Disturbance level and fire severity:
Fire disturbance generally favors western tansymustard; however, severe fire may
reduce western tansymustard's seed bank. On singleleaf pinyon-Utah juniper of west-central Utah,
western tansymustard was about equally frequent on burned plots and on plots
that were burned and chained [131]. After wildfire in a Utah juniper community
in Nevada, western tansymustard was present at postfire year 1, with abundance
peaking at postfire year 4. Sites where severe fire killed the junipers and
reduced the understory to ash were devoid of vegetation until postfire year 4,
when western tansymustard and cheatgrass established [153]. Absence of
vegetation prior to postfire year 4 on severely burned sites suggests that the seedbank was destroyed,
and western tansymustard and cheatgrass seeds were transported to the site after fire.
Fall prescribed burning in a basin big sagebrush community in east-central Oregon had no significant
effect on western tansymustard frequency in postfire year 1 or 2 [148]. See the Research Project Summary of this work for more information on fire effects
on western tansymustard and 60 additional forb, grass, and woody plant species.
The Research Project Summary
Nonnative annual grass fuels and fire in California's Mojave Desert
also provides information on prescribed fire and postfire response
of western tansymustard and other plant community species.
The following description of western tansymustard provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available (e.g. [27,49,56,75,110,141,162,172,175]).
Western tansymustard is a native winter annual. Mature plants are 2.3 to 3.3 feet (0.7-1 m) in height. Stems are coarse, glandular, and sparsely to densely pubescent, depending on the subspecies. They are 1 to several at the base, branching towards the top. Leaves are basal or cauline. Basal leaves are biwestern and may be as long as 4 inches (10 cm), becoming western and smaller up the stem. The inflorescence is a glandular raceme of perfect flowers that spreads out 45o to 90o from the stem. The fruit is a dehiscent, 2-capsuled silique 4-20 mm in length, bearing 1 to 20 small seeds per capsule. Seeds are 0.5 to 1 mm long; their average mass is 0.12 mg [40,49,56,75,91,141,162,175]. Tansymustards (Descurainia spp.) have a short taproot [125,186].
Stand structure: Plant communities where tansymustard is important have an open structure, with sparse to no overstory. For example, structure of a Colorado pinyon-Utah juniper burn in northeastern Utah consisted of the skeletons of burned conifers and a ground cover of western tansymustard and other annual forbs [51]. Excepting communities dominated by cheatgrass (e.g., [83,159,190]), descriptions of open-canopy stands dominated by annuals are sparse in the literature. Further studies of stand structure (including year-to-year fluctuations due to climate changes) in such communities would increase understanding of fuel loading, fire behavior, and current fire regimes in communities with a large component of western tansymustard and other annuals.
Western tansymustard is native to North America. It is distributed from Yukon, western Nunavut, and southern Quebec to southern California, Texas, central Florida, and Sinaloa, Mexico [75,89,181,184]. It does not occur in Alabama, is extirpated from New Hampshire and West Virginia, and is rare in Vermont and Ohio [89]. The subspecies' distributions show considerable overlap, with no apparent geological or ecological correlation to occurrence [166,175]. Plants database provides a distributional map of western tansymustard and its subspecies.
The following biogeographic classifications demonstrate western tansymustard could potentially be found. These lists are speculative and may not be accurately restrictive or complete.
Fire adaptations: Western tansymustard establishes from soil-stored seed after fire [51,186]. Animal, wind, or machinery transport from off-site may provide additional, minor sources of seed [189] or introduce western tansymustard to burns where it was not present in the soil seed bank. Fire creates conditions favorable for western tansymustard establishment (bare soil, open canopy, reduced growth interference) [12,139]. As a shade-intolerant, invasive species [149], western tansymustard thrives in the early postfire environment [139]. It typically has only a few years to increase its population and replenish the seed bank before postfire successional species recover and the canopy closes [36]. Western tansymustard may persist on burns that retain an open canopy [150].
