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Vulpia octoflora

• INTRODUCTORY
• DISTRIBUTION AND OCCURRENCE
• BOTANICAL AND ECOLOGICAL CHARACTERISTICS
• FIRE ECOLOGY
• FIRE EFFECTS
• MANAGEMENT CONSIDERATIONS
• REFERENCES

INTRODUCTORY

AUTHORSHIP AND CITATION FEIS ABBREVIATION NRCS PLANT CODE COMMON NAMES TAXONOMY SYNONYMS LIFE FORM FEDERAL LEGAL STATUS OTHER STATUS
	Photo by Kitty Kohout, courtesy of the Univ. of WI-Stevens Pt.

Other authorities do not recognize sixweeks grass infrataxa [80,137,176,232]. Varieties are based on relative hairiness of the lemmas [57,232], and hairy and glaucous lemma types are not geographically segregated in sixweeks grass. Examples of both types may occur within a single population, or even on the same plant [232].

The genus Vulpia is distinguished by annual life form and cleistogamous breeding habit, while Festuca is perennial and chasmogamous [133]. Not all systematists support the separation of these closely aligned genera [80,176,224,232].

DISTRIBUTION AND OCCURRENCE

• GENERAL DISTRIBUTION
• ECOSYSTEMS
• STATES/PROVINCES
• BLM PHYSIOGRAPHIC REGIONS
• KUCHLER PLANT ASSOCIATIONS
• SAF COVER TYPES
• SRM (RANGELAND) COVER TYPES
• HABITAT TYPES AND PLANT COMMUNITIES

Although sixweeks grass is widespread, it is uncommon in some vegetation types and is not documented in every ecosystem or plant community where it grows. It may occur in some vegetation types that are not listed below.

Sixweeks grass is documented in several vegetation types besides those listed above. It grows in shadscale (Atriplex confertifolia), Joshua tree (Yucca brevifolia), and riparian California broomsage (Lepidospartum squamatum) scrub in southeastern California and Nevada [21,22,27]. In Pinnacles National Monument, California, sixweeks grass was present on mostly barren sites with cheatgrass (Bromus tectorum) and woven-spore lichen (Texosporium sancti-jacobi), a rare lichen that occurs on xeric, undisturbed sites [141]. Sixweeks grass occurs in a saguaro-yellow paloverde (Carnegiea gigantea-Cercidium microphyllum) community on the Tonto National Forest, Arizona [42]. Sixweeks grass and red brome (B. rubens) were the 2 most productive annuals in a vegetation survey in Saguaro National Park, Arizona (Esque and Schwalbe, unpublished data cited in [63]). Sixweeks grass also occurs in silver sagebrush-western wheatgrass (Artemisia cana-Pascopyrum smithii) in north-central Colorado [129], and in sand dropseed (Sporobolus cryptandrus) communities of Nebraska [228].

Sixweeks grass was probably an important component of pristine California prairie, growing in the interspaces between dominant bunchgrasses such as purple needlegrass (Nassella pulchra). It may have been more prevalent on dry sites than on wet sites. California prairie is almost entirely replaced by annual grassland, which is dominated by nonnative annual grasses such as ripgut brome (B. diandrus) [92].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

• GENERAL BOTANICAL CHARACTERISTICS
• RAUNKIAER LIFE FORM
• REGENERATION PROCESSES
• SITE CHARACTERISTICS
• SUCCESSIONAL STATUS
• SEASONAL DEVELOPMENT

Sixweeks grass is a native winter or spring annual with a decumbent to erect growth form. Culms are solitary or in small tufts [77,176,206,208,232] that are 3.1 to 23 inches (8-59 cm) tall. Leaves are cauline. The blades are 1 to 2 mm wide and 0.8 to 4 inches (2-10 cm) long. The inflorescence is a narrow, compact panicle 1 to 6 inches (3-16 cm) long. Spikelets are 4.5 to 10 mm long [57,86,97,176,208,232]. The lemmas are awned [67]; awns are 3 to 25 mm long [97,176,208]. The seed is a caryopsis measuring 1.7 to 4 mm in length [77,176]. Mean seed mass for greenhouse-grown plants from the northern Mojave Desert was 0.30 mg/seed [55], while mean seed mass of field samples collected in northern Arizona was 0.49 mg/seed. The Arizona seeds averaged 3.29 × 0.58 mm in length and width [180]. Sixweeks grass roots are fibrous and shallow [165]: maximum rooting depth for Colorado plants was about 5.9 inches (15 cm) [78]. Maximum root depth for sixweeks grass harvested in late June in the Powder River Basin of Wyoming averaged 6.7 inches (17 cm); range was 6.3 to 7.1 inches (16-18 cm) [5]. Sixweeks grass's root:shoot ratio averaged 0.08 g/g for greenhouse-grown plants [55]. Sixweeks grass may form mycorrhizal associations [4].

Breeding system/pollination: The Vulpia genus is cleistogamous [133].

