Japanese and European Beech

[Aiki_Joker]

Only thing I can think of here is that the physical barrier stops cambium callusing for enough time for the roots to initiate(?). Its unlikely to be copper toxicity but worth bearing in mind that the copper metal is indirectly toxic. Cambium may 'prefer' not to callus but to root above the toxic substance instead. Thus moving away from it. Copper metal becomes toxic by dissolution (corrosion). Copper ions are known to inhibit metabolic activity in plants by binding organics inside vessels forming large, stable complexes that get stuck in the xylem and phloem, blocking them up and retard metabolic processes.

 

[Aiki_Joker]

I used a technique I read in a bonsai magazine, from an article published in Japan for Japanese beech, on a European beech, and it worked. To be done in mid-February in zone 8 (at leat one month before budbreak):

Great result! Was the copper wire heavily corroded or completely dissolved when you checked these roots?

 

[Sifu]

There is a video (I think it's in Dutch language, unfortunately) about air layering an European beech and it looks pretty simple
part one:

part two: