I did not make up that word thank god ! Whoever did should be punished.
It means responding by growth pattern to a physical stimulus. It is a key element in arborsculpture. I'm sharing it with you today because one of my trees is showing off its efficiency in trunk building. While researching the phenomena, I came across a great web based tree science course, this is session 4 .
Two poplars were grafted together about 10 ten years ago. After the trees stopped moving in the east west direction, the need for an equal amount of wood in that area was decreased. Over time a concave shape appeared on the inside area of the joined trees. Trees are natures most efficient builders only adding material to the locations that serve it's structural needs.
The rainbow colored glass was added about 4 years ago.
A different explanation is offered by Bob Wulkowicz. "Transport Shadow effect"
The parts of the tree beneath the graft is shadowed and therefor grows less.