Minneapolitans love their urban forest, one of the grandest of any city in the nation. I cringed when I saw one of the last of our grand elm trees on the parkway with the orange ring around it a month ago. A couple of weeks ago it was taken down. It was a sad reminder of when, a few years ago, we had to take down the beautiful elm that stood in our yard for 85+ years.
As much as most of us love our trees, it is amazing that we have as many as we do in the city and that they manage to survive considering the conditions we expect them to grow in and the abuse to which we subject them. We plant them in ‘concrete,’ walk, drive and park over their root systems, tear the roots up when we till up our lawns, prune their roots when we repair the streets or driveways, pound nails into them, whip them with the weed whips, prune them for the electrical lines, strangle them with ‘twinkle lights, carve our initials in them, chain objects to them and generally neglect them. And then we are surprised when they fail to thrive or fall over in a storm.
Some foresters call them “sidewalk coffins” — those small planters stuck in the middle of an urban sidewalk or parking lot in which a tree is supposed to grow. While the amount of root space may be sufficient for a young sapling, these planters do not provide sufficient space for the roots to spread out as the tree grows. The result is short-lived, unhealthy trees. Even a medium size tree needs 300 cubic feet of soil, considerably more than sidewalk planter, to thrive. In ideal conditions the moisture absorbing root system, the critical root zone, of a tree will spread out 1 to 1.5 feet from the trunk for each inch in trunk diameter. For example a moderate size tree with a 6-inch diameter would have a root system that reaches out 6-9 feet from the trunk. Even if the roots of a tree in a “sidewalk coffin” were able to grow out that far they would be under the sidewalk, street or parking lot and water would not be able to reach them. Usually what happens is that the roots, in an effort to reach moisture, will encircle or girdle the trunk of the tree increasing the risk of tree failure; girdling roots restrict the flow of water and nutrients within the tree. A tree with stem girdling roots usually takes about three years to die. Root confinement is also the reason we should not be trying to grow 70 to 80-foot tall trees in 10-foot wide boulevards.
Soil compaction is one of the most common killers of urban trees. Heavy traffic over the root system compacts the soil so that oxygen cannot reach the roots and the roots are literally suffocated. Heavy construction equipment parked over the critical root zone can lead to tree failure. Another construction threat to trees is root pruning. Roto-tilling to establish a garden or to renovate a lawn can threaten a tree if the critical root zone is torn up. While it may not cause the immediate death of a tree, it certainly will stress a tree, leaving it more vulnerable to disease and insects. In the event you must have sidewalk or driveway repair or construction near a tree there are some steps you can take to protect it:
- Thoroughly water your tree before and after the construction work takes place.
- Do not allow heavy equipment or supplies to lie on top of the tree roots.
- If possible place 2 to 3 inches of mulch over the root zone of your tree.
- Do not leave construction debris in the soil. It will constrict root growth and change the chemistry of the soil.
Another common problem with our urban forest is our tendency to plant inappropriate trees in inappropriate places. One example is the Colorado Blue Spruce that people tend to like for its blue green foliage. Colorado blue spruce planted in our climate live hard, grow fast but die young. They are too tall, too dense and have too shallow a root system to stay vertical for long. They are not native to Minnesota and are very susceptible to such stressors as drought, excessive heat, and compacted soils and are vulnerable to infection. When selecting trees to plant we need to consider all the existing environmental conditions, the physical characteristics of the trees and the maintenance that the tree will require. Another thing to avoid is planting forest trees on our boulevards and under electrical lines.
Dutch elm decimated the canopy on the parkway and the Emerald Ash Borer is taking down many of the trees on our boulevards and in our parks. Green ash is susceptible to Ash Yellows, Oak to Oak Wilt and the newly introduced Sudden Oak Death that has devastated eastern seaboard cities. Basswoods, of which Lindens are one, are one of the primary decay-prone trees. All trees are more susceptible to disease when they are stressed by less than ideal growing conditions like those mentioned above. But secondary stressors also contribute to the failure of trees.
Whenever you introduced a wound to a tree you are opening up the door to a pathogen or decay. We wound our trees by stapling or nailing signs to them, banging car doors into them, carving into their bark, tying them to tightly to support stakes, constantly whipping them with weed whips and even pruning them improperly.
We should remember that trees are our friends. They provide aesthetic, economic and most importantly environmental benefits. Trees and forests sequester carbon dioxide and other greenhouse emissions from the atmosphere. Nearly ½ of a tree’s dry weight is carbon.
The woodlands of Minnesota store, on average, 75 tons of carbon per acre. In the U.S. trees and forests annually sequester almost 11% of greenhouse emissions. We need to treat them with the respect they deserve and help them to thrive in order to maintain our urban forest.
I think that I shall never see
A billboard as lovely as a tree
Perhaps unless the billboards fall
I’ll never see a tree at all
-Ogden Nash – 1933