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Urban Trees

Explore the history of urban tree landscaping and learn about the findings from research studies conducted in Anchorage. Discover the importance of soil volume, tree location, curbs, mulching, and irrigation for the health and survival of urban trees.

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Urban Trees

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  1. Urban Trees The Dirt(y) Truth (As we know it) Dwayne Adams, FASLA

  2. First, some history • Initial “Streetscaping” started around 1980 • Mountain Ash on 5th Avenue • Birch on 6th Avenue • Concrete pavers

  3. Anchorage International Airport-1980 • 1000 crabapples planted • 73 survive today • Problems: • No maintenance buy-in • Voles • Long-term resistance to street landscaping

  4. 4th Avenue-1983 • 4” Curb to protect tree from snowmelt • Considered tripping hazard in winter • Were removed and flat curb installed

  5. Providence Drive-1984 (Chan & Chan) • Wide median • Rock with concrete planters for annuals • Generally very successful with some exceptions: • Rock filled in with sand • Annual plantings are expensive to maintain • Some of median plantings were crowded

  6. 5th/6th Avenue Street Improvements-1989 • Sloped curb • Larger rooting volume

  7. Other Notable Projects-1987 • Anchorage 5th Avenue • Canopy issues • Salt issues • Crowding problems • Limited rooting volume

  8. Research-Intern Projects • Land Design North conducted research over 2 different periods • 1991-388 trees sampled • 1995-2712 trees sampled

  9. 1991 Study • Anchorage CBD only • Generally young trees from 1980s w/in CBD • Ed Gross-intern from University of Idaho • Evaluated on a “tree health rating” of 0-12: • Species • Distance from curb • Exposure (N, S, E, W) • Curb type • Grate opening size • Proximity to glazing

  10. 1991-Findings • No discernible pattern of survival related to species for those trees used in the CBD. • For distance from curb • Trees should be greater than 3.5 feet from the face of curb.

  11. 1991 Study continued • Exposure • There was no correlation of exposure (N, S, E, W) to health • Curbs • Trees with raised or sloped curbs did much better than trees with flat or no curbs

  12. 1991 Study-continued • Plant Pit Size • There was no discernible difference in the effect of plant pit size on the health of the trees • Young trees may have been a factor in this

  13. 1995 Study • Conducted by Greg Huber, intern • 2712 plants surveyed • Health assessment based on scale of 0-10

  14. 1995-Items evaluated • Plant species • Distance from plant to curb • Drainage of snowmelt • Wind intensity • Sideslopes • Height of rootball above roadway • Solar orientation • Adjacent structures • Road speed • Planter types • Mechanical damage

  15. 1995-Roadways evaluated • Minnesota Drive • Northern Lights Blvd. • Benson Blvd. • Spenard Road • Arctic Boulevard • Old Seward Highway • Lake Otis Parkway • International Airport Road

  16. 1995 Study-continued • Species • Prunus (sp.) do better • Health is directly correlated to moose damage • No difference found after discounting moose damage

  17. 1995 Study-continued • Distance from curb • Average rating of plants within 2 feet of pavement was 5.5 • Those within 10 feet of the pavement was 6.25 • Those outside of 10 feet had an average rating of 7

  18. 1995 Study-continued • Drainage • Plants within 15 feet of the roadway, with drainage towards the roadway, had a health rating of 7.5 • Those within 15 feet of the roadway, with drainage neutral or towards the plant had a health rating of around 7 • Those greater than 15 feet from the roadway were not affected by runoff and may improve with runoff toward the tree

  19. 1995 Study-continued • Planter type • Elevated beds and planters had an average health rating of 8 • Non-mulched beds, medians, or trees in lawns had health ratings of 6.5

  20. 1995 Study-continued • Damage • 50% of plants in medians exhibited damage • 50% of plants in lawns exhibited damage, generally girdling due to mowers/weed whips • Spruce trees in particular were damaged by plowed (thrown) snow

  21. 1995 Study-continued • Irrigation • Disregarding damage, irrigated plantings had an average health rating of 8.8 • Non-irrigated plantings had a health rating of 7.7

  22. Summary of Study Findings • Trees on CBD streets should be greater than 3.4 feet from curb • Trees on other streets should be 10 feet away from roadway edge • Elevate plants if near roadways • Get salt away from the trees with curbs • Plants in lawns need mulching, not lawn • Irrigate

  23. Let’s talk dirt • Individuals such as James Urban, FASLA have greatly changed the way the industry looks at plants • It’s all about dirt, water, and air

