Besides the similarity of stature, staminate inflorescences, and flowers, the plants herein considered within Zuckia occupy similar, relatively high-saline, gypsiferous, and seleniferous substrates. The major difference between the lone taxon, Z. arizonica, on which the genus initially rested, and G. brandegeei sensu lato, is to be found in the fruiting bracteoles, which in Z. arizonica are mainly four-ridged and narrowly two-winged, and accommodate the mostly horizontal arrangement of the achenes.
Inclusion of Grayia brandegeei, with its laterally flattened (rarely three- or four-winged) fruiting bracteoles and vertical achenes, within Zuckia, with its architecturally differing bracteoles, requires explanation. At first glance, the six-ribbed bracteoles around a horizontal achene appear to be both distinctive and diagnostic. The bracteoles, however, merely accommodate the shape of the achene, and what is apparently very distinctive is only a structural modification. Examination of the fruiting bracteoles of all taxa previously included within Grayia brandegeei demonstrates existence of a rather wide array of bracteole morphology. Bracteoles are typically laterally compressed and samaralike. However, even on the same individual plant, there occur three-winged bracteoles, and in some bracteoles there are evident veins on one or both surfaces. The veins appear in the same position as the lateral ribs on the transversely flattened bracteoles of the arizonica phase of the species. There is also considerable variation in the morphology of the transversely flattened bracteoles, ranging from merely oval with a hint of the wings, the lateral ribs lacking altogether, to very definitely winged and with two or more lateral ribs. The overall similarity of the plants, their growth habit, and their preference for fine-textured, saline, often seleniferous substrates rather narrowly confined within the Colorado Plateau indicates a rather close relationship best reflected in their alliance.
In spite of the similarity of substrate inhabited by the three taxa, their geographic ranges are mainly discrete. This is due at least partially to the autecological differences in the habitats they occupy, and to actual geographical separation, even though apparently sympatric when mapped.
This description provides characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification are available (e.g., [5,10,40]).
Aboveground description: Siltbush is an erect, low-growing shrub. It is often described as nearly herbaceous or suffrutescent, since stems are produced annually from a persistent, gnarled woody base. Stems are slender, densely pubescent, without thorns, and may reach 2.6 feet (0.8 m) tall [3,5,6,8,28,40]. Leaves are alternate, entire to lobed, and measure 0.3 to 3 inches (0.8-8 cm) long by 0.4 to 1.8 inches (1 to 4.5 cm) wide [5,6,40]. Siltbush produces both male and female flowers, but production is temporally separate. Female flowers occur individually or in short spikes, and male flowers are often clustered in groups of 2 to 5 in spikes [3,10,40]. Siltbush fruits are one-seeded utricles that are flattened or 6-keeled and may be winged and at maturity [16,40]. The bracts that enclose siltbush seeds have been described as papery [19] or extremely hard [3]. Siltbush seeds are small. On average, 491 cleaned seeds are required to make up a gram of weight [1], and seeds are often just 2 mm broad [3].
Belowground description: In the available literature (2008), descriptions of belowground siltbush growth were generally lacking. However, unpublished data reported by Shaw and others [31] suggested that siltbush root systems can be "extensive" after a single growing season, and it is generally accepted that most chenopods (Chenopodiaceae) have deep root systems and with pulses of moisture produce ephemeral roots [18].
Siltbush varieties: Descriptions and comparisons of the siltbush varieties are provided in the following references: [30,36].
Fires are considered historically rare in salt desert shrubland habitats, which are arid, usually dominated by chenopods, and occupy saline soils [41]. However, weather events that encourage production of annual species that provide fine fuel in the interspaces between shrubs may increase burning potential. After an El Niño event in Utah and other nearby desert areas, many separate fires occurred. Fires were fueled by increased production of nonnative annuals [41]. While increased nonnative grasses were reported in some salt desert shrublands of Utah, Sanderson and Stutz [29] indicated in a 1994 report that siltbush habitats rarely supported dense cheatgrass (Bromus tectorum) growth.
Without increased fine fuel production, salt desert shrublands, and more specifically siltbush stands, are dominated by widely spaced shrubs and lack the fuel continuity needed to support fire spread [25]. Based on the LANDFIRE Rapid Assessment Vegetation Models, there is little evidence of fire in salt desert shrublands, and because of the discontinuity of fuels, any fires are typically small (tens to hundreds of acres). For the most part, researchers and experts suggest that the fire-return interval for salt desert shrublands was greater than 200 years and as high as 1,000 years [11,12,13].
The Fire Regime Table summarizes characteristics of FIRE REGIMES for vegetation communities in which siltbush may occur. Follow the links in the table to documents that provide more detailed information on these FIRE REGIMES. Find further 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".
