Page 1 of 24 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 2 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 3 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 4 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 5 of 24 Figure 4 Soil compaction around an urban tree. Figure 5 Soil compaction diagram. Page 6 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 7 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 8 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 9 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 10 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 11 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 12 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 13 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 14 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 15 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 16 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 17 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 18 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 19 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 20 of 24 Figure 23 Tree trench.