Urban Trees and Green Infrastructure.pdf - R. leBrasseur, PhD

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1.0 Urban Trees and Health – an Introduction Growing trees successfully in an urban environment is difficult. Most trees do not survive their first two years, and the average lifespan in urban environments has been estimated to be 7-10 years. The primary reason for this poor survival rate is the inadequate rooting environment found within many urban sites; especially newly constructed areas, where soil and fill are often mixed and compacted by heavy construction traffic and covered with a layer of topsoil. Trees are an important part of ecological and human wellbeing, particularly in an urban environment such as cities. Trees provide many diverse social, environmental, and economic benefits. When viewing a tree in its entirety, there are many parts which are not readily seen, each serving different role and function. Figure 1 Diagram of the parts of a tree. Trees help make happier and healthier places to live. Urban trees provide social, recreational and psychological benefits to people. Urban trees add to the identify of urban communities foster social cohesion among residents. They provide spaces to gather and share, to recreate and play, to relax and disconnect. Urban trees beautify cities adding an important element to monotonous concrete Page 1 of 24 structures and have been shown to improve physical and mental health by decreasing high blood pressure and stress and encouraging walking and recreation. Urban trees have been shown to reduce crime and increase safety. Urban trees also form an effective sound absorbing barrier to help reduce unwanted urban noise pollution. Urban trees contribute to climate change mitigation. They absorb CO2 and clean the air of pollutants while returning O2 to the atmosphere. Urban trees cool the air and reduce ‘heat island effect’ by not reflecting the sun’s radiation back into the atmosphere. They reduce wind speed in cities. Urban trees add to the concept of an ‘urban forest’ and play an important role in increasing urban biodiversity or providing plants and animals with favourable habitat, food and protection. Urban trees regulate water flow and play a key role in preventing floods and erosion and reduce dangerous sewer overflow during storms by intercepting and storing water in their root soil areas and slowing down water with their leaves and branches. Additionally, urban trees filter and clean the water of pollutants and other chemicals through their leaves and root soil areas. Urban trees add to a more sustainable city environment, reducing energy costs like air conditioning and other resource uses. Urban trees can increase property value, by up to 20 percent, and attract tourism and business. This comprehensive list of benefits is often discussed of in terms of ecosystem services, or the benefits trees provide, and many have a monetary value assigned to them. Urban street trees are a common sight in cities. Different from trees or groups of trees within urban parks, plazas, and gardens, urban street trees are often seen along city sidewalks and residential streets. Typically set in a row, not in a group, these singular urban street trees provide all the benefits listed prior as well as others. Research has also shown a 60% reduction in particulates from car exhaust fumes on streets lined with trees. Street trees have been shown to increase road safety by improving driver attention and reduce speeding. They create safer walking environments by separating pedestrians from vehicles and are sometimes placed in medians in between streets. Urban street trees add to the ‘greening’ effect of cities creating places to sit and screening unsightly features. They also further assist in water management and require less drainage infrastructure along urban streets such as curb inlets and underground pipes. Page 2 of 24 Figure 2 Example of urban street trees. SOURCES: Source - many including McPherson, Nowak, & Rowntree, 1994; Killicoat, Puzio, & Stringer, 2002; Scott, McPherson,& Simpson, 1998; Kuo & Sullivan, 2001; Anderson & Cordell, 1988; Price, 2003; Tyrväinen, Pauleit, Seeland, & de Vries, 2005; Naderi, 2003; Kuo, 2003; Westphal, 2003; Ewing, 2003; Foster and Blaine, 1978; Skiera and Moll, 1992; Nowak et al., 2004; Alberty,, Pellett, and Taylor, 1984; Bassuk, Whitlow, 1985; Brady, 1990; Craul, 1992; Lindsey, Bassuk, 1992 2.0 The Difficulties of Maintaining Proper Health of Urban Trees Removing, placing and growing trees in their non-natural environments such as cities and along urban streets is and continues to be a difficult task. Many conflicts arise, particularly between urban trees and human safety as well as infrastructure such as pipes and utilities. Sidewalks are an important of human urban life and allow residents to walk safely from one location to another in the city - to meet friends, shop, stroll and enjoy nature, recreate, and others. In urban conditions, maintaining human safety and tree health is a primary concern. Trees have a unique way of growing; their roots are the primary part to receive air, water and nutrients which enable growth. For many decades, the typical root structure knowledge was roots which matched the depth and width of the tree as shown in figure 3, Far left. This was shown to be wrong (sometime around 1990); there are significantly more lateral roots which grow along or at the surface of the planting area, Figure 3, Near left. The survival of urban trees depends on the health of their roots. Roots supply water and nutrients to the shoots, and get back sugar and other compounds they need to live and grow. Roots also store food, synthesize hormones, and provide structural support. Page 3 of 24 Figure 3 Root structure diagram Roots grow where the conditions are most favorable. These factors include soil, water, nutrients, air, and areas of unobstruction or blockage such as compacted soil, rock, and curbs or concrete walls. Other factors affecting root growth are temperature, presence of contaminants, and the soils ability to hold on to important nutrients and resist acidification (i.e. cation exchange). Root distribution or root spread is a key component of tree health in urban areas. Roots will seek out these most suitable conditions; this is typically found close to the soil surface where soil is the loosest or least compacted, and water, nutrients and oxygen are most readily available. Roots tend to not grow down in urban conditions, as was common thought. Soil conditions are often regarded as the most important condition for root growth and urban tree health and characteristics include its texture, structure and fertility or nutrient capacity. Soil texture or the