Sap flow measurement

What is sap flow?

Sap flow is the movement of water in roots, stems and branches of plants. Typically measured in sapwood xylem, or the water-conducting tissue of the plant, sap flow provides information on water status and water use of plants.

In the field of ecophysiology, sap flow is important in understanding plant transpiration and drought responses. In agriculture, it is important in the measurement of plant water use for irrigation decision.

How to measure sap flow in plants?

Sap flow is measured by inserting sensors into the xylem tissue of the plant. The sensors are then used to trace known amount of applied heat. In some plants, sensors are attached to either the stem or the trunk. A robust mathematical approach to trace this heat is using heat ratio method.

How the heat ratio method works?

The heat ratio method (Burgess, Adams, Turner, Beverly, Ong, Khan and Bleby, 2001) is based on the temperature ratio created between probes positioned symmetrically above and below a line heater. A pulse of heat is provided by a heater needle located in the sapwood area, with a downstream and upstream needle that are accurately spaced to measure the heat difference over time. This measurement, combined with data on the sapwood area allows for the calculation of the sap flow.

In both SFM1 and SFM1x, there are two temperature probes and the central heater probe. A controlled heat pulse is emitted from the central heater probe, which the temperature probes measure as the heat is transported by the sap moving up or down the xylem. The sensors subsequently measure the sap flow velocity, with calculations performed within the SFM1x to derive the plant water use.