Seed production: Under favorable conditions, sixweeks grass produces "numerous" small seeds. In the greenhouse, sixweeks grass showed higher rates of seed production than either red brome or pinnate tansymustard (Descurainia pinnata), even though its resource uptake (water and nitrogen) was less. Seed yield can vary greatly across years. A North Dakota study found 283 sixweeks grass seedlings/kg emerged from hay harvested in 1978, while no sixweeks grass seedlings emerging from hay harvested in 1980 and 1981 [183]. Declines ― or total failures ― in the seed crop may occur in drought years; however, sixweeks grass seed production rebounds quickly when climate is favorable [55]. Drought does not necessarily mean seed crop failure. The Great Drought of 1939 to 1941 was the most severe Great Plains drought of the 20th century. During the end of the Great Drought, when associated perennial grasses were mostly dead, sixweeks grass produced "abundant" seed crops in eastern Kansas and Nebraska [3].

Seed dispersal: Seed is dispersed primarily by wind [3] and sometimes on waterways [74]. The seed awns provide a mechanism for possible animal dispersal, although animal dispersal is not documented. Sixweeks grass may also be dispersed as a contaminant in hay. In North Dakota an equal number of sixweeks grass seedlings (142 seedlings/kg hay) emerged from year-old, unstored hay and year-old hay that was stored in an unheated shed [183].

Seed banking: As an annual with dormant seeds, sixweeks grass depends entirely on its seed bank for regeneration [1,54,66,98,109,132,226]. Most seeds remain dormant until soil moisture levels are adequate for germination [109]. Even in years of high germination, some viable sixweeks grass seeds remain ungerminated in the seed bank. This reproductive strategy is a hedge against high seedling mortality and poor seed set in any given year [55,168]. For example, sod slabs (20 ft²), removed from mixed-grass prairie and kept in watered flats in a greenhouse, supported 344 sixweeks grass seedlings over a 4-year study period (1929-1932). On 35-cm² quadrats on open prairie during the same period, mean sixweeks grass seedling emergence was "nearly a hundred" in 1929 and 7 in 1931, with no sixweeks grass seedlings present in either 1930 or 1932 [23]. In a blue grama-buffalo grass (Bouteloua gracilis-Buchloe dactyloides) community on the Central Plains Experimental Range of Colorado, sixweeks grass seed had a "relatively homogeneous spatial distribution." Seeds were more numerous on coarse soils, and seed density varied across sample months and years. Seed bank density was [43]:

Walters [226] found sixweeks grass was "notably dominant" in the seed bank of a creosotebush-yellow paloverde-white bursage (Cercidium microphyllum-Ambrosia dumosa) community near Phoenix, Arizona, but not in aboveground vegetation. Near Reno, Nevada, more viable seeds were found in the litter layer of a basin big sagebrush/cheatgrass (Artemisia tridentata ssp. tridentata/Bromus tectorum) community than in soil. Density (viable sixweeks grass seeds/m²) was [248]:

Grazing may affect seed concentration either negatively or positively. In a seed bank inventory of Colorado pinyon-juniper (Pinus edulis-Juniperus spp.) and blackbrush/Indian ricegrass (Coleogyne ramosissima/Achnatherum hymenoides) communities of southern Utah and northern Arizona, sixweeks grass seed was much more prevalent on ungrazed relict plots (225 seeds/m²) than on heavily, moderately, and lightly grazed plots (125, 110, and 90 seeds/m², respectively) [14]. However, on mixed- and tallgrass prairies where sixweeks grass occurred with highly palatable perennial grasses, sixweeks grass density was greater on sites with heavy and moderate cattle grazing compared to lightly grazed sites [105].

Germination requires warm temperatures or mechanical scarification of the seed coat to break dormancy [14,55]. Seed dormancy ensures sixweeks grass population survival across unfavorable growing seasons [45]. As an annual, sixweeks grass is sensitive to climate fluctuations and only germinates in years when sufficient moisture is available in upper soil layers [36,101,105]. For example, a Kansas vegetation survey was conducted during (1939 and 1940) and after (1941) the Great Drought. Percentages of available soil moisture in the top 6 inches (20 cm) of soil during sixweeks grass's growing seasons were [36]:

In this study, sixweeks grass was present in blue grama-buffalo grass-sand dropseed communities. Sixweeks grass density (plants/m²) and mean seed yield (lbs/acre) during that time were [36]:

Even though sixweeks grass requires soil moisture during the germination and establishment period, it is highly drought tolerant. Germination and establishment may occur in dry years if sparse rains are favorably timed [3,151,227]. Weaver and Albertson [3] found that sixweeks grass was "abundant" during the Great Drought, when perennial grasses were dying out. Hylton and Bement [105] compared 20-year rates of sixweeks grass establishment on the Central Plains Experimental Range with 20-year weather data (1941-1960) and greenhouse studies. They found best germination (93%-96%) in the greenhouse occurred with constant 68 °F (20 °C) temperature. Alternating day/night temperatures of 59/77 °F (15/25 °C), which closely approximated fall field conditions, produced good germination rates (66%-79%). Sixweeks grass field densities were highest in 1941 and 2nd-highest in 1958. Weather data from the same time period show that temperature and moisture conditions in late August and early September of 1940 and 1957 closely paralleled those producing most favorable germination in the greenhouse [105].

In a greenhouse experiment, DeFalco and others [55] found thinning established seedlings promoted new sixweeks grass germinants.

