Our gardening discussions give us a chance for lighthearted, upbeat fun each week, but it's difficult to put a humorous spin on a tree that's headed for that big landscape in the sky.

Around this time five years ago, our gardening column, "The mystery of the murdered tree," investigated visible injury to the base of tree trunks. Now, five years later, I decided to revisit one of the trees we photographed at the time, to see if the tree recovered from its wounds.

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In early July 2013, while driving around town, I noticed damage on trees ranging in age from newly planted to some that were quite large. The very-visible trunk and bark damage was slightly above ground level, and as I wrote at the time, the lower bark mutilation appeared to be caused by lawn mowers scraping the trunk and string trimmers whipping against the bark.

The biology of a tree trunk explains why such damage is so serious. The outer visible bark layer is the tree's protective armor, safeguarding the lifeblood of the tree, the cambium layer. This thin, greenish-white layer, immediately inside the bark, is where the tree's growth occurs. Tissues around this thin layer conduct water and nutrients up and down the tree. Farther inside the tree trunk are the rings of wood that structurally support the tree.

If the outer protective bark is damaged and the cambium layer injured, the tree is figuratively left to bleed to death. Whether death comes slowly or quickly depends upon the depth and circumference of the injury.

Years of tree growth can be ruined in seconds by damage from a mower or trimmer. The damage is also cumulative. A little nick this week, a little scrape next, and soon the damage is irreversible.

The damage that began as bark injury can cause deep bark cracks, dead branches, an overall decline in vigor, and possible death over time. A weakened tree is also more susceptible to attacks by insects, disease, and winter injury.

As a second opinion, this from Purdue University: "One of the most dangerous pests of trees is humans, especially humans with equipment. The tree trunk is protected by bark, which guards a very important plant transport system that moves nutrients and water between the roots and leaves to keep the tree alive. Damage to the bark and to this transport system can affect tree health and the tree could die. No matter what size the wound is, the damage done is irreversible."

What's the best way to prevent damage while mowing and trimming? Circle the tree with wood product mulch 5 feet in diameter, 5 inches thick, and kept 5 inches away from the trunk.

What about the tree we photographed five years ago? Has it recovered from its mower and trimmer injuries? A return to the scene showed a sad state of decline, and unfortunately, the tree is close to death.

In 2013, although the trunk was injured, the tree's leafy canopy was full and normal-looking. About 95 percent of the tree's branches are now dead bare, with only a few weak leaves clinging, and the trunk is severely rotten. A sad end to a tree only a dozen years old, a tragedy so preventable, with so many years of tree growth wasted.

Don Kinzler, a lifelong gardener, worked as an NDSU Extension horticulturist and owned Kinzler's Greenhouse in Fargo. Readers can reach him at forumgrowingtogether//growingtogether.areavoices.com.