We are approaching maple syrup season.   Sap is collected from Late February to April.   Forty gallons of sap boil down to approximately 1 gallon of maple syrup.  I tapped my sixty-foot-high maple tree several years ago and again this year, then inserted a pressure gage.  I recorded pressure, ambient temperature and tree trunk temperature at various times with the following results.

1.)   Temperature below freezing, pressure was usually zero (-1 psi minimum.)

2.)  Temperature above freezing, pressure maximum– 9 psi.

3.)  At end of season, pressure reduced to zero over a period of 2 days

  

After reviewing literature and applying our vapor pressure model for osmotic flow, we suggest the following:

Water and nutrients are absorbed by the roots.   Vapor flows from the roots to the top of the tree through the sap layer from cell to cell and between cells.   Flow is from a higher water concentration (higher vapor pressure) to a low water concentration (lower water vapor pressure).   Water evaporates from the upper surfaces.   When the temperature falls below freezing, the vapor freezes to ice.     With warmer temperatures, the ice melts to liquid form.  It temporarily exists as a liquid during the day and refreezes at night.    At the end of the season, with warmer days and nights, the liquid returns to its equilibrium condition as a vapor.  

Thus, we suggest that this temporary non-equilibrium condition which exists during the spring freeze thaw season produces the liquid sap which we refine to maple syrup.