Why Do Maples put Sugar in the Sap in Winter?

[Note–here is a webinar I made on this topic, plus how to make maple syrup.]

You may know I’m a tree physiologist and that my research has been in how a plant gets water to go up the tree, but it wasn’t until I started making maple syrup that I started thinking about maple sap. The sap we collect comes out of the wood, and it has sugar in it. Those two facts mean automatically that something very different is happening in maple wood in the winter than in the summer.  The sap “shouldn’t” have sugar in it. I read up about it and talked with a few other physiologists who have worked on this issue (and a great many who haven’t), and here is the current understanding.

I think it’s best to start with summer movement of water.

In the summer…

Water moves upward in the tree from soil to atmosphere through transpiration. Transpiration is basically evaporation of water from specialized parts of a plant. The huge majority of the water you give your houseplants and gardens is used for transpiration.

We will use the term “sap” now.  We will use sap in the botanical sense, as the liquid that moves in the wood and leaves. [We do not mean the resiny materials that come out of a cut in a conifer or the milky latex that comes out of something like a dandelion.] The wood (the botanical term is xylem) is an elegant material made of mostly dead cells that function as pipes, with valves in them that keep air from getting in and blocking sap movement. The wood of different types of plants is quite distinct. I taught wood identification for twenty years: we can identify most plants just by looking at their wood.

Surprisingly, water has high tensile strength, meaning it is strong when you pull on it (as long as no air gets in). You have experienced water’s high tensile strength when you drink through a straw or use a siphon to empty the fish tank. If the water had poor tensile strength, say, like sand, then sucking on the straw would not draw more of it up. Instead, the water is like a wire, so when you suck on it, it pulls more water into the straw. When water evaporates from leaves or needles during transpiration, that process pulls more water up the tree.

Why do plants transpire?  Basically, for a trade: water evaporates out, but CO₂ (carbon dioxide) diffuses in. This trade happens at the pores in their leaves called stomata, that plants open when they can afford to lose water and when the sun is out. Otherwise, the stomata are closed. Plants then use that CO₂ to manufacture sugars. The loss of water while gaining CO2 is similar to the dry mouth you get after sleeping all night with a stuffed nose; you’ve lost water from your mouth, but you gained gas (in your case, oxygen) that you needed to survive.

The process of using the CO₂ to make sugars is photosynthesis, which is as close to alchemy as reality gets.  A gas (carbon dioxide) gets held in place next to some parts of water molecules, gets zapped with solar energy, and we end up with a solid: gas goes to solid. And the solid is one on which almost all the rest of life on Earth depends.  If there were a year without photosynthesis, I think most living forms larger than a grape would go extinct.

That alchemic solid is a simple sugar. Plants first make the simple sugars, and then, using more chemistry, turns them into cell wall (like wood, which is cellulose, hemicellulose, and lignin), cellulose (celery strings and cotton thread), and all the goo’s, petals, roots, stems, fruits, seeds, bark, and anything else you can think of that a plant has or makes. Only a tiny quantity of material in the plant is from the soil (maybe 5% of its weight if you dry out the water).  To live, people, bugs, fungi, cows—99.9999% of all living things—rely on that carbon and on the products it has been incorporated into (like sugars and fats and a lot of forms combined with proteins—our vitamins).

You’ve heard a lot about carbon—carbon sequestration, carbon credits, rising levels of CO₂ in the atmosphere contributing to warming. This carbon of photosynthesis is all part of the same story. The short story is that when we burn calories, we release CO2 back out to the atmosphere, and when we burn firewood or “fossil fuels” (meaning fuel that is basically old plant material), we also released CO₂.  CO₂, it turns out, is one of the gases that help keep heat in the atmosphere, which isn’t a good thing at this point (that is, too much heat is being kept in). For that reason, people are talking about how much carbon we can grab out of the atmosphere to avoid more overheating. That carbon is carbon that we are releasing, through our activities. Talk of carbon credits and carbon trading is talk about letting someone release a lot, but “paying” for it by supporting another activity that sequesters a lot.

