Brooks, Matthew Lamar. 1998. Ecology of a biological invasion: alien annual plants in the Mojave Desert. Riverside, CA: University of California. 186 p. Dissertation.
Brooks, Matthew L. 1999. Alien annual grasses and fire in the Mojave Desert. Madrono. 46(1): 13-19.SPECIES INCLUDED IN THE SUMMARY:
For the prescribed fire study, three 2.25-ha study
sites were located in the western (35o14'30"N,
117o51'15"W), central (35o07'30"N, 117o07'45"W),
and southern (34o41'30"N, 117o57'30"W) desert regions. For mapped locations of the
34 fuel and 3 fire study sites, see .
Soils were gravelly to cobbly, well-drained, sandy loams over granitic parent materials. Average annual precipitation across fuel and fire sites was approximately 150 mm, with 87% occurring as rainfall from October through April. Winter rains before the spring and summer studies were 200% of normal. Mean minimum and maximum temperatures from the nearest weather station are 19 °C and 34 °C in August and 0 °C and 7 °C in December, respectively . All sites were located on alluvial bajadas, none were subject to livestock grazing, and none had burned for at least 25 years .
PREFIRE PLANT COMMUNITY:
Study sites were within a creosotebush-white bursage community. Winterfat, burrobush, and Anderson wolfberry were common shrub associates . Historical stand structure of the community was widely spaced shrubs with little annual plant biomass in shrub interspaces . Stand structure and species composition of the community differed from historical conditions, with annual species occupying many of the interspaces between shrubs. Nonnative red brome was the most common dominant annual (see Fuel load results). The nonnative grasses Arabian schismus and common Mediterranean grass dominated some study sites and codominated with red brome on others. Native bristly fiddleneck, pinnate tansymustard, and lacey phacelia were dominant forbs. Annuals with minor coverage in the community included nonnative Chilean chess, cheatgrass, small sixweeks grass, cutleaf filaree, and native sixweeks grass .
Study sites were on the following vegetation classifications:FRES30 Desert shrub 
Methods: On each of the 34 fuel study locations, 25 frequency and cover measurements per location were taken beneath large (>50-cm diameter) shrub canopies and on interspaces between large shrubs (>1 m between shrubs) [4,6].
Fuel load results: Spring and summer frequencies were highest for nonnative annual brome and schismus species, intermediate for cutleaf filaree, and lowest for most native annuals. Spring and summer cover was highest for the bromes, with schismus providing 2nd-highest coverage. Native forbs provided intermediate cover, and cover was relatively low for cutleaf filaree compared to native forbs. Native annuals other than bristly fiddleneck, pinnate tansymustard, and lacey phacelia provided least cover. Prefire, growing-season coverage and frequency are given below. Data are means; Bromus (red brome, Chilean chess, and cheatgrass), Schismus (Arabian schismus and common Mediterranean grass), and Vulpia (sixweeks grass and small sixweeks grass) spp. data are pooled :
|Cover (%)||Frequency (%)|
|Native-nonnative annual mix|
|other native annuals||10d||4cd|
Prescribed fire study: Fires were conducted on 16, 22, and 24 August 1995 on the western, central, and southern sites, respectively. Plot size was 2.25 ha; plots were located where bromes, schismus or mixed brome-schismus dominated and contributed the highest load of dead fine fuels. Fine fuels were ignited with a drip torch in a continuous flame line on the upwind border of each site. Fuel loads were :
Fine fuels (kg/ha)
Dominant annuals (>25% relative cover)
Interspace fire spread (m/min) (x)
Area burned (% of 2.25 ha)
|Beneath canopy||Interspace||Beneath canopy||Interspace|
|western||800||100||brome-schismus||schismus||1 (1)||50, patchy|
|southern||700||200||brome||brome-schismus||12 (8)||50, continuous|
There was no cloud cover during the fires. Other weather data at the beginning and end of the fires were :
|Wind direction||Wind speed (km/hr)
Fire behavior —
Site differences: The western Mojave site had large amounts of fuels beneath canopies and moderate amounts in interspaces. Relative humidity was low and wind speed high. Fifty percent of the site burned in many small patches. The central Mojave site had the lowest amount of fine fuels and wind speed and highest relative humidity. Fire did not spread beyond ignition points. The southern Mojave site had the greatest amount of fine fuels both beneath shrub canopies and in interspaces. Relative humidity was low and wind speeds moderate, so fire spread rapidly across interspaces. Fifty percent of the area burned in large, continuous areas [4,6].