percentage of sand, silt, clay, and organic matter also includes smaller amounts of water and air. These soil structure characteristics determine the soils nutrient, aeration, and moisture capacity as well as vulnerability to soil compaction. Soils in urban conditions are already heavily modified, often through prior removal, grading, construction, or other disturbances. Stony, compacted, or wet soils will cause roots to die back or turn away. Obstructions or barriers such as concrete walls, curbs and building foundations or other infrastructure will have similar results. When trees are placed in urban conditions and along streets, their soils tend to become compacted due to the constant weight of hardscape as well as the pedestrian and vehicular traffic in and around the tree root area, see Figure 4. These compacted soils lack adequate pore volume or empty spaces in which to store and process air, (drain) water and nutrients, see Figure 5. Page 4 of 24 Figure 4 Soil compaction around an urban tree. Figure 5 Soil compaction diagram. Page 5 of 24 Most urban trees and street trees grow in small areas or planting holes with limited rooting space and with soil considered compacted to heavily compacted. These conditions restrict root growth, decrease air volume, and limit water drainage or the ability water to move through soil, and makes it harder for trees to develop safely and successfully, see Figure 6. Figure 6 Images of compacted soil, left, where roots become flattened and compressed; and healthy porous soil, right, where roots are evenly distributed. There are many negative impacts which result from urban trees trying to maintain their best health within improper growth conditions and compacted soils. These include stunted growth, limited or smaller root systems, higher susceptibility to disease and insects, overall weakness, as well as root damage to infrastructure such as utilities, pipes and street furnishings, all of which add to aesthetic, maintenance and safety concerns. Tree root stress includes oxygen stress, water stress, nutrient stress, temperature stress, and compaction stress. Tree roots must find and use water and oxygen wherever they can find it. Roots coming out of a newly planted root balls into compacted soil tend to grow from a depth of 12 or 18 inches upwards, see Figure 7. This is because, in an urban environment, a small gap will develop over time between the top of the compacted soil and the underside of the sidewalk where moisture and air collects. Trees and their roots require these for growth, thus they will migrate towards this area directly underneath the sidewalk or hardscape, see Figure 8. Page 6 of 24 Figure 7 A new urban tree being delivered and prepared for installation. Figure 8 Tree roots growing under sidewalk area seeking air and water and nutrients for growth and health. Page 7 of 24 As the roots grow, they increase in diameter and raise the sidewalks and nearby curbs, creating safety hazards and accessibility limitations, See Figure 9. The horizontal or lateral roots that are causing the problems are the same roots that keep a tree stable against storms and wind. Eventually the sidewalk will have to be replaced because it has been lifted by the root growth. Figure 9 Tree root damage to hardscape or sidewalks including cracking and heaving. When roots seek optimal conditions, they tend to wrap around and break pipes seeking air and water causing significant damage, see Figure 10. Roots of urban trees also tend to ‘girdle’ if not given proper conditions. Girdling roots occur when the roots of a tree grow next to or around the trunk instead of away from the tree, see Figure 11. Figure 10 Tree root damage to underground pipes and utilities. Page 8 of 24 Figure 11 Tree girdling. Improper tree health leads to overall weak tree structure where limbs and branches become brittle and can fall off, in storms, from wind - potentially damaging people and infrastructure, see Figure 12; maintenance and arborist treatment are required and sometimes tree removal. Figure 12 Tree weakness due to poor tree health. Page 9 of 24 2.1 Urban Tree Difficulties Summary Trees face many diverse problems to survive within urban conditions. Urban areas for the most part are not designed with trees in mind; they are often treated as if they were an afterthought to an environment built for cars, pedestrians, buildings, roadways, urban drainage, sidewalks and utilities. As discussed, an adequate soil volume is central for tree root growth and overall health as soils are where nutrients, water and air are held and allows for root growth, water, air, and nutrient access. Generally, research has concluded that trees require two cubic feet of soil per every square foot of crown diameter or projection (see figure 1 prior). For example, a tree with a canopy diameter of 20 feet, the crown projection would be 314 square feet and the tree needs 628 cubic feet of soil to support it. At a standard root hole depth of 3’, this would be 20’ x 10’. Thus, a typical 4’ x 5’ tree opening in urban sidewalks is wholly inadequate. Overall, most trees in an urban condition are considered to be growing at some level of stress; they are doing their best to survive in a non-natural environment and challenging conditions, see Figure 13. Further impacts to urban trees come from urban insects and pests, ice damage, winter salt application, pet fouling, vehicle damage, improper pruning, maintenance and treatment, over or under watering due to inappropriate, broken or lack of irrigation, unsuitable tree species selection, lack of effective urban drainage systems, pollution and chemical particles, and others. Figure 13 Diagram illustrating the stressors and impacts found in urban street tree conditions. Page 10 of 24 The damage caused by these impacts and improper growing conditions such as compacted soil is a slow process and it may be several years before urban tree shows any outward signs of decline, see Figure 14. Urban trees are an important component of our life, and given the numerous benefits, it is critical we provide them with the best conditions in which to survive, thrive, and reduce damage and conflict. Figure 14 Examples of urban tree impacts. Upper left - scale on a tree trunk. upper right - utility line tree trimming; bottom left - inadequate drainage around an urban tree planting; bottom right - drought effects in tree leaves. SOURCES - many including Day, 1991; Francis, Parresol, & Marin de Patino, 1996; Lesser, 2001; Grabosky & Bassuk, 1995; Dobson, 1995; McPherson and Peper 1995; Taylor and Brar, 1991; Coder, 2000; Nicoll and Armstrong, 1998. Page 11 of 24 3.0 Methods of creating healthy growing environments for urban trees There are many techniques and strategies for improving the health and growing conditions of urban trees, some have had better results than others, but overall none have solved all the aforementioned problems together or fully effectively. The following section shall discuss the most common approaches and illustrate how these techniques have addressed some of the important concerns to tree root growth and health. 