Published on 03/02/05

Dumb as a stump? Think again -- trees are smart

By Kim D. Coder
University of Georgia

Are black mulberry trees smarter than people? You've probably met some people you have suspicions about.

Many people think humans are the most complex creatures on Earth. And we are complex. We carry around a huge amount of genetic matter that tells our bodies what to do and how to do it.

But we don't even come close to a tree like the black mulberry.

Trees are smart. They've developed defenses to competition and pests, adjusted to stress and modified growth because of changing environments.

All the while, trees are stuck in one place. They can't get out of the way of damaging agents or move to better sites. Trees must figure out how to survive and make it on their own without moving.


To do this, they carry responses in their genes for every environmental event they may encounter. As its world changes, a tree reads its genetic material to see how to respond. It reacts to changes through predetermined responses recorded in its genes.

Tree genes, like animal genes, are a part of larger units called chromosomes. As animals or plants reproduce, the chromosomes are shared or traded. The chromosomes contain the genes that allow living creatures to respond to their environments.

People have one set of 46 chromosomes matched in 23 pairs. Trees can have many more or fewer chromosomes in their cells. Three genetically simple trees are the redbud, red alder and river birch. They have 12, 14 and 14 chromosomes, respectively. If chromosome number is a gauge of complexity, these trees are much less complex than people.


Other trees, though, like American basswood (82 chromosomes), white ash (138) and silver maple (78), could be considered much more complex than people. Green ash and the fringe tree each have 46 chromosomes, just as people do.

Of course, counting just the number of chromosomes doesn't tell the whole story.

In people, each of the 23 chromosome pairs is unique. Each cell has only one copy of each pair. Trees may have many sets of the same chromosome pairs.

For example, southern magnolia has three complete sets of chromosome pairs in each cell. Sassafras (two sets), redwood (three), weeping willow (two) and black gum (two) all have more than one set of chromosome pairs.

Some tree species have different races in which each has a different number of chromosome pairs. Yellow birch and red maple can each be found with two, three or four chromosome sets. Yet all members of each species look similar.

Most of our conifers are fairly simple, with 22 to 24 chromosomes. Pines have one set of 12 chromosome pairs, as do spruce, fir and hemlock. Ginkgo also has 24 chromosomes, like pine. Giant sequoia can have 44 chromosomes.

Finally, though, the black mulberry -- are you ready for this? -- has 308 chromosomes in 11 complete sets.

Does this mean the black mulberry is better prepared for its environment or smarter than people? Judge for yourself.

(Kim Coder is a professor in the University of Georgia's Warnell School of Forest Resources.)

Kim Coder is a forester with the University of Georgia Warnell School of Forestry and Natural Resources.