Chemistry and Physics: Why Do Maple Trees Drip?

What kind of gardener are you? An indoor gardener? A gourmet gardener? A flower gardener?

In any case, I’m sure you’re looking forward to spring as much as I am, and one of the first signs of this magnificent season is my annual visit to… the sugar shack! Quebec tradition oblige: the log cabin, the checkered tablecloth, pork in all its forms, maple syrup dumplings and, of course, maple taffy.

But here’s the eternal question: will I survive my partner’s flatulence after he’s stuffed his face with pea soup and baked beans?

…

Uh… no, actually, I meant why do maple trees leak? Yes, that was THE question!

Sugar Chemistry

Where does the sugar in sap come from? As gardeners, you probably know that plants photosynthesize. When carbon dioxide is transformed into oxygen, we have a carbon that… has to go somewhere! CO2 can’t just turn into O2, the carbon molecules (the C) have to go somewhere.

Here’s the simplified formula:

The plant uses this carbon to produce carbohydrates (glucose). It’s sugar that the plant uses to make leaves, seeds, roots – in short, to grow! But it’s not this sugar that ends up in maple water: photosynthesis for maple trees takes place in summer, not winter!

When autumn arrives, the trees begin to transform their carbohydrates into starch. This is a very stable, compact form of sugar that the trees store in their roots for the winter. It’s their energy reserve for regrowing leaves the following year and, no, it’s still not the sugar found in maple water.

In spring, the starch is transformed into sucrose: (another) simpler sugar that will circulate through the tree to the branches and which, you guessed it, is none other than our famous maple water. That’s because sugar has taken so many different forms before reaching our plates!

Why Use Maple Water?

This process is the same for many trees, so why do we harvest sugar water from maple trees in particular? Quite simply, it’s the tree with the highest concentration of sugar in its sap. It’s not for nothing that one of our species is called the sugar maple.

In comparison, the white birch has such a low sugar concentration that it takes four times as much sap to produce the same amount of syrup. We’re talking about reducing some 160 liters of birch water for just one liter of syrup!

The Physics of Trees

That’s all well and good, we understand why it’s sweet, why it’s in spring, but why does it drip? That’s a simple question of physics.

When it’s cold, materials contract slightly, and when it’s hot, they expand and soften. This is why rapidly heating a dish just out of the freezer causes it to split: part of the dish tries to expand, but the rest is still frozen from the cold. Another example? A heated piece of plastic is much more malleable, even elastic, than when it’s cold.

Mind you, all this happens at the molecular level: your baking dish won’t seem stiffer or smaller when cold!

What About Wood?

The same process affects wood: when the temperature drops, it contracts; when it warms up, it expands. Now imagine all the tiny vessels that carry sap through the tree trunk. At night, when the temperature drops below zero, the wood contracts, making those tiny vessels a little wider. During the day, when the sun warms the trunk, the wood expands and compresses these same vessels. The water circulates upwards, but when the vessels are suddenly compressed, the pressure in the tree increases to the point where the slightest hole in the trunk, the slightest broken branch will let out… maple water!

If the pressure in the tree doesn’t increase, the water continues to flow towards the branches, which is why it’s important to have several days of night frost and daytime thaw: it’s the sudden and repeated increase in pressure in the trees that allows us to harvest their delicious nectar.

All that’s left to do is boil this water to evaporate the liquid and concentrate the tree’s natural sugars. Tada! Introducing maple syrup!

By the way: it’s been a long time since we harvested by boiler. Maple groves are all automated, with systems of pumps and pipes that connect the trees together. The boilers you see on your way to the sugar shack are often there just for show, and the “real” producing trees are further away in the forest. Hand harvesting is still done, but on a small scale, in your backyard, on a dozen or so trees.

Maple Syrup at Risk

It’s easy to see why Canada is the ideal place for maple syrup production: frankly, our absurd temperature variations must serve a purpose! But now comes a new set of challenges for maple syrup producers…

Climate Change

Climate change makes harvests highly unpredictable. Some years are exceptional, others very bad. Sometimes the trees flow very early, and others later… In fact, the weather has never been so stable that production was 100% predictable, but with the rapid changes in climate, harvesting could become increasingly uncertain, or even impossible for growers far to the south who have increasingly colder winters.

Diseases and Parasites

Diseases and parasites are a real threat. Ever heard of the emerald ash borer? Spruce budworm? Unfortunately, climate change and accidental species introductions are responsible for the destruction of entire forests. Studies have shown that the impact of these epidemics is even greater in low-diversity forests, and with a maple pest rampant in southern North America, we can only hope that it doesn’t migrate here, otherwise the future of monoculture maple would be in jeopardy.

Aging Forests

Aging forests are also a huge problem right now. Many operations have felled young trees to make forest travel easier. But here’s the thing: old maple trees produce much less maple water, and some growers are seeing their harvests dwindle year after year, regardless of the weather. Now that the problem is becoming better known, the practice of cutting down trees too small for harvesting is tending to decline, but how long will it be before they’re big enough to replace the older trees? Several years!

In short, the future of maple syrup is mysterious, and maple syrup producers face many challenges… But you’ve heard of challenges with plants, right? Maple tapping is just another form of gardening… on the scale of a forest!

So tell me, what is YOUR #1 use for maple syrup? On a banana-peanut butter pancake? In a cocktail with fir-tree gin? With soy sauce to marinate your pork tenderloin? As for me, I can boast that I’ve won my partner’s heart with my maple salmon recipe: in a hot BBQ, the maple turns to caramel! But it takes a real talent to achieve this without overcooking the fish (not that I’m bragging)!