CPC Plant Profile: Cotton's Milkvetch
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Plant Profile

Cotton's Milkvetch (Astragalus cottonii)

Astragalus australis var. olympicus with developing pods. Photo Credit: Mark Sheehan
Description
  • Global Rank: G2 - Imperiled
  • Legal Status: N/A
  • Family: Fabaceae
  • State: WA
  • Nature Serve ID: 147601
  • Date Inducted in National Collection: 03/08/1989

Even though Astragalus australis var. olympicus is native only to high reaches in the Olympic Mountains of Washington, it is not immune to the impact that modern humans have had on the ecosystem. With an unfortunate lack of foresight, 12 mountain goats were introduced to the Olympic Mountains in the 1920s to provide animals for sport hunting. The original population of 12 has now grown to over 1,000 individuals. Mountain goats are not part of the native ecosystem in the Olympic Mountains, and consequently, the flora of the region is not adapted to the stresses that goats put on the plants. Astragalus australis var. olympicus is no exception. While the Olympic Peninsula encompasses only 8% of Washingtons land, it contains 27% of the states rare flora. Olympic National Park has an even higher concentration of rare plants: 19% of the rare plants here are found in just over 2% of the state (Schreiner et al. 1994). This concentration of rare flora exists because small islands of habitat were not covered by ice or water during the last ice age. Mountain flora isolated in this habitat could not interbreed with other populations, and eventually evolved into its own distinct taxa. Isolated high in the Olympic Mountains during the last ice age, this particular variety of A. australis evolved a unique trait: highly inflated seedpods. These seed pods have been likened to "mini greenhouses," increasing the temperature surrounding the developing seeds. This variety has not been isolated long enough to evolve into a separate species, but it is possible that this species will become distinct enough to become its own species if it continues to be isolated and cannot interbreed with other populations of A. australis (Schreiner et al. 1994).

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Updates
  • 08/21/2020
  • Propagation Research

Seed germination studies at The Berry Botanic Garden. Seeds were scarified and then subjected either to no cold stratification or 8 weeks of cold stratification followed by either constant 68F (20C) or alternating 50F/68F (10C/20C) temperatures. 100% of the seeds without cold stratification germinated regardless of temperature regime. 80% of seeds subjected to cold stratification germinated regardless of subsequent temperature regime (BBG File).

  • 08/21/2020
  • Reproductive Research

Pollinator exclusion in the field significantly reduced fruit set but seed set per fruit was not significantly affected (Kaye 1989).

  • 08/21/2020
  • Demographic Research

Extensive research on the ecology, reproduction and demography of Cottons milkvetch (Kaye 1989). Pollinator exclusion in the field significantly reduced fruit set but seed set per fruit was not significantly affected (Kaye 1989). Demographic monitoring from 1985 to 1988 within permanent plots revealed a significant decrease in population numbers. The cause of the decline is not known. Mountain goat grazing and wallowing was one speculative explanation. Low snowpack and summer drought was favored by the researcher as the most likely cause (Kaye 1989). Transition matrix modeling projected that populations would continue to decline (Kaye 1989). In order to evaluate the effects of mountain goats, ten permanent plots were established. All Astragalus australis var. olympicus plants were mapped and counted in these plots, and grazing or physical injuries were recorded from 1985 to 1991. Monitoring results revealed that A. australis var. olympicus was grazed in more than half of the sub-populations, and that injuring was sporadic but sometimes intense. Sub-populations were more or less stable during the six-year period, but the study was not long enough to determine long term trends (Schreiner et al. 1994).

  • 08/21/2020
  • Orthodox Seed Banking

Seeds from three locations stored at The Berry Botanic Garden.

Nature Serve Biotics
  • 05/02/2017

Endemic to higher elevations in the northern Olympic Mountains of Washington State. Threatened by non-native mountain goats, which trample the native vegetation. Potentially threatened by over-use of the area by hikers.

Edward Guerrant, Ph.D.
  • 01/01/2010

Disturbance (herbivory and trampling) from non-native mountain goats (WNHP 1999) Trampling by hikers (WNHP 1999)

Edward Guerrant, Ph.D.
  • 01/01/2010

As of 1996: 10 populations on the Olympic Peninsula. Surveys were conducted between 1981 and 1996. Numbers ranged from as low as 9 to as high as 2000, with a total of approximately 5,300 individuals (WNHP 2000).

