I am a student looking for methods to estimate repair costs when trees damage public works structures such as sidewalks, roads, power lines, street lights, water, sewer & gas lines, etc. My project's focus is the valuation of an urban forest. Most trees in the dataset are an asset and I'll apply the CTLA method, but some trees are considered a liability because they have damaged infrastructure. How can I derive this "public works cost of repair" factor for liability trees? Any ideas?

Reply to post by Angel Spell, on April 09, 2001 at 22:28:38:

I think it depends o how you define or classify the liability.

First if the L trees are only those currently causing extensive damage, you might just look at current cost of repair, maybe of removal, but that might be in a separate budget.

Harder is if there is another class, maybe the superset of which the current ones are a subset, which are not causing damage today, but might in future.

In a very general sense you would want to do a present value analysis of all future costs and benefits. In this case you are dealing only with marginal or incremental infrastructure costs rather than regular capital and operating costs. You ultimately want to get the NET liability. You also want to deal with the remaining life of infrastructure in any tree interaction setting. If the infrastructure is wasting away or depreciated you may not properly charge 100% of replacement against the tree.

Also the discount rate selected will influence the analysis. Long life, publically owned assets should be discounted using less that typical commercial interest rates. Some suggest the municipal borrowing rate. Others suggest an even lower "social discount rate."