How The Sap Runs

16:31 minutes

a bucket of what looks to clear water (it's the sap) attached to a tree. two tubes collecting the sap hang from the tree
Sugar maple sap collected near Green Bank, West Virginia. Credit: Ira Flatow

Maple tapping season is underway in the sugar maple stands of the United States. Warm days and below-freezing nights kick off a cycle of sap flow crucial for maple syrup production. But why is the flow of sap so temperature dependent in sugar maples?

University of Vermont maple researcher Abby van den Berg explains how ice crystals in the trees’ cells power sap flow, while Yale University’s Craig Brodersen tackles how other trees and plants move gallons of fluid per day from roots to leaves—all without using any energy at all.

Watch our SciFri Macroscope video featuring Abby van den Berg to learn more about the science of collecting maple sap.

Further Reading

  • Read a paper about how maple sap moves in maple trees in the Maple Syrup Journal.
  • Check out these 3D views of plant xylem anatomy from Craig Brodersen’s lab at Yale University.
  • Listen to this past interview on Science Friday with Selena Ahmed who studied how climate change is affecting the taste of maple syrup.

Segment Guests

Abby van den Berg

Abby van den Berg is a research assistant professor in plant biology at the University of Vermont’s Proctor Maple Research Center in Underhill Center, Vermont.

Craig Brodersen

Craig Brodersen is an Assistant Professor of Plant Physiological Ecology at Yale University.

Segment Transcript

IRA FLATOW: If talking about soil as you’re thinking spring, what about sap? That’s right it’s that precious time of the year when the sap is flowing in the sugar maples, and maple syrup producers are hurrying to harvest before the very important cycle of freezing nights and warm days come to an end. Why does sugar maples need that kind of weather, that particular weather, to produce the sap we love? And what about other trees which move many gallons of water from their root systems to their leaves every single day all without spending any energy at all? When you think about it, it’s kind of an engineering marvel, isn’t it? 

Well here to geek out with us about the hydraulics of a tree trunk from sugar maple to redwood, Doctor Abby Van Den Berg, a research associate professor at the University of Vermont’s Proctor Maple Research Center in Underhill, Vermont. Welcome to Science Friday. 

ABBY VAN DEN BERG: Thank you. Thank you for having me. 

IRA FLATOW: And Craig Broderson is an assistant professor of plant physiological ecology at Yale University’s School of Forestry and Environmental Studies in New Haven. Welcome to Science . 

CRAIG BRODERSON: Thanks for having me 

IRA FLATOW: Abby, I know it’s a busy time of the year for you researching maple SAP production, right. 

ABBY VAN DEN BERG: Absolutely. 

IRA FLATOW: I mean, it’s just a short period that you have. 

ABBY VAN DEN BERG: It is very, very short, six weeks, sometimes longer, sometimes shorter. We never know. 

IRA FLATOW: And what are the ideal conditions for getting maple sap for syrup? 

ABBY VAN DEN BERG: As you described earlier, really the ideal and the required conditions are nights where the temperatures are below freezing followed by days or multiple days where the temperature is above freezing. 

IRA FLATOW: Well, OK, take us inside the tree and tell us why that condition is so important. 

ABBY VAN DEN BERG: It’s important for a couple of reasons. The first is there needs to be sap there in the first place. So the below freezing temperatures are what actually enables the process of the water to be drawn up from the soil through the roots and up into the higher parts of the tree. So that’s what gives us water to sap in order to collect in the first place, but also there’s a little bit of magic of the freezing and thawing causing the enzymes to get active to load sugar from the cells where it’s stored in the wood into that sap that’s been drawn up into the tree. So there’s that sort of dual process going on. 

IRA FLATOW: Dr. Broderson, just to be clear Abby’s talking about what sugar maple trees are doing when there are no leaves. What’s going on the rest of the year? 

CRAIG BRODERSON: The rest of the year is a little bit different in that the movement of water up to the top of the canopy of a big tree, whether it’s a redwood or whether it’s a sugar maple once it’s grown, [INAUDIBLE] of water coming out of the leaves and establishing a pressure gradient that is basically pulling the water out of the soil up through the roots and through the trunk out the branches and then ultimately out the little teeny tiny pores in the underside of the leaf that are this stomata that it can open and close. 

IRA FLATOW: Now I knew that if you try to use a straw and suck up water it’s only going to go a certain height, right. You can’t get it any higher due to atmospheric pressure. So how does the– what trick does a tree know to get it all the way to the top? 

CRAIG BRODERSON: Yeah, so plants have figured out a really remarkable way of making this work in really tall trees in particular. And so to get it up there what’s going on is as the xylem, the cells that make up the wood, so the pipes, the plumbing of the plants during the development of those cells, they start out filled with water and then will eventually start out as a living cell and then the cells that are conducting the water will go through it sort of a programmed cell death. And so they’ve got water in them to start with. And then they eventually die, and then we’ll start to transport water to the top of the canopy once the leaves start to photosynthesis. And this is all contingent on this sort of continuous column of water that goes all the way from the roots to all the way up to the leaves. 

