Thursday, July 6, 2017

What is a Tree Worth?

Drouin's trees are valuable for many reasons.

Trees provide habitat for urban wildlife. They reduce energy costs by producing shade in Summer and protection from cold winds in Winter. Properties in treed streets sell more easily and a tree on the nature strip can add thousands of dollars to the sale price of a house. Trees absorb carbon, airborne pollutants and noise. They 'exhale' oxygen, mitigate water run-off and reduce the level of the water table. Well treed suburbs and towns are known to have lower health costs and reduced crime rates.

Fortunately, in Australia and here in West Gippsland, we are catching up with many other places around the world and beginning to think of urban trees as assets rather than liabilities.

But, just how do you calculate the monetary value of a street tree? (If you 'Google' that question, you get hundreds of very technical answers.)

An example relatively close to home is the Urban Forest Tree Evaluation Fact Sheet from the City of Melbourne which provides a formula for providing the monetary amenity value of a tree;
Value(V)=Basic Value(B) x Species(S) x Aesthetics(A) x Location(L) x Condition(C).

Basic value (B) is taken from an internationally acceptable set of figures which provides a monetary figure for each square inch of basal trunk area.

The Species factor (S) is a score determined by analysis of the rate of growth in a particular environment.

The score for Aesthetic value of a tree, (A) is determined by its impact on the landscape if it was removed.

The Locality score (L), is evaluated from a range of localities such as undeveloped bush or open forest to the city centre or main boulevard.

Condition (C) is calculated from a table that lists factors like 'solid and sound trunk' to 'advanced infestation, decay or dieback', etc.

So, let's apply V = B x S x A x L x C to a well-known Drouin tree, (hope you're keeping up?), the Bill Kraft Giant in Albert Rd, (E. cypellocarpa).

    Base Value B
This tree's diameter at breast height is 223cm. The given table for determining the Base Value does not go this high, but extrapolation from the table gives a Base Value B = $248,200.

Species Factor S
Eucalyptus with a long life span, S estimated as 0.9. (could be higher).

Aesthetic Value V
A solitary feature specimen tree, A = 1.0

Locality L
Outer area/residential street, L = 1.5

Condition C
Rating from table C = 1.0

Determining some of the above values is a subjective process but I believe I have been conservative. The basic value figure is an estimate made in 2012 and was extrapolated from a table which did not cover a tree with trunk diameter as large as this.

Thus, the value of the Bill Kraft giant Mountain Grey Gum in Albert Rd is …

V = B x S x A x L x C
   = 248,200 x 0.9 x 1.0 x 1.5 x 1.0
   = $335,070

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