The storm that roared across Minnesota and into the Twin Cities on Friday cut power to over 615,000 homes, flooded basements and damaged cars.
One of the more visible casualties of the severe winds and torrential rains was the metro area's urban forest. Thousands of trees of all shapes and sizes fell onto houses, streets and boulevards.
• When trees crush homes and cars, who's responsible?
• Thousands still without power
• Photos from the storm's aftermath
• Xcel Energy power outage map
Lee Frelich, director of the Center for Forest Ecology at the University of Minnesota, spoke with Steven John of MPR News about what led to such extensive tree damage. An edited transcript of the conversation follows.
STEVEN JOHN: What was it about this storm that affected so many trees?
LEE FRELICH: The wind occurred over a wide area and apparently there were gusts in the range of 70-80 mph, and that's getting to the level where you can expect significant tree damage.
JOHN: Does the height and age of the tree determine whether or not it will come down?
FRELICH: Yes. Tall trees are sticking up there where the winds are higher. They have a bigger crown, or more sail for the wind to push on. Older trees, taller trees have thicker trunks that are less flexible.
If a tree isn't flexible in the wind, that puts all the force at the roots and so the tree is likely uproot or snapped off at the base.
JOHN: Does the wind pattern also determine whether a tree will topple?
FRELICH: The direction will often have a big influence on the outcome for a particular tree. If a tree is in a wind tunnel between building in the city and the wind happens to lined up with that. Or, for trees that are growing on city streets between the street and the sidewalk and so the roots are forced to grow in only two directions, and then a wind comes which is perpendicular to that and there are no roots holding the tree in that direction and the tree can tip over much more easily than if the wind was blowing parallel to the street.
JOHN: Does the type of soil make a difference?
FRELICH: Saturation of soil will allow the tree to sway back and forth in heavy winds and kind of jerk the root system. Usually the bigger roots will snap one at a time and eventually after several gusts of wind the tree will go over. When the soil is loose from being saturated that process is exaggerated, especially in soils that have a lot of silt and clay. In sandy soils, it's not such a big problem. The roots go down deeper in sandy soils and a lot of the excess rain drains away. We have a lot clay soils in Minneapolis. A lot of trees were uprooted for that reason.
JOHN: There are many species of trees that make up the urban canopy in Minneapolis. Is one more susceptible than the other to damage?
FRELICH: Basswood or linden trees had a lot of damage. Lindens tend to be hollow. We also have a long history of that species being planted too deep. A lot of ash trees went down for the same reason. Blue spruce get damaged because they have a shallow root system, so a lot of the time they don't hold on that strongly.
JOHN: Is there anything a homeowner can do to take better care of their trees so they're not so susceptible to damage during storms like these?
FRELICH: A lot of trees that blow down in urban areas are damaged in the wind because they weren't planted properly, especially trees that are planted too deep. This causes rot at the base of the trunk.
Trees that have girdling roots: If the tree was grown in a container in a nursery and has roots that go around in a circle - as those grow to bigger diameters it can actually choke off the base of the trunk and make it more likely to snap.
Hitting the tree with lawn movers or snow plows or other mechanical equipment can knock off a chunk of bark early in the tree's life and fungi that rot the tree can enter them. Many years later the tree can blow over and people don't even remember why it was that rot got into the tree.
JOHN: Was there anything surprising about this storm and the damage?
FRELICH: I was surprised to see how many trees weren't damaged. It was amazing to me that we have so many trees that withstand a heavy storm like that and come through it just fine and live for many more years. If the winds had been 100 or 110 mph it probably would have taken down the entire urban forest canopy. The winds were marginal in terms of doing major tree damage and I think we're very lucky.