Prescribed Burning on Upper Sonoran
Rangelands
Dan Robinett
Naturally occurring wildfires played an important role
in shaping desert grasslands in southern Arizona (Pase
1977, Leopold 1924). Lightning from summer thunderstorms, Indians using fire to hunt or clean up favorite
campsites, or accidental starts from their campfires were
common prior to the turn of the century (Bahre 1965,
Leopold 1924). Natural fire frequencies for these grasslands are estimated at between 10 and 20 years (Pase
1977, Wright 1980). Periodic fire in these ecosystems
would certainly favor perennial grass species over shrubs
and cacti, helping to maintain their grassland aspect in
the past (Wright and Bailey 1982, Wright 1988). A variety
of human caused impacts since settlement has greatly reduced the frequency and spread of wildfires (Bahre 1991,
Leopold 1924, Swetnam 1988). Over the last 100 years
very little of Arizona’s desert grassland has burned.
On one ranch in the Altar Valley near Tucson, prescribed fire has been used successfully since 1985 to reestablish grassland conditions. In this area elevations range
from 2,900 to 3,400 feet. Average annual precipitation is
12 inches, equally distributed between the summer and
winter seasons (NOAA 1992). Soils are in the aridic moisture regime and the thermic temperature regime. They
are developing in light-colored sandy loam alluvium of
moderate age. Hayhook soil series is typical of this area
(USDA 1993).
Present day plant communities are dominated by velvet
mesquite (Prosopis) with understories of burroweed
(Isocoma), cacti (Opuntia spp.) and annual and perennial
grasses and forbs. These communities now more closely
resemble Sonoran desert vegetation of the Arizona upland
subdivision than the grasslands they were once (Brown
1982). Technical range site descriptions for Deep sandyloam and Sandyloam upland range sites in a 10 to 13 inch
precipitation zone show potential plant communities
dominated by perennial grass (USDA 1988, 1991).
Prescribed fires in the hot, dry fore-summer (May-June)
kill burroweed and top kill mesquite and cacti species.
Management of grazing includes two years of deferment
following the burn and rest-rotation thereafter. The combination of the fire and managed grazing allows native
warm season perennial grasses to regain dominance of
the understory. Repeat fires will be needed to actually kill
mesquite trees on these sites. On the 54,000 acre Anvil
ranch large pastures were fired in 1985, 1987, 1989, and
1991. About 10,500 acres of rangeland was actually
burned out of a total of nearly 20,000 acres, leaving a mosaic of burned and unburned areas. The typical response
of these plant communities is illustrated by plot readings
of a trend monitoring transect installed in 1981 on Hayhook soils. The transect uses a pace-frequency format to
record species composition and point sampling for cover.
The area was burned in 1985.
Abstract—Naturally occurring wildfires played an important
role in shaping Sonoran grasslands in Arizona. Anglo settlement
of the region caused a variety of impacts changing vegetation
from grass to shrub. In an area transitional between semi-desert
grassland and upper Sonoran desert, the reintroduction of fire in
these ecosystems is resulting in a return to perennial grass dominance. Sandyloam range sites at elevations of 3,200 feet and with
average annual precipitation of 12 inches have plant communities dominated by velvet mesquite, burroweed, cacti, annual and
perennial grasses and forbs. Photographic evidence from the turn
of the century shows these areas as open grassland. The use of
prescribed fire in these plant communities has resulted in successful treatment of about 10,500 acres on one ranch in the Altar
valley. Fires in May and June top killed mesquite and killed
burroweed, allowing grasses to establish and thicken. A prescription and burning technique has been developed to safely burn
these plant communities. Good management of grazing after the
burns allows perennial grasses to flourish once again. The combination of prescribed fire and managed grazing in this area has
resulted in many benefits including wildlife habitat, watershed
values, reduced erosion and increased forage for livestock.
In Pima County, southern Arizona, an area transitional
between semi-desert grassland and upper Sonoran desert
occupies several hundred thousand acres of rangeland.
Photographic evidence from the turn of the century shows
much of this area to be open grassland (Humphrey 1987).
Repeats of the same scenes today show plant communities
dominated by shrubs, trees and cacti.
Anglo settlement of the region caused a variety of impacts that helped to bring about these changes (Bahre
1991, Hastings and Turner 1965). Cultural impacts like
grazing, haying, farming and woodcutting interacted with
natural factors like drought, flooding, fires, earthquakes
and changing precipitation patterns to speed the change
from grassland to shrubland in the last 100 years.
Climate in this region is dynamic with two distinct
growing seasons and two sources of moisture for storms.
Documentation of recent climate change has been made
in flood records of major streams in the region (Webb and
Betancourt 1992). On at least one site which has been
fully protected from any cultural influences, mesquite
has invaded a Sonoran desert plant community in the
last 85 years (Turner 1990).
In: Roundy, Bruce A.; McArthur, E. Durant; Haley, Jennifer S.; Mann,
David K., comps. 1995. Proceedings: wildland shrub and arid land
restoration symposium; 1993 October 19-21; Las Vegas, NV. Gen. Tech.
