Diagnostic Characteristics
ASTRAGALUS is one of the largest and most complex genera of flowering plants, with about 1,500 species primarily of the Northern Hemisphere (Barneby 1989). ASTRAGALUS DESERETICUS has been assigned to section Argophylli, a group of 36 species of western North America, all xerophytic perennials (Barneby 1964). Among experts, however, there is some disagreement over whether it should be placed nearer to A. ARGOPHYLLUS of the subsection Argophylli (Barneby 1964, 1989) or to A. PURSHII of the closely related subsection Eriocarpi (Welsh and others 1987). A. DESERETICUS differs from the parapatric A. ARGOPHYLLUS var. MARTINII in that the pods are densely longhirsute and tomentulose (vs. strigulose to strigose, the vesture not concealing the surface of the valves), relatively small (10-12 mm vs. 13-32 mm long), and with fewer ovules (1416 vs. 25-44 in number).
Occurring sympatrically with A. DESERETICUS are the less closely related A. UTAHENSIS (foliage more closely silvery pubescent, flowers lavender, pods larger and densely shaggy villous with hairs 4-8 mm long) and A. CALYCOSUS (foliage more closely silvery-pubescent, leaflets smaller, pods with short appressed hairs).
Barneby (1989), who has seen only pressed specimens of A. DESERETICUS, erroneously observed that it "differs ... from all near relatives ... in the stiff peduncles which carry the ripe pods aloft in a ring around the central tuft of leaves." In fact, A. DESERETICUS in the wild resembles its closest congeners in having fruiting peduncles that are humistrate (spreading over the ground surface) due to the weight of the pods.
Ecology
The period of vegetative growth and reproductive activity begins after the annual snowmelt, usually by about mid-April (Swenson and others 1981). Flowering and seed set occur in May and June (Barneby 1989). The pods are deciduous at maturity, and while lying on the ground they dehisce at the apex to release the seeds. Even plants that are well established begin to lose many of their lower leaves as soil moisture becomes critical with the onset of hot, dry summer weather. As the current season's stems die back, either in response to late summer drought or cold weather in the fall, new vegetative buds are initiated at the caudex, just at soil level. Insulated by snow cover from rare episodes of severe cold, these buds generally survive the winter to emerge and elongate into new shoots the following spring.
Several widespread species of ASTRAGALUS (namely, A. CIBARIUS, A. UTAHENSIS, A. PECTINATUS, A. KENTROPHYTA var. TEGETARIUS, and A. MISER var. BLONGIFOLIUS) are highly self- incompatible and appear to be obligate outcrossers (Green and Bohart 1975; Karron 1987a, 1989; Geer and Tepedino 1993). The breeding system of ASTRAGALUS DESERETICUS presently is not known, but species with restricted ranges and few individuals are likely to be self compatible, according to evolutionary theory (Harper 1979; Karron 1989, 1991). Self- compatibility has been demonstrated for A. ROBBINSII var. JESUPII, an endemic of the Connecticut River banks in New Hampshire and Vermont (Thompson 1991); for two restricted ASTRAGALUS taxa from western Colorado, A. OSTERHOUTII and A. LINIFOLIUS (Karron 1987a, 1989); and for A. MONTII, a high elevation limestone endemic on the Wasatch Plateau of central Utah (Geer and Tepedino 1993). However, the rare A. TENNESSEENSIS and A. MONOENSIS (an eastern California endemic) are both obligate outcrossers (Baskin and others 1972, Sugden 1985). ASTRAGALUS TEGETARIOIDES and the newly described A. ANXIUS (both local endemics in the western Great Basin) are similarly incapable of setting seed without pollinators and are likely self-incompatible (Meinke and Kaye 1992).