Seedling establishment: Sixweeks grass population numbers can vary greatly from year to year [75,125,160]. Germination and seedling establishment are strongly tied to climate fluctuation, with best establishment occurring in relatively wet years that have plentiful precipitation during germination and growth. In a garden study, Lippert and Hopkins [132] reported 35 sixweeks grass seedlings/m² in soil samples collected from a buffalo grass-blue grama community in Kansas. Study time was 73 days, and the garden site was watered [132]. In a creosotebush-white bursage study in Nye County, Nevada, where sixweeks grass is a winter annual, mean seedling emergence of sixweeks grass varied with precipitation [24]:

A 1992 Montana study showed ample seeds and good seedling establishment, but poor crop maturation. The researchers estimated the seed:plant ratio of sixweeks grass on the Fort Keogh Livestock and Range Research Laboratory by counting seeds in soil samples and measuring seedling density and mature plant biomass. Seed, seedling, and mature (standing crop) measures of the sixweeks grass population ranged as follows [110]:

Nurse plants may facilitate sixweeks grass establishment in arid climates. Creosotebush is a common nurse plant for sixweeks grass and other annuals in the Mojave Desert (review by [71]).

Growth rate is rapid in dry years, but may be slow compared to most annuals if favorable weather extends the growing season. DeFalco and others [55] suggest sixweeks grass has adapted to low-nitrogen soils by slowing growth; thus, it has less demand for nitrogen compared to most annuals. Plant community structure and type may also affect seedling establishment and growth rates. In adjacent plant communities of the Mojave Desert of California, sixweeks grass establishment and growth varied as follows [26]:

Asexual regeneration: Because it is an annual, sixweeks grass does not sprout from the root crown after it produces seed. It dies. However, annual grasses may die back and sprout from the root crown when wet weather follows a short-term dry period during the growing season [104].

Aspect where sixweeks grass grows varies depending upon climate and surrounding vegetation. On morainal mountain grasslands of the Blackfoot Valley of western Montana, sixweeks grass occurred on low, dry, south- and southeastern aspects of the moraines [24]. However, in knob and kettle topography on south-central Washington, sixweeks grass grew on north-facing slopes but not south-facing slopes [179]. In a sand bluestem-prairie sandreed (Andropogon gerardii var. paucipilus-Calamovilfa longifolia) prairie in the Nebraska sandhills, sixweeks grass is most common in depressions [28]. It is reported on lowlands, alluvial flats, and bajadas of the Southwest [138].

Elevation: Sixweeks grass is most common at low elevations. Elevational range by state is:

Soils: Sixweeks grass is adapted to a wide range of soils, but is most common on coarse-textured soils [208,224]. The soils are often disturbed [77,80,86,224]. Rydberg [187] described sixweeks grass as "characteristic of sandhill regions of Nebraska and Kansas," occurring on sandy riverbanks, draws, and canyon bottoms. Sixweeks grass also grows in loamy and rocky soils [97,104,113,129,137]. Slender sixweeks grass grows on dry, "ruderal" sands on Fire Island National Seashore, New York [59] and on coastal beaches and dunelands of Connecticut [99]. In the Central Great Plains of Kansas, sixweeks grass occurs on "mature, well-developed" soil profiles [2]. It grows on disturbed rocky, sandy, and sandy clay soils in Texas, where parent materials include limestone [57]. Sixweeks grass occurs on shale-derived soils in southern Illinois [139] and on gypsum-derived and alluvial soils in the eastern Mojave Desert [147].

Sixweeks grass occurs on sites disturbed by fossorial rodents [66]. In eastern Colorado it was more common in black-tailed prairie dog towns than on undisturbed blue grama-buffalo grass prairie [25]. Carlson and Crist [40] found sixweeks grass was most common on northern pocket gopher mounds and heavily grazed (≥60% utilization) cattle pastures on the Central Plains Experimental Range of north-central Colorado, and least common on lightly grazed pastures.

Because it is an annual, sixweeks grass cover may change from year to year without a major change in its successional status, particularly on open sites where light and space are not limiting. For example, in a Colorado pinyon (Pinus edulis)-Utah juniper community in northeastern Utah, Austin [12] noted little change in overall plant community composition and species frequency after 10-year remeasurements. Sixweeks grass showed 4% frequency in 1974 and 7% frequency in 1984 [12].

Fire: Sixweeks grass probably held an early seral position in fire-prone ecosystems prior to European settlement [173]. It is still common on burns in early postfire succession [54], but may occur in nearly equal numbers on similar unburned sites that are open. It was, for example, noted on both burned and adjacent unburned sites in a threetip sagebrush-mountain big sagebrush (Artemisia tripartita-A. tridentata var. vaseyana) community near Pocatello, Idaho [1]. Sixweeks grass was seeded in after a 1958 broadcast burn on a clearcut in southwestern North Carolina. By 1995, the site had succeeded to black locust-yellow-poplar (Robinia pseudoacacia-Liriodendron tulipifera) forest. Sixweeks grass frequency and biomass declined as the canopy closed [62]. See Fire Ecology for information on fire regimes of plant communities supporting sixweeks grass and Fire Effects for more information about sixweeks grass response to fire

Old fields: Sixweeks grass is a common component of old-field succession in palouse and shortgrass prairies [47,52]. In old-field succession on a bluebunch wheatgrass (Pseudoroegneria spicata)-Sandberg bluegrass community near Clarkston, Washington, sixweeks grass showed trace coverage and frequency on young (1-12 years since tilling) and old fields (39-52 years since tilling), and 2% coverage and 82% frequency on untilled sites [52]. It was a component of old fields that had been plowed 53 years before a vegetation survey on the Pawnee National Grasslands and the Central Plains Experimental Range of Colorado. The fields were succeeding to blue grama-buffalo grass communities [44]. In an old-field study in the Cross Timbers region of Texas, sixweeks grass formed 17% cover 1 year after cultivation [61]. Sixweeks grass may be invasive on tallgrass prairies where overgrazing and/or drought has reduced cover of tallgrass species [229].

