Sap Flow in Small Stems outlines how the SFM1 Sap Flow Meter can be installed on stems with a diameter of at least 1cm for precise measurements.

The SFM1 Sap Flow Meter uses the Heat Ratio Method (HRM)
The sensors measure heat pulse velocity by obtaining the ratio of downstream sapwood temperature to upstream sapwood temperature following the release of a heat pulse.
HRM Accuracy
Aim: To measure sap flow in a rose bush with a narrow diameter stem and to correlate measured data with environmental variables (vapour pressure deficit and solar radiation).
Methods: Species: Rose, cultivar Grimaldi approximately 0.6m tall, stem diameter 1.4cm
Sap Flow measured using Heat Ratio Method (Burgess et al . 2001*).
Linear regression of Sap Flow versus VPD and Solar Radiation used to assess accuracy of data.
Cover needles with foam insulation in order to avoid external heat influences.
Results:
- Basic interpretation of data on visual inspection:
- Diurnal and nocturnal sap flow
- Warm and Cool Weather

Analysis from Sap Flow Tool software: (www.sapflowtool.com) Indicated the rose plant used on average 0.12 Litres of water per day.

Results
Environmental Drivers of Sap Flow: Vapour Pressure Deficit (VPD).
Below is an example of sap flow data from HRM sensor inserted in a narrow rose stem over a seven day period.
Results
Environmental Drivers of Sap Flow: Solar Radiation.
Below is an example of sap flow data from HRM sensor inserted in a narrow rose stem over a seven day period.
Results
Statistical Analysis
Note: Data are sum of diurnal sap flow, VPD and solar radiation; data were square – root transformed to meet assumptions of normality; p – value less than 0.05 indicate data are significantly correlated; larger R 2 value in Sap Flow vs VPD indicates VPD is a stronger driver of sap flow than solar radiation in this rose plant at this location during this weather period.
Summary
- With careful experimental design and installation, sap flow of stems with a diameter of at least 1cm can be accurately measured
- Needles must be insulated to buffer external heat influences such as direct sunlight
- Sap flow data show a significant relationship with environmental drivers Vapour Pressure Deficit and Solar Radiation
- The rose plant in this example was found to use on average 0.12 Litres of water per day under temperate summer weather conditions