This is a good question. Tapping is to drill a hole to collect sap. The short answer is that with good tapping procedures, taking sap does not harm the trees. What are good tapping practices?
The season for collecting sap begins when the snow is still deep on the ground and the days start to go above freezing. The freeze-thaw cycle is like a pump for sap, so when days and nights are both warm, the season is over. That means that sugarmakers have to tap their trees before the season, when both days and nights are freezing and the sap is not running.
The guidance for tapping trees is to wait until they are 12 inches (30 cm) in diameter, which takes about 45 years of growth. If we were to hurt a tree, it would take half a human lifetime to replace it, so our job is stewardship of the forest, not to try to be smarter than nature. Most of the trees in our sugarbush are between 80 and 350 years old.
Trees heal themselves. If, for example, a large branch is knocked off in a storm, a tree can heal itself and continue to grow for another century or more. Tapping does much less harm to a tree than losing a branch.
The current guidance is 1 tap for a tree 12 to 18 (30 to 45 cm) inches, 2 taps for a healthy tree over 18 inches (45 cm). Old tapping guidance was 3 taps over 30 inches (90 cm), but recent research with modern sap collection systems has not shown enough of a benefit to add a third tap regardless of diameter. We have a few trees 39 inches (1 meter) in diameter or more in which we put in three taps.
Each tap hole is 7/16 inch (1.1 cm) in diameter and 1 1/2 to 2 inches (3.8 to 5cm) deep. Sap moves in the xylem layer of the tree, a layer under the bark about 2" thick that conducts sap. As soon as the hole is drilled, it is exposed to bacteria in the air and the tree starts to heal the hole. In weeks, the hole can seal itself enough to protect the tree. In a year, the hole can been refilled.
In one season, a tree can spare enough sap to make half a gallon of syrup per tap. That is 4 to 8 percent of the total sap in a tree. Sap can be replaced by groundwater, so it is not a zero sum equation. Our town gets an average of 89 inches of snow per year and 43 inches of rain. Each square foot of snow holds about a gallon (3.8 liters) of water. Maple roots can spread 3 times wider than the branch canopy, so a tree can draw water from a very large area.
Modern sap collection systems use vacuum to increase the sap per tap. Counter-intuitively vacuum keeps trees healthier than conventional gravity systems, whether buckets or tubes, because it keeps the hole cleaner and more free of bacteria.
When days and nights are both warm, the sap stops flowing and the sugaring season is over. Trees gain their energy from photosynthesis, not relying on sucrose pulled up from the roots. At that time, we pull the taps from the holes. That allows the trees to heal themselves.
Sugaring is an act of forest conservation. Other alternatives for land use are logging or converting the land to other agricultural uses which could require clear cutting. By leaving the maple trees standing, we preserve the forest and habitat for birds, large and small mammals, and many plants. Research has shown that thinning out non-maples reduces the productivity of maple trees. Our forest has many different species of trees.