Monitoring of xylem sap flow in trees by a non-intrusive, laser-based heat tracing technique and comparison with MRI flow imaging

Repeatable field measurements of water use by single trees or entire stands are of great interest to forest ecologists. Heat has been widely used as a means to evaluate sap flow rates for over 70 years; however the techniques used are intrusive and intrinsically disrupt the flow of sap to be measured. The only truly non-intrusive system developed to date is magnetic resonance imaging (MRI), but this is clearly not suitable for use in the field. We have therefore developed a non-invasive approach using a fibre-delivered, near-infrared laser source (lambda=812 nm) for the pulsed heating of a small portion of the stem and an IR camera for monitoring the surface temperature in the region surrounding the point of heating. Heat flow occurs by a combination of forced convection (sap flow) and conduction, and so analysis of this changing temperature field can provide a measure of sap flow velocity.