FIRE REGIMES: In wet years, the dried skeletons of western tansymustard and other annuals provided the fine, flashy fuels that helped spread fire in presettlement desert ecosystems. Year-to-year fluctuations and longer-term southern oscillations resulted in variable cover of annuals and their subsequent fuel loads [164], so fire return intervals in presettlement desert ecosystems were highly variable. Historic fire return intervals in desert ecosystems ranged from 10 to 100 or more years [8,23,114,133,171,194]. Expansion of annual alien grasses has dramatically changed FIRE REGIMES and plant communities over vast areas of western rangelands by creating an environment where fires are easily ignited, spread rapidly, cover large areas, and occur frequently [190]. Cheatgrass, red brome, and Mediterranean grasses (Schismus spp.) provide large amounts of persistent flammable fuels [20,21,116]; consequently, fire may return every 10 years or sooner in desert systems infested with exotic grasses [135,180]. Western tansymustard's role in facilitating establishment of cheatgrass and other weedy species in dry environments by providing litter (and subsequently, more mesic conditions for germination and seedling growth of other species) [34,35] needs further investigation.
Historic fire return intervals ranged from 10 to 70 years in sagebrush-dominated ecosystems [8,23,114,133,171,194]. Fire was historically uncommon in salt-desert shrubland ecosystems [22,84,129]. Native annuals such as western tansymustard, lacey phacelia (Phacelia tanacetifolia), and small fescue (Vulpia microstachys) provided fuels, but frequent breaks in the continuity of fuels hindered fire spread [20,21].
The following table provides fire return intervals where western tansymustard may be an important component of the vegetation. For further information on fire regimes in these communities, see the FEIS summary on the dominant species listed below.
Community or Ecosystem Dominant Species Fire Return Interval Range (years) maple-beech-birch Acer-Fagus-Betula > 1000 [173] California chaparral Adenostoma and/or Arctostaphylos spp. 133] bluestem prairie Andropogon gerardii var. gerardii-Schizachyrium scoparium 100,133] silver sagebrush steppe Artemisia cana 5-45 [73,140,183] sagebrush steppe A. tridentata/Pseudoroegneria spicata 20-70 [133] basin big sagebrush A. tridentata var. tridentata 12-43 [148] mountain big sagebrush A. tridentata var. vaseyana 15-40 [8,23,113] Wyoming big sagebrush A. tridentata var. wyomingensis 10-70 (40**) [171,191] 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 [133] plains grasslands Bouteloua spp. 133,183] blue grama-needle-and-thread grass-western wheatgrass B. gracilis-Hesperostipa comata-Pascopyrum smithii 133,146,183] blue grama-buffalo grass B. gracilis-Buchloe dactyloides 133,183] grama-galleta steppe B. gracilis-Pleuraphis jamesii 133] cheatgrass Bromus tectorum 135,180] California montane chaparral Ceanothus and/or Arctostaphylos spp. 50-100 [133] curlleaf mountain-mahogany* Cercocarpus ledifolius 13-1000 [9,151] mountain-mahogany-Gambel oak scrub C. ledifolius-Quercus gambelii < 35 to < 100 blackbrush Coleogyne ramosissima 133] California steppe Festuca-Danthonia spp. 133,161] juniper-oak savanna Juniperus ashei-Q. virginiana < 35 western juniper J. occidentalis 20-70 Rocky Mountain juniper J. scopulorum < 35 cedar glades J. virginiana 3-7 creosotebush Larrea tridentata 133] wheatgrass plains grasslands Pascopyrum smithii 133,140,183] Great Lakes spruce-fir Picea-Abies spp. 35 to > 200 northeastern spruce-fir Picea-Abies spp. 35-200 black spruce P. mariana 35-200 [30] pine-cypress forest Pinus-Cupressus spp. 7] pinyon-juniper Pinus-Juniperus spp. 133] Mexican pinyon P. cembroides 20-70 [120,163] shortleaf pine P. echinata 2-15 shortleaf pine-oak P. echinata-Quercus spp. 173] Colorado pinyon P. edulis 10-400+ [42,55,93,133] slash pine P. elliottii 3-8 slash pine-hardwood P. elliottii-variable 173] longleaf-slash pine P. palustris-P. elliottii 1-4 [124,173] longleaf pine-scrub oak P. palustris-Quercus spp. 6-10 [173] Pacific ponderosa pine* P. ponderosa var. ponderosa 1-47 [7] interior ponderosa pine* P. ponderosa var. scopulorum 2-30 [7,10,105] red pine (Great Lakes region) P. resinosa 10-200 (10**) [30,46] red-white-jack pine* P. resinosa-P. strobus-P. banksiana 10-300 [30,69] loblolly pine P. taeda 3-8 loblolly-shortleaf pine P. taeda-P. echinata 10 to 173] galleta-threeawn shrubsteppe Pleuraphis jamesii-Aristida purpurea < 35 to < 100 eastern cottonwood Populus deltoides 133] aspen-birch P. tremuloides-Betula papyrifera 35-200 [30,173] quaking aspen (west of the Great Plains) P. tremuloides 7-120 [7,60,112] mesquite Prosopis glandulosa 111,133] mesquite-buffalo grass P. glandulosa-Buchloe dactyloides < 35 Texas savanna P. glandulosa var. glandulosa 133] mountain grasslands Pseudoroegneria spicata 3-40 (10**) California oakwoods Quercus spp. 7] oak-hickory Quercus-Carya spp. 173] oak-juniper woodland (Southwest) Quercus-Juniperus spp. 133] northeastern oak-pine Quercus-Pinus spp. 10 to 173] oak-gum-cypress Quercus-Nyssa-spp.-Taxodium distichum 35 to > 200 [124] southeastern oak-pine Q.-Pinus spp. 173] coast live oak Q. agrifolia 2-75 [57] canyon live oak Q. chrysolepis <35 to 200 blue oak-foothills pine Q. douglasii-P. sabiniana <35 Oregon white oak Q. garryana 7] bur oak Q. macrocarpa 173] oak savanna Q. macrocarpa/Andropogon gerardii-Schizachyrium scoparium 2-14 [133,173] shinnery Q. mohriana 133] interior live oak Q. wislizenii 7] cabbage palmetto-slash pine Sabal palmetto-Pinus elliottii 124,173] blackland prairie Schizachyrium scoparium-Nassella leucotricha < 10 Fayette prairie S. scoparium-Buchloe dactyloides < 10 little bluestem-grama prairie S. scoparium-Bouteloua spp. < 35 tule marshes Scirpus and/or Typha spp. < 35 [133] elm-ash-cottonwood Ulmus-Fraxinus-Populus spp. < 35 to 200 [30,173] *fire return interval varies widely; trends in variation are noted in the species summaryInteractions with flixweed tansymustard: Interactive effects of native and exotic annuals, fire, and climate are poorly understood, and research is needed in this area. Although western and flixweed tansymustard are taxonomically and ecologically similar, little is known of possible interference of native western tansymustard by exotic flixweed tansymustard in postfire environments. Limited studies are inconclusive. Floyd-Hanna and others [43,44] noted the presence of both species after the 1996 "Chapin 5 Fire" at Mesa Verde National Park. Flixweed tansymustard was more frequent on study plots than western tansymustard at postfire year 1. Flixweed tansymustard was not observed on study plots at postfire year 2, while western tansymustard increased. Tansymustard frequencies (%) on burn sites were as follows [44]:
1997
Gambel oak/Utah serviceberry* mountain shrubland 3 16 Gambel oak-Colorado pinyon-Utah juniper 1 12 Colorado pinyon-Utah juniper 2 101998
Gambel oak/Utah serviceberry mountain shrubland 31 0 Gambel oak-Colorado pinyon-Utah juniper 24 0 Colorado pinyon-Utah juniper 27 0 *Amelanchier utahensisInteractions with cheatgrass: On dry sites where fire or other disturbance has consumed the litter, western tansymustard may pioneer, building up litter for subsequent cheatgrass establishment [34,35]. Postfire climate may greatly affect relative coverage of annual species. West [177] noted that in the Curlew Valley of northwestern Utah, western tansymustard and clasping pepperweed were common in the extremely wet period of 1983-1984, an El Niño year. The 2 forbs dominated and were the main source of fuels in shadscale (Atriplex confertifolia) and Gardner's saltbush (A. gardneri) communities; bottlebrush squirreltail (Elymus elymoides) dominated the understory of winterfat communities. Although cheatgrass typically follows tansymustard and other mustards successionally [138,139], during the El Niño event cheatgrass was "less abundant ... than it had been before or since." Cheatgrass dominated the site after El Niño passed [177].
