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How long will your tree live, and what do trees tell us about climate?

"Growing Together" columnist Don Kinzler gets answers about trees from North Dakota State University Extension Forester Joe Zeleznik.

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Annual growth rings on old logs can show a historical record of past droughts and floods.
David Samson / The Forum

FARGO — Imagine being 400 years old, still alive, and you were a youngster when the Mayflower landed in 1620. Area trees needn’t use their imaginations — some are living it.

What’s the average lifespan of a tree? That’s a figure made elusive by too many variables like tree type, weather, moisture, soil type and presence of insects, diseases and human misconduct.

We do know, however, that trees in the Upper Midwest can live to ripe old ages. North Dakota Extension Forester Joe Zeleznik says Ponderosa pines in the North Dakota Badlands have lived for more than 500 years. Two trees killed by wildfire in 2004 date back to the 1670s. A cottonwood growing along the Little Missouri river is nearly 400 years old.

Bur oak is another of our region’s long-lived tree species. Zeleznik says most currently standing oaks aren’t older than settlement times of the late 1800s, though. Early settlers needed building materials and fuel, and many oaks standing at that time were harvested for log cabins and firewood.

Very few oaks survived the period of early area settlement, so most currently living oaks got their start in the 1870s and 1880s. The oldest living bur oak in North Dakota escaped earlier lumbering, and began life in 1676. It's in the Lake Metigoshe area, and is now 346 years old.

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NDSU Extension Forester Joe Zeleznik examines the tree growth rings from a log section from Fargo's oldest house.
David Samson / The Forum

Old trees aren’t just interesting novelties — they’re sources of information. With each growing season, trees add a layer of new wood. In the cross section of a tree trunk, these layers, termed annual growth rings, are distinctly visible.

Annual growth rings can vary in appearance from year to year. During growing seasons with plentiful moisture and favorable weather conditions, more wood is added, making that year’s growth ring wide. During years hampered by drought, growth rings are very narrow. As trees grow, they’re recording each year’s weather, and old trees contain within their trunk a trove of climate data recorded on their growth rings.

How do foresters examine tree rings? When a tree dies or is cut down, the cross section of a tree trunk becomes visible. Zeleznik’s lab is filled with tree “cookies” that he’s cataloged, which are the inches-thick rounds cut from tree trunks.

The annual growth rings of living trees, and the climate record they hold, can also be examined without chopping down the tree. Foresters use an increment borer, which drills into the tree and extracts a thin tube-shaped cross section of the tree, which shows the annual rings without harming the tree.

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NDSU Extension Forester Joe Zeleznik shows the tools used for taking out a tree bore sample to count growth rings.
David Samson / The Forum

The weather data held within old trees can contribute to climate information that isn’t available elsewhere. Written weather records for much of the Upper Midwest date back only to the 1880s. A 400-year-old tree can provide records for our region back to the 1600s, giving a much broader view of droughts, rainfall and climate patterns.

What if you can’t find old trees to examine? Luckily, there are parts of old trees still existing in unique places, waiting to be discovered. One such place Zeleznik has found is the logs used by early settlers to build their log homes. The growth rings are still present and evident in the old logs’ cross sections.

The region’s few still-standing log cabins are not only gems historically, but the past climate record their logs reveal is a surprising bonus. For example, Zeleznik has examined the tree growth rings in the logs of Fargo’s oldest home, the Hector log cabin. The last ring of the log examined dates back to 1871 and the first to 1785, showing the oak tree was 86 years old when it was cut down for building material.

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Log cabins provide historic climate data, since the growth rings in the logs recorded the weather while the trees were alive.
Contributed / Joe Zeleznik

Zeleznik also finds old oak trunks buried in river mud, where they toppled, sometimes centuries ago. This past summer’s drought brought the Red River to knee-deep levels, revealing logs that are often inaccessible. Zeleznik has slowly boated miles of the river searching for long-buried logs from which sections are cut and studied.

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Oak logs are preferred because they’re long-lived, the wood is more reistatant to decay and and they’re a species whose growth rings show years when flooding occurred during the tree’s life. One oak log discovered in the river has been radiocarbon dated to the 1600s.

As an example of what the oaks along rivers can reveal, Zeleznik is always looking for tree ring signs of the Red River flood of 1826, which was recorded by early settlers at Winnipeg, to see if that record flood extended south to the Fargo area.

Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at donald.kinzler@ndsu.edu.

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