Great question.
Often you'll here terms like ...
20% thin, 20% reduction, 25% of canopy etc.
In my opinion I consider it foliage, in absence of foliage I consider it SURFACE AREA at the tips as that is what leaves are all about.
This attached link to a pic will also help you.
http://www.treeworld.info/attachment...davidevans.jpg
Lets say you have a 100' tall tree and some-one tells you to do a 25% reduction.
Do you simply make the tree 75' tall? I hope not.
Here's another, with a 20% thin do you remove 20% of the branches? I hope not.
Trees use leaves and for the greater part it's the leaves surface area that photosynthesize and make food, provide shade, transpire, and make that wind resistant sail.
Imagine a tree with the traditional apple on a stick canopy. Lets say the canopy is 40' diameter like a big ball.
Lets say we want a 25% reduction. So now do we just make it a 30' diameter ball?
Lets look at this.
The formula for calculating surface area of a sphere is 4πr2, however I found an online calculator here
Volume and Surface Area of a Sphere - Geometry Calculator
40' tree canopy surface area is
5027'2
30' tree canopy surface area is
2827'2
So in the two answers above you see that using the "linear" method of reduction actually resulted in a
44% loss of canopy surface area (leaves and sail).
Now, we want a 25% reduction in canopy surface area.
The answer is 25% x 5027'
2=1257'
2 or leave the tree with a 3770'
2 canopy surface area.
To achieve that we need a 34.64' big ball canopy.
Hope this made sense.
PS: This thread may be destined to the facts area if enough research is done.