The sugar that the plant makes moves around a plant in a tissue called the phloem, which on a stem, is that gushy, slimy cellular also called the inner bark. 

So back to sap movement in the summer. The stomata open in the leaves, and as the CO₂ diffuses in, water transpires out. Because of water’s tensile strength, when water dries out of a leaf, the molecules of water, all hooked together, pull the sap up through the leaf, which pulls it through the stem, which pulls it through the root, which pulls it out of the soil.

Sap goes upward. It goes upward through the wood. And the sap has no sugar in it. 

This brings us to the winter.

In the winter …

The process of sap exudation we are talking about is what occurs in maples and walnuts, only.  In this process, the sap goes down the tree into our waiting buckets.  It does not go up. If someone cuts down a frozen maple tree in winter, the sap comes from the cut, top of the tree downward, not from the stump up.

How? Why?  

First, maple and walnut trees are the only kinds of plants that have been shown to have a specialized cell in the wood that is gas-filled in the fall before the plant encounters freezing temperatures. It’s a “fiber cell.” Many species have fiber cells, but fiber cells in these species are unique in that they are gas-filled. The rest of the cells in the wood are mostly full of water.

When the temperature drops below freezing, twigs and fine branches freeze before the trunk (because of its size) and the roots (because of the insulation by the soil). And in the twigs and fine branches, the water that rims the specialized fiber cells will freeze first, because there is less water to freeze than in the other wood cell types.  When their walls freeze, they are, essentially, dry, and so their dryness attracts more water to the freezing front. The water can flow from the water-filled wood cells because that water is still liquid.  If the freeze is slow, then those fiber cells become packed with ice. If the freeze is fast, then not much ice will have formed in the fiber cells before the other cells have frozen, cutting off water movement. 

When the frozen twigs and branches thaw, assuming the rest of the tree is thawed, if you drill a hole into the trunk and put a tube in it (a “spile”), then liquid “falls” down the tree and pours out through the spile. That’s what is collected to make maple syrup. It takes 40-60 ounces of this exudate to make one ounce of syrup. Put another way, it takes 10-15 quarts of sap to make one cup of syrup.  

Why does the water “fall” out?  1) Because it was lifted up into the cells through freezing, and now there is nothing to hold it there. 2) Because bubbles in those special fiber cells push the sap out and continue pushing it out for a long time.  Number 2 needs some explanation.

When the fiber cells thaw and water comes out, they have bubbles in them. These bubbles form because when the sap froze, it compressed the gas that was already inside the fiber cells, so when the sap thaws, that gas, that used to occupy the entire inside of the fiber cell, has to share space with the water. Those bubbles push the water out of the fiber cells. 

The boundary between the bubble of gas and the sap, is an interface. That interface is squeezing in. Think of the gas bubble as the air inside a volleyball. That air is pressurized, and the ball itself (the interface) is pushing inward on the air. If the volleyball were a little permeable to the air, then the air would get out.  Well, the water is permeable to the air. As the bubble squeezes, gas re-dissolves into the water.

Models show that within a few hours, those bubbles should re-dissolve—but observations show that the bubbles last for a few days—which is important to the maple syrup industry, because those bubbles help propel the sap out.  But how do the bubbles last for several days?

The answer has to do with the sugar.

The plant exudes (places) sugar into the sap of the other wood cells, but, essentially, there is a membrane that keeps the sugar from getting into the fiber cells: water can move in and out of fiber cells, but sugar cannot.  That sugar in the other wood cells causes an osmotic pull on the water in the fiber cells; that is, the water in the fiber cells “wants” to be pulled outward to dilute the sugar in the other wood cells. That osmotic pull is sufficient to take some of the pressure off the bubble surfaces so that the bubbles re-dissolve much more slowly than it otherwise would—and so the bubble is able to push sap out of the tree for a longer period. 

