Trille penché

As with other North American members of the genus Trillium, overabundance of White-tailed Deer (Odocoileus virginianus) is the primary threat impacting the species. The species is long lived and is preferred by White-tailed Deer, with herbivory concentrated on the largest, most reproductively active individuals within a subpopulation (Augustine and Frelich 1998). In southeastern Minnesota, heavily browsed forests show herbivory rates ranging from 7 - 46% of all plants and 24 – 77% of reproductive plants (Augustine and Frelich 1998). This level of grazing intensity has been noted to result in long-term declines in the rate of flowering following high intensity grazing over a period of more than five years.

Though the herbivory patterns observed in Nodding Trillium subpopulations are relatively localized, White-tailed Deer populations have increased to unprecedented levels throughout much of eastern North America, including large portions of Nodding Trillium’s range (Russell et al. 2001). The preference for Trillium species, particularly those in reproductive stage classes, among browsing White-tailed Deer has been noted in a range of locations including Illinois, Wisconsin, Minnesota (Augustine and Frelich 1998), and Pennsylvania (Knight et al. 2009). Though not documented specifically for Nodding Trillium, high levels of herbivory caused by overabundance of deer have been shown to cause population declines in the related species T. grandiflorum (Augustine and Frelich 1998, Goetsch et al. 2011, Knight et al. 2009, Rooney and Gross 2003, Webster et al. 2005) and T. catesbaei (Jenkins et al. 2007). Rates of decline exceeding 0.5% annually have been documented for T. grandiflorum in several areas of its range (Augustine and Frelich 1998, Knight et al. 2009). Comparable rates of decline would qualify Nodding Trillium for threatened status under criterion A.

Historically, habitat loss has restricted the range of the species. At the western extent of the species’ range, it is present only in fragmented remnant mesic forest stands with much of its previous habitat having been replaced by agricultural fields (Dyer and Baird 1997). At its eastern margin, expansion of urban areas and disturbance associated with human activities has caused habitat loss over the course of the past century. Habitat loss and fragmentation is likely to exacerbate the impact of herbivory by White-tailed Deer through increase of deer populations associated with increased carrying capacity of forest environments bordering agricultural areas, and decreased capacity for White-tailed Deer population management in urban areas (Jenkins et al. 2007, Jenkins and Webster 2009). Competition with invasive plant species is not thought to be severe, though garlic mustard (Alliaria petiolata) has been observed at sites in New York and Michigan.

The species is considered Endangered in Illinois and Indiana (where some sources list the species as extirpated), and Exploitably Vulnerable in New York (USDA, NRCS 2019). In Illinois, the species is known to occur at a single location in Cook County where habitat restoration efforts are geared towards conservation of the species (Hintz 1995). Additional research is necessary to determine the population trends and impact of White-tailed Deer overabundance in the species’ range. Though it is not specifically protected in much of its range, the species incidentally occurs in a number of protected areas (IUCN and UNEP-WCMC 2019). The species is frequently observed in several conservation areas in Michigan and in the western portion of the Adirondack Forest Preserve in New York. The species is included in ex situ collections at 32 sites (BGCI 2019).