SUMMARY OF STORM DAMAGE

to URBAN TREES IN MINNESOTA

1995-2005

 Gary  Johnson, Extension Professor
Urban and Community Forestry
University of Minnesota

Data Collection
Forty-two (42) tree species were documented as damaged by wind and/or ice loading events in Minnesota between March, 1995, and October, 2005.   One thousand five hundred eighty-four (1584) individual sample units (damaged trees) were examined and the following data was collected from each sample unit:

• date damaged
• location (community/address)
• siting (lawn, blvd., etc)
• species
• size
• type of storm*
• type of damage
• any preexisting conditions that were associated with the failure/damage
• other comments.

The Types of Damage were collapsed into three general categories.

               Category 1:                           Category 2:                      Category 3:
             Total Failure                         Stem Failure                  Canopy Failure

  The tree failed at or below       Failed between the          Failed at or between the
         the groundline.                ground line and the first            first set of true
                                                  set of true branches            branches and the
                                                                                          terminus of the canopy
           

            *In the case of severe wind-loading events (i.e., straight-line winds, tornados) individual sample units were those determined to have been out of the direct path of the wind.

Results:

I.          Most commonly damaged species (all storms, all types of damage):
SPECIES                                            % OF TOTAL
                        Green Ash                              18
                        Littleleaf Linden                      14
                        Colorado Spruce                    11                                           
                        American Elm                           8
                        Silver Maple                             8
                        Hackberry                                7
                        Sugar Maple                             6

II.        Most common type of damage (all storms):
DAMAGE                                          % OF TOTAL
                        Total Failure                              54
                        Canopy Damage                        29
                        Stem Failure                              16
                        Multiple Failure/Damage              1

III.       Most common pre-existing conditions:

• For all damage, separating preexisting conditions:

CONDITION                                     % OF TOTAL
        Decay  Only                                      13
        Stem Girdling Roots (SGR)                12
         Included Bark Only                            4
         Root Problems (other than SGR)        3
        Codominant Leaders Only                   4
        Construction Damage Only                  1

• For all damage, combining preexisting conditions:

CONDITION                                     % OF TOTAL
        Decay+Codominance+Inclusion        16
        Codominance+Inclusion, Inclusion     13       
                  and Codominance.
         SGR + Decay                                  13
           
IV.       Most commonly damaged size (d.b.h.) ranges (all storms, all damage):
SIZE (d.b.h.) RANGE                                   % OF TOTAL
                        10-15 inches                                        24
                        > 25 inches                                          22
                        6-10 inches                                          18
                        20-25 inches                                        16
                        15-20 inches                                        17
                        < 6 inches                                            3

V.        Most common sites (for all storms, all types of damage):
SITE                                                   % OF TOTAL

   Park                                                         42
 (parks, schools, campuses, golf, courses)

Boulevard                                                    30
(tree lawns, with or w/o sidewalks) 

Lawn                                                          28
(residential and commercial private greenspaces)
           
VI.       Sorted Statistics:

• Types of Damage with (most common) Species Ranking:

1          TOTAL FAILURE              %
            Littleleaf Linden                     18
            Green Ash                             15
            Colorado Spruce                   12
            Pines                                      06
            Silver Maple/Crabapple          04

2          STEM FAILURE                 %
            Hackberry                              22
            Norway Maple                        7
            Littleleaf Linden                       7

3          CANOPY FAILURE           %
            Green Ash                              16
            Sugar Maple                           12
            Hackberry                              11
            American Elm                         11
            Oaks                                      10

• Commonly Damaged Species with Chronic Problems:

• Green Ash: 78% of those that suffered stem and canopy failure had included bark and decay as preexisting conditions for failure.

• Littleleaf Lindens: 76% of those that suffered total failure had 4 inches or more of soil over their first main-order roots and had stem girdling roots causing stem compression.

• Colorado Spruce: 83% of those suffering any type of failure had NO preexisting conditions for failure.

• Common Preexisting Conditions Associated with Failures:

• For all trees that failed totally, and were located outside of the storms' centers, 32% had 4 inches or more of soil over their first main-order roots and had stem compression from stem girdling roots.

• For all trees that suffered canopy failure, 78% of the trees had included bark, codominant leaders, codominant leaders with included bark, and/or decay associated with the inclusion and codominance.

• Total Tree Failures in Boulevards.
• As a percentage of all Total Tree Failures = 24%.

• Most commonly damaged tree size categories:

6-10" d.b.h.     29%
>25" d.b.h.      26%
20-25" d.b.h.   16%
10-15" d.b.h.   14%
15-20" d.b.h.   14%
<6" d.b.h.          1%

• Tree size categories and most common species damaged.

6-10"               Littleleaf linden          40%     (green ash 25%)
>25"                Green ash                    50%     (Am. elm 39%)
20-25"             Green ash                    45%     (Am. elm 36%)
10-15"             Littleleaf linden          40%     (no close second)
15-20"             Green ash                    50%     (linden 30%)

4.         Boulevard trees suffering Total Tree Failure

            A.  Due to stem compression from girdling roots and associated with deep entombment of roots:
                                   

• As a percentage of all Total Tree Failures in boulevards = 26.0%.

• The percentage of Littleleaf lindens that failed at compression points from stem girdling roots = 68%

• Trees in the 6-10" category that failed at compression points from stem girdling roots = 53%.

B.  Due to site limited root plates (boulevards too narrow to accommodate a proportional root system for the size of the tree) =  74%.