Vulpia octoflora



INTRODUCTORY


 

Photo by Kitty Kohout,
courtesy of the Univ. of
WI-Stevens Pt.

AUTHORSHIP AND CITATION:
Howard, Janet L. 2006. Vulpia octoflora. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [].

FEIS ABBREVIATION:
VULOCT

NRCS PLANT CODE [216]:
VUOC

COMMON NAMES:
sixweeks grass
six-weeks grass
common sixweeks grass
eight-flower sixweeks grass
sixweeks fescue
eight-flowered fescue

TAXONOMY:
The scientific name of sixweeks grass is Vulpia octoflora (Walt.) Rydb. (Poaceae) [49,57,77,97,111,112,133,188,230,245]. Varieties recognized by some authorities are:

Vulpia octoflora (Walt.) Rydb. var. glauca (Nutt.) Fernald [49,57,67,77,133], slender sixweeks grass
Vulpia octoflora (Walt.) Rydb. var. hirtella (Piper) Henrard [49,57,68,97,112,133,134,137], hairy sixweeks grass
Vulpia octoflora (Walt.) Rydb. var. octoflora [49,57,77,97,112,133,134,137], sixweeks grass

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].

SYNONYMS:
Infrataxa―
Sixweeks grass:
Festuca octoflora Walt. [67,80,113,137,176,224,232]
    =Vulpia octoflora (Walt.) Rydb. var. octoflora [57,77,97,112,137]

Slender sixweeks grass:
Festuca octoflora Walt. var. tenella (Willd.) Fernald [67,68,137]
    =Vulpia octoflora (Walt.) Rydb. var. glauca (Nutt.) Fernald
Festuca gracilenta Buckl.
    =Vulpia octoflora (Walt.) Rydb. var. glauca (Nutt.) Fernald
Festuca tenella Willd.
    =Vulpia octoflora (Walt.) Rydb. var. glauca (Nutt.) Fernald [57,67,77]
Vulpia octoflora var. tenella (Willd.) Fern. [134]
    =Vulpia octoflora var. glauca [57,67,77]

LIFE FORM:
Graminoid

FEDERAL LEGAL STATUS:
None

OTHER STATUS:
Sixweeks grass is endangered in Vermont. Slender sixweeks grass is endangered in New Hampshire [216].

DISTRIBUTION AND OCCURRENCE

SPECIES: Vulpia octoflora
GENERAL DISTRIBUTION:
Sixweeks grass is widely distributed in North America, occurring from British Columbia east to Quebec and south to Baja California Norte, Mexico, the Texas panhandle, and central Florida [111,134,232,240]. Of the varieties, slender sixweeks grass is most common in the northern United States and southern Canada, although its distribution extends to the southern United States. Hairy sixweeks grass occurs in the western United States, Baja California, and Florida. The typical variety (V. o. var. o.) is most common in the Southeast, but occurs across southwestern Canada and all of the United States except the extreme Northeast [111,134]. Grass Manual on the Web provides a map of sixweeks grass's distribution in Canada and the United States. Plants database provides distributional maps of sixweeks grass and its varieties in the United States.

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.

ECOSYSTEMS [76]:
FRES10 White-red-jack pine
FRES11 Spruce-fir
FRES12 Longleaf-slash pine
FRES13 Loblolly-shortleaf pine
FRES14 Oak-pine
FRES15 Oak-hickory
FRES16 Oak-gum-cypress
FRES17 Elm-ash-cottonwood
FRES18 Maple-beech-birch
FRES19 Aspen-birch
FRES20 Douglas-fir
FRES21 Ponderosa pine
FRES23 Fir-spruce
FRES28 Western hardwoods
FRES29 Sagebrush
FRES30 Desert shrub
FRES31 Shinnery
FRES32 Texas savanna
FRES33 Southwestern shrubsteppe
FRES34 Chaparral-mountain shrub
FRES35 Pinyon-juniper
FRES36 Mountain grasslands
FRES38 Plains grasslands
FRES39 Prairie
FRES40 Desert grasslands
FRES42 Annual grasslands

STATES/PROVINCES: (key to state/province abbreviations)
UNITED STATES
AL AK AZ AR CA CO CT DE FL GA
ID IL IN IA KS KY LA ME MD MA
MI MN MS MO MT NE NV NH NJ NM
NY NC ND OH OK OR PA RI SC SD
TN TX UT VT VA WA WV WI WY DC

CANADA
AB BC ON PE PQ SK

MEXICO
B.C.N.