Abiotic influences: Sixweeks grass was important in drought-induced succession from 1934 to 1940. During the Great Drought, Weaver and Albertson [3] wrote that "the density and extent of (sixweeks grass) stands was astonishing. Bunches consisting of 20 to 30 stems growing out of the dead crowns of little bluestems were found regularly. Two hundred such aggregations of stems in a single square meter were common." After the drought ended, sixweeks grass became less important except on disturbed sites and small openings. The authors placed sixweeks grass in the "2nd weed stage" of tallgrass prairie succession: it established after ruderal weeds such as Russian-thistle (Salsola kali) and narrowleaf goosefoot (Chenopodium leptophyllum) but before native perennial grasses [3].

On the Central Plains Experimental Range, sixweeks grass density and cover were significantly (p<0.05) greater on fine-textured soils compared to coarse-textured soils. Study sites were denuded by hand-pulling aboveground and sieving plant organs from soil to a depth of 4 inches (10 cm), then allowed to recolonize without further treatment [43].

Late succession: Sixweeks grass occurs in late-successional communities with open ground, such as pinyon-juniper; small openings within closed-canopy communities; and in small disturbances in mostly undisturbed grassland [145]. In a study on northern Wyoming's shortgrass prairie, sixweeks grass occurred in small disturbed areas within lands that had been ungrazed for 40 or more years [191]. Sixweeks grass was found on both undisturbed shortgrass prairie and reclaimed stripmine sites in the Powder River Basin of Wyoming [4,5]. In another Wyoming shortgrass prairie study, sixweeks grass occurred on ungrazed sites and sites experiencing heavy domestic sheep use. The authors suggested that plentiful fall precipitation when sixweeks grass was establishing was more important than level of grazing disturbance in determining sixweeks grass density [125]. Lippert and Hopkins [132] found sixweeks grass on "climax" buffalo grass-blue grama communities of Kansas. In southern Utah and northern Arizona, sixweeks grass occurred on an ungrazed, relict mesa of Glen Canyon National Recreation Area, and on heavily grazed sites on the nearby Navajo Reservation [107]. It was also present on a remote ungrazed, relict site in the Grand Canyon that had not burned since 1889 [194]. On desert grassland of Canyonlands National Park, Utah, sixweeks grass showed 1% or less cover on a site that had been lightly grazed for many years prior to the area being made a Park (which was 10 years prior to the study), and 1% cover on an ungrazed relict area [120]. In chamise (Adenostoma fasciculatum) chaparral of California, sixweeks grass occurred on undisturbed sites beneath chamise and in experimental clearings [71].

FIRE ECOLOGY

• FIRE ECOLOGY OR ADAPTATIONS
• POSTFIRE REGENERATION STRATEGY

Fire regimes: Sixweeks grass occurs in many different ecosystems and plant communities, so it experiences a wide range of fire regimes. In closed-canopy ecosystems such as maple-beech (Acer-Fagus spp.), where hundreds of years may pass without fire, sixweeks grass presence is most likely after canopy-gap disturbances. As long as it is at least sparsely represented in the prefire community, sixweeks grass may become more important when fire does occur in plant communities with long fire return intervals. Sixweeks grass is most important in open-canopy forests, woodlands, grasslands, and shrublands (see Distribution and Occurrence). Fire is important in retaining open structure in most of the communities where sixweeks grass is common. Ponderosa pine (Pinus ponderosa) and oak (Quercus spp.) woodlands, for example, are maintained by frequent understory fire [8,225]. Fire intervals in pinyon-juniper (Pinus-Juniperus spp.) woodlands vary greatly, but fire is important in eventually opening the canopies. Pinyon-juniper communities experience understory burns when fire is frequent, but typically have moderate- to long-interval, stand-replacing fires [152,211]. Frequent fire in annual grasslands and palouse, plains, and mixed-grass prairies maintains the grasslands by preventing invasion of woody plants and reducing litter [169,169,185,205,242]. Fire plays a more variable ecological role in shrublands where sixweeks grass is important. Some of the shrublands (e.g., chamise and other chaparral types) depend on moderate-interval (30-100 years), stand-replacing fire [169]; others are adapted to mixed-severity fires (e.g., big sagebrush) [8,38,148,192]; while many desert shrubland types (such as creosotebush) are poorly adapted to fire [33]. Descriptions of fire regimes of communities where sixweeks grass is important follow.

Desert shrub: Fire is infrequent in pristine creosotebush-white bursage, Joshua tree, and saguaro communities. Discontinuity of fine fuels in most years hinders the spread of fire, which was historically uncommon to rare [31,33,35,103,162]. In most years, pristine stand structure of these Southwestern desert shrub communities is widely spaced woody plants, some perennial bunchgrasses, and bare interspaces [29,33,34,186]. During wet winters and springs, annuals such as sixweeks grass increase fuels loads. Biomass accumulations from native annuals following an exceptionally wet growing season may provide enough fine fuels to carry a fire in desert ecosystems that otherwise rarely burn [26,212].

