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Understanding Sap Flow

Timothy Wilmot
University of Vermont Proctor Maple Research Center

The maple syrup industry exists because, during the leafless state, sap flows from fresh wounds in the wood of maple trees, and because that sap contains a relatively high level of dissolved sugar. The flow of sap is the result of positive sap pressure, a weather-related phenomenon. Many features of the sap flow mechanism are unique to maple.

Sugar in maple sap:

The large amount of sugar contained in the sap during the spring sets maple apart from other trees. During the growing season, sugars produced in the leaves by photosynthesis are stored as starch for current and future growth and to maintain cells of the tree during the night and during the long dormant season. When the weather turns cold, some of the starch is converted into sucrose and enters the sap, where it will remain mostly frozen until the spring. This accumulation of extensive amounts of sugar in the sap of maples allows stored reserves to move rapidly to the growing cells of the buds and bark once the temperature warms sufficiently. Sucrose concentration of the sap is normally 50% to 100% greater in the spring than in the fall.

Sap Pressure:

Sap pressure is the pressure of the sap inside the tree compared to the outside atmospheric pressure. For most of the year, sap pressure is negative or zero and sap will not drip from a wound. Negative pressure, or suction, occurs during the growing season when water vapor evaporates from leaves. Because there is a continuous column of water from the leaf, through the wood of the tree and into the roots, the evaporation of water from leaves actually pulls up water from the soil. This is the process of transpiration, and it occurs in all trees. During the maple sap flow season, there are alternating periods of positive and negative sap pressure.

The spring sap flow mechanism:

Maple sap is under high positive pressure during certain weather conditions, because of the structure of maple wood. Unlike the wood of most trees, the vessels, which are the sap conducting tubes, are surrounded by air-filled fiber cells. In the spring, when the temperature drops below freezing, frost begins to form on the inside of each fiber cell. This begins in the fine branches of the crown; their smaller diameter causes them to freeze first. The accumulation of frost in these billions of cells pulls water up through the still unfrozen trunk from the roots and soil. At this point the sap is under negative pressure, or suction. Without a sufficient drop line, unfrozen sap would also be pulled from tubing back into the taphole. As ice forms inside the fiber cells, the trapped air bubble in each one is compressed.

Eventually, the suction stops as more of the wood freezes. When the temperature later rises above freezing, the sap, pulled up into the crown by frost accumulation, now falls toward the tree base, additionally pushed by the expansion of compressed air bubbles. Sap pressure is at a maximum soon after the thawing begins, assuming that the air is warm enough for the outside of the trunk to thaw. Maximum pressures of 25 psi or greater are commonly seen at this time. Without another freeze to repeat the cycle, the sap pressure will eventually drop to zero. This happens because the air bubbles in the fibers will, in time, reach atmospheric air pressure. Additionally, sap from the vessels is withdrawn to refill winter-desiccated wood and lost to transpiration of fine branches.

Sap flow:

Positive sap pressure forces sap out of the taphole (and any other recent wound in the wood). Flow rates of sap have been shown to be directly proportional to pressure when the tree is fully thawed. Early in the season, however, high pressure may be generated through freezing and thawing of fine branches, but flow may be less than predicted because the trunk of a large tree remains partially frozen. We have found that the timing and rate of sap flow is highly variable from year to year. Average flow rates as high as 1 gallon per taphole per hour under gravity have been recorded; however, in some years, the maximum average flow rate is less than half of this. Maximum flow rates usually occur about halfway through the sap flow season: after trees have fully thawed, but before tapholes have started to dry out. Beginning and ending dates of the sap season in Underhill, Vermont, are unpredictable, and have been offset by a month or more from one year to the next during the 8 years that we have been recording this data.

Common myths about sap flow in maple:

  1. Sap flow does not start until the soil thaws out. Actually, forest soils are rarely frozen at any depth if there has been snow cover for most of the winter. Snow is an excellent insulator. In addition, loose and decaying leaves on the forest floor provide a good layer of insulation to the roots beneath them.
  2. Sap flow in maple occurs because of root pressure. There is some flow caused by root pressure in most trees, but the pressures generated this way are nowhere near as high as the pressure that comes from the branches, as described above. Root pressure does not occur until the soil starts warming, which is usually after the maple sap season has ended.
  3. How sap flows from maple is still a mystery to scientists. There are some aspects of the mechanism that are still in debate, but there is general agreement about the way positive pressure is generated, and how that pressure is responsible for sap flow.

The relationship between two tapholes in a tree:
Do more taps result in more sap collected?

When we put more than one taphole in a tree, we assume that sap will flow to each hole separately; in other words, sap will not flow horizontally toward one hole and away from the other. The potential for horizontal, or lateral, flow in maple wood has been the subject of several investigations by researchers. Over the years, various contradictory solutions have been described, from the early work of D.H. Jones, author of the “rule of 86,” who, using relatively primitive instruments, reported considerable potential for horizontal flow, to researchers who injected dyes in trees and reported no possibility of horizontal flow.

I investigated this question by installing single tapholes in trees of various diameters, and then using sensors located some distance from the taphole to detect changes in pressure in the wood. After recording the pressure in various parts of the tree with the taphole closed, (the tapholes had stopcocks that could seal them off) the taphole was opened. The presence of an open taphole under 15” of vacuum will cause the sap pressure of the surrounding wood to decrease; any portion of the wood that dropped in pressure indicated that sap was moving from this location towards the taphole. Using these methods, it was determined that sap was being pulled toward the taphole from a much greater distance than had previously been reported. Sap appears to move rapidly from the wood above and below the taphole, but also, more slowly, from the sides. Tapholes on opposite sides of trees 24”dbh or smaller probably share the sap during most sap runs. This is an ongoing study, and final results have not been reported yet.

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