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Why Old Trees are Good Trees

Knotted tree trunk in Salt Lake City's Liberty Park, 2022.
Brian Albers
Knotted tree trunk in Salt Lake City's Liberty Park, 2022.

I’m the first to admit it. Getting older has its drawbacks. I can’t run as fast nor dance as wildly as I did when I was 20. But there are some bonuses to getting older. My favorite? I can take a reflective walk — instead of a bone-crunching mountain bike ride — when I visit our foothills.

Aging has benefits in the world of trees, too. In the past, timber companies viewed dead trees as economically worthless. But ecologists have learned that snags — big, old, standing trees — become "biodiversity hotspots," providing unique habitats that sustain wildlife and expand the carbon storage capacity of our forests.

In North America, more than 100 species of animals need snags for their food, nests and shelter. 85 species of birds — called cavity-nesters —hole up in snags. Their presence helps control insect pests. A single snag-dwelling swallow can eat up to 1,000 mosquitoes a day.

If you were a woodpecker, which you choose to make your home: a young tree or an old tree? Excavating a bird-sized hole in a tree softened by heart-rot fungi is much easier than drilling into the solid trunk of a young tree. Heart-rot fungi doesn’t kill the tree, but just softens the resistant center. But it can take those fungi decades, even centuries, before that softening begins.

However, sustaining those elements is now a challenge, since rotations of forest harvesting are getting shorter, and the number of snags is getting smaller.

In response, foresters have developed the practice of “veteranisation,” accelerating the aging process of younger trees to create “tree veterans” in our landscapes. Although intentionally damaging younger trees sounds odd, these practices speed the development of habitats offered by older trees.

Techniques of veteranisation include bark stripping and breaking off branches. The weirdest — but most efficient — practice is to introduce heart-rot fungi into healthy trees. Foresters actually climb up snags, drill holes in the trunk and insert plugs of the fungi into the holes, hurrying the process of decay, and making cavity-excavations easier for home-seeking wildlife.

It’s kind of comforting for a snag-to-be like me to know that as we age, we can still enrich our own habitats.

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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