Why does sap run?

[bairdswift]

I'm looking for a simple reason how a maple produces sap.

[Bucket Head]

Simple reason; Mother Nature.

A little less simple: ground water is taken up by the tree, upon freezing. Then, when temperatures go above freezing and the water (sap) thaws, along with gases the tree produces pushing it, sap exits the tree through any wound. The internal pressure of the tree forces the sap out, until the internal and external pressures equalize and the flow stops. There is a term for the high springtime sap flow of a maple, but it escapes me at the moment.

Steve

[maple flats]

The sap runs as Bucket said, but the real reason is that the tree's developed internal pressure is pushing sap to the uppermost leaf buds to nourish them for opening. We just intercept a small portion of that sap for our addiction.

[DrTimPerkins]

Very simply, sap runs out of wounds due to gravity (water flows downhill).

Slightly more complicated and close to what Steve wrote, freezing causes contraction of bubbles inside the fibers of trees, pulling water up through the roots into the wood vessels where it moves into the fibers and freezes (kind of like frost on the inside of your window when it is cold outside). When the temperature warms, the frost melts, the bubbles expand, and gravity and gas bubble expansion create a pressure which pushes sap out of wounds (exudation).

The likely reason for this pressurization and exudation is probably not (directly at least) to nourish the new buds (although that is a result of the process), but rather is to refill emboli (gas bubbles) that are formed within the wood vessels during freezing weather. Gas bubbles (emboli) reduce the ability of the "pipes" in a tree to move liquid (kind of like vapor lock in a carburetor), and so this problem must be corrected in order for water transport/transpiration to be effective. Different trees do this in different ways. Some just grow new wood quickly in the spring (oaks and other ring-porous species). Some just make really tiny vessels that are less prone to embolize. Others prevent gas bubbles from forming in different ways or trap the bubble in a small space so it redissolves in the spring (some conifers). Some use root pressure to refill vessels (birches). Other species (including maple) create a stem pressure to force out bubbles and to redissolve them. All of these different strategies of northern tree species "fix" the pipes and make them functional again (or replace them) at the end of the winter.

There is a much more complicated and detailed explanation of the process. The version you read in some places (including the North American Maple Producers Manual) isn't terribly far off, but it is a little bit incomplete and imprecise in some ways, in that they largely (or completely) ignore the osmotic potential component of sap exudation (meaning that sugars and a semi-permeable membrane are required/involved at the microscopic level). Dr. Mel Tyree, the former UVM PMRC Director and USFS scientist (and the foremost tree water transport expert in the world), as well as a graduate student at the Univ of Alberta and I are currently writing a scientific review of the process of sap exudation and embolism. In the current version it is about 23 pages long. Doubtful that many folks will really want to read it (except some scientists), but after that, we are planning to rewrite the paper for a more general audience.

[Bucket Head]

Hello Dr. Tim,

Is the stem pressure, or exudation, unique to maple trees? I vaguely remember reading somewhere that the high sap flow rate was rare, as only a handfull of plants and tree species moved sap like that. Is that anywhere near true or is my memory failing quicker than I thought? What other trees move sap in the volume of the maple in the spring? Thanks.

Steve

[spud]

When living in Alaska we had a friend that tapped over 1000 birch trees. I also tapped a few in my yard just for fun. There were day's when we would get 3-5 gallons of sap out of a 8 inch white birch.

Spud

[DrTimPerkins]

Is the stem pressure, or exudation, unique to maple trees?

No, there are only a few other woody genera that also produce stem pressure in the spring. Exudation from maple and walnut are the best known. Birches produce root pressures through a different mechanism. There are other types of flows caused by wounding of the phloem (inner bark) during the growing season, and another type of flow called guttation in some herbaceous plants during the growing season. The flow from maple (in the springtime) is due to unique aspects of the wood anatomy (small, air-filled fibers and semi-permeasble membranes between vessels and fibers) and physiology (osmotic sugars in high concentrations). The flow rates from maple (and especially from sugar maple) are high compared to others, probably due to the higher sugar content of the sap.