For large plants, the fundamental sap flow sensor tends to follow one of two basic designs: the thermal dissipation method (Granier, 1985) or the heat-pulse method (Burgess et al., 2001). What is the next step? What are the biggest limiting factors with the current sensor designs (e.g. price, power requirements, heterogeneity between sampling locations, representative measurements of individual species). Should sap flow remain measured thermometrically or is there a better proxy for transpiration/vegetation storage?
Hy Tyler,
What about ultrasound ?
Ultrasonic acoustic emission (UAE) in trees are often related to collapsing water columns in the flow path as a result of tensions that are too strong (cavitation). However, in a decibel (dB) range below that associated with cavitation, a close relationship has been found between UAE intensities and stem radius changes. UAE continuously recorded from stems of mature field-grown trees of Scots pine (Pinus sylvestris) and pubescent oak (Quercus pubescens) at a dry inner-Alpine site in Switzerland over two seasons. The averaged 20-Hz records are related to microclimatic conditions in air and soil, sap-flow rates and stem-radius fluctuations detrended for growth (ΔW). Hence a candidate for sap flow?
CHeers,
Frank
Frank, thanks for the response. Is this the Zweifel-Zeugin (2008) paper? Their findings are quite interesting. It would be interesting to see if piezoelectric measurements are as representative for larger trees (>23 cm bole diameter).
Hi Michael. Thanks for the response. I have heard of the heat deformation method before, but was not aware of the paper you sent, so thanks for the article! I would argue that HFD does not give us a new sensor design beyond the thermal dissipation and heat pulse methods (i.e., it is still comprised of just a heater probe and temperature probes); rather, it is a novel configuration of existing sensor types. It is theoretically based, which I like, but it still requires little-known field/tree specific parameters (e.g., thermal conductivity of the woody matrix), most likely stochastic in nature, which leads to being as reliable as the simpler empirical method, in my opinion. Plus, with four sensor probes and constant heat requirements, this method has higher sensor maintenance cost than both the thermal dissipation and heat pulse methods. As an aside, someone once asked me if it would be possible to use a passive solar-powered heating element for monitoring sap flow. While I'm not sure this would ever work, it would be remarkable if it could be done.
Hi Tyler,
Sorry for the delay in reply. Had to to go to the Alps for a hike.
But I added the paper of Zweifel and Zeugin on ultrasound and tree growth. The technique is promising but still some work to do before sap flow can be isolated from the signal. Nice topic for sure.
Cheers,
Frank
No TDP method is still the best Jerry, the best!
Cheap (if you make the sensors yourself) and very reliable.
I agree with JC! Cheap and reliable TD means more replication, which captures more of the inherent variation in your study system. Enough replicate probes, and you can actually understand the causes of variation. Sufficient replicate probes and you can at least get a decent mean value. Where more expensive systems are found, there will always be trade offs on replication. Future efforts should be aimed at post-processing for improved accuracy, and energy efficiency. Unfortunately there is no “plug-and-play“ system that works equally well in all applications without smart installation and smart post-processing.
Dear Tyler,
Regarding the TDP method you can have a look to a recent open access paper published in the Sensor Journal by Pasqualotto et al. titled " Calibration of Granier-Type (TDP) Sap Flow Probes by a High Precision Electronic Potometer".
As regards the heat pulse (HP) method, I used it during my PhD and I should say that, even if probes were more expensive and delicate (the thermistors are inserted in thin teflon tubes) than TDP probes, overall the HP method worked very well during three years of experiment in umbrella pine in a Mediterranean coastal pinewood. With the HP method problems might arise for a misalignment of the probes (parallax error), xylematic flow perturbation due to the wound (also in TDP), incorrect estimation of sap flow due to radial variability (also in TDP), incorrect estimation of sap flow (especially in the early morning) due to thermal differences between sap in the trunk and sap arising from the soil (a problem this affecting differently HP and TDP methodologies but always related to temperature differences between the reference and the heated probes).
Kind regards,
Maurizio
Here some links:
Article Water table salinity, rainfall and water use by umbrella pin...
Article Calibration of Granier-Type (TDP) Sap Flow Probes by a High ...
Thesis Relazioni idriche e risposta ecofisiologica del Pino domesti...
I still think that stable isotope tracing is an interesting approach (see Schwendenmann et al. 2010,Tree Physiology, 30 (7), 886–900).
My former colleague, Maurits Vandegehuchte, obtained a PhD on improving sap flow sensors, at the laboratory of Kathy Steppe. The SapFlow+ sensor they designed (https://doi.org/10.1111/j.1469-8137.2012.04237.x) is far more advanced than the currently available commercial sensors, while basically as expensive as these sap flow sensors. The sensor will now be commercialized, it's best to contact Kathy Steppe (kathy.steppeATugent.be) for more details.
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