BLM PHYSIOGRAPHIC REGIONS [19]:
1 Northern Pacific Border
2 Cascade Mountains
3 Southern Pacific Border
4 Sierra Mountains
5 Columbia Plateau
6 Upper Basin and Range
7 Lower Basin and Range
8 Northern Rocky Mountains
9 Middle Rocky Mountains
10 Wyoming Basin
11 Southern Rocky Mountains
12 Colorado Plateau
13 Rocky Mountain Piedmont
14 Great Plains
15 Black Hills Uplift
16 Upper Missouri Basin and Broken Lands

KUCHLER [124] PLANT ASSOCIATIONS:
K002 Cedar-hemlock-Douglas-fir forest
K005 Mixed conifer forest
K009 Pine-cypress forest
K010 Ponderosa shrub forest
K011 Western ponderosa forest
K012 Douglas-fir forest
K016 Eastern ponderosa forest
K017 Black Hills pine forest
K018 Pine-Douglas-fir forest
K023 Juniper-pinyon woodland
K024 Juniper steppe woodland
K026 Oregon oakwoods
K027 Mesquite bosques
K028 Mosaic of K002 and K026
K029 California mixed evergreen forest
K030 California oakwoods
K031 Oak-juniper woodland
K032 Transition between K031 and K037
K033 Chaparral
K034 Montane chaparral
K035 Coastal sagebrush
K036 Mosaic of K030 and K035
K037 Mountain-mahogany-oak scrub
K038 Great Basin sagebrush
K039 Blackbrush
K040 Saltbush-greasewood
K041 Creosote bush
K042 Creosote bush-bur sage
K043 Paloverde-cactus shrub
K044 Creosote bush-tarbush
K045 Ceniza shrub
K046 Desert: vegetation largely lacking
K047 Fescue-oatgrass
K048 California steppe
K050 Fescue-wheatgrass
K051 Wheatgrass-bluegrass
K053 Grama-galleta steppe
K054 Grama-tobosa prairie
K055 Sagebrush steppe
K056 Wheatgrass-needlegrass shrubsteppe
K057 Galleta-threeawn shrubsteppe
K058 Grama-tobosa shrubsteppe
K059 Trans-Pecos shrub savanna
K060 Mesquite savanna
K061 Mesquite-acacia savanna
K063 Foothills prairie
K064 Grama-needlegrass-wheatgrass
K065 Grama-buffalo grass
K066 Wheatgrass-needlegrass
K067 Wheatgrass-bluestem-needlegrass
K068 Wheatgrass-grama-buffalo grass
K069 Bluestem-grama prairie
K070 Sandsage-bluestem prairie
K071 Shinnery
K074 Bluestem prairie
K075 Nebraska Sandhills prairie
K076 Blackland prairie
K081 Oak savanna
K082 Mosaic of K074 and K100
K083 Cedar glades
K084 Cross Timbers
K085 Mesquite-buffalo grass
K086 Juniper-oak savanna
K087 Mesquite-oak savanna
K089 Black Belt
K096 Northeastern spruce-fir forest
K097 Southeastern spruce-fir forest
K098 Northern floodplain forest
K100 Oak-hickory forest
K101 Elm-ash forest
K102 Beech-maple forest
K103 Mixed mesophytic forest
K104 Appalachian oak forest
K106 Northern hardwoods
K107 Northern hardwoods-fir forest
K108 Northern hardwoods-spruce forest
K109 Transition between K104 and K106
K110 Northeastern oak-pine forest
K111 Oak-hickory-pine
K112 Southern mixed forest

SAF COVER TYPES [64]:
12 Black spruce
16 Aspen
40 Post oak-blackjack oak
42 Bur oak
43 Bear oak
44 Chestnut oak
45 Pitch pine
46 Eastern redcedar
50 Black locust
57 Yellow-poplar
63 Cottonwood
66 Ashe juniper-redberry (Pinchot) juniper
67 Mohrs (shin) oak
68 Mesquite
70 Longleaf pine
71 Longleaf pine-scrub oak
72 Southern scrub oak
73 Southern redcedar
75 Shortleaf pine
80 Loblolly pine-shortleaf pine
81 Loblolly pine
82 Loblolly pine-hardwood
83 Longleaf pine-slash pine
84 Slash pine
85 Slash pine-hardwood
204 Black spruce
210 Interior Douglas-fir
217 Aspen
220 Rocky Mountain juniper
222 Black cottonwood-willow
229 Pacific Douglas-fir
230 Douglas-fir-western hemlock
233 Oregon white oak
234 Douglas-fir-tanoak-Pacific madrone
235 Cottonwood-willow
236 Bur oak
237 Interior ponderosa pine
238 Western juniper
239 Pinyon-juniper
242 Mesquite
243 Sierra Nevada mixed conifer
244 Pacific ponderosa pine-Douglas-fir
245 Pacific ponderosa pine
246 California black oak
248 Knobcone pine
249 Canyon live oak
250 Blue oak-foothills pine
255 California coast live oak

