Science, You’re Super: Sap!

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By Aryn Henning Nichols • Photos by Benji Nichols
Originally published in the Spring 2013 Inspire(d)

Whether the discovery of sap was a happy (sappy?) accident or not, it has definitely revolutionized breakfast forever. Pure maple syrup is a gift from…well…the maple trees (thanks, trees!).

But just what is sap, and why do we only “tap the sap” in the spring?

Let’s get science-y right away: Xylem and phloem are the transportation systems of vascular plants ­– water and nutrients in (or up) the xylem cells, and sugars – sap – out (or down and around) the phloem calls. (1)

A plant has roots to help it absorb water, but a mature tree’s leaves can be 100 feet above the ground. This is where the xylem is put into action, circulating water and dissolved minerals to the leaves. Also, fun fact: When someone cuts an old tree down, the rings you see – one for every year – are the remains of old xylem tissue (it dies and develops anew each year). (1)

But we’re really here to talk about the phloem. Most plants have green leaves, where the photosynthesis happens. Photosynthesis creates sugars – that’s the sap! – that every cell in the plant needs for energy. You can think about sap kind of like a food for the tree and its buds and leaves. The leaves produce sugar during the summer and in the spring, when the tree draws water from the ground, the water and sugar mix inside to create sap, which helps new buds grow. (2) The phloem system transports the sap throughout the plant or tree, and is what brings it to the sap tap in the spring. (1)

The sap in sugar maple contains a high concentration of sugar compared to the sap of other trees, which why so many people go to the lengthy process of collecting it and making it into that delicious syrup. But don’t the trees need the sap? Luckily, it has been estimated that tapping removes 10 percent or less of the tree’s sugar, an amount too small to hurt a healthy tree under normal environmental conditions. (3) Also, once the buds and leaves start to open in the spring, most of the sugars have already served their purpose for the trees. (2)

So is it sap season only in the spring?

Spring is when the temperature fluctuations are just right (in a good sap year, anyway) to create a good flow of sap (although some folks may also harvest sap in fall, it’s more of a rarity). Early in the season, when the maple trees are still dormant, temperatures rise above freezing during the day but drop back below freezing at night – this creates a pressure in the tree that causes the sap to flow out through a wound or tap hole. During cooler periods, or at night, when temperatures fall below freezing, suction develops, drawing water into the tree through the roots. This replenishes the sap in the tree, allowing it to flow when it’s warm again. Too hot or too cold temperatures during the short, six-week “sap season” reduces the amount of sap flow and makes for a “bad year” for maple syrup producers in that region. (3) A really good maple tree can produce sap for 100 years, and one healthy tree can produce up to 15 gallons of sap a year. (2)

The most common use of maple sap is to process it into maple syrup. To make maple syrup, the excess water is boiled from the sap. It takes 40 parts maple sap to make 1 part maple syrup (10 gallons sap to make 1 quart syrup)! (4)

Although Vermont produces most of the nation’s maple syrup, you can check out the process locally at Green’s Sugar Bush (1126 Maple Valley Road, Castalia, Iowa) or see if there’s a sugar bush near you that allows visits!

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Aryn Henning Nichols enjoys heading out to her old stomping grounds for pancakes at Greens Sugar Bush in the spring. The line is often long – stretching all the way down the driveway – but that’s often the fun! You stand out in the spring air and chat with the person next to you about nothing or everything. Or the weather – it is the Midwest!

1. www.biology4kids.com/files/plants_xylemphloem.html
2. minnesota.cbslocal.com/2012/03/27/good-question-whats-in-tree-sap/
3. maple.dnr.cornell.edu/produc/sapflow.htm
4. www.tapmytrees.com/coprsap.html

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