Fire as a control agent: There are no published studies on using fire to control western tansymustard, but given its strong response to increased light and nutrients and open ground, fire alone is unlikely to provide control. If western tansymustard is already onsite in the seed bank, or as a few plants, fire is likely to increase the species' importance in the early postfire community.Moisture regime varies from moist to dry [90]. Western tansymustard is most common on dry, open, or disturbed sites [27,49,56,108,119,172]. For example, it occurs in dry washes in the Mojave Desert [17], on stabilized sand dunes and sandy flats in Wyoming [40], and on disturbed shorelines of the Great Lakes region [172].
Soils: Western tansymustard is common on sandy, gravelly, and eroded soils [27,28,40,54,75,76,81,108,141,172]. Hinds and Sauer [76] found western tansymustard was associated with eroded soils on the Arid Lands Ecology Reserve of Washington. Western tansymustard tolerates alkaline and limestone soils [27,28,50,75,119,172]. In the San Bernardino National Forest, California, western tansymustard was a constant species on singleleaf pinyon (P. monophylla)-Utah juniper sites previously mined for limestone, and on noncarbonate soils [50].
Aspect/topography: Western tansymustard occurred on both cool northern and warm southern exposures following fall prescribed fire in a pinyon-juniper community in the Green River corridor of Utah [53].
Elevational range by state is:
Arizona <7,000 feet (2,100 m) [91] California 8,200 feet (2,500 m) [75] Colorado 4,000-8,000 feet (1,200-2,400 m) [65] New Mexico 4,500-7,500 feet (1,400-2,300 m) [110] Utah 2,500-10,700 feet (750-3,250 m) [175] Wyoming 6,000 to 7,200 feet (2,000-2,200) [40]Western tansymustard is toxic to livestock, causing blindness, staggering, and loss of ability to swallow [98,130,158]. All parts of the plant contain poisonous levels of nitrate [41,158]. The seeds also contain isoallyl thiocyanates and irritant oils [41]. Continued ingestion of large quantities western tansymustard over long periods of time are required before symptoms appear. Livestock may consume western tansymustard in quantity if other forage is sparse [98].
Large grazing animal use of western tansymustard varies, but is generally heaviest for new growth. On the Jornada Experimental Range of south-central New Mexico, cattle grazed western tansymustard in March (31% of diet) and December (8% of diet) [62]. Cattle on the Arid Land Ecology Reserve of eastern Washington used it lightly in spring [168]. Desert mule deer in Arizona and New Mexico consumed western mustard lightly (1-5% of diet) to moderately (6-15% of diet) in winter and spring [99,155,156]. A review by Kufeld and others [102] notes Rocky Mountain mule deer use western tansymustard in spring, summer, and fall. Pronghorn in northeastern Colorado preferred western tansymustard in the spring, and ate it in trace amounts in summer [152].
Small mammals consume western tansymustard. Grazing rodents and lagomorphs eat western tansymustard [88,107,187]. Black-tailed jackrabbit in southern Idaho showed greatest use in August (12.4% of the total diet) and least in June (2.2% of total diet) [38]. Granivorous rodents also use western tansymustard. In sand shinnery oak-honey mesquite/threeawn (Quercus harvardii-Prosopis glandulosa/Aristida spp.) communities of New Mexico, capture/stomach analyses trials showed Ord's kangaroo rat, spotted ground squirrel, and northern grasshopper mouse ate western tansymustard seeds [13]. On the Arid Land Ecology Reserve, eastern Washington, western tansymustard seeds formed 8% of the Townsend ground squirrel diet [87].
Western tansymustard is a larval food for several species of butterfly [82] including the desert orangetip (Anthocharis cethura pima), a sensitive subspecies in Region 3 (Southwest) of the U.S. Forest Service [6].
