Twig age data from 12 tree species sampled in S & N Wisconsin and Upper Michigan

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 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.

Methods
The twig age method is simple, non-destructive, requires no advance set-up, and rapid to implement. After identifying a study area with saplings present, observers select which deciduous species to sample (the method has yet to be extended to conifers). Site data include location, observations on canopy composition or forest type, and start/stop times. The observer then moves through the site consistently, avoiding recovering the same ground. Observers record twig ages on saplings of the taxa selected between 20-200cm tall. A sample of 60-120 saplings typically requires an observer to cover ~0.5-1ha in about an hour. The area covered and speed of sampling depend on the design and intention of the study and the density of suitable saplings. If these are scarce, observers may need to sample most. Where they are abundant, observers can move more freely sampling only a subset. For more precision, observers can sample additional saplings. To cover more of a region or compare sites that differ in some way, observers can add more plots.

For each sapling, observers record sapling height and age two live terminal twigs available to deer (between 20-180cm high). Using two twigs from different branches reduces sampling variance but the sapling is taken as the unit of replication. To age twigs, observers begin at the terminal end (supporting green leaves) then follow the twig back to its mother branch, counting the number of terminal bud scale scars that encircle the twig. Twig age is then one plus the number of terminal bud scale scars counted. Twigs lacking such scars are one-year old while twigs with one terminal bud scale scar are two. Aging extends to a maximum of 5 years as older twigs are both rare and more difficult to age accurately. Parent branch ages and any evidence of browsing they may show are ignored. Observers may also wish to record fresh browsing on this year’s twigs (those data are not used here). Data for each sapling thus consist of date, species, height, two twig ages, and (optionally) the presence of recent browsing.