A new rapid and efficient method to estimate browse impacts from twig age

- Twig age provides an efficient way to assess deer impacts and habitat conditions.
- Terminal bud scale scars provide estimates of the interval between browsing events.
- The method is simple, quick, and sensitive to deer density and species of Acer.
- The method has higher power than measuring sapling height or recording browse.

High densities of white-tailed deer restrict the regeneration of tree species, reduce understory cover and diversity, enhance invasions of exotic species, and facilitate the spread of human and deer diseases. Deer managers often base management decisions on estimated deer densities and carrying capacities, generating controversy. It may be simpler, cheaper, and more appropriate to manage deer populations in relation to their effects on local habitat conditions. Here, we describe a method based on the mean ages of woody twigs on saplings exposed to browsing. Growth can be traced back at least five years on deciduous species using terminal bud scale scars, generating estimates for the minimum interval between browsing events. We applied this method to three species of maple (Acer) growing in canopy gaps in- and outside a 5-7 year-old fenced deer exclosure in a mature forest in upper Michigan. Maples are palatable to deer but resprout readily after browsing. The method was simple and efficient to implement in the field with negligible among-observer variation. Mean twig age responded sensitively to differences in deer impacts among species, across the fence, and over time (all p < 0.002). Acer rubrum and pensylvanicum had lower mean twig ages than A. saccharum reflecting higher rates of browsing. Twig age showed a larger deer effect size and r2 values than the sapling height or browse indicators (Cohen's d = 34.85 vs. 1.39 and 9.55 for height and browse; r2 = 0.556 vs. 0.154 and 0.331). Twig ages declined with height outside the fence while rising inside, providing a second independent indicator of deer impacts. Twig ages provide a direct indicator of deer browse on regenerating trees with lower sampling variance and higher independence from local environmental conditions than height or browse incidence. We should next test the twig age method in other contexts and species to confirm that it is an efficient, sensitive, and reliable indicator of deer impacts and habitat conditions.