FRANKLIN, W.V.—Get to know your sap.
“We’re going to try to think like a tree," said industry expert Michael Rechlin, Ph.D. of Future Generations University.
During a recent webinar, The Science Behind the Sweetness of Sap Flow, Rechlin explained the hows and whys of the annual miracle of sap flow.
"We’re going to try to figure out from a tree's perspective why that sap drips out of a tree,” he said.
His talk covered evolutionary biology, chemistry, anatomy, tree physiology, physics and biology.
The sap flow process is the result of trees response over many eons to cold weather, specifically long, hard winters.
“Life originated in the tropics and as trees, like people and animals, moved north they had to find solutions to the problem of winter,” Rechlin said.
Visually, using lettuce leaves, he showed how live cells burst and wilt when subjected to freezing conditions.
In another simple experiment, using a water-filled bottle, he showed how water expands when frozen, and in this case broke the vessel it was in.
“Trees have that same problem,” Rechlin said. “They’ve got water in them.”
“It’s going to expand when it freezes. It’s going to break cells in there. Trees had to evolve a mechanism to get around this problem. This is a long way to figuring out why sap flows in the spring, but it’s very important to understand this if you want to know what's going on in there,” he said.
In a third experiment he filled a plastic bottle, which could expand, and froze it.
But the ice wasn’t clear and had a milky color because as water freezes gas bubbles come out of solution.
“That’s why when you take an ice tray out, there’s a whitish hue in the center of the ice cubes,” Rechlin said.
Frozen gas bubbles are also a problem for trees.
With a micro-photo, he showed the various components of a piece of red maple wood starting with vessels that allow for sap flow.
“They’re responsible for bringing water up from the roots up to the leaves in the springtime,” Rechlin said. “They’re also what you tap into when you drill a hole in a tree. They’re responsible for sap flow.”
Vessels are evenly distributed throughout the annual ring. Such trees are considered diffuse porous wood.
“Any diffuse porous hardwood, theoretically, will flow sap in the springtime,” he said.
Another component, fibers, which have thick-celled walls, give wood strength.
Rays, unlike vessels and fibers, are live cells that extend from the outside to the inside of a tree.
“Trees are in a fix because live cells freeze and burst, and dead cells in vessels are filled with gas bubbles. So what did trees do? They figured out a way to solve these problems, not quickly, but they figured it out,” Rechlin said. “This allowed them to move from the tropics into northern regions.”
Unlike water, which broke the glass bottle when frozen, he showed how maple syrup did not break the bottle, but stayed in liquid form, when put in a freezer overnight.
“The reason for that is, the same reason we put antifreeze in our car in winter,” Rechlin said. “Dissolved solids in a liquid lower the freezing point. Sugars in maple syrup lower the freezing point of syrup.”
Trees make sugars, a sugary antifreeze, all summer through photosynthesis and store them in live cells so they don’t freeze in winter. That was the answer to one problem.
“But trees also have these dead cells whose vessels are filled with gas bubbles,” Rechlin said. “When water in these dead cells thaws, gas bubbles are going to cause a problem. Why? It's a problem because in summer those vessels are needed for something else. At this point the tree needs to absorb water through its roots and transport that water (carrying nutrients) up through its stems and out to its leaves. In springtime, trees need to be ready to do this.
If a tree broke bud without clearing its water column of gas bubbles, water above the bubble would move on out, but water below couldn’t and the system would functionally be broken.”
Gases under pressure dissolve in a liquid. When pressure is released, gas flows out.
Rechlin demonstrated this by shaking a soda bottle and showing how gas (carbon dioxide) and liquid flows out.
“Trees are faced with the problem of how to get gas bubbles in their vessels back into solution. The answer is through pressure,” Rechlin said. “By adding pressure to the system those gases will dissolve in a liquid.”
When trees are tapped, liquid (sap) and gas both come out.
“You basically pop the top on the soda bottle by putting a hole in the tree and releasing the internal pressure in the trunk of the tree,” Rechlin said.
“During sugaring season, pressure in a tree builds as temperatures rise during the day, and falls again at night when it's cold. Sap flows out of the tree during pressure buildup, so it can clean out its vessels,” Rechlin said.
The cyclic combination of warm days and cold nights are essential for a good season.
“What the tree is doing is recharging,” he said.
When you take sap out, it has to bring water up to be ready for next day's sap flow. You need those positive pressures during the day, followed by negative pressures at night so the tree can recharge. If you don't get that freezing period, to recharge, it will flow less and less and eventually dribble on off.
With his detailed presentation, Rechlin provided the answers to four basic questions.
* Why is sap sweet? Because a trees stores sugars and starches all summer and is ready to move it on in spring.
* Why does sap flow when you put a hole in a tree? Because it's a pressurized system. By putting a hole in the tree, what's under pressure is going to flow out.
* To keep sap flowing what has to happen next? You have to have the negative pressure, freeze period, for the tree to recharge.
* Why does it happen best in spring?
“Because that's when you have most of these freeze-thaw cycles,” Rechlin said.
The session was the second in a multi-part webinar series, Out of the Woods: Enriching Your Maple Business, made possible through a partnership with Ohio State, Penn State and Future Generations universities.
The discussion, partially funded with a grant from the U.S. Department of Agriculture's Agricultural Marketing Service, proved valuable for veteran and novice sugarmakers alike by providing helpful reminders and a basic, easy-to-understand explanation about why trees produce sap.
Rechlin previously worked for the West Virginia Department of Agriculture as a maple commodities specialist and has the same job with his current employer, Future Generations University in Franklin, West Virginia.
He also has been a forestry professor and natural resources professor at several other colleges.