it difficult to sustain their livelihoods due to the disappearance of ironwood. Arizona's native plant laws prohibit transportation of ironwood, but do not require its preservation in place.
The Ironwood in Pima County: The ironwood has been identified as a habitat associate in Pima County for the endangered cactus ferruginous pygmy-owl. Urbanization in northwest Tucson is resulting in the loss of ironwood trees. Pima County's Native Plant Preservation Ordinance, adopted in 1998, requires a determination of the health of ironwoods on a site. A minimum of 80% of "specimen" ironwood trees must either be preserved or transplanted during development. The state's largest known ironwood occurs in Child's Valley in western Pima County.
References
Arizona Register of Big Trees, 1996. Arizona's Champion Trees, 1996 Edition.
Lipsky, Andrew A., Non-Game Branch, Arizona Game and Fish Department.
1999 website at www.gf.stat.az.us/frames/fishwild/ngame-d/htm
U.S. Fish and Wildlife Service, 1997. Federal Register, Volume 62, Number 46, March 10.
Determination of Endangered Status for the Cactus Ferruginous Pygmy-Owl in Arizona
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