By Jennifer Langille
Every spring, something exciting happens in the forests of eastern North America—maple trees begin to release their sweet sap, the essential ingredient in making maple syrup.
I have always heard that maple sap flows because of warm days and cold nights, and I’ve always wondered: Why? What makes maple trees special? What exactly is sap, and why does temperature matter?
The answer is more fascinating and complex than I ever imagined.
Tapping Into the Mystery
This year, I finally decided to learn about maple sap for myself—I tapped six maple trees on my property. As I inserted the first spile, or tap, I was amazed! The instant I inserted the tap, sap began flowing, as if it had been waiting for the moment to release its sweet, hidden treasure. Watching the clear liquid drip into my collection bucket felt like witnessing a small miracle of nature. But what was happening inside the tree?
The Hidden Circulatory System of Trees
Like all plants, trees have a vascular system that transports water, nutrients, and sugars throughout their structure. It functions somewhat like the circulatory system in animals. There are two primary types of transport tissues:
- Phloem carries the sugars produced through photosynthesis in the leaves down into the roots for storage, and back up to the leaves, depending on the tree’s needs. Phloem transport can occur in both directions. Picture a network of interwoven pipes made of living, hollow cells joined end-to-end.
- Xylem moves water, minerals and stored sugars from the roots up to the branches and leaves. This movement within the xylem is unidirectional: upwards. The network of interwoven pipes is similar to the phloem, but is made up mostly of dead cells – except for specialized living cells which play a key role in sap movement.
During winter, deciduous trees, including maples, shed their leaves and stop photosynthesizing. To survive, they rely on stored sugars in their roots. In early spring, before new leaves emerge, these stored sugars must travel back up via the xylem to nourish the tree—and that’s where sap flow begins.
Why Do Maples Produce Sap?
Not all trees produce a sap that flows under pressure like maples do. Maple trees, and particularly sugar maples (Acer saccharum) are particularly special because of their unique wood structure.
Within the xylem, sugar maples have specialized structures that allow gases to expand and contract, creating pressure changes that drive sap movement. These pressure differences occur due to temperature fluctuations between night and day.
You might wonder if harvesting sap stresses or harms the tree. Sugar maples store more carbohydrates than they need for immediate survival, allowing for a sustainable sap harvest. When tapped correctly, a healthy maple can provide sap for decades without harm.
Trees begin to heal the wound from a tap within six weeks, and healthy trees are completely healed within two years.
The Role of Temperature
For sap to flow, the weather needs to alternate between freezing nights and warmer days. Here’s how it works:
- Cold Nights: When temperatures drop, the gas in the tree’s fibers contracts, creating a suction effect that pulls water up from the roots – similar to how sipping on a juice box through a straw draws liquid upward.
- Warm Days: As temperatures rise, the gases in the xylem expand, increasing pressure inside the tree. This pressure pushes sap outward, much like what happens if you squeeze a juice box and liquid flows out of the straw.
Without this freeze-thaw cycle, sap flow slows or stops altogether. When nighttime temperatures become too warm (or when leaves begin to develop and transpire,) the daytime root pressure isn’t strong enough to push the sap up and out of a tap.
Furthermore, warm, sunny days enhance both the temperature effects and pressure-driven movement. Higher atmospheric pressure on sunny days helps “push” sap upward through the tree’s vascular system, enhancing flow, while lower pressure on rainy days reduces this push, slowing movement. As a result, sap flow tends to be stronger on bright, sunny, high-pressure days.
What Influences Sugar Content?
The sugar content of maple sap varies depending on several factors:
- Tree Species: Sugar maples have the highest natural sugar content, around 2%, while other maple species produce lower-sugar sap. The trees I have tapped are red maples (Acer rubrum), whose sugar content is around 1.6% (Diet Sap, anyone?)
- Weather Conditions: A good balance of cold nights and warm days leads to better sap flow and sugar concentration. Optimal temperatures are between -6 C and -11 C at night with between +4C and +10C during the daytime.
- Soil and Tree Health: Healthier trees with access to rich soil nutrients tend to produce sweeter sap.
Fun Facts About Maple Sap
- Sap is composed of:
- Mineral nutrients from the soil;
- Sugars manufactured through photosynthesis and stored in root tissues;
- Hormones that help regulate tree growth;
- Water that has been absorbed from the soil and recycled from within the tree.
- On average, it takes about 40 litres of sap to produce 1 litre of maple syrup as the water is boiled away, leaving behind the concentrated sugars.
- Indigenous Peoples of North America have been using the sugar maple sap for generations, long before European settlers arrived.
- A “sapsicle” is an icicle made from frozen sap that forms at the tip of a broken twig
- A large maple tree can move 200 litres of sap per day from its roots up to its canopy over a height of 30 metres!
I collect the sap from my six trees each day. Although I now understand the science of sap flow, the process of maple syrup production is still beyond me. My ever-resourceful neighbour has both the knowledge and the infrastructure to produce maple syrup on a small scale, so I leave that part to him. Here is what goes on:
The sap buckets are emptied into a large evaporator pan that fits on top of a wood burning stove. The pan holds approximately 50 litres of sap. The large surface area increases heat transfer, and the water portion of the sap evaporates until it is reduced and begins to have a syrup-like consistency.
The reduced sap is poured into a large pot and heated with a propane burner. Here the sap boils – see how it is clear initially, then turns darker as the remaining water evaporates further and the sugar caramelizes. Read more about this chemical process here.
My neighbour pours a small amount of reduced, boiling sap into this test cup and measures the sugar content with a hydrometer, looking for a reading of 66% sugar for the syrup to be “finished.”
And – TA-DA! The final product, delicious on pancakes – and in dressings, sauces, baked goods, tea…the list goes on! (I am suddenly finding so many uses for maple syrup!)
A Sweet and Scientific Tradition
Now that I understand the science behind maple syrup, I appreciate this seasonal ritual so much more. The connections between temperature, tree physiology, and natural pressure changes is incredible!
This year, as I watch the sap flow from my own trees, I feel connected to a process that has been happening for centuries. The sweet gift of maple syrup is about much more than the taste. It’s a testament to the magic of nature, the science within the trees, and the awe of spring’s return.
March 2025
2 Responses
Jennifer, I certainly enjoyed your article. It was so interesting. I learned a lot!! I had never heard exactly how this was done. ….only saw the taps and knew the process continued. We take so much for granted in what we eat and drink . Mother Nature it certainly complicated!!Thank you for sharing your expertise.
Thank you, Mary! I’m so glad you enjoyed the article. It was fun to research and write!