The Hackberry: A Lesson in Bark

Do you remember looking at the hackberry tree last fall?  It's the big tree in our outdoor classroom by the entrance to the parking lot.  The hackberry is the tree whose leaves had hackberry leaf galls.  I promised I'd come back to the hackberry because of its unique bark.

The stem (ok, trunk) of our hackberry tree with its great hackberry bark.

The hackberry is generally an unnoticed tree in most parts of the U.S., but here in Nashville, it's so common it could be our city's official tree.  Hackberries have a huge influence on our lives here.  They provide wonderful shade for us in the summer and help keep our city air cooler and cleaner.  Hackberries can grow big and strong in the toughest of urban conditions as long as there is enough water.  They also are among the most common wild tree outside the city.  Hackberries provide lots of wildlife habitat and food.  Hackberries often have hollow portions which provide excellent homes for birds and small mammals.  The trees' berries, which are technically edible to humans, are a good source of winter food for birds.  You may have noticed when the late winter robins and starlings arrived a couple weeks ago that there were lots of purple bird bombs (bird droppings) on cars and sidewalks.  Most of those were the result of birds eating hackberries.  Nashville has felt the downside of hackberries too.  Hackberries can be a bit brittle, especially if they are hollow inside.  Tornadoes and great storms sometimes break off hackberry limbs, which can land on power lines or houses. 

Close-up of the edge of a ridge in hackberry bark showing the layers unique to hackberry bark.

To really appreciate the hackberry, you have to get up close and personal to it.  Specifically, look at its bark.  The bark of hackberries varies widely from almost completely smooth to almost completely bumpy, but there are two things it always has in common.  First, hackberry bark is always the same steely whitish gray color.  Second, the bark always has at least some bumps or ridges, which are made of layers and look somewhat like topographic maps.  No other bark that I have ever seen has layered bumps like this.

Our giant hackberry tree with very rough bark for a hackberry.

We have investigated bark before when we looked at scars in the sourwood tree, but let's dive in a little deeper - there's more good stuff here.  Bark is a plant organ.  Organs are structures that accomplish some function in an organism.  You may be more familiar with animal organs like the brain, the stomach, the skin, the lungs, etc.  Bark has two basic functions in plants: it protects the stem and it transports food all around the tree.  We have two separate organs for protection and food transport in our bodies.  Our skin provides protection from the outside, and our blood vessels transport blood around the body.  Blood carries digested food and many other things all around the body.  Let's investigate bark's two functions a little closer.

First: protection.  Bark seals off the tree from the environment.  It prevents the tree from drying out in the heat or getting soggy in the rain, just like our skin protects us.  Bark also keeps out insects and diseases, also like skin.  The stuff in bark that forms a seal against the world is called cork.  Cork is a spongy, softer material found in most types of bark, and it is waterproof due to the presence of a wax called suberin.  Some trees make more cork than others, and humans harvest cork for sealing bottles from the corkiest tree - the cork oak.  In most trees, the cork is interspersed with harder material in the outermost part of the bark.  The ridgy bumps as well as the smooth parts of hackberry bark both contain enough cork to protect the hackberry tree.

The second function of bark is food transport.  Trees and plants use the sun to make their energy in a process called photosynthesis.  Photosynthesis is the name for the chemical reaction that plants do to make sugar, and that chemical reaction is powered by sunlight.  Plants' basic food is sugar, which they can use for energy (just like you do) or for building other necessary plant parts.  Trees do photosynthesis in their leaves, but they need food in all parts of the plant.  The sugars from photosynthesis combine with water in the tree to form a liquid called sap, and liquid sugar is easy for trees to move around.  Tiny tubes in the bark transport dissolved sugars in the form of tree sap from the leaves to the rest of the plant, which is very similar to how the tiny tubes called blood vessels transport blood (which contains dissolved sugars too!) all around your body. 

If you've ever tasted maple syrup, you have tasted the concentrated tree sap taken from maple bark.  Maple syrup is sweet because maple trees' leaves did photosynthesis using the sun to make sugar.  Unfortunately the way I've explained this makes me think of maple syrup as tree blood, but that's really not quite true.  Blood is way more complex than sap, and blood has many more functions in our bodies than sap has in trees, but that's a story for another day.