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Progress toward this goal can be evaluated by assessing tree health over time through ground surveys or by <br />record keening. Community tree inventories that include health ratings may contain all of the data necessary <br />for evaluating progress toward this goal. Otherwise, sample plots can be established to obtain data on tree <br />health and/or improper or prohibited cultural practices. For example, jurisdictions that prohibit topping might <br />survey specifically to determine the incidence of this particular problem. For certain health problems affecting <br />large areas with many trees, remote sensing methods (e.g., color infrared photos, multispectral satellite <br />imagery) may be useful. <br />3. Establish and maintain an optimal level of age and species diversity. <br />The trees that make up the urban forest have finite life spans and must 6e removed as they die. Living tees <br />may also be removed when their health, appearance, or structural integrity decline substantially, or when they <br />conflict excessively with utilities and structures. The likelihood that a tree will need to be removed for one <br />reason or another increases as the tree grows older and larger, If areas are planted to a single species at one <br />time, a large percentage of the tees will need to be removed over a short time period when they reach the <br />end of their useful life. This results in a rapid reduction in canopy cover, and the loss of many of the benefits <br />provided by the urban forest. This undesirable situation is less likely to occur if the urban forest is composed <br />of a variety of tree age classes and species. <br />Serious pest outbreaks and epidemics can arise in communities in which large areas are planted to a single <br />susceptible species or variety. These outbreaks can seriously impair the overall health, appearance, and <br />longevity of the urban forest. Species diversity and genetic diversity within species helps stabilize the urban <br />forest by buffering it from pest and disease epidemics. Many insect pests and plant pathogens can only attack <br />one or a few tree species. The reproduction and spread of many tree pests and diseases will be slowed if the <br />community forest contains a diverse mix of tree species. Furthermore, if an especially virulent disease or pest <br />problem does develop on a given species, species diversity ensures that the condition of the entire <br />community forest is not jeopardized. <br />The ordinance provisions that most directly address this goal require the development of and adherence to a <br />complete urban forest or street tree master plan. To address this goal, the plan should provide For species <br />diversity in new tree plantings, a significant change from single species blocks that are common in many <br />communities. The master plan should also describe how removal and replanting throughout the community <br />can be phased to attain a good mix of tree maturities. <br />Specific provisions: <br />o Designate administrative responsibilities <br />o Develop a comprehensive management plan <br />o Planting requirements <br />o Conservation of forest and woodland resources during development <br />In order to evaluate progress toward this goal, information is needed on the distribution of tree species and <br />age classes within the urban forest. For public tees, this information can typically be extracted from a good <br />