Winter Tree ID

Identifying deciduous trees in the winter can be challenging (unless you live in a climate where trees don’t have a need to shed their leaves, in which case you don’t really have a winter.  Or if you’re looking at an evergreen)- during the growing season many people rely partially on features of the tree that are no longer readily visible when the tree is dormant.  But with a little bit of practice, winter ID of trees can actually be a more reliable way to discern between species that are closely related: using leaf shape alone can lead to misidentification as leaves have a certain level of phenotypic plasticity (that is, leaves don’t always look typical).

More practically, it’s handy to be able to ID a tree in the winter for planning purposes!  What tree is that in the corner of the yard?  If you can’t remember off the top of your head, you can either wait ‘til leaf out to see what kind of leaves sprout, or you can ID using the bark and the twigs!

The things you’ll need in order to accomplish the ID are a section of twig from out at the tip, and a look at the bark of the tree.  Until you’ve got a ton of practice, it might also help to have an ID key.  The Peterson Field Guide to Trees and Shrubs has a handy winter silhouette guide in the front section, and I regularly use the “Deciduous Trees of Minnesota: a Winter Key” when stumped (tree pun!), put together by Thomas Morely from the University of Minnesota Botany Department circa 1975.  Is it new?  No.  Does it need to be?  Also no.

So let’s get started!  I chose a tree outside of the Tree Geek HQ (the tree specifically is a Green Ash.  I knew this going in, so it’s kind of like cheating- but this is science, so we’ll actually call it a control).   To begin with, lets look at the bark.  It’s a medium furrowed bark, with a grayer color towards the root flair- fairly typical of a Fraxinus pennsylvanica– but dark furrowed bark is also found on a lot of other trees.  We’re going to have to dig deeper.
DSC_1311When ID-ing a tree mid winter, the most important features are out at the tip of the twigs.   Make sure you find a good sample with a representative terminal bud (the set of buds growing at the very tip of a twig) and axillary buds (any buds growing below that).
DSC_1317This is a pretty good representation of a Green Ash twig.  You can see that the twig has a terminal bud that is actually composed of three buds.  Also notice that the axillary buds are growing in an “opposite” habit, meaning that the leaves of an Ash tree will grow on the opposite side of the branch from each other, rather than an “alternate” pattern- some examples of trees with alternate axillary buds include Locusts, and Oaks.  Botanists call these leaf growth patterns phyllotaxis– the greek for “leaf” and “arrangement”.  Another important aspect of the buds is the scale pattern.  Ours appears to be rough- which is in line with the genus Fraxinus (these things are tiny but still fairly observable).  The next thing you’ll want to look at the bundle scar underneath the axillary buds (or where an axillary bud used to be):
DSC_1324Bundle scars are the exposed veins where the tree used to draw energy produced by the chlorophyll in the leaves, and they should be relatively easy to spot.  On this Ash, looking at it in person it’s pretty apparent that there is only one visible scar- very typical of Ash species, and it will likely be crescent or dot shaped1.

So at this point we’ve figured out that we’re looking at an Ash Tree, and getting it down to species is a little bit more challenging.  You might even need to take some measurements.  According to Thomas Morely’s Winter Key, a main difference between White Ashes (F. americana) and the other Ash Trees in our range (Minnesota for the Tree Geeks HQ) is that the crescent shaped bundle scar is 1up to 1.5 mm deep in F. Americana versus .8mm deep in F. pennsylvanica and F. nigra.  We’ve got a relatively flat crescent.  That means we’ve got the latter.

Alright.  We now know we’re looking at either a Black or Green Ash.  To recap, the axillary buds are growing in an opposite pattern, there is but one apparent bundle scar, and the bundle scar is a relatively shallow crescent shape.  Time to go back to the bark!  Our tree is relatively grayish, with darkening bark towards the base:


Classic Green Ash.  Green Ash trees also tend to grow in relatively well drained areas.  Black Ashes tend to grow in damper, less well drained areas.  This tree is on a rise between two parking lots outside our offices, nothing wet about it (except the snow, which is technically water).  So there you have it- Green Ash.

Not all trees are that easy, and some trees will be really hard to identify unless you have the capability to take the a sample somewhere and dissect it- most people don’t want to cut tiny twigs in half in the frozen north, anyway.  Another key feature for twig identification is known as the “pith”:

It’s that center part of the twig in white.  Some trees have a solid pith, and some have what’s known as a chambered pith, where there are hollow areas (chambers!) between the spongy tissue inside.   Walnuts are a common species with a chambered pith, if you’re dying to cut some trees open.  Another feature you might have to use is the lenticels, or pores, on the twig- they appear as little white bumps shown below.  The relative distance between buds and new versus old growth is also a good indicator of which species you may be looking at within a genus.  It can get sort of complicated, but practice makes perfect!

 So go out and practice, if you don’t have Green Ash in your area, keep your eyes peeled.  We’re going to be running a series of a winter ID guides as the season progresses.  Give us a shout with any questions you might have, either in the comments section or on our Tree Geek Hotline!  We’d also love to hear if you have any suggestions for trees you’d like more information about identifying once the leaves have dropped!  Thanks, fellow Tree Geeks!

1.Morely, Thomas.  “Deciduous Trees of Minnesota: A Winter Key”.  Department of Botany, University of Minnesota.  St. Paul, Minnesota.  1975

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