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The sap flow through trees.

April 29, 2003

The transport of water through trees
Trees take up water from the soil by their root hairs and transport it through the vessels of the stem towards the leaves. At leaf level this water evaporates and comes into the atmosphere through little pores in the leaf epidermis, called stomates. This process is also known as transpiration. The water transpired through the leaves must be replaced by new water molecules. This water can either be pushed from the bottom or pulled from the top. The first process, called root pressure, is however not sufficient to push the water to the top of tall trees. Moreover, root pressure occurs only during low transpiration rates, and in the beginning of the spring when there are no leaves yet (see further). So, this means that water must be pulled up. Indeed, the water evaporated from the leaves exerts a negative pressure (a "pull" or a tension), which is transmitted all the way down the stem to the roots. In this way, water is withdrawn from the roots, pulled up the vessels and distributed to the cells that are losing water to the atmosphere. It is also this tension that rules out the positive root pressure as a crucial factor.

The exploitation of birch sap
During the growing season trees photosynthesize, meaning that inside the leaves carbon dioxide from the atmosphere is combined with water into sugars and this with the aid of solar energy. These sugars are then translocated and used for the growth of the tree. In the autumn, the tree stores extra reserve nutrition for the waking up the following spring. In the spring, the tree transforms the stored nutrition back to sugars. When dissolved materials, such as sugars, fruit acids and minerals, cumulate in the sap, the sap start to flow from the roots towards the top of the tree (root pressure). Typical for the birch tree is that this sap flow, due to root pressure, is commercially exploited. The timing of the birch collection depends on the weather conditions of each year. The collection period is extremely short, only approximately 3 to 5 weeks per year. Generally the sap starts to flow in April, and the collection period is over when the leaves start budding. At that time, the birch sap turns milky white and becomes bitter to the taste, due to the appearance of yeasts in the sap.
In contrary with other tree species, birch sap can be commercially exploited because its sugar content (especially glucose and fructose) is about 0.5 to 1.5 %, and thus higher than that of other trees. To make a thick syrup, the birch sap must be boiled down to decrease its volume by 30 to 40 times. Birch syrup contains also vitamins and minerals, including vitamin C, potassium, manganese, thiamin and calcium. All these substances of the sap have their own important functions, also in the human body.

Present scientific research: the study of the dynamic sap flow
The sap flow in birch trees is known by the general public, because it is commercially exploited. However, sap flow does not only occur in birch trees; it occurs in all trees. During the growing season, sap flow is needed to transport minerals from the soil to the leaves. Moreover, sap flow maintains the transpiration stream to cool the leaves and to protect them against overheating by the intercepted solar energy. So, the term "sap flow" does not only reflects the water transported through the birch tree due to root pressure, but it is the water transported through any tree during the entire growing season. Sap flow can be measured in a rather simple way by means of sap flow heat balance sensors.

The fact that the water flow through the tree is a very dynamic process was shown by sap flow measurements carried out during the solar eclipse of 11 August 1999 at branches on different heights and at stem level of a 75-year-old beech tree of 27 m high growing in the forest of Gontrode, located near Ghent (Belgium). During the day of the solar eclipse, the moon obscured the sun from 9.07 h GMT till 11.46 h GMT. At the moment of the maximum eclipse the solar radiation energy was only 2 % of the value which one would measure on a day without a solar eclipse, meaning that the natural light above the forest was almost completely turned off. This influenced the sap flow through branches and stem, and during the period of the solar eclipse the tree used 32 liters less water in comparison with the normal water use (128 liters) during a nice summer day in August without a solar eclipse. Moreover, comparison of the sap flow through the stem and the branches at the moment of maximum eclipse showed that the water uptake by the roots exceeded the water losses through the leaves (zero at the moment of maximum eclipse). This indicated the dynamic flow of water through the beech tree.

To study this dynamic water flow through trees in more detail, an experimental set-up was created in the growing chamber of the Laboratory of Plant Ecology. Young 3-year-old beech and oak trees were chosen as model trees. A complex set of physiological processes (transpiration rate, sap flow rate at root, stem and branch level and diameter fluctuations at stem and branch level) were measured continuously, together with the micrometeorological variables (radiation, temperature and relative humidity) controlling these physiological processes. This experimental set-up could prove that the water transport through trees is indeed a very dynamic process: not all the water taken up by the roots is immediately transpired through the leaves. So, the tree cannot be regarded as a simple tube connecting the soil with the atmosphere, and where water uptake equals water loss. Trees posses internal water reserves which they can use when the water supply from the roots to the leaves is insufficient (especially during the morning). This results in a shrinkage of the stem diameter. On the other hand, when the water uptake by the roots exceeds the water losses through the leaves, the internal water reserves, used during the day, can be refilled again, resulting in a swelling of the stem diameter (especially during the evening). With this experimental set-up the daily rhythm of the young model trees with relation to transpiration, sap flow and diameter fluctuations could be studied in detail.

Ghent University

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