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Analysis of sap flow dynamics in saplings with mini-HFD (heat field deformation) sensors

Schreel, J. D. M., Steppe, K.
Acta horticulturae 2018 no.1222 pp. 161-166
Fagus sylvatica, branches, deformation, ecophysiology, heat, hydrodynamics, sap flow, saplings, temperature
Sap flow dynamics of saplings, herbaceous species and plant parts with small dimensions have become an increasing point of interest in plant ecophysiological studies. At present, the most widely used sap flow sensors for these applications are the stem heat balance, the mini-HRM (heat ratio method) and the mini-HFD (heat field deformation). Each sensor has its own advantages and drawbacks, but when a high temporal resolution is needed in addition to measurements of positive and negative sap flow, only the mini-HFD suffices as the original heat balance method cannot distinguish between positive and negative sap flow, while the mini-HRM uses heat pulses rather than continuous heating partly sacrificing its temporal resolution. As such, the non-invasive mini-HFD is promising for studying hydraulic redistribution in saplings. However, when calibrating the mini-HFD by forcing water through cut branches problems arose during higher negative flows. The ratio used in the commercially available HFD was not optimal for the mini version. To resolve this shortcoming a new calculation method has been developed based on three temperature differences measured with the mini-HFD sensor. In this paper, the mini-HFD has been calibrated for flow rates ranging from -20 to 20 g h-1, whereby sensors were installed on beech (Fagus sylvatica L.) saplings. First results are promising and indicate a better calculation method for sap flow dynamics measured with the mini-HFD sensor.