Dry sixweeks grass provides little fine fuel biomass in most years because it crumbles so quickly after senescence. During prescribed burning on the Mojave Desert, native annual vegetation including sixweeks grass was unable to sustain fire despite abundant growth after above-average winter rains [33]. Nonnative, invasive annual grasses including red brome, have schismus have increased fuel loads from historic levels. While native annual grasses mostly grow in the protective shade of shrubs, nonnative grasses also grow in shrub interspaces, increasing fuel continuity and fire frequency and severity on invaded sites. Ecological consequences are serious, as most southwestern desert plants are poorly adapted to frequent and/or stand-replacing fire [33,34]. Nonnative annual grasses outcompete sixweeks grass on most desert shrub sites (see Invasives). Sixweeks grass became uncommon after the 1940s, when common Mediterranean grass (S. barbatus) invaded the Mojave Desert and coastal grasslands of California (Clarke, O, personal observation cited in [32]). For detailed information on fine fuel production of sixweeks grass, red brome, schismus, and other Mojave Desert annuals, see the Research Project Summary Nonnative annual grass fuels and fire in the Mojave Desert.

Sagebrush/bunchgrass: Prior to the 1890s, probably only a few grass species occupied a prominent position in early postfire sagebrush communities of the Great Basin. Sixweeks grass and small sixweeks grass (Vulpia microstachys) were among the most important of these early postfire annuals. Generally, native Vulpias would increase for a few years, then be suppressed by recovering bunchgrasses such as bluebunch wheatgrass, bottlebrush squirreltail (Elymus elymoides), and Idaho fescue (Festuca idahoensis), and by shrubs such as basin big sagebrush (Artemisia tridentata ssp. tridentata) and rabbitbrush (Chrysothamnus spp.) [173]. Historic fire return intervals in sagebrush ecosystems were variable, ranging from around 20 to 100 years. Most fires were mixed-severity and of small extent, although more widespread fires occurred on some sites [100,242,243]. Cheatgrass and medusahead (Taeniatherum caput-medusae), nonnative annual grasses, have altered fire regimes and successional patterns in some sagebrush communities. Fine fuel loads from dry cheatgrass and/or medusahead can support fire-return intervals as short as 3 to 6 years [173,236].

Short- and mixed-grass prairies: Fires were frequent in presettlement prairie ecosystems. Widespread plains grassland fires were often noted in historical records from the early 1880s [89], and presettlement fires probably occurred every 35 years or less. Agriculture, urbanization, and fire exclusion have greatly lengthened fire return intervals on the Great Plains [169,185,242]. A few shortgrass prairies mostly retain their historical composition and structure, although few are absolutely pristine. A wet-year survey of a relict threadleaf sedge (Carex filifolia)-bluebunch wheatgrass-blue grama community in south-central Montana found sixweeks grass and cheatgrass occupied interspaces between bunchgrasses only "occasionally" [244].

Pinyon-juniper woodlands: Fire return intervals in pinyon-juniper woodlands vary greatly, depending upon fine fuel loads and stand density. Historical fire return interval for western juniper (Juniperus occidentalis) woodlands is estimated at 10 to 30 years. Livestock grazing in pinyon-juniper tends to increase fire return intervals by reducing fine fuels and increasing woody fuel density (review by [243]). Cheatgrass invasion in some pinyon-juniper communities has greatly decreased fire return intervals and increased fire severity [121].

Annual grasslands: Because they are dominated by nonnative annuals, annual grasslands have no "natural" fire regime. There are no data and few historic records of presettlement fire return intervals in pristine California prairie. Probable mean fire intervals (estimates of fire intervals that are derived from historical or very limited physical evidence) for California prairie are frequent: approximately every 1 to 2 years. Probable mean fire intervals for annual grasslands are every 20 to 30 years [205].

Chaparral: Historic fire return intervals in chamise and mixed-chaparral range from 10 to 90 years [169,210]. Intervals between fires were longer in communities dominated by nonsprouting shrubs, such as bigberry manzanita (Arctostaphylos glauca), than in communities dominated by sprouting shrubs such as chamise [115].

Coastal sage scrub chaparral: Documentation of historic fire intervals in coast sage scrub is lacking. Current fire return intervals vary widely. Total area burned strongly correlates with precipitation during the previous winter, with heaviest burning occurring after wet years. Fire is rare following drought [149]. Vogl [223] estimated an average fire interval of 20 years for lightning-ignited fire in chaparral adjacent to coastal sage scrub. Fire severity is generally higher in coastal sage scrub than in seral chaparral due to higher litter loading and the higher percentage of terpenes in coastal sage scrub vegetation [81,136]. For a California sagebrush-eastern Mojave buckwheat (Artemisia californica-Eriogonum fasciculatum) community on the Cleveland National Forest, California, fire records show that stand-replacing fire occurs at approximate 28-year intervals. Sixweeks grass is noted in early postfire succession in the community [238].

Fuels: Because it usually constitutes less than 5% of the total vegetation and crumbles rapidly upon drying, sixweeks grass typically contributes little fine fuel biomass during the fire season [220].

