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Xylem Sap Moon

by Christopher J. Yahnke

“It is said that our people learned to make sugar from the squirrels.” - Robin Wall Kimmerer, Braiding Sweetgrass

Braiding Sweetgrass by Robin Wall Kimmerer is not a linear book. You can jump in anywhere and learn, and as I read it, every new chapter, new story, new lesson that I read was my favorite. That was, until I read the chapter “Maple Sugar Moon,” after which every new chapter was my second favorite. If you have not read Braiding Sweetgrass, stop wasting your time on this amateur blog post and go find a copy, and if it's early spring, start with Maple Sugar Moon.

My wife and I started practicing frugal February. To the best of our ability, we only cook and eat what we have in the house. During the year, we froze 100 pounds of strawberries and blueberries that we picked, collected bushels of apples for the crispers in the refrigerator, bought blocks of Wisconsin cheddar cheese, and we have a well-stocked pantry. This is coincident with the Hunger Moon, which Kimmerer says precedes the Maple Sugar Moon in the Anishinaabe lunar calendar. In subsistence cultures, this is the time when food stores are dwindling, and wild game is scarce.

Red maple tree showing snowmelt pattern in front of the author’s house. The open ground is south facing. Photo by Chris Yahnke.

In March, the days get longer, the sun higher, and the snow melts in a very particular way. Circles of bare ground first appear at the base of trees like this red maple in our yard. I learned from Kimmerer, a teacher who did her graduate work in botany just down the road from me in Madison WI, that the dark bark of the trees absorbs the radiant heat from the sun and reflects it back, melting the snow at the base of the tree. This is the signal that it's time to tap your maple trees.

The Canadian naturalist Marie-Victorin wrote that Native Americans discovered maple syrup and taffy thanks to a red squirrel. One Iroquois legend goes that, on a spring day, a squirrel climbed a maple tree, bit into a branch, and drank. A young man looking up from under the tree wondered why the squirrel was doing this and decided to imitate the squirrel. He made a slash in the tree with a knife. Learning about maple sap from squirrels is one example of how Traditional Ecological Knowledge (TEK) is centuries ahead of western science. In 1992, Bernd Heinrich published a paper in the Journal of Mammalogy entitled Maple Sugaring by Red Squirrels. Heinrich observed chisel-like grooves made by red squirrels on sugar maple trees at 22 sites in Vermont and Maine. Who knew squirrels intentionally chiseled maple trees to harvest the sap? Likely every Native American in the northeast. Heinrich noted that the squirrels did not drink the sap immediately but returned to lick up the more concentrated sap after allowing for some of the water to evaporate. While Heinrich rarely observed taps on trees other than sugar maples in his study, naturalists in the northeast have observed squirrel taps on 23 different species of trees.

This illustration by Scott Cameron is from Maple Moon, a wonderful children’s book by award-winning Canadian author Connie Brummel Crook.

Maple syrup is produced by boiling down xylem sap that flows in the early spring. Xylem sap carries sugars from the roots to the dormant buds to wake up the leaves for summer photosynthesis. Conversely, the sugar produced in the leaves from photosynthesis flows through the phloem to other parts of the tree and is not tapped by squirrels but is tapped by sapsuckers, woodpecker-like birds that bore holes deep into the trucks of trees to reach the phloem sap. Squirrels have been tapping maple trees for xylem sap as long as there have been both squirrels and maple trees. Recently, squirrels have also started tapping maple tappers.

Maple trees are not the only trees to produce sap. The black birch pictured here shows evidence of squirrel tapping. Photo by Linda Speilman.

Maple syrup production is a hobby for some and a livelihood for others, but Vermont is considered the epicenter of maple syrup production. In Vermont, the Maple Sugar-making Moon is called Sogalikas, and is fourth in the 13-month lunar year, following Moose-hunting Moon. Like the Iroquois and the Anishinaabe, the Abinake people of this region learned about maple sugar from the squirrels, and quickly imitated them to collect sap. Many hobbyists I’ve met have mentioned squirrels visiting their sap bags or gnawing on their tubing.

The spring of 2019 was a particularly problematic year for larger sugarbushes in Vermont and Maine. Like the Native American people, squirrels face their largest food shortages at the end of winter and the beginning of spring. One sugar producer in Vermont noted that squirrels were not particularly more abundant the spring of 2019, but they were more destructive of the tapping equipment to get at the leaking sugar. So, what changed? I think it may be related to mast.

In 2017, there was a large simultaneous masting event, with sugar maple, white pine, red oak, and red and white spruce all producing abundant mast (seeds that serve as squirrel food). This mast event led to an extreme abundance of squirrels in 2018, according to Vermont’s Chittenden County Forester, Ethan Tapper (that’s his real name – he must come from a long line of tappers, or at least Tappers). A mild winter in 2018 likely resulted in high squirrel survivorship entering spring, but with little of that 2017/2018 mast left on the landscape and squirrel competition high – maple sap from Vermont tappers was a good option.

I personally observed another mega mast year in 2019 that resulted in high squirrel numbers in 2019 and early 2020, on my Snapshot Wisconsin camera and in a small mammal trapping activity.

During the pandemic, I played around with photoshop. I photoshopped several squirrels from my Snapshot camera into a single image along with a selfie from when I was going to check my camera. The thousands of images of squirrels in 2019 and 2020 dropped in 2021. Photo composite by Chris Yahnke.

In 2020, I conducted my field mammalogy course remotely from the Kemp Natural Resources Station in northern Wisconsin. (I had been trapping small mammals on the same two grids of hemlock-maple forest with my students from University of Wisconsin-Stevens Point since 2010 and didn’t want to skip a year just because there was a global pandemic). I taught lectures from the 1922 boathouse classroom over Lake Tomahawk and set and checked 120 Sherman live traps each morning by myself. For comparison, in a normal year, there would be 16 students doing the work. The pandemic’s timing couldn’t have been worse: the year that I did all the work myself, the granivorous white-footed mouse population exploded. The only other people at the field station with me in 2020 were Jalene LaMontange and her undergraduate research student Jessica Barton from DePaul University. Jalene studies mast and reminded me that masting occurs over large spatial scales. Sure enough, NEON’s small mammal data reported high small mammal abundance in 2020, at stations in New Hampshire, Wisconsin, and Michigan, suggesting that there was ample food on the landscape in the spring of 2020, possibly resulting in lower incidences of squirrel conflict on maple sugar farms. Data from NEON, Snapshot Wisconsin, and Kemp all show squirrel and small mammal numbers down in the past two years.

White-footed mouse captures in northern Wisconsin mixed Hemlock-Maple forest (Yahnke, unpublished data). Each year involved 480 trap nights.

So, pay attention to the helicopter maple seeds in the spring, the number of spruce and pine cones ornamenting the trees, the sound of acorns raining on your gutters. Lots of squirrel food is eventually converted into lots of squirrels. And lots of squirrels on our college campuses, in our neighborhood parks and our backyards, gives us more opportunities to thank a squirrel for the gift of maple syrup.


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