Species effect on fire behavior: Among grass species, fire spread was most even, extensive, and rapid where bromes and schismus codominated. Slow, patchy fires resulted when schismus dominated interspaces; rate of spread was 1 m/minute and flame lengths were 5 to 10 cm in interspaces with schismus. In contrast, where bromes dominated interspaces fire spread approximated 12 m/minute, with flame lengths up to 30 cm. Only brome fuels produced flame lengths sufficient to consume large amounts of plant biomass. Cutleaf filaree and native annuals including bristly fiddleneck, pinnate tansymustard, and lacey phacelia did not contribute substantially to fire spread due to low frequency and cover [4,6].FIRE EFFECTS ON PLANT COMMUNITY:
Fire had no direct effect on the annuals, which were already dead at time of burning. Heat generated by bromes ignited and consumed dead stems and litter of native annuals, while most native stems and litter did not burn with schismus-fueled fire [4,6].FIRE MANAGEMENT IMPLICATIONS:
Annual litter also contributes to fine fuel loads, and thick litter layers often develop where nonnative bromes and schismus are present . Thick litter leads to high fire temperatures, long flame residence times, and continuous burn patterns (Brooks, unpublished data in ). Because plant litter decomposes slowly in deserts, and grass species can be among the slowest to decompose (review by ), brome and schismus litter accumulation may facilitate fire spread in the Mojave Desert .
With the exception of bristly fiddleneck, native annuals did not greatly affect fuel loads. Small nonnative forbs such as cutleaf filaree, and annual grasses besides bromes and schismus, did not contribute substantiality to fuel loads either. Prefire coverage and frequency of native sixweeks grass and nonnative small sixweeks grass were low, so the Vulpia species were not important fuels [4,6]. By summer, dead plant matter from these annual grass species was mostly crumbled and scattered, while biomass of dead brome and schismus was not .
Other fire management implications: This study showed that nonnative bromes fuel hot, fast-moving, continuous fires, whereas schismus fuels cooler, slower-moving, patchy fires. However, schismus can facilitate fire spread between brome patches . Schismus can, and has, fueled fires in low-elevation Mojave sites. Several wildfires in the 1990s that exceeded 40 ha were fueled mostly by schismus species .
The nonnative grass/fire cycle is documented for cheatgrass in the Great Basin (e.g., [2,14,17]), but to date (2006), is less well documented for the nonnative grasses that dominate some southwestern desert ecosystems. This study provides documentation for a nonnative grass/fire cycle in the Mojave Desert. As in the Great Basin, previously burned areas in warm (Mojave) and hot (Sonoran and Chihuahuan) desert ecosystems that are infested with nonnative annual grasses may be more susceptible to fire than unburned areas. The grass/fire cycle is a serious ecological threat in southwestern deserts because native shrubs such as creosotebush and white bursage are poorly adapted to frequent fire [2,7,13]. The grass/fire cycle is most likely to occur on high-elevation sites in the Mojave Desert. High-elevation desert sites provide more mesic conditions for red brome establishment, and high-elevation red brome populations survive drought better than bromes on low-elevation desert sites (Minnich 1998, personal communication cited in ).
A review by Rundel and Gibson  that preceded this Research Project Summary noted that nonnative bromes, especially red brome, may be exceedingly dense in years of above-average rainfall in the Mojave Desert. For example, Rock Valley, Nevada, is a relatively undisturbed site due to its proximity to the Nevada Test Site, which has restricted access. Inventories showed that before 1960, red brome density in Rock Valley was 14 plants/m² or less. In 1976, density averaged 91 plants/m². In 1988, a wet year, red brome density averaged 2,034 plants/m²: 25 times the 1988 density of all native annuals combined .
In this Research Project Summary, Brooks [4,6] emphasizes that minimizing outbreaks and spread of nonnative annual grasses and large forbs is critical for reducing fire occurrence and spread in the Mojave Desert. Human ignition sources should be minimized, especially where nonnative annuals are abundant and topography is conducive to fire spread. Limiting or restricting livestock grazing and off-road vehicle use may reduce rate of nonnative seed spread. Monitoring roadways, washes, and other corridors for nonnative invasion, and controlling new outbreaks, may slow rate of nonnative grass invasion. On infested sites, controlling nonnative grasses in interspaces can reduce fuel continuity and reduce fire spread [4,6].
Sensitive species: The Mojave Desert contains several rare plant and animal species. Some of these species are endemic to the region [10,15], and fire regimes altered from very patchy, infrequent fires to frequent, mixed- and high-severity fires are endangering Mojave Desert habitats . For example, a genetically distinct population of the federally threatened desert tortoise occurs in the Mojave Desert. Frequent fires fueled by red brome are degrading desert tortoise habitat in some areas [3,5].
|Common name||Scientific name|
|white bursage||Ambrosia dumosa|
|white burrobush||Hymenoclea salsola|
|Anderson wolfberry||Lycium andersonii|
|bristly fiddleneck||Amsinckia tessellata|
|pinnate tansymustard||Descurainia pinnata|
|lacey phacelia||Phacelia tanacetifolia|
|Asian mustard||Brassica tournefortii|
|flixweed tansymustard||Descurainia sophia|
|cutleaf filaree||Erodium cicutarium|
|shortpod mustard||Hirschfeldia incana|
|tumble mustard||Sisymbrium altissimum|
|London rocket||Sisymbrium irio|
|sixweeks grass||Vulpia octoflora|
|red brome||Bromus rubens|
|Chilean chess||Bromus trinii|
|Arabian schismus||Schismus arabicus|
|common Mediterranean grass||Schismus barbatus|
|small sixweeks grass||Vulpia microstachys|
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