Twig Age 2.0: Adjusting twig age data for differences in palatability among taxa to track ungulate browsing over sites and time

Abstract

Ungulate browsing threatens forest tree regeneration and diversity across forests in North America and other temperate regions but is inadequately monitored. This reflects both inadequate commitment and the lack of efficient, reliable methods to accurately track these impacts across sites and over time.The twig age method estimates time: how long twigs grow before being browsed, providing a direct measure of herbivory. The original method aged twigs on maple (Acer) seedlings, but it works well in other deciduous species. Given ungulate preferences for some species, it is important to adjust twig ages to correct for these.Using multiple datasets from many species and sites across the upper Midwest, I confirm the power and sensitivity of the twig age method for assessing and comparing white-tailed deer impacts over time, among species, and across sites. Twig ages respond differently to seedling heights when deer are present, providing an additional indicator. I extend the original method to rank and adjust for differential browsing among species in two ways: using least square means from statistical models and using ratios to adjust values to anAcer saccharumstandard. These adjustments match, supporting the use of either to accurately track shifts in browse pressure over time and across sites even when these differ in species composition.The adjusted twig-age method is highly efficient, allowing a sole worker to score 100+ seedlings/saplings within an hour to accurately gauge real-time ungulate impacts at a site. The reliability, power, and versatility of adjusted twig age data set a standard for assessing browse pressure and provide an efficient vehicle to expand programs to monitor deer impacts across landscapes.