A few studies provide measures of sixweeks grass fuel loads or fuel loads in plant communities where sixweeks grass is a component of the vegetation. On undisturbed palouse prairie in eastern Washington, annual grasses—with sixweeks grass the most common annual grass—composed less than 2% of total biomass from late May to early June. Live shoot biomass ranged from 50 to 69 g/m², and mostly consisted of perennial bunchgrasses [182]. In a "drier than normal" year, McColley and Hodgkinson [141] found sixweeks grass air-dry production averaged 8.3 lbs/acre on Idaho fescue-threadleaf sedge palouse prairie of east-central Washington. Potvin and Harrison [174] measured September biomass of sixweeks litter at 2.1 gm/m² on the Arapaho Prairie of Nebraska. Whisenant [236] provides the following mean fuel loads for a rubber rabbitbrush (Chrysothamnus nauseosus)/cheatgrass-Sandberg bluegrass-sixweeks grass site on the Snake River Plains of Idaho:

Fine fuels in pinyon-juniper can vary greatly. In a review, Wright and others [243] gave an average production of western juniper communities in eastern Oregon, which often contain sixweeks grass, at 600 lb/ac, with yields on sites with good soils and high precipitation as high as 1,400 lb/ac.

Leaf area index is a variable in some fuel models (e.g., FARSITE). Hazlet [91] provides leaf area indices for northern Colorado blue grama-buffalo grass communities containing sixweeks grass.

The following table provides fire return intervals for plant communities and ecosystems where sixweeks grass is important. Find fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

FIRE EFFECTS

• IMMEDIATE FIRE EFFECT ON PLANT
• DISCUSSION AND QUALIFICATION OF FIRE EFFECT
• PLANT RESPONSE TO FIRE
• DISCUSSION AND QUALIFICATION OF PLANT RESPONSE
• FIRE MANAGEMENT CONSIDERATIONS

However, heat or charate treatments had no effect on sixweeks grass germination in another laboratory experiment [20].

Intense heat generated in some shrubland fires can kill shallowly buried seed and seeds in litter. In field and greenhouse studies of the seed bank of a basin big sagebrush community near Reno, Nevada, Young and Evans [248] found 4 times as many viable sixweeks grass seeds in litter than buried in soil, where seeds were less vulnerable to fire. In a greenhouse study, soil cores from unburned chamise plots produced 122 sixweeks grass germinants/300 cm² of soil, while samples taken from an adjacent site that had recently burned produced no germinants [233]. A vegetation survey also illustrates this trend. An eastern Mojave buckwheat-California sagebrush costal sage community in the Santa Monica Mountains of California experienced wildfire on 18 June 1978, and a portion of the site reburned in June 1979. The 1978 fire had relatively long burnout times (2-minute exposure to 9 kcal/sec/m²); reaction intensity of the fire was modeled at 120 kcal/sec/m². The site was a southeast-facing, xeric slope where fortunately, vegetation sampling had been conducted in 1977. Prefire (1977) vegetation sampling showed 30.9% prefire coverage of sixweeks grass. Postfire coverage in spring 1980 was 0.5% [235].

Fire does not always reduce sixweeks grass's soil seed bank. In a seed bank study at La Purisima State Historic Park northwest of Los Angeles, Davis and others [53] found approximately equal numbers of sixweeks germinants from pre- and postfire soils. Soil samples were collected before and after prescribed burning in a maritime chamise community and moved to a greenhouse for seed germination. Mean sixweeks grass germinant volume was 1 plant/250 cm³ in unburned soil and 0.85 plant/250 cm³ in burned soil [53].

Postfire increases in sixweeks grass coverage and frequency are usually not large. Minor postfire increases in sixweeks grass may be statistically insignificant but biologically important. Doubling sixweeks grass production may provide important cover on disturbed sites [3] and enhance wildlife food webs. Sixweeks grass is, for example, highly important in the diet of the Townsend's ground squirrel, a keystone species [161] in sagebrush ecosystems (see Seed use). A doubling of sixweeks grass seed production may greatly benefit the rodent and its predators.

Positive fire effect: Sixweeks grass was the most common annual in maritime coast live oak and chamise communities 1 to 5 years after fire in Santa Barbara County, California [54]. In a survey of postfire vegetation in chamise and mixed-chaparral communities of California, Sweeney [210] found sixweeks grass was most common at postfire year 1, with abundance "decreasing somewhat" on older burns. Daubenmire [52] reported that a July 1961 wildfire ""benefited" sixweeks grass near Clarkston, Washington. The fire occurred in an old field succeeding to a bluebunch wheatgrass-Sandberg bluegrass community. Percent cover of sixweeks grass was low under all conditions, but relative frequency was high in early postfire years [52]:

Biomass of sixweeks grass greatly increased (547%) on open sites after prescribed fire on the Tonto National Forest, Arizona. Plant response to 3 fires in a saguaro-desert ironwood (Olneya tesota)/white bursage/cholla (Opuntia spp.) community was compared. Study sites included an unburned control and 2 adjacent burn sites. One of the adjacent sites burned in a wildfire on 26 May 1980; the other adjacent site was burned under prescription on 12 June 1981. For each treatment, density and biomass were measured on 2 microhabitats: open/shrub (shrub interspaces or areas with low shrubs) and shade (beneath saguaro or desert ironwood trees). Changes in sixweeks grass density were not significant (p<0.05) across treatments on open/shrub sites. Changes in biomass were significant, with sixweeks grass on the wildfire site showing biomass reduction and other sites showing increases. Changes in density and biomass were not significant on shaded sites. Mean sixweeks grass density and biomass were [41]:

Mean precipitation for the growing season (December-March) is 5.0 inches (127 mm, measured from the nearest weather station). Growing-season precipitation during the fire study years was 10.8 inches (275 mm, 216% of normal) in 1979-1980; 3.5 inches (90 mm, 71% of normal) in 1980-1981; and 6.06 inches (154 mm, 121% of normal) in 1981-1982 [41].

