If spring is marked by the return of life to our somnolescent winter woods, then the flow of sap is really that first and most prominent signifier that spring is here! When I tapped trees with kids out at the Field School, I had one rule: No Sugar Maples. All trees have sap, even conifers, and many of them produce delicious sap that can be tapped in much the same way as sugar maple. And so it became an experiment to find new flavors, to figure out which trees have sap that floweth freely and when this sap flows. We took inspiration from yellow-bellied sapsuckers, which tap and eat sap from at least 174 different species across their range (more on how sapsuckers survive winter). Some species were a total bust and had no sap flow (e.g. musclewood, apples, black locust, tamarack, etc.). But some were incredibly delicious. Black walnut with its rich carmel-y flavor was easily my favorite, though I also enjoyed the birches.
Animals are similar cued into the spring sap flow, and a variety of animals will consume the sap from damaged trees. However, it’s surprisingly rare for animals to actively create holes on a tree in order to directly consume either xylem sap (sugars, minerals, hormones that flow from the roots to the leaves) or phloem sap (sugars, minerals, hormones as well, but usually different ones that flow from where the sugars are produced to where they’re stored and/or consumed).
Insects: phloem sap feeding is restricted in the insect world to just the Hemipterans (an order that includes true bugs, aphids, planthoppers, cicadas, scales, shield bugs, etc – source.) and xylem sap feeding isn’t all that much more widespread (source). Most of these insects have long probosces (like the sucking mouthpart of a mosquito) and go after younger tissues or trees with thin bark (like beech scales, the vector for Beech Bark Disease).
Birds: It’s primarily woodpeckers (downy, hairy, acorn, etc. – source) that create holes to feed on sap. Of the bunch, sapsuckers are easily the most reliant on sap (there holes are always in horizontal rows, as in the photo above). Other birds (like hummingbirds) will steal sap from sapsucker tap holes and have to defend these. While hemlock is one of the most frequently tapped trees by sapsuckers, I’ve never been able to extract any sap from it.
Mammals: In mammals, it’s mostly squirrels that eat sap (bats and porcupines have also been observed feeding on sap). And I’ve seen both red and gray chew branches to create tap holes and then return to these repeatedly to lick up the sap.
As I was getting excited about my own sap buckets filling up, last week I noticed that a Norway maple was oozing sap. When I looked at the source of sap, sure enough there were multiple bite marks on the branch. I regularly find these bite marks (2 pairs of grooves that cut into the xylem) close to the main trunk on both the upper and lower surface of maple branches, typically on limbs less than 2” in diameter (note that these are different from the territorial chewing/scent marks red squirrels make on the upper surface of hemlock branches – see image at the very bottom). I’ve found these sugaring bite marks on boxelder, Norway maple, silver maple, red maple, and sugar maple. Most research focuses on sugaring in red squirrels (e.g. source), but most of what I see in the Burlington area has been from gray squirrels. And sure enough, the Norway maple in the photo above had been tapped by a gray squirrel. I put a trail camera up on the tree and got plenty of videos of the gray squirrel returning regularly to lick up the sap (see the video below). It’s surprising to me that squirrels don’t make greater use of sap in the spring since it’s such a reliable source of energy.