Maple Sap Exudation: How it Happens
Maple Syrup Journal. 4(1) :10-11•
Melvin Tyree
during the full period of the ex- There is renewed scientific in- the next day, the frozen sap melts otherm, i.e., as long as water terest in understanding how maple and falls down the tree under remains to be frozen. sap moves in maple trees in the gravity and it is also given a little bit While the above observations spring. The results of recent in- of a shove by compressed air bub- have been repeated many times, vestigations at the University of bles (see below). How do we know they are at the same time puzzling. Glasgow (Scotland) and University this happens? If you think about it for a moment of Toronto (Canada) are surprising. A number of scientific obser- you will probably remember that Many casual visitors to a sugar vations have been made both in water expands as it freezes; that is bush operation might first think the sugar bush and under well why ice floats on water. Well, if that maple sap is pushed up the con-trolled laboratory conditions. water expands while it freezes then tree from the roots; apparently this Measurements of the rate of sap why isn t sap pushed out of the is not true! This is easily shown flow into and out of tap holes or in- branches while they freeze? We false by anyone with a spare maple to and out of excised branches have in fact observed sap move- tree that can be sacrificed to have been made at the same time ment in other species of trees. Sap curiosity. Next spring, during the as very careful measurements is indeed pushed out during freez- sap season, after a freezing night were made of sapwood tempera- ing and sucked up during thawing and while the tree is still frozen, cut ture. It is easy to tell the precise in all the trees we have investigated it down as near the ground as moment that ice begins to form in except for maple which does the possible. When the temperature sapwood, because water does opposite! rises above freezing you will not always start freezing at 0 C The reason for the apparently observe little or no sap flow from (= 32 F). Instead water will often anomalous behaviour of maple the stump of the root stock, but sap "super cool", i.e., drop a few trees has to do with the unique pro- will drip from the freshly cut surface degrees below 0 C before it starts perties of maple sapwood. The of the tree you just felled. The sap to freeze. When water does freeze sapwood of all trees consists of that we collect apparently flows an enormous amount of heat is water conduits (called vessels in from the branches. Of course there released which very rapidly raises hardwoods) which provide a can t be an endless supply of sap the sapwood temperature to 0 C. path-way for water movement up in the branches. It must first get This heat is called the latent heat trees during the growth season. there from below. So what goes of freezing and the rapid rise in Surrounding these conduits are on? temperature is called the ex- billions upon billions of living cells What does happen is this: On otherm. Under both field and lab and dead wood fiber cells, In most frosty cold nights the sap is sucked conditions the rate of sap flow has trees the wood fiber cells are all up the tree as the branches freeze. been monitored while the stem water filled, but in all the maples While the sap is being sucked up temperature was slowly falling. As these wood fiber cells are gas all the tap holes are, of course, the temperature approaches and filled. So when maple branches sucked dry. The tree freezes from drops below 0 C some sap is begin to freeze, frost begins to form the outside in and the smallest slowly sucked in by the stern (or if inside the gas filled spaces of the branches freeze first. The sap that the tree is already exuding sap wood fibers much like frost will form is sucked up through the portions then the rate of exudation slows); on the inside of the windows of your of the sapwood not yet frozen adds but at the precise moment of the home on a cold night. Water con- to the ice crystals growing in the exotherm (when ice just begins to tributing to the frost comes from colder parts of the tree. If the form), sap is rapidly and vigorously the water conduits and is replaced temperature rises above freezing sucked up. Water uptake continues by the same kind of capillary forces placed by pure water sa.2 move- maple trees freeze slowly the that causes water to flow into a ment is much reduced. But frost in- volume of sap uptake is greatly sponge. (In other species of trees duced sap uptake occurs even in enhanced and the yield of sap the where the wood fibers are water maple branches collected in the next day (if the temperature rises filled there is no room for frost for- summer when the sugar content above 0 C) is much greater than mation so as the water freezes is very low. These conflicting when the tree freezes fast. The the unfrozen portions of the sap results need to be clarified. reason for the reduced sap uptake are pushed out.) When the maple These investigations into the during a quick freeze is that the sap tree thaws out the next day the basic biology of sap movement in in the xylem conduits freezes solid excess sap accumulated as frost maple trees are of considerable and prevents water movement to falls down the tree under the academic interest to biologists, but the wood fibers; so there is not influence of gravity. The sap is you might think at first glance that enough time for much frost build also pushed by the pressure of the they have little direct application to up inside the gas filled fibers. gas bubbles in the fiber cells. The the sugar bush operator. For- Observations of this kind can be gas filled spaces are under tunately we think that such used to predict days of good sap pressure be-cause as frost forms studies may be of real benefit to yield. inside the gas spaces the frozen the commercial operator. If we water displaces and compresses can better understand the the gas bubbles. biological mechanism for sap flow The story is a little more com- and the climatic conditions under plicated than I have indicated here which it occurs we might be able and there are a few observations to suggest management in the scientific literature that have techniques to in-crease maple yet to be reconciled with this sap yield; or we might be able to explanation. For example, some predict years when yields might older scientific studies conducted be best. In this regard we do have at the University of Vermont seem another relevant observation. to indicate that sap uptake and ex- When maple trees freeze rapidly udation occur in maple branches the amount of sap sucked up is only when maple sugar is present reduced (if it freezes very rapidly in the sapwood. If the maple there can actually be a net sap sugars are flushed out and re- exudation). But when