Burn surveys showed that late-season prescribed fires and wildland fires for resource benefit favored sixweeks grass in Dinosaur National Monument, Colorado. Sixweeks grass and/or cheatgrass formed a "major component" of the postfire vegetation on 5 of 6 burn sites. The management objective was to reduce the Wyoming big sagebrush component of the vegetation and increase native grasses including sixweeks grass [172].

No fire effect: With or without fire, sixweeks grass typically shows less than 5% cover on most sites. On open or already disturbed sites, fire may have little effect on sixweeks grass's relative coverage. It was noted as a minor grass (<5% cover), for example, on both burned and unburned big sagebrush/bluebunch wheatgrass sites in southeastern Washington [130,131]. Near Pocatello, Idaho, it was noted on both burned and unburned sites 1 year after prescribed fire in a threetip sagebrush-mountain big sagebrush community. Although the dominant species in the seed bank, sixweeks grass was uncommon on both burned or unburned plots. Relative sixweeks grass seedling density and cover at postfire year 1 were [1]:

After the 1979 Ship Island Fire on the Frank Church-River of No Return Wilderness in Idaho, differences in sixweeks grass coverage on burned vs. unburned plots at postfire years 1 and 2 were insignificant (p=0.05), ranging from 0.8% to 4.7% [209]. Similarly, April 1981 vegetation sampling conducted after an October 1980 prescribed fire in an Arizona saguaro-yellow paloverde community showed no significant (p<0.05) differences in sixweeks grass cover on burned and unburned plots [42].

Negative fire effect: Fire may not always benefit sixweeks grass in the short term. On the Arapho Prairie preserve in Nebraska, vegetation analysis was conducted in 1982 after an October 1981 wildfire. Biomass (g/m²) of sixweeks grass on burned and adjacent unburned plots was [153]:

A Nebraska sandhills prairie rangeland showed a decrease in sixweeks grass after May 1976 prescribed burning [28]. Data were not provided.

Longer-term fire effects: Many fire studies follow sixweeks grass's response for only postfire years 1 and 2; however, sixweeks grass may occur in later stages of postfire succession providing there are open microsites available. Sixweeks grass showed 4% frequency 4 years after prescribed fire in a redberry juniper community on the Rolling Plains of Texas [127]. Sixweeks grass was present on both burned and unburned blackbrush sites 6 years after wildfire in southwestern Utah and 14 years after wildfire on the Spring Mountain National Recreation Area in southern Nevada. Cover and frequency data were not given [213].

Seed bank: Fire may reduce but does not deplete the soil seed bank. Over several to many years, postfire seed production replenishes sixweeks grass's seed bank. In a study of seed bank composition and fire effects to seed on different-aged burns, Zammit and Zedler [251] collected soil-litter samples from chamise-desert ceanothus (Ceanothus greggii) stands on the Sky Oaks Biological Field Station and the adjacent Cleveland National Forest near San Diego, California. Soil-litter samples were placed in flats; dry straw burned over half the samples (temperatures of 300-570 °F (150-300 °C) for 1.2-2 min.); then all samples were tested for germination in a greenhouse for 6 months. The researchers found density of germinable sixweeks grass seed was significantly (p<0.05) higher in soil from a 62-year-old stand compared to soil from chamise-desert ceanothus stands of other ages. Sixweeks grass seed density (seeds/dm²± SD) was [251]:

Fire and grazing: Cool-season annual prescribed burning and grazing reduced sixweeks grass on the Flint Hills of Kansas. After repeat (1950-1966) annual spring burns and continuous cattle grazing (5 acres/animal unit) on big bluestem-little bluestem-grama (Bouteloua spp.) mixed-grass prairie, annual grasses (sixweeks grass, bromes (Bromus spp.), and little barley) showed less basal cover on burned sites compared to unburned sites, with mid-spring burning most harmful to the annual grasses. Data were collected in the fall of 1966 and pooled for annual grasses. Annual grasses had 7.6% basal cover on unburned sites; 2.1% cover on early spring-burned (20 March) sites; 1.6% cover on mid-spring burned (10 April) sites; and 2.6% cover on late-spring burned (1 May) sites. Bluestems also decreased under this burning-and-grazing prescription, while gramas and Kentucky bluegrass (Poa pratensis) increased [6].

Keeley and others [116] compared postfire establishment and succession on 14 chaparral burn sites in southern California. They noted sixweeks grass had high coverage on only 1 burn: a heavily grazed chamise chaparral site in its 9th year of postfire succession [116].

