Trille blanc

The primary threat to White Trillium subpopulations is the large increase in White-tailed Deer (Odocoileus virginianus) populations over the past 50 years (Russell et al. 2001). High levels of herbivory have been shown to reduce the number of reproductive plants relative to nonreproductive plants as deer preferentially browse on the largest, reproductive individuals in a given subpopulation (Anderson 1994, Augustine and Frelich 1998, Knight 2007). The impact of deer predation varies seasonally. Early spring herbivory results in recruitment failure for the year and decreases likelihood of flowering the following year where late season herbivory is less likely to impact fecundity in subsequent years (Knight 2007). Herbivory levels exceeding 15% of large 3-leaf stage and reproductive individuals within a subpopulation are predicted to result in population declines in western Pennsylvania (Knight et al. 2009).

Observed levels of herbivory in studied populations vary considerably. Rates of herbivory (among all age classes) within observed subpopulations range from 0% - 60% in western Pennsylvania (Knight et al. 2009). This study demonstrated that eight of the twelve observed subpopulations were under herbivory pressure exceeding the 15% threshold, indicating ongoing population declines. Subpopulations in northern Wisconsin and western upper Michigan have observed rates of herbivory among all size classes ranging from 0% - 30% (Rooney and Gross 2003). In southeastern Minnesota, annual rates of observed herbivory range from 24% - 77% of reproductive plants (Augustine and Frelich 1998).

Some uncertainty remains regarding the impact of White-tailed Deer overabundance across a regional scale. The areas where demographic studies specific to White Trillium have been conducted correspond to regions where the broader impact of White-tailed Deer overabundance is comparatively well characterized. By contrast, less is known about the impact of White-tailed Deer overabundance in the southern Appalachians (Russell et al. 2001). The population trends observed in the northern range of White Trillium are expected to occur in southern regions where impacts of overabundance are less well understood, but additional monitoring, and exclusion studies are necessary. Elk (Cervus elephus) reintroduction efforts in eastern North America have had varying levels of success over the course of the last century, but herds exceeding 10,000 individuals are now present in southeastern Kentucky (Popp 2017). Large herds in Kentucky have been noted to cause declines in Trillium subpopulations in the area and are likely to cause minor declines in other areas where Elk reintroduction efforts are successful. At present, declines attributable to Elk predation are thought to be very localized in scope.

Though the species is self-compatible (Irwin 2000), it may become pollen limited in fragmented forest habitats, and areas where heavy predation pressure reduces floral density, though this impact is not thought to alter population trajectories (Knight 2004). However, forest fragmentation is correlated with increases in deer populations and may result in greater predation pressure within remaining forested areas (Augustine and Frelich 1998, Horsley et al. 2003). Soil acidification resulting from industrial air pollution may cause significant and irreversible declines in populations, particularly on the Allegheny Plateau (Thompson and Sharpe 2005). Other threats to the species include habitat loss caused by development, competing invasive plants, logging, and mining. The intensity and  scope of these threats varies across the expansive range of the species. Rooting from feral hogs may impact the southern margins of the species' range.

The cumulative impact of threats to the species is not well understood. Population trends differ between subpopulations at a local level in response to predation pressure, but the threats impacting populations are pervasive throughout the species’ range indicating that range-wide declines are likely. The impact of declines due to White-tailed Deer herbivory is also not linear through time. Declines primarily impact reproductive individuals resulting in subpopulations dominated non-reproductive three-leaf stage individuals (Knight et al. 2009). Over time, reductions in the overall fecundity of the population resulting from demographic changes are likely to result in population decline.

The species is classified as Exploitably Vulnerable in New York (USDA, NRCS 2018). Though no specific protections exist for the species in much of its range, the species incidentally occurs in a number of protected areas (IUCN and UNEP-WCMC 2018).

Additional monitoring of population is necessary to better characterize the species' population trajectory. As the primary threat to the survival of the species is overabundance of White-tailed Deer, effective conservation measures must focus on management of these populations through changes to game management practices, or re-introduction of natural predators.