Losing the slow race: How deer hinder growth, survival, and regeneration of juvenile hemlocks (Tsuga canadensis)

Abstract

White-tailed deer can limit both growth and survival in slow-growing, shade-tolerant tree species. We monitored 1,100 juvenile hemlocks (Tsuga canadensis) for up to 20 years at 54 sites in northern Wisconsin and Michigan to document causes of recruitment failure. These sites include public and private forestlands and First Nation reservations with distinct approaches to forest and wildlife management. We assessed deer impacts experimentally by protecting half of the juveniles from deer and observationally by tracking unprotected individuals at sites with various deer densities. We estimated local deer and hare presence and density using repeated pellet counts, which best predicted the survival and growth of juvenile hemlocks. Caged individuals out-survived unprotected ones (35.2 vs. 15.0y estimated half-lives). For the fastest-growing quartiles, transition times through the “molar zone” of vulnerability to deer browse (heights 5–200 cm) were 33.4 and 77.7y for protected vs. unprotected juveniles. Deer browsing strongly reduces both survival and growth, allowing us to compare estimates of these processes and their ratio among sites and cohorts. The “Regeneration Ratio” of half-life to growth averaged 1.06 for the top quartile of protected hemlock juveniles vs. 0.19 for juveniles exposed to deer. Ratios below 1.0 suggest risks to hemlock regeneration while values below 0.50 signal likely failure. Forest and wildlife managers must limit deer impacts if hemlock trees and stands are to remain viable components of the region’s forests.