Fantastic, we made it to here!  Thanks for bearing with me.

Now, let’s see the explanations again, but this time with diagrams.

The wood of maple has three types of water transport cells—vessels, tracheids, and fibers. It also has parenchyma cells, which are the ones that have the sugar stored in them.  Water moves mostly in the vessels (which are large in diameter compared to the other wood cell types) but also in the tracheids. I don’t know if the fiber cells have water in them in the summer.

In the summer, water is pulled upward in the trees and out to the atmosphere in transpiration.  Sap only moves in the “sapwood,” which is the outer two inches or so of a bigleaf maple tree.

The trees can open and close the stomata to control the rate of transpiration, but they cannot perform photosynthesis when the stomata are closed because they need the CO₂ to diffuse into those holes (the same holes where water diffuses, or dries, out of the tree). 

In the winter, the vessels and tracheids have water in them, but the fibers do not.  When the temperature falls below freezing, the fiber cells, which are gas-filled, freeze first, freezing just the little water that is on their cell wall, then pulling more water in by thermodynamics, possibly until they reach a maximum volume of frozen water. The other cell types freeze, too. If the freeze is “hard” (sudden, cold), the vessels and tracheids will freeze so fast that a lot of water won’t be able to flow into the fiber cells.

Now there’s a thaw.  Everything melts. Water has been pulled upward (vertically) into the tree, and now it has a tendency to drop back downward. At the same time, it is being pushed by the bubbles that form in the fiber cells.  Those bubbles are stabilized by the parenchyma cells depositing sugars in to the vessels and fibers.

That sugar cannot cross into the fiber cells because there is an “unpitted” cell wall between the fiber cells and the other wood cell types. The cell wall, lacking fibers, has been shown to work as a membrane that lets water diffuse both ways, but sugars cannot.

Why don’t we have much of an industry for maple syrup in the West?

From my understanding of the physiology, there would be three simple answers.

1) The western species (bigleaf maple) has a lower sugar concentration in its sap than does sugar maple (60:1 needed in the West, 40:1 in the East) so we need more sap in the first place.

2) Because we boil our western sap down more to get to the same sweetness, we end up with a higher concentration of the other “mapley” flavors. The market is not set up to favor syrup with a lot of color and flavor, although arguably, ours tastes better. I find it stronger and tangier–so it probably would have somewhat different applications in our cuisine.

3) We have way fewer freeze-thaw cycles. The left side of the figure below this paragraph shows the native range of bigleaf maple—it brows up and down the west coast. The right side shows that over bigleaf maple’s range, we have many fewer annual freeze-thaw cycles–075, compared to 100-125 in the Northeast. Those periodic freezes are needed to bring the water up into the fiber cells–to “load” the canopy with potential sap, so to speak. Those periodic thaws are needed to “let that sap down.”

What’s in it for the tree?

The wood that conducts the sap in the summer is much, much more efficient if it does not have pockets of gas in it. Those fiber cells are probably not a problem because as I understand their morphology, they are “cul-de-sacs,” not “highways” for the water–that is, they are storage pockets that are not on the main line.

But gas pockets do get into the wood, inevitably. In some species, the gas pockets (called embolisms) get in mostly during the dry, hot time of the year when they are transpiring (to photosynthesize, and/or to keep cool) without much water in the soil. The tension in that “string” of water becomes very high, and air bubbles get pulled in. Those are called drought-enduced embolisms.

The embolisms we are talking about here are different. They are freeze-induced embolisms. In places in which there are freezes, it is inevitable that the vessels will end up with some gas in them. The water in them had gas dissolved in it. When it freezes (in the vessels) the crystals do not have gas in them, but when they thaw, the gas is still present, and it takes a while to re-dissolve. If the plant has sunny weather before the embolisms have re-dissolved, then when the tree transpires, it will dry out–water will not be pulled up because the “string” is broken by the presence of that water.