Palatability/nutritional value: Western tansymustard is palatable and nutritious. In the Mojave desert of California, domestic sheep showed strong preference for western tansymustard early in the growing season [137]. In free-choice trials, Montana cattle preferred western tansymustard to flixweed tansymustard. From the rosette to bud stage, average protein and phosphorus content of western tansymustard in Montana was 22.4% and 0.38%, respectively [136]. Nutritional content of western tansymustard forage collected in the great Basin of Nevada and Utah was [157]:
Cal/kg 3,660 protein (%) 27.2 carbohydrates (%) 63.3 fat (%) 0.50 ash (%) 2.9 moisture (%) 6.1Cover value: No information
Western tansymustard is important in desert ecosystems. It is common in
pinyon-juniper (Pinus-Juniperus spp.) woodlands and big sagebrush (Artemisia
tridentata) and silver sagebrush (A. cana) steppes [63,128,167]. Common associates in sagebrush steppes are rabbitbrushes (Chrysothamnus
spp.), winterfat (Krascheninnikovia
lanata), Sandberg bluegrass (Poa secunda), and bluebunch wheatgrass (Pseudoroegneria
spicata) [127,128,138,139,145]. Western
tansymustard often occurs in early seral desert systems with other annuals
including cheatgrass (Bromus tectorum), sixweeks fescue (Vulpia octoflora), Russian-thistle (Salsola kali),
tumblemustard (Sisymbrium altissimum), and flixweed tansymustard (Descurainia
sophia) [128,138,145]. On burns in northeastern
Nevada, for example, western tansymustard occurred in Utah juniper (J. osteosperma), big sagebrush, and
black greasewood (Sarcobatus vermiculatus) communities. Fire had removed the overstory dominants, and the early
postfire communities were dominated by western tansymustard and exotic
cheatgrass, Russian-thistle, and clasping pepperweed (Lepidium perfoliatum)
[153]. Also in northeastern Utah, western tansymustard was dominant in an early postfire
Colorado pinyon (Pinus edulis)-Utah juniper community. Associates were
primarily other annuals including cheatgrass, false flax (Camelina microcarpa),
tumblemustard, and prairie pepperweed (L. densifolium)
[51].
Western tansymustard also occurs in mesquite (Prosopis spp.),
salt-desert shrubland, blackbrush/Indian ricegrass (Coleogyne
ramosissima/Achnatherum hymenoides), and desert grassland communities
[24,63]. In velvet mesquite (P. velutina)
woodland of southeastern Arizona, western tansymustard occurs with Wright's buckwheat (Eriogonum
wrightii) and plains lovegrass (Eragrostis intermedia) [15].
Associates on the Desert Tortoise Research Natural Area in the Mojave Desert of
southern California include creosotebush (Larrea tridentata), white
bursage (Ambrosia dumosa), and other annuals, the most common being
cutleaf filaree (Erodium cicutarium), red brome (Bromus madritensis
subsp. rubens), and Mediterranean grass
(Schismus spp.) [20,21].
Western tansymustard is not as
important in plains grasslands and communities further east as it is in desert
shrublands [79]. It is
usually listed as a "waste place" or disturbance-indicator weed in the Great Plains and
ecosystems to the east (e.g., [27,49,56,108,119,172]).
Western tansymustard seeds have a flavor similar to commercial black mustard (Brassica nigra) seeds, and are used as a spice [27]. Native Americans ate western tansymustard greens and made gruel and pinole from the ground seeds [91,143]. The Navajo used the ground seeds in baked goods [170], and the Pima made a beverage from the seeds [143].
Horticulturalists plant western tansymustard in butterfly gardens to attract orangetip, white checkered, and white cabbage butterflies [82].
Western tansymustard establishes from soil-stored seed after fire [51,53,66,178]. Animals and vehicles can transport seeds to burns [153]. Fire creates the open, disturbed ground favored for western tansymustard establishment [76,96,149]. Biological factors in the early postfire environment may also favor establishment of western tansymustard. Keeley and others [94,99] found western tansymustard and other annual forbs showed significantly (p<0.01) greater germination when treated with an infusion of either charred chamise wood or chamise foliage.
