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. 

Winter, Spring, Summer, Abscission

It's happening everywhere right now!  Plants are chopping off their own organs, and they are piling up in yards all over town!  How come no one is worried about this epidemic of leaf death??!!  Well, it happens every year, so I'm pretty sure the plants are going to recover.  Still, why on earth would plants get rid of their most important organs?  That's what we'll address in today's post.

Closeup of leaf abscission zone on sourwood.

In the picture above, you can see the color difference between the pale pink of a leaf petiole (technical term for a leaf stem), and the bright red of a sourwood twig.  The line between those two differently-colored plant parts is called the abscission (ab-SIZH-uhn) zone. 

Fresh leaf scar where the abscission zone dissolved and the leaf fell off.

This time of year, the layers of abscission zones are changing.  One layer is hardening and filling up with a corky substance called suberin.  Suberin is waterproof and heals what would otherwise be a wound where the leaf falls off.  The leaf scar in the picture above is dry and not losing sap because suberin has sealed the wound.  The second layer in the abscission zone is made of thin-walled, weak cells that self-dissolve when the plant is ready to shed its leaves.  Abscission zones are usually quite noticeable this time of year on any plant that is in the process of losing its leaves.  Take a look at the next two pictures and find the abscission zones.

Sourwood leaves and petioles (stems) about to undergo abscission.

The abscission zone is at the base of the leaf petiole where it attaches to the twig.

It is extremely unusual for living organisms to shed any part of themselves except for the production of offspring.  Some lizards have tails that fall off to distract predators, and many plants lose their leaves in the fall - but I can't think of other examples of falling-off body parts.  Of course, most organisms constantly rebuild their outer-coverings and some organisms can replace body parts that are bitten off, but voluntary amputation is strange, indeed! 

The loss of body parts comes at a huge cost.  Plants work all summer to catch enough sunlight to grow more leaves and get bigger, and leaf abscission every fall would seem to waste that energy.   But as with the lizards that lose their tails, there are also benefits.  Lizards' bodies escape to live another day and regrow another tail.  Plants benefit from shedding leaves by not having to maintain those leaves during the winter.  Leaves are tender tissues that would become disfigured and die when frozen.  Try putting some lettuce leaves in the freezer over night and then take them out to thaw.  You will notice they turn to mush when they return to room temperature.  In order for plants' leaves to survive winter, they would have to be tough, like holly, magnolia or spruce leaves, which take much more energy to produce.  Plants with leaves that survive freezing grow more slowly than ones that shed their leaves.

Dogwood with remnants of chlorophyll along veins and lots of anthocycanins (red pigment).

Plants have many ways to minimize the costs of losing their leaves.  They move all available nutrients out of their leaves and down into their roots to save the food for the next growing season.  Leaves fall near the plant that grew them and decompose, releasing their nutrients into the soil and further increasing the amount of nutrients recovered by the plant.  In this way, deciduous plants grow their own mulch.  Some plants, like walnut trees, even deposit compounds in their leaves that suppress the growth of competitor plants as the leaves decompose throughout the winter and spring. 

Rainbow of fall colors.

As leaves senesce (slow down and die) in the fall, they turn the variety of amazing colors we are so familiar with.  Plants' normal color is green, due to the most important compound in the world: chlorophyll.  Chlorophyll is the substance in plants that allows them to absorb sunlight and use the energy from sun to make food, a process called photosynthesis.  In the fall, chlorophyll breaks down, revealing other colorful substances plants use for photosynthesis: xanthophyll (ZAN-tho-fill), a yellow pigment, and carotene (CARE-oh-teen), an orange pigment.  As temperatures drop, some plants make anthocyanin (AN-tho-SIGH-uh-nin), a red pigment that helps the plants store sugars for winter.  Some plants reveal tanins (TAN-ins) in their leaves in the fall.  Tannins are brown in color and are thought to be waste molecules produced by plants.  They have a bitter flavor, though some tannins are pleasant, including the ones found in tea leaves.

Leaf scar on a buckeye showing scars where the leaf veins were sealed off with suberin.

So leaf abscission is a trade-off that works in parts of the world with four seasons.  Plants in the tropics and plants in colder regions keep their leaves.  Tropical plants don't have to deal with cold, so they don't shed their leaves unless there is a yearly dry season.  Plants nearer the poles of the planet don't have a long-enough growing season to start from scratch every year, so they have to grow slowly and produce evergreen leaves and needles.  We lucked out, and we get to see the beautiful fall colors that accompany leaf abscission.

How to Read Bark Scars in Sourwood Trees

Sourwood trees are among the first to turn colors in the Fall.

The sourwood trees in our outdoor classroom are the first to put on their fall colors for the season.  Sourwoods are wonderful, smallish trees with beautiful foliage and interesting bark.  They are named for the sour taste of their leaves, which you can experience if you touch a bit of torn leaf to your tongue.  The leaves contain oxalic acid, which tastes pleasantly sour (all acids taste sour).  Tasting the leaf is not harmful, but the leaves are not considered edible and shouldn't be eaten.

The small orange-leaved tree in the picture is one of our sourwoods.