Postfire rehabilitation: After a 1996 wildfire in a Utah juniper/basin big sagebrush community in Utah, the burn was chained and seeded with a mix of native and nonnative rangeland grasses before the next growing season. The mix did not include sixweeks grass. Sixweeks grass showed 1% frequency at postfire years 1 and 2 [166]. The treatment did not include a comparative control.

Research Project Summaries: The following Research Project Summaries provide information on prescribed fire use and postfire response of plant community species including sixweeks grass:

• Nonnative annual grass fuels and fire in California's Mojave Desert
• Vegetation response to restoration treatments in ponderosa pine-Douglas-fir forests of western Montana

MANAGEMENT CONSIDERATIONS

• IMPORTANCE TO LIVESTOCK AND WILDLIFE
• VALUE FOR REHABILITATION OF DISTURBED SITES
• OTHER USES
• OTHER MANAGEMENT CONSIDERATIONS

Wild ungulates and herbivorous small mammals utilize sixweeks grass. Bison, pronghorn, and desert mule deer graze it lightly in spring [122,170,171,196]. Black-tailed jackrabbits consume sixweeks grass (review by [218]). Near Twin Falls, Idaho, they grazed sixweeks grass heavily in April and lightly in June [65]. Black-tailed and white-tailed prairie dogs also graze sixweeks grass [117,118].

Seed use: Granivorous rodents consume sixweeks grass seeds in summer and fall and may graze new foliage early in the growing season (review by [84]),[69,157,202]. On the Eastern Colorado Range Station, plains pocket gopher consumption peaked during seed set (August) and seedling establishment (November) [157]. In a northern Arizona study, Reichman [179] found Merriam's kangaroo rats and Arizona, Bailey's and rock pocket mice consumed sixweeks grass seeds more than expected based upon availability. A study on the Snake River Birds of Prey Area showed that Townsend ground squirrels consumed sixweeks grass in early March (25% frequency in stomach contents) and heavily in May (100% frequency), when seed was curing [246]. Granivorous songbirds and upland game birds, such as chukar and sharp-tailed grouse, consume sixweeks grass seeds (review by [84],[207]). Relative importance of sixweeks grass forage is largely unstudied for most birds, and sixweeks grass may be very important for some species. It was the top-ranking food for lesser prairie-chickens on a sand sagebrush (Artemisia filifolia)-shortgrass prairie in Oklahoma [108].

Palatability/nutritional value: Sixweeks grass is not highly palatable to large herbivores. Palatability is rated as poor for elk, mule deer, and pronghorn [58]. Palatability ratings for livestock generally range from poor to fair [58,84,101,143]. Kelso [117] rated sixweeks grass's forage value as poor to good for cattle and horses and poor to fair for domestic goats and sheep. In a review, Gullion [84] rates sixweeks grass palatability as fair for cattle and domestic sheep. Loosely rooted, sixweeks grass tends to tear out by the roots when grazed, leaving bits of soil that livestock will not consume [207]. Cattle on the Central Plains Experimental Range, Colorado, avoided areas with concentrations of sixweeks grass, and tended to drop sixweeks grass plants inadvertently included in a bite [101,104].

Few studies have measured the nutrient content of sixweeks grass. Nutritional value of plants collected in Suton County, Texas, was [73]:

In Arches National Park, Utah, Belnap and Harper [17,18] found sixweeks grass growing on cryptobiotic soil crusts was more nutritious than sixweeks grass on unstable blow-out sands:

Sixweeks grass seed from northern Arizona averaged 4,132 calories/g and 2.02 calories/seed. [180].

Cover value: Sixweeks grass provides poor cover for small mammals and birds [58].

Weaver and Alberston [3] credited sixweeks grass as an important soil stabilizer during the Great Drought. In eastern Kansas and Nebraska, it "covered the dead bodies of dead dominants." They write that had this grass been removed, much of the landscape would have lost 95% of its litter and live plant cover [3].

Rangeland: Sixweeks grass is not invasive on well-managed rangelands. Stubbendieck and others [207] stated that it is "usually not a serious weed."

Since it is relatively unpalatable, sixweeks grass is generally favored by moderate to heavy grazing [105,107,178,215]. Valone and Kelt [219] found significantly (p=0.05) more sixweeks grass on ungrazed plots than on cattle-grazed plots in an acacia (Acacia spp.)-dominated desert shrub community of southeastern Arizona. Sixweeks grass increased after long-term summer cattle grazing (0.2 AUM) on the Crescent Lake National Wildlife Refuge, located in Nebraska sandhills prairie [28]. Even short-term intensive grazing may increase sixweeks grass cover [231], and moderate to dense sixweeks grass cover is sometimes used as an indicator of poor or declining rangeland conditions [143,250]. However, total sixweeks grass coverage may be slight even on overgrazed rangelands. Dyksterhuis [61] reported 2.5% to 3.0% sixweeks grass coverage on old fields in the Cross Timbers region of Texas and Oklahoma. The old fields were subjected to year-round, unrestricted cattle grazing. Sixweeks grass was not present on rangelands in good to excellent condition, had 1% cover on fair-condition sites, and 6% cover on poor-condition rangelands [61].

Fertilizer: Nitrogen application on plains grasslands may increase coverage of sixweeks grass and other annual herbs at the expense of blue grama and other palatable perennial grasses [90,101,128].

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