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Identifying At Risk Trees By Craig M. Greco As landscape professionals it is important to be able to identify at-risk trees as an integral part of our everyday valuation and assessment of our client's property. It is also our responsibility as professionals to inform clients of potential risks, to educate them, and to provide them with appropriate recommendations for proper management. At risk trees can have a tremendous impact on project planning, and early identification can not only increase the accuracy of project estimates and outcomes, but can also enhance the reputation of a contractor as a more credible and reliable individual. What constitutes an at risk tree? Any tree may fail, but fortunately, only a few actually do. Location plays a vital role when considering a tree at risk. To be considered an at-risk tree, a failure must be a direct threat to property and/or safety. In other words, it must have two things--a fault and a target. For example, a leaning tree in a forest would not be considered a risk, whereas the same tree adjacent to a play set or house would be considered at-risk. The urban environment is a complex setting that can compound stresses on a tree. Trees subject to poor growing conditions like insufficient or nutrient lacking soil, and drought may be at risk. Also, trees may be at risk if subject to construction damage, wounding, and improper pruning. Species is also a factor when considering a tree at risk for a potential hazard. A general rule of thumb to consider is, the faster the species grows; the greater risk for a failure. For example, cottonwood, poplar, pear, and willow trees are fast growing and are generally regarded as weak species more prone to developing risk factors. Conversely, oaks, hickories, and beeches are considered slow growing and as most slow growing species, they can build defenses (stored reserves) for the future seasons. How do I identify an at risk tree? Rarely does a tree develop into it's true potential. This is obviously caused by the environment the tree has been subjected to. Trees are complex organisms, much like us, and they do suffer from biotic (those caused by living organisms) and aboitic (those not caused by living organisms) factors. Observing a tree in a systematic manner and evaluating each component individually provides for a more thorough and accurate assessment. The thorough observer must not only question the obvious signs and symptoms, but also look for camouflaged ones. What follows is a general working overview of tree anatomy and physiology from the ground up. Roots Roots comprise roughly 50% of a tree's mass, but rarely do we understand the environment (rhizosphere) they need to sustain growth and vigor. Soil pH, organic matter, and nutrient levels may be altered or depleted in most urban settings. It is not uncommon for soil assays in an urban environment to vary tremendously from one sample to the next even if the samples are taken from only a few feet apart. Construction debris, soil contaminants, and soil compaction are common findings in this setting. Soil compaction is a particularly insidious and difficult problem in high use and in previous construction areas. Oxygen that is vital to a trees' survival is depleted in a compacted soil environment, and the tree slowly suffocates. Construction grading, changing drainage, adding fill, or removing soil near or from under the canopy of the trees may cause damage to the tree and is often evident by overall decline. Many times it takes several years for the symptoms to appear, and it is often too late to save the tree. Root Collar The root collar, or trunk flare is the widened area of the trunk of the tree from which the buttress roots begin. This is the transition zone between root and stem tissue. Over time, soil or mulch can build up against the root collar or even upon to the trunk proper and trap an excessive amount of moisture against the trunk. This is most likely in a tree with flat or absent root flares. The problem with this scenario is that the tree's trunk is not designed to absorb water like roots are. If allowed to progress, these persistently wet conditions may cause the bark to weaken and eventually die, creating an ideal environment and entry point for damaging pathogens like root rots and other fungi. Additional symptoms of a buried root collar are stunted or premature foliage turning color, stunted internodal growth, and dieback within the crown of the tree. The presence of conks--fungal fruiting structures--near the trunk of the tree or within the drip line of the canopy is often another sign of decay and structural compromise. Girdling roots are roots that wrap around buttress roots or around the trunk of the tree choking the plant of its nutrients. A seasonal indicator of a tree with a girdling root or a root collar disorder is if one side of the crown loses its leaves first in the early autumn. Trunk There are several important items to note when looking at the trunk, or bowl, of a tree. Cracks, peeling bark, and fruiting bodies (mushrooms) can all be signs of a tree with potential hazards. During the summer months thousands of trees are struck by lightning. The severity of the damage and injury are variable and appear to be influenced by the moisture content of the wood, the initial charge of the voltage and the species of the tree. A thin, splintered strip of bark running parallel to the wood fibers down the entire length of the tree or trunk may be stripped, burned or shattered off. The internal tissues (cambium) may be scorched without apparent evidence, and the root system may be significantly damaged as a direct result the strike. It is also important to evaluate the trunk for areas of stained bark that may indicate underlying decay and to look for frass (excrement of wood boring insects) within the bark or at the base of the root collar. A method for identifying heartwood decay and rot is to use a rubber mallet to knock on the bark around the circumference of the tree. When performing a mallet test, a hollow "thump" is often heard when tapping over an area of internal decay. Vines growing along the trunk of the tree could be a risk to the overall health and vigor to the tree as they may be hiding cracks, cankers, or a lightning strike. It is also important to note that the mere presence of some vines and plants may compromise the health of the tree, as they may be parasitic. Mechanical injury is also a significant route of entry for decay and insects. Even with apparent surface healing of an injury, decay may lurk behind the wound. Limbs Limbs most frequently fail as a result of excessive limb weight, decay, or poor attachment to the trunk. Frequent defects that encourage the potential for limb or branch failure also include, storm damage, improper pruning cuts and the resultant decay, cankers, and right or obtuse branch angle attachments. Attachment failures occur at the point of union where the trunk and branch are directly joined--a failure of this union often results in a bark tear that strips a large portion of the trunk. Branches that have narrow angles of attachment and co-dominant leaders may fail at the point of attachment if there is included bark in the crotch. Several species have this inherent predisposition, pear, apple, linden, and maple to name a few. Branch or limb failures may be due to a single factor, but are generally multifactorial. Crowns An ideally shaped crown would appear perfectly symmetrical with all of the limbs emerging from the trunk at wide angles and positioned evenly around the tree. In reality, trees are complex organisms in complex environments, and their peculiarities are a reflection of the seasons that they have weathered and the environment they have endured. Leaning trees develop a bowed trunk, which in turn creates an asymmetric crown. Although this can be visually aesthetic, when subjected to natural phenomena or even if left alone--the chances of failure are increased without crown weight reduction or artificial support. Parasitic plants such as mistletoe are common on some trees in the southern states. These plants establish themselves on twigs and branches of their host and extract water, nutrients and food by way of a parasite nutrient-uptake organ. The overall effect of the parasite on a tree is a reduction of vigor of the affected branch; sometimes, if the population of this plant is significant enough, the parasite can actually kill the tree. Limbs or branches that have fallen and are lodged or suspended by other limbs or branches are always a risk. Hanging branches often show up after high winds or strong storms and are generally an indication that the tree needs a quality crown cleaning to remove dead, dying, diseased, or conflicting branches. Management Corrective pruning and proper maintenance practices are most beneficial when a tree is young. A properly trained young tree will develop into a structurally strong mature tree that requires minimal pruning. When a tree has a healthy root system, it is better able to retrieve adequate water and nutrients for increasing vigor and to and building reserves for the upcoming years. Proper nutrient management is also vital for increasing sustainability of a tree. Maintaining the proper pH level, soil chemistry, and organic matter is just as important as proper physical maintenance. More established and mature trees might benefit from consultation with a professional ISA Certified Arborist with practical knowledge and field experience to assist you in your long-term management plans. Craig M. Greco is the Plant Health Care Coordinator for the Mid-Atlantic region for The Care of Trees and is a certified arborist.

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December 10, 2019, 7:06 pm PDT

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