Maple Syrup is Antifreeze

TRANSCRIPT

Overwintering animals in Canada have all kinds of tricks to dodge the worst of it when the weather gets dramatic.

Hibernating, retreating to a den, grouping together for warmth, huddling temporarily in the shelter of an evergreen…

But… what about that evergreen? Or, any of the plant species who have to go through the same winter gauntlet without the benefit of dens, or hiding, or retreating to warmer climes. They just have to brace against it and tough it out.

Well, it should come as no surprise that they too have evolved a lot of smart strategies to let them do just that. And today we’ll be looking at one species from a particularly Canadian angle. The sugar maple. Because if it wasn’t for their winter survival strategy, you guessed it - we would not have maple syrup.

Through spring and summer, trees like the sugar maple are working hard. Their leaves, through photosynthesis, are converting sunlight into energy the tree can use. 

Among these byproducts of photosynthesis are starches, derived from CO2, that the tree stores in its branches. These starches are inert for now, but they will become critical later on.

As the growing season comes to an end and the temperature lowers, trees like the sugar maple begin an intense preparation for winter. Starting in August and biologically cued by the shortening days, they start to produce abscisic acid. This does two very important things for them.

One, it stops the flow of nutrients out to the leaves and seals them off from their stems. This is the beginning of the process that has those leaves changing colour, dying and falling off in autumn. The tree’s growing season is over and it needs to pare back and preserve its energy rather than wasting it on leaves during the short days and weaker sun of winter. The tree is entering dormancy, its own plant version of hibernation.

Two, this hormone increases the permeability of cell membranes. Which is interesting. Why would it wanna do that? We’ll come back to this.

But first - it’s worth asking what exactly the tree is getting ready to protect against. It may seem obvious but: what exactly is it about freezing that is so dangerous? Why can’t a tree - or any living thing - just freeze and then thaw and be OK?

Because ice cubes float.

Water is actually pretty unusual in that, when it freezes, it expands and gets less dense - by about 9%. This causes a lot of annoyances for us every winter. One of the most common is pipes freezing. If water filling a pipe expands by 9% as it turns to ice, it has nowhere to go but out and can crack or burst the pipe. 

But if it helps, the alternative would be way worse.

If water followed the pattern of getting more dense and contracting when it froze, ice would sink, and bodies of water would freeze solid from the bottom up rather than keeping an insulating layer at the surface. Goodbye freshwater ecosystems. And if layers of ice built up on the bottoms of deep bodies of water, they would be insulated from the warming of seasonal change, which might lead to a runaway loop… it’s just all bad. A bursting pipe now and then is probably a small price to pay.

And what, after all, is a tree but a pipe? They’re 60% water. And this ice expansion is a problem for them on a micro and macro level. Each individual living cell’s water content can freeze and burst its cell walls, killing it - that’s what makes freezing so dangerous for all living things.

And the tree itself, as one big pipe, can be put under enormous pressure from aggregate internal freezing. Despite the mitigations we’ll be talking about, this still can and does happen. On especially cold nights in winter if you stand in a quiet forest, you can often hear gunshot cracks. You’re hearing trees burst like the pipes in your poorly insulated basement.

But let’s return to the micro. The goal of the sugar maple in winter is to prevent its individual living cells from bursting due to freezing and expansion. 

How does it do that? Back to the abscisic acid it released. We know it does two things: cuts off nutrients to leaves so they’ll die and fall, and increases permeability of cell membranes.

So again, why increase permeability of cell membranes? Because that lets water drain out of those cells and into the nutrient channels in the tree. It intentionally dehydrates its cells. This leaves them with much less liquid inside, which means less expansion, which means less chance of bursting the cell walls. The nutrient channels have more room for the water to expand harmlessly.

Good start. But that water in those nutrient channels is still going to freeze, and expand. When it does, those ice crystals are going to press up against the living cell walls and once again threaten to burst them.

But the sugar maple’s ready for that. It releases a second hormone which increases the elasticity of the cell walls. Now when a sharp ice crystal jabs the cell, rather than bursting, the cells are much more able to deform, flex and bend around them.

And we’re not done yet. The cell dehydrating process leaves those cells with less liquid in them. But they still need some amount of water to keep them viable. That water can still freeze - so we’re not out of the woods yet, so to speak.

To ensure this reduced liquid in the cells doesn’t freeze and cause damage, we return at last to those starches the tree has been banking all year. Now it’s time to put those starches to work.

The sugar maple releases an enzyme called amylase that converts these starches into sugars, and those sugars are delivered to the inside of the cells. The tree is producing its own antifreeze, or ‘cryoprotectant’.

Why does sugar work as an antifreeze? Ice forms around a nucleus at low temperatures as molecules of water kind of click into place in a crystal lattice. As more molecules of water encounter the edges of this lattice, they click in too. This expands the edge and it becomes a runaway growth of ice.

Sugar molecules, extremely well-dissolved in water, physically interrupt that ability to click in. A water molecule can’t click into the lattice if there’s a sugar molecule in the way. So the presence of sugar arrests that runaway growth and makes it much harder for ice to form. 

And the colder it gets, the more starch the tree converts. They can concentrate sugar inside their cells to the point where the liquid won’t freeze even at -30 degrees celsius. 

This amazing combination of strategies - dehydration of cells, increased elasticity of cell walls, starch conversion and sugar dissolution - can keep sugar maples healthy through the deep freeze of winter.

But I promised maple syrup. So we’re not done yet.

In the spring, a whole new process starts. The sugar that has been keeping the cells from freezing all winter is invaluable for a tree about to enter a rapid growing season. It needs that fast, abundant energy.

So after the threat of freezing is over, the sugars are released back into the tree’s circulation system.

This can only start to happen when the ground thaws. That lets the tree start taking up water from its roots again.

The water mixes with the sugars from the cells to become sap. That sap ‘runs’ at a very specific time of year when the temperature oscillates in a freeze / thaw cycle, lowering at night and raising during the day. This temperature differential produces a pressure differential in the tree that creates natural suction, a sort of respiratory effect that pulls sap up during the night and lets it flow down during the day. 

That sap, and those sugars, are used to grow new buds, blossoms and leaves as the tree enters a new photosynthetic growing period, sucking up sunlight and CO2, using last years’ converted starch as energy to drive the accumulation of new starches that’ll be used next winter… and the cycle continues. An incredible multi-pronged, or many-branched, approach to let the sugar maple stand tall against the worst of winter.

And of course, we sneak in at this point as well. If done properly, harvesting sap from a sugar maple is not a huge burden on the tree, as evidenced by operations that responsibly harvest from the same stands of trees for years while they continue to grow.

When we tap the tree, we’re also exploiting that natural respiratory pressure cycle so when the sap runs, some of it drains out of the tap, collects in a bucket and gets boiled down into one of our national symbols. Maple syrup. 

Who knew we were all pouring antifreeze on our pancakes.