Western tansymustard is common in early postfire communities [121]. It typically occurs in the earliest, annual-dominated stage of postfire succession in pinyon-juniper and sagebrush communities [51]. In a mountain big sagebrush community in Idaho, frequency of western tansymustard was more than 100% greater (p <0.05) in fall-burned compared to unburned control plots [109]. Although fire creates an open canopy and bare mineral soil, which favors tansymustard establishment, western tansymustard is not an obligate "fire follower." Any area with bare ground, open sunlight, and a seed source is vulnerable to western tansymustard invasion [71,72]. In greenhouse trials, there was no significant difference in emergence of western tansymustard seedlings from unburned, "lightly" burned, and severely burned soils. Greenhouse emergence of western tansymustard (seedlings/m2) from soil samples collected after prescribed burning near Burns, Oregon was as follows [25]:
Control "Cool" burn "Hot" burnBasin big sagebrush site
5.5 4.5 7.3Wyoming big sagebrush site
8.8 21.3 15.3Fire may not increase postfire cover of western tansymustard [190]. In his classic study of postfire succession of western tansymustard and other annuals in big sagebrush, Piemeisel [138] wrote "the mere statement that a field has been burned is not sufficient information to foretell what the effect will be on the succeeding plant cover." For example, a 1977 June wildfire burned rough fescue (Festuca altaica)-bluebunch wheatgrass mountain grassland in Missoula, Montana. In fall 1977, spring 1978, and summer 1978, western tansymustard was present in low amounts on burned and unburned sites, with no significant differences (p <0.05) between western tansymustard cover on burned and unburned sites [4]. Piemesel [138] stated that site grazing history, postfire weather patterns, and level of postfire cheatgrass cover will affect cover and relative abundance of tansymustards and other early successional forbs.
Because it is an annual, western tansymustard population size is highly dependant upon year-to-year variations in postfire climate [123]. Following 1964 wildfires in northeastern Nevada, western tansymustard frequency fluctuated greatly for the next 3 years (41% in 1965, 6% in 1966, and 66% in 1967; pooled from 32 plots on 3 burns) [153]. Western tansymustard was found on fall-burned plots in postfire years 4 and 11 (but not other years) in a pinyon-juniper community in the Green River corridor of Utah [53].
Western tansymustard cover generally decreases with postfire succession. In Colorado pinyon-Utah juniper stands in Mesa Verde National Park, western tansymustard showed 44% frequency on a 4-year-old burn. It was not present on 30- or 90-year-old burns [33]. However, western tansymustard may remain a component of postfire vegetation as long as the site remains open.
Fuels: It is unclear how western tansymustard contributed to fuel spread in the Southwest when presettlement FIRE REGIMES were functioning. Researchers have speculated that following a wetter-than-average growing season, dead, dry native annuals provided the fuels that carried infrequent spring or summer fire [55]. In a creosotebush-white bursage community in the Mojave Desert of southern California, however, experimental fires set in August 1995 were fueled entirely by red brome, cheatgrass, and Mediterranean grasses (Schismus spp.). Precipitation the preceding winter was 200% of average, so cover of annuals was high (~36% for all annual species). Where exotic grasses dominated, mean spring (prefire) cover of annuals was approximately 58%, whereas mean spring cover where native annuals dominated was approximately 30%. (At 30%, western tansymustard cover matched overall forb cover.) Fire did not spread in areas dominated by native annuals due to low fuel loads [20]. This suggests that in desert shrublands of the Southwest, presettlement fires were probably small and patchy.
POSTFIRE REGENERATION STRATEGY [160]:
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 REGIMES: 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"
As an annual, western tansymustard reproduces solely from seed [24,26,66,76].
Breeding system: Mustards (Brassicaceae) are cross-pollinated. Selfing also occurs [74].
Pollination is insect-mediated. The sepals contain nectaries that attract insect pollinators [74].
Seed production: No information
Seed dispersal: Neither fruits nor seeds have specialized appendages for dispersal [92,95], and most western tansymustard seed falls near the parent plant [147]. Wind, water, machinery, and animals transport seed long distances [115,147]. Animals disperse tansymustard seeds when the sticky seedcoat adheres to feathers or fur [192].