SRM (RANGELAND) COVER TYPES [199]:
101 Bluebunch wheatgrass
102 Idaho fescue
103 Green fescue
104 Antelope bitterbrush-bluebunch wheatgrass
105 Antelope bitterbrush-Idaho fescue
106 Bluegrass scabland
107 Western juniper/big sagebrush/bluebunch wheatgrass
109 Ponderosa pine shrubland
110 Ponderosa pine-grassland
201 Blue oak woodland
202 Coast live oak woodland
203 Riparian woodland
204 North coastal shrub
205 Coastal sage shrub
206 Chamise chaparral
207 Scrub oak mixed chaparral
208 Ceanothus mixed chaparral
209 Montane shrubland
210 Bitterbrush
211 Creosote bush scrub
212 Blackbush
214 Coastal prairie
215 Valley grassland
301 Bluebunch wheatgrass-blue grama
302 Bluebunch wheatgrass-Sandberg bluegrass
303 Bluebunch wheatgrass-western wheatgrass
304 Idaho fescue-bluebunch wheatgrass
305 Idaho fescue-Richardson needlegrass
306 Idaho fescue-slender wheatgrass
309 Idaho fescue-western wheatgrass
310 Needle-and-thread-blue grama
311 Rough fescue-bluebunch wheatgrass
312 Rough fescue-Idaho fescue
314 Big sagebrush-bluebunch wheatgrass
315 Big sagebrush-Idaho fescue
316 Big sagebrush-rough fescue
317 Bitterbrush-bluebunch wheatgrass
318 Bitterbrush-Idaho fescue
319 Bitterbrush-rough fescue
320 Black sagebrush-bluebunch wheatgrass
321 Black sagebrush-Idaho fescue
322 Curlleaf mountain-mahogany-bluebunch wheatgrass
323 Shrubby cinquefoil-rough fescue
324 Threetip sagebrush-Idaho fescue
401 Basin big sagebrush
402 Mountain big sagebrush
403 Wyoming big sagebrush
404 Threetip sagebrush
405 Black sagebrush
406 Low sagebrush
407 Stiff sagebrush
408 Other sagebrush types
411 Aspen woodland
412 Juniper-pinyon woodland
413 Gambel oak
414 Salt desert shrub
415 Curlleaf mountain-mahogany
416 True mountain-mahogany
417 Littleleaf mountain-mahogany
418 Bigtooth maple
419 Bittercherry
421 Chokecherry-serviceberry-rose
501 Saltbush-greasewood
502 Grama-galleta
503 Arizona chaparral
504 Juniper-pinyon pine woodland
505 Grama-tobosa shrub
506 Creosotebush-bursage
507 Palo verde-cactus
508 Creosotebush-tarbush
509 Transition between oak-juniper woodland and mahogany-oak association
601 Bluestem prairie
602 Bluestem-prairie sandreed
603 Prairie sandreed-needlegrass
604 Bluestem-grama prairie
605 Sandsage prairie
606 Wheatgrass-bluestem-needlegrass
607 Wheatgrass-needlegrass
608 Wheatgrass-grama-needlegrass
609 Wheatgrass-grama
610 Wheatgrass
611 Blue grama-buffalo grass
612 Sagebrush-grass
613 Fescue grassland
614 Crested wheatgrass
703 Black grama-sideoats grama
704 Blue grama-western wheatgrass
705 Blue grama-galleta
706 Blue grama-sideoats grama
707 Blue grama-sideoats grama-black grama
708 Bluestem-dropseed
709 Bluestem-grama
710 Bluestem prairie
713 Grama-muhly-threeawn
714 Grama-bluestem
715 Grama-buffalo grass
716 Grama-feathergrass
717 Little bluestem-Indiangrass-Texas wintergrass
718 Mesquite-grama
719 Mesquite-liveoak-seacoast bluestem
720 Sand bluestem-little bluestem (dunes)
721 Sand bluestem-little bluestem (plains)
722 Sand sagebrush-mixed prairie
724 Sideoats grama-New Mexico feathergrass-winterfat
727 Mesquite-buffalo grass
728 Mesquite-granjeno-acacia
729 Mesquite
730 Sand shinnery oak
731 Cross timbers-Oklahoma
732 Cross timbers-Texas (little bluestem-post oak)
733 Juniper-oak
734 Mesquite-oak
735 Sideoats grama-sumac-juniper
801 Savanna
802 Missouri prairie
804 Tall fescue
805 Riparian
810 Longleaf pine-turkey oak hills
812 North Florida flatwoods

HABITAT TYPES AND PLANT COMMUNITIES:
Sixweeks grass is most frequent in desert grassland, desert shrub [138], sagebrush (Artemisia spp.) [5], short- and mixed-grass prairie [118,229,229], and annual grassland communities [92]. Even in those communities, it is usually a minor grass (<5% cover) [3,61,214,217,234]. Coverage may be more extensive on disturbed or open sites. A botanical survey in Zion National Park, Utah, showed 40% sixweeks grass occurrence in open singleleaf pinyon (Pinus monophylla) communities [87]. Hairy sixweeks grass is common in burned singleleaf pinyon-juniper (Juniperus spp.) woodlands of California [97]. Sixweeks grass was common on heavily grazed bison rangelands in 2 presettlement (1810) surveys of shortgrass prairies [89].

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].

