An ode to bur oaks

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We who live among trees tend to take them for granted — or are affronted when storms blow them down, as happened in the Twin Cities in late August.

I have had a love affair with bur oak trees that started when I grew up on a small farm in northern Minnesota in the 1950s. If you don’t know what a bur oak is, take a drive along West River Road south of the Lake Street Bridge in Minneapolis. It is lined with gorgeous old bur oaks. They are well known in rural Minnesota, since they are found in every county. They can live more than 300 years.

Bur oaks are common on our farm in northwestern Minnesota, including next to our house. When I was young, I fell asleep to the sound of wind flowing through their branches and leaves outside my open window; sounds words can’t describe. It is the same now. Twenty-five years ago, we put bur oak wainscoting that we harvested from fallen bur oaks in our living room.

I am a wildlife biologist. My love of bur oaks expanded in college and later in my career. I learned its scientific name: Quercus macrocarpa, one of my favorite Latin phrases. Several years ago, a small tornado toppled a large bur oak on our neighbor’s farm. We counted the annual growth rings. We discovered it was a sapling that began growing in about 1842, not long after Fort Snelling was founded by Europeans.

The recent violent thunderstorms and high winds blew down many trees in the Twin Cities. I drove down both East and West River Roads along the Mississippi River the next morning, worried about the trees, especially the bur oaks. I marveled at how few of the bur oaks had been damaged.

Many trees have specific adaptations for surviving in dry ground or wetter ground. Bur oaks are unusual — they can survive both wet and dry conditions. In fact, they can even survive being flooded during the growing season. This is uncharacteristic of upland trees.

Bur oaks have a thick bark that allows them to adapt to hot prairie fires. The first explorers in Wisconsin were shocked when they found large areas with bur oaks that were evenly spaced. (Now, the areas are called “oak savannas.”) The explorers thought the spacing meant they had found evidence of European settlement deep into the unknown wilderness. They found no such people, but were but a bit afraid, thinking they were dangerous and lurking nearby.

Prairie grasses grow between these bur oaks. Scientists have discovered the reason for the even spacing. Hot prairie fires kill some oaks. They decay and fall down, leaning against an adjacent tree. The next fire was intense because of the dead tree so the fire then killed the one it leaned against. The long-term result was bur oaks became fairly evenly spaced — perhaps a bit farther apart than their 80-foot height.

Bur oaks are key to an astounding bit of Canadian research. It has yielded an annual precipitation record of about 575 years in the Red River Basin. The Red River has had immense floods, some of which have threatened Winnipeg. The research is based on analysis of bur oak annual tree ring records. Tree ring patterns are unique, just like fingerprints. The rings demonstrate dry and wet conditions: During wet years tree rings are wider; in dry narrower.

Canadian scientists matched tree rings from living trees to logs from old houses and finally to bur oak logs buried in Red River sediment from ancient large floods. They matched bur oak tree rings all the way back to the 14th century. In other words, it was as if they had a tree that was 575 years old, since tree ring patterns are annual — and unique. That and other knowledge about the width of tree rings in modern trees can be compared to the last 150 years of annual precipitation and thus be extrapolated to judge current annual precipitation.

Bur oaks can reliably predict extreme and rare large summer floods. When flooded during the growing season, a unique tree ring appears that year. Bur oaks normally grow in uplands not subject to flooding. If a mature bur oak tree — a tree that has grown in uplands — is flooded long enough to result in the unique tree ring, it means there was an enormous, extreme and rare summer precipitation event where floodwaters reached far into dry uplands.

The result is that Canadian scientists were also able to predict extreme flood events and long dry periods during the past 500 years. Many of those past events are far more extreme than the precipitation records since settlement that is part of the basis of the “100-year flood boundaries.” Bur oaks made this research possible.

Most bur oaks grow from forgotten acorns buried by squirrels. Gray squirrels, flying squirrels, red squirrels and fox squirrels all love bur oak acorns. Of all the oak species in the U.S., bur oaks have the biggest acorns, acorns eaten by squirrels, wood ducks, deer and, when boiled, traditionally by Native Americans.

Please, as you walk or drive under our lovely bur oaks, give them the credit they deserve; perhaps even a good kiss on their bark and a pat of respect for their long and interesting lives.

Paul Stolen, of Fosston, Minn., is a retired biologist.