Seed banking: Western tansymustard has soil-stored seed [24,26,66,76]. Soil samples from a needle-and-thread grass-blue grama (Hesperostipa comata-Bouteloua gracilis) community in Yellowstone National Park supported a mean of 13 western tansymustard germinants/m2 in the greenhouse [26] .
Germination: Western tansymustard has a temperature-dependent afterripening requirement that lessens with time and exposure to cold temperatures [192]. It is adapted to establishing in soils that may dry quickly. The seedcoat forms a thin layer of mucilage after wetting; the sticky layer helps germinating seeds retain water [189,192]. Best germination occurs with ample soil moisture, however. Seedling establishment is closely tied to favorable moisture levels at time of germination, and western tansymustard cover fluctuates from year to year [31]. In undisturbed creosotebush-white bursage in the northern Mojave Desert of Nevada (where western tansymustard germinates in winter), spring density of western tansymustard ranged from 18 plants/m2 (the driest winter) to 212/m2 (the wettest winter) over 6 years [16].
In a study on germination of California chaparral species, heat treatments had no significant effect on germination of western tansymustard; however, germination of western tansymustard seeds was significantly (p <0.1) enhanced by treatment with an infusion of chamise (Adenostoma fasciculatum) foliage, and by application of charred, finely ground chamise stems to the seeds [96].
Seedling establishment/growth: Although western tansymustard establishes on open, dry sites, establishment may be facilitated by shrubs, especially big sagebrush, which has been shown to increase water content of surface soil through hydraulic lift [144]. In Theodore Roosevelt National Park, North Dakota, western tansymustard showed better establishment when in close proximity to big sagebrush plants compared to density of western tansymustard away from big sagebrush crowns. Silver sagebrush crowns also enhanced western tansymustard establishment, although the number of tansymustard plants was considerably less in the silver sagebrush community compared to the big sagebrush community. The average number of western tansymustard growing within a circle around sagebrush stems (using the stems as the center of the radii) varied as follows [67]:
Species 0-30.4 cm away from stem base 30.4-43 cm away 43-52.7 cm away Big sagebrush 92 plants 60 plants 23 plants Silver sagebrush 6 plants 1 plant 0 plantsGrowth is enhanced on wet, nitrogen-rich sites. On the Jornada Experimental Range of New Mexico, western tansymustard showed best growth (peak biomass=3.2 g/m2) on sites fertilized with nitrogen and given 25 mm of supplemental water/per week [61]. Nitrogen-fixing plants may increase western tansymustard cover (review by [106]).
Western tansymustard is an early seral species. It occurs on open, disturbed sites such as railroad rights-of-way [40,172], overgrazed rangelands [14,19], and early seral burns [32,33,51,53]. Western tansymustard and other annuals are common following disturbances such as fire or grazing. They become increasingly scarce as the canopy closes [3,32,33,51,104]. Western tansymustard does not form mycorrhizal associations [14,134]; thus, it can occur on sterile sites and on sites undergoing primary succession.
A common pattern of succession in disturbed sagebrush steppe begins with Russian-thistle. Western tansymustard, flixweed tansymustard, and/or tumblemustard follow successionally. Russian-thistle may in turn reinvade the mustard stands with grazing disturbance, but more commonly, the mustards are succeeded by cheatgrass [68,78,78,139,185]. In desert environments, cheatgrass usually requires litter for successful germination and establishment. The dried skeletons of western tansymustard and other annual forbs may facilitate cheatgrass establishment by providing litter [34,125]. Western tansymustard and other annual forbs achieve greater cover in early seral sagebrush communities when cheatgrass is absent [193]. For information on the interactions of western tansymustard and cheatgrass, see FEIS reviews on flixweed tansymustard and cheatgrass.
Western tansymustard and the exotic flixweed tansymustard are apparently equally invasive, and fill similar ecological niches [118]. They sometimes codominate in early seral communities [2].
