This document summarizes the key characteristics of an effective urban forest for stormwater management. It discusses 10 factors: 1) providing large volumes of soil, 2) maintaining species diversity, 3) irrigating trees with small storms from hard surfaces, 4) storing roots and stormwater under pavement, 5) planting and maintaining trees correctly, 6) growing canopy and trunk rapidly, 7) developing dark green canopies, 8) allowing trees to reach maturity, 9) covering hard surfaces with canopy, and 10) rigorously sweeping and composting leaves. The document provides examples and research to support the importance of each factor for maximizing stormwater interception and management benefits from urban trees.

1. PRESENTED BY: Peter MacDonagh Kestrel Design Group/Univ. MN
L. Peter MacDonagh PLA, FASLA, RHS, ISA
Kestrel Design Group
University of Minnesota
November 2016: Partners in Community Forestry
Indianapolis, IN: Type II Storms; 42.4” Annual
Precipitation; 39% Urban Tree Canopy (Pre-EAB)
State of the Science using Trees for Stormwater
Management……
BIG TREES for Cleaner Rivers, Lakes, Bays

2. Images from Fairfax County Park Authority, Fairfax VA
An Upside Down World

3. TSS (Total Suspended Solids): Sand, Silt, Clay, Organics
Sand
Silt
Clay
TSS

4. NITROGEN & PHOSPHOROUS IN WATER…
PhosphorousNitrogen
250
LBS
1 Pound 1 Pound+ +

5. Huge
Pipe
Medium
Pipe
Flood Control
Volume
Rate
Water Quality
Localized Flood
Control
EMERGING PARADIGM
OLD PARADIGM
“Nearly all of the associated problems result from
one underlying cause: loss of the water-retaining and
evapotranspiring functions of the soil and vegetation
in the urban landscape.”
EPA - 2008
Image from Fairfax County
Park Authority, Fairfax VA
Big Trees For Stormwater Management? YES

6. WHAT DOES A SUCCESSFUL STORMWATER
URBAN FOREST LOOK LIKE?
It’s Trees:
1) Have Large Volumes of Air Filled Soils…
2) Species Diversity
3) Irrigated by Small Storms from Hard Surfaces…
4) Store Roots & Stormwater under Pavement…
5) Plant Trees Small & Maintain Correctly…
6) Grow Canopy & Trunk Rapidly
7) Grow Dark Green Opaque Canopy
8) Live to Maturity…
9) Cover Hard Ground Surfaces with Canopy…
10) Rigorously Sweep, Compost, Reuse Leaves
The Kestrel Design Group, Inc.
Copyright 2016
References:
Trees: Urban; Ball; McPherson; Xiaou; Watson; Green; Gilman; Bassuk; Grabowski; Smiley; Shigo.
Stormwater: Hunt; Weinstein, Schueller; Bannerman; Kim; Graham; Davis; Hsieh; Hong; Ermillio.

7. Requirements to grow a healthy tree
Trunk
Flare
Zone of rapid root taper
Sufficient soil volume
Water in
Water out
Room for
canopy
growth
Image: James Urban

8. Norway Maples
(Acer platanoides);
Zone 8; Type II
Storms; Germany
KEY #1: Have Large Volumes of Air Filled Soil
Soil Volume and Canopy Size?

9. Walt Disney World Orlando FL
USDA Zone 9; Type III Storms; 51”
Annual Precipitation
• Evaluated 1,127 Parking Lot Trees: 1-30 Years
Old; 21 Spp.
• Define Tree Success: Good, Fair, Poor,
Dead
• Relationships: Soil Vol & Tree Condition
• Test Applicability: Soil Vol
Recommendations
• GOOD CONDITION
• 100% of Trees in 1,500 CF (Cubic Feet)
• 95% of Trees in 1,000 CF
• 84% of Trees in 500 CF
• 65% of trees in 100 CF
KEY #1: Have Large Volumes of Air Filled Soil
>1000Ft3= a 95% Success rate

10. KEY #2: SPECIES DIVERSITY
(<5% UTC per GENUS)
9
Image to the right from the Forest History Society, Inc. at
http://www.appalachianwoods.com/appalachianwoods/history_of_the_american_chestnut.htm
American Elms:
Dutch Elm Disease
American Chestnut
Chestnut Blight
We Had Big Urban Trees & Lost Them
SuperTrees that Can Grow in Any Media
BUT Can’t Survive Monocultures!
American Ash:
Emerald Ash Borer

11. Grate with catch basin
& Distribution pipe
Water Harvesting
In urban spaces
2nd Ave. Calgary CA
KEY #3: Irrigated by Small Storms from
Hard Surfaces

12. 1) Infiltration + 2) Evapotranspiration (ET) + 3) Interception
Full ET credit for a mature tree
is given IF 2 c.f. of soil is provided
per s.f. of canopy
KEY #3: Are Irrigated by Small Storms from Hard Surfaces
Stormwater Credits Using Properly Planted Trees for Stormwater: Minnesota 2014
http://stormwater.pca.state.mn.us/index.php/Trees
Volume based performance goals for:
• New development
• Redevelopment
• Linear Development
• Sites with Restrictions