Sixweeks grass is not usually named as a dominant species in vegetation classifications. Burzlaff [39] describes 3 sixweeks grass unions and 1 sixweeks grass-forb union on the Nebraska sandhills prairie. Daubenmire [51] defines a plant union as "the smallest structural unit in the organization of vegetation, consisting of 1 or more species that make similar demands on their environment." A winterfat/Sandberg bluegrass (Krascheninnikovia lanata/Poa secunda)-sixweeks grass community was identified on the Snake River Birds of Prey Area of southern Idaho [247]. Sixweeks grass is important in rock outcrop communities of the Cross Timbers region, where a survey showed 11% sixweeks grass presence and 5.4% mean cover on a yellow stonecrop (Sedum nuttallianum)-foliose lichen rock outcrop community in south-central Oklahoma [46]. Sixweeks grass and poverty oatgrass (Danthonia spicata) dominate pristine shale barrens of southern Illinois [139].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Vulpia octoflora
GENERAL BOTANICAL CHARACTERISTICS:
This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for species identification are available (e.g. [86,97,134,176,245]). Lonard [134] provides a detailed key for distinguishing the varieties.

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].

RAUNKIAER [177] LIFE FORM:
Therophyte

REGENERATION PROCESSES:
Because it is an annual, sixweeks grass regenerates only from seed [1,54,66,98,109,132,226].

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 ft), 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-cm 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]:

Date Site Density
(sixweeks grass seedlings/m)
July 1984 coarse-textured 181
fine-textured 283
Sept. 1984 coarse-textured 2,143
fine-textured 1,147
Nov. 1984 coarse-textured 8
fine-textured 0
March 1985 coarse-textured 38
fine-textured 8
May 1985 coarse-textured 1,140
fine-textured 400
July 1985 coarse-textured 1,653
fine-textured 596
Sept. 1985 coarse-textured 91
fine-textured 45
Nov. 1985 coarse-textured 242
fine-textured 98

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/m) was [248]:

  Sept. Nov.
litter 4,350 900
soil (0-2.5 cm depth) 1,200 300

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/m) than on heavily, moderately, and lightly grazed plots (125, 110, and 90 seeds/m, 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]:

1939 1940 1941
April May June April May June April May June
6.2 0.5 2.9 7.3 4.0 5.3 19.2 24.0 16.9

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

1930 1940 1941
Density Yield Density Yield Density Yield
0.0 0.0 0.0 0.0 14.0 9.0

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/m 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]:

 

Mean precipitation (mm)

 
Year Sept.-Oct. Nov.-Dec. Jan.-Feb. Total (Sept.-Feb.) Density*
1971 0 37 8 45 2.960
1972 0 41 0 41 0.530
1973 40 29 70 141 5.005
1973 3 24 35 63 3.890
1975 25 35 5 63 10.108
1976 5 4 98 108 7.652
*plants/m

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]:

Date Seed density Seedling density (plants/m) Standing crop mean,
1992 (g/m)
March 1992 0-1,865 ---- 0-4
June 1992 ---- 0-1,090 ----

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]:

  Joshua tree/blackbrush blackbrush Utah juniper (J. osteosperma)/
blackbrush
1969 1970 1971 1969 1970 1971 1969 1970 1971
density (plants/0.1 m) 0.22 0.46 0.06 0.14 1.42 0.35 0 0 0.04
frequency (%) 10 14 4 9 23 10 0 0 4
height (x, cm) 7.3 3.0 4.5 2.2 3.1 5.7 0 0 3.0

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].

SITE CHARACTERISTICS:
Sixweeks grass is most common on disturbed sites and open areas. It is reported on roadsides, fields, and "waste places" in the Southeast [176,245]. It also occurs on dry stream channels [201], and is common on animal-disturbed sites such as seed harvester ant mounds and prairie dog towns [25,57]. Common locales for sixweeks grass in Michigan include dunelands, lakeshores, roadsides, and oak woodlands [224]. Sixweeks grass favors plant communities with open structure [77,86], occurring on coastal and desert sites in Baja California [240]. Since is drought tolerant and grows on xeric sites that few other grasses can tolerate, it is sometimes a xeric-site indicator [94,139]. An Arizona field experiment showed that sixweeks grass had the lowest relative water requirement (as a ratio of water used:dry-matter production) of 5 southern Arizona annuals [142]. At the end of the Great Drought (1941), Weaver and Mueller [229] found sixweeks grass in "abundance" on depleted tallgrass prairies of eastern Kansas and Nebraska.

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:

Area Elevation
Arizona <6,500 feet (2,000 m) [113,239]
     Sawtooth Range 1,600-1,750 feet (490-530 m) [138]
California <7,000 feet (2,000 m) [97,156]
Colorado 3,500-8,500 feet (1,100-2,600 m) feet [88]
Nevada 1,800-6,600 feet (550-2,000 m) [112]
New Mexico 5,000-6,500 feet (1,500-2,000 m) [137]
Utah 2,490-5,510 feet (760-2,290 m) [232]
Baja California low elevations [240]

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].

