Tree Appraisal
Different methodologies and
strategies exist for appraising trees in a developed environment; TreeWorks
employs the widely-used Cost Approach established by the Council of Tree
and Landscape Appraisers’ Guide for Plant Appraisal, 9th Edition
(CTLA). In the Cost Approach, one of
two formulas is applied. If the
appraised tree is of a size (trunk diameter) that can be replaced out of the
stock of a regional nursery, the Replacement Cost Method is used;
alternatively, if the tree is too large to be replaced, the Trunk Formula
Method is used. Both methods use
the same basic formula; the formula encapsulates the idea that the value of a
given tree is equal to the local cost of buying and installing a replacement
tree of similar size and species.
This base value, which represents an “ideal” instance of the tree (in perfect
health and desirable location) is modified downward by the actual condition and
location scores of the appraised tree.
Constants
All of these values are set by regional committees of the International Society
of Arboriculture (ISA), and can be customized in TreeWorks to match local
preferences and variances:
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Replacement Tree DBH
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The average trunk diameter of the largest tree normally available for most
species from a local nursery
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Installed Tree Cost
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The cost to purchase and install the largest normally-available replacement tree
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Unit Tree Cost
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The average cost in dollars per square unit (in or cm) of trunk area, for such a
replacement tree
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Species Rating
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Quality of species.
Each species is assigned a rating, and
these values are contained in the species table in the geodatabase
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Variables
These three variables are taken directly from each tree’s attributes:
·
DBH
·
Condition
·
Location
Replacement Cost Method
Appraised Value = Installed Tree Cost
x Species Rating x
Condition x
Location
Note that in the above formula,
the Installed Tree Cost is a standard (see above) dollar value, and the three
modifiers are percent values (i.e. 0 to 1), so any modifiers of less than 100%
adjust the dollar value downward by that much.
Trunk Formula Method
The trunk formula Method follows the above formula,
but adds value to the installed tree cost based on a larger trunk area:
Appraised Value = Basic Tree Cost
x Species Rating x
Condition x
Location
Where:
Basic Tree Cost = (Trunk Area
Increase x Unit Tree Cost) +
Installed Tree Cost
Where:
Trunk Area Increase = (Trunk Area of Appraised Tree – Trunk
Area of Replacement Tree)
(Trunk Areas are calculated using the trunk diameter (DBH and
Replacement Tree DBH) values)
Additionally, for trees larger than 30 inches in DBH, trunk area for the purpose
of this appraisal calculation was adjusted downward, based on the idea that a
tree’s value would not increase proportionally to its trunk size beyond a
certain point—conceptually along the lines of “diminishing returns”.
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