3.1 Urban tree selection Generally, there are only 10 - 20 tree types being selected for urban tree systems as described in southern Canada, and most of them are special hybrids or varieties specifically modified and selectively grown to survive these unnatural urban conditions. Characteristics included smaller root areas, disease resistance, drought tolerance, alkaline tolerance, and reduced messiness such as fruit, seed, and leaf droppings. Problems remain, however, in fulfilling their healthy growth and long-term success. 3.2 Structural soils designed to avoid compaction Structural soils are soils that are specially designed with stone mixed into soils that are modified to provide nutrients, pore space or open pockets to accommodate air and water movement and root growth while also allowing for compaction to support pavement and traffic on the surface. The rocks within the structural soil provide the load-bearing support for pavements and civil-engineers assist in determining proper structural soil mixes. Additionally, the larger pores within the soil max often allows the infiltration or percolation of water down into the soil and thus tree root area, see Figure 15. Also called manufactured soil or manufactured tree pit soil. However, these structural soils often lack nutrients and over time the soils eventually become compacted which affected proper tree root growth and health. Structural soils require certified testing by a pavement civil engineer as the cost of failure is significant. Thus, the soil mixes tend to be excessively focussed toward structural integrity and less so on tree root growth and health. Figure 15 Structural soils in urban tree planting. Page 12 of 24 3.3 Air and watering tubes Compacted soils impeded water and air intake to the root soil areas. Typically, these urban planting areas have PVC circular tubes with holes in them inserted vertically into the ground next to or in the root ball zone growth area. This could include 1 to 4 tubes, typically 2” to 4” in diameter about 2’ - 4’ deep, see Figure 16. This allows ambient air and rainwater to enter the root growth area as well as a place for excess water to leave. Alternatively, water and fertiliser could be added manually if needed. These are often called soil aeration tubes or soil watering tubes. Irrigation of urban trees, particularly street trees, is not common given the expense and history of damage. Root bubblers or direct irrigation, installed below the surface at the root growth area, are prone to root damage and deterioration. Furthermore, these tubes often fill with growing roots, thereby blocking any air and water passage. Though they are inexpensive, their value of delivering air, water and nutrients is only most effective in the first 1-2 years of tree growth. Figure 16 Soil aeration and watering tubes Page 13 of 24 3.4 Root pruning Root pruning refers to the roots being cut and removed to not cause hardscape conflicts or upheaval followed by a sidewalk or hardscape re-installation, see Figure 17. Other terms include root shaving, or root cutting. Root pruning is a very technical process, requiring an expert, which is not always done. Root pruning is not a long-term solution; roots grow-back and the problem remains. Root decay is common and the labor hours for this technique is expensive. Tree growth is stunted and limited resulting in weak plant structure, often exhibiting poor health and prone to wind damage, disease, and other stressors. Figure 17 Root pruning. Page 14 of 24 3.5 Tree root control barriers Root control barriers inhibit or deflect tree root growth away - often down - from areas where tree roots would do significant damage such as sidewalks and infrastructure such as utilities and pipes. Common names are biobarriers, root deflectors, root traps, and geotextiles. A simple root control barrier is root mesh or screens of welded metal sheets or woven fabrics (e.g. copper mesh screens, woven nylon). The small screen hole size allow limited root growth and strong screen material strangles and girdles the root as it grows. See Figure 18. Advantages include water and air permeability. Disadvantages are limited large root development leading to potential instability of tree, root girdling, downward root growth - not lateral, and poor overall health. Soil compaction is still frequent. Figure 18 Tree root control barrier, root trap, or screen. The most common root control barrier is a thick fabric or plastic / PVC barrier, though concrete walls were also used in some conditions with the tree control barrier laid next to it. This is most often used in urban conditions in tree planting areas next to sidewalks or roads. The plastic or fabric ‘wall’, redirects root growth away or down or away from sidewalk or utilities. Because downward growth is not guaranteed, some designs included vertical ribs to further direct root growth downward. See Figure 19. Sometimes a root control barrier could be placed directly under the hardscape or horizontally to prevent upheaval from roots. The fabric or woven types could wrap pipes and utilities and some allowed for water to pass through them. Advantages are that they are relatively cheap, easy to install and could be installed linearly such as along street tree trenches or pits. However, they could be large and heavy-duty. Urban street conditions often require cumbersome panels over 4’ in depth. They inhibit air and water flow and root growth redirection causes additional tree health issues. Unnatural root growth leads to improper health and instability. Soil compaction is still frequent. Page 15 of 24 Figure 19 Tree root control barrier - plastic Quite often, the root control barrier is impregnated or has added a strong herbicide to inhibit root growth such as trifulralin. Advantages are that the chemical generally keeps the root tip from growing, thus containing the spread of the entire root system. Disadvantages include root girdling and loss of chemical effectiveness over time. Unnatural root growth leads to improper health and instability. Trees are more prone to wind damage, disease, and other stressors. Soil compaction is still frequent. Tree control root barriers are often not effective because tree roots continually seek out air, water and nutrients typically found in nearby, non-compacted soils. As shown in Figure 20, tree root growth continues