Where the sugar is stored
Carbohydrate made by the leaves in summer is stored over winter, largely as starch, in living ray cells in the sapwood. As the wood warms in late winter, the tree converts this store to sucrose, which moves into the xylem (the water-conducting wood) and becomes the sugar in sap. Sugar maple sap averages about 2–3% sugar.
What moves the sap
Unlike birch, maple sap flow in late winter is not driven by root pressure. The leading explanation is a freeze-thaw cycle in the stem:
- Cooling: gas in the xylem contracts and dissolves; stem pressure falls and the stem absorbs water.
- Freezing: ice forms in the wood, compressing and trapping the remaining gas.
- Thawing: the ice melts, the compressed gas expands, and positive pressure pushes sap out of any wound or taphole.
Sugar matters to the mechanism itself, not just the flavour: sap flow has been found to depend on the sap containing enough sucrose. Models now combine ice melting, gas dissolution and osmosis to reproduce sap exudation. See Ceseri and Stockie (2012) below.
Sugar content and yield: the Rule of 86
A traditional estimate, credited to C. H. Jones (1946): divide 86 by the sap’s sugar percentage to estimate how many units of sap make one unit of syrup. At 2% sugar, 86 ÷ 2 ≈ 43. At 3%, about 29. Sap sugar is measured with a sap hydrometer or refractometer. Later research has refined the formula, so treat it as an estimate.
How MapleSAP uses this
The MapleSAP app (in development) is being built to combine forecast conditions such as freeze-thaw, sun, temperature swing, snowpack, soil thaw and recent streaks into a daily read on sap-flow potential. That is a planning aid built on these ideas, not a guarantee of any run. See the app presentation.
Open questions
- Exactly how gas, ice and osmosis share the work in different trees and conditions.
- The chemistry behind late-season “buddy” off flavours is still being studied.
Sources
- University of Vermont Libraries, Sugar Maple: Sugaring — Physiology. libraryexhibits.uvm.edu
- S. G. Saupe, College of St. Benedict / St. John’s University, Biology of Maple Sap Flow (course notes citing Kozlowski & Pallardy, 1997). faculty.csbsju.edu
- M. Ceseri and J. M. Stockie, A mathematical model of sap exudation in maple trees governed by ice melting, gas dissolution and osmosis (2012). arxiv.org
- University of New Hampshire Cooperative Extension, sap sugar content and the Rule of 86 (fact sheet). extension.unh.edu
- Cornell Cooperative Extension of Warren County, Maple Syrup Production. warren.cce.cornell.edu