Musings on Maples on Climate Change

(Originally published in March 2013)

By Laurel Sundberg

Interpretive Naturalist
Lowry Nature Center

          Once upon a time in Minnesota an early sign of spring was a bucket hanging on the trunk of a massive old Sugar maple. These gnarled old giants were tapped and sap was collected and cooked into syrup or sugar. In fact, European settlers learned to collect and cook maple sap to sugar from American Indians.

          Fast forward to the present day; you will still see buckets hanging on Sugar maples in late winter, but the tradition is changing. Specifically the weather that creates our sugaring season is changing. The season we have to tap these trees, with nights below freezing and days in the 40’s, is shortening. And with climate conditions being less favorable for Sugar maple as a species, the forests in our area are filled with fewer giant old maples.

          Sugar maples are native to North America, ranging from the New England, north into Canada, and southwest to Missouri. Minnesota lies on the west edge of this range. Historically we were smack-dab in the middle of Sugar maple heaven; a region known as the Big Woods. But many of the woods have fallen to farming and development. The maples that remain are showing changes in reaction to our warmer winters. 

          Many of our current studies on maple sugaring come from New England, a region with strong economic ties to syrup production. In the past 100 years the average New England temperatures have warmed 2-4 degrees F. Similar changes are taking place in the Midwest. With warmer winters, timing the collection of sap becomes very tricky. Sometimes we completely lack weather conditions needed for sap flow. In 2012 sap collection became nearly impossible because we just never had the right combination of winter-into-spring weather. 

          Sap flow is complicated and not fully understood…but here’s a stab at it. Cold recharge (CR) is needed to get trees producing sap. This equates to weeks of below freezing temperatures before having a thaw. Historically this CR cycle would occur during December, January and February. During CR the sap in the limbs turns to ice and creates a low pressure area and pulls more sap into frozen zones from the roots. How does frozen stuff pull sap? Don’t ask; that’s where the math gets fuzzy and chemistry equations fly around. CR also gets the tree to convert starch into sugar to fuel spring bud break. When warm weather melts the frozen sap, it gets pushed in all directions and we can collect it.

The sap itself is changing. 60 years ago Sugar maple sap in the eastern US averaged between 2.5-3% sugar. That percentage is dropping. During the last decade, in the same region, sugar contents were measured at 2-2.5% on average. This represents a significant decline in the vigor of Sugar maple trees. Trees are more stressed by environmental factors and produce less sweet sap then 60 years ago.

          There is visual confirmation of this change using satellite photography. A pigment called anthocyanin is tied to starch production in maples. High starch content produces high anthocyanin; which we see visually as red pigment in the leaves. High anthocyanin produces brilliant orange-red fall colors. And satellite photography over the past decade shows that the brilliant orange-red is dimming.   

What does all of this mean? That’s the million dollar question; only time will tell. In the short term it gives us a reason to celebrate the cold weather this winter. Long term, you might want enjoy those Sugar maples while they still grow here in MN!

We’re all hoping for a better sugaring season than 2012. We would love to have you visit or volunteer during the season. Don’t forget to pick up some tapping supplies while you’re here. You could try your very own maple sugaring experiment this spring!