SUCCESSIONAL STATUS:
Sixweeks grass is a ruderal species favoring open sites and bare ground [93,112,113,145,165]. It is shade intolerant [139], and its presence is documented on disturbed sites across its distribution [197,198,221]. It was present in black spruce (Picea mariana) plantation plots in New Brunswick, but did not occur in undisturbed, naturally regenerated forest plots [221]. Slender sixweeks grass is noted in early duneland succession on Fire Island, New York [59] and on innundated streambanks of Colorado [74]. Beetle [16] lists it as an "invader" on shortgrass prairie near Laramie, Wyoming. A vegetation survey 3 weeks after the May 1980 eruption of Mt. St. Helens in Washington showed no effect of ashfall on sixweeks grass, which was setting seed even in areas where compacted ash was >1 inch (3 cm) thick [135].

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].

SEASONAL DEVELOPMENT:
Sixweeks grass is a cool-season species that establishes in late fall or early spring, flowers in spring or early summer, then matures, sets seed, and dies by late spring or early summer, depending on geographic location [39,39,106,165,208,230]. It is a winter or spring annual in the deserts of the Southwest [33,55,109,138]. Growing season generally extends for several months in wet years, but may be 6 weeks or less in dry years [155,208]. Phenology of sixweeks grass across its distributional range is shown below.

Area Germination and seedling emergence Flowers Seeds disperse Plants die
Arizona ---- spring [113] ---- ----
 Sonoran Desert ---- April-June [155] ---- ----
California ---- April-June [156] ---- ----
     Mojave Desert fall-winter [15,109] April-June [155] ---- ----
Carolinas ---- April-June [176] ---- ----
Colorado Sept.-Nov. [104,157] mid-May-early June [56] early July-Aug.  [104,129,157] early fall [104,157]
Nebraska ---- June-July [203] ---- ----
Nevada ---- April-June [112] ---- ----
New Mexico ---- May-July [137] ---- ----
     Chihuahuan Desert Feb.-March March April [119] ----
North Dakota Sept.-Dec.; May-Aug. [106] ---- ---- ----
Texas ---- April-May [57] ---- ----
Cross Timbers January [61] ---- ---- ----
Great Plains March [193] April-June [80] ---- ----
Intermountain Region ---- May-June [49] ---- ----
Ozarks ---- ---- June [48] ----
Baja California ---- April-June [240] ---- ----

On blue grama-buffalo grass sites in Colorado, sixweeks grass cover declined as plants matured and died. It showed 1.4% and 5.6% cover in May (on black-tailed prairie dog town and undisturbed sites, respectively), 0.3% and 1.7% cover in June, and 0.1% cover (both sites) in August and September [25].

FIRE ECOLOGY

SPECIES: Vulpia octoflora
FIRE ECOLOGY OR ADAPTATIONS:
Fire adaptations: Sixweeks grass establishes from soil-stored seed after fire, typically as a component of early postfire vegetation [1,54,66,98,109,132,173,226]. No other means of postfire regeneration are documented, although establishment from off-site wind-, animal-, or water-dispersal seed is possible.

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/m, 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/m 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:

Fire frequency (#/year) Fine fuel frequency (%) Fine fuel quantity (lb/acre) Elevation (feet) Burn age when sampled
0.6 50 705 3,570 6

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. For further information, see the FEIS review of the dominant species listed below.

