Effects of chronic herbivory and historic land use on population structure of a forest perennial,Trillium catesbaei

Question: How have long-term herbivory and past land use impacted the population structure of Trillium catesbaei, a long-lived rhizomatous herb? Location: Western Great Smoky Mountains National Park, Tennessee, USA. Methods: We examined T. catesbaei populations at three sites: (1) Cades Cove (CC), an area of intensive historic land use that has been maintained as open fields and woodlots with a history of chronic deer herbivory, (2) Whiteoak Sink (WOS), a reference area with similar land-use history, geology, and soils that has succeeded to closed-canopy forest with relatively low levels of deer herbivory, and (3) Leadbetter Ridge (LBR), an area of primary forest that has never received significant anthropogenic disturbance. Trillium catesbaei is the most common Trillium species at the three study sites, but smaller in stature, shorter lived, and more of a habitat generalist. Results: Chronic herbivory in CC has created a highly-truncated age structure with no plants older than 9 years, while plant ages at the other sites were more evenly distributed. Compared to WOS, plants in CC were younger at a given height and more likely to flower when younger. Across all life stages, populations at CC contained 68 × fewer plants than WOS. The age structures of WOS and LBR were similar. Compared to published age estimates for other Trillium species, our results suggest that T. catesbaei is relatively short lived within the genus. Conclusions: Chronic herbivory had pronounced effects on the population structure of a perennial herb. Other long-lived herbaceous species may exhibit similar truncated age structures and flowering by younger and smaller plants. Habitat generalist species within a genus, such as T. catesbaei, that are able to reproduce more quickly may persist longer under chronic herbivory. However, chronic herbivory has likely caused the loss of herbaceous species from CC and may eventually cause the local extirpation of T. catesbaei populations.