Articles
DETAILED ANALYSIS OF RAW TEMPERATURE DATA MEASURED WITH THE HFD-METHOD
Article number
846_6
Pages
77 – 84
Language
English
Abstract
The heat field deformation (HFD) method for sap flow measurements is based on the analysis of symmetrical (dTsym) and asymmetrical (dTas) temperature differences measured in axial and tangential directions around a linear heater in a tree stem.
These temperature differences characterise the deformation of the heat field around the heater caused by the ascent of sap.
Beyond sap flow calculated from the measured temperature differences, they are very informative for tree physiology studies.
Symmetrical temperature difference is also known as the sap flow index (SFI) which may be used as a stress indicator.
The ability to measure this SFI with a HFD-sensor at several stem sapwood depths allows analysing the impact of different xylem layers in avoiding drought stress.
Another important feature of the SFI is its relationship with daily diameter changes.
For an apple tree it was shown that SFI is proportionally related to the diameter changes during the day and inversely related during the night.
Changes from proportional to inverse relationships occurred during SFI maxima, usually recorded on cloudless days in the early morning and in the evening.
Analysis of SFI in different trees showed that the period between the two daily SFI maxima and the corresponding decrease in SFI are usually larger for the outer xylem layers (in which higher sap flow rates are recorded) than for the inner xylem.
This is also true for the same (e.g. outer) xylem layers with increasing height along the tree stem.
This may reflect higher relative internal water storage consumption from highly conducting tissues.
SFI behaviour in large roots may differ from that typically observed for the aboveground part of a tree due to the high complexity of flow through the root tissues driven by transpiration and hydraulic redistribution towards the soil.
The ability of SFI to detect abrupt changes in sign under freezing or thawing conditions opens another possibility to use SFI for characterizing the internal plant water status and resistance of trees to frost.
Additional information can be obtained when comparing relationships between both measured temperature gradients (dTsym and dTas) and their ratio.
This might help to understand redistribution of flow within the stems xylem which is still largely unknown and insufficiently understood compared to hydraulic redistribution in the soil by roots.
These temperature differences characterise the deformation of the heat field around the heater caused by the ascent of sap.
Beyond sap flow calculated from the measured temperature differences, they are very informative for tree physiology studies.
Symmetrical temperature difference is also known as the sap flow index (SFI) which may be used as a stress indicator.
The ability to measure this SFI with a HFD-sensor at several stem sapwood depths allows analysing the impact of different xylem layers in avoiding drought stress.
Another important feature of the SFI is its relationship with daily diameter changes.
For an apple tree it was shown that SFI is proportionally related to the diameter changes during the day and inversely related during the night.
Changes from proportional to inverse relationships occurred during SFI maxima, usually recorded on cloudless days in the early morning and in the evening.
Analysis of SFI in different trees showed that the period between the two daily SFI maxima and the corresponding decrease in SFI are usually larger for the outer xylem layers (in which higher sap flow rates are recorded) than for the inner xylem.
This is also true for the same (e.g. outer) xylem layers with increasing height along the tree stem.
This may reflect higher relative internal water storage consumption from highly conducting tissues.
SFI behaviour in large roots may differ from that typically observed for the aboveground part of a tree due to the high complexity of flow through the root tissues driven by transpiration and hydraulic redistribution towards the soil.
The ability of SFI to detect abrupt changes in sign under freezing or thawing conditions opens another possibility to use SFI for characterizing the internal plant water status and resistance of trees to frost.
Additional information can be obtained when comparing relationships between both measured temperature gradients (dTsym and dTas) and their ratio.
This might help to understand redistribution of flow within the stems xylem which is still largely unknown and insufficiently understood compared to hydraulic redistribution in the soil by roots.
Publication
Authors
N. Nadezhdina
Keywords
diameter fluctuation, frost, radial and circumferential variations, vector, water storage
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