The Calibration and Use of Capacitance Sensors to Monitor Stem Water Content in Trees

The overall goal of this methodology is to continuously monitor the volumetric water content in the stems of mature living trees. This method can help answer key questions in the field of ecohydrology, such as how vegetation responds to moisture limitation, including when water stress begins and how quickly drought recovery occurs. The main advantage of this technique is that it allows for automated measurements of biomass water content and C2 at subhourly timesteps.

The implications of this technique extend toward remote sensing because measurements of above-ground water content are necessary for the partitioning of remotely-sensed data products and to above-and below-ground components. Though this method can provide insight into drought response it can also be extended to other phenomena, such as monitoring fuel water content for forest fire ignition prediction. This is my undergraduate Rio Mursinna, who will help us with selecting a branch and harvesting branch segments.

This is Kelly Malone, an undergraduate research assistant in my lab. After selecting a tree for instrumentation as described in the text protocol, proceed with collecting about four meters or more of wood from the trunk or a branch that is at least six centimeters in diameter. Larger diameters are better.

Next remove any attached branches, leaves, lichen, or any other loose material. Then slice the sample into cylindrical sections that are about 15 centimeters long. Make at least 25 such sections.

Then label each segment and record each segment's average diameter and length. Also record each segment's volume, assuming a cylindrical shape. Set aside a segment from the center and from the end for the baseline analysis.

Then place one-third of the remaining segments in a water bath to rehydrate, and put the other two-thirds in a drying oven set to 60 degrees Celsius to dry. To begin, connect a capacitance sensor to a data-logging device and to a computer for real-time sensor readout visualization. Set it to collect data on 30-second intervals.

Begin with analyzing the segments saved for the baseline analysis. Begin with drilling two new sets of holes for the sensor in each segment. Make the hole diameter slightly smaller than the tines.

Place the hole sets about two centimeters apart vertically and 150 degrees apart radially. Next, weigh the segments to the nearest hundredth of a gram. Then seal the ends of the segments with plastic wrap so they do not dry out.

Now clean the tines of the capacitance sensor with alcohol and insert them into one set of drilled holes such that the tines are completely buried in the wood. Next, wait a few minutes for readings to stabilize. Then collect 10 measurements over five minutes.

Use the average value for future analyses. Now, gently remove the sensor and clean the tines with alcohol. When the output readings return to zero, take 10 measurements from the second set of holes.

After taking measurements, remove the plastic and place the segments in the oven. During the first two days of drying, there is a lot of moisture loss, so take measurements twice a day. Later, take measurements just once per day.

During each measurement session, only drill and analyze one or two segments. For the rehydrating segments, measure them once daily until each has been measured. Always dry them off before taking measurements.

Once these segments have been fully rehydrated, transfer them to the oven to desiccate. To begin, select a location for the sensor installation on the tree of interest and record the stem diameter at that location. The standard pairing of sensor locations it to place one about a half meter above the ground into the trunk, and place the second just below the first major branching split.

At the selected locations, use a draw blade to remove the bar to expose the cambium to create a flat surface where the sensor will be installed. The sensor must sit flush against the surface with no parts of the tines exposed, and the measurements must only come from the water content in the xylem, excluding the water content of the bark or phloem. Drilling appropriately-sized holes at the correct alignment and the correct depth is essential for the functionality of this method.

If the sensor meets with too much resistance, carefully back the sensor out and widen the holes. Next drill the holes for the tines. In soft wood, use a drill bit slightly smaller than the diameter of the tines, whereas in hard wood, use a bit closer to the true size of the tines.

Prior to installing a sensor, clean it with alcohol. When attempting to insert it, if there is too much resistance, use the drill to widen the holes slightly. Ultimately, all three tines should be fully inserted with the body of the sensor flush against the tree trunk.

Use a silicone-based sealant to seal the sensor to the tree and prevent infestation. Then cover the sensor with reflective insulation to avoid external heating, and connect all of the sensors to a power source and to a compatible data logger. Using the laboratory-based technique with stem segments, calibrations were performed for five common Eastern forest tree species, representing a variety of wood densities and xylem structures.

Calibration curves different by as much as 97.7%for populus grandidentata and acer rubrum, demonstrating to the need to perform species-specific calibration to obtain accurate VWC measurements. To evaluate the observed diurnal dynamics, the rate of change in total water content was compared to sap flux data, collected simultaneously in the same tree using Granier-style thermal dissipation sensors. Next, three acer rubrum trees were studied in the field.

While total stem water storage is dependent on the volume of the trunk, all three individuals exhibited patterns of recharge and replenishment, consistent with seasonal trends and sap flux and available moisture in the top meter of soil. Stem-stored water, sap flux, and soil moisture were all at their lowest during the end-of-summer extended interstorm period. From this data, the drought recovery time for acer rubrum is estimated at 10 days.

After watching this video, you should have a good understanding of how to calibrate, install, and operate a capacitance sensor to monitor water content in a living biomass. Following this procedure, additional sensors could be added along the branching structure to perform a whole-tree water balance. After its development, this technique paved the way for researchers in ecohydrology and land-surface modeling to evaluate model performance on the basis of ecosystem water storage and energy flux.