The structure of the ASTRAGALUS flower, like that of other papilionaceous legumes, indicates an adaptation to pollination primarily by large bees, which land on the keel petals and "trip" the flower while pressing under the banner (also known as "standard") to reach a nectary located at the base of the ovary. Based on previous studies of both widespread and rare ASTRAGALUS species in the Great Basin and elsewhere (Baskin and others 1972, Green and Bohart 1975, Sugden 1985, Karron 1987b, Thompson 1991), the most frequent pollinators are bumblebees (BOMBUS spp.) and other polylectic bees (i.e., generalists capable of using a variety of floral resources). According to Karron (1987b), there are two possible explanations for the association between generalist pollinators and restricted ASTRAGALUS species: (1) pollinator specialization is unlikely to evolve or be maintained since a small plant population can only sustain a limited number of forager individuals; and (2) the rare ASTRAGALUS taxa speciated from widespread taxa that are themselves generalist pollinated.
Seed set in the rare ASTRAGALUS LINIFOLIUS is limited by low rates of pollinator visitation (Karron 1987b), suggesting that livestock grazing and other land management practices that reduce the size of pollinator populations may adversely affect reproductive success in A. DESERETICUS. Sugden (1985), studying pollination biology of the restricted A. MONOENSIS, noted that bumblebees (BOMBUS spp.) usually nest in abandoned rodent burrows and concluded that grazing may decrease the number of existing and potential nest sites by causing these burrows to collapse. Pesticides used in statewide programs to control grasshoppers and other insect herbivores may also be harmful to bee populations (Harper 1979, Senft 1990). Because bees have low fecundity, their populations may not recover for many years following such episodes (Karron 1987b).
Pre-dispersal predation of ASTRAGALUS seeds by several types of insect larvae has been reported (Green and Bohart 1975). High seed parasitism has also been noted in A. SCAPHOIDES, a local species of southwestern Montana and adjacent Idaho (Lesica 1987), and in the rare A. OSTERHOUTII (Karron 1989). Preliminary field observations in late May 1992 indicate that seed predation occurs infrequently in A. DESERETICUS, but insect populations probably fluctuate from year to year in relation to climatic and other factors.
Seeds of ASTRAGALUS DESERETICUS evidently lay dormant over the winter and germinate in the spring when soil moisture and temperature conditions are optimal for germination and seedling establishment. Many legume seeds possess a hard outer coat which prevents germination by inhibiting water absorption. Baskin and Quarterman (1969) planted untreated seeds of A. TENNESSEENSIS and found that very few had germinated after one year. Maximum seed germination was obtained only after: (1) mechanical scarification of both the impermeable outer seed coat and a tough inner seed coat; and (2) leaching of an inhibitory substance(s) from the embryo. Similar seed germination requirements have been reported for other ASTRAGALUS species, including A. LENTIGINOSUS var. MICANS (Pavlik 1987a) and A. LINIFOLIUS and A. LONCHOCARPUS (Karron 1989). Assuming that seed dormancy is well developed in A. DESERETICUS and a large seed bank exists at the Birdseye site, these factors would: (1) increase the effective size of the population; and (2) ensure continuation of the population (without immigration) if seed set or seedling establishment fails in any given year (Baskin and Quarterman 1969, Baskin and Baskin 1978).
ASTRAGALUS DESERETICUS undoubtedly depends on favorable weather and soil moisture conditions during the growing season, especially in the critical period of initial seedling establishment. Seedling mortality is probably high, and establishment may be completely unsuccessful in some years. Baskin and others (1972) reported that only 21.5 percent of wild A. TENNESSEENSIS seedlings survived their first summer due to soil moisture stress in their open, rocky habitat. They also found that established plants required several years to reach reproductive maturity, leading them to conclude that very few seedlings reach the adult stage. Another study (Pavlik 1987a) determined a 95 percent mortality rate for germinules of A. LENTIGINOSUS var. MICANS (a threatened dune endemic in eastern California), with anecdotal observations suggesting successful establishment on the order of once every 2 to 4 years. Seed dormancy was well developed in this taxon, and the half-life of established plants was 2.7 years, suggesting an adaptive strategy whereby high seedling mortality is offset by long-lived seeds and high adult survivorship.
