Tapping a maple tree for sap collection creates an internal zone of discolored (non-functional) wood as the tree compartmentalizes the wound. Sap does not flow through such discolored areas ever again, therefore it is important that maple producers preferentially tap into white functional wood to achieve the highest level of sap production.
Tapping Guidelines are designed to limit the amount of non-functional wood in a tree stem. Maple producers see the results of overtapping or cluster-tapping as an increase in the probability of drilling into a discolored zone of wood. Different practices (# of taps per tree, tree growth rates, diameter and depth of taphole, etc.) affect the proportion of the tree that becomes compartmentalized. Overtapping can lead to a high probability of hitting discolored wood, resulting in reduced sap flow.
When tapping trees (assuming they have been in production for at least 20 years), what rate of hitting old tapholes (indicated by the presence of yellow/brown wood shavings on the bit) would you consider acceptable)? Put another way, what probability of hitting discolored wood would cause you to change your tapping practices (reduce tapping intensity, lengthen drop lines) to avoid hitting discolored wood from old tapholes in the future?