This will be a two-part discussion, as I’ve asked some syrup producers to add to the dialogue.
From the perspective of an arborist, I approach the question by looking at how the spigot (spout) hole affects tree behaviour. Assume we are tapping a tree with at least an 18” diameter, and drilling approximately 5 cm into the tree in order to place the spigot: we are drilling into various layers of tree tissue. First, the bark is the layer of transport cells that carries carbohydrates manufactured in the leaves to the living cells throughout the tree. Just inside the bark is the vascular cambium (which creates those annual rings we use to count the age of the tree). The cambium contains of a bunch of different kind of cells, including fibrous support cells (for strength), dead cells, living cells which store starch (converted to sugar in key temperatures), and chains of vessels (the “plumbing” system) which carry water from the roots to the leaves and rest of the tree. By cutting into these vessels, we access the sap flowing through the vessels, which is sweetened by the contribution of starch-storing cells.
Now, trees don’t heal like humans. Trees compartmentalize wounds; that is, they wall off a damaged area to limit the extent of a wound. The vascular cambium dies off above and below the hole, but at the end of the sugaring season, “wood repair” tissue called “callus” will begin to fill the hole--if, of course, the spigot is removed! In essence, the taphole will result in a wound and will become a very small, nonfunctional area of the tree. However, the tree should be able to deal with the wound, or else, it should certainly be able to function without this very small area. A loss of sap does not affect the tree.
Maple syrup producers have been tapping trees for hundreds of years, so it is clearly in their best interest to follow a standard of practice that is sustainable for their industry. I’ve heard about a North American Maple Syrup Producers Manual, and would be interested to find out what the regulations for managing tapping consist of.
Now, the question to be answered: how far away should you drill new holes from old tapholes?