We started tapping maple trees on campus this week. I would guess that most of my readers are familiar with the general process of making maple syrup – you tap the trees to collect their sap and boil it down to get rid of the excess liquid and concentrate the sugar. The Ojibwa Indians in this area were already collecting sap to make syrup and sugar before the first European settlers arrived, and there’s even a town south of here called “Sugar Camp” because the site was known as a center of maple tapping activity. But why do we tap the trees at a specific time of year? What exactly is going on with the sap?
Well, first, what is sap? Duh, we all know it’s the sticky liquid in trees, but it has an important function. Sap flows through a tree’s xylem and phloem (remember your high school biology?), bringing moisture and nutrients from the roots up to the leaves and taking sugar and other carbohydrates produced in the leaves back down to the roots for storage. This is roughly (very roughly) analogous to an animal’s circulatory system.
The ideal time to tap a tree to collect sap is early in the spring when temperatures are above freezing during the day but below freezing at night – basically right now, if you live here in northern Wisconsin. The most commonly accepted theory for why sap flows at this particular time of year is that as temperatures rise and fall each day, carbon dioxide within the tree’s tissues expands and contracts, creating a cycle of pressure that causes sap to flow out of any wounds in the tree during the day and suction that causes water to be drawn up from the roots at night. This is a separate process from the water tension and cohesion that usually drives the movement of moisture up and down the tree.
Sugar maples and other closely related species have particularly high amounts of sugar in their sap, which makes them ideal for making syrup, but some sources I found suggested that another reason maples are particularly suited for tapping is that the structure of their internal tissues makes this pressure cycle more pronounced in maples than in other trees. Even with the relatively high amount of sugar in sugar maple sap, it takes about forty gallons of sap to make one gallon of syrup.
So, have you ever done your own maple syruping? One of the very first posts on this blog was actually about the syruping process, way back in March 2010. I can tell you this much, real maple syrup is way better than the cheap fake stuff in the grocery story.