Twig age 2.0: A novel and reliable method to track variation in ungulate browsing over species, sites and time

Abstract Ungulate (deer) browsing threatens forest diversity and tree regeneration, but without efficient and reliable methods, we cannot monitor these impacts. Traditional methods mostly measure the size, density or growth rates of seedlings/saplings, all of which respond sensitively to local conditions, inflating variation and potential bias. The twig age method uses bud scale scars to estimate the time interval between bouts of browsing. The original method measured browse only on maples ( Acer ) at a single site. Here, I here extend the method using twig age data from 12 tree species distributed across 60 sites in the Upper Midwest, USA to assess its efficiency and compare how deer browse impacts vary across an exclosure fence, among species, across sites, and over time. Because deer prefer to browse on certain species, I develop and compare two methods to adjust for deer preferences to ensure accurate comparisons among sites that differ in species compositions. The twig age method is highly efficient, requiring less than an hour and minimal equipment to obtain an informative sample (90+ saplings). Exclosures confirm that twig ages respond sensitively and rapidly to deer browsing with more power than methods based on seedling/sapling size. Twig ages also covary with height under deer browsing, adding another indicator. To adjust for differential browsing among species, one can use either ratios of twig ages to an internal standard (e.g. Acer saccharum ) or 2‐way models to adjust for species effects when comparing sites. These approaches yield identical results. These adjusted twig age data provide a reliable and accurate way to measure differences in deer browse impacts over species, sites and time. Twig ages provide a powerful tool to sensitively track variation in local deer impacts over time and across sites and habitats. Its efficiency means forest and wildlife managers can rapidly expand deer impact monitoring programs at low cost. Applying this method broadly over successive years should soon identify adjusted twig age thresholds for predicting successful sapling recruitment and tree regeneration.