We have two sourwood trees.  Above you can see the location of one sourwood - it's the orange-leaved small tree in the center of the picture.  See if you can find the second sourwood tree when you visit the classroom.

Lenticels in young bark of the sourwood tree.

Sourwood bark is wonderful - it has so many visible features that give clues to what the tree is doing and what it has gone through during the tree's life.  A lot of people think tree bark is a dead part of the tree, but the opposite is true: tree bark is a living, important tree tissue that changes as trees grow.  Bark is mostly responsible for moving sugars (a tree's food, made from photosynthesis) between the leaves and roots.  Bark is filled with phloem tubes for transporting the sugar.  The above picture of a young twig contains tiny spots called lenticels.  Lenticels are tiny holes in the bark to allow air to get into and out of the inner tissues of the twig.  Compare the above twig to the one below.

Sourwood twig with cicada damage.

The twig in the picture above is about the same age as the one in the previous picture, but something looks wrong!  This giant gash in the bark is the healed wound cut into the bark by one of last year's cicadas.  Cicadas cut into young bark and lay their eggs in the gash where the developing offspring can feed on tree sap.  The living bark responds by slowly growing a scar to heal the wound and seal off the wood, which is what you see above.  

Older twig with young bark splitting as the twig grows larger.

Bark naturally stretches and tears and re-heals to allow tree twigs and trunks to increase in girth.  The twig above shows the first tears in young bark as the twig is getting thicker through the years.  The stretch marks get bigger as the tree gets bigger, and large branches and trunks might have deep furrows in the bark.

Scar from where a branch broke off the tree.

When branches fall off or are broken off, the bark around the broken area swells up and heals over the scar.  The scar above looks like a pretty big scar, so I suspect the branch that used to grow here was torn off unevenly.  Notice the larger tears in the normal bark above and below the branch scar.

Large gash in bark that is healing over - possibly damage from planting the tree.

Here is an even bigger scar from some major damage to the trunk.  Something cut into the bark of this tree.  Perhaps it was damaged as it was being transported or planted here.  Such an injury can weaken or kill a tree, because it can let diseases into the tree, just like a wound in our skin can become infected.  I wish more people realized this so they wouldn't carve their initials into trees' bark.  Nevertheless, this tree appears to be healing from its damage.  You can see exposed wood through the gash in this bark.  If the wound to this tree were to have cut through the bark all the way around the tree, the tree would have died, since the bark would be unable to move sugars up and down the tree.  Plant managers who need to kill trees use this technique - it's called girdling a tree.

Check out the bark on our sourwood trees and look for lenticels, branch scars, normal tears in the bark, and possible injuries to the bark.  Then take a look at other types of trees and see if you can read the scars in the bark.  Can you tell where branches used to be?  Can you see how the bark split at the tree got bigger?

Click to zoom in and see how the leaf veins connect.

When you check out the bark on our sourwoods, be sure to look at the leaves too.  Sourwood leaf veins are large, and it's easy to see the network of how the veins connect.  Also be sure to look for the remnants of flowers, now turning into fruits, at the ends of some of the branches. 

How Trees Work

Trees' general strategy as plants is to grow slowly, put a lot of time into building a structure that gets leaves closer to the sun, and eventually out-compete the fast-growing soft plants and shrubs that can't get off the ground.  The part that makes a tree a tree, namely the wooden trunk, serves as both support for holding up leaves and the means transportation of materials from the leaves to the roots and back. 

This is a tree.

 Tree trunks are strong because their cells have surrounded themselves with lignin, the hard material in wood.  There are two basic tissues in tree trunks: wood (the inside), and bark (the outside).  Guess which one of these materials is alive?  You'll have to wait just a minute to find out.  The wood part of a tree trunk contains mostly lignin, so it's very strong.  The bark contains more suberin and much less lignin.  Suberin, the subject of a previous post, is a softer substance, and if you have ever squeezed a wine cork, you know the texture of suberin.  Wine corks are cut from the bark of the cork oak. 

The wood of a tree is produced by plant cells growing very long, surrounding themselves with lignin,  leaving a few tiny holes in each end to connect to the next cells, then dying and leaving behind hollow tubes of lignin.  Yes, the interior of trees is mostly dead.  The hollow tubes connect down to the roots and up to the leaves.  The tubes are so narrow that water can pull itself up through the tubes by capillary action.  Capillary action occurs because water is pulled more by the chemical attractions of water to the lignin than by the pull of gravity.  Capillary action works in trees as long as the lignin tubes are very thin and the tree isn't too tall.  You may remember from 9th grade Biology that the material which transports water in plants is called xylem.  In fact, wood is almost entirely xylem.

Bark, the living part of a tree trunk, is composed of mostly phloem and suberin.  Phloem transports sugars, the tree's food, dissolved in water.  If you look at bark under a microscope, you would see that it, like wood, is composed of microscopic tubes running up and down the trunk.  Unlike xylem, phloem tubes can run up or down, depending on the season.  In the summer, the phloem is busy transporting extra sugars and nutrients down into the roots for winter storage.  In the early spring, phloem brings that stored sugar back up to provide energy for the new spring growth.

