Plant Species Biology Summary for Tennessee Milk Vetch
Illinois Department of Natural Resources
Division of Natural Heritage

The following information is derived largely from monitoring records of the Illinois Department of Natural Resources but some may come from the literature and other sources. It relates to the biology of the species in Illinois.

Species: Astragalus tennesseensis, Tennessee Milk Vetch

Compiler: John Schwegman

Date of Most Recent Update: January 23, 1996

Location of Monitored Populations: Manito Prairie, Tazewell Co. Illinois.

Plot Type and Size, Population Size: Protractor plot 3 meter radius for 180 degrees (14.14 square meters). Additionally the entire population of 5 stands was censused annually.

Monitoring Dates that are Basis for this Analysis: Early to mid May annually form 1985 to 1995 inclusive for the demographic plot and annually (except for 1983) from 1981 through 1995 for the total population census.

Variations from Normal Conditions while Monitoring: A severe drought was under way when the plot was established in 1985 with severe drought and excessive summer heat again in 1987 and 1988. Heavy rains and floods occurred in the fall of 1987 and the growing seasons of 1990 and 1993 were very wet and above average in rainfall.

Range of Natural Communities Inhabited: Upland gravel prairie.

Range of Plant Communities Inhabited: Arenaria stricta-Muhlenbergia cuspidata-Opuntia humifusa in the demographic plot.

Range of Soil and Substrate: Gravel to loamy gravel and sand.

Range of Slope and Aspect: Northwest to West on a gentle slope measured at 13 degrees at the demographic stand but also rarely growing on nearly level upland.

Range of Shading or Crown Cover: Mostly full sun but sometimes growing next to the prairie edge where it gets some shade.

Flowering Dates in Illinois: Late April to early June.

Fruiting Dates in Illinois: Late summer to fall but seeds may not be released from pods forseveral years.

Known Phenotypic Variation: None.

Known Pollinators in Illinois: Bumblebees.

Reproductive Mode: Seed.

Known Conditions for Sexual and Asexual Reproduction: No asexual reproduction. Most seedlings were seen the spring following the wet summers of 1990 and 1993 indicating that moist warm conditions may stimulate seed for germination. Germination occurs from late April through May in thinly vegetated openings in the dry prairie community. Survival seemed to be best when seedlings were adjacent to or mulched by low sheltering vegetation. Seed germination seemed to have been enhanced by spring burning in 1992 and 1994. Seed pods are thick and spongy (styrofoam-like) and light in weight. They open at one end in the fall or winter after maturing and depend on wind and water to move them and cause seed dispersal out the pod opening. Many pods never move and depend on the rotting of the pod to release seed, taking up to 3 years.

Known Diseases: No problem diseases were noted.

Known Grazers and Parasites: One large plant had inflorescences and leaves grazed by what appeared to be deer in early May of the drought year 1987. No other grazing was noted. Digging by small mammals (rodents ?) uprooted and killed 2 plants in one year and 1 in another.

Known Problems with Exotic Species: None noted.

Known Response to Fire: This species sends up new growth late in the fall and early in the spring that is damaged by fire at these times, but no permanent damage was noted after the 4 burns that included the plot during the monitoring. Seed pods of this species are thick and spongy and can lie around for years before deteriorating and releasing seed. During this monitoring, a fire burned and destroyed seed in some pods. Evidence from 1992 and 1994 indicates that fire can stimulate seed germination.

Known Mortality Factors: Most mortality of mature plants was due to tiny (2.5 mm) black-headed white insect larvae that bore through the stem just above ground level, turning it to mush and causing the plant to wilt and die. This death usually occurs in May. Seedlings and small plants seem to die from drought or uprooting by wind before they reach large enough size to withstand these forces. A few juvenile plants may have died from drought also. A major mortality of mature plants occurred during the very wet summer of 1990 for unknown reasons. This followed the appearance of many new young plants that spring.

Known or Suspected Responses to Environmental Stress: Young and juvenile plants may be killed by drought and heat. The largest die-off of mature plants occurred in the very wet summer of 1990 which followed a three-year drought period. It is not known whether wet conditions increased vulnerability to disease or pest insects or whether it increased competition from associatedvegetation. Of 13 plants that flowered in the drought year of 1987, only 4 plants developed pods and some of these had few seed.

Known Shifts in Plant Vigor: Two large flowering plants had a reduction in number of inflorescences following the severe drought year of 1987 and during the drought year of 1988 but returned to high inflorescence numbers in 1990. Drought also reduced seed set following flowering, often eliminating it altogether.

Plant Longevity: Twenty-nine plants that survived their seedling year were tracked from seedlings or juveniles to death. After adding one year for the seedling year of plants first spotted as juveniles, the average life span of these plants is 3.58 years. Six plants lived for 7 years, 2 for 5, 2 for 4, 6 for 3 and 13 for 2. Of 11 plants still alive at the end of the study, 1 was 5 years old, 2 were 4, 2 were 3, 4 were 2 and 2 were first year seedlings. Twenty-nine of 45 marked seedlings died in their first year for a seedling mortality rate of 64%.

Known "Resting" (of Perennial Herbs): No marked plants were observed to fail to emerge one year and then return in a subsequent year.

Summary of Apparent Factors Regulating Size and Health of Populations:

Boring insect larvae infest lower stems killing many mature plants. Drought kills seedlings and some young plants not well enough established to withstand its stress. Very wet summers lead to the death of many large plants for unknown reasons but also lead to increased seed germination and seedling establishment the following spring. Drought reduces seed set. Fire can destroy seed still in pods but scarifies and promotes germination of some seed as well.