Chel Anderson is a North Shore naturalist. She lives here in Cook County and joins us periodically to talk about phenology or what’s going on in the woods right now. Welcome back, Chel.
Well, it’s already starting. Cold nights, warm days; that means the sugar maple sap is flowing.
Anderson: It does, and we can all look forward to the products to come. We have a lot of maple syrup producers here for our relatively small population, and so we have a lot of experts for you all to call upon to really talk about the whole maple syrup production end of things, so maybe we’ll talk about the mechanics of sap flow, because sap flow as happens in maples is not really very common. There are very few species that actually have the kind of sap flow that maples, sugar maples, red maples, have, and it maybe be something that people don’t have a lot of understanding.
So, I understand that birch does?
Anderson: Yeah, birch are another species that do up here, but they’re really the only two groups of species that really have this kind of sap flow.
Why is that?
Anderson: Well, why? That’s always a really big question that I’m not sure any of us can really answer. It’s to do with their evolution as species that has made them different from other tree species, but the mechanics that they have evolved to have that create a sap flow are quite interesting. First of all, a misconception to clear up to start with is that we all learned in grade school and biology that sap wood in the tree conducts the water from the roots up to the rest of the tree and that the nutrients, the carbohydrates and other nutrients that the photosynthesis creates is conducted by another set of tissue called the phloem into the tree, back to the roots and into other parts of the tree. Well, in the case of the maple, that isn’t exclusively true, and that’s why the sap of the maple is sweet, because at this time of year, it’s actually absorbing sugars that were created by the photosynthesis during the growing season and stored in the tree in the form of sugars, sucrose, and that’s being absorbed into the sap and being carried by the xylem, by the sap wood. So, it’s a little different from what is happening in most trees. So, like other trees, things flow in both directions, up and down, but in the case of trees that actually have a sap flow, like maples, something forms called positive pressure, suction and osmotic pressure. Those three things are the mechanical aspects of why sap flows out of your tap in a tree or out of a wound. Let’s say, a big branch has broken off of a tree in a winter storm or something, or a beaver cut a birch tree down in the fall and now in the spring all this sap is falling out.
I’ve seen red squirrels nibble on branch ends of maple trees and lick the sap.
Anderson: Exactly. Red squirrels: really smart.
They are going to take over the world. Red squirrels and cockroaches.
Anderson: Yes, and they’re so energetic besides.
Let me ask you. When you talk about sugars and storage of sugars, now, do those sugars help the tree do something besides just produce sap?
Anderson: Oh sure. They’re part of the nutrients that the tree, that the photosynthesis--
They’re using it?
Anderson: They are using it, but in the fall, as things slow down, the tree can’t use all of the nutrients, carbohydrates that are being created, so they end up being stored in the cells in the tree and the tree freezes up and nothing is happening, until we start getting the weather cycle that you just described, warm days and cold nights. And, with the warm days, when the tree warms up and the cells in the tree warm up, there is carbon dioxide within the cells of the sap wood that gets released out into the spaces between the cells. And there’s also carbon dioxide in the sap itself and as it thaws and becomes liquid some of that escapes into those intercellular spaces and that creates pressure inside the tree.
You talk to a lot of tree tappers, and there are good years and not-so-good years. Sometimes that might be related to the weather, but is it also related to what happens to the tree, like, in the fall? Are there things that can affect the amount of sugars stored quite apart from the weather?
Anderson: What makes for good years and bad years is probably primarily due to these weather patterns and the access of trees to a good source of moisture to replace the sap that flows during the course of a warm day. So, once the pressure builds up, then sap will flow out of your tap in the tree. As the cold night comes, if the night gets cold, then the carbon dioxide actually shrinks and contracts, because it’s cool, so it contracts in between the cells of the tree, and some of it is reabsorbed into the now-freezing sap and cooling sap. And, when that happens, that creates a suction that draws water from the roots up into the tree to fill—so it’s a suction effect that happens. Then, when it warms up the next day, then the sap will flow again. Well, if you didn’t have a good source of moisture, of water, to move up from the roots, maybe a year when we don’t have a really good snow cover, and so the roots are down in frozen ground for the most part, there isn’t as much access to ground water. Those would be years when having a really strong flow would be much more difficult to create on a daily basis, because there wouldn’t be that access to a good resupply of the water. And then, of course, if it doesn’t get cold at night, then you don’t get that suction created and sap can continue to flow during the night, but then, of course, there’s less and less pressure, because you aren’t replenishing in the way that suction replenishes the fluid aspect of the sap.
Anderson: Yeah, and we don’t have to do anything!
Chel Anderson, botanist and plant ecologist. Thanks for helping us understand maple sugar.
Anderson: You’re welcome.
Photo courtesy of Sébastien B. via Flickr.