A tree will die if the bark is cut all the way around the tree.  The tree below has been girdled.  It is a white poplar, a weedy tree, that is probably interfering with the native plant restoration going on in Lincoln Park.  The ecosystem manager probably also applied an herbicide to the bottom cut on this girdle so it would be pulled down into the roots.  White poplars are notorious root-sprouters, and if you don't kill the roots, you will have new mini-trees coming up all over the place.

A girdled tree.

This tree has also been girdled, but not by humans.

A girdled green ash.

Bark is infinitely variable in its patterns and characteristics.  Here's my favorite bark - white birch.  You can tell a lot about a tree by looking carefully at its bark.  This will be the subject of a future post!

Beautiful birch bark.

Caught in a Web of Suberin

It's melon season.  We're searching through the melon fields to collect the ripe ones on the farm right now.  Testing for ripeness is an art, and it varies for each type of melon.  The cantaloupes are perfectly ripe when the stem easily peels off the top of the fruit.  We farmers refer to this as 'full slip', as in, "That melon's ready to go - it's full slip."  The reason melons from our farm (and, to be fair, other local farms) taste infinitely superior to supermarket melons is that mass-market melons are harvested at half-slip or even earlier, so the fruit flesh doesn't mature into as complicated of a combination of sweet and aromatic notes.  In addition, supermarket melons are treated with a compound to prevent their rinds from molding.  Unfortunately this treatment also prevents the melons from smelling like melons.  Go buy a melon from the farmer's market or your local organic farmer (my favorite is here: Fresh Harvest Cooperative).  Even if you hated canteloupe as a kid, like I did, you will not be able to stop eating a delicious local ripe cantelouple. 

The farm I'm working on grows a variety of melons.  Here is what I harvested one day this week:

Genetic diversity in melons on our farm.

The picture above contains two varieties of watermelons, cantaloupes, an heirloom variety of cantaloupes, and honeydews.  All the types of melons above except the watermelons are types of muskmelons.  Muskmelons are tropical melons originating from Africa or the Near East.  They require a long warm growing season.  They are in the same plant family (Cucurbitaceae) as squash and cucumbers, so they have to deal with squash bugs, too. 

Have you ever noticed the netting on the rind of cantaloupes?  If not, take a look at the cantaloupe below.  If you did notice the netting, did you ever wonder what it was doing there?  Not me, but when I started reading about cantaloupes for this post, I found out the most fascinating thing about that netting.

Netting on cantaloupe rind.

The netting on cantaloupe rind is made of a substance called suberin.  Suberin is a fantastic molecule produced by many plants.  It is waxy and waterproof.  On the cantaloupe rind, anywhere the rind cracks as it grows, the cracks are quickly filled in with suberin.  The suberin helps keep the juiciness in and the moldiness out.  It's basically very much like a scab on our skin, but not gross.  Fortunately for me, but not for the farm owner, suberin fails on occasion and a crack becomes too large to grow a suberin scab.  These cracked melons cannot be sold, but they are fine to eat as long as they don't get moldy and aren't inhabited by an ant colony.   I've been stuffing myself with melon culls for two weeks now.

But back to suberins.  You may already know suberins from their most famous location.  Cork is pretty much solid suberin.  Cork is the almost-outmost layer of bark of the cork oak (Quercus suber).  It provides waterproofing for the cork oak, enabling the cork oak to survive in the very dry areas around the Mediterranean Sea.  It provides the same function when humans peel it from those trees, cut it into cylinders and cram it into wine bottles. 

You will probably be surprised to learn that roots of almost all plants contain microscopic layers of suberin under their surfaces.   This seems ludicrous at first, since roots are supposed to absorb water. You'd think the last thing a plant would want is a layer of waterproofing around their roots.  In roots, suberin helps the plant regulate what can get into the plant.  There are doors and windows in the suberin layer, and those openings let in water plus the minerals the plant needs.  The openings keep out anything the plant doesn't need, such as too much salt, harmful bacteria or lots of other things that could be dissolved in groundwater.  Mangrove roots are highly suberized to keep out the salt of the ocean water in which they grow.

 Suberins are actually a group of complicated molecules.  All suberins have a similar structure, but they can vary slightly in their components.  Suberins are long polymers of smaller molecules, arranged variously but allowing crosslinks.  These molecules have a lot in common with oils, which makes them hydrophobic and water-repellent.  A little-known fact about suberins is that they have their own facebook page. 

Many varieties of melons don't have suberin netting, including most honeydew melons.  Their rind is smooth and net-free. The lack of suberin is not due to an inability to make suberin or an absence of the gene for suberin.  In melons lacking netting do not express the gene for suberin in their rinds.  The difference between netted and non-netted melons is a genetic difference, but not in what the DNA says.  The difference is in how the DNA is read.  Imagine the DNA for melons as a book.  If you read the pages in a different order, you get a different story.  For honeydews, the suberin chapter is skipped over altogether, except in the root cells.  (Source)  I don't know why honeydew melons don't have cracks on them.  Their rind must be able to expand around the fruit without cracking. 

I'll leave you with this picture of last week's melon harvest. 

Nice melons!  Ready for the market.