Trees provide many important benefits in the urban environment. But, all too often, trees are perceived to be incidental attributes in the landscape, and their needs are poorly understood by design professionals, the public, and the politicians. New Trees: The planting site and its suitability Let?EUR??,,????'???s start with new trees. The first aspect to consider is the planting site and the tree species to be used. Is there room for the tree to grow here? Will the fully-grown tree interfere with overhead power or utility lines? Will it obscure traffic patterns There is no point in spending a significant amount of money on the installation if the tree has to be cut down a few years later. In fact, it may be much better to design trees in movable pots, which become a disposable landscape, that can be replaced periodically at less cost, and with more lasting benefits. Soil: Quality is key If the site is suitable then we must consider what the tree needs to grow. Many newly planted trees die young simply because they don?EUR??,,????'???t have enough good quality soil. At the time of planting it is easy to believe that the small nursery tree will be fine in a soil pit that seems to be so much bigger than the root ball. The problem is that the soil has to provide several crucial factors. First of all the soil has to be reasonably fertile, that is, it has to provide the nutrient supply for the tree. Next, the soil texture has to be just right. A soil with a high silt content has much less pore space than a sandy loam. A really gravelly soil may have too much pore space. The pore space is critical; it provides the only means by which gases can flow into (oxygen) or, out of (carbon dioxide) the soil. Pore space also determines how well water can move through the soil and help to make soluble nutrients available for the roots to utilize. A coarse soil drains very fast, so the water does not spend much time close to the roots; a very fine soil may be waterlogged, which in turn diminishes the ability of the soil to exchange gases. Waterlogged soils lead to anaerobic conditions, the roots rot away, and eventually the entire tree dies. Next, we need to consider the physical placement of that soil in the planting area. Even if the ideal soil texture and fertility is used, it will be completely useless if the soil is heavily packed down. Excessive soil compaction squashes the individual soil particles, eliminating the available soil pore spaces, and thus, eliminating opportunities for gas exchange and water infiltration. Compaction also makes it much harder for the roots to develop, since they are constantly blocked by physically impenetrable conditions. From an engineering viewpoint, dense, well-compacted soils are ideal, since they are strong and support large loads under compression. From the tree?EUR??,,????'???s point of view densely packed soil tends to be lethal. One way to compromise on the tension between engineering requirements, and the biological needs of the tree, is to use structural soils. These have been widely debated in recent years. They consist of large aggregate, which has very large pore spaces, and a mixture of soil components that can fill in these large pore spaces. Conceptually, the soil mix can be compacted to meet engineering needs, while still retaining enough pore space to permit gas exchange, and water infiltration. Care is needed to create the right mix of ingredients, and deliver and place it in the ground with the right consistency. Even with the perfect soil, there is still one other critical factor - soil volume. Trees need enough soil volume to sustain them over their predicted life span, which typically would be many decades. Many urban trees are planted - at considerable expense - without enough soil volume to support them. If the roots have exploited the available soil, and by extension, the available nutrient pool within that soil, and there is nothing else for them to use, then the tree starts to decline and eventually, it dies prematurely. In effect, it starves to death. Irrigation Irrigation is a further issue that seems to be neglected. The only effective way to assess water needs is to undertake a detailed assessment of the soil, climate, and species used. Ideally, the roots should be deep enough that they can survive drought periods, and extensive enough that they can use a large area of moist soil. There are designs available to install soaker hoses, or large plastic tubes filled with water, that form a ring around the tree. The area inside the ring is filled with water perhaps once a week, or every other week, and this then soaks deeply into the soil. The overall volume of water used may be the same, perhaps less, but the results can be more effective. Consult a standard text such as Arboriculture. Integrated Management of Landscape Trees, Shrubs, and Vines. 3rd Edition to better understand these design issues. Installation Having designed the planting area with enough good quality soil to permit the tree to grow properly, we still need to consider the tree itself. Selecting high quality nursery stock is very important. Nursery practices vary in quality. The best trees are those that have been grown to encourage two key elements: i) a uniform and fibrous root system free of girdling roots, and ii) a branch structure that is evenly distributed with the main scaffold branches properly spaced and well attached to the trunk. The first decade of the tree?EUR??,,????'???s life will often determine its future form. Poorly developed branching patterns become the broken out limbs of the future. Inadequate root systems offer little or no structural support. It should also be remembered that a balled and burlapped tree has just lost upwards of 80% of its root system when it was dug up. The remaining 20% of the roots have to provide the nutrient supply to keep the tree alive, as well as develop enough physical support to keep the tree standing. The book ?EUR??,,????'??An Illustrated Guide to Pruning?EUR??,,????'