in the direction the barriers were supposed to prevent, often around or under them. Figure 20 Tree root control barriers do not redirect root growth effectively. Page 16 of 24 3.6 Tree pits This technique is still often used in urban conditions. They are essentially underground boxes. They could be simply dug holes with gravel or undisturbed soil at the edges, but in urban conditions, they are most often concrete and precast or poured (i.e. constructed) on-site. The concrete could be on all 4 sides or just 2. Tree pit sizes vary but most often 4’, 6’, or 8’, square and rectangular in shape. They are often used when there are utilities underground and the goal is to prevent damage from roots. Utilities are often in a separate area or utility corridor, but not always. This pit is often lined with a type of root control barrier. This pit could then be fitted with soil aeration and soil watering tubes. Tree pits are often called tree boxes, underground tree boxes, tree wells, or concrete planter boxes. See Figure 21. Advantages include they are easily sourced materials and limited subsurface conflicts with infrastructure and poor soils. Disadvantages include limited tree pit volume for root growth thereby not allowing for horizontal or lateral growth of roots. They require oversight and maintenance for water and nutrients. Tree growth is stunted and limited resulting in weak plant structure, often exhibiting poor health and prone to wind damage, disease, and other stressors. Overall, there is limited water coming into the tree growth area and this promoted urban water runoff. Roots break through concrete or other material causing safety hazard and damage to nearby infrastructure. These often require manufactured tree pit soil. Often the only porous or non- hardscape surface in an urban condition or street, tree pit’s limited water drainage results in a saturated tree rooting zone, though some may have drainage pipes in the root growth area. Figure 21 Tree pits. Page 17 of 24 3.7 Raised tree boxes This technique is often used in urban conditions. They are essentially aboveground boxes or ‘raised’ tree boxes. The most common material is concrete and could be precast or constructed on- site, though metal, wood, and plastic types are available. This box is also typically lined with a root control barrier. Box sizes varied but most often 4’ or 6’ square; height varies as well but infrequently over 4’. These are often used when there are utilities or very poor soil underground. Common names are elevated planters or raised tree boxes. See Figure 22. Advantages include they are relatively cheap and easy to install and eliminates most underground conflicts. Disadvantages are that the tree box volume is limited; tree girdling occurs and growth is stunted resulting in a structurally weak tree, often exhibiting poor health and prone to wind damage, disease, and other stressors. They require considerable oversight and maintenance for water and nutrients. Tree box drainage is typically not accounted for, though small holes could be provided. They are not the most visually pleasing. Tree roots often break through the bottom corners of the concrete causing safety hazard and damage to nearby infrastructure. Soil compaction is still frequent within the raised tree box. Figure 22 Raised planter boxes. Page 18 of 24 3.8 Tree trenches and tree corridors This technique is also often used in urban conditions. This technique is essentially a larger or longer tree pit whereas there is a continuous channel of soil material in which for tree roots to grow, the goal to provide larger volume for root growth. Essentially, they connect the individual tree pits with soil underneath the hardscape, sharing soil space. Other names include continuous tree pit, subsurface tree corridor, underground tree corridor, continuous soil trench. Similar to tree pits, the most common material is concrete in urban conditions, and could be precast or constructed on-site. Length of trench varies. This is often used when there are utilities underground and the goal is to prevent damage from roots. Utilities are often in a separate area or utility corridor, but not always. This trench is often lined with a type of root control barrier and fitted with soil aeration and soil watering tubes. The trenches are filled with underground utilities and pipes then with the soil medium for tree root growth. Concrete or other hardscape could then be placed over this trench area. See Figure 23. Advantages include a larger tree root area and reduced underground conflicts with infrastructure and poor soils. Disadvantages are that they are expensive and still have a limited tree pit volume for root growth such as lateral root growth, particularly radially or out in multiple directions from the tree trunk not just in the tree trench direction. Compacted soils are typical. Tree growth is stunted and limited resulting in weak plant structure, often exhibiting poor health and prone to wind damage, disease, and other stressors. Overall, there is limited water coming into the tree growth area and this promotes urban water runoff. Roots break through concrete or other material causing safety hazard and damage to nearby infrastructure. This technique requires low-compacted or manufactured tree pit soil to provide better tree root growth, but this is not always provided, resulting in compacted soil. Often the only porous or non-hardscape surface in an urban condition or street, the tree trenches limited water drainage results in a saturated tree pit, though some could have underground drainage pipes in the root growth area and manufactured soil which allow water to move through the tree root growth area. Even with the use of porous pavements, tree trenches required irrigation and fertiliser treatment. They still require oversight and maintenance for water and nutrients. Repair to infrastructure or utility under the hardscape and/or planting area requires significant work and can damage tree health. Page 19 of 24 Figure 23 Tree trench. Page 20 of 24 3.9 Suspended pavement tree trench This technique is becoming more common in urban conditions. Similar to the tree trenches or corridors as mentioned prior, but with the inclusion of removable hardscape panels such as concrete sidewalk at the surface level. These trench cover panels can be removed for access to utilities, infrastructure, etc. as well as the tree soil corridors. Subsurface drainage is often coordinated, limiting saturation of tree root growth area. These strategies serve to raise the hardscape above the soil, thus significantly reducing compaction of soil. These are also referred to as a sidewalk bridge, pavement bridge, or suspended walk. The construction required for