Many species of ASTRAGALUS in the Intermountain region are poisonous to livestock (Barneby 1989). Some ASTRAGALI are primary selenophytes (i.e., species which concentrate the toxic element selenium in their tissues and return it to the soil in soluble form that can be taken up by preferred forage plants). Many others synthesize nitrotoxins to which cattle and sheep are particularly susceptible. In addition, a few species produce an alkaloid compound (known as locoine or swainsonine) causing the disease called "locoism" found primarily in horses. The lack of the characteristic "snakelike" odor in A. DESERETICUS and the absence of other primary selenophytes in the area of the Birdseye occurrence indicate that the species is not a selenium accumulator (M. Ralphs, pers. comm. 1992). A recent assay for swainsonine in seeds of A. DESERETICUS also yielded negative results (Ralphs 1993). Both A. ARGOPHYLLUS and A. PURSHII contain low levels of nitrotoxins (Williams and Barneby 1977), and A. DESERETICUS probably contains them also, based on its phylogenetic relationships (Ralphs 1993).
The apparent restriction of ASTRAGALUS DESERETICUS to a single locality raises the question of whether it is a relatively "new" species on the scale of geologic time or a relict population of an older species that was once more widely distributed. Barneby (1989) noted that "proliferation [of ASTRAGALUS] by adaptive radiation into arid and otherwise hostile microhabitats appears to be a relatively recent phenomenon that has not yet run its course. An ability to colonize new unstable habitats in progressively dry climates has hastened evolution of the genus and incidentally given rise to many sharply differentiated but geographically restricted genotypes." Thus, based on the pattern of speciation in the genus as a whole, A. DESERETICUS is most likely a localized neoendemic.
Many endemics in ASTRAGALUS are restricted to inhospitable substrate conditions or limited by the abundance or absence of some particular soil mineral such as selenium, gypsum, or lime (Barneby 1964). One wonders then if A. DESERETICUS is restricted to some confining ecological niche or whether it is for some reason unable (or perhaps has not had sufficient time) to expand into other areas of suitable habitat. The absence of primary selenophytes such as STANLEYA PINNATA, ASTRAGALUS BISULCATUS, and XYLORHIZA spp. from the area of the Birdseye occurrence indicates that seleniferous soils are not a factor. A nearby site (east side of U.S. Highway 89 about 1 mile south of Birdseye) with surface geology mapped as the Moroni Formation (Witkind and Weiss 1985) has soils that are "strongly calcareous" (Swenson and others 1981, p. 77). However, soil pH at the A. DESERETICUS site has not been tested. Franklin (1990; pers. comm. 1992) reports that A. DESERETICUS is apparently specific to the Moroni Formation and that soils on other outcrops in the vicinity are more clay-rich and not as sandy as those at the Birdseye site.
Upper slopes within the Birdseye occurrence are steep and dominated by outcrops of poorly consolidated bedrock. ASTRAGALUS DESERETICUS occurs very sparsely on these slopes, as the erosion rate generally exceeds the rate of soil formation and there is little available rooting substrate. Middle slopes are moderately steep and have a thin mantle of loose, sandy soil overlying the parent material. ASTRAGALUS is more abundant in this topographic position, but erosion rates are high, creating an element of habitat instability which appears to limit the size (and probably the life span) of individual plants. Lower slopes (i.e., those closest to the highway) are more gradual and also more stable, with deeper soils. Here A. DESERETICUS cover is at its maximum, and the plants are generally much larger (and probably longer-lived) than on midslopes. Large and vigorous plants are also found on the adjoining west-facing road cuts above the highway.
Intolerance of shade is very common in the genus ASTRAGALUS (Barneby 1964), and A. DESERETICUS is seemingly excluded from the denser pinyon-juniper tracts characteristic of slopes in the vicinity of the Birdseye site. Within its area of occurrence the species is only rarely found on north-facing exposures (Franklin 1990). However, there appears to be little likelihood that the Birdseye population would ever decline significantly via competitive exclusion by the woodland dominants, since its habitat exists in a state of "perpetual succession" due to high soil erosion rates on steep slopes.