Edward Guerrant, Ph.D.
  • 01/01/2010

Extensive research on the ecology, reproduction and demography of Cottons milkvetch (Kaye 1989). Pollinator exclusion in the field significantly reduced fruit set but seed set per fruit was not significantly affected (Kaye 1989). Demographic monitoring from 1985 to 1988 within permanent plots revealed a significant decrease in population numbers. The cause of the decline is not known. Mountain goat grazing and wallowing was one speculative explanation. Low snowpack and summer drought was favored by the researcher as the most likely cause (Kaye 1989). Transition matrix modeling projected that populations would continue to decline (Kaye 1989). In order to evaluate the effects of mountain goats, ten permanent plots were established. All Astragalus australis var. olympicus plants were mapped and counted in these plots, and grazing or physical injuries were recorded from 1985 to 1991. Monitoring results revealed that A. australis var. olympicus was grazed in more than half of the sub-populations, and that injuring was sporadic but sometimes intense. Sub-populations were more or less stable during the six-year period, but the study was not long enough to determine long term trends (Schreiner et al. 1994). Seed germination studies at The Berry Botanic Garden. Seeds were scarified and then subjected either to no cold stratification or 8 weeks of cold stratification followed by either constant 68F (20C) or alternating 50F/68F (10C/20C) temperatures. 100% of the seeds without cold stratification germinated regardless of temperature regime. 80% of seeds subjected to cold stratification germinated regardless of subsequent temperature regime (BBG File).

Edward Guerrant, Ph.D.
  • 01/01/2010

No active management. Seeds from three locations stored at The Berry Botanic Garden.

Edward Guerrant, Ph.D.
  • 01/01/2010

Monitor known occurrences for population trends (WNHP 1999). Conduct field searches to locate new populations (WNHP 1999). If population sizes continue to decline, seedling recruitment may be increased by removing mountain goats from the area, controlling pre-dispersal predators, sowing extra seeds, and tending established plants (Kaye 1989).

Edward Guerrant, Ph.D.
  • 01/01/2010

Collect and store seeds from across range. Determine propagation and reintroduction protocols.

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Photos
Nomenclature
Taxon Astragalus cottonii
Authority M.E. Jones
Family Fabaceae
CPC Number 390
ITIS 25479
USDA ASCO13
Common Names Cotton's milkvetch
Associated Scientific Names Astragalus australis var. olympicus | Astragalus australis var. cottonii | Astragalus cottonii | Astragalus cottoni | Astragalus olympicus | Atelophragma cottonii
Distribution Olympic Peninsula of Washington
State Rank
State State Rank
Washington S2
Habitat

South-facing talus slopes, ridges, and knolls comprised of calcareous substrates derived from sea-floor sediments (limestone) with a pH nearly always higher than 6. The region is characterized as sub-alpine, with an elevation of 4800 to 6000 ft (1460-1830m). The sub-alpine prairie association includes species such as Phlox diffusa, Festuca idahoensis, Allium crenulatum, and Lomatium martindalei.

