Journal of Arboriculture 31(3): May 2005 117 availability (e.g., the fact that a poplar tree in general is easier to produce [cheaper] than an oak tree [more expensive]). Health Factor (H) It is assumed that a nursery tree is in optimal condition. For this model, this means that an existing tree can never exceed the health status of a newly established nursery-grown tree. At best, the damaged tree that is to be assessed may keep a condition rating of 100%, but most commonly the health factor will be lower than 100%. The rating of tree health is derived from the CTLA (2000) and Mitchell (1974) systems. It includes a plant rating, determined by the sum of the rating scores of each of five factors: roots, trunk, scaffold branches, small branches and twigs, and foliage and/or buds (Table 2). Each rating uses a scoring system ranging from 0 to 5, where 5 represents no health problems and/or structural problems. A rating of 5 should be used only after sufficient inspection in the tree. A rating of 0 indicates that the tree is dead or clearly dying. The sum of the five ratings is divided by the maximum amount (25), which gives a value of H between 0 and 1.00. The CTLA (2000) formula uses a rating at each of the five factors, from 1 to 4. We chose to use a broader scale, to further minimize the effects of each individual rating. Similar to CTLA (2000), a checklist for each of the five factors follows the model. The rating should be based on the actual condition of the tree, apart from the damage that led to the assessment. Thus, the rating is not a reflection of general or genetic behaviors of the species, but a clear reflection of the health of the actual tree. Location Factor (L) The location of a tree may raise or lower its value. Location may accentuate the positive characteristics or limit the negative in relation to natural/ecological adaptation, architecture, aesthetics, visibility, and environmental factors. In principle, it does not matter whether the damaged tree is self sown or planted. As in the case for the Health Factor, a rating scale from 0 to 5 is used for each of five factors, with a maximum sum of 25. The sum is divided by 12.5 to make it possible for the Location Factor to act positively on the overall rating (giving a factor of 0 to 2.00) and to minimize the effect of each individual rating (Table 2). Half points may be used for each of the five ratings. A rating of 2.5 may be used if the rating is neutral, which means that the tree does not contribute either positively or negatively to its location. The general values of a species are not assessed. For example, it is not important that the species has beautiful flowers. But if the assessed tree has flowers that contribute to the quality of the tree in the specific location, that should be acknowledged in the assessment of the Location Factor. Therefore, species does indirectly influence the Location Factor. The natural/ecological adaptation rating is an assessment of the tree’s vulnerability to de-icing salts, air pollution, or similar unavoidable abiotic stress factors. The better the tree can adapt to the specific stress factors on the specific location, the better the rating that should be given. An old, historically and/or culturally important tree will be valued high. The general level of adaptation should be assessed in relation to the specific local climatic and soil conditions. The tree should be assessed in relation to the architec- tural context, in which it could contribute either in a positive or a negative way. There may be locations in which a tree negatively counteracts and influences the overall architec- tural context. In such situations, the rating of the architec- tural context should be negative. However, in many cases, the positive influence of the tree is high (e.g., in a systematic planting where each individual tree plays an important role). The tree as a guide for traffic (e.g., as an orientation mark) (Lynch 1988) is important for this specific rating. The tree should be assessed in relation to its aesthetic features. These may be flowers, bark, fruits, foliage, and other features, in relation to the specific location of the tree. The more attractive the features, the higher the rating. The visibility factor is related to the number of people who potentially enjoy the tree. In principle, a tree gains a high visibility rating if there are many people in the area. A tree with a high visibility rating typically is located in an urban setting. The environmental factor is related to the climatic and environmental consequences that the damaged tree has on its specific location. These consequences may be in providing shade or light, and protection against wind, dust, and other factors. Under Danish conditions, only a few species are rated as problematic according to the Danish Asthma and Allergy Association (DAAA 2004): hazel (Corylus spp.), elm (Ulmus spp.), alder (Alnus spp.), and birch trees (Betula spp.). Age Factor (A) Based on growth, the Basis Value of a tree will increase through its entire lifetime. The Health and Location Factors will influence this value; however, the result will often be that the tree will gain a relatively high value even in its old age. In the Danish model, a high age of the tree will influ- ence the overall value of the tree in a negative way. Koch, who introduced an age factor in a German tree appraisal model (Koch 1987), acknowledges the presence of this factor. The Age Factor is effective when the tree has exceeded half of its life expectancy. As the tree nears its total expected lifetime, the Age Factor will lower its overall value substantially (see Figure 1). In addition to low Health Factor scores, the Age Factor ensures that trees with serious health problems will be reduced in value due to a decreased life expectancy. ©2005 International Society of Arboriculture
May 2005
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