Tree diversity is key

Congratulations to Cloquet city officials for getting Tree City status. I believe the last time we received the designation was during mayor Arlene Wolner's tenure. She was a dedicated tree person as I am, and we worked together to get the designation for Cloquet. At that time, requirements included having a tree board and a tree ordinance. We organized both and received the Tree City designation for several years.

As part of the effort, one of my graduate students conducted a survey of the street trees in Cloquet as a research assignment. He came up with an interesting list of trees that had been planted on our city boulevards. It seems that after the 1918 fires, people badly wanted trees growing again after they had largely been eliminated. There were quite a few boxelder trees planted, as they were fast-growing, although relatively short-lived. They did provide instant greenery to the city. Our tree survey found a number that had survived. I remember we even found a catalpa tree that had been planted and survived. This is largely a southern species and was far north of its natural range.

The boxelder trees were likely the parents of some of today's boxelder trees in the city. As Mike Creger noted in his article, these are home to boxelder bugs. As he also noted, these insects are relatively harmless to the tree but give off a nasty odor when squished. I grew up in southern Minnesota, and they could be a real nuisance, especially in the early spring.

In the past the major hardwood growing in the eastern United States was the American chestnut. These fine trees dominated the eastern forests, grew over 100 feet in height and were 2-3 feet in diameter. They were largely eliminated by the chestnut blight.

Chestnut trees were planted in the arboretum at the Cloquet Forestry Center, but after growing to a certain height, they would be affected by the blight and the crowns would die. They sent up several new shoots from the stump but they stopped reproducing after a number of years and they all died.

The same has been seen with the American elm. They were widely planted along streets in many cities, but Dutch elm disease has killed many of them. Green ash - considered to be resistant to most insects and diseases - were planted as a replacement, but recently the emerald ash borer has arrived in northern Minnesota and threatens their survival.

The ash are being replaced with maples such as the red maple with its brilliant red leaves in the fall, and sugar maple, which adds yellow to the fall landscape.

There are a number of maple cultivars that are also being used. Silver maple is not recommended because of its size and invasive root system, which can cause problems with sidewalks and sewer and water lines.

Oak is another widely planted species on the boulevards, mostly from the red oak family. Oak wilt is a major threat to them since it spreads from tree to tree through root grafts.

Currently, it looks like the maples, oaks and the littleleaf lindens - which are a cultivar from the basswood family - seem to be favored for planting along our streets.

Writer Al Alm is a forestry professor emeritus, and worked at the Cloquet Forestry Center on and off for three decades.