Western tansymustard may persist into late succession if the canopy remains open [150]. For example, western tansymustard was common (20% frequency) in a relict, 80-year-old Colorado pinyon-Utah juniper burn in northeastern Utah. It was less common, but present (2% frequency), on another relict Colorado-pinyon-Utah juniper site that had not experienced stand-replacement fire for at least 150 years. Structure at both sites was very open, with few, scattered trees [52]. Sagebrush communities may also retain open conditions into late succession [171,188]. In the Columbia River Basin of south-central Washington, western tansymustard occurs in big sagebrush communities through all stages of succession but is most common in early successional stages. It is the most common native annual on newly disturbed sites and old fields [18]. Western tansymustard occurs in pristine, late-successional communities. For example, it occurs in undisturbed, climax mountain meadows (Idaho fescue-bearded wheatgrass (Festuca idahoensis-Elymus caninus)) in Grand Teton National Park [174].
The scientific name of western tansymustard is Descurainia pinnata (Walt.) Britt.
(Brassicaceae) [27,49,56,75,89,110,119,150,184]. Currently recognized infrataxa are:
D. p. subsp. brachycarpa (Richards.) Detl. [27,56,89,110,119], western tansymustard
D. p. subsp. filipes (Gray) Detl. [89], western tansymustard
D. p. subsp. glabra (Wooton & Standl.) Detl. [75,89,91,110], western tansymustard
D. p. subsp. halictorum (Cockerell) Detl. [27,56,75,89,91]
D. p. subsp. intermedia (Rydb.) Detl. [56,75,89], western tansymustard
D. p. subsp. menzeisii (DC.) Detl. [75,89], Menzies' tansymustard
D. p. subsp. nelsonii (Rydb.) Detl. [89], Nelson's tansymustard
D. p. subsp. ochroleuca (Woot.) Detl., western tansymustard
D. p. subsp. paysonii Detl. [89,91], Payson's tansymustard
D. p. subsp. pinnata [27,89], western tansymustard
Infrataxa designations are somewhat arbitrary [27,175]. There are
intergradation and introgression among the subspecies; consequently, the
subspecies are often
difficult to distinguish [27,75].
Descurainia pinnata is a species of flowering plant in the family Brassicaceae known by the common name western tansymustard. It is native to North America, where it is widespread and found in varied habitats. It is especially successful in deserts. It is a hardy plant which easily becomes weedy, and can spring up in disturbed, barren sites with bad soil. This is a hairy, heavily branched, mustardlike annual which is quite variable in appearance. There are several subspecies which vary from each other and individuals within a subspecies may look different depending on the climate they endure. This may be a clumping thicket or a tall, erect mustard. It generally does not exceed 70 centimeters in height. It has highly lobed or divided leaves with pointed, toothed lobes or leaflets. At the tips of the stem branches are tiny yellow flowers. The fruit is a silique one half to two centimeters long upon a threadlike pedicel. This plant reproduces only from seed. This tansymustard is toxic to grazing animals in large quantities due to nitrates and thiocyanates; however, it is nutritious in smaller amounts. The flowers are attractive to butterflies. The seeds are said to taste somewhat like black mustard and were utilized as food by Native American peoples such as the Navajo.
Descurainia pinnata is a species of flowering plant in the family Brassicaceae known by the common name western tansymustard. It is native to North America, where it is widespread and found in varied habitats. It is especially successful in deserts. It is a hardy plant which easily becomes weedy, and can spring up in disturbed, barren sites with bad soil. This is a hairy, heavily branched, mustardlike annual which is quite variable in appearance. There are several subspecies which vary from each other and individuals within a subspecies may look different depending on the climate they endure. This may be a clumping thicket or a tall, erect mustard. It generally does not exceed 70 centimeters in height. It has highly lobed or divided leaves with pointed, toothed lobes or leaflets. At the tips of the stem branches are tiny yellow flowers. The fruit is a silique one half to two centimeters long upon a threadlike pedicel. This plant reproduces only from seed. This tansymustard is toxic to grazing animals in large quantities due to nitrates and thiocyanates; however, it is nutritious in smaller amounts. The flowers are attractive to butterflies. The seeds are said to taste somewhat like black mustard and were utilized as food by Native American peoples such as the Navajo.