Ecological Relationships

Astragalus australis var. olympicus may represent a relictual population of A. australis that existed in the Olympic Mountains during the Pleistocene. It remained during glacial advances, and was stranded in suitable habitats as the climate changed and adjoining populations receded (Kaye 1989). This species is set apart from other Astragalus species by distinctive inflated pods, which may be an important dispersal adaptation. In addition to the explosive expulsion, consumption or simply dispersal by gravity common to flat pods, inflated pods can also be dispersed by wind (Ridley 1930 in Schreiner 1994). The inflated pods may also act as """"mini greenhouses,"""" raising the temperature around the developing seeds.It appears that soil chemistry determines the broad-scale distribution of Astragalus australis var. olympicus, while competition and other environmental factors determine it local distribution (Kaye 1989). An association with VAM mycorrhizae (O'Dell in Kaye 1989) may help it survive in high pH and calcium soils, which are functionally deficient in phosphorous (Lesica and Antibus 1985 in Kaye 1989).The structure of Astragalus australis var. olympicus plants suit them to areas of poorly developed, unstable soils. The tap root branches to form spreading lateral roots. This species may be subjected to a fair amount of sliding and downhill creep. There is probably considerable frost heaving of soils in late winter and early spring due to the sparseness of vegetation. This may keep other species from becoming established and competing (WNHP 1999).Astragalus australis var. olympicus is an herbaceous perennial that holds over-wintering buds just below the soil surface. The species does not appear to reproduce vegetatively. Plants usually begin to bloom in early June and peak in late June. A few fruits dehisce (break open) in late July while still on the plant, but most seeds are dispersed in September after fruits have fallen (Sheehan and Kaye 1986 in Kaye 1989).Experimental insect exclusion reduced overall fruit set but not the number of seeds per fruit. This indicates that Astragalus. australis var. olympicus is typically out-crossed but genetically self-compatible. A mechanical barrier, not a genetic one, may interfere with self-pollination. Inbreeding depression from selfed seeds was not tested, but selfed seeds from other rare Astragalus species do display decreased fitness. Self-compatibility in A. australis var. olympicus is consistent with the view that small populations survived in the glacial refuges during the Pleistocene, and suggests that it may be able to survive future bottlenecks (Kaye 1989).Bumblebees (Bombus appositus, B. birafius nearcticus, and B. occidentalis occidentalis) and a solitary bee (Osmia spp.) are the primary visitors to Astragalus australis var. olympicus flowers. These bees are capable of tripping the pollination mechanism, and are observed to be relatively faithful to A. australis var. olympicus (Kaye 1989). Despite abundant flower production, seed set was observed to be low in monitored plants. Low seed set was primarily due to predation, seed abortion, and lack of fertilization. At the sites where weevils were present, predation accounts for the greatest loss of ovules within fruits. Fertilization (or lack of) is not usually a limiting factor (Kaye 1989). Seeds do not possess highly specific germination requirements. They germinate equally as well in light and darkness. Rates of germination tend to decrease with decreasing temperature and moisture availability, but some seeds are able to germinate near environmental extremes. Most seeds require seed coat scarification to break dormancy. Frost heaving; soil slumping; wind; gnawing by insects, ingestion by rodents and birds; and fungal hyphae may cause scarification in nature. A weevil, (Tychius spp) was the only adult insect observed to be common on buds, flowers and immature fruits. Larvae reared from fruits produced only Typhius weevils (Kaye 1989). A patchy but locally abundant distribution may make A. australis var olympicus more susceptible to predation than one with a sparse distribution. A. australis var olympicus seems to have few defenses against the Tychius weevil (Kaye 1989). Seed predation appears to have a negative effect on population growth, but is not responsible for the rarity of the taxon. Some populations occupy habitats prone to rock slides and surface disturbances by introduced mountain goats. Therefore, seed production is crucial for replacement of individuals within populations and for dispersal to new sites (Kaye 1989). Kaye (1989) suspects that plants do not reach reproductive age until they are five to fifteen years old. Low observed rates of seedling establishment and dominance of large reproductive individuals indicate that recruitment is low. Low seedling establishment is typical of plants at high elevation (Bliss 1971 in Kaye 1989). Both summer drought and winter freezing disturbances probably restrict survival of A. australis var. olympicus to specific microsites (e.g.: rock margins) where drought and freezing disturbance is reduced (Kaye 1989). While growth rates are most sensitive to the number of reproductive individuals, population viability relies on seedling recruitment. In order for projected declines to be reversed, increases in seedling recruitment must be sustained (Kaye 1989). Monitoring of grazing impact revealed that A. australis var. olympicus was grazed in more than half of the subpopulations surveyed, and that injuring was sporadic but sometimes intense. While it is not expected that mountain goats consume or trample the last of the individuals, they may increase risk of extinction by fragmenting populations or habitats (Screiner et al. 1994).

Pollinators
Common Name Name in Text Association Type Source InteractionID
Reintroduction
Lead Institution State Reintroduction Type Year of First Outplanting

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