Trillium nivale Riddell
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Basis for Listing
The status of Trillium nivale is somewhat equivocal. Although it grows in fifteen states and is considered "secure" nationwide, populations are considered "scattered to rare across its range" (NatureServe 2009). The species is listed as endangered or threatened in half of the states in which it occurs, and it is watch-listed or special concern in the remainder of states. First documented as "local and rare" in Minnesota in the late 19th century (MacMillan 1892), Minnesota's T. nivale populations vary wildly in size, with many of the largest located in vulnerable areas. The horticultural popularity of this species among wildflower gardeners is no assurance of its perpetuation, and actually places natural populations, in states where it is not protected, at greater risk. Given its rarity and vulnerability, T. nivale was listed as a special concern species in Minnesota in 1984.
Trillium nivale is a small plant, up to 10 cm (4 in.) tall. Its flowers are erect, facing up above the whorl of leaves. Petals are white, up to 3.5 cm (1.4 in.) long, and up to 1.5 cm (0.6 in.) wide (Case 2002). Both the sepals and leaves have a slight bluish-green cast. Trillium nivale can be distinguished from all other Minnesota trilliums not only by its earlier flowering period, but also by its whorl of 3 small, oval leaves with prominent parallel veins. Also unlike other members of the genus in Minnesota, the tips of the leaves in T. nivale are rounded. Leaves can be up to 4.5 cm (1.8 in.) long and 3.4 cm (1.3 in.) wide. Immature plants may exhibit 1 or 2 leaves (Martin 1935), which are recognizable by their shape, color, and vein pattern similar to flowering plants.
Most plant communities in which T. nivale occurs are classified as mesic hardwood forests dominated by Acer saccharum (sugar maple), Tilia americana (basswood), and Quercus spp. (oaks). Among canopy oaks, Q. rubra (red oak) prevails in the Paleozoic Plateau and only Q. macrocarpa (bur oak) is present in the drainages of the Cottonwood and Des Moines rivers. Populations along the Cottonwood River extend onto riparian terraces where the canopy has been lost due to Dutch Elm disease. The number of associated spring ephemerals declines from southeast to west and northwest. Sanguinaria canadensis (bloodroot) and Allium tricoccum (wild leek) most consistently co-occur with T. nivale throughout its Minnesota range. Similar spring groundlayer associates are found in slope and floodplain terrace habitat, but floodplain terraces tend to be dominated by Laportea canadensis (wood nettle) later in the summer.
Biology / Life History
Trillium nivale is a perennial spring ephemeral; one of a group of plants that emerge, photosynthesize, flower, and set seed during the brief time when leaves of forest canopy trees have not fully developed. The species thus experiences sunny or partially shaded conditions at the time it is above ground, but later in the summer, the habitat may be heavily shaded. Although March flowering is reported in states east of the Mississippi River, Minnesota populations of T. nivale tend to flower from early April through mid-May. Individual plants are reported to reach sexual maturity at three and to live to at least eight years of age (Nesom and La Duke 1985). The longevity of populations is not known, but one Minnesota population was first observed in 1908.
Conservation / Management
Minnesota is one of the strongholds for T. nivale, but many of the largest populations occur in places where they are subject to rapidly expanding residential development and/or increased recreational use. Although only one population is in the Twin Cities Metropolitan Area, eight of the state's populations are in the rapidly expanding development perimeter of Mankato. Several more are wholly within the city limits of Jackson. In all these cases, development, intensive land use, direct conversion to lawns and recreational uses, and potential expansion of Rhamnus carthartica (common buckthorn) and Alliaria petiolata (garlic mustard) are potential threats. The impact of canopy loss is not known, but may vary between harvested slope forests and floodplains where Ulmus spp. (elms) have died but other tall vegetation shades the ground during the summer months. The potential impact of large white-tailed deer (Odocoileus virginianus) populations, trail erosion, and exotic earthworms is unknown, but all three factors are known to be present at some T. nivale sites. The potential threat of competition from R. cathartica and A. petiolata is increasingly likely.
Conservation Efforts in Minnesota
Very few T. nivale populations occur in sites where protection of rare features is the primary land management goal. The three populations that occur in state Scientific and Natural Areas or private preserves are among the state's smallest. Three populations occur in county and local parks, one of which supports the largest overall population in the state. At the present time, no special management on behalf of the species is known to occur at any of these sites. Potential threats from Alliaria petiolata and erosion are greatest in areas where trails pass through T. nivale populations.