13. Swales-Expanding soil volume for trees
KEY #4 Store Roots & Stormwater under
Pavement

14. 13
• Pond liner to Remove Outside Catchment Variables
• Runoff from street directed via a catch basin & sump into distribution pipe into the Silva Cells (see A)
• Underdrains with upturned elbows slow water, denitrifies, then directs runoff into the Wilmington’s MS4 (see B)
• Profile by Jonathan Page, NCSU Biological and Agricultural Engineering
OUTIN TREATMENT AREA
A - New catch basin with sump
along curb line at upslope end
of system
A - 6” pipe
conveyance to Silva
Cells
A - Distribution pipe B – Loam in
Silva Cells
B - 1 ft (30 cm) upturned
elbow in underdrain
B – Underdrain connected to
existing catch basin
NCSU Research
KEY #4: Store Roots & Stormwater under Pavement

15. Percent of Runoff Treated
Significantly less bypass is expected at typical Silva Cell installations
because:
1) Pond liner was used so no exfiltration was possible – for typical
Silva Cell installations pond liner is NOT so exfiltration is possible.
2) Drainage area to these Silva Cell systems (1 tree per 0.1 acre)
was significantly greater than typical installations
of the runoff was treated by
the Ann St Silva Cell system
KEY #4: Store Roots & Stormwater under
Pavement
NCSU Research

16. KEY #4: Store Roots & Stormwater under Pavement
NCSU Silva Cell Monitoring Results
• NCSU performance
monitoring study in
Wilmington, NC
• Removal rates at or
above peer mean
bioretention mixes
• Particularly good
nutrient removal
– Nitrogen: 72-74% removal
vs. typical 14% leaching
(nitrates)
– Phosphorus: 35-60%
removal vs. 70% leaching
Source: Page, J.L., R.J. Winston, and W.F. Hunt, III. 2013. Field Monitoring of Two Silva
Cell™ Installations in Wilmington, North Carolina: Preliminary Monitoring Report.
N P MetalsTSS

17. Actual conditions: Average street tree
has access to
between 0.9 m3 (32 ft3) to
1.8 m3 (64 ft3) of soil.
MSP MN: Honeylocust (Gleditsia triacanthos) Zone 4;
Type II Storms; 31” Annual Precipitation
A Special Tree…..
Spilled Diesel?
Hours of Idling Machines?
Concrete & Sheetrock Soil Amendments?
String Trimmer Bark Treatments?
Deep Trunk Immersion?
Once Yearly Watering?
Salt Spray Foliar Feedings?
&
Ran out of Money?
KEY #5: Plant Trees Small & Maintain Correctly
Pride & Joy

18. …Contract Grow…
…Expensive Than Cheap…
KEY #5: Plant Trees Small & Maintain Correctly

19. Bare Root Gravel Bed Nursery…
…Cheap and Cheap…
KEY #5: Plant Trees Small & Maintain Correctly

20. Deep Holes are for Bodies not Trees…
KEY #5: Plant Trees Small & Maintain Correctly

21. Shallow Saucers are for Trees…
KEY #5: Plant Trees Small & Maintain Correctly

22. Water Deep at the Trunk
See Peterson’s 3:2:1 System
SAWS
KEY #5: Plant Trees Small & Maintain Correctly

23. No…the Most Important Tool for
Pruning…
KEY #5: Plant Trees Small & Maintain Correctly

24. Tree grate tripping hazards
Courtesy of Jim Urban
KEY #5: Plant Trees Small & Maintain Correctly

25. KEY #5: Plant Trees Small & Maintain Correctly

26. KEY #6: Grow Canopy & Trunk Rapidly
Primer To Plant Response
Dense Canopy
Dark Color
Twig Extension
Trunk DBH Increase

27. Willow Oaks:
40mm (19 inch) DBH
21.7m (91 feet) Tall
19m³ (700ft³) of loam soil / tree
98% survival rate (167/170)
Designed by Don McSween
USDA Zone 8; Type III Storms;
42” Annual Precipitation
E. Thomas Smiley et al 2009, 2010; Bartlett Tree Laboratory
KEY #7: Grow Dark Green Opaque Canopy &
#8: Live to Maturity
29 Year Old Trees in Suspended Pavement
Charlotte, NC – 1985
Trade & Tyron St. 29 years

28. KEY #7 Grow Dark Green Opaque Canopy & #8: Live to
Maturity
45 Year Old Trees Loam under Suspended Pavement
Christian Science Center, Boston, MA
Little Leaf Lindens
700 c.f. of loam per tree average,
but connected
100% Success Rate
Sasaki& Assoc. 1968
Zone 5; Type III; 44”

