Yes, this is the big trick right :) We spend a lot of time talking with people about this because it takes some thought.

I'm going to do a video on this subject this week, I promise. It comes up so often I really need to do it. For now - yes you need to know the leaf location and age if possible, and yes you cannot really force the plant to behave so you have to embrace variability! Get as many measurements in as wide a range of light conditions as possible. That's your best bet. Generally in a given population, I try to get 3 plants and 3 leaf locations at a minimum (minimum!).

I'm having the same problem guys. I am ready to start collecting measurements but am still a bit cautious as im looking at trees and measurements throughout the canopy will be tricky even for a tree climber. I am wondering if 4 measurements from the lower canopy skirt at separate positions North, south, east and west - total 16 measurements per (evergreen) tree per season for a year? total per year 64 measurements. How does this sound Greg?

I think that you have the right idea. However, while direction can have an impact on light intensity and photosynthesis, so can the age of the leaf/stem that you are measuring. I would suggest adding some measurements from older and newer leaves/stems. This may increase your total data points per tree from 16 to 32, but might also give a better indication of partitioning of nutrients within the tree, across different seasons.