Asclepias amplexicaulis Sm.
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Basis for Listing
The concern for Asclepias amplexicaulis is a result of its natural rarity in Minnesota, the specificity of its habitat needs, and the land-use trends that are putting pressure on the habitat of the few remaining populations. These factors seem to be compounded by the natural demographics of this species, which may require relatively large tracts of intact habitat. This is indicated by the widely scattered spatial distribution of individuals and the relative infrequency of seedlings and juveniles observed at known sites in Minnesota. Asclepias amplexicaulis was listed as a state special concern species in 1984, however, elevating its status to threatened is currently being considered.
Asclepias amplexicaulis occasionally reaches 50 cm (1.6 ft.) in height and has large, opposite, leathery leaves. True to its common name, the leaves have no petiole (stalk), and appear to clasp the stem. The flowers are green with a pinkish or purplish tint and, except for their large size, are similar in structure to flowers of other milkweeds. Unlike most milkweeds however, the flowers occur in a single umbel at the top of the stem rather than in multiple umbels in the axils of the leaves.
Asclepias amplexicaulis occurs exclusively in dry, sandy, sparsely vegetated soil in savannas and upland prairies. It requires full sunlight and minimal competition from other perennials. In today's landscape, these are often small remnants of larger habitats that have been fragmented by human activities.
Biology / Life History
Like all milkweeds, A. amplexicaulis is a perennial although it is not known how long individuals actually live. The flowers are pollinated by a number of insect species and the seeds are spread great distances by the wind. The thick waxy leaves are an adaptation to the hot, dry habitat in which the species lives, allowing it to survive summer-long droughts and still produce viable seeds.
Conservation / Management
Asclepias amplexicaulis is a plant of prairies and open savannas where the trees are few and widely spaced. These conditions developed under a regime of periodic wildfire started by lightning strikes. This favored herbaceous vegetation and discouraged woody vegetation. Since wildfires are essentially a thing of the past, prairie and savanna habitats are frequently degraded by encroaching woody vegetation, which shades out herbaceous plants. The survival of A. amplexicaulis requires that the original open conditions be maintained or recreated, preferably with a program of controlled burns conducted in early spring (before the plants have emerged from winter dormancy) on a 3-8 year rotation. It is also known that A. amplexicaulis does not compete well with invasive non-native species, especially sod-forming grasses such as Bromus inermis (smooth brome). Controlling such invasives may require close monitoring and an ability to respond quickly to newly developing threats.
Conservation Efforts in Minnesota
A number of remnant habitats that contain small populations of A. amplexicaulis are in state ownership and are being managed to maintain natural habitat conditions. At this time, the effects of such management on this rare species are not being closely monitored. However, there is reason to believe the species is responding favorably.
Woodson, R. E. 1954. The North American species of Asclepias L. Annals of the Missouri Botanical Garden 41:1-208.