29. Bloor St.
Toronto, ON; CA
13cm (5”) diameter in 30 years
This tree is a potted plant…
Big Bonsai
30 rings
KEY #8: Live to Maturity

30. Stormwater Interception Hackberry vs Age of Tree
150 Gal.
Year 5
Images from http://www.tankwatersolutions.com.au/rainwater_tanks.php
5000 Gal.
Year 40
KEY #9: Cover Hard Ground Surfaces with Canopy
What’s So Great About Big Trees? Interception

31. MacDonagh 2014: Unpublished
53 Miles2
KEY #9: Cover Hard Ground Surfaces with Canopy
Minneapolis Chain of Lakes Correlative Study

32. In the Late 1800s,
American Elm (Ulmus
americana) made up 90%
of the boulevard trees in
Minneapolis
USDA Zone 4…..30.5” Annual
Precipitation…..Type II Storms
•1963: First Dutch Elm Disease
Detected in Trees
•1972: Elms Dying in the
Thousands
•1977: 31,000 Elm Trees Removed
•1978: 20,000 Elm Trees Removed
•2004: 10,000 Elm Trees Removed
•2005-2015: 2,700 Elm Trees
Removed Annually Almost 300,000 elm trees
have been removed from
the entire City, with an
average DBH of >30
inches.
Since 1977, 63,700 Elm
trees have been removed
in the boulevards of
Minneapolis
KEY #9: Cover Hard Ground Surfaces with Canopy
American Elm Tragedy: Minneapolis
http://princetonamericanelm.blogspot.com/

33. Total street tree population 120,676
-American Elms: 9.9% of total street trees
-30.75% of total street tree stormwater
benefits (total tree stormwater benefits) =
3,400,000 cf
= 78 acre feet
= Candlestick Park: 78 feet Deep Water
-3142 total elms >36” DBH = 2.6% of total
street tree population = 1,163,000 cf
= 27 acre feet
Tree size Stormwater Interception
DBH
(cm)
DBH
(inches)
cubic
meters/
tree/yr
Gal/tree/
yr cf/tree/yr
38 15 5.4 1,427 191
114 45 63 16,640 2,225
American Elms
Source: Minneapolis Municipal Tree Resource Analysis,
McPherson et al, 2005, and personal communication
KEY #9: Cover Hard Ground Surfaces with Canopy
Modelled Stormwater Value to Minneapolis
Benefits of Large Street Trees

34. 33
KEY #9: Cover Hard Ground Surfaces with Canopy
Relationship of Tree Species Diversity and Water Quality
There is a correlation to loss of tree canopy and water clarity
Following the removal of Elm trees (during the late 1970s and early 1990s), there was a
marked decrease in water clarity depth in the Chain of Lakes, yet building development
stopped in 1953 throughout the contributing sub-watershed around Lake Calhoun.
Lake Calhoun Lake Clarity Depth Over Time in METERS
DataCollectedfromtheCitizenLakeMonitoringProgramforLakeCalhoun,
MinnesotaPollutionControlAgency
RemedialAction
Meters

35. KEY #9: Cover Hard Ground Surfaces with Canopy
EAB: A Predictable Pattern of Losses
0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
PercentMortality
Years After First EAB Infestation
Ash Mortality from EAB
Based on data from Dr. Dan Herms, The Ohio State University
MPRB 5000 Ash Trees being Removed Annually; Average Ash >18” DBH

36. KEY #9: Cover Hard Ground Surfaces with Canopy
Relationship of Tree Species Diversity and Water Quality
Elm Canopy Loss
Potential Ash
Canopy Loss
Dutch Elm Disease
& Emerald Ash
Borer

37. KEY #9: Cover Hard Ground Surfaces with Canopy
40% by ’40…for Minneapolis
40% Urban Tree Canopy by 2040

38. Tree Leaves & P…What to Do???
…Not This…
KEY #10: Rigorously Sweep, Compost, Reuse Leaves

39. Tree Leaves & P…What to Do???
…Do This…
KEY #10: Rigorously Sweep, Compost, Reuse Leaves

40. WHAT DOES A SUCCESSFUL STORMWATER
URBAN FOREST LOOK LIKE?
It’s Trees:
1) Have Large Volumes of Air Filled Soils…
2) Species Diversity
3) Irrigated by Small Storms from Hard Surfaces…
4) Store Roots & Stormwater under Pavement…
5) Plant Trees Small & Maintain Correctly…
6) Grow Canopy & Trunk Rapidly
7) Grow Dark Green Opaque Canopy
8) Live to Maturity…
9) Cover Hard Ground Surfaces with Canopy…
10) Rigorously Sweep, Compost, Reuse Leaves
References:
Trees: Urban; Ball; McPherson; Xiaou; Watson; Green; Gilman; Bassuk; Grabowski; Smiley; Shigo.
Stormwater: Hunt; Weinstein, Schueller; Bannerman; Kim; Graham; Davis; Hsieh; Hong; Ermillio.