I am currently doing a project which involves using the CTLA plant appraisal method. As there is no regional plant appraisal committee in England i am working out my own set of calculations. Has anyone got any ideas on how to set up a species rating or ideas on what elements i could use ie maintenance of different species? thanks

Reply to post by Judith Latham, on October 04, 2001 at 09:32:44:

From my perspective, you have to start with an understanding of the underlying theory. First, value is broadly defined as the current worth of future benefits. That is value is the sum of all future benefits discounted to present value. Second, the CTLA methods are depreciated replacement cost approaches to value. They indicate value, the sum of benefits, by suggesting that whatever those benefits are, they can be replaced for a current cost. Now that cost is for a tree in idealized condition. To the extent the tree is not ideal the cost is depreciated. Species is a depreciation fator.

So IMO the principal thing the Species rating should reflect is how the benefits stream is reduced by a species characteristic. This includes at least three items. First beneficial life... an oak may have a useful life of hundreds of years. A birch, 60 years. So in very simple terms a birch would be rated less than an oak. Now, depending on the discount rate you select, benefits that are too far out will discount to zero in a period that may fall even within the shorter life, so there would not be a direct linear penalty. A second thing would be maintenance needs. I think you'd have to assume that even a 100% tree has some maintenance needs so want to look at dsiproportionately high needs. The idea is that the cost of these maintenance needs will reduce the benefits stream. A third thing would be risk. For example in the US, American Elm is a beautiful and desireable tree but is susceptible to Dutch Elm Disease. The benefits stream would have to be discounted for the risk that it will be cut short by disease.

Now, not to make it too simple for you, you can look at maintenance costs two ways: One is a deduction from the benefits stream as discussed above, a burden. But the other is inverse. You can argue that the willingness to maintain is an indication that the tree is desireable... elms being an example where many communities invest in significant exclusion, sanitation, prophylactic and theraputic programs to beat the disease problem. Maintenance might be a capitalized part of value rather than a deduction.

IMO the Species factor may have been overstressed. It does serve a valid purpose but may be less important than Condition and Location. In fact in drafts of the 9th Edition CTLA Guide it was proposed that it be eliminated, which would be the same as saying all Species should be rated at 100%.

So if I were to develop Specs ratings I think I'd want to see really negative factors to significantly reduce the rating.

Good luck.