By Wendy Diaz EMGV
If you are hiking in the forest this winter or even ‘intentionally walking your own yard’ (recent post on https://durhammastergardeners.com) and want to identify a tree, the best way to identify the species in the wintertime is to use the morphological features of the tree that botanists utilize. To start with, all you need is a twig!
I recently attended a Tree Identification In Winter Workshop by Matt Jones, Extension Agent, Horticulture at the Chatham County Center in Pittsboro where I was introduced to such precise methods and I practiced identifying twigs using the ‘tools of the trade’1. To differentiate between character traits of tree species one can use the North Carolina State University (NCSU) Dichotomous Key (Key) developed by Dr. Alexander Krings for our particular geographical region of the Piedmont2. Dichotomous keys start by presenting the user with two sets of plant characteristics in a couplet form, then the user has to select one of the two choices, which more accurately describes the specimen in hand1. The user’s choice leads them to the next couplet and so on until they reach a species name and there are no more choices to make. The Key gives the species name in Latin to avoid confusion from the many common names a tree species can have. An easy way to obtain the common name if you are not familiar with the Latin species name is to use the online NCSU Plant Toolbox3.
The features of the tree which are visible in winter and which are most diagnostic of each species are based on the woody plant morphology which does not include the bark of the tree. Bark can be diagnostic for some trees with very pronounced characteristics like the smooth bark of a beech (Fagus grandiflora) for example but depending on the age of the tree or the position of the bark on the trunk, different bark texture may be observed on the same tree which can lead to confusion and it is not always diagnostic. For the typically difficult genus Quercus or oak, it also helps to look for acorns around the base of the tree as the acorn’s morphological features can differentiate between species.
The main morphological features included in the Key are the following: the arrangement and position of buds, bud scales, arrangement of leaf scars, pith anatomy, number of vascular bundle scars, stipular scars and armament. A hand lens, a set of pruners and a bright area to work in will also help in discerning these relatively small species-specific character traits on the twig. It is essential that the user know the vocabulary4 of the Key, such as but not limited to:
Petiole – The stalk that joins the leaf to the stem.
Alternate – The arrangement is at alternating points from one side of the stem to the other or staggered.
Opposite – The arrangement is on the same spot on stem but on opposite sides of the stem.
Leaf scar – A visible thickened crescent mark on stem where the leaf was attached.
Pith Anatomy: The core of the stem can be homogenous (solid and uniform), diaphragmed (sections), chambered (hollow sections) or excavated (hollowed out).
Diaphragmed: Horizontal breaks in the core of the stem and each section is filled with material.
Bud scale: A modified leaf that forms a protective covering over the bud.
Valvate bud scale: The scale forms two parts of the coating like a clamshell.
Bud Scale Scar: The concentric rings formed by bud scales from the previous year’s terminal bud.
Stipular scars: A pair of appendages found on many leaves where the petiole meets the stem, tiny and attached around the stem and can be slit-like or ring-like.
To practice my newly acquired knowledge, I cut a small twig from a tree in my front yard and brought it inside with, to my surprise, a caterpillar.
The first step was to determine to which structural group the tree belongs by deciding if the leaf scars were alternate or opposite. The leaf scars were alternate .
For the second step, the stem had to be sliced longitudally so it’s pith anatomy could be examined. The second couplet asked if the pith was chambered/diaphragmed or homogeneous. The pith of this stem was diaphragmed so the next step using the Key was to proceed to Group 2 where the couplet asked if the stipular scars were encircling the twig or if they were absent.
The stipular scars were clearly encircling the twig so that lead to the next couplet.
This couplet described the bud as either cap-like and pointed apex or valvate and rounded apex. These buds were valvate (two parts) with a rounded apex guiding me to the final lead, which is the species name of Liriodendron tulipifera or Tulip Poplar!
The NCSU Dichotomous Key can be a little daunting if you are not a botanist, especially if the tree species you are trying to identify requires multiple couplets and it is at the end of the Key such as Carpinus caroliniana (American Hornbeam). I found clicking on the + symbol next to each couplet, which links to a photo of an example image, is a great visual aid and helped me on whether or not to continue on to the next couplet or backtrack. Practice makes perfect, but if you get frustrated with the Dichotomous Key, you can always wait for spring and the leaves to come out or identify your trees with leaves by attending the companion workshop offered by Matt Jones in September 5.
Online tool: NCSU Botanist’s Little Helper: 2. https://projects.ncsu.edu/cals/plantbiology/ncsc/TwigID/
Online tool: NCSU Plant Toolbox: 3. https://plants.ces.ncsu.edu
Terminology: Plant Identification Terminology An Illustrated Glossary James G. Harris Melinda Woolf Harris second edition
Advanced reference: Woody Plants of the Southeastern United States: A Winter Guide, Ron Lance, University of Georgia Press