?? provides an excellent source of advice about pruning to establish a healthy tree. One of the most common problems with nursery grown trees is that they have either been planted too deep in the nursery, or at the planting site. Trees should have one system of roots, so if a second system is found, higher up the trunk, we know the tree has been buried too deeply. On site, planting the tree too deeply simply buries the active roots at a depth where they cannot adapt fast enough to survive. They die back, rot away, and the tree dies. When planting street trees, considerable expense is often involved by the placement of elaborate grates around the tree. Their use is designed to leave a larger area of soil open to allow greater water infiltration and gas exchange capacity. While that part may work, it is very common to find the trees completely girdled by the grate, simply because there has been no follow up maintenance to cutback the grate as the tree trunk grows. Often, an expensive feature is installed to highlight the tree, but if insufficient soil volumes are used, or not enough care is taken with the installation and follow up maintenance, the entire effort is wasted. Similar problems arise when staking trees. Stakes are designed to help the tree establish roots in the first few years, but after that, the stakes need to be removed. Many expensively installed trees are seriously damaged or killed by the stake supports. In both cases simple maintenance would eliminate most of these problems and ensure that the investment is protected. Another common problem is that designers place grass right up to the base of the tree. This creates serious problems when the lawnmower comes by and damages the tree trunk, or the hired help uses the weed eater to trim the grass the mower missed, and strips off the bark at the base of the trunk. Both issues inflict physical injury on the tree and shorten its lifespan. Physical Injury to the Tree Trees react to physical damage by attempting to control the influence of the wound. Typically, the wounded tissue dies, and then undergoes physiological changes that lead to the onset of decay. The tree tries to contain the decay, which develops as a column up and down the trunk, or along the branch, by developing internal walls of special cells. The ability of the walls to contain the decay varies by species. But, regardless of the species, physical injury induces cellular changes, and as the rest of the tree develops, these damaged areas can become areas of decay, or even cavities. A healthy tree can devote all of its energy into growing big and strong, and staying that way for a long time. A wounded tree has to divert energy into dealing with the wound, and consequently, less energy is available for growing. Newly planted trees that have lost a large amount of root area, and then get damaged by careless installation, have to divert most of their energy into simply surviving, which is why many freshly planted trees stagnate for the first few years - they are fighting competing demands to grow new roots, new foliage, contain wounds, and at the same time develop enough energy to set new buds for the following year. It may be several years before the true affects of planting are seen, which is one of the reasons for a landscape installation performance bond that lasts longer than one year. Wounding also occurs when the landscape around existing trees is modified. Many people fail to appreciate that a tree?EUR??,,????'???s rooting system is very extensive. A rule of thumb is that the radial extent of the canopy foliage is the minimum equivalent to the rooting system Young trees that are established and healthy, may be better able to tolerate some ground disturbance closer to the tree trunk. That happens because tree roots start at the junction with the trunk, and undergo rapid tapering down to the fine root hairs. In a small tree, the root hairs are relatively close to the tree trunk; shearing them off still allows the option for them to re-grow. In a mature tree, these fine roots are much further away, so cutting close to the trunk cuts through much larger roots, and these have less ability to re-grow and provide the necessary fine root structures that control nutrient uptake. It is possible to carefully remove soil in crucial points to locate roots and work around them; a tool such as the Air Spade is ideal for this purpose due to its low cost and portability. If you are unsure, then working outside the drip line is a good approach to minimize damage to the tree. In some situations, that is simply not possible. Sidewalks are a good example. There is never enough room to accommodate all of the needs. For new install-ations, consider the use of structural soils and large underground vaults of soil. For existing installations, it may be difficult to integrate engineering needs with the existing trees. Generally, unless there are knowledgeable people involved, the trees lose out. Effective protection of existing trees means having a sturdy fence at or beyond the drip line, and ensuring that the fence is maintained in place in good condition, before activity starts, and throughout the entire construction period. It is also important to ensure that the final landscape installation, often undertaken once the fences are removed, does not itself cause serious damage. Even small machines like a Bobcat can compact soil and damage the roots. Finally, the existing soil grades around an established tree cannot be changed when the adjacent area is re-developed. Cutting the grades down within or close to the drip line will cause physical injury to the roots leading to decay and subsequent decline, possibly even blowdown. Cutting the grades down beyond the drip line can be accomplished with care, but note that a large excavation into the ground seriously changes the water table around the tree, generally drastically lowering it. Not all tree roots can adapt fast enough to the change, and as a result, the trees decline rapidly, and in some case die out. Raising the grade within the drip line buries the existing roots, thus cutting them off from the surface where gas exchange can take place. As a consequence, the roots die back and rot away. An easy way to see if the tree has been buried, is to examine the base of the tree and look for the trunk flare. A tree trunk with no flare at its base has been buried. Remarkably, many trees will survive physiologically on a very small root system - that is, they are demonstrably still alive. However, structurally, these trees may be very unsafe, since the original supporting root system has died, and only a small frill of new roots exists to keep the tree alive. Grade changes are best avoided. Summary Trees in the urban landscape provide many benefits, including air purification, slope stability, aesthetic enhancement of the built environment, and habitat for other species. In order to gain these benefits we need quality trees, good nursery practices, careful preparation and delivery, good installation, thorough aftercare and follow up, and much better design practices. The design professions, which includes engineers, architects, landscape architects, and planners all need to better understand tree requirements. Many clients pay a large amount of money to have healthy trees installed with the goal of seeing them develop into mature trees. Sadly, a high percentage of the efforts fail due to a lack of understanding and care. Hopefully, as more people learn about effective tree care, we will see more success and less damage. Your work today is the legacy for tomorrow. Handle it with care. Dr. Julian A. Dunster is a Consulting Arborist, based in British Columbia, Canada