Community or Ecosystem Dominant Species Fire Return Interval Range (years)
maple-beech-birch Acer-Fagus-Betula spp. >1,000 [225]
California chaparral Adenostoma and/or Arctostaphylos spp. <35 to <100 [169]
bluestem prairie Andropogon gerardii var. gerardii-Schizachyrium scoparium <10 [123,169]
Nebraska sandhills prairie A. gerardii var. paucipilus-Schizachyrium scoparium <10 [169]
silver sagebrush steppe Artemisia cana 5-45 [96,175,242]
sagebrush steppe A. tridentata/Pseudoroegneria spicata 20-70 [169]
basin big sagebrush A. tridentata var. tridentata 12-43 [192]
mountain big sagebrush A. tridentata var. vaseyana 15-40 [8,38,148]
Wyoming big sagebrush A. tridentata var. wyomingensis 10-70 (x=40) [222,249]
coastal sagebrush A. californica <35 to <100
saltbush-greasewood Atriplex confertifolia-Sarcobatus vermiculatus <35 to <100
desert grasslands Bouteloua eriopoda and/or Pleuraphis mutica 5-100 [169]
plains grasslands Bouteloua spp. <35 [169,242]
blue grama-needle-and-thread grass-western wheatgrass B. gracilis-Hesperostipa comata-Pascopyrum smithii <35 [169,185,242]
blue grama-buffalo grass B. gracilis-Buchloe dactyloides <35 [169,242]
grama-galleta steppe B. gracilis-Pleuraphis jamesii <35 to <100
blue grama-tobosa prairie B. gracilis-Pleuraphis mutica <35 to <100 [169]
cheatgrass Bromus tectorum <10 [173,236]
California montane chaparral Ceanothus and/or Arctostaphylos spp. 50-100
paloverde-cactus shrub Parkinsonia microphylla/Opuntia spp. <35 to <100 [169]
curlleaf mountain-mahogany* Cercocarpus ledifolius 13-1,000 [11,195]
mountain-mahogany-Gambel oak scrub C. ledifolius-Quercus gambelii <35 to <100
blackbrush Coleogyne ramosissima <35 to <100 [169]
California steppe Festuca-Danthonia spp. <35 [169,205]
black ash Fraxinus nigra <35 to 200 [225]
juniper-oak savanna Juniperus ashei-Quercus virginiana <35
Ashe juniper J. ashei <35
western juniper J. occidentalis 20-70
Rocky Mountain juniper J. scopulorum <35 [169]
cedar glades J. virginiana 3-22 [85,169]
creosotebush Larrea tridentata <35 to <100
Ceniza shrub L. tridentata-Leucophyllum frutescens-Prosopis glandulosa <35 [169]
yellow-poplar Liriodendron tulipifera <35 [225]
wheatgrass plains grasslands Pascopyrum smithii <5-47+ [169,175,242]
northeastern spruce-fir Picea-Abies spp. 35-200 [60]
southeastern spruce-fir Picea-Abies spp. 35 to >200 [225]
black spruce P. mariana 35-200 [60]
pine-cypress forest Pinus-Cupressus spp. <35 to 200 [8]
pinyon-juniper Pinus-Juniperus spp. <35 [169]
Mexican pinyon P. cembroides 20-70 [152,211]
shortleaf pine P. echinata 2-15
shortleaf pine-oak P. echinata-Quercus spp. <10 [225]
Colorado pinyon P. edulis 10-400+ [72,79,114,169]
slash pine P. elliottii 3-8
slash pine-hardwood P. elliottii-variable <35 [225]
longleaf-slash pine P. palustris-P. elliottii 1-4 [158,225]
longleaf pine-scrub oak P. palustris-Quercus spp. 6-10 [225]
Pacific ponderosa pine* P. ponderosa var. ponderosa 1-47 [8]
interior ponderosa pine* P. ponderosa var. scopulorum 2-30 [8,13,126]
pitch pine P. rigida 6-25 [37,95]
loblolly pine P. taeda 3-8
loblolly-shortleaf pine P. taeda-P. echinata 10 to <35 [225]
galleta-threeawn shrubsteppe Pleuraphis jamesii-Aristida purpurea <35 to <100
eastern cottonwood Populus deltoides <35 to 200 [169]
aspen-birch P. tremuloides-Betula papyrifera 35-200 [60,225]
quaking aspen (west of the Great Plains) P. tremuloides 7-120 [8,83,146]
mesquite Prosopis glandulosa <35 to <100 [144,169]
mesquite-buffalo grass P. glandulosa-Buchloe dactyloides <35
Texas savanna P. glandulosa var. glandulosa <10 [169]
mountain grasslands Pseudoroegneria spicata 3-40 (x=10) [7,8]
Rocky Mountain Douglas-fir* Pseudotsuga menziesii var. glauca 25-100 [8,8,10]
coastal Douglas-fir* P. menziesii var. menziesii 40-240 [8,154,184]
California mixed evergreen P. menziesii var. menziesii-Lithocarpus densiflorus-Arbutus menziesii <35
California oakwoods Quercus spp. <35 [8]
oak-hickory Quercus-Carya spp. <35 [225]
oak-juniper woodland (Southwest) Quercus-Juniperus spp. <35 to <200 [169]
northeastern oak-pine Quercus-Pinus spp. 10 to <35 [225]
oak-gum-cypress Quercus-Nyssa-spp.-Taxodium distichum 35 to >200 [158]
southeastern oak-pine Quercus-Pinus spp. <10 [225]
coast live oak Q. agrifolia 2-75 [82]
canyon live oak Q. chrysolepis <35 to 200
blue oak-foothills pine Q. douglasii-P. sabiniana <35
Oregon white oak Q. garryana <35 [8]
bear oak Q. ilicifolia <35 [225]
California black oak Q. kelloggii 5-30 [169]
bur oak Q. macrocarpa <10 [225]
oak savanna Q. macrocarpa/Andropogon gerardii-Schizachyrium scoparium 2-14 [169,225]
shinnery Q. mohriana <35 [169]
chestnut oak Q. prinus 3-8
post oak-blackjack oak Q. stellata-Q. marilandica <10
black oak Q. velutina <35 [225]
interior live oak Q. wislizenii <35 [8]
blackland prairie Schizachyrium scoparium-Nassella leucotricha <10 [225]
little bluestem-grama prairie S. scoparium-Bouteloua spp. <35 [169]
elm-ash-cottonwood Ulmus-Fraxinus-Populus spp. <35 to 200 [60,225]
*fire return interval varies widely; trends in variation are noted in the species review

POSTFIRE REGENERATION STRATEGY [204]:
Ground residual colonizer (on-site, initial community)
Initial off-site colonizer (off-site, initial community)
Secondary colonizer (on-site or off-site seed sources)

FIRE EFFECTS

SPECIES: Vulpia octoflora
IMMEDIATE FIRE EFFECT ON PLANT:
Fire occurring when plants are mature kills sixweeks grass [53]. If fire occurs early in the growing season, annuals such as sixweeks grass may sprout from the root crown [104]. Fire in any season may reduce the seed bank [53]. Seed in litter or lying on the soil surface is most vulnerable to fire kill [248].