suspended trenches varies. It could be simply framed hardscape at surface level held in place by 4 long metal screws into the soil. It could be poured concrete pilings or piers which similarly held up the concrete slab, often without the need of a metal frame. See Figure 24. It could also be poured linear concrete walls or headwalls which serve as the lateral support for the surface hardscape panels to be suspended or held up, see Figure 25. Figure 24 Example of concrete piling or pier to support hardscape. Page 21 of 24 Figure 25 Concrete headwalls to support hardscape. Advantages include that they maintain low-compacted soils. They allow for the integration of mixed-use underground trenches or corridors for utilities and tree root growth areas. Minimal maintenance is required within the tree growth area, though aeration and watering tubes are required. Disadvantages include they are very expensive and sometimes difficult to construct. Tree growth is stunted and limited resulting in weak plant structure, often exhibiting poor health and prone to wind damage, disease, and other stressors. Overall, there is limited water coming into the tree growth area and this promotes urban water runoff. Irrigation is often required. Special mechanical equipment is required to remove the suspended panels in order to access the utilities, etc. under the suspended hardscape, this also then requires the re-filling of trenches with material and together could damage or injure tree roots. The hardscape in the panels are prone to damage which was expensive to repair. SOURCES: ibid; Craul 1992; Wagar and Franklin 1994. Page 22 of 24 4.0 Summary - Urban Tree Health and Green Infrastructure In summary, though there were many techniques developed over time to address the many difficulties of maintaining urban tree health and tree roots growth, failure and safety hazards ultimately continue. And though some techniques are better than others and address multiple concerns, none are able to provide cost effectiveness, longevity, adequate soil volume, soil quality, nutrient, water & air management, and human safety comprehensively. In urban environments such as cities, there is competition for resources such as clean air, clean water and open space. Trees have remained a priority, and rightly so, however, as shown, conflicts persist. Since 2004, a new, integrated discipline combining principles of urban design, ecological science, and landscape architecture called green infrastructure began to evolve in response to these concerns and conflicts. Green infrastructure recognises the larger natural system of urban trees, often purposefully removed in traditional urban design such as water and air movement and tree growth characteristics, many discussed prior. Specifically, prior to green infrastructure techniques, water systems, soil systems, and utility systems were deliberately separate: Stormwater fell and was quickly moved into drains or inlets and then to larger pipes in order to get water off the streets and sidewalks as quickly as possible. Utility lines and other infrastructure such as drainage pipes underneath the hardscape had separate corridors or trenches to prevent root damage. Specially designed structural soils were placed under hardscape and other urban areas with great effort, then water and air systems constructed to encourage tree growth. This convoluted process often left the tree as the least-supported element within an urban environment. Stormwater management is an important component of urban design and green infrastructure, and thus requires a brief discussion. In urban conditions, stormwater or water from rain and other weather events such as snow melt, is considered a nuisance and hazard and must be regulated. Urban hydrology has developed as a sub discipline of civil engineering. Urban environments are considered impervious or not allowing fluid to pass through them. They are designed to convert surface precipitation such as rain and ice melt on the streets, parking lots, roofs and sidewalks to stormwater runoff which can then be controlled and managed by grey infrastructure such as channels, pipes, and sewers and moved to centralised collections such as dams and reservoirs. However, large volumes of stormwater runoff leads to flooding, sewer system overflow or malfunction, and damage to the overall grey infrastructure system, sometimes resulting in significant impacts to humans and their health. Human development in and around cities as well as more frequent and extreme storm events have rendered existing grey infrastructure systems inadequate - the pipe-size diameters installed decades ago are now too small, groundwater and coastal water levels have risen, and aquifers are becoming drained or polluted. The costs to repair, replace, and upgrade gray infrastructure is very expensive and the solutions are temporary and not comprehensive to the overall water system’s needs. Page 23 of 24 Green infrastructure techniques help to direct the stormwater to planted areas instead of pipes and drains and allow infiltration and retention. Stormwater infiltration is defined as the process by which water enters the soil and stays part of the overall water cycle, see Figure 26. For instance, rainwater which falls on sandy soil which has many open spaces between the soil particles will allow water to infiltrate or percolate down into the soil more than that of clay soil, which has few pore areas. Figure 26 Diagram of the natural vs urban water cycle. (The term evapotranspiration simply refers to the evaporation or movement of water into the air, which can occur through various mechanisms such as soils, trees, plants, rivers, ponds and others.) Green infrastructure stormwater management strategies began developing in wet regions such as the pacific northwest of North America. They primarily served to divert water from the grey infrastructure to nearby planting areas or grassy basins and ponds. The integration into urban tree planting was not common until the 2010’s. As green infrastructure techniques evolved in urban environments, it created underground areas for the storage of stormwater, which decreased flooding and sewer overflow, as well as delivering a steady supply of air and water to nearby trees, thereby reducing subsequent damage caused by roots seeking them out. In urban environments where stormwater runoff volume is challenging to manage, trees and the systems in which urban trees grow is a significant component of green infrastructure planning and design. They route rainfall to various components of the hydrologic cycle. Losses can occur via canopy interception loss, transpiration, improved infiltration, and possible benefits with regard to deeper percolation along