I consider Espalier to be extreme modifying and controlling a plant, just like bonsai. I currently am riding herd on a particular cultivar, the Sycamore Fig or 'Biblical fig' (Ficus Sycamorus) that will go 50 meter with 25 cm leaves (150 feet and 10 inch leaves) if in natural conditions. BIG tree. I am ultra super dwarfing these
trees. In espalier, you are seriously modifying and training that tree to an environment that is not natural for it. It may be modifying the environment (against a south facing wall plus a heated building) that gives it advantages...
We are considering espaliering some trees to both increase the apparent grow zone 1 or 2 zones warmer, PLUS allow us to build protective 'caps' to assist in defying the weather and protect against sudden frost snaps and hail (both prevalent here in early spring and late fall) to extend growing season.
An espalier guild would be dedicated to the unique criteria and growing conditions of the more extreme growing situation. I would consider Espalier a step above coppice-ing, and two below bonsai. All are modifying nature and natural growing conditions to what we wish, need, and desire.
