Beautiful lawns don't always mean beautiful and healthy trees. I recently visited a home where an Austrian pine had blown over on one of our recent windy days. It didn't seem too windy to me but the tree must have felt it was too windy to allow it to stay anchored in the ground. The tree had fallen over pulling out its undersized pancake-shaped root system.
This tree was at the bottom of a gentle slope where water tended to puddle. With the pancake-like root system fully exposed you could see where many of the roots had rotted in part due to the wet soil conditions. Trees develop "sinker" roots, roots designed to penetrate a foot our more into the soil to help anchor the tree in place. This tree did not have sinker roots. They may not have been able to develop due to wet, oxygen-starved soil. Roots require oxygen to develop. Pine also typically have two sets of roots. One set is at ground level and the other is as much as 18 inches below ground. The lower set of roots also helps keep the tree anchored in position. This tree lacked the lower set of roots due to either being severed when dug in the tree nursery or they rotted off due to the wet, oxygen-starved soil.
In addition to the wet soil conditions, this tree has a stem girdling root. SGRs often develop due to being planted too deep either in the nursery field where they are grown or when they are planted. When a tree is planted, any SGRs need to be severed and removed. In this case the tree had been in the ground for several years before the SGR was severe enough to have starved the roots of the food the needles were producing. Sugar, starch, and hormones move down the tree in the phloem, the inner part of the bark. When a root encircles the trunk, the phloem is compressed and these products cannot reach the roots.
So did the tree die from an oxygen-starved roots or did the roots die because the SGR prevented sugars and starches produced by photosynthesis in the needles from reaching the roots? No matter which was more critical, saturated soil or a stem girdling root, the tree was a goner.
Tom Lynch asked me a while ago why many of the trees in the park he walks every day were losing limbs. He also wondered if the amount of limb damage he was seeing was normal. The answer to the last question is no; the dropping of limbs in that park was unusual.
The first question as to why the trees were dropping limbs is directly related to the failure of the Austrian Pine I just discussed. The loss of roots. The root loss in this park was caused by a contractor cutting trenches to install a new sprinkler system.
When roots are severed, limbs start to die as water and nutrient-uptake is restricted. This die-back may take a number of years to appear due to the stored water and nutrient reserves in the trunk and branches. As these reserves are depleted, branch die-back occurs. Dead and dying branches are common in many of the trees in this park.
When I called about the trenching, I was told emphatically, "We do it all the time and it doesn't harm the trees." The damage is not obvious in the first couple of years after trenching due to the tree's stored reserves, but the damage will appear at a later time.
Having said this, I need to tell you about a Hackberry tree Mike Vendegna, Grand Junction City Parks Superintendent, looked at when he was the City Forester. (Mike, by the way, was not the person I spoke to about the trenching. He knew the trouble the trenching was going to cause.) The lot where the hackberry was located (it was dead) had been trenched in the spring to install a sprinkler system. The concerned homeowner wanted to know if the tree was just taking a nap. Mike and I both identified the cause of the death of the tree as trenching. We determined hackberry trees were much more sensitive to root damage than most other trees.
Keep in mind that the roots, branches, and leaves are intimately connected. If you damage what is below ground, you will have an effect above ground. If you are going to trench around a tree, try to determine the amount of root damage that will occur. Since root spread can be three times or more the height of an established tree, the amount of root damage due to construction can be quickly estimated.
Soon to be retiring, Dr. Curtis E. Swift is the area horticulture agent with the CSU Extension. Reach him at Curt.Swift@mesacounty.us, visit WesternSlopeGardening.org, or check out his blog at http://SwiftsGardeningBlog.blogspot.com.