Desmodium cuspidatum var. longifolium (Torr. & Gray) Schub.
Big Tick Trefoil
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Basis for Listing
Desmodium cuspidatum var. longifolium (big tick trefoil) occurs in mesic forests in the southeast portion of the state. This region comprises areas that have traditionally been called the Big Woods and the Paleozoic Plateau. Nearly 90% of the forests in these areas have been cleared for agriculture or housing, and many of the remaining fragments have been degraded by livestock grazing and timber cutting. This makes any remaining high-quality forest fragment very important to biodiversity conservation. For this reason, most of these fragments have been studied carefully by the Minnesota Biological Survey, even though they frequently have been less than 40 ha (100 ac.) in size. Results of the Survey have not been encouraging; fewer than ten sites were found to harbor D. cuspidatum var. longifolium. For this reason, this species was listed as special concern in Minnesota in 1996. Subsequently, population trends were perceived to be worsening, so its status was elevated to threatened in 2013.
Desmodium cuspidatum var. longifolium generally produces a single stem that grows to a height of about 1 m (3 ft.). The leaves are pinnately compound, with 3 leaflets each. The leaflets are 6-12 cm (2.4-4.7 in.) long, each with a sharply pointed tip. The inflorescence is a simple raceme or a sparsely branched panicle, with small pink flowers. The fruit is a thin and indehiscent pod covered with hooked hairs and divided into transverse segments. Each segment holds one seed and eventually separates from the others (Isely 1998).
Desmodium cuspidatum var. longifolium is very closely associated with mesic hardwood forests, particularly the type dominated by Quercus spp. (oaks), Acer saccharum (sugar maple), and Tilia americana (basswood), (Southern Mesic Oak-Basswood Forest). Within such habitats, it appears to be rather mobile, possibly seeking out small canopy gaps or temporary edges where there is filtered sunlight rather than continual shade.
Biology / Life History
Desmodium cuspidatum var. longifolium is a short-lived perennial forb, adapted to the shade or filtered sunlight that is characteristic of mature hardwood forests. The flowers are small but brightly colored and pollinated by flying insects, most commonly bees. A typical plant produces about 240 flowers. The species is self-compatible but still requires pollinator visits for fruit set to occur. Flowers open shortly after sunrise, remaining open for approximately one day, and exhibit an explosive dehiscing or tripping mechanism of pollen release. Flowers that are not pollinated within 24 hours of opening are aborted (Skogen et al. 2010). The seed pods are covered by small hooked hairs that catch in the fur of mammals (like Velcro).
Desmodium cuspidatum is comprised of two rather indistinct varieties. Variety cuspidatum occurs in the easterly portion of the species’ range, and variety longifolium occurs in the western portion of its range (Isely 1998).
As a legume, D. cuspidatum is known to fix atmospheric nitrogen with the nodules on its roots. This is believed to give it a competitive advantage in nitrogen-poor soils. However, there is some evidence that nitrogen levels in soils are increasing with global warming and may shift the competitive advantage away from nitrogen-fixing plants (Skogen et al. 2011). It has been hypothesized that this is the cause of a perceived decline of D. cuspidatum var. cuspidatum in northeastern North America (Skogen et al. 2011).
Demographic studies have shown that, compared to other species of Desmodium, both recruitment in populations and reproductive success of D. cuspidatum var. cuspidatum is low, though causes for this are unknown (Skogen et al. 2010). How this might apply to Minnesota populations of D. cuspidatum var. longifolium is also unclear.
Conservation / Management
The conservation needs of D. cuspidatum var. longifolium are similar to those of other rare forest species, mainly habitat protection. This means excluding logging and livestock grazing. Another, more insidious, threat is invasive species, particularly Rhamnus cathartica (common buckthorn), Lonicera spp. (Eurasian honeysuckle shrubs), and Alliaria petiolaris (garlic mustard). These species are capable of invading a forest fragment, reproducing so quickly, and forming such a dense growth that nearly all native species are crowded out. Yet another threat is the over-abundance of White-tailed Deer (Odocoileus virginianus), which are concentrated in forest habitats and can cause considerable damage to native vegetation.
Best Time to Search
The best time to search for D. cuspidatum var. longifolium is when the plants are in flower or fruit, from early July through September.
Conservation Efforts in Minnesota
Desmodium cuspidatum var. longifolium is known to occur at Lake Louise and Forestville/Mystery Cave state parks and at Chub Lake Wildlife Management Area, all of which may provide some level of protection. However, no known attempt has been made to assess the conservation needs of this species within these areas or to determine the effects of current management practices on its status.
Welby Smith, MN DNR, 1988, 2008, and 2018
Coffin, B., and L. Pfannmuller, editors. 1988. Minnesota's endangered flora and fauna. University of Minnesota Press, Minneapolis. 473 pp.
Isely, D. 1998. Native and naturalized Leguminosae (Fabaceae) of the United States. Brigham Young University Press, Provo, Utah. 1,007 pp.
Isely, D. I. 1955. The Leguminosae of the north-central United States II. Hedysareae. Iowa State College Journal of Science 30(1):33-118.
Minnesota Department of Natural Resources. 2005. Field guide to the native plant communities of Minnesota: the eastern broadleaf forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 394 pp.
Minnesota Department of Natural Resources. 2006. Tomorrow's habitat for the wild and rare: An action plan for Minnesota wildlife, comprehensive wildlife conservation strategy. Division of Ecological Services, Minnesota Department of Natural Resources. 297 pp. + appendices.
Minnesota Department of Natural Resources, Division of Ecological Resources. 2008. Rare species guide: an online encyclopedia of Minnesota's rare native plants and animals [Web Application]. Minnesota Department of Natural Resources, St. Paul, Minnesota. Accessed 1 July 2009.
Minnesota Department of Natural Resources, Division of Fish and Wildlife. 1995. Statement of need and reasonableness in the matter of proposed amendment of Minnesota Rules, Chapter 6134: endangered and threatened species. Minnesota Department of Natural Resources, St. Paul, Minnesota. 336 pp.
Ownbey, G. B., and T. Morley. 1991. Vascular plants of Minnesota: a checklist and atlas. University of Minnesota Press, Minneapolis, Minnesota. 320 pp.
Randall, J. M., and J. Marnelli. 1996. Invasive plants: weeds of the global garden. Brooklyn Botanic Gardens, Brooklyn, New York.
Skogen, K. A., K. E. Holsinger, and Z. G. Cardon. 2011. Nitrogen deposition, competition and the decline of a regionally threatened legume, Desmodium cuspidatum. Oecologia 165(1):261-269.
Skogen, K. A., L. Senack, and K. E. Holsinger. 2010. Dormancy, small seed size and low germination rates contribute to low recruitment in Desmodium cuspidatum (Fabaceae). The Journal of the Torrey Botanical Society 137(4):355-365.