A Model of the Tapping Zone

MODEL OVERVIEW AND BACKGROUND INFORMATION The tapping zone of a maple tree is defined as the area around the circumference of the stem that can be utilized for sap collection. For sap collection with tubing, it can be thought of as a radial band of wood based at the point on a tree where the dropline meets the lateral tubing of the sap collection system. The physical boundaries of the tapping zone are defined by the depth of the taphole, the length of the sap dropline, and the circumference of the tree (Figure 1). Each year, tapping for sap collection not only permanently removes a small amount of wood from the taphole itself, but, in addition, the tree's response to the taphole wound1 generates a "stained" column of wood extending above and below the taphole that remains permanently nonconductive to sap (Figure 2). However, stem growth also adds new conductive wood to the outside of the tree each year. This outward growth functionally shifts the tapping zone outward over time so that some of the nonconductive wood generated by previous tapping is embedded deeper into the tree and is thus no longer within the tapping zone boundaries. Thus, the volume of the tapping zone of a particular tree, and the relative amounts of conductive and nonconductive wood within it over time, are dependent on the tree's size and growth rate, as well as tapping practices tapping depth, spout size, dropline length, and the number of taps.2 And, these volumes can be calculated for any given time using these input parameters (Figure 3). Based on these premises, we developed a model that calculates the proportions of conductive and nonconductive wood in the tapping zone of a tree over time given user-input values for tree diameter and tapping practices. (The model is described in greater detail on the following page.)