118 Randrup: Development of a Danish Model for Plant Appraisal However, there may be old and decaying trees that should be acknowledged for their ecological and/or cultural values. Such trees will usually be valued high in the Location Factor in relation to the natural/ecological adaptation, since they have survived on the specific location for a very long time and have shown suitability for the specific growing conditions. Also, a tree that may be considered to have a low Health status in general terms may be considered high in ecological terms. Such trees may be located in an area where safety is not the main issue, and a longer life expectancy may be considered. Thus, there will be a smaller reduction due to the Age Factor in comparison to a tree that has no significant ecological value. To estimate the Age Factor, the actual as well as the expected total age of the tree is needed. Several methods may be used to estimate the actual age of the tree. In cases for which the tree is to be removed, the growth rings of the tree may be counted. In other cases, documentation of when the tree was planted can be assessed. However, in most cases, the age of the tree must be assessed as a professional estimate, which requires experience and knowledge of tree growth related to the local area. Mitchell (1974) has provided a helpful guideline to estimate a tree’s age using a nondestructive method. The expected age may be difficult to assess unless the tree is very close to the end of its total expected lifespan. There- fore, a list of the most common species and their life expect- ancies under urban conditions has been developed. The list is based on a similar list made by Pribbernow and Fritzon (1980). However, it is important to make individual judgments for each tree and use the list as a guideline only. In many cases, it is primarily a question of judging whether the tree has lived more than half of its total expected life. If not, there is no reduction based on the Age Factor. In the assessment of the Age Factor, it should be evaluated whether the tree is hazardous, and, if so, it should probably be felled. The Age Factor is derived mathematically based on the relationship between the estimated remaining number of years the tree would have lived had it not been damaged, and half of the life expectancy of the tree. To estimate the decrease in value as the tree gets older, the square root of this relationship is used (Table 2). Hypothetically, this relationship ensures that the tree will get a value of 0 if the tree is already dead, and that the expected total lifespan equals the actual age of the tree. In Figure 1, the actual effect of the Age Factor is shown. Examples A formula has been developed to facilitate the use of the model (Table 3). Three cases from the city of Copenhagen are described to illustrate the use and effectiveness of the model on different trees with different health statuses, total life expectancies, and different locations. ©2005 International Society of Arboriculture Figure 1. Age Factor influence on VAT03. From Randrup et al. (2003). The Age Factor has an influence on the total tree value when the tree has reached half of its expected total lifespan. However, it does not have a major impact until the tree approaches senescence. Tree 1: Tilia cordata (Figure 2). The tree is estimated by the city planners to be 47 years old, and it has a stem circum- ference of 147 cm (58.8 in.). The establishment costs for a new tree on this location are rather high (approximately US$3,300) because it is in a paved area, and because it is assumed that a new tree will be planted using structural soils. The tree is assessed as healthy, with an overall Health Factor of 0.82. The tree is located on a small square in the center of Frederiksberg, one of the most populated and busiest areas of the city. The square functions as a focal point for traffic from three different directions, and it shades a bus stop and a small local hot dog dealer. The Location Factor is high (1.56). It is assumed that the tree may live up to 120 years in this location. Thus, the tree has not yet reached half of its life expectancy, and no deduction is made for the Age Factor. The total value of this tree is US$7,600. Tree 2: Aesculus hippocastanum (Figure 3). The tree is estimated to be approximately 75 years old, and it has a stem circumference of 240 cm (96 in.). The establishment costs for a new tree on this location are similar to tree 1 (approxi- mately US$3,300) because it is in a paved area, and it is assumed that a new tree will be planted using structural soils.
May 2005
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