Here’s a tree that will soon disappear from western Pennsylvania, a victim of the emerald ash borers that arrived here in 2007.

White ash (Fraxinus americana) is easily identified by its twig with the chocolate-brown bud.  The twig is stout, the leaf buds are opposite each other, the leaf scar is a horseshoe shape under each leaf bud, and all the buds are chocolate brown.   (Click here for definitions of twig anatomy.)

It’s easy to find these buds in our area.  There are many white ash seedlings now because the trees have put out a lot of seed while they’re under attack.

Opposite leaf buds are a good marker for the ash because most trees have alternate leaves.  The main species with opposites are maples, ashes, buckeyes and dogwoods.  I learned to remember opposite leaves with the acronym MAB DOG (Maple, Ash, Buckeye and Dogwood).

White ash bark is distinctive too.  Its deeply ridged and the ridges join to form long diamond shapes as shown below.

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Unfortunately, larvae of the emerald ash borer kill the tree by tunneling under the bark and damaging the phloem and xylum.  Often this causes the ridges to slowly separate from the bark.  Woodpeckers hear the larvae (amazing!) and chip away at the ridges to get at the bugs.  The result is that a dying tree has pale patches where the ridges fell off.  Infected trees try to survive by sending out sprouts near the ground.  You can see both effects on the trunk below.

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If you examine the chipped bark closely you may find the D-shaped exit hole of the emerald ash borer.  (Thanks to Dianne Machesney for this photo.).

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Learn the white ash now.  Sadly, it won’t be with us much longer.

(photos by Kate St. John, except for the one noted by Dianne Machesney)

Today is the first entry in this winter’s Wednesday tree series.

Though I mentioned we would identify trees by their twigs I can’t resist starting the series with a tree that’s really easy to identify by its bark.

This is the Common Hackberry (Celtis occidentalis), a member of the Hemp family.  It produces small berries that ripen in autumn.  Some berries fall to the ground, others persist on the tree into the winter and provide a good food source for birds.

For me the easiest way to identify young hackberry trees is by their bark.  (Bark is at eye level!)

Hackberry bark looks as if someone glued lumpy pie-crust ridges onto the originally smooth gray surface.  You can see these odd ridges in the photo above.

A second very distinctive trait is the witch’s broom, easy to see when the leaves are off the trees.  Not all hackberries have these bundles of malformed twigs but when you see them in combination with the lumpy bark you can be sure you’ve found a hackberry.

Here’s a close-up of a witch’s broom.  Not only do the twigs clump at one spot but there are woody lumps at their base.

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As the trees mature the pie-crust lumps grow farther apart and sometimes look as if they’ll peel off the trunk.

Hackberries are easy to find in Schenley Park, especially near the Greenfield Bridge.

(three photos by Kate St. John.  Photo labeled UGA5188076 is by Whitney Cranshaw, Colorado State University from Bugwood.org )

On my walks through Schenley Park I’ve started to notice the trees again.  Not that they’ve been missing — far from it! — but ever since the songbirds arrived last spring the trees were mere bird accessories, places for birds to find food, nest and perch.

Now the migratory songbirds have left and the deciduous trees have gone through a great transformation.  They’ve dropped their leaves and seeds and stripped down to trunks and twigs.  They’re easier to see and they’re easier to identify.

Back in the 1990’s I took a class on winter tree identification at Chatham University’s Rachel Carson Institute where I learned that twigs and bark tell you almost all you need to know to identify a species.  In class we used the booklet pictured above, the Winter Tree Finder by May Thielgard Watts and Tom Watts.   It’s easy to carry and contains a step by step key for identifying twigs, much like Newcombs Wildflower Guide does for flowers.

In late October when I noticed the trees again, I got excited and took my Winter Tree Finder to Schenley Park.  Soon I began taking pictures of bark and twigs.  It didn’t take long before I’d dreamed up a Wednesday series on Winter Trees.

Today is the first entry but it’s just an introduction.  Before I start showing you the trees next week you may want to do two things:

1. First,  familiarize yourself with the anatomy of a twig (shown below).  I’ll be using the terms highlighted on this illustration from Clemson University.  Click on the twig picture to read the definitions and learn more.
2. Second, you may want to get your hands on the Winter Tree Finder so you can explore for yourself.  (You can buy it on Amazon by clicking on the book cover above.)

Next Wednesday I’ll show you the first tree in the series, all of which grow in southwestern Pennsylvania.  I know they do because I found all of them in Schenley Park.

(Cover of Winter Tree Finder from Amazon.com.  Image of twig anatomy linked from Clemson University’s Familiar Trees of South Carolina)