CPC Plant Profile: Whited's Milkvetch
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Plant Profile

Whited's Milkvetch (Astragalus sinuatus)

A close-up view of the white flowers of Astragalus sinuatus and the dry soil that it inhabits. Photo Credit: Washington NHP
Description
  • Global Rank: G1 - Critically Imperiled
  • Legal Status: N/A
  • Family: Fabaceae
  • State: WA
  • Nature Serve ID: 133554
  • Date Inducted in National Collection: 04/04/1991

Astragalus sinuatus is found only within a ten square mile (26 sq. km) area in Central Washington. Although much of the historic range of this species has been converted or destroyed by agriculture and grazing, there is quite a bit of suitable habitat remaining. Scientists do not know why A. sinuatus has not spread to these areas of seemingly suitable habitat. Whited's milkvetch presents quite a challenge to researchers and seed conservationists. The seed pods of this species are small and incredibly tough. It may take a pair of pliers to crack the fruit open and extract the seed. For all the work involved, the pods have only one or two (rarely more) seeds inside (and sometimes they are completely empty). In the wild, pods open at one end, releasing seeds as they shake in the wind or roll down the hill. Often, the pods are so tough that they do not open until weathered and rolled along the ground. Despite the tough pod, the seeds often fall victim to seed predation or larval damage. Certain insects can bore a tiny hole through the pod and the seed coat and deposit an egg within the seed. As the larvae develops it consumes the contents of the seed.

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Updates
  • 08/27/2020
  • Seed Collection

Collect and store seeds from all known populations. Determine optimum germination procedures. Determine propagation and reintroduction protocols.

  • 08/27/2020
  • Propagation Research

Germination trials at the Berry Botanic Garden revealed that 8 weeks of cold stratification, followed by either a constant 68F (20C) or alternating 50/68F (10/20C) temperature yielded slightly higher germination rates (67%) than the same temperature treatments with no cold stratification (40% at 68F and 50% at 50/68F) (BBG file).

  • 08/27/2020
  • Orthodox Seed Banking

Seed from one population is stored at The Berry Botanic Garden

Nature Serve Biotics
  • 05/02/2017

Very narrowly endemic to a small creek drainage along the western edge of the Columbia Basin in central Washington. The species' habitat is threatened by fire suppression, fire management, non-native species invasions, grazing, and agricultural development.

Edward Guerrant, Ph.D.
  • 01/01/2010

Years of fire suppression has lead to increased vegetative cover. Now, in the event that a fire does start, this increased fuel load will lead to hotter burning fires, which can kill native vegetation. (WNHP 1999). Invasion by weedy annual species (WNH

Edward Guerrant, Ph.D.
  • 01/01/2010

As of 1995: 8 populations (one with 2 sub-populations) all in an area less than 10 square miles. Populations range in size from 5 to 2,900 individuals and from acre to 80 acres. The total population in 1994 was approximately 5,000 (Gamon 1995).

Edward Guerrant, Ph.D.
  • 01/01/2010

Germination trials at the Berry Botanic Garden revealed that 8 weeks of cold stratification, followed by either a constant 68F (20C) or alternating 50/68F (10/20C) temperature yielded slightly higher germination rates (67%) than the same temperature treatments with no cold stratification (40% at 68F and 50% at 50/68F) (BBG file).

Edward Guerrant, Ph.D.
  • 01/01/2010

All populations have been subject to grazing by domestic livestock, especially cattle and horses, at some point since at least 1900, possibly as early at the 1880's (Gamon 1995). One population is within a ""Natural Area Preserve"" managed by the Washington Department of Natural Resources (Gamon 1995). The area is fenced to exclude livestock. A portion of another population on BLM land is designated as an ACEC (Area of Critical Environmental Concern). There is a grazing lease on the land, although the lessee has not grazed livestock there since around 1985 (Gamon 1995). All other sites are used periodically as range land for livestock (Gamon 1995) Seed from one population is stored at The Berry Botanic Garden

Edward Guerrant, Ph.D.
  • 01/01/2010

Research ways to reduce big sagebrush cover other than fire that benefit Astragalus sinuatus (WNHP 1999). Determine why individuals appear to reproduce successfully, yet seemingly suitable habitat remains unoccupied (WNHP 1999). Determine factors influencing limited distribution.

Edward Guerrant, Ph.D.
  • 01/01/2010

Collect and store seeds from all known populations. Determine optimum germination procedures. Determine propagation and reintroduction protocols.

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Nomenclature
Taxon Astragalus sinuatus
Authority Piper
Family Fabaceae
CPC Number 491
ITIS 25681
USDA ASSI5
Common Names Whited's milkvetch | Whited milk-vetch
Associated Scientific Names Astragalus sinuatus | Astragalus whitedii | Homalobus sinuatus | Homalobus whitedii | Phaca sinuata
Distribution WA: EC (Eastern Cascades)
State Rank
State State Rank
Washington S1
Habitat

Astragalus sinuatus grows on rocky in the sagebrush/bunchgrass shrub steppe community. Associated species commonly include Lupinus sulphureus, Erigeron linearis, Phlox longifolia, Woodsia oregana, Balsamorhiza sagittata, Lomatium dissectum, Lithophragma bulberifera and Astragalus purshii. Astragalus sinuatus grows on southwest to southeast facing slopes which makes them warmer and drier than the reported temperatures. The region is very harsh and dry with reported winter lows around 20F (-6.6C) and summer highs around 87F (30.5C). Precipitation averages only 9 inches (22.9 cm) per year, most of which falls from October to June (fall to spring).

Ecological Relationships

The region that Astragalus sinuatus occupies is subject to occasional fire, localized erosion, slope movement, and frost heaving. Astragalus sinuatus does not seem to take full advantage of, or require these localized disturbances (Gamon 1995). Although plants may show a short-term positive response from fire and other mild disturbances, it seems unlikely that they are required for long term survival of the population. It is found in early, mid and late successional stages. Population numbers become stabilized as the climax ecosystem is reached (Gamon 1995).Occasional fire (30-90 year interval) probably played a role historically in maintaining open habitat for this species. Now, fire may lead to increased success of weedy annuals (WNHP 1999). Other disturbances can also have a negative impact as they may increase the number of weedy species which may eventually out-compete A. sinuatus (Gamon 1995).The plant actively grows for only a short period every year. Leaf emergence is most likely initiated in late March or early April. The plant is fully leafed-out by mid April, just when floral buds appear. Most flowers bloom between late April and early May, and the majority of fruits develop by mid-May. Seed is fully mature by the middle to end of July. Most leaves begin to senesce by June, and in summer, the only identifying features remaining may be the empty seedpods (Gamon 1995). Astragalus sinuatus is sensitive to fluctuating precipitation levels. During drought years, many plants remain dormant, never emerging from the ground. Those that do emerge may not flower (Gamon 1995).Knowledge of the reproductive biology of Astragalus sinuatus is scant. It is believed that they are a predominantly out-crossing species, although some members of the genus Astragalus can self-pollinate as well. Bumblebees have been observed in and around the plant (Gamon 1995). The incredibly tough pods do not naturally split along the """"seam"""" as a pea or a bean would. They open only at the end, and seeds may fall out by shaking in the wind or as the pod rolls downhill. Pods may open more as they are weathered and rolled along the ground. Seeds must be scarified (have their surface scratched slightly), before they will germinate. Most seeds are dispersed close to the parent plant or slightly downhill from it. A. sinuatus is more common lower on slopes (Gamon 1995).Like other members of the Pea Family (Fabaceae), Astragalus sinuatus can develop a symbiotic relationship with nitrogen fixing bacteria that live in their roots. This gives them a competitive advantage over other species in nitrogen poor soils and increases the available nitrogen in the soil (Gamon 1995).Cattle do not seem to eat Astragalus sinuatus plants. They do, however, compact the soil, accelerate soil erosion, and contribute to an increase in weedy, annual, non-native species, such as cheatgrass (Bromus tectorum) (Gamon 1995). Areas most heavily used by cattle have lower population densities and lower seedling recruitment than areas less heavily used by livestock.

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

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