IRA FLATOW: This is Science Friday from WNYC Studios. Abby, what’s going on– how does a maple– a sugar maple tree deal with this. You said that the freezing is important. Does ice in the sugar maple contribute to sucking up this sap. 

ABBY VAN DEN BERG: Yes, so the process that Dr. Broderson was talking about is basically the movement of water up from the soil through the roots and out through the leaves is driven by the evaporation of water. But in sugar maples during the leafless period this movement of water is driven by the freezing of water instead. So we have those vessels where the sap actually– sort of, the pipes that sap water move through in the plant in sugar maple those vessels are surrounded by these fiber cells that are actually hollow. 

And when the water, the liquid water, the sap in the vessels begin to freeze, ice crystals begin to form on the outsides of those neighboring fiber cells. And the growth of those ice crystals is actually what creates the negative pressure, the tension that provides that driving force for water uptake. So freezing of water instead of evaporation of water. 

IRA FLATOW: All right, number 844-724-8255 if you’d like to talk about the tree sap. We love talking about stuff like this. And let me get– we’ll be taking a break in a couple of minutes. Let me get a couple of more questions in. Craig, this is a whole lot different than how animals move water around, right. 

CRAIG BRODERSON: Yeah, it’s a fundamentally different way of moving large volumes of liquid around an organism. So in human systems, human vasculature, we’re talking about positive pressure with a heart that is using a lot of energy to do all those contractions to move the blood throughout our circular system. And the conduits, the veins and the arteries in our body, they’re somewhat elastic and so they can accommodate those differences in pressure that arise. 

And so in plants at least in the part that’s transporting the water, again, it’s under, as Dr. Van Den Berg mentioned, it’s under negative pressure tension that arises as a consequence of the evaporation of the water out of the leaves. And so the pressures that we’re talking about or the negative pressures, the tension that we’re talking about, the xylem turn out to be pretty large. And so instead of needing to be able to expand outward like our vascular system does, they need to be really structurally sound so that they don’t buckle, they don’t collapse or implode because of the really significant negative pressures that arise in xylem. 

IRA FLATOW: Yeah, when we were all in grade school, we did an experiment in science class. We took a stalk of celery and put it in colored water. And we watched the coloring move up the stalk of celery. And people talked about capillary action. I didn’t hear you or Dr. Van Den Berg say a word about capillary action. Is it not– is it not useful in a tree? 

CRAIG BRODERSON: We think that capillary action and, in particular, the properties– the surface tension properties of water are actually really important for maintaining this continuous column of water. And so what the plant is sort of fighting against is the weight of all the– and gravity that’s acting on the water column. And it’s supported by the essentially a wet surface film on the inside of the leaves, and this is where the– all that– a lot of that tension is supported from the little– tiny little meniscus. So if you do these measurements in high school where you’re looking at a graduated cylinder and you’re always measuring from that meniscus, the smaller the diameter of that pipe the higher the water can rise. 

IRA FLATOW: We’ll be talking more with Dr. Abby Van Den Berg and Craig Broderson after the break, our number, 844-714-8255. You can also tweet us @scifri. If sap is your subject, stay with us. 

This is Science Friday. I’m Ira Flatow. We’re talking sap science, hydraulics, the mysteries of how trees move all the hundreds of gallons of water they use each day with Dr. Abby Van Den Berg from the University of Vermont and Dr. Craig Broderson from Yale. Our number 844-724-8255. You can also tweet us @scifri. Let’s go to the phones. Let’s go to Cincinnati. Hi, Carrie. 


IRA FLATOW: Hi there, go ahead. 

CARRIE: Yeah, I have a question. We have– well, my son and I have been tapping maple trees the past couple of years, and we also tap black walnut trees and make black walnut syrup, which is very tasty. But we heard a rumor that you could tap Sycamore trees and that you could make syrup from that and that it tasted, kind of, like butterscotch. And I was wondering if they knew anything about that, and if so, if you have to tap them in a different depth than you would tap a maple or a black walnut tree. 

ABBY VAN DEN BERG: Well, that’s a really interesting question. And the fact that you tap walnut trees is also super interesting, because they happen to be one of the few closely related tree species to sugar maple that have that very unique anatomy with the hollow fiber cells that allow this positive pressure to happen and allow us to tap them and collect sap. Sycamore trees are– yes, it appears that they may be able to be tapped, but we’ve really don’t have a lot of good data on when they should be tapped and how they should be tapped. I know there are a few people experimenting with that in West Virginia and my colleagues in New Hampshire as well, so I guess the real answer is stay tuned. 

CRAIG BRODERSON: Craig, you have anything you can add to that? Sure. There are– as Abby mentioned, there are a few other species that people are starting to play around with. It’s, sort of, a niche market at the moment, but it’s certainly expanding is as people are getting more interested in doing this on their own and sort of exploring other options for different species. 

IRA FLATOW: Abby, is this so interesting– so easy to do that can try– you can try this at home? 

ABBY VAN DEN BERG: Tapping maple trees to collect sap? Absolutely. If you live in a place where you have a maple tree, be it sugar maple or red maple or even something crazy like box elder, as long as you have those breezy nights followed by warm days, there is no reason why you shouldn’t try this at home. You can make very, very tiny quantities of syrup on your kitchen stove, as long as you aren’t afraid about removing wallpaper or making a giant mess. It’s something that everyone should try at least once. 