root channels and water table management. Overall, green infrastructure strategically brought together the varied elements of urban street tree conditions together, creating an integrated and mutually supportive system to encourage safe root growth and tree health while minimising public safety concerns. Strategic design and placement or urban elements including trees and utilities within the principles of Green Infrastructure is proven to be cost effective and flexible while maintaining the many benefits of trees in urban environments safely. Page 24 of 24 The State of Urban Tree Planting Construction Techniques and Strategies A History of Strategies and Techniques to Decrease Tree and Sidewalk/Pedestrian Conflicts within Urban Tree Plantings TECHNIQUE OUTCOMES Tree Root Control Barriers Inferior Tree Health - brittle branches, weak stem structure, root growth UP not wide (creates hazard) Poor Root Resipration - less photosynthesis, transpiration and respiration - hydraulic failure (inadequate water update) - root growth is down when health wants them to spread out Poor Soil Conditions - soil compaction = shallower root growth (lateral) - limited water infiltration Chemical Intrusion - these are constructed with polythene membranes, polyvinyl chloride, and other petrochemicals poisonous to tree health - Soil Aeration Tubes at Rootzone of Tree (e.g. Rootwell) Not natural or grounded in ecological principles Roots follow the oxygen and water within these plastic tubes of varying diameter - not proper tree root structure to promote health - somewhat beneficial for severely distressed trees, but only a treatment Biobarrier Trifulralin - a strong herbicide to inhibit root-tip cell division and thus overall growth, leads to improper health Page 1 of 3 TECHNIQUE OUTCOMES Raised Planter Boxes Tree pit root management - limited growth zones, stunted tree growth, poor health, dangerous - requires irrigation system Sidewalk Structure Design Increased tree pit zones, manufactured tree pit soil - compacted soils, tree upheaval, poor tree health - no or limited water infiltration Tree Corridor Sidewalk Structure Permeable Pavement Strategies Design - not effective, too much maintenance Subsurface Root Areas - does not allow for horizontal growth (individual pits, albeit larger in size) - utility corridors part of this which require access and maintenance, and limiting overall tree growth area Integrated Sidewalk Corridors - Tree Allowed for suspended panels to sit above tree growth/soil area which had pavement elements on Pits and Utilities with Pavement them. These were often pre-cast headwalls or could be cast-in-place. - very expensive, limited infiltration Continuous Urban Sidewalk Tree - similar tree root growth and health issues (less compaction) Trenches - requires irrigation (not integrated into hydrological system) Suspended pavement panels via 8” - 12” subsurface concrete walls. Pre-Cast trench cover panels which could be removed but primarily for access to utilities, etc. and not soil subsystem Page 2 of 3 A History of Strategies and Techniques to Handle Urban Stormwater TECHNIQUE OUTCOMES Curbside Inlets direct water into Dangerous and Wasteful Storm Sewer System - surface water dangerous in extreme conditions (to people, plants, etc.) - loss of asset to tree health (does not re-use water as a resource for trees) Sheet Flow of water off paved directly Poor water quality into open planting areas -water flow captures the petrochemicals, debris, etc. on paving surfaces Soil erosion - water flow not regulated - water flow focussed around trunk area, not root tips, this creates poor health and trunk mould Soil Filters and Fabrics Added to soil matrix and subsurface structures of soil pits - requires significant maintenance - irreplaceable once deteriorated/damages - sieve sizes typically too large to be effective Biobarrier Trifulralin - a strong herbicide to inhibit root-tip cell division and thus overall growth Leads to improper health Page 3 of 3 Term Explanations Poor Tree Health - brittle branches, weak stem structure, root growth UP not wide (creates hazard) - tree stress, structural instability Poor Root Resipration - less photosynthesis, transpiration and respiration - hydraulic failure (inadequate water update) - Poor Soil Condition - soil compaction = shallower root growth (lateral) - limited water infiltration Root Girdling - the radial or circular growth of roots within confined areas Page 1 of 14 The State of Urban Tree Planting Construction Techniques and Strategies Prior Strategies and Techniques to Decrease Tree and Sidewalk/Pedestrian Conflicts within Urban Tree Plantings TECHNIQUE OUTCOMES No Treatment The most common location in urban conditions for tree root and growth damage are the cracked and heaved sidewalks directly adjacent to trees (Nicoll and Armstrong 1998). There were no underground trenches for roots to expand or special soils, just small concrete sidewalk boxes in which the trees were placed. Tree root growth is not a linear and known science - it occurs in a random and wild manner as there are a myriad of factors impacting tree growth, whether urban condition or other. Most urban trees in this condition were considered of average to poor tree health. Roots grow towards air and water and/or better soil. This can be sideways and up which result in breaches to the sidewalk, creating dangerous conditions. This root growth can also be down where they often wrap and break pipes seeking water and air. Advantage: Inexpensive and easy to install. Disadvantage: Root growth is down when health wants them to spread out. Damage to utility pipes. Requires proper placement by arborist, often not done so. The damage and impacts from this design technique were many including: Aesthetically unpleasant. Repairs to sidewalks, utilities, street lights, furniture, etc. Safety liability. Accessibility (wheelchair, etc.) restricted. Expensive cycle of maintenance and repair and often tree removal. (McPherson and Peper 1995). Poor tree health. Root girdling. Poor root respiration. Poor soil conditions. Page 2 of 14 TECHNIQUE OUTCOMES Tree Root Control Barriers Three main types of root control barriers per Coder, 1998: traps, inhibitors, and deflectors 1. Root traps or screens Screens, welded metal sheets or woven fabrics (e.g. copper mesh screens, woven nylon). Screen hole size allow root tip growth but strong screen material strangles and girdles the root as it grows. Hole size varies from 1/16 to 1/26 of an inch square. Advantage: Water and air permeability for soil health. Disadvantage: Limits large root development leading to potential instability of tree. The damage and impacts from this design technique were many including: Poor tree health. Root