Throughout its range, siltbush often occurs on steep dry hills or slopes [8,35,40]. The extensively studied siltbush stand near Sterling, Utah, occupied slopes of 52% [24].
Soils: Siltbush habitats are rarely described without mention of soils. Typically siltbush stands occur on saline or seliniferous soils [40]. Soil textures in siltbush habitats can be fine-textured heavy clays, clay loams, or silts [10,35,36,40]. Pendleton and Meyer [23] indicated that siltbush was most common on heavy clay soils from decomposed shale with a "high shrink-swell capacity". Collotzi [3] found siltbush most often where the pH ranged from 7.4 to 7.7 on silty clay loam soils of the Chipeta formation. Siltbush populations near Sterling, Utah, occurred on slopes with little soil development [24].
Climate: The restricted distribution of siltbush suggests a fairly narrow climatic tolerance. Siltbush occurrence on salt deserts implies outstanding drought tolerance, since it maintains physiological processes in high-salt soils [35]. The climate in siltbush habitats has been described for the Orange Cliffs area of Utah, which experiences weather typical of the Colorado Plateau and represents nearly the center of the siltbush range. Nearby meteorological stations report precipitation averages of 5.2 to 11.6 inches (132-295 mm), maximum temperatures of 63.4 to 69.4 °F (17.4-20.8 °C), and minimum temperatures of 36.7 to 41.8 °F (2.6-5.4 °C) [32]. Average winter temperatures reported from siltbush habitats in Baggs, Wyoming, were much lower than minimums reported for Orange Cliffs (see Germination).
Weather can affect siltbush flowering. A series of rainstorms in early June delayed flowering, especially of male flowers, in the Sterling, Utah, population [22].
Elevation:
Elevational ranges reported for siltbush by state State Elevation range (feet) Arizona ~5,000 [10] Colorado 5,000-6,500 [8] Utah 4,200-8,010 [40]Livestock and game animals browse siltbush, especially in the spring [17]. Thrips and Lepidopteran larvae "heavily" utilize siltbush seeds [24], and seeds that persist on the plant through the winter are often removed by small mammals and bird seeds [16]. Mice and other small rodents consume siltbush seedlings. Seedling protection may be necessary for revegetation on some sites [17].
Palatability/nutritional value: Palatability of mature plants has been rated as "moderate" (Stutz, personal communication cited in [30]).
Siltbush primarily regenerates sexually through seed production and seedling establishment, but "a limited amount of vegetative spread also occurs" [21].
Pollination and breeding system: Siltbush flowers are wind pollinated, and plants are monoecious [24]. Although many thought that siltbush was dioecious [6,18,40], an in-depth breeding system study of a population near Sterling, Utah, determined that siltbush is monoecious. However, male and female flowers on the same plant are almost completely separated in time, so plants "functionally approach dioecy" [22]. Siltbush shrubs are either protogynous or protandrous. Erroneous reports of siltbush as dioecious were likely the result of insufficient specimen collection. Nearly all herbarium specimens were collected before female flower development or after fruit was produced and male flowers had died [24].
Temporal separation of male and female flowers indicates that cross pollination predominates. In artificial fertilization experiments, significantly fewer self-pollinated than cross-pollinated flowers produced viable fruits (P=0.0006) [22,24].
Seed production: Siltbush seed production can be sporadic, and typically protogynous plants produce more viable seed than protandrous plants [19,24]. McArthur and Monsen [16] reported that siltbush seed viability is generally low, which they attributed partly to harsh, arid growing conditions. Predation by thrips, Lepidopteran larvae [24], small mammals, and birds [16] can also reduce siltbush seed production.
In a siltbush population near Sterling, Utah, researchers found that breeding system (protandrous vs. protogynous), site conditions, and precipitation affected flowering and seed production. Protandrous plants produced significantly fewer fruits/stalk (P=0.0088) and viable seeds/stalk (P=0.05) than protogynous plants. Delayed fruit production on protandrous plants likely contributed to poor seed development. Protandrous fruits matured several weeks later than those of protogynous plants, when moisture stress was likely greater [21,24]. The importance of moisture availability to seed production was determined when multiple sites and years were compared. Protogynous plants located below a seep produced nearly double the amount of fruits and seeds/plant than those on a site that did not receive additional moisture. Both protogynous and protandrous plants produced double the number of seeds in a year when winter and early spring precipitation was 28% more than the previous year [21].
Seed dispersal: Wind and gravity are the primary siltbush seed dispersal agents [30]. Dispersal is considered slow, and fruits may remain on the plant through the winter [31]. Although birds and mammals were identified as seed predators (see Seed production), they were not noted in the available literature as important dispersers.
Seed banking: It seems unlikely that siltbush seed persists in the soil seed bank. In laboratory experiments, nearly all siltbush seed collected in Utah and Wyoming germinated after relatively short periods of cold exposure (see Germination), suggesting that persistence in the soil is unlikely unless some dormancy mechanism failed to materialize in the experiments [19]. Collotzi [3] reported that his collections of siltbush seeds were enclosed in "extremely hardened" bracts that required hot-water soaking for seed release. While extremely hard bracts were not described elsewhere in the available literature (2008), bracts have been linked to the dormancy of siltbush seeds (additional information provided in Seed dormancy).
Siltbush seeds survived less than 15 years of storage in an open warehouse in Escalante, Utah. Over a 25-year period in the warehouse, temperatures ranged from -21.8 to 101 °F (-29.9 to 38.3 °C). After the first 4 years of storage, siltbush germination ranged from 86% to 92%, but germination decreased significantly in subsequent years (P<0.05). After 5 years of storage, germination was 57%; after 7 years, 13%. No siltbush seeds germinated after 15 or more years of storage [33].
Germination: Prevailing climates affect siltbush seed germination requirements, and bracts that enclose siltbush seeds function in seed dormancy. After germination studies on seed collected from many sites, researchers concluded there is "strong selection pressure for adaptive germination-timing strategies in response to climate". However, regardless of temperature, seeds without bracts germinated better than seeds enclosed in bracts (also see Seed dormancy). Generally, germination was best at 59 °F (15 °C) and 86 °F (30 °C). Seeds from Antelope Valley, Utah, where January temperatures (31 °F (-0.6 °C)) were the warmest of all collection sites, were least sensitive to germination temperature. Seeds collected from Baggs, Wyoming, with the lowest average January temperature (17 °F (-8.3 °C)), germinated best at 86 °F (30 °C). Eight weeks of cold exposure were sufficient to encourage germination of seeds from sites where average January temperatures ranged from 25 to 28 °F (-3.9 to -2.2 °C). Researchers concluded that seeds from warm-winter sites were relatively nondormant, probably germinated with fall moisture, and survived as seedlings through the relatively mild winter. Seeds from cold-winter sites may germinate at high temperatures soon after dispersal but largely remain dormant until early spring [19].
Seed dormancy typically decreased with seed age and was controlled by enclosing bracts. Fifty-three percent of 4-month-old seeds were dormant, 24% of 24-month-old seeds were dormant. Experiments on seeds with and without bracts that were kept moist with water, bract leachate, or saline water revealed that physical inhibition from intact bracts was important to seed dormancy. However, another inhibitory factor was possible, since a 16 mmhos/cm concentration of leachate suppressed germination more than saline water with similar conductivities and osmotic potentials [23].
Seedling establishment/growth: Siltbush seedlings are most common in plant interspaces [23], and although they reportedly grow quickly, predation by small mammals occurs [16]. In the field, siltbush seedlings did not occur beneath the parent plant, where bracts and litter accumulated. The researcher suggested that these accumulations may, through dissolved salts or other inhibitory compounds (see also Seed dormancy), inhibit or suppress seedling establishment [23].
Vegetative regeneration: Layering is the only type of vegetative regeneration described in the available siltbush literature (2008). When studying siltbush near Sterling, Utah, Pendleton [24] wrote that "in some cases, it was difficult to determine what constituted an individual plant, whether one large plant whose lateral branches had become buried or several separate plants." Layering was also observed on a steep clay site in Colorado, although discrete shrubs were most common (Pendleton 2008, personal communication [25]).
The scientific name of siltbush is Zuckia brandegeei (A. Gray) S.L. Welsh
& Stutz ex S.L. Welsh (Chenopodiaceae) [5,9]. It is common to see the scientific species name
spelled brandegei throughout systematic and other literature [6,20], but the correct
spelling of the last name of the first person to collect siltbush is Brandegee [2,36].
Infrataxa:
Zuckia brandegeei var. arizonica (Standl.) S.L. Welsh
Zuckia brandegeei var. brandegeei (A. Gray) S.L. Welsh & Stutz ex S.L. Welsh [6]
Zuckia brandegeei var. plummeri (Stutz & S.C. Sand.) [4,36,39]
Throughout this review, siltbush varieties are identified using scientific names.
Hydbridization:
Drobnick and Plummer (1966, cited in [1]) reported that siltbush and shadscale (Atriplex confertifolia)
hybrids occurred naturally, and in artificial pollination experiments, Blauer and others [1]
found that viable seed was produced when fourwing saltbush (A. canescens)
received siltbush pollen.