The gas bubbles in the fibers are likely to be adaptations that help re-fill the embolisms in the vessels by adding a little positive pressure to the vessels to help their bubbles re-dissolve.

That mechanism has been well-described for other species, but with the pressure coming from sugars stored in the roots or lower stems, but the mechanisms of generating pressure in the canopy, as far as we know (and as far as know) is unique to maples and walnuts.

Interestingly, in the summer of 2018, I made a presentation on this mechanism, with a bunch of my own questions, to a couple hundred colleagues at an international conference on plant water transport (Gordon Research Conference on Plasticity in Plant Vascular Systems: Roles, Limits and Consequences). Only three or four of the participants knew even the basics of maple sap exudation, which didn’t surprise me–there is a lot to know out there, and this information may seem obscure.

But I find it a good reminder that we interpret “what something is for” (like a vessel, tracheid, or fiber, with its various morphologies) without actually knowing the whole story of “what it is for” for the plant itself.  We are imperfect observers. If we think of good questions, the scientific method helps us get closer and closer to reality. For example, I spent my career thinking about the wood adaptations and water movement in the growing season only. 

Sources of information I consulted:

Amegilo T, Ewers FW, Cochard H, Martignac M, Vandame M, Bodet C, Cruiziat P. 2001. Winter stem xylem pressure in walnut trees:  effects of carbohydrates, cooling and freezing. Tree Physiology 21: 387-394.

Bruce D. 2003. Production and quality of sap from the bigleaf maple (Acer macrophyllum Marsh) on Vancouver Island, British Columbia. MS thesis, Department of Geography, University of British Columbia and University of Victoria. 112 pages.

Carlquist S. 2014, Fibre dimorphism: cell type diversification as an evolutionary strategy in angiosperm woods. Botanical Journal of the Linnean Society 174: 44-67

Cirelli D, Jagels R, Tyree MT. 2008. Toward an improved model of maple sap exudation: the location and role of osmotic barriers in sugar maple, butternut and white birch. Tree Physiology 28: 1145-1155.

Holtta T, Dominguez Carrasco MDR, Salmon Y, Aalto J, Vanhatalo A, Back J, Lintunen A. 2018. Water relations in silver birch during springtime: how is sap pressurized? Plant Biology 20: 834-247.

Millburn JA, O’Malley PER. 1984. Freeze-induced sap absorption in Acer pseudoplatanus: a possible mechanism. Canadian Journal of Botany 62: 2101-2106

Perkins TD, van den Berg AK. 2009. Maple syrup–production, composition, chemistry, and sensory characteristics. Advances in Food and Nutrition Research 56: 101-143

Tyree MT. 1983. Maple sap uptake, exudation, and pressure changes correlated with freezing exotherms and thawing endotherms. Plant Physiology 73: 227-285

Tyree MT. 1995. The mechanism of maple sap exudation. pp. 27-45. In Proceedings of the 1^st International Symposium on Sap Utilization, Bifuka, Hokkaido, Japan. M Terazawa, CA McLeod, Y Tamai, eds. Hokkaido University Press.

Tyree MT. 1984. Maple sap exudation: how it happens. Maple Syrup Journal 4(1): 10-11.

Vazquez-Cooz I, Meyer RW (2006) Distribution of libriform fibers and presesence of spiral thickenings in fifteen species of Acer. IAWA Journal 27: 173-182

Vazquez-Cooz I, Meyer RW (2008) Fundamental differences between two fiber types in Acer. IAWA Journal 29: 129-142

Weigand KM (1906) Pressure and flow of sap in the maple. American Naturalist 40: 409-453

Wheeler EA (1982) Ultrastructural characteristics of red maple (Acer rubrum L.) wood. Wood and Fiber 14: 43-53

The line drawings of maple anatomy are adapted from the Cirelli et al. paper.

I also had several conversations about this with Mel Tyree and Teemu Holtta.