girdling. Poor root respiration. Poor soil conditions. Other names: root mesh. Page 3 of 14 TECHNIQUE OUTCOMES 2. Chemical inhibition or chemical Chemicals are added to the root barriers (plastic) or fabrics (textiles) (i.e. Root Deflectors, see #3 repulsion below) through chemical impregnation or application (spray). The most common products are cupric carbonate (CuCO3) and trifulralin which act as root growth inhibitors . Trifulralin - a strong herbicide to inhibit root-tip cell division and thus overall growth. Advantage: Chemical generally keeps the root tip from growing, thus containing the spread of the entire root system. Can wrap pipes and other infrastructure to prevent damage, often permeable to allow water and air flow. Disadvantage: Loses effectiveness over time as they are impacted by soil temperature and moisture; leads to improper health, instability. The damage and impacts from this design technique were many including: Poor tree health. Root girdling. Poor root respiration. Poor soil conditions. Other names: biobarriers. Page 4 of 14 TECHNIQUE OUTCOMES 3. Root deflectors This was the most widely used form, and still is today. They come in many materials but most common is plastic - polyethylene or PolyVinyl Chloride (PVC). This is the technique most often used in urban conditions in tree planting areas next to sidewalks or roads. Simple plastic ‘wall’, redirects root growth away or down from infrastructure areas such as sidewalk or utilities. Because downward growth is not guaranteed, and this technique still resulted in spiral girdling of tree roots, some designs included vertical ribs to further direct root growth downward. Advantage: Relatively cheap and easy to install. Can be installed linearly such as along street tree trenches or pits. Disadvantage: Large and heavy-duty; urban conditions often required cumbersome panels over 5’ in depth. Plastic rolls not typically used in urban conditions. Inhibits air and water flow. Does not guarantee root growth is directed down. Other names: biobarriers. Concrete Walls were also a common type of root deflector. Made of concrete 6” or more thick and 2’ or more deep. Advantage: Easy to source material, easy to install. Disadvantage: Expensive, degrades over time. Tree roots can damage structural integrity of concrete wall and can grow under/around wall. Page 5 of 14 REFERENCES: Gilman, E. F. (1996). Root barriers affect root distribution. Journal of Arboriculture, 22, 151-154. Yau, Peter. Planting urban trees in paved areas [online]. Trees and Natural Resources, Vol. 38, No. 1, Mar 1996: 15-20. Appleton, B.; Horsley, J.; Harris, V.; Eaton, G.; Fox, L.; Orband, J.; Hoysa, C. (2002) Trees for Parking Lots and Paved Areas. In Virginia Cooperative Extension; Publication 430-028; Virginia Polytechnic Institute and State University: Blacksburg, VA, USA. Costello, L. R., Elmore, C. L., & Steinmaus, S. J. (1997). Tree root response to circling root barriers. Journal of Arboriculture, 23(6), 211. Soil Aeration Tubes and This technique was often used in urban conditions. The goal was to allow for the delivery of air and Soil Watering Tubes water to the root area via vertical tubes. Tubes are vertically placed in and around the root zone of the tree to a depth entering the root growth medium or soil area. Air is provided, water can be provided through irrigation system for manual application. The watering tubes were used to deliver fertilizer and other nutrients. Other names: root snorkels. Advantage: Relatively cheap and easy to install. Disadvantage: Not adhering to the natural process of tree growth, not grounded in ecological principles. Requires oversight and maintenance, eg. cleanout. Studies done on the use of passive evaporation tubes and their ability to aerate the soil found that there is very little evidence of success. - not proper tree root structure to promote health. - somewhat beneficial for severely distressed trees, but only as a treatment. - no documents research proving efficacy to delivery oxygen and improve tree health. - most effective for compacted soil conditions with slow draining characteristics. Page 6 of 14 TECHNIQUE OUTCOMES REFERENCES: The Myth of Root Snorkels, Linda Chalker-Scott. https://blog.lawneq.com/are-root-watering-aeration-tubes-effective/ Root Channel Tubes This technique was often used in urban conditions or highly compacted soils where small planting pits were provided. Tree roots are directed to grow in PVC pipes or tubes of varying diameters, often with holes and wrapped with a root control fabric or mesh. A healthy soil mix can be added to these pipes. Tree roots follow the oxygen and water within these plastic tubes of varying diameter. Typicall, directs root growth away from infrastructure and/or hardscape. Other names: Root tubes, root pipes. Advantage: Relatively cheap and easy to install. Allows roots a place to grow. Reduces damage to nearby hardscape and infrastructure. Disadvantage: Not adhering to the natural process of tree growth, not grounded in ecological principles. Requires oversight and maintenance. Stunted tree growth, unhealthy tree. Roots break through tubes causing damage to nearby infrastructure. REFERENCES: http://hort.ifas.ufl.edu/woody/urban-sidewalk-channel2.shtml Page 7 of 14 TECHNIQUE OUTCOMES Raised Planter Boxes This technique was often used in urban conditions. The most common material was concrete and could be precast or constructed on-site, though metal types were infrequent. This box was also often lined with a type of root control barrier. Sizes varied but most often 4’, 6’ or 8’ box shaped, height varied as well. 2”, 4” or 6” thick concrete. This was often used when there were utilities underground and the goal was to prevent damage from roots. Other names: Elevated planters, raised tree boxes. Advantage: Relatively cheap and easy to install. No subsurface conflicts such as soil, infrastructure, water. Disadvantage: Tree box volume is limited. Requires oversight and maintenance for water and nutrients. Often no the most aesthetically pleasing. Tree growth is stunted and limited, not structurally strong plant, often exhibiting poor health - prone to wind damage. Roots break through concrete or other material causing safety hazard and damage to nearby infrastructure. Page 8 of 14 TECHNIQUE OUTCOMES Tree Pits This technique was often used in urban conditions. The most common material was concrete and could be precast or constructed on-site; this concrete could be on all 4 sides or just 2. Sizes varied but most often 4’, 6’ or 8’, square and rectangular in shape. 