Rep. INT-GTR-315. Ogden, UT: U.S. Department of Agriculture, Forest
Service, Intermountain Research Station.
Dan Robinett is an Area Range Conservationist, Soil Conservation
Service, 2000 E. Allen Road, Bldg. 320, Tucson, Arizona 85719-1596.
361
Species
Burroweed
Mesquite
Opuntia
Santa Rita threeawn
Arizona cottontop
Annual grama
Basal cover
Production (lbs./ac. air dry)
Percent frequency
1981
1984
1989
1993
60
4
4
5
0
76
1
20
75
3
7
17
1
91
2
100
71
6
5
38
1
90
5
200
5
5
5
67
4
37
8
400
Fire weather is monitored on site for two or three days
prior to burning and each day while burning. Coordination is made with all the appropriate state, federal and
county agencies to comply with all regulations, procedures
and standards. Actual firing is done by ranch personnel
with ranch equipment. Soil Conservation Service personnel act as fire managers, monitoring fire weather and behavior and advising the burn boss (rancher) to stay within
prescription. Standby assistance from several agencies,
including the Arizona State Land Department, the
Buenos Aires National Wildlife Refuge, the Bureau of Indian Affairs and local volunteer fire departments, is almost always on hand. Large blocks (2,000 to 3,000 acres)
usually take 5 to 6 days to burn.
The first summer rains arrive in this area in early July.
In the first year the remnant perennial grasses on site recover and set seed. Annual grasses and forbs cover the
burn areas. As the years go by, and with managed grazing, the perennial grasses expand and fill in the vacancies
left by dead shrubs. Top killed mesquites take about 8 to
10 years to recover to their pre-burn canopies giving an
additional advantage to grasses.
The reintroduction of fire in Sonoran desert grassland
ecosystems appears to be a viable and needed undertaking. Safe techniques have been developed. The economics
to the land owner appear to be good, especially when federal cost-share programs can be used. Not only is grass
production increased, but also variability in forage production is reduced as the forage base changes from browse
and annual plants to one of perennial grass. Burning also
opens up shrubby areas, making livestock handling easier
and more efficient. Public values improve in many ways
and wildlife habitat is improved. A mosaic of burned and
unburned areas greatly increases edge in treated pastures. Small herbivores increase tremendously as inedible
shrubs are replaced by grasses and forbs. Birds increase
in numbers and species in response to increased seed and
insect production on burned areas. Improved covers of perennial grass reduce soil erosion, water runoff, and subsequent flooding and sedimentation downstream. The visual
quality of these rangelands improves as the monotonous
cover of mesquite and burroweed is broken up by open
grassy areas.
Prescribed fire allows land managers to treat large areas at low cost in relatively short periods of time. Since
1985 nearly 20,000 acres of Sonoran rangeland has been
improved by the combination of prescribed fire and managed grazing. Wildlife habitat, watershed values, recreational uses and grazing are all enhanced.
Perennial grass production has quadrupled on this area
and basal cover of perennial grass has increased from 2 to
8 percent in the 8 years since the burn. Research on similar soils on the Santa Rita Experimental Range has
shown a decrease in soil erosion and water runoff on areas
where shrubs have been controlled and grass cover has increased (Martin and Morton 1993).
Direct costs to the land owner for the burns average
about $1.50 per acre. Deferment costs for the 24 month
nonuse period after the fire is about $2.50 per acre in this
area. The additional carrying capacity which has developed 8 years after burning is worth about $1.50 per acre.
Depending upon the lifespan of this treatment, the practice appears to be a marginal to good investment for the
land owner. The use of federal cost-share dollars (Agricultural Conservation Program) will help make it an attractive investment.
A safe technique and burn prescription has been developed over the years to minimize both the costs and the
risks associated with this practice. Burn area perimeters
are fired in the late afternoon to evening and allowed to
carry several hundred feet into the interior. The burned
edge is mopped up very early the next morning before
dust devils become active. These small whirlwinds are the
main reason for fire escapes in these flat areas. After the
perimeter has been burned, the interior is burned in the
daytime using headfires to get the hottest fires possible.
These daytime burns also loft smoke columns to where
high altitude winds can quickly disperse them. This is an
important air quality consideration in an area close to a
large city like Tucson.
Fine (one hour) fuel loads consist of annual grasses
(95% dry) and burroweed (75% dry) and range from 3,000
to 4,000 pounds per acre. Ten to twenty mesquite trees
per acre contribute to the fuel load. Prescription parameters call for the hottest fires possible to control shrubs.
Burning is done in the hot dry season of May and June.
Daytime temperatures range from 90 to 105 °F. Nighttime temperatures are from 65 to 90 °F. Relative humidities are usually less than 15 percent during the day and
recover to 25 percent by evening. Dead fuel moisture values (10 hour) range from 2 to 6 percent. Burns are conducted with light prevailing winds of 2 to 8 miles per
hour. Under these conditions daytime fires move at a rate
of 20 to 30 chains (1,300-2,000 ft.) per hour with flame
lengths of 10 to 15 feet. Backfires move at rates of 2 to 3
chains (130-200 ft.) per hour with 2 to 3 foot flame
lengths.
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363