  24. Soil Volumes • James Urban recommends that soil be apportioned to tree size

  25. Soil Volumes • A 4-inch caliper (diameter) tree needs 200 cubic feet of soil to survive These trees showed signs of stress 8 years after planting, died 3-5 years later

  26. Northern Soils-Acid Buildup • Roots and decaying organisms release carbon dioxide • photosynthesis which releases oxygen occurs in leaf tissues • Carbon dioxide in combination with other materials create acidic compounds • Since weathering in northern locations is slow, acids persist • The persistence of the hydrogen ions prevents the availability of positively charged nutrients

  27. Temperature and Plant Growth • Enzyme activity within the plant slows with cold. Activity doubles with every 10 degrees C, up to 30 degrees C. • Solubility of gases is limited by cold. Toxic gases may build up in soils and in the plant. • Water is harder for plants to pick up at lower temperature. Viscosity doubles from 25 degrees C to 0 degrees C. • Heat balance is disrupted with the difference in soils and air temperature, creating freezing of cells.

  28. Soil Temperature and Plants • The “whiplash curve” explains the limitations of soils on temperature • Biologic activity slows dramatically below 50-degrees Fahrenheit

  29. Effect of the whiplash curve • Soils are too cold to have much biologic activity below 24 inches • Pancake roots • Limited ability of small areas to provide adequate rooting media

  30. Road and Walkway Construction • Non Frost Susceptible (NFS) soils have low levels of fines, hold little or no water • Plants need soil fines to hold nutrients and water • Plants are not going to get nutrients or water from roadbeds or walkways

  31. Soil Composition • The best balanced soil is a loamy soil of approximately 5% organics by weight • Municipal standards: • Organic Matter 15-25% by dry weight • Silt 25%-45% by dry weight • Sand 35%-55% by dry weight

  32. Salts and Trees • Increased winter tourism and liability concerns have increased use of salts on downtown sidewalks

  33. Salts and Trees • Salt affects: • Retention and acquisition of water • Disrupts chloroplast functions • Affects ion exchange • Limits detoxification

  34. So What Have We Learned? • It’s about • Soil Volumes • Air and water • Soil Temperatures • Soils mixes • Salts

  35. The “Best” Scenario • Elevate the plant • Greater than 3.5 feet from road • Give it rooting capacity • Give it air, light, water

  36. When Space Is Not Available “Bridged Planters” provide more extensive plant root zones Very expensive- $10,000 per tree

  37. Ways to make this less expensive • Fairbanks Rabinowitz Courthouse • Topsoil was placed • Concrete placed on top of topsoil-topsoil then settled • Engineers and contractors fear slump of concrete

  38. Bridged Plantings vs. Heated Sidewalks • Heated sidewalks leads to a disparity in the rootball temperature vs. crown temperature

  39. Dena’ina Convention Center • Will have heated sidewalks • Maples will be planted on F Street in bridged planters

  40. Newer Technology • James Urban system • DeepRoot Silva Cells • Allows AASHTO H20 loading with cast in place concrete Image from Ian Wilson, Urban Forestry Supervisor and Terry Barton, Park Planner, City of Kelowna, BC Pacific Northwest Trees Magazine, Volume 28, No. 3, Fall 2007.

  41. DeepRoot Silva Cells Trench is fit to size requirements of trees Image from Ian Wilson, Urban Forestry Supervisor and Terry Barton, Park Planner, City of Kelowna, BC Pacific Northwest Trees Magazine, Volume 28, No. 3, Fall 2007.

  42. DeepRoot Silva Cells DeepRoot cells and irrigation lines are placed Image from Ian Wilson, Urban Forestry Supervisor and Terry Barton, Park Planner, City of Kelowna, BC Pacific Northwest Trees Magazine, Volume 28, No. 3, Fall 2007.

  43. DeepRoot Silva Cells Final Product Image from Ian Wilson, Urban Forestry Supervisor and Terry Barton, Park Planner, City of Kelowna, BC Pacific Northwest Trees Magazine, Volume 28, No. 3, Fall 2007.

  44. Bridged Plantings vs. Heated Sidewalks • Isolation of the rootball is needed • Insulation is required between the heated sidewalk and plant pit

  45. Summary of Lessons • Setback from and elevation above the road are good ideas. • Protection from mechanical damage is important • Salt runoff is an increasing problem • If you must place trees in sidewalks, provide volume, air, and water

  46. Urban Trees The Dirt(y) Truth (As we know it) Dwayne Adams, FASLA

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