IRA FLATOW: Can the maples, the sugar maple tree survive if they don’t freeze, if you’re not in the freezing temperatures in the winter? 

ABBY VAN DEN BERG: That is a very good question. I think if there is no freezing, if there’s no dormancy, I think the tree will probably have bigger problems than it’s freeze thaw, the lack of freeze thaw in the spring, for example. Not having winter dormancy creates a whole host of other issues for trees that are adapted to that kind of climate and environment. 

IRA FLATOW: Because I keep hearing– I live in New England, and I keep hearing about in Vermont the climate changes change the micro systems, the weather systems, and the maple trees are heading to Canada because it’s not cold enough in the winter. 

ABBY VAN DEN BERG: Well, the maple trees are definitely already in Canada for sure. 

IRA FLATOW: We know about it. We know about all the maple syrup up there. 

ABBY VAN DEN BERG: I think it will be quite a long time before the species migration goes to that length that we don’t have any maple trees here or any freeze thaw conditions. I think a long time before that happens, we might have one long season or two very short seasons in the fall and the spring. And there are also– you can do this type of sap collection and maple syrup production really with any species of maple. 

Sugar maple as has always been favorite for that because its sugar content in its sap is relatively high relative to other species, but you can tap a red maple and make maple syrup for example. And red maples are adapted to a far wider range of climate conditions and growing conditions than sugar maple are. So it will be a while before we see this disastrous consequence of climate change. We do see the season changing and we have to adapt our practices right now as an impact of climate change, but technology and practices have allowed maple producers to really adapt to what’s already happening. 

IRA FLATOW: Craig, what effect does drought have on trees? 

CRAIG BRODERSON: Drought ends up being a big issue. And so with trees, generally the way we kind of talk about it is the trees sort of have to pick during drought. They can either die of starvation or die of thirst. And so the starvation part is that when plants sense that the atmosphere is drier, whether the soil starts to dry out, the little pores in the underside of the leaf will close. 

And so those are opening and closing primarily to let CO2 in so that the plants can do photosynthesis. And if the plant is sensing that it’s dry, it’s going to close down this stomata in order to minimize water loss. And so when they do that, they’re no longer able to eat. They’re no longer able to do photosynthesis. 

And so as a consequence, they start burning through all the stored carbon that’s on the inside– and inside of the plant. And there’s a finite amount of those resources that the plant has to draw from. And in particular, the thing that we’re seeing now is that sort of season after season, year after year of drought, sort of, minimizes and draws down a lot of those stored carbohydrates. And if a plant isn’t bringing in more carbon than it’s spending, then that ends up being a pretty big problem. And so the plants will shut their system out, and they, kind of, have to wait it out until it rains again. 

IRA FLATOW: Let’s go to Paul in Durham, North Carolina. Hi, Paul. 

PAUL: Hi, very good. Thank you. Appreciate your show so much. I know time is short. We’re in a very wet period. I’m just curious, given the hundreds and hundreds of gallons that trees can use with negative pressure and maintaining it during normal times, in what way do they adjust their liquid appetite or metabolism in periods of drought? 

CRAIG BRODERSON: During the drought, so there are a number of different strategies that plants have come up with. There’s a huge range. There’s tons and tons of different types of species– different species. And so some of them will adjust the ratio of the roots to the amount of foliage that’s on the top of the canopy. 

So in severe droughts one of the symptoms that happens after the stomatal closure takes place is they’ll actually start shedding their leaves to prevent additional water from evaporating out of the plant. And then once the water comes back into the soil, one of the things– first things they’ll do is start to grow new roots to access all that water. 

IRA FLATOW: So considering that stream of water that runs all the way up from the top to the bottom of a plant, the worst thing that could happen could be an air bubble in that stream. 

CRAIG BRODERSON: That’s right. So the water that’s in the xylem sap is under tension. And so as a consequence, it’s a negative pressure in the liquid. And as a consequence, it’s call what we– being meta stable in that it, sort of, wants to become– change from the liquid phase to the gas phase, because it’s below the vapor pressure for water. 

And so these bubbles can arise from basically the separation of the water molecules. These cavitation events that lead to a bubble, and then those bubbles have a tendency to propagate through the vascular system of the plant. And that’s when the plant really gets into trouble. 

IRA FLATOW: So if we as engineers knew how the plant did it, we could make our own devices. 

CRAIG BRODERSON: Well, it depends on how quickly we want to transport water. So the actual rates of water transport up the tree can be high in some species, but it’s probably not at the rate that we would need it to move for running our faucets. 

IRA FLATOW: Thank you very much. Very interesting conversation. Craig Broderson is an assistant professor of plant phys– professor of plant physiological ecology– that’s a long term– at Yale University School of Forestry and Environmental Studies in New Haven. Dr. Abby Van Den Berg is a research associate professor University of Vermont’s Proctor Maple Research Center in Underhill, Vermont. Thank you both for taking time to be with us today. 

ABBY VAN DEN BERG: Thank you. 


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