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
It is unclear whether heat breaks seed dormancy and enhances sixweeks grass germination. In an outdoor pot study, Odion [163] found a significant positive effect of heat on seedling emergence. At Vanderburg Air Force Base, California, pre- and postfire soil samples were collected from a maritime chamise community that had not burned for at least 75 years (at Site 1) or 50 years (at Site 2). Prefire soil cores were collected in fall, and both study sites were burned under prescription soon after. Postfire soil samples were also collected [163]. Fire intensity was relatively high (see [164] for fire data). Laboratory treatments for prefire soil samples were a control, heat (100 C for 7 min.), and heat with chamise charate. Seedling emergence was counted for pre- and postfire soils samples and in the field on prescribed burn plots. Mean number of germinants/m emerging from soil samples and on burn plots was [163]:

Prefire soil samples
   Control 4.4
   Heat 74.8*
   Heat and charate Site 1 Site 2
74.8 492.8
Postfire soil samples 8.8 101.2
Field emergence on burn 0.1 0.5
*significant at p>0.05

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 cm 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/m); reaction intensity of the fire was modeled at 120 kcal/sec/m. 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 density was 1 plant/250 cm in unburned soil and 0.85 plant/250 cm in burned soil [53].

PLANT RESPONSE TO FIRE:
Sixweeks grass establishes from soil-stored seed in early postfire plant communities [1,48,54,190,238]. It often reaches greatest coverage and frequency in the 1st few years after fire, then declines [54,210]. To date (2006), there are too few fire studies to access sixweeks grass's "typical" response to fire. Some studies show an increase after fire [3,41,52,54,102,172,210]; some show little overall change in sixweeks grass abundance before and after fire [1,42,130,209]; and a minority show a decrease [28,153]. Because it is an annual with its establishment rate closely tied to growing-season precipitation, short-term fire studies that do not provide comparative pre- and postfire precipitation data may provide little insight into sixweeks grass response to fire. Postfire drought will probably impact sixweeks grass's ability to compete for water against deep-rooted perennials. Studies characterizing sixweeks grass response to fire in the short term may show trends, however. As an annual, sixweeks grass's response is most likely linked to fire effect on the seed bank, with severe surface and ground fires most restricting its ability to recover from fire.

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]:

  Postfire year 2 Postfire year 4 Postfire year 12
Burned sites Cover (%) 2 1 trace
Frequency (%) 85 62 2
Unburned sites Cover (%) 0 1 1
Frequency (%) 0 28 30

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]:

Microhabitat Variable No Fire 1980 Wildfire 1981 Prescribed Burn
1981 1982 1981 1982 1981 1982
open/shrub density (plants/m) 121 123 20 34 88 153
  biomass (g/m) 0.5 1.5* 1.2 0.7* 0.6 9.7*
shade density 84 8 40 35 99 23
  biomass 0.3 0.3 0.5 1.0 0.3 1.5
*significant change across treatments (p<0.05)

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]:

  Burned site Unburned site
Relative seedling density (%) 0.651 0.239
Cover (%) 0.5 0.16

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/m) of sixweeks grass on burned and adjacent unburned plots was [153]:

  June July August October
burned 1.2 2.7 1.9 1.0
unburned 4.7 3.3 2.3 7.4

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/dms) was [251]:

Stand age
(years since fire)
10 17 36 62 86
Burned flats 0.50.7 0.0 0.30.5 2.21.5 0.50.7
Unburned flats 0.20.4 1.21.9 0.20.4 4.04.5 0.40.6

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
Fire season: Few studies have documented seasonal effects of fire to sixweeks grass. However, a study in mixed chamise-mission manzanita-bushrue (Xyloccus bicolor-Cneoridium dumosum) in San Diego County, California, found no significant differences in mean herbaceous cover on spring-burned (10 May 1993) and fall-burn (28-29 October 1993) sites, but overall herbaceous diversity was greater on fall burns. The spring following burning, sixweeks grass was infrequent on spring burns, but at 0.5% frequency and 0.08% cover, it was still 1 of the 10 most common herbs on fall burns [20]. Owensby and Smith [167] reported that in tallgrass prairie, spring burning is most damaging to sixweeks grass because it is actively growing at that time. Prescribed spring fire on a Kansas prairie reduced annual grasses, including sixweeks grass, little barley (Hordeum pusillum), and bromes (Bromus spp.), from 1.5% to 0.3% cover [167].

