DOI: 10.5061/dryad.8gtht76z5
title: Twig age data from 12 tree species sampled in S & N Wisconsin and Upper Michigan

description: 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.
description: 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.
description: # Twig Age 2.0 Supporting Data
 [https://doi.org/10.5061/dryad.8gtht76z5](https://doi.org/10.5061/dryad.8gtht76z5) ## Description of the data and file structure These data sets are in 3 groups: 1. Data from Huron Mountain Club N of Marquette, MI:  TwigAgeData-HMC-Exclos-2022.csv & HMC-AcerTwigAge-2019.csv 2. Data from 10 sites in S Wisconsin:  TwigAge-2019-S.Wisc-11-Sites.csv 3. Data from 40 sites in N Wisconsin:  TwigAge-N-Wisc-Data-4289-sdlgs-40-sites.csv & N-WI-40-Sites-Locs-Time.csv Note regarding Missing Values:  In the data, some values are missing.  In such cells, we use the code "NA" to stand for "not available."  These data (e.g. a second twig on some saplings or the time it took to sample a particular site) were never collected. Note regarding units for Twig Age:  These are always in years. ### Files and variables #### File: TwigAgeData-HMC-Exclos-2022.csv **Description:**  Data from the deer exclosure near Mummy Mountain at the Huron Mountain Club ##### Variables * Location:  site location * Exclos:  whether the seedling was IN or OUTside the fenced exclosure * Species:  the species of seedling sampled * Height:  height to tallest twig in cm * Twig Age:  the age in years of the twig based on counting terminal bud scale scars (1 - 5 years) * Browse:  whether the seedling shows signs of fresh browse #### File: TwigAge-2019-S.Wisc-11-Sites.csv **Description:**  Twig age data from the 10 (not 11) sites in S Wisconsin ##### Variables * Location:  site name * Date:  date data were collected * Genus:  of tree seedling sampled * Species: of tree seedling sampled * Taxon:  genus or species name * Height (cm):  of seedling sampled * Twig 1:  age in years of twig 1 * Twig 2:  age in years of 2nd twig on that seedling * Browse?:  any fresh browse present (on this year's twigs)? * AveTwigAge:  mean of Twig 1 and Twig 2 #### File: HMC-AcerTwigAge-2019.csv **Description:** Twig age data obtained in May, 2019, from near the exclosure at the Huron Mountain Club ##### Variables * Exclosure (0 = outside the fence, 1 = inside the fence) * Genus of sapling measured * Species sampled * Height of sapling in cm * Twig 1 (yrs) - age of first twig sampled in years * Twig 2 (yrs) - age of second twig sampled in years * Browse? - whether sapling showed contemporary browse damage #### File: N-WI-40-Sites-Locs-Time.csv **Description:**  twig age data from 40 sites dispersed across N Wisconsin ##### Variables * Site:  site name * Site:  site name * Property:  Landowner * Latit North:  Latitude (decimal degrees)  * Longit West:  Longitude * Azimuth 1:  orientation of transect for planted seedlings * Month:  Month data were collected * Season:  season data were collected * Twig Age Date:  date of data collection (e.g., 14 Mar. 2023) * Lat+Long:  Sum of latitude and longitude * Minutes:  how many minutes it took to sample the site * N:  how many seedlings were sampled * Mean TA:  mean twig age * Mean Ht:  mean seedling height * ATA1 - TA LS Mean:  least-square means for that site (from 2-way ANOVA of twig age over site and taxon) * SM Mean TA:  mean sugar maple twig age at that site * ATA2 - Site Wtd SM Mean:  2nd adjusted twig age estimate, based on ratios to sugar maple twig age * Browse Index:  overall index of browse intensity at that site * Comm Palatab:  Community palatability score = 5 - adjusted twig age #### File: TwigAge-N-Wisc-Data-4289-sdlgs-40-sites.csv **Description:**  raw twig age data from the 40 sites in N Wisconsin ##### Variables * Site Code:  code from original site selection process  * Locale Name:  name of site * Month:  month of data collection  * Genus:  genus of seedling sampled * Species:  species of seedling sampled * Taxon:  genus or species name (some species lumped) * Height (cm):  height of seedling * Twig 1:  age in years of 1st twig sampled * Twig 2:  age in years of 2nd twig * Browsed:  evidence of fresh browsing on seedling? * TwigAge:  mean of twig ages 1 & 2 * \# Twigs: number of twigs sampled ## Code/software n/a ## Access information Other publicly accessible locations of the data: * none Data was derived from the following sources: * All data were collected personally by the author