Washington Evening Journal

Neighbors Growing Together | May 25, 2017

Tracks and Tales: A Sticky Subject

By Pam Holz

“Around the heartwood, we have the … ?”
“What’s its job?”
“Bring water up!”
“Around the sapwood is the … ?”
“Inner bark!”
“What does it do?”
“Bring food back down to the roots!”

I’ve repeated this early elementary tree program enough times that all I need to prepare is to go through a mental checklist.  I never thought too much about it, and the other parts of the tree, till my boss suggested I write my column on maple syrup.  
As I turned over the idea in my mind, I thought about what I knew personally about trees and about maple syruping.  In the process, I discovered something didn’t fit.  
Moving in from the outside of the tree, we first encounter bark.  The outside portion acts as protection from the weather, insects, disease, and other harm.  The inner ring of bark, though, known as the phloem, is essential.  Once food is made in the leaves during photosynthesis, the phloem transports those sugars down to where the energy is needed, such as the roots.  Since this layer is incredibly thin, in many trees, you don’t need to cut a deep ring around the tree to kill it.  Such carvings, even if it’s one’s initials, cut off those crucial food transport cells.
At first glance, tapping into this layer for sweet maple syrup sap makes the most sense.  By definition, sugars already move through this space.  However, I also know the tap for syrup is screwed further into the wood than the phloem.  Not only that, the tree isn’t exactly making sugars during the leafless winter.  That means the tap must be placed into the water-transporting xylem.  It’s this point that confused me; syrup is not made from water.   
The next layer after the phloem is the cambium.  Another thin layer, the cambium creates new phloem and xylem cells.  The bulk of the tree, the woody growth, consists of xylem cells, or simply, the sapwood.  Name-wise, it makes sense to get sap from the sapwood.  However, the purpose of the xylem is to bring water and nutrients from the roots to the leaves.  That definitely does not sound very sweet.  
If the tree is large and old enough, it might have heartwood.  This dead wood is older xylem now used for storage of food resources.  I didn’t think syrup taps went that deep nor that these stores would be syrupy sweet.  I discovered a mystery:  where exactly does maple syrup sap come from and why?  
The answers lie in the type of tree used (maple) and the nature of winter.  Apparently even the maple syrup industry did not fully understand the process till recently themselves.  
At the close of summer, maples begin storing starches in their xylem tissue.  I imagine these reserves give energy to the trees for spring growth.  At winter’s end, the starches break down into sugars which dissolve in the sap.  The freeze/thaw cycle of warm days and cold nights, plus the specific distribution of gases and fluid in the wood create sap flow.  During the warm days, sap is pushed upward or outward, if the tree is wounded, such as with a tap.  At night, the sap freezes.  It may be the gases within the trunk, expanding and contracting with thawing and freezing, that create the sap movement.  
Few trees have sap flow and in many of those, their sap may not be sweet enough.  A different process makes sap flow in birches, and syrup can be made from these trees.  Fortunately for maple syrup lovers, sugar maple, the main maple for syrup, is widespread and common enough to support our native, organic, and sustainable sugar resource.