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:

FIRE MANAGEMENT CONSIDERATIONS:
Prescribed and wildfires are most likely to benefit or have no effect on sixweeks grass coverage and frequency. Typically a minor grass favored by disturbance, sixweeks grass is well adapted to establish in most early postfire environments. Regarding fire season, spring prescribed burning may reduce sixweeks grass coverage the most, at least in the short term. Widespread fire that burns into the soil, killing seed, can reduce sixweeks grass's ability to recover from fire. Nonnative annual grasses, which outcompete sixweeks grass (see Invasives), can also greatly impact postfire coverage and frequency of sixweeks grass. A postfire reduction in sixweeks grass is likely if annual bromes, schismus, and/or lovegrasses are present before fire or represented in the seed bank.

MANAGEMENT CONSIDERATIONS

SPECIES: Vulpia octoflora
IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Overall forage value of sixweeks grass is slight due to its sparse growth [112]; however, sixweeks grass provides seasonal forage when green [208]. Productivity of sixweeks grass varies greatly, depending upon precipitation during the establishment period (see Fuels for sample production rates). In the Cross Timbers region of Oklahoma and Texas, Dyksterhuis [61] found light cattle use of sixweeks grass in March, heavy use in April, and light use in May; after that, it was not utilized. Cullen and others [50] noted that cattle on the Gudmundsen Sandhills Laboratory near Whitman, Nebraska, did not graze sixweeks grass in summer.

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]:

  Preflower Dough stage
Date collected 23 March 26 April
Protein (%) 18.9 (high)* 9.36 (fair)
Ether extract (%) 3.01 2.05
Crude fiber (%) 21.86 27.34
N-free extract (%) 41.32 47.33
Water (%) 7.04 7.21
Ash (%) 8.48 6.71
K2O (%) 2.63 1.22
CaO (%) 0.59 (good) 0.62 (good)
 MgO (%) 0.32 0.22
P2O5 (%) 0.79 (good) 0.43 (fair)
*(overall ratings for livestock)

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:

  Cyanobaterial-
Collema
spp. sands
Blow-out sands Significance (p)
N (mg/g) 22.5 19.5 <0.05
P (mg/g) 2.5 1.4 <0.0001
K (mg/g) 18.8 16.4 <0.05
Ca (mg/g) 6.5 5.2 <0.0001
Mg (g/g) 1.5 1.3 <0.01
Cu (g/g) 11.0 10.4 NS
Fe (g/g) 300.3 149.4 <0.01
Ma (g/g) 60.3 74.0 NS
Na (g/g) 61.5 59.8 NS
Zn (g/g) 43.0 33.0 NS

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].

VALUE FOR REHABILITATION OF DISTURBED SITES:
Sixweeks grass is an important component of several endangered communities including California prairies, desert grasslands, and shale barrens [141]. It may regenerate naturally on some sites. As of 2006, commercial seed was apparently unavailable [216]. On shale barrens of southern Illinois, now reduced to 5.9 acres (2.4 ha) as a result of fire exclusion and oak (Quercus spp.) encroachment, sixweeks grass was represented in the soil seed bank on restored (prescribed burn) sites, but was not found in soil samples from unrestored (unburned) sites [139]. It was also present on an unreclaimed lignite surface mine in east-central Texas [200]. In Colorado, natural sixweeks grass regeneration occurred on both ends of dirt road succession: on new roads created by military vehicles and on long-abandoned roads in old fields [197,198].

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].

OTHER USES:
Native Americans in the Southwest traditionally ate roasted sixweeks grass seeds [150].

OTHER MANAGEMENT CONSIDERATIONS:
Invasives: Red brome, cheatgrass and other exotic annuals may outcompete and displace sixweeks grass and other native annuals on some sites [15,130,159,189]. Previously common in the Mojave Desert, sixweeks grass became uncommon after invasion of the nonnative annual grasses red brome, Arabian schismus (Schismus arabicus), and common Mediterranean grass [30]. In a big sagebrush steppe community in the Powder River Basin of Wyoming, Allen and Knight [5] found that sixweeks grass had shallow roots compared to associated annual herbs, so it was unable to extract water from deep soil layers. Nonnative tumble mustard (Sisymbrium altissimum), Russian-thistle, and red brome, and native common pepperweed (Lepidium densiflorum) showed greatest maximum root depths; sixweeks grass showed least maximum root depth of 5 nonnative and 2 native herbs [5]. DeFalco and others [55] suggest that sixweeks grass is best able to compete with red brome after extended drought, because red brome's lack of seed dormancy tends to deplete its soil seed bank (see Seed banking). Desert sites where sixweeks grass may compete with or even outcompete red brome include hummocks and nitrogen-poor soils [33].

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].

Control: Applications of either atrazine or simazine controlled sixweeks grass on the Central Plains Experimental Range. Houston and Hyder [101] recommend either spring or fall application. Since sixweeks grass establishment is largely dependent upon weather conditions, they also recommend spraying only when needed, and caution against routine annual control [101].

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