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Heartwood: A Tree's Storage Closet

The heartwood is darker, mid-section of the tree.
Wikimedia Commons
The heartwood is darker, mid-section of the tree.

When you look at the cross section of the trunk of an old tree, you see that it has two zones: the sapwood, which is the younger wood that’s closest to the bark — and the heartwood, which is the darker, central part of the trunk.

The sapwood is the living transport system of the tree. It houses the vertical highways where water, nutrients and sugars move between roots and leaves.

The heartwood is more like the tree’s storage closet. And, just as we can move things in and out of our bedroom closets, heartwood is connected to the rest of the tree. Sapwood cells make protective compounds, like resins and tannins, and then move them to the heartwood to help the tree resist attacks from insects, fungi and bacteria.

Researchers at Humboldt State University are documenting how climate change affects redwood forests. Nighttime temperatures are warmer than they used to be, which stunts the redwood trees’ growth and wood production. But the trees now use the sugars they produce to make larger amounts of fungicide. And where do they store them? In their heartwood closets.

This decay-resistant heartwood also contributes to the structural complexity of old-growth trees. And that fosters needed habitats for organisms that live with redwood trees, like cavity-nesting birds and insect-eating bats. So, the production of heartwood promotes both greater amounts of disease-resistant wood and higher biodiversity of the forest as a whole.

It’s an important reminder that we need to protect our old-growth forests, and move those previously-harvested forests towards becoming old forests again.

So the next time you store something valuable in your closet, think about how heartwood stores vitally important compounds that protect forests — and the life that exists within them.

Dr. Nalini Nadkarni is an emeritus professor of both The Evergreen State College and the University of Utah, one of the world’s leading ecologists and a popular science communicator. Dr. Nadkarni’s research and public engagement work is supported by the National Geographic Society and the National Science Foundation. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. @nalininadkarni
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