Day One: Beginning Research and Duke Visit

Today marked the official beginning of my project.  The first order of business was to find some articles that seemed promising to the topic I was researching.  I found an article reviewing the different methods employed currently to measure whole-tree water use.  They included a heat balance method, a sector balance method, a heat dissipation method, and a heat pulse method.  Although I thought that the heat pulse method seemed very promising, it turned out that the heat dissipation method made popular by Granier (1987) was the best tradeoff in terms of quality of measurement versus price of equipment and robustness.  My review of the article is as follows:

Measurement of sap flow in plant stems: Smith D. M. et al. 1996

Measuring sap flow is a method of determining the water uptake rate and the transpiration rate of a plant.  Sap flow can be measured several different ways, with different methods for plants of different sizes.  Transpiration is a measure of the amount of water lost by a plant or tree.  Determining transpiration is important because it can help us determine whether there is climate change in the area. 

Methods of measuring Sap Flow:

• Stem Heat Balance Method:
• This method of measuring sap flow can measure stems ranging in diameter from 4mm to 125mm.  A flexible heater with layers of shielding are wrapped around the plant, and two thermocouple pairs measure the temperature gradients  and . 
• Gauges should be installed in strategic locations on the tree trunk in order to have the most accurate measurements.  Loose bark and other obstructions should be removed around the gauges in order to not interfere with the device.
• Constant power is supplied to the heater in commercially available systems and the voltage is adjustable.
• Trunk Sector Heat Balance Method
• This method is used for tree trunks with diameters larger than 120mm.  Heat is applied internally to the a segment of the trunk rather than to the entire circumference of the trunk.  The equation  describes the measurement of sap flow using this method.
• P is electrical power dissipated as heat
• Qv is heat lost vertically
• Qr is heat lost radially
• Ql is heat lost laterally
• Qf is heat lost by convection in the moving sap stream
• Heat Pulse Method
• Sap flow rate is measured by determining the velocity of a short pulse of heat carried by the moving sap stream
• Only used on woody stems
• The heater and sensor probes are drilled into the sapwood.  Heat is pulsed into one sensor (the heat is released for one or two seconds) and the heat is carried by the moving sap and affects the downstream sensor.  The temperature of the sensors will be equal again at some time within 60 seconds.  The velocity of the heat pulse can therefore be described by:
• Correct orientation of the gauges in this method is also essential.  Accurate spacing is required so that the measurements are correct. 

Applications:

These methods can be extrapolated to an entire stand of trees by way of measuring how much influence on tree’s sap flow measurement has on the entire stand.  Transpiration in a Douglas-fir stand was evaluated using the heat pulse method combined with measuring the contribution of each tree to the stand. For example, the average percent of plot sapwood for a dominant crown was 19.9%, while a suppressed crown was 8.7%.  

I rewrote my summary going a little more in depth about each technique of measuring sap flux:

There are several ways to measure transpiration for small plants, however, measuring transpiration in larger trees is more difficult.  Main methods of measuring transpiration include stem heat balance, trunk sector heat balance, heat-pulse, and thermal dissipation:

The stem heat balance method involves wrapping a heater and cork around the entire stem/trunk of the plant being measured.  The cork serves to protect the heater from being influenced by external heat and radiation.  A thermocouple installed slightly above the heater serves as a reference point to measure change in temperature.  Change in temperature, , is calculated.  The total heat applied to the tree can be split into three parts: vertical heat loss by conduction in the stem, radial heat loss by conduction, and heat uptake by the moving sap stream.  The heat uptake by the moving sap stream is the only value we want, so the other two components are calculated and subtracted from the total heat applied to the tree to arrive at the total sap flow value.  This measuring technique is only useful on herbaceous or woody stems less than 120mm in diameter. 

The trunk sector heat balance method, in contrast, can be used on plants greater than 120mm in diameter.  It shares the main principles as the stem heat balance method; the main difference is that only a section of the circumference of the plant is measured.  Five electrodes are embedded in a portion of the circumference of the tree and an electrical current is applied.   is measured between the five electrodes, and the total heat equation is calculated again.  In this method, heat lost radially is split into two calculations.  The first is radial heat lost to heartwood; the second radial heat lost to adjacent sapwood.  Mass flow of sap is then calculated and scaled to the entire cross section of the plant at that point. 

The heat-pulse method is significantly different from the first two methods.  In the heat-pulse method, three probes are inserted into the full depth of the sapwood.  There is a singular heater probe and two reference sensor probes containing miniature thermistors.  Heat is applied to the heater probe for a set amount of time, and the time-to-reference probe is measured.  Velocity of the heat pulse is measured, and then converted into sap velocity.  Expanding this measurement to the entire cross section of the tree will result in sap flux at a point.

Finally, the thermal dissipation method is used to determine sap flux and transpiration in plants.  The thermal dissipation method is only applicable when the depth of sapwood in a tree exceeds 20mm; however, there is no maximum depth constraint for this method.  Two probes are inserted into the trunk of the tree – one heater and one reference probe.  The heater probe is inserted 10-15cm above the reference probe and is constantly heated.  Changing sap flux rates will change the temperature gradient between the two probes.  Measuring this number will allow one to conver

In the afternoon, Dr. Oishi and Henry came over to NCSSM to look at the installation site.  The wires were not installed yet, so we couldn't do much setup outside.  Instead, we installed important software on my computer.  The power supply was also not present - Dr. Oishi said that he would go get one for me.  We talked a lot about theory behind measuring sap flux, as well as some problems that may arise.  One of the oaks may be either dead or close to it, according to maintenance, and so some of the probes may not take data as well as we want them to.  Dr. Oishi gave me a list of articles to read for our next meeting on Wednesday.  I looked at the PlantCam next to the dogwood, and it seems to still be taking pictures just fine.