4” or 6” thick concrete. This was often used when there were utilities underground and the goal was to prevent damage from roots. Utilities were often in a separate area or utility corridor, but not always. This pit was often lined with a type of root control barrier. This pit was often fitted with soil aeration and soil watering tubes. Other names: Tree boxes, underground tree box, concrete planter box. Advantage: Easily sourced materials. Limited subsurface conflicts with infrastructure and poor soils. Disadvantage: Tree pit volume for root growth is limited, does not allow for horizontal growth. Requires oversight and maintenance for water and nutrients. Tree growth is stunted and limited, not structurally strong plant, often exhibiting poor health - prone to wind damage. Roots break through concrete or other material causing safety hazard and damage to nearby infrastructure. Often requires manufactured tree pit soil. Limited water infiltration, saturated tree pit. Research has suggested that tree roots grow toward areas under sidewalks as the soil under these areas have better soil and temperature conditions for tree growth (Craul 1992; Wagar and Franklin 1994). REFERENCES: Craul, P. J. (1992). Urban soil in landscape design. John Wiley & Sons. Wagar, J. A., & Franklin, A. L. (1994). Sidewalk effects on soil moisture and temperature. Journal of Arboriculture, 20, 237-237. Page 9 of 14 TECHNIQUE OUTCOMES Tree Trenches and Tree Corridors This technique was often used in urban conditions. This technique is essentially a larger or longer tree pit whereas there is a continuous trench of soil material in which for tree roots to grow, the goal to provide larger volume for root growth. The most common material was concrete and could be precast or constructed on-site. 4” or 6” thick concrete. Length of trench varied. This was often used when there were utilities underground and the goal was to prevent damage from roots. Utilities were often in a separate area or utility corridor, but not always. This trench was often lined with a type of root control barrier. This trench was often fitted with soil aeration and soil watering tubes. Some tree trenches did not require subsurface concrete walls, but most did in an urban condition. If they did not have a concrete wall, they most often utilised a root control barrier. The concrete trenches were filled with infrastructure (e.g. underground utilities) then with the soil medium for tree root growth. Concrete or other hardscape could then be placed over this trench area, often requiring a low-compaction soil. Other names: Continuous tree pit, subsurface tree corridor, underground tree corridor, continuous soil trench. Advantage: Larger tree root area. Limited subsurface conflicts with infrastructure and poor soils. Disadvantage: Expensive. Limited water infiltration. Requires irrigation. Still limited tree root growth and health impacts. The non-soil substances or heavily compacted soils that satisfy hardscape load- bearing requirements further limit lateral root growth. Repair to infrastructure or utility under the hardscape and/or planting area requires significant work and can damage tree health. Page 10 of 14 TECHNIQUE OUTCOMES REFERENCES: Jim, C. Y. (2003). Protection of urban trees from trenching damage in compact city environments. Cities, 20(2), 87-94. Jim, C. Y . (1998). Urban soil characteristics and limitations for landscape planting in Hong Kong.Landscape and Urban Planning 40, 235–249. Jim, C. Y. and Ng, J. Y. Y .(2000). Soil porosity and associated properties at roadside tree pits in urban Hong Kong. In Soils of Urban,Industrial, Traffic and Mining Areas III: The Soil Quality andProblems: What Shall We Do?, (eds) W Burghardt and C Dornauf., pp 629–634. University of Essen, Essen, Germany Page 11 of 14 TECHNIQUE OUTCOMES Suspended Pavement Tree Trench This technique was becoming more common in urban conditions in 2005. Similar to the tree trenches or corridors as mentioned prior, but with the inclusion of removable hardscape panels at the surface level. These trench cover panels could be removed for access to utilities, infrastructure, etc. as well as the tree soil corridors. Subsurface drainage was coordinated, limiting saturation of tree root growth area. Similarly, trenches were typically constructed with 4”, 6”, 8” or 12” thick reinforced subsurface concrete walls (i.e. headwalls) or concrete pilings. These served as the lateral support for the surface concrete or hardscape panels to be suspended or supported ; width and depth varied. These were often cast-in-place or could be precast. The suspended panels were typically metal and specially designed to include various hardscape materials - brick, pavers or paving stones, or concrete. Permeable pavers were not common at this time for this suspended application. This technique is different than the typical tree trench prior and applies to those pre-supported panels, not pavements placed directly on the trench surface, most often via the concrete walls or pilings. This technique then specified soil be loosely placed in the tree pit area or tree trench area before installing the slabs or suspended pavement panels on the pilings. A drainage system was usually accounted for within the tree trench to move excess water away from the root growth areas. These techniques were not integrated within the existing hydrological system. Other names: Sidewalk bridge, pavement bridge, suspended walk. Advantage: Maintained low-compacted soils. Allowed for the integration of mixed-use of corridors for infrastructure and tree root growth areas. Minimal maintenance of tree growth area, though aeration and watering tubes required. Disadvantage: Very expensive. Limited water infiltration. Requires irrigation. Still limited tree root growth and health impacts. Requires special equipment to remove the suspended panels, required the re-filling of trenches with material. Could damage or injure tree roots when accessing panels and infrastructure. Hardscape prone to damage and breaking in panels, expensive to repair. Page 12 of 14 Prior Strategies and Techniques to Handle Urban Stormwater TECHNIQUE OUTCOMES Curbside Inlets direct water into Dangerous and Wasteful Storm Sewer System - surface water dangerous in extreme conditions (to people, plants, etc.) - loss of asset to tree health (does not re-use water as a resource for trees) Sheet Flow of water off paved directly Poor water quality into open planting areas -water flow captures the petrochemicals, debris, etc. on paving surfaces Soil erosion - water flow not regulated - water flow focussed around trunk area, not root tips, this creates poor health and trunk mould Soil Filters and Fabrics Added to soil matrix and subsurface structures of soil pits - requires significant maintenance - irreplaceable once deteriorated/damages - sieve sizes typically too large to be effective Page 14 of 14