Upper Sonoran desert on fans and terraces associated with granite and granite-like rocks

Scattered about the study area from Rincon Valley to San Manuel, this EU is flat: about 90% has a slope of less than 18%, and more than half of this has a slope of less than 5%. Elevations range from 800 m (2600 ft) to 1200 m (3900 ft). At two of the four sites surveyed, the dominant vegetation is predominantly mesquite, 2 to 5 m tall, with 10-14% cover; at the other two sites, whitethorn was dominant, typically, 1.5 m tall, with 10-14% cover.

At one site, the whitethorn shared dominance with creosote, but in general the creosote was a minor associate. Prickly pear was more common, often giving 1-4% cover. Grasses gave less than 5% cover at two sites, 5-9% at one, and 15-25% at another. Saguaros and palo verde, hallmarks of the Sonoran desert, typically give less than 1% cover.

About 10-20% of this EU is developed lands, in either the Catalina Foothills, San Manuel, or near the town of Catalina.

Photo Point 2, near the intersection of the Catalina Highway and Snyder Road, 2800 ft, March, 2008. Mesquite dominates, with a cover of 10-14%, while a variety of cactus combine for an additional 15-24% cover. Saguaros are obvious yet uncommon. So are grasses: less than 5% cover of the annual three-awn.

Photo Point 29, near the mouth of Cargodera Canyon on the NW side of the Catalinas, elevation 3100 feet, May, 2008. Mesquite dominate, with 10-14% cover, while catclaw and fairyduster are common associates. Saguaro and palo verde are uncommon, and restricted to southern exposures. Grasses provide 15-24% cover, and include Lehman’s love grass, spider grass, and purple three-awn.

Photo Point 181, about 2 miles SW of San Manuel, 3400 feet, October, 2008. Although the foothill palo verde is the stand-out, its overall cover is less than 4%. Likewise, mesquite is common but not abundant, while creosote and whitethorn co-dominate with each giving 5-9% cover. Grasses combine for less than 5% cover. Here, at the edge of the EU, the Sonoran desert is transitioning to the Chihuahuan. There are no saguaros at the photo point.

Desired Conditions

Desired conditions are adapted from Desert, mid-scale desired condition statements in March 2010 Coronado National Forest planning documents.

On moderately steep hillsides and fan piedmont where alkaline soils are formed in alluvium from mixed parent materials, the predominant species are foothill paloverde, saguaro, prickly pear, cholla, barrel cactus, ocotillo, false mesquite, triangleleaf bursage, bush muhly, slender grama, curly mesquite, spidergrass, tangelhead, purple threeawn, janusia, ayenia and globe mallow. Annual forbs and grasses, an important part of this plant community, fluctuate with precipitation from nearly nothing in dry years to several hundred pounds per acre in wet years. Plant basal cover ranges from 1 to 3 percent, and cover by plant litter ranges from 10 to 75 percent. Canopy cover ranges from 1 to 20 percent for grasses, 1 to 15 percent for forbs, and 5 to 20 percent for shrubs and succulents. Tree canopy cover is 1 to 10 percent.

On moderately steep hillsides and fan piedmont where non-alkaline soils are formed in alluvium from mixed parent materials, the predominant species are foothill paloverde, saguaro, prickly pear, cholla, ocotillo, whitethorn acacia, creosote bush, false mesquite, range ratany, desert zinnia, bush muhly, black grama, slim tridens, fluff grass, janusia, desert senna, and twinberry. Annual forbs and grasses, an important part of this plant community, fluctuate with precipitation from nearly nothing in dry years to several hundred pounds per acre in wet years. Plant basal cover ranges from 1 to 3 percent, and cover by plant litter ranges from 5 to 45 percent. Canopy cover ranges from 1 to 10 percent for grasses, 1 to 10 percent for forbs, and 5 to 20 percent for shrubs and succulents. Tree canopy cover is 1 to 10 percent.

On nearly level floodplains, low stream terraces and canyon bottoms where soils are formed in recent alluvium from mixed parent materials, the predominant species are foothill and blue paloverde, mesquite, catclaw acacia, desert willow, desert hackberry, wolfberry, big bursage, burrobrush, desert honeysuckle, bush muhly, sand and spike dropseed, sideoats grama, tanglehead, spidergrass, and mesa threeawn. Annual forbs and grasses fluctuate with precipitation from nearly nothing in dry years to several hundred pounds per acre in wet years. Plant basal cover ranges from 2 to 5 percent, and cover by plant litter ranges from 5 to 45 percent. Canopy cover ranges from 10 to 20 percent for grasses, 1 to 15 percent for forbs, and 5 to 15 percent for shrubs and succulents. Tree canopy cover is 10 to 15 percent.

Historic Mean Fire Return Interval (Schussman et al. 2006 literature review)

> 250 years for warm desert communities

Primary treatment options (estimated representative project size range):

• Mechanical treatments for fuel reduction and to maintain openings (up to 2,500 acres per project)
• Herbicide treatment for exotics and shrub management to maintain openings and reduce fire spread (up to 1,000 acres per project)

Supplemental treatment option (estimated representative project size range):

• Thinning around developments for safety-related fuel reduction (up to 500 acres per project)

Other Vegetation Classifications: