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Mead’s milkweed (Asclepias meadii) is a late-successional perennial, rhizomatous herb found primarily in virgin tall grass prairies, prairie hay meadows, and glades (Tecic et al. 1998, USFWS 2003). Occasionally Mead’s milkweed has been reported from prairie remnants, sandstone barrens, and sandstone ledges (Voight and Mohlenbrock 1964, Tecic et al. 1998). Seasonal growth begins in mid to late April with a single, slender, unbranched stalk, 20-40 cm (8-16 in) tall that is glabrous but covered with a whitish, waxy covering. Leaves are opposite with a herringbone venation, broadly ovate, 5-7.5 cm (2-3 in) long, 0.9-5 cm (3/8 – 2 in) wide, hairless and covered with a whitish, waxy covering. When in flower (late May to early June), a single umbel is located at the top of the stalk. The umbel is comprised of 6-15 greenish-cream colored flowers. Successful sexual reproduction results in green fruit pods in late June with mature seeds having formed by mid-October (Morgan 1980, Kurz and Bowles 1981, USFWS 2003).
Life History and Population Dynamics
Mead’s milkweed is a late-successional prairie species (Bowles et al. 1998, Bowles and Bell 1998) that occurs in mesic to dry mesic upland tallgrass prairies (Freeman 1988, USFWS 2003) in full sun. The species may persist in a vegetative state in partial shade for long periods of time until destroyed by chance impacts from animals or pathogens. Mead’s milkweed is also known from glade and barrens habitat (USFWS 2003). Mead’s milkweed has low reproductive rates and does not produce flowers every year (Thurman and Hickey 1989). Betz (1989) found that flowering plants only produced seed pods approximately 6.4% of the time. In contrast, Kettle et al. (2000) determined pod formation at a rate of 15%. Further, some estimates have suggested successful fruit production may be as low as 15% (Kurz and Bowles 1981).
Mead’s milkweed is an obligate out crossing species and is pollinated by insects. Pollen clusters together in pollinia, and its seeds are wind-dispersed from follicles (Betz and Lamp 1992, Betz et al. 1994, Tecic et al. 1998). Mead=s milkweed usually reaches reproductive maturity in three to eight years from seed under cultivated conditions (Betz 1989), but may require as long as 15 years (Bowles et al. 2001).
Slow maturation appears to be an important life-history strategy and has sustained the species in hay meadows where mowing results in the removal of fruits before they mature and release seeds (Bowles et al. 1998, Tecic et al. 1998). The establishment of seedlings is often infrequent, but is essential for the establishment of new populations, and may be necessary for long term population viability (USFWS 2003). Mead’s milkweed also spreads vegetatively producing ramets from underground rootstock. Underground rhizomes can grow to 1 m (39 in).
In southern portions of its range, Mead’s milkweed begins flowering in late May and mid-June in the north (USFWS 2003). Stress from extreme events, such as drought, are known to cause flower loss, wilting, and may result in plants being reduced to sterile or juvenile conditions. Mead’s milkweed, similar to many milkweeds species, is self-incompatible and sensitive to inbreeding depression. Self-incompatible species usually require out crossing between sexually compatible plants for production of viable seeds (Shannon and Wyatt 1986, Kahn and Morse 1991, Broyles and Wyatt 1993). As a result, inbreeding depression occurs in populations with very small numbers. It is believed that small populations of Mead’s milkweed with low numbers of genotypes will have a reduced reproductive capacity. In fact, viable seeds have not been found in most populations east of Kansas and western Missouri. Seed production may also be reduced due to high rates of pod abortion and loss of pollinators.
Pollination in Mead’s milkweed is carried out by small bumblebees and miner bees. Individual pollen grains adhere to each other in a paired mass referred to as pollinium. The pollinium is then transported by bees, which can retain the mass for up to 6 hours (Morse 1980). Following successful pollination and seed formation, seeds are then wind dispersed from follicles (Betz 1989, Betz and Lamp 1992, Betz et al. 1994). Wyatt and Broyles (1994) suggested that the slow turnover of pollinium, in addition to the flying capabilities of bees, contributes to long distance pollen transfer. Hayworth et al. (2001) concluded that long distance pollen transfer and wind dispersed seeds have likely resulted in the large neighborhood sizes and low levels of genetic variation observed across the range of this species.
Total reproductive success in Mead’s milkweed is low (Betz 1989, Thurman and Hickey 1989, Bowles et al. 1998, Tecic et al. 1998) and the species has been extirpated from many sites in the eastern portion of its range. Additionally, many populations throughout the range are small and contain only a low number of individuals (Bowles et al. 1998, Tecic et al. 1998, Watson 1998), often consisting of genetic clones. Watson (1998) postulated that the species may remain dormant some years due to environmental factors. This theory is supported by the observations of Betz and Hohn (1978) who noted populations may fluctuate from year to year, with individual plants flowering successively for several years only to then disappear completely for a few years. This phenomenon has been observed in at least one southern Illinois population (Elizabeth Shimp, USFS, pers. comm. 2005).
Bowles et al. (1998) and Tecic et al. (1998) have reported a reduction in genetic diversity in prairie populations managed as hay meadows compared to those managed with prescribed fire. Currently, only two populations of Mead=s milkweed are known to reproduce sexually and produce viable seed on a regular basis: Rockefeller Prairie in Jefferson County, Kansas and the Weimer Hill igneous glade in Iron County, Missouri (Bowles et al. 1998, Tecic et al. 1998). Both of these sites have been managed primarily through prescribed fire. Tecic et al. (1998) compared the genetic variability of plants at these two sites with plants from other populations in hay meadows and determined that the two fire-managed populations had more genotypes but fewer ramets than those on hay meadows. Observations by Bowles et al. (1998) also determined that while mowed sites had a higher density of ramets, burned sites had a larger proportion of flowering ramets. Therefore, burning is likely to promote flowering and enhance sexual reproduction, while mowing during the growing season prevents sexual reproduction and promotes vegetative spread (USFWS 2003).
|Appendix hm references used in habitat models for Southwest Regional Gap Analysis Project||Appendix Bibliography (list of all articles cited and what chapter cited in)|
|Appendix 8: Curricula Vitae for Part-Time Faculty Appendix 1||Draft appendix a appendix a documents to for accreditation|
|Appendix a1||Appendix L|
|Appendix 1 – References||Supplementary Appendix|
|Bibliography: All References, Including Sources and Literature Cited||Appendix User Bibliography|